MV Switchgear Bus Differential

[rockman7892]

I wanted to hear some of the opinions/experiences of others regarding bus differential (low or high impedance)on MV Metal-Clad switchgear applications. I have come across some 13.2kV Switchgear recently that had high impedance bus differential but have seen plenty of other 4.16kV switchgear that did not.

Does this typically come down to an economic decision or are there certain Switchgear applications that lend themselves more towards bus differential than others? Most of the Switchgear I am referring to is M-T-M switchgear as part of a radial distribution system.

I know the bus diff can possibly prevent bus damage and reduce arc flash for faults on the bus but I'm not sure what other considerations there may be?

 

[Mbrooke]

High impedance has a space saving advantage, especially when a large number of feeders are involved. You only need one relay, and you could theoretically string then CT to CT without having to bring all of them to a central location as with numerical low impedance protection.

 

[cranky108]

Part of the decision is arc-flash concerns, and the required PPE for different functions. The basic point is the faster the clearing, the lower the pPE required to work on it.

It's hard to put CT's in older gear, so the decision to live with it might be the right one.

Another issue is that the CT's must fit over the bushings, and in lower voltage gear, they may not fit.