Switchgear Feeder CT Placement 1

[rockman7892]

For MV Switchgear in a radial system where is the desired location for CT's on feeder circuits in relation to the bus side vs load side of the feeder breaker? I have seem them located on either side so was wondering if there is a standard or if its more of a preference thing.

I guess that perhaps on the bus side the CT's can detect any abnormalities within the breaker itself? Are there any issues with having them on the load side?

 

[davidbeach]

Ideally each zone of protection uses CTs located across the breaker from the rest of the zone so that the breaker is always in the zone of protection. In your case the feeder protection CTs should be on the bus side of the breaker and the bus protection CTs should be on the feeder side of the breaker. A fault between the CTs would then take out both zones as is necessary for a breaker fault.

 

[rockman7892]

Thanks davidbeach that make sense.

You referenced both feeder protection CT's and Bus projection CT's which indicates the presence of buss differential protection. This particular Switchgear that I am looking at does not have plans for bus differential. Are you suggesting that Bus differential should be added, or is considered the norm in MV Switchgear applications? If not it would simply be the phase CT's on the bus side of the feeder breaker.

 

[marks1080]

Ideally you have CT's on both sides of the breaker and a traditional bus differential.

It's becoming more common to use CT's on only one side of the breaker at lower voltages, especially with GIS systems and space restrictions. I guess it doesn't really matter which side of the breaker the single CT is on for the bus/feeder protections. This may be dictated by the manufacturer as well. You won't be using bus differential for this kind of set up, rather 50/51 elements (possibly with direction) and coordinated blocking signals from the feeder protections. I've had to do this for GIS gear and in that situation the CT's were on the load side of the breaker.

 

[rockman7892]

marks1080

should bus differential be used/considered on new 4.16kV metal-clad switchgear where there is adequate room for CT's?

 

[davidbeach]

Historically there wouldn't have been bus differential, but today with arc-flash considerations I wouldn't consider not having bus diff.

 

[marks1080]

My personal opinion is that I always want CTs on both sides of the feeder/bus breaker, with a full bus differential scheme.

I don't often get what I want though. I consider it common place to see metal clad gear with a CT on only one side. When I have this set up I use overcurrent on the bus with coordinated blocking signals from the feeder protection to block the bus protection for a feeder fault. I completely agree with davidbeach's comments

Some of the more recent metal clad GIS systems I've worked with have very fancy venting and arc detection systems. I haven't witnessed enough operations yet to have complete faith in these systems. I do not think they are cheap.

 

[rockman7892]

Thanks for the responses. A few questions:

1) In terms of Arc Flash mitigation does the differential become as much as a factor if the switchgear is designed to have all breakers operated remotely and thus eliminate the need for personnel to routinely interact with Switchgear?

2) For faults on the bus can the bus differential potential salvage the bus to allow for repair as opposed to bus destruction if relying on main breaker relay to trip for bus faults? If main breaker has INST with delay to coordinate with feeders will this still lead to potential bus destruction for bus faults?

3) Is bus differential applied for low level available fault currents at 4.16kV with available fault less than 5kA?

4) Why is bus differential not applied at LV switchgear where fault currents are even higher? I know modern switchgear has zone interlocking which is similar to buss diff but my understanding is that its not as quick as bus diff. Is the answer simply because the LV switchgear usually does not have the relays or CT's available to accomplish bus diff?

 

[ppedUK]

All the above responses with regard to protection integrity are all good advice, however you very often don't have a choice where to locate the CTs. Here in IEC land they are almost always on the circuit (Load) side of the breaker and cannot be accommodated on the bus side.

Also bear in mind that a CT fault (or even just a ratio change to accommodate a new feeder circuit) would require you to have a complete board shutdown to change a CT. Here in the U.K., safety rules would also not allow you to work on a protection circuit where the Primary side of the CTs had not been earthed, so work on individual protection circuits (even on metal clad switchgear) would not be permitted unless there was a complete switchboard shutdown.