major project, hvdc thesis - saurabh saxena

STUDY OF HVDC BACK-TO-BACK COUPLING

SCHEME WITH CASE STUDY OF

VINDHYACHAL GRID

Project No. – 7

Submitted as a

Major Project for the

Degree of Bachelor of Engineering

Year 2005-2006

Guided by: Submitted by:

Miss Madhu Gupta Rashmi Jain

Saurabh Saxena

Vaseem Mansuree

Department of Electrical Engineering

SHRI VAISHNAVSM

INSTITUTE OF TECHNOLOGY AND SCIENCE

INDORE

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STUDY OF HVDC BACK-TO-BACK COUPLING

SCHEME WITH CASE STUDY OF

VINDHYACHAL GRID

Major Project

A Dissertation submitted to

Rajiv Gandhi Proudyogiki Vishwavidhyalaya, Bhopal

towards partial fulfillment of the

Degree of Bachelor of Engineering

in

Electrical Engineering

Year 2005-2006

Guided by: Submitted by:

Miss Madhu Gupta Rashmi Jain

Saurabh Saxena

Vaseem Mansuree


SHRI VAISHNAVSM

INSTITUTE OF TECHNOLOGY AND SCIENCE

INDORE

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This is to certify that Miss Rashmi Jain (0802EE033D06), Mr. Saurabh Saxena

(0802EE021050) & Mr. Vaseem Ahmad Mansuree (0802EE021054) students of Final

Year (VIII Semester), Electrical Engineering Branch, working in a group have successfully

completed the required work for this semester for the major project no. 7 titled “STUDY

OF HVDC BACK-TO-BACK COUPLING SCHEME WITH CASE STUDY OF

VINDHYACHAL GRID”. This project work is a part of the syllabus prescribed by

R.G.P.V. under the subject “Major Project” for the academic year 2005-06.

Project Guide Head of Department

Internal Examiner External Examiner


SHRI VAISHNAVSM INSTITUTE OF TECHNOLOGY AND SCIENCE INDORE

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No great tasks can be completed successfully without suitable functional

environment and proper guidance. We are thankful to the board of education for

giving us a chance to apply our theoretical knowledge to develop practical skills

through this project.

We feel immense pleasure and deep feeling of gratitude towards Miss

Madhu Gupta (Lecturer, Department of Electrical Engineering) for encouraging us

in choosing this project and guiding us with constructive and valuable suggestions

and constant motivation, which not merely helped but enabled us to complete the

report. We express our gratitude towards Proff. R. N. Paul (HOD, Electrical

Engineering Department) for his guidance and timely advice for the preparation of

the report.

We are also thankful to Mr. M. C. Sahu (D.G.M., HVDC BTB

Vindhyachal, PGCIL), Mr. A. K. Pandey (Manager, HVDC BTB, Vindhyachal,

PGCIL), Mr. Praveen Ranjan (Dy. Manager, HVDC BTB, Vindhyachal, PGCIL)

for their guidance and providing functional environment during our visit to

Vindhyachal BTB station.

And finally heartfelt appreciation to all those persons, who were directly

and indirectly, helpful in completing this report.

Rashmi Jain

Saurabh Saxena

Vaseem Ahmad Mansuree

Contents

CONTENTS

Page No.

1. Synopsis

1. Aim 1

2. Objectives 2

3. Introduction 3

3.1 Introduction to HVDC 3

3.2 HVDC scenario in India 4

3.3 Selection of voltage level for HVDC transmission 5

3.4 Cost structure of HVDC 7

3.5 HVDC connection schemes 8

4. EHV-AC versus HVDC 11

4.1 Technical considerations 11

4.2 Economical considerations 13

5. HVDC back to back interconnection 15

5.1 Significance 15

5.2 Overview of operation 15

6. Substation configuration 17

6.1 Converter bridge unit 18

6.2 Converter transformer 20

6.3 Smoothing reactor 23

6.4 Filters 23

6.5 Reactive power sources 24

6.6 Transmission medium 25

6.7 DC switchgear 25

6.8 Earth electrode 25

7. Future work to be done 26

8. Utility and application of the project 26

9. Conclusion 27

2. Introductions

Contents

3. Chapter 1: Converter analysis

1.1 Thyristor valve 30

1.1.1 General 30

1.1.2 Valve design consideration 30

1.1.3 Valve firing 31

1.1.4 Recent trends 32

1.2 Choice of converter configuration 32

1.2.1 Valve rating 33

1.2.2 Transformer rating 34

1.3 Analysis of Graetz circuit 34

1.3.1 General 34

1.3.2 Analysis without overlap 36

1.3.3 Analysis with overlap 40

1.3.4 Inversion 48

1.4 Steady state equivalent circuit 51

4. Chapter 2: HVDC control

2.1 Introduction 52

2.2 Principle of DC link control 52

2.2.1 Desired features of control 53

2.3 Voltage–current characteristics for HVDC converter 55

2.3.1 Individual characteristics 55

2.3.2 Combined characteristics 57

2.4 Basic control system 58

2.4.1 Firing angle control 59

2.4.2 Constant minimum ignition angle control 60

2.4.3 Constant current control 60

2.4.3 Constant extinction angle control 62

2.5 Master control 62

2.6 Higher level controllers 62

2.7 System control hierarchy 63

2.8 Reactive power control 64

2.8.1 Introduction 64

2.8.2 Steady state reactive power requirement 65

2.8.3 Sources of reactive power 70

Contents

5. Chapter 3: Harmonics and filters

3.1 Introduction 75

3.2 Generation of harmonics 77

3.2.1 Generation on AC side 77

3.2.2 Generation on DC side 77

3.3 Characteristic harmonics 78

3.3.1 Harmonics at no overlap 79

3.3.2 Harmonics with overlap 81

3.4 Non-Characteristic harmonics 83

3.4.1 Causes 83

3.4.2 Amplification 84

3.4.3 Consequences 84

3.5 Troubles caused by harmonics 84

3.6 Means of reducing harmonics 85

3.6.1 Increased pulse number 85

3.6.2 Application of filter 85

3.7 Filters 86

3.7.1 Purpose 86

3.7.2 Classification 86

3.7.3 Cost 87

3.7.4 AC filters 88

3.7.5 DC filters 89

6. Chapter 4: Converter faults and protection

4.1 Introduction 90

4.2 Converter Faults 90

4.2.1 General 90

4.2.2 Arc-back 91

4.2.3 Arc-through 92

4.2.4 Misfire 92

4.2.5 Quenching (current extinction) 92

4.2.6 Commutation failure 93

4.2.7 Short circuit in bridge 95

4.3 Protection 95

4.3.1 General 95

4.3.2 DC reactor 96

4.3.4 Voltage oscillations and valve dampers 96

4.3.5 Current oscillations and anode dampers 97

Contents

7. Chapter 5: Case study

5.1 Introduction 98

5.2 Location of Vindhyachal HVDC Back-to-Back station 98

5.3 Technical information and data 98

5.4 Nominal ratings 99

5.5 Single line diagram 100

5.6 Equipments 101

5.7.1 Converter transformer 101

5.7.2 Thyristor valve 103

5.7.3 Smoothing reactor 107

5.7.4 Filter and shunt bank 108

5.7 System control and auxiliary power 109

5.7.1 Control hierarchy 109

5.7.2 Control modes 109

5.7.3 Block control 110

7.7.4 Station level controller 111

5.8 Other auxiliaries 112

5.8.1 Valve cooling system 112

5.8.2 D.G. Set 113

5.8.3 PLCC Room 113

5.8.4 Battery Room 114

5.8.5 Battery Charging Room 114

5.8.6 Fire Fighting System 114

5.9 Operations and maintenance 115

8. Conclusions 116

9. Bibliography 117

Synopsis 1

AIM

Study and analysis of HVDC back-to-back coupling scheme &

case study of Vindhyachal HVDC back-to-back interconnection

between Western and Northern Regions.

Synopsis 2

OBJECTIVES OF THE PROJECT

¾�To understand the basic operation of HVDC interconnection.

¾�To study and analyze converter operation.

¾�To study the various design considerations of converter transformer

¾�To study the problem of harmonics during converter operation

¾�To analyze the operation of filters and smoothing reactor

¾�To study reactive power requirement and compensation schemes

adopted in HVDC

¾�To study and analyze the control parameters governing the

magnitude and direction of power flow.

¾�To Study the coupling scheme adopted at Vindhyachal back-to-back

interconnection between Northern region and Western region.

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Synopsis 3

INTRODUCTION

1. Introduction to HVDC

Early electric power distribution schemes used alternating-current

generators located near the customer's loads. As electric power use became

more widespread, the distances between loads and generating plant

increased. Since the flow of current through the distribution wires resulted in

a voltage drop, it became difficult to regulate the voltage at the extremities

of distribution circuits. A generator connected to a long ac transmission line

may become unstable and fall out of synchronization with a distant ac power

system.

An HVDC transmission link may make it economically feasible to use

remote generation sites. HVDC transmissions make an important

contribution to controlling power transmissions, safeguarding stability and

containing disturbances.

In an HVDC transmission, electric power is taken from a three-phase

.AC network, converted to DC in a converter station, transmitted to the

receiving point by a cable or overhead line and then converted back to AC in

another converter station and injected into the receiving AC network. As the

conversion process is fully controlled, the transmitted power is not dictated

by impedances or phase angle differences, as is the case with AC.

The investment costs for HVDC converter stations are higher than for

high voltage AC substations. On the other hand, the costs of transmission

medium (overhead lines and cables), land acquisition/right-of-way costs are

lower in the HVDC case. Moreover, the operation and maintenance costs are

lower in the HVDC case. Initial loss levels are higher in the HVDC system,

but they do not vary with distance. In contrast, loss levels increase with

Synopsis 4

distance in a high voltage AC system. The following picture shows the cost

breakdown (shown with and without considering losses).

)LJXUH��9DULDWLRQ�RI�&RVWV�RI�$&�YHUVXV�'&�7UDQVPLVVLRQ�The breakeven distance depends on several factors, as transmission

medium (cable or OH line), different local aspects (permits, cost of local

labor etc.). When comparing high voltage AC with HVDC transmission, it

is important to compare a bipolar HVDC transmission to a double-circuit

high voltage AC transmission, especially when availability and reliability

is considered.

2. HVDC Scenario in India

In India, HVDC technology is new and presently only seven HVDC

links are under operation and two links are under construction. These are

Commissioned Projects

1. HVDC Back to Back station, Vindhyachal (Madhya Pradesh)

2*250 = 500 MW (Northern region and Western region)

2. HVDC Back to Back station, Sasaram (Bihar)

1*500 = 500 MW (Eastern Region and Northern Region)

3. HVDC Back to Back station, Vijag (Andhra Pradesh)

2*500 = 1000 MW (Southern region and Eastern region)

Synopsis 5

)LJXUH��,QGLDQ�+9'&�,QWHUFRQQHFWLRQ�4. HVDC Back to Back station, Chandrepur (Maharashtra)

2*500 = 1000 MW (Western region and Southern region)

5. HVDC Bipolar line; Chandrapur (MH) to Padeghe (MH)

Ratings: ± 500 kV, 1500 MW, 850 km

6. HVDC bipolar line; Talser to Kollar (Karnataka)

Ratings: ± 500 kV, 2000 MW, 1700 km.

7. HVDC Bipolar Line; Rihand (UP) to Dadri (Delhi)

Ratings: ± 500 kV, 1500 MW, 800 km

Under Commissioning

8. HVDC Bipolar line : Baliya (UP) to Bhivadi

Ratings: ± 500 kV, 2000 MW, ~ 850 km

9. HVDC Bipolar line: Arunachal Pradesh to Agra (UP)

Ratings: ± 800 kV, 3500 MW, 2000 km

Synopsis 6

3. Selection of transmission voltage

Transmission voltage is selected taking into account the line cost and

converter cost. With the increase in voltage level, converter cost increases

gradually on account of increase in voltage rating while the line cost (which

is a function of many parameters) shows the characteristics as shown in

figure.2

Optimum system voltage at which power can be transmitted most

economically is given by the minimum separation between the two curves.

That optimum system voltage may or may not be equal to the optimum line

voltage as shown in figure 3.

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Synopsis 7

2. Cost Structure of HVDC

The cost of an HVDC transmission system depends on many factors,

such as power capacity to be transmitted, type of transmission medium,

environmental conditions and other safety, regulatory requirements etc.

Even when these are available, the options available for optimal design

(different commutation techniques, variety of filters, transformers etc.)

render it is difficult to give a cost figure for an HVDC system.

Nevertheless, a typical cost structure for the converter stations could

be as follows:

)LJXUH��&RVW�6WUXFWXUH�VKRZLQJ�WKH�YDULRXV�FRVW�GHWHUPLQLQJ�SDUDPHWHUV�RI�+9'&�

Synopsis 8

HVDC CONNECTION SCHEMES

1. Monopolar Link

Monopolar HVDC system has only one conductor, usually of negative

polarity (pole) and return path is provided by permanent earth or sea. This

system is used only for low power transmission. Earth electrodes are

designed for continuous full-current operation and for overload capacity

required in the specific case.

)LJXUH��6FKHPDWLF�'LDJUDP�RI�0RQRSRODU�+9'&�/LQN 2. Homopolar Link

Homopolar link has two or more conductors all having the same

polarity, usually negative; all operates with the ground return. In the event of

fault on one conductor, the entire converter is available for connection to the

remaining conductor or conductors, which have some overload capability,

can carry more than half of the rated, power and perhaps the whole rated

power, at the expense of increased line losses.

It has advantage of lower power loss due to corona and smaller radio

interference due to negative polarity.

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Synopsis 9

3. Bipolar Link

Bipolar link has two conductors, one positive and other negative.

Each terminal has two converter of equal rated voltage. The neutral point of

one or both end is grounded. In the event of fault on one conductor, the other

conductor with ground return can carry up to the half of the rated load. The

voltage between poles is twice that of pole to earth voltage therefore its

typical rating can be expressed as 500 kV, 1500 MW.

)LJXUH��6LQJOH�/LQH�'LDJUDP�RI�%LSRODU�+9'&�/LQN�4. HVDC Back-to-Back Coupling Scheme

HVDC coupling scheme is used for interconnection between

adjacent AC networks for the purpose of frequency conversion or for

asynchronous interconnection.

