Vacuum fault interrupter 2

[sahelrahman]

There are two vista switchgears, one fed from A-side and the other one fed from B-side. These switchgears serves transformers.

Since the B vista switchgear is energized through the closed loop switch in each transformer a fault on a primary conductor will be fed direct from the utility and also through the primaries feeding the other transformers. The loop feed switch at the transformers creates a direct connection between the A and B side primary conductors without going through the transformer.

The VFI feeding the faulted line and the A side should open first and the B side VFI should also open since it will be back fed through the other transformer primary conductors. Currently both the VFIs are set as

Pick up - 200A
U2 Inverse - 0.5
Instantaneous - 1000
Definite time - 96ms

If I bring down the definite time setting of the A-side VFI to 80ms,will this solve the issue?

Thanks

 

[jghrist]

Normally, you would have an open point at an intermediate transformer so there is no closed loop feed. A fault will result in only one side tripping and not all of the transformers will lose power.

Why is it necessary that the A side open first? With the closed loop, both sides will open for a fault. Who cares if one opens 16 ms earlier?

Do your settings provide coordination with transformer fuses?

 

[ScottyUK]

You could consider a direction overcurrent scheme, if your relay allows this. You will need a set of VTs to provide a reference.


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If we learn from our mistakes I'm getting a great education!

 

[sahelrahman]

Thanks for responding..

jghirst- Yes the settings provide coordination with the transformer fuses. I don't have a good knowledge about the closed loop system and currently I am doing the coordination study using the SKM powertool analysis. My manager asked me to do this. So I am kind of confused with this. He said the A side should trip before B side inorder to protect other transformers in the loop

 

[jghrist]

A single-line diagram would help. I'm not sure what is meant by A side and B side.

 

[sahelrahman]

Jghrist...Here is the one line diagram...Please have a look at the uploaded file

 

[jghrist]

The one line shows the input to Vista Switch B as normally open and there is circuitry for source transfer in the event of loss of the normal source. I don't think that the intent was to feed any transformers or faults through the other transformer loops, but this is what will happen if the normal feeder VFI is tripped. Tripping the A feeder faster than the B feeder will not work because the other A feeders will actually be feeding the fault and will trip at the same time. Then all of the transformers will be fed from Switch B only which will have no source. The whole system will trip for a fault on one feeder.

I think Scotty is right - a directional scheme is needed, but this is not an option on the Vista switch. I believe that you need to have the B feeders normally open as well to prevent backfeeding faults. You would need to open all Switch A positions if Source A is lost and close all Switch B positions.