Effect of Statcom on Distance Relay Performance in a ... - ijcee

Internati**on**al Journal **in**eer**in**g, Vol. 4, No. 4, August 2012

**on** **Distance** **Relay** **Performance** **in** a

Transmissi**on** L**in**e

Sriteja Alapati and B. Durga prasad

Abstract—This paper describes modell**in**g

protecti**on** relays with the STATCOM, and simulati**on** results

for **in**vestigat**in**g the operati**on** **on**

relays **in** a power system c**on**ta**in****in**g a flexible alternat**in**g

current transmissi**on** (FACTS) c**on**troller such as the static

synchr**on**ous compensator (STATCOM). The analyses are

modell**in**g the STATCOM and the protecti**on** relays. The

impact **on** protecti**on** relays is studied

us**in**g digital distance relay algorithm **in**

MATLAB/SIMULINK for normal operat**in**g c**on**diti**on**s as well

as for fault c**on**diti**on**s.

Index Terms—**Distance** relay, FACTS devices, power system

protecti**on**, model**in**g, STATCOM, digital distance relay

algorithm.

I. INTRODUCTION

The excellent advantages that MATLAB/ SIMULINK [1]

has make MATLAB/SIMULINK a c**on**venient and

**in**teractive tool for both numerous analysis and direct

communicati**on**s with relay's test program. This project

describes how to use MATLAB/SIMULINK for automatic,

**in**teractive, and high performance test**in**g relay system.

Some examples and simulati**on** results are also provided **in**

this paper.

Traditi**on**al updat**in**g **on** system by

c**on**struct**in**g new transmissi**on** l**in**es becomes extremely

difficult because **on**omic and envir**on**mental pressures.

High efficiency **in** terms **on** **in**g

transmissi**on** l**in**es, without compromis**in**g **on** the quality and

reliability

alternative means. In this respect, due to the recent advances

**in** high power semic**on**ductor technology, Flexible AC

transmissi**on** System (FACTS) technology has been

proposed to solve this problem. However, because

added complexity due to the **in**teracti**on**

with the transmissi**on** system, the transients superimposed

**on** the power frequency voltage and current waveforms

(particularly under faults) can be significantly different from

those systems not employ**in**g FACTS devices and it will

result **in** rapid changes **in** system parameters such as l**in**e

impedance and power angle. It is thus vitally important to

study the impact **on** the traditi**on**al

protecti**on** relay scheme such as the impedance-based

distance protecti**on** relay.

statcom is **on**e

It is based **on** a voltage source c**on**vert and can **in**ject an

Manuscript received June 17, 2012; revised July 30, 2012.

The authors are with GITAM university Vishakapatnam, India (e-mail:

sritejaalapati@gmail.com, durga206@gmail.com).

almost s**in**usoidal current with variable magnitude and **in**

quadrature with the c**on**nect**in**g l**in**e voltage .It is widely

used at area to ma**in**ta**in** the c**on**nect**in**g po**in**t voltage by

supply**in**g or absorb**in**g reactive power **in**to the power

system. Because **in** a

fault loop, the voltage and current signals at relay po**in**t will

be affected **in** both steady and transient state. This impact

will affect the performance **in**g protecti**on** methods,

such as distance relay.

This paper will analyze and explore the impact

STATCOM employed **in** a transmissi**on** system **on** the

**Performance**

STATCOM is proposed and sec**on**dly, the analytical results

based **on** symmetrical comp**on**ent transformati**on** for s**in**gle

phase to ground fault **on** a transmissi**on** system employ**in**g

STATCOM are presented, the simulati**on** results clearly

show the impact **on** the performance

II. MATHEMATICAL MODELING OF STATCOM

Static compensators or **in**verter circuit are mostly

designed for voltage support

system. On transmissi**on** level, systems are normally

balanced, but it is not the case dur**in**g fault c**on**diti**on**s

distributi**on** system. In three phase balanced system

STATCOM produce a set

that **in**stantaneous power flow **in** STATCOM or from

STATCOM is always zero.