)LJXUH��6LQJOH�/LQH�'LDJUDP�RI�%DFN�WR�%DFN�+9'&�/LQN�Rectifier and inverter are connected to form a DC closed loop. There

is no DC transmission line and DC smoothing reactor is connected to de

Synopsis 10

loop. Rectifier and inverter are installed in the same station. The exchange of

power can be controlled, both in direction and magnitude, without can be

controlled without transferring frequency disturbances.

5. Multi-terminal HVDC Scheme

Multi-terminal HVDC scheme is used for asynchronous

interconnection of two or more AC network. This scheme offers an effective

way of large power transfer along with improvement in system stability.

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Synopsis 11

EHV-AC VERSUS HVDC SYSTEM

1. Technical Considerations

1.1 Stability of Transmission System

HVDC gives asynchronous tie and transient stability does not pose

any limit on power transfer. Line can be loaded up to thermal limit of the

line or valves. But in AC system to maintain its stability under transient

condition, it remains in synchronism.

1.2 Short Circuit Level

In AC transmission, when an existing AC system is interconnected

with another AC system, the fault level of both the system increases.

However, when both are interconnected by DC transmission, the short

circuit current is not increased so much as for DC line contributes no current

to an AC short circuit beyond its rated current.

1.3 Corona Losses and Radio Interference

For the same power transfer and same distance, the corona losses and

radio interference of DC system is less than that of AC system, as the

required dc insulation level is lower then corresponding ac insulation.

1.4 Line Loading

The permissible loading of an EHV-AC line is limited by transient

stability limit and line reactance to almost one third of thermal rating of

conductors, no such limit exists in case of HVDC lines.

1.5 Skin Effect

This is absent in dc current, as current density is uniformly distributed

across the cross-section of the conductor while the effective resistance in AC

conductor is increased due to skin effect.

Synopsis 12

1.6 Surge Impedance Loading

Long HVAC lines are loaded to less than 0.8 PN (Surge impedance

loading or natural loading of line). No such condition is imposed on HVDC

line.

1.7 Voltage along the line

Long HVAC line has varying voltage along the line due to absorption

of reactive power. This line remains loaded below its thermal limit due to

the transient stability limit. Such problem does not arise in HVDC line and it

gives almost flat voltage profile.

1.8 Reactive power requirements

HVDC line does not need intermediate reactive power compensation

like in HVAC line but it requires reactive power at converter terminals. The

required reactive power varies with the transmitted power and is about 60 %

of the total active power transferred per converter station. Usually shunt

capacitors or synchronous condenser are installed for supplying reactive

power.

1.9 Skin Effect

This is absent in DC and hence current density is uniformly

distributed across the cross section of the conductor, this leads to reduced

heating effect and optimum conductor utilization

1.10 Rapid power transfer

The control of converter valves permit rapid changes in magnitude

and direction of power flow. Limitation is imposed by power generation and

AC system. For AC line power per phase can be given as

Pac = {[ | V1 | . | V2 | ] Sin δ }/ X Watts / phase

Synopsis 13

The AC line can be loaded to transient stability limit which occurs at

δ=300 and given by,

Pac = { | V1 | . | V2 | } / 2 X Watts / phase

AC power cannot be changed easily, quickly and accurately as V1 and

V2 should kept around rated and δ can not be changed quickly, while power

flow through DC line can be given as

PDC = {Vd1 – Vd2}* Vd / R Watts / pole

By varying Vd1 and Vd2 by means of thyristor converter control and

tap changer control, PDC can be quickly, accurately and easily controlled.

Ramping rate (the rate at which magnitude of power transfer can be varied)

can be as high as 30 MW / minute.

1.11 Transmission through cables

DC transmission can be through underground or marine cables since

charging currents are taken only while energizing the DC link and are not

present continuously. In AC system, there is limit on length of cable

depending upon rated voltage. This limit is about 60 km for 145 kV, 40 km

for 245 kV and 25 km for 400 kV AC line.

2. Economical Considerations

2.1 Substation cost

Substation cost of HVDC is very high owing to costly terminal

equipments like converter, filters, converter transformer, complex control

equipments etc, while initial cost of HVDC terminal substation is very low.

2.2 Number of lines

HVAC needs at least two-three phase lines and generally more for

higher power. HVDC needs at maximum, only one bipolar line for majority

of application.

Synopsis 14

2.3 No of conductors

Bipolar HVDC transmission lines require two-pole conductors to

carry DC power. Hence HVDC transmission becomes economical over ac

transmission at long distance with the saving in overall conductor cost,

losses, towers etc.

2.4 Right of way

Right of way for DC line is low as compared to that of AC transmission

system

2.5 Cost of towers

More number of conductors require high tower strength to stand with

the mechanical forces and weight. This increases the cost of AC towers

while in case of DC tower has to carry only two lines and a compact

structure is sufficient.

Synopsis 15

HVDC BACK TO BACK INTERCONNECTION

1. Significance of HVDC back-to-back interconnection

Interconnections between grids are desirable because they not only

permit economies through the sharing of reserves, but they also make the

trading of electricity between grids possible. Interconnections allow power

consumers to benefit from generation at the site of lowest incremental cost.

On the downside, however, disturbances can easily spread from one area to

another.

Major blackouts in recent years highlight the vulnerability of large AC

systems and have shown how relatively minor malfunctions can have

repercussions over wider areas. As one link overloads it is tripped,

increasing the strain on neighboring links, which in turn disconnect,

cascading blackouts over vast areas and causing huge productivity losses for

the economy.

HVDC back-to-back coupling scheme play a significant role in

interconnecting the power systems as it not only allows the precise and

reliable transfer of power but also prevent the frequency disturbances to

transfer from one system to another. A HVDC link can fully control

transmission but does not overload or propagate fault currents.

2. Overview of Operation

A back-to-back station is system for power transfer in which both

static inverters are in the same area, usually even in the same building and

the length of the direct current line is only a few meters. Figure 10 shows the

detailed schematic diagram of HVDC back to back coupling scheme,

main components of any such schemes are as follows:

Synopsis 16

• Converter

• Converter transformer

• Shunt compensators

• Smoothing reactor

• AC Filter

)LJXUH��&RQILJXUDWLRQ�RI�EDFN�WR�EDFN�+9'&�VXEVWDWLRQ�In an HVDC coupling scheme, electric power is taken from one grid

(three-phase AC network), converted to DC in a converter station, fed to the

receiving point (inverter) and then converted back to AC in another

converter station and injected into the receiving AC network. As the

conversion process is fully controlled, the transmitted power is not dictated

by impedances or phase angle difference, as is the case with AC. Earthing is

only for reference, it does not carry any direct current and there are no

problems of galvanic corrosion of substation earth and underground pipes,

structure etc.

Synopsis 17

HVDC SUBSTATION CONFIGURATION

At HVDC converter station, conversion from AC to DC (rectification)

or DC to AC (inversion) is performed. Role of rectifier and inverter can be

reversed using suitable converter control configuration.

A point to point transmission requires two converter stations. While In

a back-to-back station, both rectifier and inverter station are usually installed

in a single valve room with the converter transformer installed on either side

of valve room (Hall) and the DC bushings are taken indside the valve hall

for connection to the valves.

)LJXUH��+9'&�6XEVWDWLRQ�&RQILJXUDWLRQ�Figure 11 shows the typical arrangement of the converter substation.

One of the main components of a converter substation is the thyristor

converter, which is usually housed in a valve hall. As seen from figure, the

Synopsis 18

substation also essentially consists of converter transformers. These

transformers transform the ac system voltage based on the dc voltage

required by the converter. The secondary or dc side of the converter

transformers is connected to the converter bridges. The transformer is placed

outside the thyristor valve hall, and the connection has to be made through

the hall wall. This is accomplished in two ways: 1) with phase isolated bus

bars where the bus conductors are housed within insulated bus ducts with oil

or SF6

as the insulating medium, or 2) with wall bushings, and these require

care to avoid external or internal breakdown.

Filters are required on both ac and dc sides since the converters

generate harmonics. The filters are tuned based on the converter operation (6

or 12 pulse). DC reactors are included in each pole of the converter station.

These reactors assist the dc filters in filtering harmonics and mainly smooth

the dc side current ensuring continuous mode of operation. Surge arrestors

are provided across each valve in the converter bridge, across each converter

bridge, and in the dc and ac switches to protect the equipment from over

voltages.

1. Converter Bridge Unit

This usually consists of two three phase converter bridges connected

in series to form a 12 pulse converter unit. The total numbers of valve in

such a unit are twelve. The valves can be packaged as single valve, double

valve or quadrivalve arrangements. Each valve is used to switch in a

segment of an AC voltage waveform. The converter is fed by converter

transformer connected in star/ star and star/delta arrangement.

Synopsis 19

)LJXUH��7ZHOYH�SXOVH�EULGJH�XQLW�The valves are cooled by air, oil, and water. Liquid cooling using

deionized water is more efficient and results in the reduction of station

losses. The ratings of valve group are limited by more permissible short

circuit current than steady state load requirement. The design of valve is

based on the modular concepts where each module contains a limited

number of series connected thyristor level.

Valve firing signals are generated in the converter control at ground

potential and are transmitted to each thyristor in the valve through a fiber

optic light guide system. The light signal received at the thyristor level is

converted to an electrical signal using gate drive amplifier with pulse

transformer.

Synopsis 20

2. Converter Transformer

The HVDC converter transformer is a very important component in a

HVDC transmission system. 25 –30 % cost of the converter station is

determined by the cost of converter transformer. In addition to its normal

application to provide transfer of power between two voltage levels, it serves

a number of additional functions like galvanic separation between the AC

and DC systems. A fairly large tapping range permits optimum operation

also for a large variation in load without loss of efficiency.

The converter transformer is generally built with two valve windings

of equal power and voltage ratings. One of the windings is connected in star

and the other in delta. With this arrangement the dominant harmonics from

the converter will be cancelled out. Windings, which are directly connected

to AC system, are termed as line windings while winding connected to the

converter is called valve windings.

The HVDC converter transformer can be built as three-phase or as

single-phase units depending on voltage and power rating. When built as

three-phase transformer there is generally one unit with the valve winding

arranged for star connection and the other delta connection. In single-phase

design the two valve windings are generally built on the same transformer

unit.

Synopsis 21

2.1 Winding Connection

The generation of harmonics is an undesirable feature in the converter

equipment and in order to minimize these, 12 pulse converter is normally

used. It is usual to arrange both star and delta connected valve windings

have a common star connected primary line winding.

2.2 Insulation Design

The insulation design of HVDC converter transformer is determine by

following factors:

1. The AC voltage distribution and DC bias voltage which is a

function of dc system voltage experienced by valve winding

2. The DC voltage experience a voltage polarity reversal when the

direction of power flow is reversed

3. The behavior of insulating materials, paper, pressboard and oil,

differs greatly in its response to DC stress than it does to AC

stress.

In the case of a system subjected to a DC stress the distribution is

determined by material dimension and their resistivity and in case of

converter transformer, as combination of AC and DC stresses occur in

practice.

2.3 Harmonics Consideration

The harmonics add considerably to the stray losses in the transformer

windings, core and structural work and due allowances must be made for

their effect. Reduction is harmonics in line side is achieved by the use of

connecting filters.

Synopsis 22

2.4 Commutating Reactance and Short-Circuit Current

Fault current in the case of converter transformer is likely to contain a

very much greater DC component than is the case for normal transformer

and unlike the in the case of fault in the conventional AC circuit fro which

the DC component decays very rapidly, for converter circuit the high DC

component will continue until the protection operates. The resulting

electromagnetic forces can therefore be very significant. These forces can be

kept within the limits by either higher impedance which result in high

regulation or, the use of tap changers, which in addition to control of valve

firing angle to control the power flow will often have up to 50% grater range

than conventional transformer, so the need to limit the variation of

impedance with tap position becomes an important consideration in

determining the winding configuration.

2.5 Configuration

THE converter transformer can have different configuration (1) three

phase, two winding,(2) single phase , three winding (3) single phase two

winding. The valve side windings are connected in star and delta with

neutral point ungrounded. On the AC side, the transformers are connected in

parallel with neutral ground. The leakage reactance of the transformer is

chosen to limit the short circuit current through any valve.

In back to back links, which are designed for low DC voltage level, an

extended delta configuration can result in identical transformer being used in

twelve pulse converter units. This result in the reduction of the spare

capacity required.

Synopsis 23

3. Smoothing Reactors

The main purpose of a smoothing reactor is to reduce the rate of rise

of the direct current following disturbances on either side of the converter.

Thus the peak current during the dc line short circuits and ac commutation

failure is limited.

For satisfactory current conversion in thyristor-converters and to

eliminate pulses from DC current waveform, a large series inductance (L) is

necessary on DC side. A DC Smoothing reactor (smoothing reactor) is a

high inductance coil connected in series with the main DC pole circuit

between Converter Bridge and DC line-pole. Due to high inductance (L) the

current (Id) stores high energy (e=1/2 L*Id2) in the reactor coil. The current

in an inductance cannot change instantaneously. Hence the fluctuations and

pulses in the direct current Id are smoothened thus the function of the

smoothing reactor is to eliminate the pulses and fluctuations in DC current

waveform, i.e. to smoothen the DC current.

The reactor blocks the non- harmonic frequencies from being

transferred between two ac systems, and also reduces the harmonics in the

dc line.

4. Filters

There are three types of filter used in HVDC System

4.1 AC Filters

Filters are used to control the harmonics in the network. The filter

banks compensate the reactive power consumed by the converters at both the

ends. For example, in CCC (capacitor commutated converter) reactive power

is compensated by the series capacitors installed between the converter

transformer and the thyristor valves.

Synopsis 24

4.2 DC Filters

The harmonics created by the converter can cause disturbances in

telecommunication systems, and specially designed dc filters are used in

order to reduce the disturbances. Generally, filters are not used for

submarine or underground cable transmission, but used when HVDC has an

overhead line or if it is part of an interconnecting system. The modern filters

are active dc filters, and these filters use power electronics for measuring,

inverting and re-injecting the harmonics, thus providing effective filtering.

4.3 High frequency filter

These are connected between the converter transformer and the station

AC bus to suppress any high frequency current.

