CT on a ground bus

[Red Wiggler]

I have seen in the past (on Medium Voltage Switchgear) a "CT" installed on the ground bar. This CT is evidently moitors the amout of current that might be "leaking" from the sheilds on the Medium Volt wire. This seems like a very important component within the gear in the event you have bad conductor. My question is what is this ground CT called? Is there a specific name for it?

Also it seems that recently I have seen this CT less and less on the gear packages that come through the office. Is this an expensive option to include in the pricing? Most protective relays should have the ability to accept this input... correct? Is it an option that is not taken frequently enough?

 

[zog]

Usually refered to as a 4 wire system. If the gear uses a 4 wire system it uses a seperate CT for GF sensing, the relay is then refered to as a 51G relay.

More often in MV systems a 3 wire system is used, all 3 phase relays share a common nuetral which is used as the input to the GF relay, in this case refered to as a 51N relay. (see attachment for wiring configuration of the relays in a 51N scheme)

 

[kingpb]

It is most likely intended for ground fault protection on the main breaker to the switchgear, but it could also be used for bus ground differential protection, for the switchgear, which is not as common.

To expand on the earlier post regarding the difference between 51N and 51G:There is 50G 51G, 50N and 51N.

When utilizing the letter "G" it is generally understood to mean utilization of a zero sequence CT. For a 3-ph, 3-wire system the 3 current carrying conductors pass through the center of a single CT. The CT measures the 3Io current. In a 4-wire system the neutral would also be run through the CT. These CT's can also referred to core balance CT. A zero sequence CT is also used on the grounded neutral circuit.

The letter "N" is understood to mean the use of 3 individual phase CT's with one installed on each of the current carrying conductors. The residual current of the CT's is picked up by the relay.

Zero sequence CT's will provide a higher sensitivity for detection of ground fault. However, due to cost savings, and where sensitivity is not a concern, if 3 phase CT's are already being used for other protective functions, it is more economical to utilize them and not add an additional zero sequence CT. Especially since microprocessor relays usually have the 50/51, and 50/51N capability in one device. The relay may also have the 50/51G as well, but again it requires an additional CT.

 

[Hameedulla-Ekhlas]

I have seen in the past (on Medium Voltage Switchgear) a "CT" installed on the ground bar. This CT is evidently moitors the amout of current that might be "leaking" from the sheilds on the Medium Volt wire. This seems like a very important component within the gear in the event you have bad conductor. My question is what is this ground CT called? Is there a specific name for it?

Also it seems that recently I have seen this CT less and less on the gear packages that come through the office. Is this an expensive option to include in the pricing? Most protective relays should have the ability to accept this input... correct? Is it an option that is not taken frequently enough?

You can call it earth fault protection CT.
and
There are two usage purpose of usage see below topic for extra information.

Current transformers are often used for the dual duty of measurement and protection.

CT is used For overcurrent and earth fault protection, with elements of similar VA consumption at setting, the earth
fault element of an electromechanical relay set at 10%
would have 100 times the impedance of the overcurrent
elements set at 100%.



CT and VT function like ?ears' and the ?eyes' of the protection system. They listen to and observe all happening in the
external world. Relay itself is the brain which processes these signals and issues decision commands implemented by circuit breakers, alarms etc.

The CTs can be classified into following types:

Measurement CTs
Protection CTs

A measurement grade CT has much lower VA capacity than a protection grade CT. A measurement CT has to be accurate over its complete range e.g. from 5% to 125% of normal current. In other words, its magnetizing impedance at low current levels. (and hence low flux levels) should be very high. Note that due to non-linear nature of B-H curve, magnetizing impedance is not constant but varies over the CT's operating range. It is not expected to give linear response (secondary current a scaled replica of the primary current) during large fault currents.

In contrast, for a protection grade CT, linear response is expected up to 20 times the rated current. Its performance has to be accurate in the range of normal currents and upto fault currents. Specifically, for protection grade CT's magnetizing impedance should be maintained to a large value in the range of the currents of the order of fault currents.

When a CT is used for both the purposes, it has to be of required accuracy class to satisfy both accuracy conditions of measurement CTs and protection CTs. In other words, it has to be accurate for both very small and very large values of current. Typically, CT secondary rated current is standardized to 1A or 5A (more common).

For part(II) I dont have information.

 

[SG-1]

If the shield wires are pulled through the CT then it is a PD sensor ( Partial Discharge ) also known as a RFCT. The gear may have an insulation monitoring system installed.

 

[zog]

If the shield wires are pulled through the CT then it is a PD sensor ( Partial Discharge ) also known as a RFCT. The gear may have an insulation monitoring system installed.

Did not think of that but you are right, it could be that too. Usually look different than regular CT for a relay. On line PD sensors and monitoring is perhaps the best new MV switchgear technology in the last decade. Neat stuff. The new hand held PD sensors are amazing, I carry one in my trunk.

 

[jdsmith]

On line PD sensors and monitoring is perhaps the best new MV switchgear technology in the last decade. Neat stuff. The new hand held PD sensors are amazing, I carry one in my trunk.

Is online PD monitoring worthwhile now from an economic standpoint as opposed to annual PD testing? In the past in our plant online PD is looked at kind of like installing dissolved gas analyzers on transformers - maybe some people can justify it but the upfront capital cost was too much as opposed to the cost of the annual gas sampling & testing program.

 

[zog]

Is online PD monitoring worthwhile now from an economic standpoint as opposed to annual PD testing? In the past in our plant online PD is looked at kind of like installing dissolved gas analyzers on transformers - maybe some people can justify it but the upfront capital cost was too much as opposed to the cost of the annual gas sampling & testing program.

Yes it is pretty neat stuff and afforadble if done during initial installation. Many PD sensors can be used going to a single module, 10 or 12 if I recall and they all go through existing comms system. We have a division that specilizes in these systems, I can post some details if anyone cares.