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European Journal of Scientific ResearchISSN 1450-216X Vol.31 No.1 (2009), pp. 59-71

EuroJournals Publishing, Inc. 2009

http://www.eurojournals.com/ejsr.htm

A New Method to Prevent Undesirable Distance Relay Tripping

During Voltage Collapse

Ahmad Farid Abidin

Department of Electrical, Electronic and Systems Engineering

Universiti Kebangsaan Malaysia , Selangor, Malaysia

E-mail: [email protected]

Tel: +603-89216590; Fax: +603- 603-8921 6146

Azah Mohamed



E-mail: [email protected]: +603-89216590; Fax: +603- 603-8921 6146

Afida Ayob



Abstract

This paper presents a new approach to prevent undesirable distance relay trippingby using the rate of change of voltage, dv/dt and voltage stability index (VSI). By using the

dv/dt, the distance relay is able to block the false trip signals due to voltage instability.

However, the dv/dt approach is inappropriate to identify between a fault and close tocollapse and may lead to undesirable tripping. Hence, an appropriate indicator based on

VSI is proposed to block such false tripping during voltage collapse. The result from the

application of dv/dt for both events has been carried out to ascertain such hypothesis. In theproposed method, the performance of the VSI is evaluated for its accuracy in

discriminating a fault and voltage collapse by comparing it with the System Status

Indicator (SSI). Then, the combination of dv/dt and VSI is applied to the distanceprotection to ensure the reliability of such approach. To illustrate the effectiveness of the

proposed approach, simulations were carried out on the IEEE 39 bus test system using the

PSS/E software. Test results show the effectiveness of the combined approach in blocking

the relay false trip signals.

Keywords: Distance relay, fault, voltage collapse

1. IntroductionDistance protective relays are widely used as the main protection scheme for transmission lines in

power systems. These relays operate based on the measured impedance calculated from the monitoredvoltage and current signals. The setting of distance relays operating zones is obtained from the line

impedance. Basically, a relay initiates a trip signal to the associated circuit breaker when a fault occurs

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A New Method to Prevent Undesirable Distance Relay Tripping During Voltage Collapse 60

within its operating zone. During a fault, the apparent impedance trajectory falls inside the operating

zone and as a consequence, the trip signal will be activated by the relay. However, many studies have

shown that undesirable zone 3 distance relay operations have initiated tripping which lead to cascadingevents and finally wide-area voltage collapse (Jonsson and Daadler, 2003; Kim et al, 2005; Jin and

Sindhu, 2008 ; Lim et al, 2008). During voltage instability, the apparent impedance seen by the

distance relay is decreased and it can enter the zone 3 operation boundary if the progressive decline ofvoltage continues. Zone 3 protection of the relay sends trip signals if the apparent impedance remains

in that zone after the elapse of zone 3 time delay.To improve reliability of relay operation, adaptive scheme for distance protection has been

introduced by many researches (Horowitz, Phadke and Thorpe, 1988, Thorpe et al, 1988: Jonsson and

Daadler, 2003; Kim, Heo and Aggarwal, 2005; Lim et al, 2008). In adaptive distance protection

relaying, an important aspect to be considered is developing an indicator that can differentiate betweenfaults and other events such as overload, voltage instability and load encroachment. The objective is to

permit the trip signals during faults and block the trip signals during other events.

The adaptive scheme of distance protection operation to avoid unnecessary tripping of distance

protection devices during voltage instability has been reported by Jonsson and Daadler (2003). Thealgorithm is based on dv/dt as an additional criterion along with apparent impedance. Nonetheless, the

adaptive scheme needs the support from communication devices to avoid undesired operation due to

load shedding events and fault clearance issues. A novel zone 3 protection schemes based oncombining the steady state component and transient component of the voltage signals has been

discussed by Kim, Heo and Aggarwal, (2005). The state diagram is deployed to facilitate the sequence

of studies that emanate from the sequence of events. This algorithm prevails over the shortcomings ofthe conventional relay by clearly distinguishing different events such as three-phase fault, load

encroachment and voltage stability. However, this algorithm is still in conceptual level. A novel

technique, based on the combination of the system status indicator and dv/dt has been introduced by

