Among the various possible methods used to achieve correct Overcurrent relay co-ordination are those using either time or overcurrent, or a combination of both. The common aim of all three methods is to give correct discrimination. That is to say, each one must isolate only the faulty section of the power system network, leaving the rest of the system undisturbed.
In this method, an appropriate time setting is given to each of the Overcurrent relays controlling the circuit breakers in a power system to ensure that the breaker nearest to the fault opens first. A simple radial distribution system is shown in Figure 9.1, to illustrate the principle.
Overcurrent protection is provided at B, C, D and E, that is, at the infeed end of each section of the power system. Each protection unit comprises a definite-time delay overcurrent relay in which the operation of the current sensitive element simply initiates the time delay element.
Provided the setting of the current element is below the fault current value, this element plays no part in the achievement of discrimination. For this reason, the relay is sometimes described as an ‘independent definite-time delay relay’, since its operating time is for practical purposes independent of the level of overcurrent,
It is the time delay element, therefore, which provides the means of discrimination. The relay at B is set at the shortest time delay possible to allow the fuse to blow for a fault at A on the secondary side of the transformer. After the time delay has expired, the relay output contact closes to trip the circuit breaker. The relay at C has a time delay setting equal to t1 seconds, and similarly for the relays at D and E.
If a fault occurs at F, the relay at B will operate in t seconds and the subsequent operation of the circuit breaker at B will clear the fault before the relays at C, D and E have time to operate. The time interval t1 between each relay time setting must be long enough to ensure that the upstream relays do not operate before the circuit breaker at the fault location has tripped and cleared the fault.
The main disadvantage of this method of discrimination is that the longest fault clearance time occurs for faults in the section closest to the power source, where the fault level (MVA) is highest.
Current grading in Overcurrent Relay
Discrimination by current relies on the fact that the fault current varies with the position of the fault because of the difference in impedance values between the source and the fault. Hence, typically, the relays controlling the various circuit breakers are set to operate at suitably tapered values of current such that only the relay nearest to the fault trips its breaker. Figure-B 2 illustrates the method.
For a fault at F1, the system short-circuit current is given by:
I=6350/(Zs+ZL1)
Where, ZS = source impedance =112/250=0.482 Ohm
ZL1=cable impedance between C and B = 0.24 Ohm
Hence, I=6350/0.725=8800 Ampere
So, a relay controlling the circuit breaker at C and set to operate at a fault current of 8800A would in theory protect the whole of the cable section between C and B. However, there are two important practical points that affect this method of co-ordination:
It is not practical to distinguish between a fault at F1 and a fault at F2, since the distance between these points may be only a few metres, corresponding to a change in fault current of approximately 0.1%.
In practice, there would be variations in the source fault level, typically from 250MVA to 130MVA. At this lower fault level the fault current would not exceed 6800A, even for a cable fault close to C. A relay set at 8800A would not protect any part of the cable section concerned.
Discrimination by current is therefore not a practical proposition for correct grading between the circuit breakers at C and B. However, the problem changes appreciably when there is significant impedance between the two circuit breakers concerned. Consider the grading required between the circuit breakers at C and A in Figure B. Assuming a fault at F4, the short-circuit current is given by:
I=6350/(ZS+ZL1+ZL2+ZT)
Where, ZS = source impedance=112/250=0.485 Ohm
ZL1 = cable impedance between C and B = 0.24 Ohm
ZL2 = cable impedance between B and 4MVA transformer = 0.04 Ohm
ZT = transformer impedance=0.07 (112/4)=2.12 Ohm
Hence, I=6350/2.885=2200 Ampere
For this reason, a relay controlling the circuit breaker at B and set to operate at a current of 2200A plus a safety margin would not operate for a fault at F4 and would thus discriminate with the relay at A. Assuming a safety margin of 20% to allow for relay errors and a further 10% for variations in the system impedance values, it is reasonable to choose a relay setting of 1.3 x 2200A, that is, 2860A, for the relay at B. Now, assuming a fault at F3, at the end of the 11kV cable feeding the 4MVA transformer, the short-circuit current is given by:
I=6350/(ZS+ZL1+ZL2)
Here, we assume Fault level at 250MVA.
I=6350/(0.485+0.24+0.04)=8300 Ampere
Now, we assume source fault level at 130MVA
I=6350/(0.93+0.214+0.04)=5250 Ampere
For either value of source level, the relay at B would operate correctly for faults anywhere on the 11kV cable feeding the transformer.
Time and Current grading of Overcurrent Relay
Each of the two methods described so far has a fundamental disadvantage. In the case of discrimination by time alone, the disadvantage is due to the fact that the more severe faults are cleared in the longest operating time. On the other hand, discrimination by current can be applied only where there is appreciable impedance between the two circuit breakers concerned.
It is because of the limitations imposed by the independent use of either time or current co-ordination that the inverse time overcurrent relay characteristic has evolved. With this characteristic, the time of operation is inversely proportional to the fault current level and the actual characteristic is a function of both ‘time’ and ‘current’ settings. Figure 9.3 shows the characteristics of two relays given different current/time settings. For a large variation in fault current between the two ends of the feeder, faster operating times can be achieved by the relays nearest to the source, where the fault level is the highest. The disadvantages of grading by time or current alone are overcome.
