connecting protection relay of power transformer

[emadshaaban1]

Dear all experts
we are swapping a step-up power transformer of (35MVA 11kv/20kv) that has been defected due to low winding Insulation , but we are facing:

The old Transformer is provided with CT in side the primary winding of the Oil Transformer connected
to relay 87T (SEPAM 2000) but the new swapped transformer isn't provided with this internal C.T.

Proposal to solve the problem :
To connect the primary input of 87T relay( Sepam 2000) to the generator out put CTs in series with generator protection 87G(ALOSTOM) , in this way two protection relays (SEPAM & ALOSTOM) are connecting in series from on CT
what do you think about this connection ?
How can we make functional test (current injection) before charge the Transformer ?

 

[davidbeach]

I think I sort of have a picture of what you're talking about, but...

By "primary" do you mean high-side or low-side. Primary and secondary can be ambiguous but high- and low-sides aren't.

If the existing generator protection uses CTs at the generator and the existing transformer protection uses CTs at the transformer, how do you protect the bus between the generator and the transformer?

If your generator and transformer protection both have the CTs wired "right", the currents through the generator relay are 180 degrees out of phase from those going through the transformer relay. Can the transformer relay compensate for reversed polarity?

 

[pwrtran]

Technically you can but is not a good practice if there is another option. In our case we have two different protections sharing the same CT circuit. But make sure the CT was rated for your application and the burdens are okay and be aware that you will lose the diversity.

 

[emadshaaban1]

Thank you for your replies ,
-Mr davidbeach 1) you may not noticed that I mentioned it's a step up transformer ,it is clear what I meant by "primary"
2) why the current at 87G and 87T will have 180 degree out of face?

Emad Shaaban
Tripoli Libya

 

[davidbeach]

Yes, I noticed, but there are some people who will argue that the high-side is the primary side, regardless. I figured you meant the low-side, but there's no ambiguity with low- and high-side.

Odds on, the CTs for the 87G are grounded toward the generator and the CTs for the 87T are grounded toward the transformer. There's the 180 degree phase shift.

 

[emadshaaban1]

DEAR MR DAVID
I tried to get what you've meant by " ground toward Gen & ground toward Trans"
can you give me more clarification
I attached to sketches of the new and old CT connection .

first sketch

Emad Shaaban
Tripoli Libya

 

[emadshaaban1]

second sketch

Emad Shaaban
Tripoli Libya

 

[davidbeach]

Your first sketch shows four wires from the CTs to the relays while the second sketch shows six. Change the second to show the wires exactly as they are, including the ground connection and the relay polarity. CT polarity is only relevant to the extent you get the physical connections correct. Now consider a current from left to right on the power system; which way does the secondary current go through each relay?

In your first sketch, is there really that unprotected gap? Or does the 87G actually use CTs on the transformer and the 87T use CTs on the generator so that the zones overlap?