design consideration for the eddy county static var compensator

8/9/2019 Design Consideration for the Eddy County Static Var Compensator

1/7

EEE

Transactions

on

Power

Delivery Vol 9

No.2

April

1994

DESIGN CONSIDERATIONS FOR THE

EDDY COUNTY STATIC VAR COMPENSATOR

757

H.

K.

Tyll, Member G. Huesmann K. Habur K.Stump,Sr. Member

W. H.

Elliott, Member

F.

E.Trujillo, Sr. Member

Siemens AG Siemens E&A Southwestern Public Services

ErlangeqGermany Atlanta, GA, USA Amarillo, TX, USA

KEYWORDS

SVC, thyristor controlled reactor (TCR), thyristor swit-

ched capacitor (TSC), reactive power compensation,

voltage control, filter design

ABSTRACT

This paper describes the steps for the design of the

Static VAR Compensator (SVC) Eddy County. The

specified system requirements on operating range, loss

evaluation and harmonic performance led

to

an SVC

configuration which contains one TSC branch, one TCR

branch for continuous reactive power control and two

double tuned filter branches. The system voltage

of

the

SVC secondary bus was optimized to 8.5 kV based on thy-

ristor equipment capabilities. The paper shows the voltage

and current stresses of the thyristor valves taking into

account system faults for the TCR branch and misfiring

effects on the TSC branch. The approach for filter design

considering the harmonic performance requirements and

resulting component ratings are shown. The Eddy County

SVC commenced commercial operation in April 1992.

INTRODUCTION

System studies conducted by Southwestern Public Ser-

vice indicated the need for reactive compensation between

50 MVAr (inductive) and

+

100 MVAr (capacitive). A Static

VAR Compensator system covering this range has been

designed and installed at the 230

kV

Eddy County sub-

station located in the southwest corner of the SPS system.

Figure 1 shows the SPS system in the area of Eddy Coun-

ty. Significant features of the Eddy County substation are

that it contains a back to back HVDC converter (rated 200

MW)

to

transfer power between the

SPS

system and

western New Mexico, it has a 345 kV connection

to

a

major generating station at Tolk, and it has a 230 kV con-

nection to generators in the Cunningham

/

Maddox area.

The purpose of this SVC is

to

provide voltage support for

the system and allow most efficient use

of

the generators

in the system.

This paper presents the SVC designed for the Eddy

County substation and highlights the main SVC design con-

siderations and procedures. The SVC secondary voltage is

optimized

to

take full advantage of the thyristors as proven

by type tests for the selected thyristor design. The circuit

configuration and SVC physical layout are presented. Filter

design considerations and techniques are discussed.

93 SM

450 7

PWRD

A

paper recommended and approved

by the IEEE Substations Committee of the IEEE Power

Engineering Society for presentation at the IEEE/PES

1993 Summer Meeting Vancouver B.C. Canada July

18- 22 1993. Manuscript submitted August 28 1992;

made available for printing April 5 1993.

PRINTED IN USA

NEW

MEXICO

\--- .

Roswell

Lubbock

Eddy County Cunningham DenverCity

To

EPE

Maddox

TEXAS

4

A

atisbad

345 V

230 kV

115

kV

Legend: - - -

- - -

HVDC Ba ck-to-Back

Station

Fig. 1 Location of the Eddy County Substation

NETWORK REQUIREMENTS

System characteristics:

Normal voltage

Max. cont. voltage

Min. cont. voltage

Max. phase voltage unbalance

Transient overvoltage

Normal base frequency

Normal frequency deviation

Min. cont. frequency

Max. cont. frequency

Short circuit power range

System harmonic requirements

Individual Voltage Distortion

Total Voltage Distortion (THD)

230 kV

1

.O

pu)

242 kV (1.05 pu)

219 kV (0.95 pu)

2

o

345 kV (1.5 pu)

60

Hz

20.02Hz

59.5 Hz

60.5 Hz

1.1

...2.0

GVA

1.O

1.5%

SVC basic data requirements

Design points

Rated inductive power -50 MVAr 1

.O

pu voltage

Rated capacitive power 1OOMVAr 1.O pu voltage

Operating range

Continuous cap. operation

0.85

to 1.1 pu voltage

Continuous ind. operat ion

0.9 to

1.1 pu voltage

Full inductive operation at voltages above

1.1

pu

0885-8977/94/$04.00 993 IEEE


2/7

758

The data listed above results in a V

/ I

characteristic as

shown in Figure

2.