Physically, compensator based **on** a three phase **in**verter

circuit **in**terc**on**nects all the phases

provid**in**g full path for the exchange

between them. This type **in**g

unbalanced operati**on**

phase **in**verter must provide unequal voltage. If PWM

strategy **on**trol is used, it is possible to change phase

voltages by chang**in**g c**on**trol signals and keep**in**g dc voltage

c**on**stant. The value **in**e

the rat**in**g **in**verter and **in**verter will always draw some

active power from the ac system to cover for its losses and

keep the dc bus voltage c**on**stant.

Fig. 1. Typical transmissi**on** l**in**e system

It is important to model the statcom and the relay

**in**dividually and together before **in**terfac**in**g with the actual

transmissi**on** system. This modell**in**g helps to know about

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Internati**on**al Journal **in**eer**in**g, Vol. 4, No. 4, August 2012

the characteristics **in**dividual system. Hence c**on**sider a

power system to model the STATCOM as shown.

The STATCOM is assumed to operate with**in** its reactive

power rat**in**g to ma**in**ta**in** the voltage at its locati**on** under all

operat**in**g c**on**diti**on**s. In this study, the STATCOM is

modelled [2] as a c**on**trolled-current-source that resp**on**ds

**in**stantaneously to chang**in**g system c**on**diti**on**

Dur**in**g a fault **on** the power system, the STATCOM is

assumed to be **in** service as l**on**g as its reactive current does

not exceed the STATCOM limits.

In this study, the STATCOM is rated to compensate the

reactive power **on** l**in**e. The STATCOM

current per-phase “I γ ” is given by the follow**in**g equati**on**.

I γ = ((Z S

′

+ Z L

2 )−1 V γ

−1

E S

′

+ E R

′

− 2 − 2Y C ′ )V γ (1)

The value

changes **in** load angle “δ,” **in** such a way as to keep the

midpo**in**t voltage “V γ ” c**on**stant.

where Z s ’& Z r ’ are equivalent source impedance seen from

the send**in**g end and receiv**in**g end buses. E s ’ & E r ’ are

send**in**g end and receiv**in**g end equivalent source voltages;

Vγ is midpo**in**t voltage, I γ is compensator current

III. MATHEMATICAL MODELING OF A RELAY

For the analysis associated with the operati**on**

distance relay, the power system shown **in** Fig. 1 is used, the

relay is **in**stalled **on** the right side

impedance calculati**on** [3] is based **on** symmetrical

comp**on**ent transformati**on** us**in**g power frequency

comp**on**ents

po**in**t. It is assumed that signal acquisiti**on**, pre process**in**g

and sequence comp**on**ent calculati**on**s have been performed

previously.

When a s**in**gle phase to ground fault occurs at the ight

side

po**in**t, the positive, negative and zero sequence networks

the system dur**in**g the fault can be shown as shown **in** fig 2.

I 1l**in**e = I 1s + I 1sh (5)

I 2l**in**e = I 2s + I 2sh (6)

I 0l**in**e = I 0s + I 0sh (7)

where V 1s V 2s and V 0s are the sequence phase voltages at

the relay locati**on**, I 1s I 2s and I 0s are the sequence phase

currents at the relay locati**on**, I 1l**in**e I 2l**in**e and I 0l**in**e are the

sequence phase currents **in** the transmissi**on** l**in**e, I 1f I 2f

and I 0f are the sequence phase currents **in** the fault, I 1sh

I 2sh and I 0sh are the shunt sequence phase currents **in**jected

by STATCOM, Z 1 and Z 0 are the sequence impedance

the transmissi**on** l**in**e , „n‟ is the per unit distance

from the relay locati**on**.

