Field testing of CT's in SF6 switchgear

[lz5pl]

We received an enquiry for commissioning of 110 kV GIS. Unfortunately we have no experience in field testing of CT's in such switchgears. Would you suggest me which parameters are usually tested ?
According to me they should be Insulation resistance measuring of secondary windings and possibly Excitation curves. Insulation resistance from primary winding and Primary current injection tests seems not to be applicable for that case.
Of course we will ask the manufacturer, but for the moment some advice from experienced colleagues will be very helpful.

------------------------
It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant

 

[JVShermco]

Typically you need to do saturation, ratio and polarity tests. Use a Vanguard EZCT tester.

 

[lz5pl]

Typically you need to do saturation, ratio and polarity tests. Use a Vanguard EZCT tester.
==================
For conventional CT's is OK, but for these in GIS probably there is no access to primary winding terminals, so how about polarity and ratio ?

------------------------
It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant

 

[JVShermco]

Just because it is gas insulated, does this mean you can not get to the primary terminations? The CTs should be wired out to a terminal strip. You can ratio it there.

 

[oldfieldguy]

On GIS gear, you have to use the REAL primary conductor path for tests. This can be rather inconvenient and require some thought, but CT's ARE CT's, and you need to verify ratio, polarity and saturation.

old field guy

 

[lz5pl]

old field guy, I think the same, but at the moment I have no idea how to find access to real primary path. Probably primary injection tester should be connected to feeder inputs and inject with closed disconnectors and CB's. Anyway I will ask GIS-manufacturer, my question here is just to ask for advise from colleagues with more field experience.

By the way, with 26-years commissioning of 6 to 400 kV substations on three continents can I join also the club of Old Field Guys ;-)

------------------------
It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant

 

[scottf]

I agree with what is said about and would add that insulation resistance (Megger Test) is also a good idea, if not just as important as the other tests mentioned. This should be done secondary winding(s) to ground/case at a minimum of 1 kV.

 

[ScottyUK]

lz5pl,

Have a look at your earthing switches. Depending on configuration they can provide a means of getting current on and off the busbar for test purposes. I've used this method before.

Obviously you will need to keep a dead and earthed section of bar between you and anything live - that sometimes limits your options for choosing a switch.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[lz5pl]

Thank you, Skotty. For similar tests in conventional switchgears I often use earthing switch closed to avoid lifting of one current lead up to the primary terminals. I hope same solution would be used in GIS too. Live parts are not a problem for that particular case - it is new installation.

------------------------
It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant

 

[oldfieldguy]

lz5pl- (That sounds like and amateur radio callsign - de kd5qi)

I have done these and ended up string a lot of temporary test conductors between incoming line bushing and such. It's not easy.

The megger test is a point I missed. That needs to be done for sure.



old field guy

 

[lz5pl]

Thank you KD5QI, I am going to inject in a similar way. Actually we have done it already in GIS, but it was 20 kV. The points I am afraid for now are the longer test leads and several contact resistances on the chain of CB's and disconnectors (GIS is in 1,5 CB's scheme). But well, let's win the contract first. I sent the offer and will wait for the results.

73! from LZ5PL

------------------------
It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant

 

[DTR2011]

As it was mentioned by ScottyUK, your access to the main bus is via the earthing switches. There are generally provisions within the switch (a removable link) to get access to the bus.

Depending on the bus configuration, you can set up one end on the line side of the main CB and then test through to other breakers.

Using the voltage method would probably be easier, especially for HV gear, as the earth switch connections will be high up. Voltage method uses small leads, vs. current which would be much larger.

Careful planning (test all CT's per phase) can speed up the job.