Shen and Ajjarapu (2007). This technique aims to improve the performance of the adaptive relay sincedv/dt solely may fail to perform when the system is close to voltage collapse. The technique considers

both the identification of the critical relays and the corresponding algorithm. However, the

effectiveness of the SSI during faults has not been verified and therefore it cannot be used to

differentiate between faults and voltage instability conditions. Moreover, this indicator is proven to becomputationally inefficient as the relays need to compute both the magnitude and phasor angle

simultaneously.A new reliable and computationally fast indicator needs to be applied to combat false tripping

relay associated with zone 3 relay. To achieve this, the VSI has been introduced as a complementary

technique for dv/dt in order to improve the reliability of distance relay. This index is developed basedon the apparent power loss which is formulated from the local voltage magnitude and current

measurements (Verbic and Gubina, 2006; Haque, 2007). The technique has been tested on the IEEE 39

bus test system to ensure the reliability of the proposed technique during faults and voltage collapse

conditions.

2. Theoretical BackgroundThe distance relay operates in three different operating zones namely, zone 1, zone 2 and zone 3, which

is referred to as multi-zone distance protection. Zone 1 provides instant tripping for faults along the

transmission line and it is used as a primary protection. This zone covers about 80%-90% of theprotected lines. The relay sends the trip signals instantaneously if a fault occurs within the coverage of

zone 1. Zone 2 distance relay provides protection coverage to the remaining part of a transmission line

not reached by zone 1 and extends into the neighboring transmission line.In order to ensure the selectivity of relay operation, the operating time of zone 2 is delayed by a

certain time referred to as zone 2 operating time. Often, zone 2 operating region is designed to cover

100% of the protected line plus 10-20% of the adjacent line. Zone 3 operating zone is the longest zone

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61 Ahmad Farid Abidin, Azah Mohamed and Afida Ayob

available for the distance relays mechanism and theoretically can extend to the remaining part of the

power system in the forward reach with plus 2-7% of reverse reach. The time delay for zone 3

operating scheme is made greater than zone 2 operating time by additional time delay.Zone 3 is used as a back up protection when faults are not cleared by the primary protection of

each transmission line within the reach of the relay. Figure 1 shows the reach setting of distance relay

zones with different time settings.

Figure 1: Distance Relay time settings at Zone1, Zone 2 and Zone 3

breaker

ZL1

Zone 3

reach

ZL2 ZL3

Zone 1

Zone 2

Zone 3

t10

t2

t3

Distance

Tripping timeat each Zone

X

The relay operating characteristics are commonly drawn in polar coordinates which form acircular shape in the R-X complex plane. The R axis corresponds to the real part of impedance Z, while

X axis corresponds to the imaginary part of impedance Z. The apparent impedance as seen by a

distance relay, Za is calculated from the following equation (Jin and Sidhu, 2008):

2

2222 i

ijij

ij

ijij

ij

a VQP

Qj

QP

PZ

++

+=

2

22

2

22 i

ijij

ij

i

ijij

ijV

QP

QjV

QP

P

++

+=

aa jXR += (1)

in which,

2

22 i

ijij

ij

a VQP

PR

+= (2)

2

22 i

ijij

ij

a VQP

QX

+= (3)

where,

Za : apparent impedance

Ra : apparent resistance

Xa : apparent reactance

Pij : active power of line flowing from bus i toj

Qi : reactive power of line flowing from bus i toj

|Vi|: the amplitude of voltage at bus i

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During normal conditions, the measured apparent impedance is outside the relay operating

zone. Once fault occurs at one of the points covered by the relay operating zone (point X in Figure 1),

the apparent impedance enters the relay operating zone. In this situation, the relay will send the tripsignal to the breaker to clear the fault. Figure 2 illustrates the polar characteristics of the three-zone

Mho type distance relay and apparent impedance trajectory before and during fault.