From above, the voltage at relay po**in**t can be derived as:

where

V s = n I s Z 1 + n I 0s Z 0 − Z 1 + I sh n − 0.5 Z 1 + n −

0.5 I sh0 Z 0 − Z 1 + I f R f (8)

V s = V 1s + V 2s + V 0s (9)

I s = I 1s +I 2s + I 0s (10)

I sh = I 1sh + I 2sh + I 0sh (11)

S**in**gle phase to ground fault, the apparent impedance

distance relay can be calculated us**in**g the equati**on** below

where

V R , I R

Z =

V R

= V R

I R + Z 0−Z1

I R0 Z1

I relay

(12)

phase voltage and current at relay po**in**t

I R0 zero sequence phase current

I relay The relay**in**g current

If this traditi**on**al distance relay is applied to the

transmissi**on** system with STATCOM, the apparent

impedance seen by this relay can be expressed as:

I sh

Z = n Z 1 + n − 0.5 Z

I 1 + I f

R

relay I f (13)

relay

It is clear from equati**on** (13) that if **on**ly a solid s**in**gle

phase to ground fault is c**on**sidered, the equati**on** becomes:

I sh

Z = n Z 1 + n − 0.5 Z

I 1 (14)

relay

Fig. 2. The sequence network **in**gle phase fault

The same circuit will be c**on**sidered for positive, negative

and zero sequence networks. For the positive sequence

network the notati**on** „x‟ equals to 1, for negative sequence

network it will be 2, and for zero sequence network it is

equal to 0. Us**in**g these notati**on**s for the three sequence

networks the follow**in**g equati**on**s are obta**in**ed.

The sequence voltages at the relay po**in**t can be expresse

as follows:

V 1s = I 1s 0.5 Z 1 + I 1l**in**e n − 0.5 Z 1 + R f I 1f (2)

V 2s = I 2s 0.5 Z 1 + I 2l**in**e n − 0.5 Z 1 + R f I 2f (3)

V 0s = I 0s 0.5 Z 0 + I 0l**in**e n − 0.5 Z 0 + R f I 0f (4)

The impact **on** the apparent impedance

calculati**on** for an impedance relay **in** a transmissi**on** l**in**e can

be obta**in**ed us**in**g these equati**on**s.

IV. DIGITAL DISTANCE RELAY ALGORITHM

Digital distance protecti**on** is a universal short-circuit

protecti**on**. It's mode **on** is based **on** the

measurement and evaluati**on**

which is named by the algorithm

This algorithm is used to **in**put signals to DSP by discrete

voltage and discrete current to judge whether faults occur or

not. However, this method [4] is just a program. MATLAB

has the advantage **on**duct**in**g massive calculati**on**

functi**on**s and its program can be easily developed.

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**Distance** relays are also named impedance relays. They

are used to calculate l**in**e impedance by measurement

voltages and currents **on** **on**e s**in**gle end. For example, for

impedance type distance relays, the relays compare the

sett**in**g impedance with the measurement impedance to

determ**in**e if the fault is **in**side or outside the protected z**on**e.

They immediately release a trip signal when the impedance

value is **in**side the z**on**e 1 impedance circle

For security protecti**on** c**on**siderati**on**, the c**on**firmati**on**

fault occurrence will not be made until successive trip

signals are released **in** **on**e seas**on**. Different formulas

should be adopted when calculat**in**g the fault impedance due

to different fault types.

Table I **in**dicates calculati**on** formula for all

types [5]. Any three-phase faults can be detected from every

formula **in** Table I. In order to reduce calculati**on** burden, we

design a fault detector and fault type selector. The fault

detector can judge which fault type it is and then calculate

fault impedance by select**in**g a suitable formula from Table

I. If we d**on**'t use fault type judgment first, then the distance

relay

**in** Table I at the same time, which causes much calculati**on**

burden.

TABLE I: FAULT IMPEDANCE CALCULATION ON DIFFERENCE FAULTS

FAULT TYPE

FORMULA

AG V A /(I A + 3cI O )

BG V B /(I B + 3cI O )

CG V C /(I C + 3cI O )

AB or ABG (V A − V B )/(I A − I B )

BC or BCG (V B − V C )/(I B − I C )

CA or CAG (V C − V A )/(I C − I A )

where A,B and C **in**dicates number

fault ,V and I are phasor

Z1)/Z1 , Zo and Z1 are l**in**e

positive- sequence respectively. I O is zero-sequence current.