Below

0.85

pu voltage the SVC shall

operate at its capacitive limit. At voltages above

1.1

pu the

SVC shall operate at its inductive limit. The operation at

1.5

pu voltage is a limited period of

3

cycles. Within the

voltage range of

0.85 to 1 . 1

pu, any operating point else-

where in the shadded area is permitted.

I

TSC+TCR+FC

I

I

I

I I I

I

I 1

I

I 1

I I

I 1

100 50

I

I 1

design point design point

TCR+FC I

I

I

I

50

Oprim

in

MVAr

Continuous

V

operation

4

1 o 0.5

0 0.5

b r i m

in PU+

~leurCaf ivofi

-

apacitive

Operation

Fig.

2: V / I

diagram of the SVC Eddy County

as seen on the HV side

4

Capacitive

Range

SVC DESIGN

Inductive

Range

Basic configuration

Figure 3 shows a single line diagram of the SVC confi-

guration selected

to

meet requirements outlined in the

previous section. SVC capacitive requirements are met by

a

76

MVAr TSC branch and a 24 MVAr filter. The filter is

chosen to shunt away harmonic currents produced by the

TCR

so

that the harmonic distortion limits

of

the system

are satisfied. The TCR branch is rated for approximately 74

MVAr to compensate the filter and absorb the required

50

MVAr from the system. The MVAr figures are referred

to

the

230

kV bus and include the effect of the

12

SVC

transformer.

The TSC branch includes two surge arresters. An arres-

ter labeled CC in Figure

3

is connected across the capa-

citor to limit voltage in case the thyristor misfires at the

worst possible time. A second surge arrester labeled SR in

Flgure

3

connected across the TSC valve and reactor

protects the thyristor against overvoltages. Neither of these

arresters operate during normal SVC operation. They

protec the TSC branch from faults and misfiring transients.

Figure

4

shows the SVC operating diagram. There is an

overlap range of approximately

10

degrees where the TSC

may be either off or on depending upon past history. This

overlap is controlled by a hysteresis type effect and avoids

control instability at the TSC switching time even for the

case of large system frequency deviations as specified.

230 kV, 60 Hz

TSC

DF1

A

LTSC = 0.58 mH

C T ~ C

765 pF

LI = 0.81 mH

L2 = 73pH

C1

= 10000 pF

Cp

= 368pF

I

DF2

TCR

A

L3 =

0.20

mH

4 = 50pH

C3

= 1753 pH

C4 = 441 pH

R

= 7.5 Q

LTCR = 7.5 mH

Fig. 3: Single line diagram and data

of the SVC Eddy County

Fig.

4:

Operating diagram of the SVC Eddy County


3/7

759

The TCR reactors consists of two coils per phase in-

stalled in a double stack arrangement. Reactors for the

TSC are installed

side by side. The filter arrangement

consists of two double tuned filters (DF). The first is tuned

to the 3rd and 5th harmonics and the second is tuned to

the 7th and ll.5th harmonics. The second DF includes a

resistance to achieve a high pass characteristic for the

higher harmonics.

Filter branches are tuned very accurately to keep the

harmonic distortion to a minimum and within specified

limits. Filter reactors have been designed with taps to

enable the inductance to be adjusted. This allows initial

mistuning due to reactor and capacitor manufacturing

tolerances to be compensated. Filter reactors are installed

in a triple stack arrangement. Hence, it is very important to

consider the influence of mutual coupling between coils to

achieve proper tuning. More information about the filter

design procedure is given in the Filter Design section.

Operatinq losses

formula.

The SVC losses are evaluated according

to

the following

LOSS-Cost

=

$Cl

x

A + $C2 x B + $C3 x C

A, B, C

$C l, $C2, C3

operating ranges

/

points

cost figures for A, B, C

$C1 = $1800

$C2

=

$56

$C3 = $56

A Average power loss in kW for SVC output on 230 kV

bus from max. inductive range to

0

MVAr

B Maximum power loss in kW for SVC output on 230 kV

from 0 MVAr to the net filter range

C Average power

loss

in kW for SVC output on 230 kV

from the net filter range to 100 MVAr.

