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Solution to Close-in Fault Problem in Directional Relaying

A. K. Pradhan, P. Jena

Abstract- Directional relays use voltage as the polarizing quantity.

When three phase faults occur near to the relay bus the available

voltage becomes nearly zero and this creates problem in estimation of

the fault direction. The capacitor coupling voltage transformer

(CCVT) subsidence transients add to this problem. The memory

voltage used as the polarizing quantity at these situations is a

compromise. This paper highlights these issues and proposes a simple

solution using the power flow direction in addition to other

information. Performance of the technique is evaluated through

simulation in PSCAD.

Key words- Fault, Digital Relay, Directional Relay, Phasor Estimation,Subsidence Transient

I. INTRODUCTION

IRECTIONAL relaying is widely applied in line protection to

enhance the sensitivity and reliability of the protection schemes

[1]-[4]. Current and voltage phasors or the derived sequence

components are used to estimate the fault direction where voltage is

used as polarizing quantity. When three phase faults occur near thesensors the available voltage to the relay becomes substantially low

and it puts challenge in correct voltage phasor estimation. Due to

subsidence transients with CCVT, at these situations the

performance of directional relay is not reliable. As a measure, the

low voltage polarizing quantity is substituted by any suitable

memory signal such as prefault positive sequence voltage [1], [3].

However this approach is also not reliable which is demonstrated in

the following example.

A 132 kV, 50 Hz three phase two-source system as shown in

Fig. 1 is simulated using PSCAD. A directional relay is located at

bus B and power flow direction at a situation is from bus A to bus

C. Three phase faults are created at Fx and Fy sides of the relay

and results are shown in table 1 (phasors computed through onecycle DFT with 1 kHz sampling rate). This indicates that the

phase angle difference between positive sequence fault current and

voltage is positive for Fx side faults and negative for Fy side faults.

This rule is applied in such a directional relaying with the angle

difference be restricted to rad.

Next three phase fault is created in Fx side very close to the relay

bus B. The voltage waveforms are shown in Fig. 2 where thesystem voltage collapses but corresponding CCVT outputs still

show voltage. The results of different phasors are provided in table

2 where it is observed that if the low voltage at relay bus is

considered as the polarizing quantity the angle difference is

negative for Fx case and positive for Fy case which contradicts to

the earlier result in table 1. On the other hand if prefault positive

sequence voltage becomes the polarizing voltage the results in last

column are in accordance with the rule; providing accurate

direction estimation.

TABLE I RESULTS ON DIRECTIONAL RELAYING

The power flow direction is then changed to bus C to A and simil

faults are created in Fx and Fy sides. The phasor results a

provided in table 3 where it is observed that with prefault volta

as the polarizing quantity, for faults in Fx side the angle differen

is negative and for Fy side it is positive which is aga

contradictory. A solution to this problem is proposed in the ne

section.

D

Fault current

phasor

Fault voltage

phasor

Fault

position

Mag

(A)

Angle

(rad)

Mag

(A)

Angle

(rad)

Angle

Difference(rad)

Fx 19.2 0.51 17.2 -1.88 2.39

Fy 17.4 -2.61 30.3 -1.73 -0.88

Fault current

phasor

(I1)

Fault voltage

phasor

(V1)

Prefault

voltage phasor

(VB)

Fault

position

Mag

(A)

Ang

(rad)

Mag

(A)

Ang

(rad)

Mag

(A)

Ang

(rad)

I1- V1

(rad)

I1-

(rad)

Fx 28.6 0.30 3.6 2.88 62.8 -1.61 -2.58 1.91

Fy 16.6 -2.95 4.6 2.87 62.8 -1.61 0.46 -1.34

Fault current

phasor

Prefault voltage

phasor

Fault

position

Mag

(A)

Angle

(rad)

Mag

(A)

Angle

(rad)

Angle

Difference(rad)

Fx 17.6 2.11 63.5 -1.61 -2.56

Fy 16.1 -0.39 63.5 -1.61 1.22

Fig. 1. A two-source system

A B

Fx Fy

~

Ipre

~C

Fig. 2. (a) Line voltage during close-in fault at relay bus

(b) CCVT output during close-in fault

Time (sec)

Voltae(V)

0.24 0.26 0.28 0.3 0.32 0.34 0.36-100

-50

0

50

100

Voltage(V)

Time sec0.24 0.26 0.28 0.3 0.32 0.34 0.36

-1.5

-1

-0.5

0

0.5

1

1.5x 10

5

(a)

(b)

TABLE II RESULTS ON CLOSE-IN FAULT

(POWER FLOW DIRECTION A TO C)