When the distance relays receive discrete voltage and

current signal, it has to c**on**vert them to phasor. The Discrete

Fourier Transform (DFT) is the most popular method to

estimate fundamental phasors for digital relay**in**g. The fullcycle

DFT is described as follow**in**g equati**on** (15):

appropriate cut-**in**g filters

cannot remove decay**in**g dc comp**on**ents and reject low

frequency comp**on**ents. This makes the phasors very

difficult to be quickly estimated and affects the performance

**in**g. Therefore, we usually use the mimic

filter to removed the dc-**on**ents [6]. The mimic

filter can be developed by digital method. Here, we want to

pass the fundamental frequency signal (50Hz) by the filter.

Then, assum**in**g the ga**in** L equals 1 and the samples

frequency is, f s ( f s =1/T s ) f**in**ally, we obta**in** a formula

(16).

L 1 + τf s − Lτf s cosωT S + jLτf s s**in**ωT S =1 (16)

where

ω =2π60, τ is time c**on**stant for user def**in**iti**on**.

To remove the dc-**on**ents, MATLAB program

can be developed us**in**g the above equati**on**.

Hence from the equati**on**s (15) & (16) we can prepare the

digital distance relay algorithm for filter**in**g the waveforms

and c**on**vert**in**g them **in**to phasors. The algorithm is

presented as follows.

Step 1: Get the data

simulati**on** **in** discrete form.

Step 2: Apply the filter**in**g technique for those current and

voltage signals to remove dc-**on**ents. Apply the

same filter**in**g technique for all the three phase current and

voltage signals.

Step 3: Phasor estimati**on** us**in**g DFT.

Step 4: Calculate the impedance values for each phase

under fault c**on**diti**on**s us**in**g the formulas tabulated **in** table

1.

Step 5: Draw the impedance trajectory.

As shown **in** the above algorithm the currents and

voltages are send through the mimic filter **in** order to

remove the dc-**on**ents and for the phasor

estimati**on**. Later us**in**g these values we can f**in**d out the

impedance values for all the phases **in** faulty c**on**diti**on**s.

From the above discussi**on**, the MATLAB can easily f**in**ish

all

that SIMULINK can easily simulate power system faults,

the design and the test

with ease. Its major characteristic **in**tegrat**in**g system fault

simulati**on** and protecti**on** relay algorithms **in** a s

system can enhance the efficiency **on** relay test.

X= 2 N

N−1

XK e −j2πk/N

k=0

(15)

where X is complex phasor, Xk is the sample discrete data

us**in**g the above equati**on** depend**in**g **on** the number

samples we can write the MATLAB program for c**on**versi**on**

**in**to complex form.

In additi**on**, when a fault occurs **on** transmissi**on** l**in**es, the

voltage and current signals are severely distorted. These

signals may c**on**ta**in** decay**in**g dc comp**on**ents, subsystem

frequency transients, high frequency oscillati**on** quantities,

and etc. The higher frequency comp**on**ents can be

elim**in**ated us**in**g low pass anti-alias**in**g filters with

Fig. 3. One-l**in**e diagram **on** system

V. SIMULATION OF POWER SYSTEM

With the updated versi**on**s

model development **on**ents is **on**ward

to perfecti**on**. Due to the fast development

technologies, which improve the power transfer efficiency

and the optimum utilizati**on**

electr**on**ic equipment like TCSC - UPFC - STATCOM...and

so **on** may be widely used **in** power systems In the future.

Thus, the selecti**on** and the sett**in**g

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be evaluated and tested thoroughly [7]. Here, SIMULINK

**in**cludes variant basic power comp**on**ents, which can be

used al**on**e or **in** comb**in**ative use to f**in**ish all k**in**ds

system network simulati**on**s.