5. Reactive Power Sources

Converter station requires power supply that is dependent on the

active power loding. Fortunately, part of this reactive power requirement is

provided by AC filters. In addition, shunt capacitors, synchronous

condensers and static VAR system are used depending on the speed of

control desired. The control of various bus voltage is achieved by supplying

and absorbing the reactive power requirement of respective bus bars by

means of series or shunt compensation. Compensation of reactive power

means supplying/ absorbing reactive volt- amperes. The compensation on

AC side is provided by the following means:

• AC filter capacitors

• AC shunt capacitors

• Synchronous condensers

• Static VAr sources ( SVS )

Synopsis 25

6. Transmission Medium

Transmission medium is not required in Back to Back configuration

as it has ideally zero length and practically only few meters. HVDC cables

are generally used for submarine transmission and overheads lines are used

for bulk power transmission over the land. The most common types of

cables are solid and the oil-filled ones. The development of new power cable

technologies has accelerated in recent years, and the latest HVDC cable

available is made of extruded polyethylene

7. DC Switchgear

This is usually a modified AC equipment used to interrupt small Dc

current. Dc breaker or metallic return transfer breaker are used, if required

for interruption of rated load current.

8. Earth Electrode

Earth electrode is used for providing the return path for the direct

current. This is used in case of Bipolar, Monopolar and Homopolar

configuration but is not required in Back-to-Back system. It is usually

located 5 – 25 KM away from the station to avoid the galvanic corrosion of

substation earthing.

Synopsis 26

UTILITY & APPLICATION OF THE PROJECT

Utility

To make the back-to-back coupling system more familiar to people

Applications

1. As an asynchronous tie between two regional grid for precise and

reliable power transfer.

2. For interconnecting the two or more systems operating at different

frequencies.

3. In some cases it may be for transferring the energy generated by

windfarms to the backbone network

FUTURE WORK TO BE DONE IN THE PROJECT

¾�Detailed study and analysis of converter

¾�Analysis of control schemes adopted in converter station.

¾�Study of reactive power requirement in converter operation and

compensation schemes adopted.

¾�Study of protection schemes in HVDC

¾�Study of Valve halls and switchyard

¾�Case study of Vindhyachal HVDC back-to-back interconnection

between northern and western grid.

Synopsis 27

CONCLUSION

In the present scenario of energy crisis it becomes very

important to effectively utilize the available energy. In a way to achieve

this, HVDC back to back Interconnection play very important role in

connecting the power systems by providing the asynchronous tie and

enabling the precise exchange of power along with consolidation to the

stability of the existing AC network.

.

In HVDC Back-to-Back coupling, the great advantage of

avoiding synchronization between AC power systems & to grid helps in

power transfer smoothly. Our study on the subject should reveal new

facts, which will be helpful in power system stability analysis.

With a lot of advantages over conventional (AC) connection

schemes, due to very high cost consideration of equipments required

and complexity of operation, the realization of scheme is restricted to

few places in our country and is scarcely used in other countries except

the developed countries except the developed countries.

Introduction

INTRODUCTION

While the very first practical applications of electricity were based on

direct current, this technology was quickly replaced by three-phase

alternating current because of various advantages. Still, in spite of the

principal use of alternating current in power systems, there are some

applications for which direct current is the better not only from the point of

view of technical performance but, even taking into account the economic

consideration.

With today’s power systems being operated closer to their stability

limits, and particularly in view of the vulnerability of AC system to faults,

there is an increasing need to understand how the HVDC technology can

play in important role in improving the dynamic performance of the existing

AC network. The thesis is organized as follows:

First of all the thesis gives, a brief overview of HVDC technology,

various transmissions schemes and presents a technical comparison with the

existing AC system. This also gives some introduction to HVDC Back-to-

Back interconnection and typical substation configuration and shows the

significance of the project from the utility and application point of view.

Secondly, it reviews with the basic converter elements i.e thyristor valve

followed by the analysis of converter and concludes with the equivalent

electrical model of a HVDC scheme.

With the various control strategies adopted for the efficient power

transmission It also cites the necessity of reactive power requirement and

Introduction 29

various compensation techniques. The next chapter presents the harmonics

generation phenomenon in converter operations, their effects and

introduction to AC and DC filters for elimination of those harmonics. Also

brief study of faults during converter operation and protection schemes is

carried out

Case-study of Vindhyachal HVDC Back-to-Back station demonstrates

the practical implementation of the so far theoretically known operation and

control of link. And finally a conclusion has been drawn citing the

significance of the technology for the existing AC system presenting the

back to back interconnection as a solution to many challenging situation.

Converter Analysis

Chapter 1

CONVERTER ANALYSIS

1.1 THYRISTOR VALVE

1.1.1 General

A thyristor Valve is made up of number of devices connected in series

to provide the required voltage rating and also of devices connected in

parallel to provide the required current rating. Device ratings, transient

overvoltages and protection philosophy determine number of series and

parallel-connected thyristor.

The valves are usually placed indoor in a valve hall for the protection

purpose and are base mounted in single, double or quadric-valve

configuration. These are usually air insulated and cooled using air, water, oil

or Freon. The water flowing in ducts cools heat sinks and damping resistor.

1.1.2 Valve design consideration

The valve design must consider the voltage and current stresses that

occur during normal and abnormal operating conditions such as over voltage

(which may occur due to switching action or as a result of external cause) or

over current, which may arise from short circuit across a valve or a converter

bridge.

The losses in a valve includes

i. The losses during on-state and switching losses

ii. Damper and grading circuit losses

iii. Losses due to auxiliary power requirement of cooling

Converter Analysis 31

The reduction in short circuit ratio (SCR) tends to reduce the maximum

value of fault current in a valve. The low SCR can also result in non-

sinusoidal voltage at the converter bus, which can give rise to commutation

failures.

The valve can be subjected to high stress commutation resulting from high

di/dt the discontinuous conduction can also result in high over voltages

across a valve. The control of electrostatic and electromagnetic fields

surrounding a valve is essential to avoid corona discharge and interference

with sensitive electronic circuits.

1.1.3 Valve firing

The basic valve-firing scheme is shown in Fig.1.1. The valve control

generates the firing signals. Each thyristor lever receives the signal directly

from a separate fiber-optic cable making each thyristor level independent.

)LJXUH��9DOYH�ILULQJ�VFKHPH�


The valve control unit also indicates many monitoring and protective

function. The return pulse system coupled with short pulse firing scheme is

used in present day valve control unit. A separate light guide is used to send

a return pulse whenever the voltage across a thyristor is sufficient and the

power supply unit is charged. If at that time, firing pulses are demanded

from the valve control, the light signals are sent to all the thyristor control

units simultaneously.

During normal operation, one set of the light pulses are generated in a

cycle for each valve. However, during operation at low direct current, many

light pulses are generated due to discontinuous current.

1.1.4 Recent Trends

The recent developments are expected to improve reliability and

reduce the cost of HVDC valves. These are mainly:

• Development in high power semiconductor devices these include

direct light triggered thyristor and metal oxide semiconductor

controlled thyristor.

• Better cooling techniques such as forced vaporization as a means of

reducing thermal resistance between the heat sink and the ambient.

• Suspension of quadrivalve assembly from ceiling to withstand

seismic forces.

1.2 CHOICE OF CONVERTER CONFIGURATION

The configuration for a given pulse number is selected in such a way

that both the valve and transformer utilization are maximized. The basic

commutation group defines a converter configuration and the number of

such groups connected in series and parallel.


�)LJXUH��&RQYHUWHU�PDGH�RI�VHULHV�DQG�SDUDOOHO�FRQQHFWLRQ�RI�FRPPXWDWLRQ�JURXS�

If there are ‘q’ valves is a basic commutation group and r of these are

connected in parallel and s of them are connected in series, then

p = q*r*s …………1.1

1.2.1 Valve Rating

The valve voltage is specified in terms of peak inverse voltage (PIV)

it has to withstand, The ration of PIV to the average DC voltage is an index

of valve utilization. The average maximum DC voltage across the converter

is given by

Vd0 = VT��(P�VLQ��T) …………1.2


The peak inverse voltage (PIV) across a valve can be obtained as

follows:

The valve utilization factor is given by

PIV / Vd0 = 2��>VT�VLQ��T�@�� (for q even)

= ��>VT�VLQ��T�@ (for q odd)

For a six-pulse Graetz circuit, valve utilization factor comes out to be

1.047, which is one of the min. VUF obtained for various combinations.

1.2.2 Transformer Rating

The current rating of a valve is given by

IV = ID /[r ¥T@ Where, ID is the DC current assumed to be constant. The transformer rating

on the valve side (in volt ampere) is given by

STV = p EM IV / ¥�

The transformer utilization factor (STV/ Vd0 ID) for q = 3 is obtained as

1.481, while for Graetz circuit it is equal to 1.047. Thus it is clear from the

above discussion that both from valve and transformer utilization

consideration, Graetz circuit is best circuit for six pulse converter.

1.3 ANALYSIS OF GRAETS CIRCUIT

1.3.1 General

Converters used in HVDC system are of various types six pulse, twelve

pulse etc, here we are considering a six-pulse converter, whose circuit

diagram is shown in the figure 1.3 with the notation adopted. Following

assumptions are made regarding the voltage source, current nature,

frequency etc to simplify the analysis:


Assumption

1. Power source (or sink) consisting of balanced sinusoidal EMFs of

constant voltage and frequency in series with equal lossless

inductances.

2. Constant ripple free direct current

3. Valves with no forward resistance and infinite inverse resistance

4. Ignition of valve at equal interval of one-sixth cycle (600)

�� )LJXUH��%ULGJH�&RQYHUWHU��VFKHPDWLF�FLUFXLW�IRU�DQDO\VLV�The instantaneous line-to-neutral EMFs are taken as:

……… 1.3


Corresponding line-to-line emfs are

………1.4

1.3 ANALYSIS WITHOUT OVERLAP

At any instant, two valves are conducting in the bridge, one from the

upper commutation group and second from the lower commutation group.

The firing of the next valve in a particular group results in the turning off the

valve that is already conducting.

One period of AC supply voltage has six intervals corresponding to

conduction of pair of valves.

)LJXUH��%ULGJH�&RQYHUWHU�ZLWK�9DOYHV��DQG��&RQGXFWLQJ�Fig 1.4 shows the typical waveforms of the converter if the ac

inductance LC is neglected. In the top graph the ac line-to-neutral voltages

are drawn in thin lines and, in heavy lines, the potentials of the positive and

negative dc terminals with respect to ac neutral. The middle graph shows the

ac line-to-line voltages and, in a heavy line, the instantaneous direct voltage

VD (or Ud). The bottom graph shows the constant dc current and, in a heavy

line, the ac line current Ia.


At any given instant, one valve of the upper commutation group and

one of the lower rows are conducting. Therefore, the instantaneous direct

voltage at any time equals one of the six line-to-line voltages. The instant at

which the direct voltage changes to another line-to-line voltage is controlled

via the firing angle ‘.¶�

)LJXUH��9ROWDJH�DQG�FXUUHQW�ZDYHIRUPV�ZLWKRXW�RYHUODS��D��SRVLWLYH�DQG�QHJDWLYH�GF�WHUPLQDO�SRWHQWLDOV��E��LQVWDQWDQHRXV�GLUHFW�YROWDJH��F��SKDVH�D�OLQH�FXUUHQW��

Average direct voltage Vd (or Ud)

It is assumed that the valves are fired at equal intervals. Hence, Ud

consists of six identical segments of 600 width each, and so the average

direct voltage can be found by averaging the direct voltage over any 600

interval. LCC models average direct voltage is given by


……………1.5

Where

……………1.6

is the so called ideal no-load direct voltage.

DC voltage harmonics

The dc voltage waveform contains a ripple whose fundamental

frequency is six times the supply frequency. This can be analyzed in Fourier

series and contains harmonics of the order

h = np

Where,

p is the number of pulse and n is integer.


The rms value of hth

order harmonic in DC voltage is given by

Vh = Vd0 * ¥��>��K2 – 1) Sin

2 .@1/2

/ [h2 – 1] ………………1.7

$OWKRXJK�.�FDQ�YDU\�IURP��-1800 , the full range cannot be utilized. In

order to ensure the firing of all the series connected thyristors, it is necessary

to provide a minimum limit of .�greater than zero. Also in order to allow for

the turn-off time of a valve, it is necessary to provide an upper limit less than

1800. The delay angle .� is not allowed to go beyond (180

0 - �) where ��is

called the extinction angle (also called margin angle). The minimum value

of extinction angle is typically 100, although in normal operation as an

inverter, it is not allowed to go below 150 or 18

0.

AC current waveform

As it is assumed that the direct current has no ripple (or harmonics).

The AC currents flowing through the valve (secondary) and primary

windings of the converter transformer contains harmonics.

)LJXUH��$&�FXUUHQW�ZDYHIRUP�The waveform of the current in a valve winding is shown in fig.1.6

The rms value of the fundamental component of the current is given by

I1 = ¥�� ,D …………1.8

Whereas the rms value of the current is

I = ¥�� ,D … ………1.9


The Power Factor

The AC power supplied to the converter is given by

PAC = ¥��(LL I1�FRV�3� �¥��(LL * (¥��)* ID�FRV�3��««««��

Where,

FRV�3�� the power factor of ac side

ELL line to line voltage on AC side

I1 rms value of fundamental component

ID value of direct current on DC side

The DC power fed must match the AC power ignoring the losses in

the converter. Thus, ignoring losses we get

PDC = VD*ID = 3¥��(LL*I1 cos .�� ………... 1.11

Where,

VD value of direct voltage on DC side

Equating the above two equations we get

FRV�3� �FRV�.�� …………1.12

From the above equation it is clear that the reactive power

UHTXLUHPHQW� YDULHV� LQ� GLUHFW� SURSRUWLRQ� WR� ILULQJ� DQJOH� .�� +HQFH� LW� LV�recommended to operate the rectifier at low firing angle with suitable safety

margin.

1.3.3 ANALYSIS WITH OVERLAP

1.3.3.1 General concept of overlap

Because of the AC source inductance and converter transformer

leakage reactance, transfer of current from one phase to another can’t be

instantaneous but requires finite time called commutation time or overlap

time � where � is the overlap angle. In normal operation it is less than 600:

typical full load values are 20 - 250.


With the increase in overlap angle, number of conducting valves at a given

time increases.

)LJXUH��7LPLQJ�GLDJUDP�Each interval of the period can be defined by two subintervals. In the

first subintervals two valves are conducting and in the second subintervals,

three valves are conducting. As the overlap increases to 600, there is no

instant when only two valves are conducting. As the overlap angle increases

beyond 600, there is a finite period during an interval when four valves

conduct and the rest of the interval during which three valve conduct. Thus

there are three modes of the converter as follows:

1. Mode 1 – Two and three valve conduction (� < 600)

2. Mode 2 – Three valve conduction (� = 600)

3. Mode 3 – Three and four valve conduction (� > 600)

For the simplicity of analysis, we will discuss here only Mode-1 of

operation, which is most usually encountered during the operation.