Figure 5 shows the operating losses of the SVC as a

function of its reactive power output. The average losses of

the overall operating range based on the nominal ca-

pacitive power are 0.45%. The cost of losses evaluated

according the given formula amounts to

$

0.5 Mio. A loss

versus reactive power characteristic is shown in Figure 5

as dorred line. Comparable average losses would amount

to 0.43

Y

which is only slightly lower.

But converted to loss cost according the given formula

an amount of $ 1.1 Mio would arise. The difference of $

0.6 Mio justifies the higher installation cost using a TSC

branch.

Losses

1 F 100 Ind.

00 80 60 40 20 20

40

60

Reactive

Power

[MVAr]

Fig. 5: Operating losses

of

the SVC Eddy County

Physical Layout

Figure 6 shows the physical layout of the SVC system.

The branches (TCR, TSC and filters) are connected

to

the

secondary bus by removable links. This arrangement

allows failed branches to be disconnected

so

that SVC

operation can be resumed in a degraded operating mode.

Removable links used for branch connections allow a

lower cost design than disconnect switches. In addition to

lower hardware costs, removable links require less space

in the station than switches. Total area for the SVC equip-

ment is only 50

m

by 44.5 m.

Filters

k

Fig. 6: Layout of the SVC Eddy County

The SVC control building contains the open loop control

(PLC), closed loop control (regulation)[4,5], protection

system consisting of main and back-up protection, thyristor

valves with Valve Base Electronic (VBE), cooling system

for the thyriztor v&es and AC

/

DC distribution which

includes a battery unit. In addition the building contains

switchgear control cubicles and a transient fault recorder

from the

SPS

Company. The control room is airconditioned

and the thyristor valve rooms are ventilated to keep the

temperature within the allowed limits.

The thyristor valve cooling system is a single circuit

cooling system with outdoor water

/

air recooler fan banks.

To avoid freezing

of

the water during extreme cold weather

conditions a water

/

glycol mixture is used. The glycol con-

tent is approx. 30 . This will prevent the cooling system

from freezing at temperatures down to 5 degree F when

the SVC is not in operation. The cooling system includes

redundant pumps

(100

o redundancy). Two water / air

heat exchangers provide 100

oo

redundancy for

temperatures up to 92 degree F.


4/7

760

DesiQncalculations for the thyristor valves

49.7 V.

25 kV

valve voltage

- 25 kV1

10 kA

29.0 V'

-

22.5

kA

15.8 kV

0 kV

10

kAr

T 7

Nominal secondary voltage for the SVC was chosen as

8.5 kV to make optimum use of the thyristor capabilities.

Three phase thyristor valves consist of three thyristor

modules stacked one above the other. Figure 7 shows a

three phase TSC valve. The Eddy County TSC valve con-

tains

14

hyristor levels and the TCR valve contains

7

thy-

ristor levels. The thyristors each have a current carrying

capacity of

4200

Arms and a peak blocking voltage of

5.6

kV. In the Eddy County design, rated TSC current is 3200

Arms

and rated TCR current is

3300

Arms. These design

ratings are below the

4200

Arms capability of the thyristors

and allow for additional stress due

to

short time overloads

and thyristor misfiring [l] In all cases, the junction

temperature remains well below 120 degrees C.

Thyristor modules applied in the Eddy County SVC

design have been extensively type tested for other

applications

[2].

est values are based on a draft paper of

the ClGRE working group

14.01

TF

02.

hat proposes tests

which are more applicable

to

real SVC valve stresses than

those which have been proposed by IEC

146.

F r u n t

v i e S i d e V i e w

, W a t e r t u b e s

n u b b e r c a p a c i t

W a t e r o o l e d

snubber

r e s i s t

Thyristor electronics

Fig.

7:

Drawing of a three phase TSC valve

Valve stresses in the Eddy County SVC have been cal-

culated with the NETOMAC

[3]

digital electromagnetics

program. Worst current, recovery voltage, and di

/

dt

stresses for the TSC valves occur during a misfiring event.

