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 Post subject: CT Locations for MV Switchgear
PostPosted: Tue Jun 23, 2009 7:57 am 
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Joined: Wed Jan 28, 2009 2:19 pm
Posts: 70
Location: Georgia
My experience so far has been with systems 600 volt and less. I have some questions that I am hoping you guys can help with. At one of our facilities there is a utility owned transformer 43.8 kV / 4.36 kV. We own the outdoor switchgear for the secondary side of the transformer.

Our switchgear consists of (2) branch circuit breakers…no main. Both breakers are identical (Allis Chalmer MA-250C-1, 1200 amp).

The first breaker is operated by (3) Westinghouse relays. (1) CO-9, 50-51 relay on phase one. (1) CO-9, 50-51 relay on phase three. (1) CO-9, 51 relay on the ground.

The second breaker is operated by (3) Westinghouse relays. (1) BL-1, 49-50 relay on phase one. (1) BL-1, 49-50 relay on phase 2. (1) 50 relay on the ground. There is a 4th relay (Time Mark 3 phase voltage unbalance monitor).

I understand that the ground and voltage unbalance relays are not considered during an arc flash study, so for the purposes of this discussion I am ignoring them.

1. Since only two of the three phases on each breaker are monitored by relays, is it OK to model these as 3 phase for the purpose of arc flash?

2. Where would I look to find the CT’s associated with each relay? I pulled the back cover off of the switchgear to expose the load conductors leaving the breakers. I found one CT per breaker. All three phases passed through this single 50:1 CT. I was unable to find additional CT’s.

3. I have been told you could determine the CT winding by looking at the bus rating on the switchgear sections and by looking at the amp range on the relay. Is this true?

Any help y’all can give is greatly appreciated.


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PostPosted: Tue Jun 23, 2009 6:49 pm 
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Joined: Thu Jan 10, 2008 8:49 pm
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Location: New England
This is a tough one.

1) The center leg has no overcurrent. You are relying on a phase to phase, or three phase fault to trip the phase relays, or a single phase to ground fault on the center leg to trip the ground fault. The most conservative way would be to calculate the trip times for both the phase devices and the ground fault device and take which ever is greatest. For the phase to ground scenerio this will inflate the number considerably, but would be accurate for three phase bolted faults. There is no equation for single phase modeling, and I am reluctant to guess if it would fall by 1.73.

2) The CT that you see all three phases passing through is the ground fault relay. If all three phases are equal current, this CT reads zero. It basically see the difference between the phases. Under a ground fault one leg would be very high while the others would be near normal. For the phase CTs, I don't know this gear specifically but usually they are on the bus itself. If you don't see the CT from the point its bolted to the line side of the breaker to the point it leaves the gear, then its on the bus. The CT ratios can be any nominal value. You have to look at the construction print, read it off the nameplate of the CT, or verify it by testing. Measure the CT current going through the wire leads to the relay. The relay is setup for 0 to 5 amps. Next measure the actual current in the 4.16KV cable with a clamp on ampprobe. Follow proper safety precautions and let someone who knows how to do this perform the task. Divide 5 by the relay lead current. Take this result and multiple the actual current cable. The result is the ct ratio. Example, you measure 2.5 amps on the relay and 100 amps on the actual cable. 5/2.5=2.0. 2.0 x 100 =200. The ct is a 200:5 ratio ct. I'd round to the nearest nominal CT as the are only found in nominal ratios. If the cable read 92 amps and you multipled by 2 and got 184. There is no 184:5 CT unless it was specially made, which just wouldn't happen, so its a 200:5.

3. The bus rating won't tell you anything about the CT. A typical bus is 1200A because the typcial relay is 1200A. The CT is selected for the actual load. Its not unusual to have a 1200A bus and a 200:5 CT or a 400:5 CT. If you only have one cable leaving per phase it probably is not a 1200:5 CT.


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