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ZSI Time Margins

ZSI Time Margins

ZSI Time Margins

Hi all,

Does anyone have experience with implementing ZSI protection schemes on MV distribution networks. My understanding is that a 200ms time margin is usually used between levels however I am not sure where this comes from.

Any thoughts?


RE: ZSI Time Margins

We have implemented several fast bus tripping schemes - basically ZSI at MV using digital relays. We designed much tighter intervals. The downstream relays will need perhaps a cycle to detect the fault and close a contact signalling the upstream relay to block the fast tripping. I think we allowed a few cycles of definite time delay before tripping the upstream breaker.

At 0.2 seconds, you could probably coordinate without ZSI simply with overcurrent devices, assuming 3 cycle breakers.

But possibly we're talking about different things.



RE: ZSI Time Margins

Thanks Dave, sounds like an interesting project.

That's great, I'm not so familiar with the internal time delay of the relay itself, I guess you can get it from the specs.

What do you think of these ZSI schemes? Do you think they will become more common place?

RE: ZSI Time Margins

Yes, because they are relatively simple to implement on existing switchgear - we call them the "poor man's bus differential relay".

RE: ZSI Time Margins

Haha nice.
Yeah I can definitely see the advantages.

Thanks for your advice

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