REF

[faycal bellali]

I have an existing 11/3.3kV transformer, rated at 12MVA, which has one CT in the secondary star/neutral connection. The CT is located in the transformer 3.3kV cable box and is rated at 50/5A, 25VA, Class 3. Neutral earthing of 3.3kV is by 20A NER.

We will have new switchgears at 11kV and 3.3kV side.

Vendor said that existing CT can be used to provide either SBEF or REF function, but not both.

We will also have overall transformer differential protection via new CTs in the new switchgear.

So my question is, do we need both SBEF and REF or would be ok with SBEF only since we have 87T?

 

[davidbeach]

I’m not familiar with the term “SBEF”, could you expand it?

 

[Palletjack]

Standby earth fault

 

[davidbeach]

An old high impedance REF couldn't share the CT with anything else, but with a modern transformer relay you just connect the CT to the relay and use both, either, neither, as you choose.

 

[Kiribanda]

In my opinion,
1)if you are having an electromechanical relays, then what the vendor said is correct because you have only one CT
in the NGR path to connect to REF relay.
2)But if you have a microprocessor relay such as GE-T60,SEL-387 etc, then the same single CT can be used to program
50G/51G element(s) and also 87G (REF) elements.

 

[RRaghunath]

Hi, I doubt REF protection is relevant here.
You have 20A NER which makes your 3.3kV system high resistance earthed.
High resistance earthed systems cannot have REF protection.
In this case, a 12MVA transformer @ 3.3kV is likely to have phase CTs of not less than 2000A primary rating. With 2000A rating, the available maximum earth fault current is 1%, which means the REF protection setting will have to be set lower than 1%.
Moreover, high resistance earthing allows the system to be in service with one earth fault. Thus, there is no need to have faster isolation.
SBEF protection is adequate, in this case.
Make sure you don't have a lot of power cabling on 3.kV side. With High resistance earthing, it is important to ensure that the total capacitive current is well below 20A (NER value).