Hello Everyone,

I came across a very interesting document recently from Schneider electric regarding virtual mains. It seems it's used as an AF mitigation technique in some cases.

I read there are two implementations.

The first is for systems where the HV of an XFMR has protection but the LV side does not.
Here a new set of CTs and a relay is added to the secondary. This new relay has the inst. curve enabled and is only in action if enabled through a maintenance switch when work is done below the XFMR secondary.

The document below then also says the following "The second form utilizes zone-selective interlocking with downstream branch circuit breakers in the switchgear, which eliminates the need for the maintenance selector switch."

Can anyone shed some light on how this second version works?

Does this mean the additional relay on the XFMR secondary somehow communicates with the low voltage breakers downstream? I have only heard of this technology through LVCBs with Zone selective interlocking technology built-in. So im having trouble picturing this applied through a combination of relays and LVCBs.

Any help is greatly appreciated, thanks!

https://download.schneider-electric.com ... 1910BR1302

Yes, Zone Selective Interlocking can be applied to combinations of both relays and breakers. Sometimes you will need to add some external power supply and interposing relays (don't forget to include any introduced time delays), especially between products from different families or manufacturers.

Thanks for the feedback!
How exactly is this accomplished?
What tells the upstream relay to hold for an intentional delay when the faults downstream protected by LVCBs?

Most relays can be programmed to follow an alternative trip curve based on a digital input signal, while other relays can be programmed to prevent a trip signal from being intiated.

But it is my understanding that each trip unit manufacturer might have a different control scheme used between the Main and the Feeder Breakers.
You would need to understand the nuances of each vendors scheme, i.e.; voltage requirements, wiring, etc for the controls and then utilize interposing relays and possibly power supplies to make it all work with a multifunction VM relay.

It's a shame that a standardized scheme was not developed that inherently allows for Relay/Trip Unit communications.
A scheme like that could then be employed for switchgear that currently doesn't have a main breaker.
There are a lot of facilities where mains have not been employed.
Zone Selective Interlocking Schemes ware really only meant for those that do.

Jeff