ANALYSIS WITH OVERLAP LESS THAN 600

Since a new commutation begins every 600 and lasts for angle ��the

angular interval when two valve conducts is 600 –� �� The sequence of

conducting valve is 12, 123, 23, 234, 34, 345, 45, 456, 56, 561, 61, 612 and

so on.


)LJXUH��9ROWDJH�DQG�FXUUHQW�ZDYHIRUPV�VKRZLQJ�WKH�HIIHFW�RI�RYHUODS��D��SRVLWLYH�DQG�QHJDWLYH�GF�WHUPLQDO�SRWHQWLDOV��E��LQVWDQWDQHRXV�GLUHFW�YROWDJH��F��SKDVH�D�DQG�E�OLQH�FXUUHQWV��

Consider the situation when valve 1 and 2 were conducting initially.

At &W� � ., when valve 3 is ignited, the effective circuit is as shown in

fig.1.10 with valve 1, 2 and conducting.


)LJXUH� ��(TXLYDOHQW�FLUFXLW�IRU�WKUHH�YDOYH�FRQGXFWLRQ�During this interval direct current is transferred from valve 1 to valve 3.

Hence, at beginning (&W� �.�� i1 or ia = ID and i3 or ib = 0 …………1.13

At end (&W� �.�� /):

i1 = 0 and i3 = ID …………1.14

1.3.3.2 Average Direct Current

The mesh equation for the loop N31N can be given as

The emf in this loop known as commutating EMF, which is

ea – eb = ¥��(P�VLQ�&W …………1.15

The sum of ia and ib during commutation equals direct current

and equation 1.15 becomes

¥��(P�VLQ�&W� ��/C dib / dt = 2LC di3 / dt


Integration during commutation period (from W� �. to W� �/) gives

And finally inserting the boundary conditions in the LHS of the above

equation, we get

Id = ¥��&/C��FRV�.�–�FRV�/� …………1.16

Equation 1.14 shows that i3, the current in the incoming valve during

commutation, consists of a constant (dc) term and a sinusoidal term which

lags the commutation voltage by 900. And has a crest value which is that of

the current in a line-to-line short circuit on the AC source.

From this equation the extinction angle d (and ultimately the overlap

angle �) can easily be determined for a given firing angle .. It also allows

the calculation of the ideal maximum firing angle .MAX for which the

commutation will succeed in a converter with ideal valves. Since

For any angle /

And

………… 1.17


1.3.3.3 Average Direct Voltage

During commutation the two impedances in the commutation loop act

as a voltage divider that sets the potential of the positive converter terminal

to the average of the two line voltages. It is only after the commutation that

the terminal potential recovers to the voltage of the on-going phase.

The consequence is that an area Aµ as shown in Fig. decreases the

voltage/angle-area A derived in Eq. 1.17. This results in a voltage drop ¨Ud

of the average direct voltage,

…………1.18

…………1.19

Comparison of equation 3.17 and 3.20 shows that voltage drop is

directly proportional to the DC current

¨8d � �� &�/C ID …………1.20

The total average direct voltage is thus given by

= Udi0 cos�.�– RC Id ………... 1.21


Where,

RC� �� &�/C

‘RC’ is called the equivalent commutation resistance. It accounts for

the voltage drop due to commutation. However, it is not a real ohmic

resistance and thus consumes no active power.

With Eq 1.21 the average direct voltage could also be written as

= Udi0�� FRV�.��FRV�/�� …………1.22

1.3.3.4 Equivalent circuit of rectifier

From the equation 1.22, equivalent circuit of the rectifier can be

drawn as

)LJXUH��(TXLYDOHQW�FLUFXLW�RE�EULGJH�UHFWLILHU�1.3.3.5 DC voltage waveforms

)LJXUH��'&�YROWDJH�ZDYHIRUP�IRU�.� ��


Figure 1.11 shows the waveforms of the voltage across the converter

bridge VD. The valve voltage (not shown in figure) has various jumps that

occur at the firing and the turning off of the valve. This voltage jumps results

in extra losses in the damper circuit.

1.3.3.6 AC Current Magnitude and Phase

Approximate analysis:

Due to the overlap the ac currents are no longer rectangular blocks.

Instead, their shape is that of a deformed trapezoidal Still, Eq.3.24 is a good

approximation for the fundamental frequency component of the ac current:

I1 = 2¥�� ,d ……… ... 1.23

By assumption, the converter is lossless and therefore the ac active

power must equal the dc power:

3/2 Em I1�FRV�3�§�8di0 Id�� FRV�.��FRV�/��«««��

Where,�3 denotes the angle by which fundamental component of the

line current lags the applied voltage. On simplification, equation 1.24 gives,

FRV�3 §��FRV�.��FRV�/�� ……… ...1.25

another expression for the power factor FRV�3�can be given as

FRV�3 §�8d / Udi0

§�FRV�.�– Rc Id / Udio ………...1.26

shows that with increasing load the power factor decreases and

accordingly the phase shift between the fundamental ac current and the ac

voltage increases.

Reactive power on the AC side may be found from

4� �3D�WDQ�3 ………...1.27

Where, Eq.1.26 or 1.27 gives�3. Of course, there is no reactive power on the

DC side.


1.3.4 Inversion

1.3.4.1 General

Because the valves conduct in only one direction, the current in a

converter cannot be reversed, and power reversal is obtained only by the

reversal of average direct voltage VD. The voltage then opposes the current

is called counter voltage.

Ideally the inversion occurs in the region 900� �� .� �� 0

, but in

practical case, there is always some overlap and the vaOXH� RI� .� DW� ZKLFK�inversion begins is given as:

.� ��–�/� �>��-��@�� …………1.28

Which is always less than 900.

Moreover, /�ought to be less than ��by at least an angle corresponding

to the time required for the de-ionization of the arc, which is 1 – 80.

Synchronous machines connected to AC side furnish the commutation

voltage for the HVDC inverter. If the AC system receiving power from DC

link has no generators, a synchronous condenser is used.

Notations for Ignition and Extinction Angle

In inverter theory, commoner practice is to define LJQLWLRQ� DQJOH��

and H[WLQFWLRQ�DQJOH�� by their advance with respect to the instant when the

commutation voltage is zero and decreasing. Referring to figure 1.12 this

parameters can be described.


)LJXUH��5HODWLRQV�EHWZHHQ�FRQYHUWHU�DQJOHV�The relations among the several inverter angles are as follows:

�� – .�� ………..1.29(a)

�� –�/ ………..1.29(b)

�� /�– .� ��–�� ………..1.29(c)

1.3.4.2 Equivalent Circuit

Equations for Average Direct Current and Voltage

General equations 1.16 and 1.17can be changed to inverter equations

by changing the sign of VD and putting

FRV�.� ��–�FRV�� and FRV�/� ��–�FRV��, with these results

ID = IS2��FRV��-�FRV�� …………1.30

VD = VD0 [cos .��FRV��@�� …………1.31

For constant ignition advance angle �, equation 1.31 becomes

VD = VD0�FRV��5C ID

Because the inverters are commonly controlled so as to operate at

constant current advanced angle � it is useful to have relations between ID

and VD for this condition.

VD = VD0�FRV��– RC ID


Under this condition, the equivalent commutation resistance is – RC

and is negative. This is the reason why inverter is said to posses a negative

commutation resistance. The equivalent circuit of the inverter is shown in

the figure 1.14

)LJXUH��(TXLYDOHQW�&LUFXLWV�RI�,QYHUWHU�


1.4 STEADY STATE EQUIVALENT CIRCUIT

From the ongoing discussion, equivalent circuit of the for the steady

state operation of two terminal DC link can be drawn as shown in fig 1.15.

)LJXUH��(TXLYDOHQW�FLUFXLW�RI��WHUPLQDO�'&�OLQN�The effect of leakage reactance in producing drop of direct voltage is

accounted for by the equivalent commutating resistance and subscripts ‘r’

and ‘i’ signifying rectifier and inverter.

HVDC Control

Chapter 2

HVDC CONTROL

2.1 INTRODUCTION

A well known technical advantage of HVDC is it s inherent ability for

control of transmitted power. The voltage across valve-bridge can be

changed nearly instantaneously. The speed of response of the control is

limited only by the maximum voltage available, the dynamics of the DC side

circuit and the sped of change of power which the connected AC networks

can stand

The fact that the reactive power consumed by the HVCD converter is

dependent on the values of the control angles means also that reactive power

of the converter station and the AC network can be controlled and the AC

voltage can be stabilized.

This chapter covers the control fundamentals for the HVDC converter.

Starting with the general discussion of control characteristics of the

converter, the report deals with the different stages in control hierarchy.

2.2 PRINCIPLE OF DC LINK CONTROL

By incorporating the equivalent circuit of the converter shown in

figure 1.15, the direct current ID in the DC line can be given as

……………2.1

From the above equation it is clear that

ID ∝ Voltage drop

∝ 1 / (total resistance)

HVDC Control 53

Direct voltage and thus current ID can be controlled by control of

internal voltage which then can be controlled by

1. Grid Controlling

Grid control, delaying the ignition angle . (time &�.), reduces the

internal voltage from the ideal no-load voltage VD0 by the factor FRV�..

2. Control of alternating voltage

The alternating voltage is usually controlled be tap changing on the

converter transformer.

Grid control is rapid (1 to 10 ms), but tap changing is slow (5 to 6

seconds per step). Both these means of voltage control are applied

cooperatively at each terminal. Grid control is used initially for rapid action

and is followed by tap changing for restoring certain quantities (ignition

angle in the rectifier or voltage in the inverter) to their normal values.

2.2.1 Desired features of control

The following features are desirable:

• Limitation of the maximum current so as to avoid damage to

valves and other current carrying devices.

• Limitation of the fluctuation of current due to the fluctuation of

alternating voltage.

• Keeping the power factor as high as possible.

• Prevention of commutation failures of the inverter.

• Prevention of arc back of the rectifier valves.

• In multi-anode valves, providing a sufficient anode voltage before

ignition occurs.

• Controlling the power delivered or the frequency at one end.

• Provided better voltage regulation.

HVDC Control 54

There are four reasons for keeping the power factor high, two

concerning the convertor itself and the other two concerning the ac system to

which it is connected. The first reason is to keep the rated power of the

converter as high as possible for given current and voltage rating of valves

and transformer. The second reason is to reduce the stresses on the valves

and damping circuits. The third reason is to minimize the required current

rating and copper losses in the ac lines to the converter. The fourth reason is

to minimize voltage drops at the ac terminal of the converter as its loading

increases. The last two reasons apply to any large ac loads.

The p.f. can be raised by adding shunt capacitor, if this is done , the

disadvantages becomes the cost of the capacitors and switching them as the

load on the converter varies. The p.f. of the converter itself is

FRV�3� §��FRV�.��FRV��.�� …………2.2

for rectifier and

FRV�3� §��FRV��FRV�� …………2.3

for an inverter.

In a rectifier, we can make .�= 0 for which FRV�. = 1. In an inverter it

is more difficult. In order to avoid a commutation failure, commutation must

be completed before the commutating voltage reverses at �� , hence y

must be greater than zero by some margin. Because of some inaccuracy in

the computation of � and a possibility of changes indirect current and

alternating voltage even after commutation has began, sufficient

commutation margin above the minimum angle required for de-ionization of

the mercury arc must be allowed. The easy and safe way would be to choose

a larger value of �. This way lowers the power factor and raises the stresses

on the valves.

HVDC Control 55

2.3 V-I CHARACTERISTICS OF HVDC CONVERTERS

2.3.1 Individual characteristics of Rectifier and Inverter

These are plotted in rectangular coordinates of direct current Id and

direct voltage Vd. If the rectifier be equipped with constant-current regulator,

ideal characteristics will be a vertical line AB, but in practice it has a high

negative slope which can be shifted horizontally by adjusting current

command.

If the inverter be equipped with C.E.A. regulator, then inverter

characteristics is a line with slightly negative slope (under the assumption

that commutating resistance RC2 is somewhat higher than line resistance RL)

given by

VD = VD02�FRV��5l – RC2) * ID …………2.4

Operating point of the system is the point of intersection of rectifier

and inverter characteristics.

It may be said with fair accuracy that rectifier direct current and

inverter controls direct voltage. But control of one parameter at one end

affects both the current and voltage settings due to non ideal characteristics.

HVDC Control 56

)LJXUH��D��&KDUDFWHULVWLFV�RI�&RQWURO�6FKHPH�

)LJXUH��E��$FWXDO�FRQWURO�FKDUDFWHULVWLFV�

HVDC Control 57

If the inverter voltage changes, rectifier voltage must be change by

equal amount in order to keep current constant which can be done quickly by

current regulator till .� ��Rectifier voltage can be increased further only by

taps on the rectifier transformer.

2.3.2 Combined Characteristics of Rectifier and Inverter

In many dc transmission links each converter must function

sometimes as a rectifier and other times as an inverter therefore each

converter is given a combined characteristics consisting of three linear

portions: C.I.A., C.C. and C.E.A.

)LJXUH��&RQYHUWHU�FRQWUROOHU�FKDUDFWHULVWLFV�

HVDC Control 58

With the characteristics shown by solid lines, power is transmitted

from converter-1 to converter-2 and with the characteristics shown by

broken lines, direction of transmission is reversed by reversal of direct

voltage with direction of current being the same.

Usually the current setting of constant current characteristics of the

two converter are separated by û,d called current margin to account to

maintain positive margin in spite of errors in the current measurement and

regulation as the operation of the two steep CC characteristics, with both

current regulators would be highly unstable.

There can be three modes of operation of the link (for the same

direction of power flow) depending on the point of intersection of the two

characteristics.

1. CC at rectifier and CEA at the inverter (normal mode of operation)

2. With the slight dip in the AC voltage, the point of intersection drift

which implies minimum .�at rectifier and minimum � at inverter.

3. With the lower AC voltage at the rectifier, the mode of operation

again shifts which implies CC at the inverter with minimum . at the

rectifier.

