High Impedence Differential Protection - Shunt Reactor. 1

[inpran]

The requirement to have High Impedence Differential protection is to have CTs at both the sides of the protected equipment with same CT Ratio and Magnetising Characteristics.
But have anyone of you seen it otherwise? ie, CT with same ratio but different magnetising characteristics.
If yes. What have been the consequences?

For example. I have the CT on HV Side of the Reactor with 1000/1 ratio,Vk=1000 V,Imag < 60mA at Vk. Rct=5ohm.

On the neutral side of the reactor, the CT has a ratio of 1000/1, Vk=500 V, Imag=30 mA at Vk/4, Rct=1 Ohm.

The reactor is 80 MVAr 420 kV reactor with rated current of 110 Amps at 420 kV. It is being used as a bus reactor for voltage regulation purpose.

We follow Indian Standards, which is in line with IEC Standards.

 

[FPelec]

inpran,
in my opinion having different magnetizing curves, in case of relevant DC components of current, as happens during reactor energization, the CTs could saturate at different points and then read different current value at HV and neutral side causing misoperation of differential protection.

Si duri puer ingeni videtur,
preconem facias vel architectum.

 

[inpran]

Fpelec,
That is what I thought, but the sub-station in which I'm working right now, the client has the reactor with the CT characteristics as I have mentioned above and hasn't had a single tripping case. If what you said is right, there should have been mal-operation. Is there anything that we are missing? The relays are old (static relays) and its functionalities have been checked and found ok. Are there any other criteria in high impedence differential protection?

 

[veritas]

It is not necessarily true that CT's with diff mag characteristics will result in relay operation. For a given primary current (through current)in both CT's there will be a component of the difference current proportional to the difference in mag currents. The diff relay operation threshold will need to factor this in (practically it results in a slight loss of sensitivity - due to the increased operating threshold but in many instances it can be accommodated).

Also, saturation with energisation whether it be trfr or reactor inrush is not a given. If the CT has been properly sized (correct transient factor, Ktf as per IEC60044-6) then saturation is negligible.

Regards.