Figure 2: Polar characteristics of the three-zone distance relay and apparent impedance trajectory before and

during fault

Z a be fo re

faul t

Z a du r ing

faul t

Z o n e 1reg ion

Z o n e 2reg ion

Z o n e 3reg ion

L ineI m p e d a n c e

R

X

3. Formulation of Voltage Stability Indicator (VSI) ApplicationThe developed VSI is based on the apparent loss at the measured line. It is derived by considering the

fact that during a voltage collapse, any increment of apparent power at the sending end can no longerproduce any increment of apparent power at the receiving end (Verbic and Gubina, 2006; Haque, 2007;

Smon, Pantos and Gubina,2007). The VSI is first derived by considering,

IVS = (4)

where,

S: apparent power magnitude

V : voltage magnitude

I : current magnitude

When the load apparent power is changed, both V and I also changes as denoted by,

))(( IIVVSS ++=+

IVVIIVIV +++= (5)

where,

S : change in apparent power

V : change in voltage magnitude

I : change in current magnitude

Assuming the higher order term is negligible in which 0 IV , equation (5) becomes,

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VIIVIVSS ++=+ (6)

Subtracting equation (6) from equation (4), we get,

VIIVS += (7)

Rearranging the above equation,

1+

=

VI

IV

VI

S(8)

Assuming that in the vicinity of voltage collapse, 0=S , the VSI becomes,

VI

IVVSI

+=1 (9)

4. The Proposed AlgorithmThe proposed Zone 3 distance relay adaptive settings algorithm is shown in Figure 3 in terms of a

flowchart. The relay continuously monitors the apparent impedance at a measured bus. Once the

apparent impedance enters the zone 3 relay, the relay shall distinguish between a fault and voltageinstability by using dv/dt. The change of voltage during the fault is very substantial and it may produce

very large negative value of dv/vt compared to the other events. The threshold value for dv/dt is based

on these conditions (Jonsson and Daadler, 2003):

dt

dv

dt

dv fault maximum A fault has occurred

dt

dv

dt

dv> fault maximum No fault has occurred

Since there is no study that explicitly identifies the threshold value of dv/dt fault maximum, the

threshold value in this study is identified based on the simulation of the corresponding case. For this

study, the threshold value of dv/dt fault maximum is set as -10 pu/s.

Figure 3: Flowchart of the proposed algorithm

Moni t or i ng t he apparent

i m p e d a a n c e

Enter Zone 3

VSI> th

Y es

N o

Y es

Tr ip s igna l

N odv /d t= dv /d t

Y es

N o

E x c e e dt ime l im i t

Y es

N o

C ont ro l ac t i on

F aul tm a x i m u m

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5. Simulation ResultsThe proposed approach to prevent undesirable distance relay tripping by using the rate of change of

voltage, dv/dt and VSI is tested on the IEEE 39 bus test system by using the commercial PSS/E version31 software. The test system which consists of 10 generators and 18 load buses is shown in Figure 4.

Figure 4: One-line diagram of the IEEE 39 bust test system

In the simulation, the sequence of events is set as follows:i. 0-5 second - base case power flow simulation

ii. 5 second create a three phase fault at bus 7 (200 % of relay reach )iii. 5.06 second clear the faultiv. 5.06-10 second - recovery periodv. 10 second increase the load

Six 6 different simulation cases have been studied in order to validate the proposed technique.

The six cases are described as follows:

Case 1: Three phase fault at bus 5 from 5 to 5.06s, increase load 4 at 5.5 p.u and 10s, line

outage of line 9-39 at 15 s and proximity to voltage collapse at 58.6 second.

Case 2: Three phase fault at bus 3 from 5 to 5.06s, increase load 4 at 5.5 p.u and 10s, lineoutage of line 2-3 at 10s and proximity to voltage collapse at 22.9 second.