Misfiring calculations were based on the worst case

sequence of events as follows:

- The SVC is in full capacitive operation. This leads

to

the

maximum voltage on the secondary side of the SVC

transformer.

During this operation the TSC valves are blocked and

then the valve in the TSC leg with the highest capacitor

voltage is misfired.

-

The instant of misfiring is chosen at the time of

maximum blocking voltage across the TSC valve. This

results in maximum current magnitude and highest di

/

dt stresses.

Figure

8

shows the voltages and currents in the TSC

branch for this misfiring. The magnitude of the current

pulse was below 23 kA. Recovery voltage across the

blocked valve remained below

50

kV. This is well below

the blocking voltage of the

14

hyristor level TSC valve.

-

24.4 V

20kVL

I

\

10 kA

capacitor voltage

i30.1 kV

capacitor current, 22.5

kA

Fig. 8: Voltage and current stresses of a TSC branch

due to a valve misfiring

40 k V

4 k q -valve current AB

n

n

rq

w v

valve voltage

AB

9.35kA

valve voltage

CA

Fig. 9: Voltage and current stresses of a TCR branch

due to a three phase system fault

The worst stresses for a TCR valve occurs during a three

phase fault on the high voltage bus. Voltage and current

stresses for this case are shown in Figure 9. Note that

these traces show valve current when the valve is

conducting and voltage when the valve is blocked. The

sequence of events for Figure

9

were as follows:

- Full inductive operation at 1.5per unit voltage

- Fault initiation at the time when one TCR current is at a

peak

-

Fault clearing between

4

and 5 cycles after initiation

- TCR valve blocking at the first current zero after fault

clearing.


5/7

761

that components are rated properly and not overstressed

in the extreme conditions.

The network impedance as a function of frequency must

be known for proper filter performance design. The

impedance versus frequency characteristics

of

the Eddy

County bus were computed for many system conditions.

The following parameters were varied during the system

impedance evaluation study:

-

range of generation

-

range of load

- various steps of HVDC compensation

- system configuration (lines out).

Figure 12 shows the system impedance boundary that

was determined from the impedance versus frequency

calculations. For all frequencies and system conditions, the

network impedance is somewhere within the shaded area.

Data given on the figure quantitatively defines the network

impedance region.

equiv

-

The TCR currents decay on the long time constant of the

TCR branches. Clearing of the fault at the most

unfavorable time adds an additional peak to the thyristor

currents in phases AB and CA. The TCR current in one

phase (CA in Figure 9) may be prolonged because of

delayed zero crossings. Junction temperature must remain

below 120 degrees C in this extreme case for the thyristors

to remain undamaged. Oscillation in the TCR current

during the decay period is due

to

the natural frequency of

the filters and the SVC transformer leakage inductance.

Figures 10 and 11 are examples from the thyristor type

tests which were performed at the CESl laborities in

Milano, Italy [2]. Figure 10 shows the resutts of a fault test

which is equivalent to a thyristor misfiring. Figure 11 shows

the result of a DC trapped current decay test. These tests

are more sever than the stresses computed for the Eddy

County thyristors in the TSC and TCR branches.

Calculation of the thyristor junction temperature verified

that it remains below 120 degrees C at the end of current

flow when the valve is required

to

withstand the recovery

voltaae. Hence, it is concluded that the thyristor valves in

Iv TCR

-

(Fault current test)

Fig.

11

Test results of the TCR valve type test

(DC trapped current)

Filter desiqn

Filter design is one of the most important steps in the

SVC design process. The first step in proper filter design

must ensure that the specified distortion limits are not

exceeded. The second step in filter design is

to

ensure

Fig. 12: Network impedance equivalent area

The equivalent circuit for all harmonic calculations is

shown in Figure 13. The TCR is represented as the

harmonic current source. In parallel the SVC filter and TSC

branches are connected. The TSC branch may be

switched off according

to

the SVC operation conditions.The

LV bus is connected via the transformer impedance

to

the

230

kV

system. At the 230 kV bus the HVDC filters and the

equivalent network impedance are represented. The HVDC

filter branches can be regarded according to the HVDC

operation.

