International Journal of Emerging Technology and Advanced Engineering

Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 10, October 2013)

153

Voltage Sag Mitigation in the Distributed Generation System

with STATCOM Sumeet Trivedi

1, D. Chattopadhyay

2

1M.E.Student, Power System Engg., S.S.C.E.T., Bhilai, C.G., India 2Sr.Asso.Professor, E.E.E. Deptt., S.S.C.E.T., Bhilai, C.G., India

Abstract— This paper investigates the effect of Static

Synchronous Compensator (STATCOM) on the voltage sag in

a distributed generation system. For this purpose, the DG

system of an integrated steel plant, which is connected to 132

KV grid, is taken under study. The entire system is modelled

with STATCOM based Voltage Source Converter (VSC)

PWM technique. The voltage sag improvement of the DG

system is compared with and without STATCOM. Power

System Computer Aided Design/Electromagnetic Transients

including DC (PSCAD/EMTDC) is used to carry out

simulation of the system under study and detailed results are

shown to access the performance of STATCOM based Voltage

Source Converter.

Keywords— Distributed Generation, Voltage Sag,

STATCOM, PSCAD, PWM

I. INTRODUCTION

A Voltage Sag as defined by IEEE Standard 1159-1995,

IEEE Recommended Practice for Monitoring Electric

Power Quality, is a decrease in RMS voltage at the power

frequency for durations from 0.5 cycles to 1 minute,

reported as the remaining voltage. The measurement of a

Voltage Sag is stated as a percentage of the nominal

voltage, it is a measurement of the remaining voltage and is

stated as a sag TO a percentage value. Thus a Voltage Sag

to 60% is equivalent to 60% of nominal voltage, or 288

Volts for a nominal 480 Volt system. Voltage sags can

occur on Utility systems both at distribution voltages and

transmission voltages. Voltage sags which occur at higher

voltages will normally spread through a utility system and

will be transmitted to lower voltage systems via

transformers. Voltage sags can be created within an

industrial complex without any influence from the Utility

system. These sags are typically caused by starting large

motors or by electrical faults inside the facility.

Fig.1 A balanced three phase voltage sag

Traditional methods of Voltage Control include

Transformer Tap Changers both mechanical and SCR

switched units [4], Servo-Variac technology and Ferro-

Resonant Transformers (constant voltage transformers) . In

some cases and for some applications these traditional

technologies may still be applicable and work well but in

many cases they were designed to correct problems other

than voltage sags. Industrial UPS units are widely used to

protect electronic process control equipment and to allow

for an orderly shutdown of the process but it is rarely

economic to install large UPS systems with their attendant

large battery banks for high power electrical equipment

such as high horsepower drives, extruders etc. Different

power electronics solutions are used for improving the

performance of the power system by mitigating the voltage

sag [7]. Mitigation devices are installed at the system-load

interface for power quality enhancement and prevent the

voltage dip [5],[8],[9].

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154

At present, a wide range of very flexible controllers,

which capitalize on newly available power electronics

components, are emerging for custom power applications.

Among these, the static compensator (STATCOM) and the

dynamic voltage restorer (DVR), both of them based on the

VSC principle, are the controllers which have received the

most attention. The DVR is a dynamic solution for

protection of critical loads from voltage sags / swells [6].

The DVR restores constant load voltage and voltage wave

form by injecting an appropriate voltage. Modern industrial

processes are based on a large amount of electronic devices

such as programmable logic controllers and adjustable

speed drives. Unfortunately, electronic devices are

sensitive to disturbances, and thus, industrial loads become

less tolerant to power quality problems such as voltage

sags, voltage swells, and harmonics. The dynamic voltage

restorer (DVR) has become popular as a cost effective

solution for the protection of sensitive loads from voltage

sags and swells [10].

A Static Synchronous Compensator (STATCOM) is a

static synchronous generator operated as a shunt connected

static var compensator whose capacitive or inductive output

current can be controlled in- dependent of the AC system

voltage. A STATCOM is a controlled reactive-power

source. It provides voltage support by generating or

absorbing reactive power at the point of common coupling

without the need of large external reactors or capacitor

banks. Voltage fluctuations caused by rapid industrial load

changes have been a major concern for both power

companies and customers in the area of power quality [1].

The fast response of the static compensator (STATCOM)

makes it an efficient solution for improving power quality

in distribution systems. The STATCOM have a function of

compensating reactive power, absorbing the harmonic and

compensating the voltage dip [2].

II. STATCOM

The Static Synchronous Compensator (STATCOM) is a

VSC that is shunt connected to the distribution system by

means of a tie reactance. In general, a coupling transformer

is installed between the distribution system and the

STATCOM for isolating the low voltage (STATCOM)

from the high voltage (distribution system), as shown in

Fig. The STATCOM can be seen as a current source since

it is connected in shunt with the distribution system and the

load.

