Switchgear Low Impedance Vs High Impedance Bus Differential 2

[rockman7892]

I am currently evaluating low impedance vs high impedance bus differential schemes for a 13.8kV Switchgear replacement project. Existing switchgear does not have any bus differential but new switchgear will be provided with bus differential. Switchgear is 3000A M-T-T-M with a total of about 14 breakers including all mains, ties, and feeders.

I know that both schemes provide pretty much the same response times so choosing high vs low becomes somewhat of an application consideration. From what I have seen elsewhere two primary considerations become available space (dedicated CT's for high impedance scheme) and cost (low impedance system with higher cost).

If new switchgear has space to accommodate dedicated CT's for high impedance schemes and the economics point towards the cheaper high impedance scheme, what other factors should be considered when evaluating these schemes?

Is the slope differential low impedance scheme typically more secure than the voltage restraint used in the high impedance scheme? Should preliminary saturation calcs be evaluated in the criteria for selection?

 

[submonkey]

Hi Rockman,

The factors in favour of high Z are in my opinion:

- Simpler to set, configure, prove, debug.
- Very long life - the CTs and wiring will last as long as the switchgear.
- If you use an electronic relay, you may need to factor in one replacement, but it is a simple replacement.
- Excellent stability.

The factors in favour of low Z are in my opinion:

- The ability to share CTs. If you only need two cores per bay you'll fit in better CTs.
- The ability to set backup overcurrent elements in the bus relay.
(but the settings are a royal pain to change).
- Good for fault recording from many sources (but your other relays probably do this).
- Support for complex bus arrangements (which are often a bad idea for other reasons).

If it's a utility substation where feeder settings will change often, and bus outages
are expensive/difficult, and the substation will be there for 50 years - I'd pick High Z.

If it's an industrial substation where the feeder settings will be untouched, and the
equipment has an uncertain future in 20 years - low Z may be better.

I'm generally a big fan of well constructed high Z bus protection. Very little goes
wrong with copper and steel. Electronic stuff is usually less dependable.

Thanks,
Alan

 

[davidbeach]

As I’ve looked at it, the big difference, given the relays we’d select, is that high impedance is one relay for any number of positions. As we’d pick them a low impedance relay is good for at most seven positions; eight requires a relay per phase and that takes a lot of real estate.