Fig. 13: Equivalent circuit for harmonic calculations

At a first step without knowledge about the system

impedance only a filter tuned

to

fifth harmonic was


6/7

762

assumed. Afterwards a two double tuned filter arrangement

had

to

be selected, but the total fundamental power for the

filter branch was not changed.

The worst resonance condition between the network

impedance boundary and the harmonic filters was

assumed for the calculation of the individual harmonic

voltage distortion. Harmonic voltage distortion was also

computed with the network circuit open. The total harmonic

distortion (THD) was computed with the two highest

individual harmonic distortion values from the worst case

network resonance conditions and the remaining harmonic

distortions determined with the network open circuited.

This yields to the worst possible THD results.

Harn

nonic performance is based on:

Nominal system voltage

Maximum initial mistuning due

to

tolerances

Detuning according temperature range

Assuming ma . magnitudes of TCR harmonic current

independant on firing angle

Negative sequence voltage content which gives rise

to generation of triple harmonics outside the delta

Firing angle unsymmetry which results in generation

of even harmonics

Loss of capacitor units of up to alarm level

Operating ranges of the TCR

Rating calculations for the filter components are done

seperately from the performance calculations. They are

based on similar conditions as above with the addition that

the following conditions are imposed:

- Maximum system voltage of 1.1 pu

- Increase of harmonic currents by 10°/~

- Extended detuning effect due to loss of capacitor

units up to trip level.

I -

Ambient harmonics

In principal the same equivalent circuit was used. The

limitation of the impedance area by the straight lines was

increased to 80”. The maximum impedance was set to

infinity. This leads to higher amplification factors and

results in higher safety margins for the components. The

influence

of

ambient harmonics on filter component rating

is considered assuming the infeed of harmonics from the

HVDC station at the 230 kV busbar.

kV

Fig. 14: Voltage and current stresses of the components

in the parallel circuit of DF 3

/ 5

Rating values of the components have also been

checked for transient conditions of:

- SVC energization

- Three phase system fault and voltage recovery.

-

Saturation by neighbor transformers

Figure 14 shows worst case stresses of the 3

5

DF

components during the condition of a three phase fault and

volta e recovery. Harmonic currents generated by the

HVD8 and SVC transformer results in extreme high

stresses for the filters. The voltage and current stresses of

the parallel circuit of the DF in this case determine the

actual rating values for these components.

CONCLUSION

A Static Var Compensator meeting the, -50 MVAr to

+

100 MVAr compensation range required by the

Southwestern Public Service Company has been designed

and installed at the Eddy County substation near Artesia,

New Mexico. Basic requirements on voltage and reactive

power were determined for Eddy County by computer

studies and project ions of future SPS needs. Costumer

load, fuel expense, voltage regulation, generation, energ

losses, and system reliability were considered in the SV

design specification.

A basic configuration consisting of a TSC branch, a TCR

branch and two double tuned filter branches has been

designed to meet the system requirements. An econo-

mical and efficient design has been achieved by applying

thyristors with surge arrester protection, where necessary,

that have been previously type tested for conditions more

severe than those that can exist in the Eddy County SVC.

Loss curves and evaluation has indicated favourable SVC

efficiency. Filter branches in the SVC were designed to

meet the SPS performance specifications and component

ratings were selected to exceed stresses imposed by the

system. Close working cooperation between Southwestern

Public Service Company and the SVC supplier during all

stages of the project (studies, design, installation and

commissioning) made it possible to meet all SVC

requirements and

to

put the SVC into commercial

operation within the scheduled time frame.

REFERENCES

G.Thumm, H.Tyll, ”A Closer Look at Thyristors in

SVC applications”, Siemens Energy and Automation,

Vol. 1 pp. 12-17, 1989

B. Endres, G . Thiele,

I.

Bonfanti, G. Testi, ”Design

and Operational Testing on Thyristor Modules for the

SVC Kemps Creek”, IEEE Transaction on Power

Delivery, Vol.