By controlling the magnitude and the phase angle of the

output voltage of the VSC, both active and reactive power

can be exchanged between the distribution system and the

STATCOM. Being a shunt connected device, the

STATCOM mainly injects reactive power to the system.

Fig.2 STATCOM connected to a power system

The contribution of the STATCOM to the load bus

voltage equals the injected current times the impedance

seen from the device, which is the source impedance in

parallel with the load impedance. The ability of the

STATCOM to compensate the voltage dip is limited by this

available parallel impedance. In addition, the device should

be installed as close to the sensitive load as possible to

maximize the compensating capability. The STATCOM

also can be used in the function: power factor correction,

mitigation of load fluctuation (including voltage flicker) [3]

and active filtering.

A STATCOM functions as a shunt-connected

synchronous voltage source which accounts for its superior

functional characteristics, better performance, and greater

application flexibility. Concerning the non-linear operating

range, the STATCOM is able to control its output current

over the rated maximum capacitive or inductive range

independently of AC system voltage. Thus, the STATCOM

is more effective in providing voltage support under large

system disturbances during which the voltage excursions

would be well outside of the linear operating range of the

compensator.

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The ability of the STATCOM to maintain full capacitive

output current at low system voltage also makes it more

effective in improving the transient stability.

Fig.3 STATCOM V-I characteristics

The attainable response time and the bandwidth of the

closed voltage regulation loop of the STATCOM are

significantly better. In situations where it is necessary to

provide active power compensation the STATCOM is able

to interface a suitable energy storage (large capacitor,

battery, ...) from where it can draw active power at its DC

terminal and deliver it as AC power to the system.

III. DISTRIBUTED GENERATION SYSTEM OF NECO STEEL

PLANT

Distributed generation (or DG) generally refers to small-

scale (typically 1 kW – 50 MW) electric power generators

that produce electricity at a site close to customers or that

are tied to an electric distribution system. There are many

reasons a customer may choose to install a distributed

generator. DG can be used to generate a customer’s entire

electricity supply; for peak shaving (generating a portion of

a customer’s electricity onsite to reduce the amount of

electricity purchased during peak price periods); for

standby or emergency generation (as a backup to Wires

Owner's power supply); as a green power source (using

renewable technology); or for increased reliability. In some

remote locations, DG can be less costly as it eliminates the

need for expensive construction of distribution and/or

transmission lines.

Distributed Generation has a lower capital cost because

of the small size of the DG (although the investment cost

per kVA of a DG can be much higher than that of a large

power plant).

It may reduce the need for large infrastructure

construction or upgrades because the DG can be

constructed at the load location. If the DG provides power

for local use, it may reduce pressure on distribution and

transmission lines [11]. With some technologies, DG

produces zero or near-zero pollutant emissions over its

useful life (not taking into consideration pollutant

emissions over the entire product lifecycle i.e. pollution

produced during the manufacturing, or after

decommissioning of the DG system). With some

technologies such as solar or wind, it is a form of

renewable energy. It can increase power reliability as back-

up or stand-by power to customers and offers customers a

choice in meeting their energy needs [12].

In this paper, we have taken under study, the distributed

power system of NECO steel plant situated at Raipur. This

system consists of six turbo generators of small size which

generate electricity utilizing the heat of waste blast furnace

gas and coke oven gas in the boilers & produce steam to

run the turbines. Out of these, TG1 to TG5 generates power

at 6.6 kV whereas TG6 generates at 11 kV. As it is clear in

the single line diagram that all the generators are ultimately

connected to 132 kV CSEB grid, so the flow of power is

possible in both the directions, as per condition.

Fig.4 Single Line Diagram of power system of NECO steel plant

connected to 132 kV grid

IV. SIMULATION IN PSCAD AND DESCRIPTION

PSCAD/EMTDC is an industry standard simulation tool

for studying the transient behaviour of electrical networks.

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156

Its graphical user interface enables all aspects of the

simulation to be conducted within a single integrated

environment including circuit assembly, run-time control,

analysis of results, and reporting [15]. PSCAD/EMTDC

use in determining the exact switching times that allows the

simulation to run at high speed and does not introduce

inaccurate results. Another valuable feature of

PSCAD/EMTDC is the accurate model provided for the

wind turbine rotors, transformers, underground cables and

other electrical and electronic devices. At present a wide

range of very flexible controllers, which capitalize on

newly available power electronics components, are

emerging for custom power applications. Among these, the

static compensator (STATCOM) and the dynamic voltage

restorer (DVR), both of them based on the VSC principle

and the SSTS are the controllers which have received the

most attention. PSCAD/EMTDC will be used to perform

the modelling and analysis of such controllers for a wide

range of operating conditions [16]. PSCAD/EMTDC’s

highly developed graphical interface has proved

instrumental in implementing the graphics-based PWM

control the STATCOM [13],[14]. The single line diagram

shown above is modelled in PSCAD.

