The overcurrent protection is used in the power system to protect the network. In this article, we discuss the Drawback of Overcurrent Protection.
The reach of the overcurrent relay depends on two factors; the magnitude of fault current and source impedance.
Both magnitude of fault current and source impedance depends on the type of fault.
The magnitude of fault current varies with the type of fault. In overcurrent protection, the overcurrent relay operates on overreach or underreach.
Let’s understand the drawback of overcurrent protection with an example.
Consider a system network with source E. The source impedance is Zin. The overcurrent relay is used to protect the line AB.
The relay is set for the pick-up current of I0. And this value is designed for the fault type L-L-G (double line to ground fault).
Now, if the fault varies from double line to ground (L-L-G) to line to ground (L-G), the current changes and relay reach changes to point B1.
The distance AB1 is less than AB. And the relay is under reach.
Similarly, for a three-phase fault, the reach of a relay changes to point B2. Therefore, the distance is AB2. And this distance is more than AB. This is an overreach of the relay.
Hence, the reach of relay changes with the type of fault which is undesirable.
Now let’s say, the relay reach is designed for the current I0 with source impedance Zin for a particular type of fault. And the reach of a relay is calculated up to point B.
If source impedance changes to Zin1, where Zin1 < Zin then the relay reach changes to point B1. And distance AB1 is less than AB. It makes the relay under reach.
If source impedance changes to Zin2, where Zin2 > Zin then the relay reach changes to point B2. And distance AB2 is more than AB. It makes the relay overreach.
Therefore, the reach of relay depends on the source impedance. As the source impedance changes, the reach of relay changes which is undesirable.
To solve the drawbacks of overcurrent protection, the distance protection scheme is used in transmission line protection.
The distance protection depends on the ratio of voltage at the relay point to fault current.
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