2.4 BASIC CONTROL SYSTEM

As the current order to the inverter is lower by the current margin that

in the rectifier, current delivered by the rectifier is higher than demanded by

the inverter, latter tries to counteract that by increasing and the counter emf

VHW�XS�DJDLQVW�WKH�UHFWLILHU�DQG�LQYHUWHU�.�WHQGV�WR�UHDFK�LWV�maximum value

determined by the minimum commutation margin and the CFC will operate

in another mode, the commutation margin control (CMC).

HVDC Control 59

)LJXUH��%DVLF�FRQWURO�RI�+9'&�FRQYHUWHU�The next step in our study of the control of a converter is to examine in

more detail how each of the three straight-line segments of the combined

characteristics can be obtained

1. Constant minimum ignition angle

2. Constant current characteristics

3. Constant extinction angle

2.4.1 Firing Angle Control

The objective of convertor firing control (CFC) system is to generate

control pulses to all valves within the convertor in correct phase position and

inside the interval�. = .�MIN to . = . MAX., The latter being determined by a

minimum commutation margin limit. The output form the CFC is issued to a

control pulse generator (CPG), which forms individual gate control pulse

signals for all the valves within the convertor.

HVDC Control 60

Types of firing control systems

The firing angle control systems can be broadly referred into two categories:

• Individual Phase Control

In this, phase positions of gate control signals are determined

VHSDUDWHO\� IRU� HDFK� YDOYH� DQG� .� RU� �� ZLOO� EH� HTXDO� HYHQ� LI� WKH� $&�network is unbalanced.

This scheme generates higher amount of harmonics.

• Equidistance Pulse Control

.-order in current control can be turned into a gate pulse signal in

correct phase position by using a phase-controlled oscillator. Firing

pulses are generated in steady state at equal interval of 1/pf through a

ring counter. This removes the risk of harmonics. This is the control

principle used in all modern HVDC systems.

2.4.2 Constant Minimum Ignition Angle Control

Normally rectifier is operated at ‘Constant minimum ignition angle’

to minimize the reactive power requirement as it varies in direct proportion

to ignition angle at rectifier side. To maintain a minimum delay angle say .0

following method is used.

Voltage across each valve is measured, if it is found less than a pre-

specified voltage say ¥��9M Sin .0 the constant current control is prevented

form igniting the valve. In practice, secondary voltage of control transformer

is used rather than the voltage across the valve by any suitable arrangement.

2.4.3 Constant Current Control

Under normal operation system is made to operate at constant current

control setting of the rectifier end. In this mode short-circuit current are

ideally limited to the value of the load current and in practice to about twice

rated current

HVDC Control 61

Constant-current control involves the following:

1. Measurement of direct current.

2. Comparison of direct current Id with the set (or reference) value IdS.

3. Amplification of difference IdS - Id called error.

4. Application of output signal of the amplifier to a phase-shift circuit

that alters the ignition angle . in proper direction for reducing error.

)LJXUH��6FKHPDWLF�GLDJUDP�RI�FRQVWDQW�FXUUHQW�UHJXODWRU�If the measured current in a rectifier is less than the set current, .�must

be decreased in order to increase FRV�. and thus raise the internal voltages of

the rectifier VDO FRV� .�� The difference between the internal voltages of

rectifier and the inverter is thereby increased, and the direct current is

increased proportionally. A decrease in . increases the algebraic internal

voltage VDO FRV�.��This means that a same constant current controller can be

used on a given converter without change of connections during both

rectification and inversion.

In practice, however, the same current setting is transmitted to both

terminals of a DC line, and the current margin is subtracted from the current

setting of the inverter; that is, the error signal for the inverter’s current

regulator is

0 = IDS – ¨,D – ID

HVDC Control 62

The current regulator is a simple kind of feedback amplifier

characterized by gain and time constant. Its differential equation is

…………2.5

Where,

V = instantaneous voltage

T = R2C = time constant

K = gain of amplifier and phase shift circuit

�0� �(UURU�VLJQDO 2.4.4 Constant Extinction-Angle Control

Each inverter must be ignited at such a time that extinction occurs at a

later time, which, how ever must be earlier by an adequate margin than the

time when commutation voltage reverses. The easy and safe way is to

FKRRVH�D� ODUJH�YDOXH�RI��7KLV�ZD\�KRZHYHU��ORZHUV�WKH�SRZHU�IDFWRU�DQG�rises the stresses on the valve. The better way is to compute the firing angle

UHTXLUHG� WR� REWDLQ� D� FRQVWDQW� H[WLQFWLRQ� DQJOH� �� 7KLV� FDOFXODWLRQ� LV� GRQH�using a analog computer obtaining input from the AC side of inverter and

current in the DC link.

2.5 MASTER CONTROL

Master control generates the current order to be issued to the current

control systems of both converter stations, from instance the power order set

by the operator, and it includes control functions for modulation of the

transmitted power when the HVDC link is used for stabilization of

connected AC network.

2.6 HIGHER LEVEL CONTROLLERS

The HVDC transmission can be used for stabilization of AC system

by modulating the power flow in accordance with the variations in some AC

HVDC Control 63

system quantities, usually frequency. The link can also be used to directly

control the frequency of an AC network connected to one of the substation.

2.7 SYSTEM CONTROL HIRARCHY

The control functions required for the HVDC link are performed using

the hierarchical control structure shown in Fig. 2.5. The master controller for

a bipole is located at one of the terminals and is provided with the power

order (PREF) from the system controller (from energy control center). It also

has other information such as AC voltage at the converter bus, DC voltage,

etc. the master controller transmit the current order (IREF) to the pole control

units, which in turn provide a firing angle order to the individual valve

groups (converters). The valve group or converter control also oversees

valve monitoring and firing logic through the optical interface; it also

includes bypass pair selection logic, commutation failure protection, tap

changer control, converter start/stop sequences, margin switching and valve

protection circuits.

)LJXUH��+LHUDUFKLFDO�FRQWURO�VWUXFWXUHV�IRU�D�'&�OLQN�

HVDC Control 64

2.8 REACTIVE POWER CONTROL

2.8.1 Introduction

The converter in HVDC stations are line commutated, which implies

that the current initiation in the valve can only be delayed with reference to

the zero crossing of the converter bus AC voltage. This result in lagging

power factor operation of the converters, requiring reactive power sources

connected at the converter bus for better voltage control. Figure 2.6 shows

the typical phase displacement of the line side current waveform phase with

that of AC voltage with the firing angle as a parameter.

)LJXUH��5HODWLRQ�EHWZHHQ�LJQLWLRQ�GHOD\�DQG�SKDVH�GLVSODFHPHQW�

HVDC Control 65

Figure 2.6 illustrates that fundamental line current lags the line to

neutral source by an angle equal to its firing angle. Hence it is advised to

keep the ignition angle low. Statistical data shows that at each converter

station, reactive power requirement is around 60% of the active power

transferred.

2.8.2 Reactive Power Requirement in Steady State

2.8.2.1 Conventional Control Strategies

Under normal operation, a DC link is operated with current control at

the rectifier side and minimum extinction angle control at the inverter. This

method of control leads to minimum reactive power requirement at both

ends.

The equation for the reactive power as a function of the active power

is conveniently expressed in terms of per unit quantities. Average bridge

voltage across the converter bridge is given by

VD = 9�FRV�.�– RC*ID (for rectifier) …………2.6(a)

VD = 9�FRV��– RC*ID (for inverter) …………2.6(b)

Where,

VD = voltage on DC side (in per unit value)

ID = current on DC side (in per unit value)

The power factor is given by

&RV�3 §��VD / 9�� FRV�. – (RC*ID/V) …………2.7

The power and reactive power in per unit are given by the following

equations:

PD = V*ID�FRV�3 …………2.8(a)

QD = V*ID�VLQ�3 …………2.8(b)

Also, the power factor of the converter can be given by equation

FRV�3 �>FRV�.��FRV��.��@�� ...………2.9

HVDC Control 66

Thus, from the above equations, variation of reactive power demand

with the active power demand as a function of firing angle can be shown as

in figure 2.7

)LJXUH��9DULDWLRQ�RI�4'�ZLWK�3'�From the figure, it is clear that

• Under normal operation (.��0), reactive power at any station is

around 0.6 times the rated active power.

• Increase in firing angle .�leads to sharp increase in reactive power

demand with the increase in active power supplied, hence it is

recommended to maintain low firing angles in steady state.

However, too low values of .�can result increased frequency of mode

shifts and too low values of � can result in increased incidence of the

commutation failure.

The reactive power is also affected by the magnitude of AC voltage.

The reduction in V leads to increase in QD, however on-load tap changer can

control V within limits.

HVDC Control 67

2.8.2.2 Alternate Control Strategies

The region of operation of a converter bridge is bounded by the limits

on the DC current and firing angle. Neglecting minimum current limit, the

operating region of a bridge in PD – QD plane is shown in fig. 2.8

)LJXUH��2SHUDWLQJ�UHJLRQ�RI�D�EULGJH�LQ�3'�4'�SODQH�Which is drawn for a constant (rated) AC voltage. This region is

bounded by three regions:

i. minimum . characteristics

ii. minimum � characteristics

iii. constant rated DC current

In general the locus for a constant DC current in part of a circle in the

3'�4'� � plane and the constant DC voltage characteristic is a straight line

passing through the origin.

The operation at constant DC voltage implies constant power factor

characteristics at the converter bus. At the rectifier, the characteristic is that

of a load with lagging power factor, while at the inverter, this can be viewed

as a generator with leading power factor operation.

HVDC Control 68

)LJXUH��6LPSOLILHG�V\VWHP�GLDJUDP��D��UHFWLILHU��E��LQYHUWHU�This is shown from the analysis of a simplified system shown if

Figure 4.10 In Fig 2.9 (a), the rectifier is shown as a constant (lagging)

power factor load while Fig. 2.9 (b) is applicable to the inverter operation.

The phasor diagram for both the cases are shown in Fig 2.10.

)LJXUH��3KDVRU�GLDJUDP��D��UHFWLILHU��E��LQYHUWHU�It can be shown by the phasor diagram that

9� �(�FRV��/��3��FRV�3�� …………2.10

Where, 3� is the power factor angle.

The power expression is given by

P = VEB�VLQ�/ …………2.11

Substituting Eq. 2.10 into Eq. 2.11, we get

P = E2�%�FRV��/��3�� VLQ�/��FRV�3�� …………2.12

From the above equation, it can be shown that maximum power transfer is

obtained when,

/� ��–�3��

HVDC Control 69

The maximum power (for 3� ��0) is given by

Pmax = 0.2887 E2 B …………2.13

This is much less than what can be obtained in the case with 3� ��-30 or V = E.

To modify the power rating, the provision of a shunt capacitor (having

succeptance, BC) at the converter bus results in the modification of the

maximum power expression from equation 2.10 to 2.13

Pmax = 0.2887 E2 B / (1 – BC/B) …………2.14

The above analysis shows that there is a need to modify the reactive

power characteristics of the converter station by either

i. choice of reactive power sources

ii. adjustment in the converter control characteristics

When the DC link involves long distance transmission, the

minimization of power losses in the line dictates operation at constant DC

voltage and flexibility of converter operation is not feasible. However, with

back-to-back links, the operation at constant voltage is not critical and

alternate converter control strategies, as shown in Fig. 4.12 can be adopted.

These are

1. Constant reactive power characteristics

2. Constant leading power factor characteristics

)LJXUH��$OWHUQDWH�UHDFWLYH�SRZHU�FRQWURO�VWUDWHJLHV�

HVDC Control 70

It is to be noted that by providing a constant reactive power source of

QN at the converter bus, the characteristics ab or a’b results in unity power

operation of the converter. Similarly, by providing reactive source of 2QN,

the power factor angle is changed from 3�to -3�� 2.8.3 SOURCES OF REACTIVE POWER

The reactive power requirement of the converter are met by one or

more of the following sources:

• AC system

• AC filters

• Shunt Capacitors

• Synchronous Condenser

• Static VAR system

These are shown schematically in Fig. 2.12

��)LJXUH��5HDFWLYH�SRZHU�VRXUFHV�DW�D�FRQYHUWHU�EXV�The voltage regulation at the converter bus is desirable not only from

the voltage control viewpoint but also from the minimization of loss and

HVDC Control 71

stability considerations. This requires adjustable reactive power source,

which can provide variable reactive power as demanded.

2.8.3.1 AC System

Figure 2.13 shows the reactive power drawn by AC system at the

inverter bus, as a function of PD. At low values of delivered power, reactive

power supplied by AC system is positive while with the increase in PD it

goes negative.

)LJXUH��5HDFWLYH�SRZHU�VXSSOLHG�E\�WKH�$&�V\VWHP�This value is more negative when the short circuit ratio (SCR) is

lower for the same amount of power transfer PD.

2.8.3.2 AC Filters

AC filters, that are provided at the converter bus for filtering out AC

current harmonics, appears as a capacitors at the fundamental frequency and

thus provide reactive power. These filters are mechanically switched and

suffer from the inability of continuous control. Also they can cause low

order resonance with the network impedance, resulting in harmonic

overvoltages.

HVDC Control 72

2.8.3.3 Shunt Capacitor

For slow variation in load, switched capacitors or filters can provide

some control, which may cause voltage flicker owing to discrete control,

unless the size of unit, which is switched is made sufficiently small.

2.8.3.4 Synchronous Condenser

Synchronous condenser provides continuous control of reactive power

and can follow fast load changes. It has following advantages

1. The availability of voltage source for commutation at the inverter

even if the connection to the AC system is temporarily interrupted.

2. Increase in SCR as the fault level is increased.

3. Better voltage regulation during a transient due to the maintenance

of flux linkages in the rotor windings.

But still there are some disadvantages to its part such as – (i) high

maintenance and cost & (ii) possibility of instability due to machine going

out of synchronism.

2.8.3.4 Static VAR Systems (SVS)

In HVDC converter station, the provision for SVS mainly helps to

have fast control of reactive power flow, thereby controlling voltage

fluctuations and also to overcome the problem of voltage instability. There

are basically three types of SVS schemes:

i) Variable impedance type SVS

ii) Current source type SVS

iii) Voltage source typr SVS

The variable impedance type is most common in power system

applications and will be described next.

HVDC Control 73

Thyristor Controlled Reactor (TCR)

)LJXUH��6LQJOH�SKDVH�WK\ULVWRU�FRQWUROOHG�UHDFWRU�Single phase TCR is shown in Fig. 2.14 By controlling firing angle of

the back-to-back connected thyristor, the current in the reactor can be

controlled. A TCR is usually operated with fixed capacitor (FC) to provide

the variation of reactive power consumption form inductive to capacitive.

