ammeter and measuring ct in protection circuit 1

[SilverfoxUK]

Gents

I need to add a PQM to my 11kV board to give me a kW value I can use for comparison purposes, I'm not looking for fiscal accuracy.

The 11kV switchboard I've inherited has CL 0.2S metering Cts used by the supply utility for fiscal measurements.

As part of the inheritance and already installed to the CL 5P10 protection Cts is an ammeter in the yellow phases and a GEC IStat in the red phase to give me an analogue signal for our measurements.

Before opening the board my intentions were simply to drop the new PQM in series with the Utility Companies fiscal meters.

Connecting into the protection relay Cts would still give me the accuracy I need without the possible polical fallout with the Utility Company but everything tells me this is a bad thing to do. The protection relays are supplying to a 50/51 O/C E/F protection relay.

Would value your thoughts

 

[davidbeach]

Getting into the utility metering CT circuits may get you into trouble; tampering and all that. No problem with a check meter in the same CT circuit as the protective relays using the protection grade CTs.

 

[che12345]

Dear Mr SilverfoxUK

1. " Getting into the utility metering CT circuits may get you into trouble; tampering and all that " as pointed out by learned Mr davidbeach.
2. " No problem with a check meter in the same CT circuit as the protective relays using the protection grade CTs " may not be a good choice. Take note that the meter may not be rated for the short-circuit current which the protective CT can produce 10-20 times the current before saturation.
3. Although installation of a set of [HV metering] CT would be ideal but would be very costly and may not be an option due to space limitation. Also may not be UL listed etc.
4. Alternatively, a set of [LV metering CT, very small in size] may be placed over the conductors [with full insulation]. Caution: over the conductors [with full insulation].

Che Kuan Yau (Singapore)

 

[FreddyNurk]

che12345 has a point, depending on your PQM it might be able to withstand the short circuit current, or it might not. Some power meters these days have a surprisingly high withstand.

Another alternative which might be easier or more difficult depending on your circumstance, replace the protection relay with one that is capable of also providing Power Quality measurements and use that instead.
You'll obviously get better accuracy if you used metering CTs but whether that is an issue for you is up to you.



EDMS Australia

 

[RRaghunath]

"CL 5P10 protection Cts is an ammeter in the yellow phases" - it seems we have problem already here. Analog ammeters cannot have short time overload withstand in excess of 10A. With 5P10 CT, the analog ammeter is in danger of catching fire when there is a severe short circuit fault in the incomer.
Coming to new PQM, I think regulations allow you to connect the meter in Check meter circuit.
Since the accuracy is acceptable, the PQM can also be connected in 5P10 CT circuit as the digital meters are as good as protection relays when it comes to short time overload withstand.

 

[waross]

I'm not looking for fiscal accuracy.

Metrosil silicon carbide varistors?
Other applications include:• Current and voltage transformer protection• Telecommunication equipment protection • Telephone line protection• EMP protection•• Protection of high impedance relays
Link

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[SilverfoxUK]

Gents

Thank you for the response.

I take note of tampering with the fiscal metering circuits and am going to stay away from that circuit. I'm good with freedom!

It has been suggested that the PQM might be robust enough to withstand a high current from the CTs
The protection CTs are 5P10 so when under fault it seems the PQM I have which will stand 20x CT for 1 sec and 100 x CT for 0.2 sec will be able to cope. Well hidden on page 587 of the PQM manual. Good call.

Great call about the ammeter - that's coming out to start a new life in the dump!

Kind regards and thanks to all who have helped.

Regards

SilverfoxUK

 

[bacon4life]

Some power quality meters have options for using split core clamp on CTs. This makes installation easier, and the auxiliary CTs saturate at a low enough current to protect meters with low current withstands.

The GE PQM has an end of life notice on the webpage. Hopefully it is a different PQM you are installing.