application of REF protection.
application of REF protection.
(OP)
We are looking at the feasability of installing a new 11000/400v transformer (max 4mva) on a customers site. The transformer will be a dry type and will be constructed as a package substation (LV switchpanel and transformer close coupled). I am considering the use of restricted earth fault protection (REF)and I understand the principle that REF is restricted to an earth fault occuring in a zone. But, I am unsure of the other principle reasons for the application or specifications of this type of protection. Is it governed by fault level, the risk of an earth fault occuring in that zone or a combination of many different considerations?
Many Thanks.
Andross.
Many Thanks.
Andross.






RE: application of REF protection.
One up from just fitting REF would be to fit a bias differential relay, this looks at the transformer as a whole, the benefit is most new digital bias diff relays have REF built in to them.
RE: application of REF protection.
Many Thanks for your reply. I understand the concept of protecting more of the TX winding and the HV/LV intertripping that you have explained. What I cannot fully understand is why some transformers are equipped with this protection and others are not? Is there a point at which a transformer of a certain rating is deemed to be of such a size that more complex protection is required to protect the equipment.Could such considerations be to reduce the risk of damage to equipment that could be more costly to repair, or to reduce the risks of more extreme damage to the power system?
Andross.
RE: application of REF protection.
RE: application of REF protection.
Usually the more expensive the plant the more is spent on protecting it in order to reduce damage and to maintain system stability.
RE: application of REF protection.
RE: application of REF protection.
RE: application of REF protection.
Regards.
Slava.
RE: application of REF protection.
In the US, most distribution transformers (MV/LV) as in the OP, would be protected by fuses, but then there are very few distribution transformers over 5 MVA.
RE: application of REF protection.
The 587 (without REF) is less than half the price of the 387. Then there is the cost of the neutral CT. If the CT is present, and you want the other extra features of the 387; then it is free.
The purpose of REF is to get the transformer off line as soon as possible for faults located near the neutral point where differential elements loose sensitivity. The sooner the fault is cleared, the more likely the transformer can be repaired. But how feasible is it to repair a dry transformer?
A consideration of fuses versus a relay may be arc flash exposure. High side fuses have long clearing times for low side faults, and don't have the "maintenance setting" that can be programmed into a relay.
RE: application of REF protection.
RE: application of REF protection.
Andross.