5,

No

3,

July 1990, pp. 1321-1328

W.Baver. K. H. Kruaer. D. Povh. B. Kulicke. ”Studies

for HVDC and SVC ‘Using the NETOMAC Digital

Program System”, IEEE CSEE Joint conference on

High Voltane Transmission Systems in China, 1987,

Paper 873C-32

H. Tyll, K. Leowald, F. Labrenz, D. Mader, ”Special

Features of the Control Svstem of the Brushv Hill

SVC”

,

Canadian Electrical Association, Power Sistem

Planning and Operation Section, Spring meeting,

Toronto 1989

K. Bergmann, B. Friedrich, K. Stump, W. Elliot,

Digital Simulation, Transient Network Analyzer and

Field Tests of the Closed Loop Control of the Eddy

County SVC”, IEEE 93

-

WM

056 -

2 PWRD


7/7

BIOGRAFIES

763

Frank E. Truiillo was born in Deming,

New Mexico, on July 17, 1954. He

William H. ( Bill ) Elliott was born in Bird

City, KS on June

1,

1942. He received a

bachelor of sience degree in electrical

engineering from Kansas State

University, Manhattan, Kansas in 1965.

He joined Southwestern Public Service,

Amarillo, TX in 1965. Mr. Elliott’s

activities have been in the field of

system and transmission operation,

system engineering and planning.

~

Currently he is working as Principal

Engineer in Electrical Operation’s. Mr. Elliott is member of

the Institute of Electrical

&

Electronic Engineers (IEEE), the

Texas Society of Professional Engineers ( TSPE ) and the

National Society of Professional Engineers NSPE

).

Klaus Habur was born in Furth, Bavaria,

Federal Republic

of

Germany on April 8,

1952. He received his education at the

Ohm-PolytechnikumiNurnbergand at the

DAG-TechnikumMlurzburg. He joined

Siemens AG 1980 as a design engineer

for electrochemical plants. Previous to

this work, he was with a consulting

engineers company. In 1985 he joined

the Reactive Power Compensation Sales

Department and worked as a project

engineer and project manager for various SVC projects.

Gabriele Huesmann was born in

Munster, Westfalia, Federal Republic of

Germany on March 18, 1963. She joined

Siemens in 1982 and received an

education as engineers assistant. After

two years work in the Transportation

Systems department, she joined the

network planning department. Since

1986 she is active in the field of

programming, SVC design and harmonic

system analysis.

Keith B. Stump was born in Richmond,

Indiana, on February 12, 1941. He

received a bachelor of Science de ree

in electrical engineering from 8hio

University in 1963, and a Master of

Science degree in electrical engineering

from Purdue University in 1965. He was

employed by Allis-Chalmers Corp. in

Milwaukee, WI, from 1965 through 1977.

He then transferred to Siemens-Allis,

Inc. in 1978 which became Siemens

Energy &Automation, Inc., Atlanta,

GA.

Mr. Stump is

currently working in the Power Systems Technology

department in the area of system simulation and analysis.

Mr. Stump is a member of the IEEE Power Engineering

Society and vice chairman of the IEEE Surge Protective

Devices Committee.

received a Bachelor of Science Degree

in Electrical Engineering from New

Mexico State University, Las Cruses,

New Mexico in 1976. He joined

Southwestern Public Service, Amarillo,

TX in 1977. Mr Trujillo’s activities have

been in the field of system engineering.

Currently he is working as Senior Design

Engineer in the System Engineering

group for SPS. Mr. Trujillo is a Senior member

of

IEEE.

Heinz

K.

Tyll was born in Hof, Bavaria,

Federal Republic of Germany on May

15, 1947. In 1968 he graduated in

Electrical Engineering from Coburg

Polytechnikum. In 1974 he received the

diplom degree from the Technical

University of West-Berlin. After joining

Siemens AG, he worked in their High

Voltage Transmission Engineering

Department since 1975 in the field of

SVC system analysis with transient net-

work analyzer and di ita1 programs. Since 1988 with the

System Engineering &oup of the HVDC and SVC Sales

Department he is responsible for SVC design and

transmission system analysis. He is member of IEEE,

ClGRE WG 38 TF 04 and 05 and also IEC WG 22F TF

05.

design consideration for the eddy county static var compensator

Documents