Directional protection scheme

Hello folks,

I am working on protection of a loop system, the system figure is attached. In this loop system, each section has two pilot protection relays. I've already achieved the protection of the system by using directional over current relay. Now in case of fault in any of the section we will generate generated a radial system. Consequently, the relays in the radial section provide further protection by working as over current relays.

This is demonstrated in the enclosed figure. The fault is cleared by R1 (as it occurs in the forward direction of R1) and R2 (as it occurs in the forward direction of R2). The opening of CB1 and CB2 generate a radial system, which is protected by R1- R4 or R4-R3 or R3-R2.

Now for this radial system, if there is a fault in between CB3 and CB4, the current of Relay 2 is zero. So during this case how system needs to be redesigned to take care of the second type of fault because, in this case, directional protection scheme is no longer valid. Any views or help on this issue would be greatly appreciated.

Thanks

 

[davidbeach]

There's a variety of ways this can be done. Not knowing how you're going about it the following may or may not be useful:

Assuming at POTT scheme with some form of echo logic:
[ul]
[li]For complete loop and fault where indicated, Relay 3 keys permissive to relay 2 but receives no permissive in return because relay 2 sees a reverse fault (relative to relay 3.[/li]
[li]With CB2 open, and a fault beyond relay 3, it sends permissive to relay 2. Relay 2, seeing no fault on either side, waits a few cycles (2-5) and then echos the received permissive signal. Relay 3 issues the trip.[/li]
[li]In the meantime, relay 3 is timing toward a trip (always include unaided local trips in case of comm failure).[/li]
[/ul]

 

[johndaniel]

Thanks for the reply. I have a question, the scheme you have suggested will work only for fault in section CB1-CB2. But what about the fault which occurs in section CB3-CB4 . In other words, could you guide me how to make POTT logic works for a looped system?
Thanks