TABLE III RESULTS ON CLOSE-IN FAULT (POWER FLOWS FROM C TO A

The authors are with the Department of Electrical Engineering, Indian Institute ofTechnology Kharagpur, INDIA-721302

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II. PROPOSED SCHEME

It is observed that the decision of conventional directional relaying

is not consistent for close-in faults when prefault voltage becomes

the polarizing quantity. Numerous cases were simulated and it is

found that for one direction of power flow such polarizing quantity

provides correct decision but not for the reverse direction of power

flow. This is due to fact that the fault current takes a different

position when the power flow direction is changed. This is clearly

evident from phasor diagrams in Fig. 3 and Fig.4. Positive

sequence fault current (I1) and prefault voltage (VB) phasors for

faults at Fx and Fy sides are available in Fig. 3. It is observed that

the corresponding phase angle differences x and y are of

positive and negative values respectively for power flow direction

from bus A to C. On the other hand x and y for power flow

direction from bus C to A (Fig.4) are negative and positive values

respectively. Thus the phasor diagrams indicate that to obtain the

correct fault direction the angle should be multiplied by -1 in

the case of reverse power flow; from bus C to A. This provision is

included in the proposed directional relaying scheme which is

shown in Fig. 5.

TABLE IV CLOSE-IN FAULT (POWER FLOWS FROM A TO C)

TABLE V CLOSE-IN FAULT (POWER FLOWS FROM C TO A )

To demonstrate the performance of the approach three phase close-

in faults are simulated at Fx and Fy sides with power flow direction

from A to C. The result for the case is provided in table 4 and it is

found that it has correctly identified the direction. Similarly correct

decisions are observed for power flow direction from C to A as

presented in table 5.

III. CONCLUSION

This paper addressees issues related to close-in-fault in direction

relaying when memory polarization (prefault voltage) is bein

applied. It proposes a solution for such low voltage situatio

using power flow direction as additional information which

being verified through simulations.

IV. REFERENCES

[1] A. G. Phadke, S. H. Horowitz, Power Systems Relaying, Research StudPress, Taunton, 1992.

[2] D. Birla, R. P. Mahwswari and H. O. Gupta, A new nonlinear directio

overcurrent relay co-ordination technique, and banes and boons of near-efaults based approach,IEEE Trans. on Power Delivery, vol. 21, no. 3,

1176-1182, 2006.

[3] J. Roberts and A. Guzman, Directional element design and evaluatio

www.selinc.com/techpprs/6009.pdf

[4] A. K. Pradhan, A. Routray and G. S. Madhan, Fault direction estimation

radial distribution system using phase change in sequence current, IE

Trans. on Power Delivery, , vol. 22 , no. 4, pp. 2065 2071, 2007.

Fault current

phasor

Prefault voltage

phasor

Power

FlowDirection

(PD)

Angle

Differenx PD

Faultpositi

on

Mag(A)

Ang(rad)

Mag(A)

Angl(rad)

Ang(rad)

Fx 17.1 -0.88 63.5 -1.61 0.73

Fy 16.2 2.65 63.5 -1.61

From

A to C

(+1)-2.02

Fault currentphasor Prefault voltagephasor PowerFlowDirection

(PD)

AngleDifferenx PD

Faultpositi

on

Mag

(A)

Ang

(rad)

Mag

(A)

Ang

(rad)

Ang

(rad)

Fx 17.6 2.18 63.5 -1.61 2.49

Fy 16.1 -0.41 63.5 -1.61

From

C to A

(-1)-1.20

Fig. 5. Flow diagram for the algorithm

Start

Compute the positive

sequence phasors of

prefault voltage and

current

Compute the

positive sequence

phasors at fault

Voltage and current samples acquisition

Fault direction estimation

Output

Fault detection

Estimation of

power

flow direction

VC

VA

I1Fy

Ipre

1I

(b) fault at Fy

VC

VA

I1Fx

Ipre

1I

(a) fault at Fx

Fig. 4. Different positive sequence currents when power flows from bus C to A

VBVB

x

y

1 1pre FxI I I = + 1 1pre FyI I I

=

VA

VC

I1Fx

Ipre

1I

(a) fault at Fx

VA

VC

I1Fy

Ipre

y

(b) fault at Fy

Fig. 3. Phasor diagram showing different positive sequence currents, I1Fx and I1Fy -

fault components only andI1' andI1'' fault currents (including load current) when

power flows from bus A to C

1I

x

VB VB

1 1pre FxI I I =

1 1pre FyI I I

= +