[Hamid]

As you know, the inrush current just appears in one side of transformer when we energize power transformers.
As you know, the inrush current just appears in one side of transformer when we energize power transformers. Therefore transformer inrush current can be cause the malfunction of differential protection; because of unbalance detected current between primary and secondary side. On the other hand, according to technical recommendation, the applied restricted earth fault protection to one side of transformer shall be check for inrush currents too. Why? Regarding activation of restricted earth fault protection system in one side of transformer ( except auto transformers ), why shall we check the existing inrush current in unbalance zero current detection process similar the overall differential protection cases?

Topic

[Abdul Hameed Pathiyil]

The inrush current flowing in the primary while energisation of the transformer is not of same magnitude in each phase, they are different in magnitude. Hence there will be phase currents of different magnitude and also it is not balanced through neutral. So there will be differential current in REF relay and hence it may operate during energisation if proper filtering/suppressing of 2nd harmonics is not done.

[Hamid]

Please note, high impedance type restricted earth fault relays just contain one signal input and individual phase current do not insert to relay. In low impedance type equally, the function of relay is related to zero sequence calculator stage of relay; therefore the phase unbalance of inrush current cannot cause relay mal-operation. So REF must be stable against through zero sequence inrush current theoretically.

[K B Santharam]

Dear All, REF protection being a current balance scheme applied to only one side of the transformer, it would appear that no second harmonic restraint is required as in the case of differential relay. However, REF relay, usually being set very sensitive (about 10-15% of CT rating) and without any intentional time delay, it is imperative that CT secondary current balance is maintained fairly accurately for all through faults, in zone phase faults, system recovery transients (after external fault has been cleared by another relay) and transformer

[Hamid]

As Mr. K B Santharam said, the DC off set inrush current can cause CT saturation in initial short time fault current which subjected to REF operations. I think mentioned facility in new numerical relays cannot cover mal- function of saturated CTs in protection system.

[Author]

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