Case 3: Three phase fault at bus 6 from 5 to 5.06s, increase load 4 at 5.5 p.u and 10s, line

outage of line 18-3 at 15 s and proximity to voltage collapse at 22.5 second.Case 4: Three phase fault at bus 8 from 5 to 5.06s, increase load 8 at 4 p.u and 10s and

proximity to voltage collapse at 12 second.

Case 5: Three phase fault at bus 28 from 5 to 5.06s, increase load 27 at 7 p.u and 5s, line outageof line 17-27 at 15 s and proximity to voltage collapse at 17.1 second.

Case 6: Three phase fault at bus 29 from 5 to 5.06s, increase load 28 at 4 p.u and 10s, lineoutage of line 28-29 at 15 s and proximity to voltage collapse at 16.6 second.

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Six different relays have been identified to operate falsely during proximity to voltage collapse.

Those relays are:

Case 1: Relay at bus 3Case 2: Relay at bus 14

Case 3: Relay at bus 4

Case 4: Relay at bus 5Case 5: Relay at bus 29

Case 6: Relay at bus 27

5.1. Result of dv/dt

Identical values of dv/dt appear during fault and voltage collapse as the decrement of voltage is also

very substantial for each situation. Hence, an additional approach needs to be considered before thetripping signals can be activated. The results in Figure 5 show clearly dv/dt for the affected relay

during fault and voltage collapse for each case. It is noticed that dv/dt for cases 1, 3, 4 and 5 which

involve the relays at Bus 3, Bus 4, Bus 5 and Bus 29, respectively, have exceeded the dv/dt thresholdvalue. The distance relay of these buses may send false trip signals if the operation of the relay only

depends on the dv/dt.

Figure 5: dv/dt for each case during fault and close to voltage collapse

-70

-60

-50

-40

-30

-20

-10

0

1 2 3 4 5 6

case

dv/dt(pu/s

fault

Close tocollapseThreshold

value for dv/dt

5.2. Comparison of SSI and VSI under Fault and Close to Voltage Collapse

The VSI is proposed as an additional technique so that the relay is capable to differentiate between a

fault and proximity of voltage collapse. Simulation results are displayed by plotting the voltage profile

at bus 4, the VSI and SSI against time as shown in Figure 6. The performance of both indicators duringfault and collapse are shown in Figure 6. The result shows that the VSI is better than SSI on detecting

the collapse as VSI is approaching zero while SSI just reaches 0.65. For fault condition, both indicatorsproduced the identical value of 13.40. The results for every case have been tabulated in Table 1. Shen

and Ajjarapu (2007) stated that the threshold value of 0.2 needs to be imposed as indicator for voltage

collapse in order to block the relay trip signals.

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Figure 6: Plots of voltage, VSI and SSI at bus 4

0

0.5

1

1.5

2

2.5

0.0 5.0 10.0 15.0 20.0 25.0

time(second)

Indicatorvalu

0

0.2

0.4

0.6

0.8

1

1.2

Voltage(p

u)

Voltage at bus 4

SSI

VSI

Table 1: VSI and SSI values during fault and close to voltage collapse

Fault Close to voltage collapseCase Relay

VSI SSI VSI SSI

1 Bus 3 6.22 3.91 0.13 0.14

2 Bus 14 1.65 1.62 0.01 0.36

3 Bus 4 13.40 13.40 0.01 0.65

4 Bus 5 1.88 1.88 0.01 0.09

5 Bus 29 1.48 1.98 0.04 0.07

6 Bus 27 0.96 0.96 0.08 0.08

Figure 7 shows VSI and SSI for each measured bus during close to voltage collapse. It is veryclear that the values of VSI do not exceed the threshold value of 0.2 during voltage collapse. However,SSI for cases 2 and 3 exceeded the threshold value. This indicates that the SSI is less sensitive than

VSI on detecting the proximity of voltage collapse. This can affect the reliability of the distance relay

operation since the blocking scheme can only be activated if the indicator value is equal or below thethreshold value.