The schematic FC-TCR is shown in Fig. 2.15.

)LJXUH��7KH�VFKHPDWLF�GLDJUDP�RI�)&�7&5�

HVDC Control 74

Thyristor Switched Capacitor

Thyristor switching is faster than mechanical switching. A reactor is

usually connected in series with the capacitor to reduce the rate of change of

the inrush current.

)LJXUH��$�VLQJOH�SKDVH�7&6�

Harmonics and Filters

Chapter 3

HARMONICS AND FILTERS

3.1 INTRODUCTION

HVDC converter introduces AC and DC harmonics that are injected

into AC system and DC line side respectively. A converter of pulse number

p generates harmonics principally of the order of

h = p*q (on the DC side) …………3.1

And

h = p*q ± 1 (on the AC side) …………3.2

Where, q is an integer.

Most of the HVDC converters have pulse number 6 or 12 and thus

produce the harmonics of the order given in table 3.1

Table 3.1 Orders of the Characteristics Harmonics

The amplitude of the harmonics decrease with increasing order: the

AC harmonic current of order h is less than I1/h where I1 is the amplitude of

the fundamental current.

There are several problems associated with the injection of harmonics

and these are listed below:

Harmonics and Filters 76

• Telephone interference

• Extra power losses and consequent heating in machines and

capacitance connected in the system.

• Over voltage due to resonance.

• Instability of converter control, primarily with individual phase

control scheme of firing pulse generation.

• Interference with ripple control system used in load management.

AC filters are invariably used to filter out AC current harmonics

which are critical. These filters are of band pass or high pass type and also

supply reactive power. DC smoothing reactor along with DC filter perform

the function of filtering DC harmonics.

In addition to the harmonics, which cause telephone interference, the

harmonics at the carrier and radio frequencies are also generated by the

converter and may require suitable filters.

Principal means of diminishing the harmonic output of converter are

1. Increase the pulse number

2. Installation of filters

In general, converters with pulse number greater than 12 are not used

as the complexity of operation and control overshadows the significant

advantages of higher pulse number. It is also found that for HVDC converter

use of filter is more economical than the use of higher pulse number (greater

than 12). AC filters serves the dual purpose of diminishing the AC

harmonics and supply reactive power at the fundamental frequency.


3.2 GENERATION OF HARMONICS

3.2.1 Generation of Harmonics in AC Side

The line current and phase voltage waveforms under the condition of

no overlap are as shown in figure 3.1

)LJXUH��/LQH�FXUUHQW�ZDYHIRUP�Line current waveform under the condition of no-overlap is the series

of equally spaced rectangular pulses with alternately positive and negative

value.

3.2.2 Generation of DC Harmonics on DC Side

DC voltage waveforms contains ripple whose fundamental frequency

is six times the supply frequency.

)LJXUH��/LQH�FXUUHQW�ZDYHIRUP�This voltage is analyzed in Fourier series and contains harmonics of the

order of h

h = n*p

Where, p is the number of pulse and n is an integer.

The rms value of the hth

order harmonic can be given as

Vh = Vd0 * ¥��>��K2 – 1) Sin

2�.@1/2 / [h

2 – 1] …………3.3


Normally, a DC reactor of large inductance is used in the DC side so

that the DC current is almost constant and can be considered free from

ripples hence it can be said that on DC side there are voltage harmonics

predominately while on AC side, current harmonics predominates.

Besides these, some harmonics also occur owing to imbalance in the

AC supply waveform, difference in firing angle etc. These harmonics can be

categorized namely Characteristics Harmonics and Non- characteristics

Harmonics

3.3 CHARACTERISTICS HARMONICS

Characteristics harmonics are those which can e predicted by

mathematical analysis and are generally predominate. These are present

even under ideal operating conditions like balanced AC voltage,

symmetrical three-phase network and equidistant pulses. Characteristics

harmonics are those of orders given by equations 3.1 & 3.2.

Assumptions:

The following assumptions are made as bases for deriving the orders,

magnitude and phases of the characteristics harmonics of a six-pulse

converter:

• The alternating voltage are three phase, sinusoidal, balanced, and of

positive sequence.

• The direct current is absolutely constant that is without ripple. Such

current would be the consequence of having a dc reactor of infinite

inductance.

• The valves are ignited at equal times interval of one-sixth cycle that

is, at constant delay angle measured from the zeros of the respective

commutating voltage. By assumption 1, these zeros are equally spaced

in time.


• The commutation inductances are equal in the three phases.

3.3.1 Harmonics at No Overlap

The wave shape of alternating voltage and currents confronting with

the assumptions made above are equidistant rectangular pulses assuming that

direct current has no harmonics, current waveforms for the primary side of

converter transformer are drawn for an ignition delay of .�without overlap

(� = 0).

3.3.1.1 For six-pulse Converter

The line current waveforms at no overlap are a series of equally space

rectangular pulses, alternately negative and positive. Fourier analysis of such

wave shape, for finding the characteristics harmonics can be carried out

under the following steps:

Consider the pulse of unit height and width w radian that is of

duration w/& seconds.

iA = (2¥��ππ) Id�>�FRV�&t – (1/5) cos 5&t + (1/7) cos 7&t

– (1/11) cos 11&t + (1/13) cos��&t -….] …………3.4

The rms value of the hth

order harmonic in DC voltage is given by

equation 3.3.

3.3.1.2 For 12-pulse Converter

A 12-pulse group in a HVDC converter is composed of two 6-pulse

group fed from sets of valve side transformer winding having a phase shift

of 300

between fundamental voltages. Since .� is same for both 6-pulse

group, the fundamental valve side currents have the same phase difference

as the voltages, and fundamental network-side currents are in phase with one

another. The schematic diagram for a 12-pulse converter unit is shown in

figure 3.3


)LJXUH��6FKHPDWLF�GLDJUDP�RI�D��SXOVH�FRQYHUWHU�Neglecting overlap, the current in the primary side of star-star

connected transformer (assuming turns ratio of 1:1) is given by the equation

3.5. Similarly, assuming that the delta-star connected transformer has turns

ratio of ¥��IA2 can be given as

iA2 = (2¥��ππ) Id>�FRV�&t + (1/5) cos 5&t - (1/7) cos 7&t –

(1/11) cos 11&t + (1/13) cos��&t -.…] …………3.5

The current IA can be given by the summation of IA1 and IA2 or,

IA = IA1 + IA2

IA = (4¥��ππ) Id>�FRV�&t – (1/11) cos 11&t + (1/13)

cos��&t – (1/23) cos 23 &W��FRV��&W�«�] …………3.6

From the above expression, it can be observed that

I10 = (2¥�� ,D

Iho = I10 / h

Where I10 and Ih0 are rms values of the fundamental component and

harmonic of the order of ‘h’. The second subscript shows that the overlap

angle � is considered zero.


The magnitude of the characteristics harmonics is also a function of the load

current. This is shown in the Fig. 3.4

)LJXUH��+DUPRQLF�PDJQLWXGHV�ZLWK�YDULDWLRQ�RI�'&�FXUUHQW�3.3.2 AC and DC Harmonics with Overlap

Because of overlap (owing to inductive nature of transformer winding

and inductance of AC network seen through the converter) valve current in

valve winding is distorted.

)LJXUH��$&�DQG�'&�FXUUHQW�ZDYHIRUP�XQGHU�RYHUODS�Thus, expressions for the fundamental component of the AC current

derived for the case with no overlap is not valid. The actual expression for

the current can be derived from Fourier analysis and is given by

I1 = [I112 + I12

2]

½ …………3.7


Where,

I11 = I1�FRV�3� �¥��,d�>FRV�.��FRV��.��/@�� …………3.8

I12 = I1 VLQ�3�� ¥��,d�>��VLQ��.�–�VLQ��/��FRV�.�–�FRV/�@��

Where, 3 is the power factor and /� �.�� From the above expression, the power factor angle can be obtained as

WDQ�3 ��VLQ��.�–�VLQ��/��FRV��.�–�FRV��/� …………3.10

The harmonic components in the AC current are also altered. These

are reduced from the value calculated with no overlap. The expression can

be given as

Ih = Ih0 [A2 + B

2 –��$%�FRV��.��@ 1/2��>FRV�.�–�FRV�/@��«««�.11

Where,

$� �VLQ�^�K�� ` ��>K��@ …………3.12(a)

B = sin {(h –�� ` ��>K�– 1] …………3.12(b)

/� �.��

The above expression is valid for ��0. For higher values of the

overlap angle, the expression given by equation 3.10 can still be used if .��

and�/�are replaced by .¶��¶ and�/¶�where,

.� �.�– 300��¶� ��0

and�/¶� �/��0

Also, from the Fourier Analysis of DC voltage waveform, we can

obtain

Vh = Vh0 [C2

+ D2 –��&'�FRV��.��@ 1/2

/ ¥2 …………3.13

Where,

&� �FRV�^�K�� ` ��>K��@ …………3.14(a)

D = cos {(h –�� ` ��>K�– 1] …………3.14(b)


3.4 NON-CHARACTERISTICS HARMONICS

The conditions postulated in the foregoing analysis of characteristics

harmonics of a converter are never fulfilled in practice. Consequently, not

only are the harmonics of characteristic order slightly changed from their

theoretical magnitude and phases, but also – and more important –

harmonics of non-characteristic order are also produced.

Harmonics of low non-characteristic order are normally much smaller

than those of adjacent characteristic harmonics in the converter itself. Filters

are usually provided for low characteristic order. For high order, the

magnitude of both characteristics and non- characteristic order are small and

approximately the same.

3.4.1 Causes

1. Unbalanced three phase alternating voltage

The time delay angle of a rectifier is usually measured from a zero of

the commutating voltage. If the three phase AC voltages are

unbalanced, their zeros are not equally spaced, and, consequently,

valves are not fired at equal time intervals.

2. Jitter in the electronic circuitry of the current regulator

Even with the balanced voltage, this may sometime cause variation

in firing angles from their normal by 10 or 2

0.

3. High gain and short time constant in current regulator

Combination of high gain and short time constant in current

regulator may cause alternate early and late ignitions.

4. Inverters normally operate on CEA control, and unbalanced three-

phase voltage can again lead to unequal timed firing.


5. Interaction of harmonics with fundamental currents

Interaction of characteristics harmonics with fundamental currents in

non-linear elements of the power system, which produces sum and

difference frequency harmonics is also suggested to be the cause of

production of non-characteristics harmonics.

3.4.2 Amplification of non-characteristics harmonics

The addition of harmonics to the fundamental three-phase voltage

wave shifts the times of voltage from zeros from the zeros of the

fundamental waves alone. These shift cause unequally spaced firings of the

valves, which, in turn generates uncharacteristic AC harmonics.

If any of these current harmonics meets a high impedance, significant

voltage harmonics of like order and are produced. This particular harmonic

is amplified by positive feedback. If the loop gain is high enough, a

harmonic oscillation of increasing amplitude is produced: this is instability.

3.4.3 Consequences

Uncharacteristic harmonics

1. Increase telephone interference, because it is not feasible to

provide adequate filtering of each order of them, and

2. In some cases, causes instability of CC control.

3.5 TROUBLES CAUSED BY HARMONICS

Troubles in the Converter and on the AC power System

1. Extra loss and heating in machine and capacitors

2. Overvoltage due to resonance

3. Interference with the ripple control unit

4. Instability of the constant current control of converters

Troubles in the Telecommunication system

1. Noise on voice-frequency telephone lines


3.6 MEANS OF REDUCING HARMONICS

3.6.1 Increased Pulse Number

In low voltage high current rectifier, high pulse number have been

used sometimes used, ranging from 24 to 108. This means of reducing

harmonics is very effective as long as all valves are in service, but it requires

complicated transformer connection. In HV high current converter for dc

transmission, problems of insulation of the converter transformer to

withstand high alternating voltage in combination with high direct voltage

dictate simple transformer connection. A pulse number of 12 is easily

obtained with simple connection of two six pulse valve groups, as we have

seen and 24 pulse can be obtained with four six pulse groups by use of a

phase shifting transformer bank in conjunction with two 12 pulse converter.

The required phase shift is 15°.

The effectiveness of 12 or 24-pulse converter in reducing harmonics is

somewhat decreased when one valve group is out of service. The 12-pulse

converter has some advantages over the six pulses converter even when one

bridges is out of service though less than when all are in service.

3.6.2 Filters

Any necessary reduction in harmonics outputs of the converter

beyond that accomplished by increase of pulses number must be done by

harmonics filter.

Filters are almost always needed on the AC side of the converter and

on the DC side also. The ac filters serves two purposes simultaneously:

supplying reactive power of fundamental frequency in addition to reducing

harmonics. The filter capacitors are required for supply of reactive power.

Thus we are led to concept of the minimum filter, which is required for

harmonics reduction only in installation where the reactive power required


by the converter can be supplied by the ac system without reinforcing the

later. A filter costing more than the minimum filter not only supplies

additional reactive power but also generally gives better filtering.

3.7 HARMONIC FILTERS

3.7.1 Purpose

The AC harmonics filters serves two purposes: (1) to reduce the

harmonics voltage and current in the ac power network to acceptable levels

and (2) to provide all or part of the reactive power consumed by the

converter, the remaining being supplied by shunt capacitor banks, by

synchronous condensers, or by the ac power system. The dc harmonics

filters serve only to reduce harmonics on the dc line.

3.7.2 Classification

The filters at a convertor station may be classified by their location,

their manner of connection to the main circuit, their sharpness of tuning, the

number and frequency of their resonance.

3.7.2.1 Location

Filters are located on both ac and dc sides of convertor. Filter on the

ac side may be connected either on the primary side of the convertor

transformer or on the tertiary winding if one is provided for this purposes.

Filters are never connected to the secondary winding.

3.7.2.2 Series or Shunt

Harmonics may be

A. impeded in passing from the convertor to the power network or

line by a high series impedance or

B. diverted by a low shunt impedance or

C. Both.


The series filter must carry the full current of the main circuit and

must be insulated throughout for full voltage to ground. The shunt filter can

be grounded at one end and carries only the harmonics current for which it is

tuned plus a fundamental current much smaller than that of the main circuit.

Hence, a shunt filter is much cheaper than a series filter of equal

effectiveness.

