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Metering testing

Metering testing

Metering testing

(OP)
Anybody here has the experince on the installation and testing of Substation metering Cabinet?

The metering cabinet is located on the secondary of 138KV/4160V Transformer. Site superviser told me he has seen so many mess-ups on the metering cabinet. To me, it is a just metering equipment. As long as the CTs and PTs are wired correctly, all the cables and grounding are installed properly, there should not be ant issues. Any other things I should pay attention? Any inputs would be much appreciated.

 

RE: Metering testing

It would help if the supervisor can provide some examples of the mess up.

Key is to meet the utility company's specs for cold or hot sequence, CT/PT type, ratio and class etc., size( dimensions) of the cabinet and any other special requirements or preference for a mfr. etc. You cannot just install whatever a mfr. will provide.

Rafiq Bulsara
http://www.srengineersct.com

RE: Metering testing

On the other hand, re-reading the post are you asking about testing or installation? They are two different things.

Rafiq Bulsara
http://www.srengineersct.com

RE: Metering testing

Installation:  

Do NOT assume that the design people "knew" what they were doing.  I (and many others) have found the design drawings to be horribly wrong.  Do spend a bit of time in a "reality check", verifying polarities, ratios, connections, etc. against the desired characteristics to be measured.

Upon finding the drawings correct, develope a commissioning test plan that will verify correct installation.  Secondary injection tests from the instrument transformers themselves are needed before energizing.  If programmable devices are used, verify the program.

Upon initial energization check the indications on the metering, including during such load tests as may be done, against other indicators such as advanced protective relay indications, etc.

If you're talking about checking metering that has been installed and in service, do you have indication that it has EVER worked correctly?  If it has, and now isn't, what changed?  Did somebody replace wiring or instrument transformers or other devices?  Did somebody "calibrate"  meter?

If it has never worked correctly, then you go back to the list for commissioning new installations.

This is fun stuff...

old field guy

RE: Metering testing

My old IEEE Red Book, ANSI Std 141-1986, Section 10.7.2 says

"The probability of error in selecting or connecting a kilowatthour meter is high, especially if instrument transformers are required."

We engineers tend to follow the Red Book.  

Check everything with the utility and someone who knows metering, there's good advice above.

RE: Metering testing

(OP)
Thanks for the valuable inputs, I will make sure the wiring is done properly as per Utility requirement and make them do function test before energizing the transformer.

RE: Metering testing

(OP)
I noticed they supplied smaller CTs that we expected. Their explanation is, these CTs have RF factor ( continuous thermal rating factor. The rated primary current of the transformer can be multiplied by RF to obtain the maximum primary current that can be carried without exceeding 30DegC ambient air temperature. Is it common for the Utility applications? These CTs are installed inside the metering cabinet. The ambient temperature inside could be higher than 30DegC.

Thanks in advance.  

RE: Metering testing

Utility is betting that the normal load will be less than rated.  This is a common application.  I'm not sure what effect it has on the metering accuracy, but any effect would probably be in the customers favor.

RE: Metering testing

We typically recommend a small ct with a TRF (Transformer Rating Factor) of 3 to 4 and size them small with a 20 amp meter.  That is because actual load usually ends up being 1/4 to 1/2 of the projected load.

Alan

RE: Metering testing

(OP)
What is the TRF? In our case, the maximum running current will exceed the continuous rating of the CT primary.Utility is relying on RF factor to multiply the continous rating of CT to obtain the required primary current. however the performance of continuous thermal rating of CT may be affectd in summer time.      

RE: Metering testing

Actually I believe it is Thermal Rating Factor.  A 5 amp ct with a TRF of 4 is rated for 20 amps.

Alan

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