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Figure 7: VSI and SSI for each case during close to voltage collapse

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

1 2 3 4 5 6

Case

IndicatorValue

VSI

SSI

Threshold value to

discrminate fault with

close to collapse

5.3. Testing and Result Verification

The performance of the proposed technique needs to be verified for each tested relay. For illustration

purpose, the operation of the relay at bus 4 will be thoroughly explained. As seen in Figure 8, theapparent impedance,Zrseen by distance relay at bus 4 is very low during the fault and close to voltage

collapse conditions.

Figure 8: Apparent impedance as seen by distance relay at bus 4

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 5 10 15 20

time (second)

Z(ohm

The apparent impedance enter the relay operating zone at both situations as Zr is very low as

shown in Figure 9. Once Zr enters the relay operating zone, distance relay sends a trip signal to thebreaker to clear the fault. However, during close to voltage collapse, the relay blocks the trip signals

where the different corrective measure such as load shedding should be taken, instead.

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Figure 9: The Impedance trajectory enters the relay operating zone during fault and collapse.

5.3.1. Testing of Distance Relay Operation Using dv/dt with VSI

Additional criteria need to be imposed to the distance relay operation to avoid such undesirable trip

signals. The application of dv/dt has been utilized in order to discriminate the fault and other events.Figure 10 shows the corresponding dv/dt of the measured voltage at bus 4.

Figure 10: The corresponding dv/dt of the measured voltage at bus 4

-40

-30

-20

-10

0

10

20

30

0 5 10 15 20

time (second)

dv/dt(pu/s

dv/dt during fault

dv/dt close to collapse

However, this approach fails to perform in the proximity of voltage collapse since the proposedfeature is almost identical for both situations (fault and collapse). By adding this criterion, the distance

relay is still not able to differentiate between the fault and collapse and it sends false trip signals as

shown in Figure 11. The trip signals in this case are represented by 1.

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Figure 11: The relay signals sent to the circuit breaker during fault and close to voltage collapse

0 5 10 15 20-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

time(seconds)

RelaySig

nals

Relay signals with dv/dt application during power systems operations

Relay trip signals

In this technique, the VSI has been utilized as a complementary technique to overcome such

problems. This indicator provides a significant feature to the relay as VSI approaches zero prior to

voltage collapse and becomes very high during a fault condition. By imposing this indicator, the

threshold values need to be assigned so that the relay can block the trip signals in the vicinity ofvoltage collapse. In this case, the chosen threshold value is 0.2. The relay tripping signals after utilizing

the VSI is shown in Figure 12. Note that the relay trip signal only appears during the fault and no tripsignals appear in the vicinity of the voltage collapse.

Figure 12: The relay signals during a fault after utilizing VSI

0 5 10 15 20-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

time(second)

relaysignal

Relay signals with dv/dt and VSI during the power systems operations

5.3.2. The Comparison of dv/dt, dv/dt with SSI and dv/dt with VSI

The operation of the corresponding relay is tabulated in Table 2. The comparison has been made withthree different techniques which are dv/dt, dv/dt with SSI and dv/dt with VSI. The relay shall block the

trip signals during close to voltage collapse. Four of the effected relays send false trip signals during

close to voltage collapse when utilizing dv/dt approach. The dv/dt of close to voltage collapse for thesecases are -10.21 pu/s, -10.22 pu/s, -11.46 pu/s and -13.36 pu/s for bus 3, bus 4, bus 5 and bus 29,

respectively. Since all the dv/dt is lower than the threshold value, the relays falsely identify that the

fault occur at the operating zone and as a consequence, the false trip signals are sent to the breakers.Further enhancement has been introduced to avoid such false tripping by combining dv/dt and

