Bus differential on MV MCC

[rockman7892]

Is it typical to include Hi Z bus differential on MV MCC’s? I know it’s a common application on MV SWGR but have not come across it much on MV MCC’s.

I’m looking at an application what has a 6.9kv MV breaker close coupled to an MV MCC.

For HI Z diff CT’s are required to be same size. With main breaker having 1200 A rating I see it as perhaps not being practical to have a 2nd set of CT’s on MV MCC contractor feeders rated at 400 A (not much physical space)

 

[davidbeach]

What else are you doing to mitigate arc-flash events?

 

[rockman7892]

Davidbeach

Good question. In this particular case the gear is Arc Resistant and will also include AF light sensing for AF mitigation. I'm curious if Buss Diff and AF light sensing are typically used in conjunction or its usually a one or the other approach? Can the AF light sensing act as a bus diff in a sense (I guess the assumption would be that a fault created a light source every time). Do these both have similar response times?

The issue is going to be fitting a second set of CT's in the MVC. There is obviously limited real-estate here compared to Switchgear to it can perhaps be a challenge to fit a dedicated HI Z CT along with phase and gnd CT's. Would a Low Z bus diff solution perhaps be more practical here with using the phase CT's for the Low Z inputs as well?

 

[jraef]

I've done a fair number of MV MCCs, I have never once been asked to put bus differential protection into it. Generally, bus sectionalizing schemes are done in the switchgear ahead of the MCCs, the MCCs are just going to the loads.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[davidbeach]

I don't work with MCCs, but for switchgear being purchased now we are including a non-redundant high-impedance bus differential and an optical arc flash detection system that is supervised by current to minimize the chances of false operation based on light leaks. We will also have a maintenance mode switch on the transformer. Additionally, if the maintenance mode is active the outgoing feeders should all be tagged as well. With all protection functional, there would be three zones of protection that would produce an instantaneous trip for an arcing fault in the gear. The optical AF protection, unlike bus diff, will not trip for a solid fault, such as what happens when somebody closes in grounds that didn't get removed.

 

[dpc]

I've never seen bus differential relaying on MV MCC. I'm not sure where the additional CTs would go.

 

[LionelHutz]

Not common. I've only seen it done for one customer.

Fiber optic for arc flash detection is rather common nowadays.

 

[rockman7892]

It sounds like its not typical on MV MCC's and I have never seen as well which is the reason I asked. Would the philosophy with MV MCC not be the same as SWGR however with CT's on line side of Main and then load side of contactors to provide fast response for faults internal to MCC? Is the fact that its not common on MCC's due to the construction on MCC's and space limitation?

As I mentioned in my OP this particular lineup has a Breaker structure close coupled to the MCC lineup (2 Main breakers / No Tie) So to make this work we'd need to have CT's on the two mains in the SWGR along with the CT's on the load side of the contactors on MCC. I don't think it would be warranted to provide Bus Diff on SWGR only which would also provide further complications with putting CT's on bus transition.

Given the fact that there will be light sending in both pieces of equipment it may not be worth the complexity here?

 

[rockman7892]

Does anyone think that it may be a good option to look into application of Low impedance Bus Diff for this application? I know SWGR usually incorporates Hi Z buss diff but in this case where there is limited space for CT's in MCC does it make sense to make use of phase CT's and provide Low Z buss diff across MV SWGR and MCC?