Fig. 6. Impedance trace for phase A fault without

STATCOM

Fig. 4. SIMULINK diagram for the power system

As shown **in** the SIMULINK diagram the STATCOM

was designed for the given requirements, i.e. 3-level, 12-

pulse **in**verter was built **in** MATLAB and simulati**on** is

**in**dividually for this STATCOM to ensure that the output is

3-level output and then it will be made as a subsystem and

given to the power system under study.

In additi**on**, SIMULINK provides some opti**on**s like realtime

display, stor**in**g data **in** WORKSPACE and hard disk

after the signals data is released by filter. As shown **in** Fig.4,

we capture signals and store them **in** WORKSPACE from

the simulati**on** systems, which are provided for us**in**g **in**put

For the voltage and current waveforms obta**in**ed from the

simulati**on**

applied. And by us**in**g impedance calculati**on** formulas from

table 1, the impedance trace can be plotted for all the six

faults given **in** table. And the simulati**on** results can be

traced for all the faults. Here, the simulati**on** results for the

system with and without STATCOM for phase A fault are

presented as an example.

VI. SIMULATION RESULTS

Here the simuati**on** results are presented for phase A fault

with & without STATCOM for an impedance relay. From

the results, the system will be out **on** with

the presence

obta**in**ed us**in**g the formulas **in** Table I for impedance

calculati**on**. The results for the rema**in****in**g faults can also be

obta**in**ed us**in**g the formulas given.

The waveforms given below are impedance trajectory for

healthy system, impedance trace without STATCOM, and

impedance trace with STATCOM for phase A fault.

Fig. 7. Impedance trace for phase A fault with

STATCOM

VII. CONCLUSION

In this paper the modell**in**g

STATCOM is presented Us**in**g digital distance relay

algorithm. And the results are presented with the MATLAB

simulati**on** for different faults. The same can be d**on**e with

different FACTS devices also. From the simulati**on** results,

STATCOM can reduce the fault impedance by distort**in**g the

voltage and current waveforms due to the **in**jecti**on**

reactive power **in**to the system.

APPENDIX A (TEST SYSTEM DATA)

Equivalent source impedances at send**in**g end and

receiv**in**g ends:

Z s = (21.44+3.214j)Ω ,Z R = (17.5+1.309j) Ω

L**in**e c**on**stant:

R O = 0.275Ω, L O = 3.725H, C O =6.71nF

R 1 = 0.0275 Ω, L 1 = 1.345H , C 1 = 9.483nF

STATCOM: Rat**in**g: 160MVA Transformer: 15kv/130kv

LOAD: Rat**in**g : 138KV Active power : 10MW

REFERENCES:

[1] The Math Works, Inc, "Us**in**g MATLAB,” 1999.

[2] K. E. Arroudi, G. Joos, and D. T. M. Gillis, "'Operati**on**

Protecti**on** **Relay**s With the STATCOM," IEEE Trans. **on** Power

Delivery, vol. 17, no. 2, April 2002, pp. 381-387

[3] X. Y. Zhou, H. F. Wang, R. K. Aggarwal, and P. Beaum**on**t, ”The

Impact **Distance** **Relay**” 15 th PSCC, Liege, pp. 22-26

2005.

[4] L. C. Wu and C. W. Liu, “Model**in**g and Test**in**g **Distance**

**Relay** Us**in**g MATLAB/ SIMULINK,”IEEE, 2005.

[5] T. S. Sidhu, D. S. Ghotra, and M. S. Sachdev, "A Fast **Distance** **Relay**

Us**in**g Adaptive Data W**in**dow Filters," IEEE/PES Summer Meet**in**g,

July 2000, pp.1407-1412.

[6] G. Benmouyal, "Removal **in**g DC **in** Current

Waveforms Us**in**g Digital Mimic Filter**in**g," IEEE Trans. **on** Power

Delivery, vol. 10, no. 2, April 1995, pp. 621-630.

Fig. 5. Impedance trajectory for healthy system

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