Ungrounded System Earth Fault Protection

I am not sure why differential is operating. Check whether interposing CT available in the protection circuit (OR) suitable phase change input given if the protection is numerical type to avoid possible spurious differential operation.

For detection of earth fault on delta side, you can introduce star PT with neutral grounded and check for neutral displacement (ANSI 67N).

I agree with the Answer#1 regarding Trafo. Differential protection tripping of the transformer due to a fault on load side.That means, it's a spurious tripping.

As regards the E/F detection of DELTA side of the Trafo.,you can have an Open Delta winding on the secondary side of the corresponding Line or Bus PT across which a Neutral Displacement Relay ( AREVA make VDG-14 or equivalent )has to be connected.You may note that ,under normal condition,the voltage across the open delta wdg. will be zero, whereas, some voltage will appear in case of E/F on the load side to actuate the NDR when it exceeds the preset value.

Cheer up!

Dear s. pradhan

thanx for reply.

i find ur ans appropriate.

sagar

That is what is commonly known as bus ground protection. It will only give an alarm in case of a single phase to ground fault. In the case where there are more than one phase to ground fault, this relay will not give any tripping.

For every outgoing feeder on the delta side, a 64 relay can be connected on the CT neutral and the bus can be grounded using a zig zag transformer. This method shall also give a suitable earth fault protection.

This is a simple one! If you have your secondary, load side, hooked up as a delta, you need 3 grounding transformers hooked up in a wye configuation with the mid point of that wye going to ground. When currnet flows to ground, in a ground fault situation, that fault will feed back into the grounding transformers. Have a C.T. reading the ground wire or use a comparator circuit to read the 3 grounding transformers to tell you when a ground fault has occured. These can get very complicated or you can keep it simple. I design these for the co-op and my own hydro stations for 3 phase generator ground faults. Delta is always easy, Wye secondaries can be a little more difficult. Your grounding transformers should be rated for the full delta voltage, Fused at no more than 1/2 amp and use a 250-400 VA max. Thats all you need.

J.R. Watts Professional Engineer