Voltage and current trasnformer fault

[stratford]

Dear all. On site we experience a fault with VT and CT failed on the generator. These VT and CT are used for excitation control, their secondaries feed the GE EX2100 exciter panel for 9FA generator. I was wondering if
1) for the VT application , i can use the secondary of another VT e.g the one used for metering or synchronization and wire it in parallel to the place of the faulty one
2) for the CT application , i can use the secondary of another CT e.g the one used for metering or synchronization and wire it in series to the place of the faulty one.
Vt and CT are made from Narayan Powertech.

 

[wbd]

It's been awhile but PT's & CT's used for metering and relay are usually of a higher accuracy class than for control PT & CT's. This would something to check as well as the burden limits on the transformers.
I would be concerned about reducing the built in seperation and reliability by using the protection transformers for control. Also, why did the PT and CT's fail? Is there a problem on the control circuit that could cause your protection PT & CT's to fail? I would be very concerned about that.

 

[waross]

If the Potential transformer is feeding power or a voltage reference to the Automatic Voltage Regulator, you can use a small dry type transformer. The accuracy is good enough with a steady load such as an AVR. I have used 75VA and 100 VA dry type transformers on generators of over one magawatt capacity.
I wouldn't try to mix systems on CTs and PTs.
respectfully

 

[ScottyUK]

One detail the OP omitted to mention is that this an HV installation - probably in the order of 200MW.

Be careful how the CT connected to the AVR behaves when the generator is delivering very high current under system fault conditions. Your AVR is key to keeping the machine stable in the event of a severe external fault so don't compromise it by using a CT with the wrong performance. I don't know the specific AVR you're talking about but there will be a published requirement for the CTs. A spare protection core could probably be used but note the use of 'spare'. Don' share the AVR CT with other things unless the OEM validates it. Don't mix CT classes within a 3-phase group.

The VTs are likely to be more-or-less interchangeable.


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