Fig.5 Three Phase Fault block and STATCOM connected to 132 kV

bus of the network

Now we connect a Three Phase Fault block in the

diagram in the 132 kV bus of the network with additional

resistance and inductance. This will create the effect of a

voltage sag instead of the fault, whose duration will be

determined by the Timed Fault Logic. To mitigate this

voltage sag, STATCOM is connected to the same bus via. a

filter, an HV breaker and a 2-wdg. transformer.

This breaker is initially in closed condition. A portion of

the network is shown below with 132 kV bus and

STATCOM & Fault block connected to it.

V. RESULTS AND DISCUSSION

A. During Normal Condition

1) When the grid connected DG system is working under

the normal conditions and the simulation is run for no fault

(or voltage sag) introduced in the system, then the three

phase grid voltage is obtained as shown below:

RE : Graphs

Time S 0.500 0.520 0.540 0.560 0.580 0.600 0.620 0.640 ...

...

...

-100

-75

-50

-25

0

25

50

75

100

Voltage (

kV

)

V132

Fig.6 Three phase voltage during normal condition

2) The per unit voltage is also constant at 1.1 pu during the

whole simulation time which is in the voltage stable zone:

Voltage in pu

Time S 0.0 1.0 2.0 3.0 4.0 5.0 ...

...

...

-2.00

-1.50

-1.00

-0.50

0.00

0.50

1.00

1.50

2.00

Voltage p

u

Vpu

Fig.7 Per unit voltage during normal condition

3) In this condition, the fault current is zero for all the three

phases:

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157

Fault Current

Time S 0.0 1.0 2.0 3.0 4.0 5.0 ...

...

...

-2.00

-1.50

-1.00

-0.50

0.00

0.50

1.00

1.50

2.00

Fault C

urr

ent

(KA

)

Fault Current Ifa Fault Current Ifb Fault Current Ifc

Fig.8 Fault current in three phases during normal condition

B. During Voltage Sag Condition

1) By using the Three Phase Fault block, a voltage sag is

introduced in the 132 kV bus voltage from 1.0 sec. to 1.75

sec.

RE : Graphs

Time S 1.0 2.0 3.0 4.0 5.0 ...

...

...

-100

-80

-60

-40

-20

0

20

40

60

80

100

Voltage (

kV

)

V132

Fig.9 Three phase voltage during voltage sag condition

2) The dip in the per unit voltage is also obtained as it

dropped from 1.1 pu to 0.85 pu for 0.75 sec.

Voltage in pu

Time S 1.0 2.0 3.0 4.0 5.0 ...

...

...

-2.00

-1.50

-1.00

-0.50

0.00

0.50

1.00

1.50

2.00

Voltage p

u

Vpu

Fig.10 Per unit voltage during voltage sag condition

3) During the same interval, a fault current is flowing in all

the three phases i.e. Ifa, Ifb, Ifc is not zero for 0.75 sec.

Fault Current

Time S 1.0 2.0 3.0 4.0 5.0 ...

...

...

-2.00

-1.50

-1.00

-0.50

0.00

0.50

1.00

1.50

2.00

Fault C

urr

ent

(KA

)

Fault Current Ifa Fault Current Ifb Fault Current Ifc

Fig.11 Fault current in three phases during voltage sag condition

C. Effect of STATCOM on Voltage Sag

1) Now we connect a STATCOM in the 132 kV bus whose

breaker is in initially closed position, and run the

simulation with the voltage sag created for 0.75 sec., we

observe that as soon as voltage sag is produced at 1.0 sec, it

is mitigated by the effect of STATCOM as it injects the

voltage during the whole voltage dip of 0.75 sec.

RE : Graphs

Time S 0.80 1.00 1.20 1.40 1.60 1.80 2.00 ...

...

...

-150

-100

-50

0

50

100

150

Voltage (

kV

)

V132

Fig.12 Voltage mitigation during fault by STATCOM

2) Also the per unit voltage of the bus is also maintained at

1.0 pu throughout the simulation run because STATCOM

is connected to the bus from the starting of the run.

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Voltage in pu

Time S 1.0 2.0 3.0 4.0 5.0 ...

...

...

-3.0

-2.0

-1.0

0.0

1.0

2.0

3.0

Voltage p

u

Vpu

Fig.13 Per unit voltage mitigation during fault by STATCOM

VI. CONCLUSION

In this paper, the effect of voltage sag on the distributed

power system of an steel plant has been studied and the role

of STATCOM on the mitigation of the voltage dip have

been found out. The simulations are carried out in PSCAD

and its results are analysed with & without STATCOM

connected in the 132 kV bus. It is clear that the presence of

STATCOM in the network helps in mitigating the voltage

sag and the bus voltage remains in the stable region.

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159

AUTHOR’S PROFI E

Sumeet Trivedi is pursuing his M.E. in

Power Systems Engg. from S.S.C.E.T.,

Bhilai.

D. Chattopadhyay is currently working as Sr. Asso.

Professor in S.S.C.E.T., Bhilai. She has done her M.E. in

Power Systems.