SSI approach. However, the relay at bus 5 still sends false trip signals while implementing this

technique. This false operation is mainly due to the SSI failure to identify the proximity of voltage

collapse at the corresponding relay. In this case, SSI is higher than the threshold value which has been

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set as 0.2. Hence, the blocking scheme has not been activated. The blocking scheme is only triggered if

SSI value is equal or lower than 0.2. As a consequence, the relay falsely assumes the fault occurs at the

associated line and the trip signal is sent to the breaker for fault clearance operation.In order to improve the reliability of the previous techniques, the relatively new indicator has

been introduced to avoid such adverse relay operation. The application of VSI as a complimentary

technique for dv/dt has been proven to be reliable as it is able to detect the proximity of voltagecollapse accurately. The testing results in Table 3 prove that the combination of dv/dt and VSI is

successful in blocking the false trip signals during close to voltage collapse.

Table 2: The operation of the corresponding relay of dv/dt, dv/dt with SSI and dv/dt with VSI

Relay Operation during close to voltage collapseCase Relay

dv/dt dv/dt with SSI dv/dt with VSI

1 Bus 3 Trip Block Block

2 Bus 14 Block Block Block


4 Bus 5 Trip Trip Block


6 Bus 27 Block Block Block

6. ConclusionThe combined use of dv/dt and VSI has been proposed as a technique to block the distance relay trip

signals during voltage collapse. Time domain simulations were first carried out under the conditions of

fault and voltage instability until the system collapses. Then, further observation was carried out toidentify the affected relay. The combination of dv/dt and VSI has been applied and tested to evaluate

its effectiveness in blocking the trip signals. The results show that the combined use of dv/dt and VSI

is proven to effectively block the trip signals during voltage collapse compared to the use of only thedv/dt or dv/dt with SSI.

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References[1] Chul-Hwan Kim, Jeong-Yong Heo and Raj K. Aggarwal, 2005 An Enhanced Zone 3

Algorithm of a Distance Relay Using Transient Components and State Diagram, IEEE

Transactions on Power Delivery 20, pp 39-46.

[2] G. Verbic and F. Gubina, 2006, A New Concept of Voltage-Collapse Protection Based onLocal Phasors,IEEE Transactions on Power Delivery 19, pp. 576-581.

[3] Gang Shen and V. Ajjarapu, 2007, A Novel Algorithm Incorporating System Status to Prevent

Undesirable Protection Operation during Voltage Instability, 39th North American PowerSymposium, pp. 373 378.

[4] I. Smon, M. Pantos and F. Gubina, 2007, An Improved Voltage Collapse Protection AlgorithmBased On Local Phasor,Electric Power System Research 78, pp. 1-7.

[5] J. S. Thorp, A. G. Phadke, S. H. Horowitz and M. M. Ekgovic, 1988, Some Applications ofPhasor Measurements to Adaptive Protection, IEEE Transactions on Power Systems 2, pp.

791-798.

[6] M. H. Haque, 2007, Use of Local Information to Determine the Distance to VoltageCollapse,International Power Engineering Conference, pp. 1 6.

[7] M. Jonsson and J. E. Daadler, 2003, An Adaptive Scheme to Prevent Undesirable DistanceProtection Operation during Voltage Instability, IEEE Transaction on Power Delivery 18, pp

1174-1180.[8] Ming Jin and Tarlochan S. Sindhu, 2008, Adaptive Load Enchroachment Scheme for Distance

Protection,Electric Power System Research Journal 78, pp. 1693-177.

[9] S.H. Horowitz, A.G. Phadke and J.S. Thorpe, 1988, Adaptive Transmission SystemRelaying,IEEE Transactions on Power Delivery 3, pp. 1436-1445

[10] Seong-Il Lim, Chen-Ching Liu, Seung-jae Lee, Myeon-Song Choi, 2008, Blocking of Zone 3Relays to Prevent Cascaded Events,IEEE Transaction on Power Systems 23, pp. 747-754.

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