application of REF protection. 1

[andross]

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.

 

[isquaredr]

REF as you say is installed to protect a specified zone. When installed to protect a transformer seperate relays are used to protect the HV & lv windings, when setup correctly the system is very stable for out of zone faults and extremely fast for in zone faults. A circuit breaker on both the HV & lv side of the tx is required with x-tripping between if both are local, intertripping if the HV cb is remote. REF can provide added protection for deep internal winding faults which standard e/f protection will not see.
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.

 

[andross]

Isquaredr.

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.

 

[davidbeach]

Historically it took a separate REF relay to provide REF protection. Now that REF is free in all but the most basic transformer relays, why not use it?

 

[isquaredr]

andross
Usually the more expensive the plant the more is spent on protecting it in order to reduce damage and to maintain system stability.

 

[ELEP]

REF is generally provided for transformers rated more than 7MVA. Also, it is more suitable for oil cooled transformers due to chances of more internal faults. I think it is not practice to provide REF for dry type transformers, the protection such as Primary fuse, OC/E/F protection are sufficient.

 

[davidbeach]

If REF comes for free in the relay, and all you have to do is put a CT on the neutral, why is there a question? I must be missing something here. Sure, if all you are doing is putting fuses on the high side and calling it good there isn't much you can do to implement REF protection. But if you have a high side breaker, and you have a relay, why would you not turn on REF protection?

 

[slavag]

I'm totally agree with ELEP and David.
Regards.
Slava.

 

[jghrist]

The question, since REF is included in transformer protection relays, is for what size transformer do you provide relay/breaker protection instead of fuses. There is no code answer, at least in the US. ANSI/IEEE C37.91 suggests 10 MVA and above for power transformers.

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.

 

[stevenal]

I must disagree that REF is free. Check out

http://www.selinc.com/tbc_protect.htm

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.

 

[davidbeach]

The SEL-587 is what I had in mind when mentioning "the most basic transformer relays". Yes, there is the issue of the cost of the neutral CT, compared to the cost of the transformer the CT cost is likely to be insignificant. Besides, if you have a relay that could accept the input from a neutral CT you are already doing backup overcurrent on the neutral anyway aren't you?

 

[andross]

Many Thanks to all who have replied to my post. I have decided to opt for the use of REF, as the extra costs involved are minimal (compared to the overall installation cost) and the extra level of protection provided will be of benefit.

Andross.