Introduction

Power System Protection deals with the protection

of electrical network’s fault currents and to prevent

such networks from the faulted area in order to

minimise damages. It ensure the safety of a system

and personnel and secure electrical equipments like

transformers, generators, transmission lines etc.

Protective device coordination is the process of

determining the "best fit" timings of current

interruption when abnormal electrical conditions

occur. The basic principle is to co-ordinate

protection so that the device nearest to the fault

must operates first i.e. the relay closest to the fault

has shortest operating time and so on.

It should be such that if any relay nearest to the

fault fails to operate, then the next up-stream relay

should operate and so on in order to islolate the

other part of the plant from the faulted part.

Abstract

The exercise is to perform protection coordination

or overcurrent relays grading for a an electrical

system and to consider following main points :

• Measurement of Fault Currents on 11kV and

3.3kV bus bar.

• Grading of P121 type over current relays by

calculating operating times.

• Plot of over current relays and fuse

characteristicscurves.

Results

The graph in Fig. 2 explains the characteristic

curves of over current relays as to when and which

relays will operate first. Fuses curves have been

plotted directly by taking the data from the data

sheet just to show their comparision with relays.

OVER CURRENT RELAYS GRADINGDr. Derek Pinches and Haseeb Aslam Ansari

Staffordshire University, Stafford, England

References

Conclusion

After carrying out the analysis of a given system it

has been found that the relay B which has the

lowest operating time will operate first if there is

any fault occurs over there. If this relay fails then

the next upstream relay D will operate after some

interval of time and so on.

Moreover Relay J which is at the top will trip the

breaker when all downstream relays have failed

and hence it provides protection coordination and

so over current grading has been achieved.

Method

In order to perform over current relays grading of a

system shown in Fig. 1, fault current at each level

of busbars needs to be calculated using the

formula :

IF = MVA

1.732 X kV

It has been shown (Mehta and Mehta, 2003) that

an inverse time relay is one in which the operating

time is approximately inversely proportional to the

magnitude of the actuating quantity. At value less

than the setting current, the relay never operates.

Therefore to grade a relay we requried a setting

current IS ,which should be taken as 10% extra of

normal FLC with a reset ratio of 95% .

IS = 1.1 x IF /0.95

The operating time of a particular relay can be

calculated using Extremely Inverse Characteristic

formula as :

t = 80 x TMS , TMS= Time Multiplier Setting

[IF / IS]² -1

Note that a grading margin of 0.4s should be added

in the operating time of all the relays in order to

have correct co-ordination, so that the relays can

have sufficient time for discrimination.

Thus, the operating times of all overcurrent relays

are tablulated as shown in Table 1.

Mehta. V., Mehta. R. 2003, Principles of Power

System, New Delhi : S. Chand Publications.

Figure 2. Overcurrent Relays Characteristic Curves

Table 1. Operating Times of Over Current Relays

Current at

3.3 kV

(from ISupto the

Fault

current

IF )

Operating

time of

Relay B

at

TMS=0.1

Operating

time of

Relay D

at

TMS=0.18

Operating

time of

Relay E

at

TMS=0.33

Operating

time of

Relay G

at

TMS=0.43

Operating

time of

Relay H

at

TMS=0.44

Operating

time of

Relay J

at

TMS=0.45

2.66 2.66 8.8 11.46 11.73 12

0.53 1 3.3 4.3 4.4 4.5

0.22 0.53 1.76 2.29 2.34 2.4

0.12 0.33 1.1 1.43 1.46 1.72

0.08 0.23 0.75 0.98 1.38 1.5

Figure 1. Single Line Diagram