MV Switchgear Hi Impedance Bus Diff with PT's in zone 1

[rockman7892]

Looking for feedback from experience with applying Hi Impedance bus diff on switchgear bus where station service CPT's are fed from bus within the differential zone.

Application under consideration is a 34.5kV 1200A SWGR lineup with high impedance bus diff wrapped around incoming, feeders and tie of M-T-M lineup. CT's on Main, Feeder, and Tie breaker are C200 1200:5 MR CT's tapped at 600:5 Tap. Buss diff relay is SEL 487Z.

Switchgear has a 50kV CPT fed from bus with primary fuse. CPT primary rating at full load is 1.44A. I'm guessing that with inrush of CPT it can have momentary inrush of 15-20A.

With 1200:5 CT's tapped at 600:5 they provide an effective C100 ratio.

Concern is weather CT ratings will be able to produce enough voltage and cause mis-operation or nuisance tripping of buss diff during CPT normal operation or inrush.

Looking to hear practical experience from others and weather Low Imp buss diff is more practical for this application. Or is high impedance diff with higher CT class preferred option?

 

[davidbeach]

Why 600:5? Put them back to 1200:5. Always use the highest ratio available for hi-z bus diff. If they're all 1200:5 go with 1200:5. You're not trying to do anything subtle with a hi-z bus diff.

Assuming the "487Z" is actually a 587Z. Also assuming that 50kV is really 50kVA.

Pick the voltage setting above what a spill current of 30A (15-20 plus margin) would produce. Also look for guidance for when arrestors are in the bus zone, that will provide some info about slowing things down just a bit.

If your ever do have a CPT primary fault, you'll probably get both the bus diff and the fuse. The fuse will at least tell you where the problem that locked out the bus was.

Depending on the number of positions in the switchgear, doing low-z bus diff can get really complicated really fast.

Another option to directly address the issues is to simply throw a CT on the connection to the CPT and remove it from the bus zone. "Problem" disappears and becomes an out of zone "fault".

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[rockman7892]

davidbeach

Thanks for response. Yes I meant 587Z and 50kVA thanks for clarifying.

Arrangement is M-T-M with only a single feeder breaker on each side of lineup feeing an identical rated transformer. So would be a total of 5 CT's to either type of buss diff relay

The load on each side of M-T-M Switchgear is a 15/22.4 MVA transformer on each side. Worst case loading on feeder breakers would be transformer operating on fans with loading of 374A. Worst case loading on main breaker would be with tie breaker closed feeding two transformers running on fans for total load of 749A. Based on that it may be feasible to use 600A SR CT's as opposed to using a 1200:5 with taps? Only potential issue there is it limits potential future expansion of switchgear (very unlikely however) given the gear rating of 1200A.

If 600A CT's are an option do you believe that is a better alternative than a low impedance diff?