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.
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.