MCAG14 Restricted Earth Fault Tripping

[warmulli]

Hi Guys,

Please could you help with any suggestions.
I got a 1000kVA 11KV dry cast resin transformer with two LV panels connected to it. It has a MCAG14 on the interconnecting LV panel but also seems to have a set of CT's connect to the other LV panel, The REF tripped around a year ago which in return activated the shunt trip on the transformer. The transformer was reset and the REF was fine for a few month and then tripped again, this tripping became more frequent. The only way to prevent the REF from tripping now is to set the REF relay from 0.025-0.1A TO 0.1-0.4A, CT ratio is 400/5. Could this tripping be down to the adjustable stabilizing resistor, There is a large data lab being fed off of the system. Could it be seeing an external earth fault due to the stabilizing resistor or due to a problem with the neutral CT.? Ps It has a test block allowing access to CT'S and Relay.

Im new to REF so your help will be much appreciated. Thanks

 

[ScottyUK]

Check the polarity of the CT on the neutral. With a relatively well-balanced load it will be almost un-noticeable. In the event of an external earth fault the relay will be very sensitive. It will not behave correctly for an in-zone fault. This is a remarkably common fault with these schemes - it should be picked up by the commissioning engineer, which tells you a lot about the quality of some commissioning engineers.


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If we learn from our mistakes I'm getting a great education!

 

[wcarter]

warmulli,

I am not overly familiar with this type of relay, but did the trips occur during a storm or other simular occurance? Perhaps a neutral to ground connection? Also, when was the last time this relay was tested? If this is an older building, a degraded grounding network can also cause issues.

 

[warmulli]

Thanks guys for you help and input, Shutting down soon for testing so will hopefully get a result. Tripping became more frequent as load has increased over time, making the possibility of a reversed polarity on the neutral CT also have notice the stabilizing resister setting have not be altered to match the increase of load over time along with an increase in return neutral current. Relay has never been tested so goanna get it bench tested.

 

[vandal06]

Scotty beat me to it. The fact that it's been tripping more frequently over time and the stabilizer resistor has not been adjusted makes a reverse CT polarity the most likely case.

Anecdote: I set one of these schemes that had the same problem. The three-line diagram and single-line diagrams for the project corroborated and showed the correct CT neutral polarity (towards ground) but in the field it was the opposite and the commissioning guys missed it. It finally mis-operated on feeder single-phase switching, which looked like an in-zone ground fault to the relay. Very embarrassing, especially considering that our commissioning department was responsible.

 

[Hoxton]

Does your commissioning team use "Primary Injection"?

 

[vandal06]

I failed to mention that was the job previous to my current position. I doubt they used primary injection, and if they did they obviously neglected the neutral CT.

At my current position our commissioning guys are very good, and they use Primary Injection and probably a couple other methods I'm not familiar with.

 

[ScottyUK]

Primary injection on a big transformer with internal bushing CTs is a major task. Even with the HV earthing switches closed it takes a high power injection source to circulate much current. Last big transformer I did was 400MVA and that test needed a containerised generator as a source and a rental company who didn't ask too many questions about what we were doing (questions like: what kind of load are you connecting to our generator? ; why are you disconnecting our AVR and using your DC power supply? ; why are you disabling our protection relay? . That sort of thing.)

Forcing current through the neutral CT can be done using judicious opening of the HV earthing switches. If you're in a power plant and can get access to the pilot exciter or the firing control circuit of a static exciter then the generator itself makes a great injection source.


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If we learn from our mistakes I'm getting a great education!