REF/STAND BY EARTH FAULT

[apprenteng]

Hallo guys,
I have a scheme whereby the neutral of a 23MVA 33/11kV trafo has only one neutral CT (BUSHING) of class X. The incomer breaker too has class X CTs. I want to use ICT to be able to use the single bushing CT for both SBEF and REF.

Is it safe to use the ICT or is the REF alone sufficient for protection of the Trafo windings and the cable to the incomer breaker.

 

[Marmite]

The REF and SBEF are two separate protection functions. The REF will protect the transformer winding and the cables to the breaker. SBEF is standby earth fault protection. It is backup protection in case of uncleared faults out on the network, or earth faults outside the REF zone. It may also protect the neutral earthing resistor if there is one, and may act as a cheap busbar protection system by tripping the LV bus section first and the transformer LV C/B second. Do you really want to compromise important backup protection for the cost of a neutral CT? You have the option to get another CT fitted in the bushing, or if there is a neutral cable terminated on the bushing you could install a slipover CT on the cable, or in the NER cable box. Lots of possibilities, and not very expensive.
Regards
Marmite

 

[apprenteng]

Thank you Marmite. I have also been contemplating the use of the 2 relays in series as shown in the Acad drawing attached. Any comments please. I cant get a neutral CT at the moment but the trafo must be energized on Friday since some clients are out of supply.

 

[ScottyUK]

Horrible idea.

The SBEF CT doesn't need to be anything special and you should have enough adjustment on the relay to allow a fair choice of CT ratio. Are you saying that you can't find any CT at all? If you have an NER then you could use a metering CT as the CT will never see high current under fault conditions.


----------------------------------

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

 

[slavag]

Apprenteng.
I'm not sure if it's a good idea use series connection for the REF function. It not seems. Are the REF HI or LI?
Actually, you have a solidly trafo grounding..
REF is good function, but not must here. 87T is enough sensetive in such confuguration. You can use REF no problem and SBEF with residual connection. In first case you can add neutral CT and reconnect SBEF relay.
Just opinion.
Best Regards.
Slava.
BTW, why PUNGLU is name of drw, one of our members is PUNGLU .

 

[SMB1]

thanks everyone
i would like to point the following:

1. REF is used because 87T doesn't cover 100% of Xfmr winding.
2. if the REF is high impedance type, deticated CTs shall be used for this function.

Best Regards
SMB

 

[apprenteng]

Thank you guys for the advice. Sorry, the trafo should be YNyn0 with the LV side solidly grounded. Punglu? Draw attention to him... Had similar configuration in a previuos thread. May be draw attention.

 

[apprenteng]

REF is high impedance

 

[slavag]

NO, you must dedicated CT for HI REF protection
Best Regards.
Slava