Question

# For an overcurrent relay coordinated in a power system network, it was observed that relay, the...

For an overcurrent relay coordinated in a power system network, it was observed that relay, the following tripping times were observed:

1- For phase-ground fault, double phase-to-ground fault and three phase to ground fault result in tripping time of 0.5141s

2- For phase to phase fault, the tripping time is 0.5389s

Why does it take more time for that relay to operate for phase-to-phase fault? Please clarify.

I have drawn characteristic of overcurrent relay. The y-axis has operating time and the x-axis has per unit fault current. As we can see as the fault current increases operating time decreases.

I have made simple calculations taking the reactances of practical power system and foundd LG fault current and LL fault current for the same system. we can see that the fault current for an LG fault is greater than that for an LL fault. Here the fault considered our dead shot circuits i.e. no fault impedance has been included while analysing the fault currents. Thus for higher currents the operating time is lesser it means that the operating time for LG fault will be lesser than LL fault.

One more factor for a constructional feature of an overcurrent relay also suggest us the reason for this if we look at the construction of an overcurrent relay. We can see a rotating disc placed between poles of an electromagnet that is energized by a coil carrying a part of current (fault current). As the magnitude of current increases the speed of rotation of the disc also increases. Greater the current, greater is the force and hence more speed. This brings the movable contact in contact with trip contact faster than that when the current is lower. As soon as the trip contact closes, the relay trips. Thus for higher currents the disc will move faster and will trip the relay faster hence taking list tripping time.

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