Shunt filter are used exclusively on the ac side. On the dc side, the dc

reactor, which is obviously a series element, constitutes all part of the dc

filter. It serves several additional functions however, that require series

connection. The remainder of the dc filter consists of shunt branches.

3.7.2.2 Sharpness of Tuning

Two kinds are used (a) the tuned filter which is sharply tuned to one

or two of the lower harmonics frequencies ,such as the fifth and seventh and

(b) the damped filter which if shunt connected , offers a low impedance over

a broad band of frequencies embracing.

3.7.3 Cost of Filters

The capital cost of ac filter is in the range of 5 to 15% of the cost of

the terminal equipment .A minimum filter is one that adequately suppress

harmonics at the least cost and supplies some reactive power but perhaps not

all that is required. A minimum cost filter may not gives adequate filtering.

About 60% of the capital cost of the filter is that of the capacitor Hence,

substantial saving are possible by judicious choice of kind of capacitor.


3.7.4 AC Filters

3.7.4.1 Types

On the basis of tuning and resonance frequency (ies) the AC filter can

be classifies as

Table 5.2 Filter configuration and impedance characteristics


3.7.4.2 Criteria for the Adequacy of AC Filter

Ideally, the criterion should be the absence of all detrimental effects

from harmonics, including telephone interference, which is the most difficult

effect to eliminate entirely. This criterion is impractical from both technical

and economics standpoints. From the technical standpoint of filter design,

the distribution of harmonics throughout the ac network is too difficult to

determine in advance. From the economic standpoint, the reduction of

telephone interference can generally be accomplished more economical by

taking some of the measures in the telephone system and others in the power

system.

3.7.5 DC Filters

The harmonics in the DC voltage across the converter contain both

characteristic and non-characteristics order. The harmonic current generated

in the line can be computed from the knweledge of harmonic voltage sources

ar the converters, smoothing reactor, DC filter and line parameters. The

effectiveness of DC filter is judged by following criteria

3.7.5.1 Criteria for Effectiveness of DC Filter

1. Maximum voltage TIF (Telephone interference factor) on DC high

voltage bus.

2. Maximum induced noise voltage (INV) in milli volts / Km in a parallel

test line one kilometer away from the HVDC line.

Converter faults and Protections

Chapter 4

CONVERTER FAULTS AND PROTECTION

4.1 INTRODUCTION

As in AC system, the faults are caused by malfunctioning of the

equipments and controller failure of insulation caused by external source.

The faults have to be detected and the system has to be protected by

switching and control action such that the disruption in the power

transmission is minimized

Apart from disrupting the normal operation, the various faults that can

occur also cause the stressing of the equipment due to overcurrent and

overvoltages. In a converter station, the valves are the most critical

equipment, which need to be protected against damage caused by the rise in

the junction temperature of thyristor, which is caused by excessive losses in

the device and sensitivity to overvoltage.

4.2 CONVERTER FAULTS

4.2.1 General

There are three basic types of fault occurring in a converter station can

be categorized as:

• Faults due to malfunctions of valves and controllers

• Arc backs (of back fire) in mercury arc valve

• Arc through (fire through)

• Misfire

• Quenching or current extinction

• Commutation failure in inverter

• Short circuit in a converter station

Converter faults and Protections 91

4.2.2 Arc Backs

The arc back is the failure of the valve to block in the reverse

direction and results in the temporary destruction of the rectifying property

of the valve due to conduction in the reverse direction. This is a major fault

in mercury arc valves and is of random nature. This is a non self clearing

fault and result in severe stresses on transformer windings as the incidence

on arc backs is common.

Thyristor do not suffer from arc-backs, which has led to the exclusion of

mercury arc valve from modern converter stations.

Causes

Among the factors that tend to increase the occurrence of the arc-

backs are the following:

• High peak inverse voltage

• High voltage jumps

• High rate of change of current at the end of conduction

• High rate of increase of inverse voltage

Consequences

Arc-back results in line-to-line short circuits, which subject the

transformer and valve to much greater current than does the normal

operation.

Cure

When an arc-back is detected, the main bridge valve should be

blocked, and the bypass valve should be unblocked as soon as possible. The

bridge should remain bypassed until the faulty valve is capable of

withstanding normal inverse voltage.


4.2.3 Arcthrough

Arcthrough is conduction in forward direction during blocking period.

Causes

It can be caused by the failure of the negative grid bias, by a defect in

the grid circuit, by the too early occurrence of a positive grid pulse, or by a

sufficiently great positive transient overvoltage on the grid or anode.

Arcthrough in Rectifier & Inverter

This arcthrough merely reduces the ignition delay angle from its

normal valve to a smaller valve or zero. Its effect on the waveshapes of

current voltage is small, while arcthrough in a inverter produces similar

effect to those produced by commutation failure

4.2.4 Misfire

Misfire occurs when incoming gate pulse is missing and valve is

unable to fire i.e. failure of valve to ignite.

Causes

It may be caused by failure of grid pulse or by low or reversed net

cathode current

Consequences

In an inverter persistent misfire leads to average bridge voltage going

to zero while AC voltage is injected into the link. This results in large

current voltage oscillations in the DC link.

4.2.5 Quenching or Current Extinction

Quenching is the extinction of current which can occur in a valve

when current through it falls below holding current. The current extinction

can result in overvoltage occurs across the valve due to current chopping in

an oscillatory circuit formed by the smoothing reactor and the DC line

capacitance.


Causes

Occurrence of transient at low value of bridge current in mercury arc

valves it may occur as a result of insufficient ionization.

Effects

Quenching of the arc soon after conduction starts has almost the same

effects as failure to begin conduction (misfire). These include a short circuit

of the DC terminals of the bridge for a short while, with the consequent

collapse of direct voltage and interruption of the alternating current.

Quenching may give a high voltage across the break, that is, across the

valve, as well as across lumped inductances.

Protection

• The tank temperature is automatically controlled to the right value.

• Protection against current chopping is employed against quenching.

4.2.6 Commutation Failure

Because of the turn-off time requirements of thyristor, there is need to

maintain a minimum value of the extinction angle defined by

�� 0 –�.�–��

The overlap angle � is a function of the commutation voltage and the

DC current. The reduction in the voltage or increase in the current or both

can result in an increase in the overlap angle which can result in ��MIN.

This gives rise to commutation failure. The current in the incoming valve

will diminish to zero and the outgoing valve will be left carrying the full link

current.

Causes

Because of

• the increased direct current

• low alternating voltage (may be caused by short circuit)


• or a combination of 1 and 2,

Commutation is not completed before the alternating commutation

EMF reverses. Thereafter direct current is shifted back from the incoming

valve to the valve that was expected to go out.

Single and Double Commutation Failure

If the causes which lead to commutation failure in outgoing valve in

first instance have disappeared, bridge operation returns to normal state.

Thus if single commutation occurs, it is self-clearing. Double Commutation

Failure is the failure of two successive commutations in the same cycle.

This occurs when the conditions caused by first commutation failure persist.

Consequences

The following are the effects of a single commutation failure.

1. The bridge voltage remains zero for a period exceeding 1/3 of a cycle,

during which the DC current tends to increase.

2. There is no AC current for the period in which the two valves in an

arm are left conducting.

3. The commutation failure in a bridge can lead to consequential

commutation failures in the series connected bridge unless the rate of

rise of current is sufficiently limited by the series connected

smoothing reactor.

Cure

1. After the occurrence of the commutation failure, the succeeding

commutation is initiated earlier by the C.E.A. control system.

2. If the failure is caused by low alternating voltage, the reappearance of

normal alternating voltage helps prevent further failures.

3. In the event of persistent commutation failure, the bridge in which

they appear should be blocked and bypassed.


4.2.7 Short Circuit in a Bridge

This fault has very low probability as the valves are kept in valve hall

with air conditioning. However, bushing flashover can lead to a short circuit

across the bridge and produce large current peaks in the valve that are

conducting. Short circuit current is significant only in rectifier bridges.

The worst case is when the short circuit occurs at the instant of firing

a valve at .� ��0. the peak current are of the order of 10 – 12 times rated

current and the thyristor valves must have surge current ratings above this

value

Protection

It is similar to arcback in many cases, and the protection against it is

the same as that against an arcback, that is,

1. Blocking the pulses when the fault current goes to zero, the valve

assumes blocking state provided the voltage across it is not too high.

2. If the valve is unable to block the forward voltage, additional loops of

overcurrents results and this can be avoided only by tripping the AC

circuit breaker.

4.3 PROTECTION

4.3.1 General

Protection of HVDC System can be divided into following three

categories;

1. DC Reactor

2. Overvoltage protection

3. Overcurrent protection


4.3.2 DC Reactor

These reactors, usually having inductance of 0.4 to 1.0 H, are

connected in series with each pole of each converter station. They serve

following purposes:

1. Prevent consequent commutation failure by limiting the rate of

increase of direct current.

2. Decrease the harmonic voltage and current in the DC line.

3. To smooth the ripple in the direct current sufficiently to prevent the

current from becoming discontinuous.

4.3.3 Voltage Oscillations and Valve Dampers

)LJXUH��&RQQHFWLRQ�RI�5&�GDPSHU�FLUFXLW�DFURVV�YDOYH�RI�D�FRQYHUWHU�EULGJH�This is a damper circuit used to limit the rate of rise of inverse voltage

and the peak inverse voltage. Such circuits are required also across thyristor

for avoiding their breaking down on inverse voltage exceeding their rated

value. Typical arrangement for valve dampers is shown in Figure 4.1


4.3.4 Current Oscillations and Anode Dampers

If ignition of valve is delayed, a positive voltage builds up across it

which collapses when the valve is ignited. Any stray capacitance across the

valve is charged to this voltage and discharges through the valve as soon as

the latter ignites. Because of inductance (either lumped or stray) and low

resistance, in the discharge circuit, the discharge is oscillatory and lightly

damped. These current oscillations have detrimental effects like radio

interference. To damp out these oscillations, Anode reactor and resistor are

added as shown in Figure 4.2

)LJXUH��%DVLF�&LUFXLW�RI�FXUUHQW�RVFLOODWLRQ��D��6WUD\�FDSDFLWDQFH�DQG�LQGXFWDQFH�RQO\��E��$QRGH�UHDFWRU�DQG�UHVLVWRU�DGGHG�

Case Study

Chapter 5

CASE STUDY

5.1 INTRODUCTION

Vindhyachal HVDC back-to-back station is the first commercial

HVDC project in India, which has been established for the asynchronous

connection between western and northern regions. On the northern side it is

connected to SSTPS via a 400 KV transmission line and on the Western side

to VSTPS via 400 KV bus extensions. Since June 1989, the station is in

commercial operation, benefiting both the regions through power exchange

as and when called for by system conditions, Built at a capital cost of

Rs. 176 Crore it constitute and important asset of Power Grid Corporation of

India Limited (PGCIL).

5.2 LOCATION OF VINDHYACHAL HVDC BTB STATION

Vindhyachal HVDC Back to Back Station is located in the Sidhi

district of east Madhya Pradesh, bordering the state of Uttar Pradesh, about

200 KM south of Varanasi. It is situated inside the NTPC/VSTPP Plant area.

5.3 TECHNICAL INFORMATION AND DATA

The total station occupies an area of 1.88 Lakhs Sq. Meter. There are

two identical blocks of 250 MW capacity each. The 12-pulse rectifier and

inverter (converters) are located inside the valve halls of the station building.

Twenty four thyristors constitute one valve and there are such 48 valves in

the two blocks. The valves are water cooled using demineralised water,

which is produced is a separate treatment unit in the valve cooling system.

Heat exchangers and cooling towers also form part of cooling system.

Case Study 99

Associated 400 KV Switchyard has total 25 bays – four converter

bays, fourteen filter bays, two shunt reactor bays, two northern connection

bays, one tie bay and two western connection bays.

5.4 THE NOMINAL RATINGS

Capita Cost : 176 Crore (1986)

Capacity : 2*250 MW

2 hr overload : 2*275 MW

Min. power per block : 25 MW

Rated DC Voltage : 70 KV

Rated DC Current : 3600 A

Converter Transformer : 400/30.5, 156 MVA

Valve Type : OCTUPLE, 3 No. per Block

Thyristor : YST 60, 192 per Valve

Filters : 11/13, 4*60 MVAr (2 each NR and WR)

: 5/24, 4*40 MVAr (2 each NR and WR)

: 3/27, 4*60 MVAr (2 each NR and WR)

Capacitor Block : 2*40 MVAr Connected to WR

Shunt Reactor : 2*93.2 MVAr (NR and WR)

Commissioning Date : Block-1; Block-2 17th

& 22th

April, 89

5

.5 S

ING

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Case Study 101

5.6 EQUIPMENTS

5.6.1 Converter Transformer

Converter transformer used in Vindhyachal HVDC BTB has

extended delta configuration. 12-pulse converter is obtained by the

combination of two extended delta connections. This lead to the

requirement of only one spare transformer which would have been two if

star-star and star-delta transformer would have been used to obtain a 12-

pulse converter. Phasor diargram of extended delta connection is shown in

the figure 5.3

)LJXUH��&RQYHUWHU�WUDQVIRUPHU�XVHG�DW�9LQGK\DFKDO�+9'�%7%�6WDWLRQ�Ratings

Type : TCA 55

Line Side ABC : 400+16

-12 *1.25% KV 156 MVA

Valve Side abc : 30.5 KV 156 MVA

Temperature Rise : Oil 500 C

Winding 550 C

Case Study 102

Winding Bushing

Terminal KV BIL KV BSL KV BIL KV BSL

ABC 1300 1080 1800 1300

abc 325 270 450 375

N 170 --- 250 ---

Impedance 400/30.5 KV 18.6% 156 MVA

480/30.5 KV 19.5% 156 MVA

340/30.5 KV 19.0% 156 MVA

Connection and Phasor Diagram

�

)LJXUH��3KDVRU�GLDJUDP�RI�FRQYHUWHU�WUDQVIRUPHU¶V�VHFRQGDU\�SKDVH�HPIV��IRU�H[WHQGHG�GHOWD�FRQILJXUDWLRQ��

Case Study 103

5.6.2 Thyristor Valve

Figure 5.5 shows the arrangement of thyristor valve in valve hall.

There are total 592 thyristors in one block. One module contains 24

thyristors in series and there are such eight layers per phase (hanging unit)

with three hanging sections (one for each phase) per block, total number of

thyristors there are, 3*8*24 = 576 thyristors.

