Paralleling core balance CTs

[ScottyUK]

Something I have never previously considered but I am being backed in to a corner:

Can two core balance CTs be paralleled and give reliable results in a sensitive earth fault protection scheme?

I have a short feeder between two 3.3kV boards comprising two paralleled 400mm² 3-core cables. One board is fed directly from the 3.3kV transformer and the other is a sub-board fed from the first. The outgoing feeder to the sub-board has GEC MCSU01 sensitive earth fault relay fed from residually connected 1200/1 CTs within the switchgear. System earth fault current is low and the relay settings are right down at minimum in order to see the earth fault current at all. The definite time sensitive earth fault relay should discriminate with the inverse time standby E/F relay on the transformer neutral to disconnect the sub-board in the event of a fault. It almost does, in theory. I didn't design this BTW, I'm just trying to make it work.

The MCSU relay is trigger-happy at these low settings and is tripping during motor starting (we have a few big motors on this board) and the relay has been disabled leaving only the standby E/F relay on the transformer neutral as the dedicated earth fault protection. I'm aware of the problems with using residually connected CTs with low relay settings, particularly with current asymmetry during motor starting and I have major doubts that the residual connection can be made to work acceptably in this application.

I don't think I can easily get a single core balance CT into the cable box without a fairly lengthy outage to re-cable and probably make new gland plates so I was idly wondering about paralleling two CBCTs instead. I've never seen it done and wondered if anyone has practical experience of it. In principle it seems to work, but in principle so does the residual connection.


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Sometimes I wake up Grumpy.
Other times I just let her sleep!

 

[davidbeach]

The residual connection only works in principle if you assume ideal CTs, but does not work in principle with real CTs.

I've never seen what you propose, so what follows in theoretical only, but...

With two CBCTs you will still have some CT performance mismatch issues, but if you have two 50:1 CBCTs in parallel, your existing setting will require 24 times as much current as provided by your 1200:1 CTs. So, if you were looking for 0.1A (a really bad setting for a residual CT connection) you would now need 2.4A. If the mismatch between the two CBCTs gives you 0.1A of noise, you now have considerable margin from your setting.

 

[ScottyUK]

Thanks David,

Existing relay is 1A nominal, lowest setting is 0.5% (5mA) which is where the trouble started. Ideal CTs... if only I knew where to buy them!

I think I know in my heart that a new CBCT encompassing all conductors is the ideal way to go. Trying to centralise the conductors from two large 3-core cables is going to be tricky in the small cable box I have to work with but at least 3.3kV XLPE cable is fairly forgiving in terms of clearances. It might be easier to go for one massive 3-core but it will be a pig to install.


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Sometimes I wake up Grumpy.
Other times I just let her sleep!

 

[davidbeach]

Scotty, let me know when you find that ideal CT...

 

[slavag]

Hi Skotty.
Of course, I don't have excperience with paralleling of
CBCT, but why not? I'm not see any theoretical problem.
Next option: relay with two separated current inputs for SEF.
Regards.
Slava