)LJXUH��7K\ULVWRU�KDOO�

Case Study 104

5.6.2.1 Rating of thyristor:

Non-repetitive reverse voltage Vrsm : 5350V

Non –repetitive forward voltage Vdsm : 4850V

Repetation voltage Wrsm,Vdrm : 3950V

Octuple valve section without hanging section

Height : 6400mm

Width : 2700mm

Length : 4250mm

Mass : 4000Kg

Thyristor module:

Length : 1200mm

Width : 900mm

Mass : 155Kg

REACTOR MODULE

Length : 550mm

Width : 930mm

Mass : 100Kg

5.6.2.2 Technical data of 6-pulse bridge converter

Rated DC voltage : 35 KV

Normal ideal no-load voltage : 41.2 KV

Maximum ideal no-load voltage : 46.6 KV

Nominal delay angle : 150

electrical

Nominal margin of commutation angle : 200 electrical

Minimum power flow per blocks is decided by the holding current

value and minimum limit is 25 MW per block.

Number of thyristor module/ octuple valve : 32

Number of thyristor module / single valve : 8

Case Study 105

5.6.2.3 Single thyristor module

Figure 5.5 Showing the arrangement of 24 thyristors in series in a single

thyristor module.

)LJXUH��6LQJOH�7K\ULVWRU�PRGXOH�GHVLJQ�

Case Study 106

5.6.2.4 Octuple valve

)LJXUH��2FWXSOH�YDOYH�DUUDQJHPHQWV��Figure 5.6 shows the arrangement of octuple valve at Vindhyachal

Grid where each thyristor shaped symbol represents 24 actual thyristors in

series as shown in figure 5.5

Case Study 107

5.6.3 Smoothing Reactor

Type : XMZ 40/3600 0.04 Henry

Rated direct voltage : 70 KV

Maximum direct voltage : 71kv

Cooling : OFAF

Temperature rise at 3600 A.

Winding : 55 K

Top oil : 50 K

Impulse level:

HV winding : 450 BIL

HV bushing : 450 BIL

MASSES:

Core and coil : 52800Kg

Tank and fittings : 1600Kg

Oil : 13500Kg

Total : 82300Kg

OIL QUANTITY:

In tank : 14500L

In coolers : 610L

Pressure : 0.43Kg /cm2

Case Study 108

5.6.4 Filters

)LJXUH��)LOWHUV�IRU�KDUPRQLF�HOLPLQDWLRQ�

Table 7.1 Values of filter parameters

Type Æ Base Filter Base Filter

Branch (harmonic

order)

5/24 11/13 3/27

Fundamental Reactive

Power (MVAr)

41.60 60 60

Resonance

Frequency (Hz)

250/1200 538/650 150/1350

Reactor Quality

Factor

200 200 200

C (µF) C1 C2

1.19 32.1

C1 C2

1.19 32.1

C1 C2

1.19 32.1

L (mH) L1 L2

26.6 37.0

L1 L2

60.6 2.25

L1 L2

12.9 114.2

R (�� R1 R2

400 2308

R

66.9

R1 R2

329 2666

Filters employed in Vindhyachal HVDC Back to Back Station are

double tuned. These can be Base filters, or auxiliary filters. Base filters are

those which are very much essential for elimination of predominated

harmonics without which the efficient operation of the unit would not have

Case Study 109

been possible. Total 8 base filters (4 for each WR and NR) are connected

to the main bus. Besides these filters, auxiliary filter are also employed

which mainly improves the efficiency of operation.

5.7 SYSTEM CONTROL

5.7.1 Control hierarchy

Control of the DC link is obtained in two steps

Step I : Valve Control

Step II : Block Control

.5.7.1.1 Valve control

Valve control unit convert the controls pulses to firing pulses. It

operates in loop in loop out (LILO) system. It sends a status check pulse to

each thyristor, and if state is favorable for firing a return pulse is obtained

which lead to generation of firing pulse.

5.7.1.2 Block control

A block control controls the performance of one block such as

start/stop order, power order etc. In case of communication failure, a

backup control is also provided

5.7.2 Control modes

There are basically two modes of operation of whole Back-to-Back

station namely:

1. Block control mode

2. Joint control mode

Under block control mode, power transfer through each block is

monitored separately and each block operates independent of each other.

While in case of joint control mode basically control operation of both

block is same but power order issued by RLDC is divided equally among

Case Study 110

two blocks and each block is set for half of the total power to be

transferred.

Joint control mode is said to be the highest control mode of

operation as in case of failure of one block power flow can be continuous

and reliability is maintained. Here it will be sufficient to understand only

one mode ie Block control mode as control procedure for each block is

same.

5.7.3 Block control

The block control, two redundant for each of the two 250 MW

converter mainly contains sequence for start/stop, power control, converter

firing control and converter protection. The units for the power stepping

are located in the block control. There are back-up control panels for each

block, located in the block control cubicles. The block controls for each

block are duplicated (redundant).

5.7.3.1 Converter sequence for start up and shunt down

This unit is automatically administrating start up and shut down. It

keeps track of breaker status, currents and voltages. It de-blocks and blocks

the valve in right order and set the correct initial firing angle.

5.7.3.2 Block power control

The block power control mainly contains the units that converts and

i/p power order to and O/p power order by dividing with a voltage that is a

function of power order and power direction. When two blocks are

controlled separately, the power order is taken from the block part of the

control desk via a data link. When two blocks are controlled together, the

power order is taken from the power order distribution unit in the station

control.

Case Study 111

5.7.3.3 Tap changer control

Tap changer control senses the voltage on AC side of the converter

transformer and changes the transformer taps accordingly to maintain the

required power transfer. Tap changer control is usually slow than grid

control.

5.7.3.4 Converter firing control

Converter firing control controls the firing order of the rectifier and

inverter side thyristors. Usually the frequency at which the two grids (WR

and NR) operates is different so same firing order frequency can not be

provided to the two units. On rectifier side, firing order is calculated by

measuring the frequency of line side of converter transformer and this

frequency is send as the reference for firing pulse generation of that side

only. Similar operation is repeated for inverter side hence continuous

monitoring of the frequency of the two sides is mandatory for the

converter firing control.

5.7.3.5 Valve control

Valve control is same as explained in section 5.7.1.1

5.7.4 STATION LEVEL CONTROLLER

5.7.4.1 Reactive power controller

It can be set for automatically switching in/out AC harmonic filters,

shunt capacitors and shunt reactors to compensate for reactive power

consumption of the converters and to control reactive power exchange

within ± 10 MVAR with Western Grid and ± 25 with Northern Grid

5.7.4.2 Damping controller

Vindhyachal STPP is connected to Western Grid by 400 KV long

transmission lines. Damping controller provides positive damping to

Case Study 112

electro-mechanical oscillations between VSTPS machines and rest of the

system disturbances, provided power flow is from North to West.

5.7.4.3 Frequency controller

Prevents tripping of VSTPP generators in case of islanded condition

at VSTPS by controlling DC power through HVDC.

5.8 OTHER AUXILARIES

5.8.1 Valve Cooling System

Efficient heat dissipation is one of the most important aspect for

reliable operation of the whole unit. It is found that 8% decrease in

ambient temperature (temp. at which valve operates) increases the capacity

of valve by 20%. Valve cooling system is employed for the dissipation of

heat generated in thyristor and snubber-circuit. Mainly it consists of two

circuits:

Fine water Circuit: It is a closed loop circuit with very low electrical

FRQGXFWLYLW\��RI�WKH�RUGHU�RI��6�FP��RI�WKH�WRWDO�ILQH�ZDWHU�LV�continuously filtered through deionizer and deoxidizer tank so as to reduce

its electrical conductivity and corrosive action on metallic pipes. This is

passed through two chambers:

1. C/A (Cation/Anion) Chamber : used for deionization

2. Deoxidizer Chamber

Raw Water Circuit: This water circuit is used for the extracting the heat of

fine water through heat exchanger. This is an open loop circuit and make

up water is continuously maintained by external reservoir.

5.8.2 D G Set

Diesel Generator of 250 KVA mounted in DG Room is used in case

of tripping of all feeders. It is automatically (and manually) operated and

Case Study 113

used to supply the power to auxiliaries requiring 415 volts 3 phase AC and

220 Volts AC supply.

Ratings

Make : NGEF in collaboration with AEG Telefunction

Type : SGBD 312 Z 4/4

250KVA, 200KW, 3 phase, 50Hz, Y- 415V, 348 A

Insulation class : F

Speed : 1500rpm

Duty cycle : S1

Exciter ratings : 44V, 2.3A

5.8.3 Battery Room

In the battery room, three types of voltage levels are obtained

namely:

• 24 V, 500 Ah: 4 banks are arranged, 2 for each block. This usually

supplies power to various relays, controlled cords etc

• 48 V, 200 Ah: Two banks, one for each block. This provide

operating voltage for PLCC purpose

• 220 V, 200 Ah: This voltage level is used for controlling outdoor

equipments, CB, grounding equipments etc.

Normally, battery voltage is utilized for the desired purpose only if its

terminal voltage is within 90 % of its limit.

5.8.4 Battery Charging Room

It has two charging mode

1. Float : voltage control mode

2. Boost : Current control mode

Case Study 114

Boost mode is under operation when it is required to charge the

batteries while float mode is normal operating mode. Normal charging

current is of the order of 500 mA.

5.8.5 Fire Fighting System

This comprises two sections namely VESDO and HALON Room.

1. VESDO

Very Early Smoke Detecting Operator: It is especially used for

Valve Halls where high protection against fire is required. It detects even a

small trace of smoke in very early stage. Detectors are mounted near each

thyristor module and high safety zones. These smoke detector send signals

to HALON room and initiates the respective fire extinguisher.

2. HALON ROOM

In HALON Room, fire extinguishers are mounted for each section

of the whole system including Valve Halls, PLCC room, Battery room, and

all important equipments including transformer, Smoothing reactor etc.

5.8.6 PLCC Room

Power Line Carrier Communication plays very vital role in efficient

control and operation of the whole system. Frequency range over which

information is transmitted is allowed by telecom department.

There are three sections in PLCC Room.

Speech

Speech section just enables the transfer of speech signals at carrier

frequency (in the range of KHz.)

Main 1 Main 1 operated tripping circuits

Main 2In case of mal-operation or failure of Main 1, Main 2 section comes

into picture instantaneously. This ensures the reliability of operation.

Case Study 115

5.9 OPRATION AND MAINTANANCE

Vindhyachal HVDC Back to Back station is in commercial

operation since june1989. The annual station availability figure remains

above the guaranteed valve of 97%. The average station utilization figure

is about 20%.

The utilization of the station has always been need based and the

factor is likely to go up in the future with improved inter regional

coordination.

According to reactive power requirement of northern and western

systems at the local level, the back-to-back station also supplies up to

100 MVAR using the available reactive power elements.

The preventive and breakdown maintenance of valve hall, control

system and bay equipment is carried out by the Operation and Maintenance

staff that constantly upgrades their skill through on the job experience. The

significant contribution of the Operation and Maintenance team include

modification in frequency controller and overhauling of 400KV SF6

breaker.

Conclusion

CONCLUSION

With the advent of power semiconductor converter application in

power system, the existing power transmission technology has undergone

rigorous changes. HVDC Back to Back interconnection is the recent

development in this area which not only improve the reliability and

technical performance of the existing system but incentives to

geographically interconnect areas with significant differentials in power

supply costs can improve the economy of system operation.

Although operation of converter generates current and voltage

harmonics which spreads along the AC and DC side, additional cost of

harmonics filters is overshadowed by associated advantages. Lagging

power factor operation of line commutated converters also requires

additional reactive power supply which is compensated by reactive power

sources. The performance can be improved by using IGBT and HVDC

Light (which is a scope of further development) which gives independent

control of active power and reactive power at both stations. Further, with

use of IGBT as converter elements (still to be analyzed), compact design

and high switching performance may be possible.

Advanced microprocessor based control and protection schemes

have reduced the probability of occurance and severity of faults in

operation to much extent and precise and reliable exchange of power have

been made possible. Hence, in terms of functionality of the new systems

and the resulting application opportunities this is a very attractive solution

to many challenging situations.

Bibliography

BIBLIOGRAPHY

Books

• EDWARD WILSON KIMBARK, Direct Current Transmission (Volume I),

John Wiley & Sons Publication

• C. L. WADHWA, Electrical Power System (Third Edition), New Age

International (P) Limited, Publishers (INDIA)

• MARTIN J. HEATHCOTE, J & P Transformer Book (Twelfth Edition), Newnes

Publication

• S. RAO, EHVAC, HVDC Transmission & Distribution Engineering (Third Edition),

Khanna Publishers, New Delhi (INDIA)

• K. R. PADIYAR, HVDC Transmission, Wiley Eastern Limited (First Edition,

Second Reprint 1993), New Delhi (INDIA)

References

• VINDHYACHAL HVDC BACK TO BACK STATION, Power Grid Corporation of

India Limited (INDIA)

• HVDC CONTROL, Asea Transmission (SWEDEN)

• VINDHYACHAL BACK TO BACK HVDC SYSTEM, National Thermal Power

Corporation (INDIA) & ABB (SWEDEN)

Papers

• Power System Interconnections using HVDC Links, (IX SYMPOSIUM OF

SPECIALISTS IN ELECTRIC OPERATIONAL AND EXPANSION

PLANNING) ,QJP� )TCJCO� #$$� $TC\KN�� )GKT� $KNGFV� #$$� $TC\KN�� ,CP�,QJCPUUQP�#$$�5YGFGP�

• 2QYGT�5GOKEQPFWEVQTU� KP�6TCPUOKUUKQP�CPF�&KUVTKDWVKQP�#RRNKECVKQPU��

4CJWN�%JQMJCYCNC��#$$�5GOKEQPFWEVQTU�#)��5YKV\GTNCPF

• HVDC Technologies – The Right Fit for the Application (2002 ABB

ELECTRIC UTILITY CONFERENCE ) Michael P. Bahrman

Bibliography 118

• High Voltage Direct Current (HVDC) Transmission Systems

Technology Review Paper, (ENERGY WEEK 2000, WASHINGTON, D.C,

USA,) G.J. Roberto Rudervall (ABB Sweden), Raghuveer Sharma (ABB

Sweden), J.P. Charpentier (World Bank ,US)

Websites

• http://www.abb.com

• http://www.answers.com

• http://www.reference.com/encyclopedia

• http://www.tocatch.info

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