Restricted Earth Fault (REF) Protection for a Cast Resin Transformer

[krisys]

As everybody is aware that the cast resin transformer has entirely different construction as compared to oil immersed transformer. Conventionally the REF protection (which is dominant in the IEC world) will be provided for the wye connected winding of the transformer. This would detect the fault at an early stage and isolate the transformer from the system in order to minimise damage to transformer. This would also necessitate a minimum repair before putting the transformer back into service. For the cast resin transformer, they claim it is fault free. However in case of any fault, the faulty limb cannot be repaired. It has to be fully replaced. So is there any point in providing the REF protection for a cast resin transformer? Your views/thoughts on this is highly appreciated.

 

[ScottyUK]

Anything which gets a fault off the system is generally a good thing.

REF is a good relay to protect against an internal short close to the neutral point which would probably not be seen by an overcurrent relay, and is effective for any in-zone fault. Today it is a fairly cheap protection function which provides rapid clearance for an in-zone fault. How big is the transformer, and how important is the load it feeds?

As for claims about things being fault-free, I would counter that they claimed that Titanic was unsinkable yet it lies on the bottom of the North Atlantic.

 

[krisys]

The Cast Resin Transformer in question is 2000 kVA, 11/0.433 kV, Dyn11. It is feeding a training center building and related facilities.

I agree that the protection functions in a given multifunction relay is quite cheap provided the particular protection function is available in the relay and it is required only to be configured. But in the case of REF, as per the industry practice it is necessary to provide a separate set of PS class CTs. As the distribution system at 415 V is 3-phase 4-wire system. Thus 4 nos. of CTs in the switchgear side and one no. CT on the transformer (usually mounted in the neutral bushing) is required.

As the neutral CT (NCT) shall be exactly matching with the Phase CTs, and the NCT supply will be part of transformer vendor, it needs additional engineering and follow up with different suppliers for the matching of CT parameters.
Additionally a high impedance relay is required to complete the scheme.

Had there been a transformer differential protection, at least the CT and relay could have been shared with REF. Since the transformer is relatively small, only REF is to be provided with all these additional components.

The additional cost is one thing. It would also add to the unnecessary complexity of the system and additional engineering coordination efforts during the project stage. Not to mention the additional maintenance and testing check list points during the equipment life time.

 

[ScottyUK]

Low impedance biased REF is a lot more forgiving of imperfect CT matching. Some relays offer the choice of both within one relay, Alstom's P143 for example. If you want to find reasons not to fit REF on a transformer this size there plenty, but there are enough justifications if you want to.

One benefit of any unit protection scheme, if your organisation is trying to manage its arc flash hazard, is that the rapid clearance time for an in-zone fault can take the arc flash level from way above the upper limit of Cat 4 to something more manageable.

 

[ppedUK]

I agree with Scotty on the "fault free" claim............don't believe it!

For a 2000kVA TX, I would certainly fit REF. I always try to persuade clients to fit REF on all but really small (<315kVA) TXs, it is so cheap to install, even if you have to fit a separate relay. You get a high speed response for in zone faults for little cost and if you earth the TX star point at the main switchboard instead of at the TX (common IEC practice), you only need 4 CTs in the whole scheme which can be easily supplied and fitted by the switchboard manufacturer. Class PX CTs are not expensive either.

Remember also that with CTs fitted at he switchboard, REF protection also provides high speed E/F protection to the TX LV cabling or busduct and those items are certainly not "fault free". REF also provides E/F protection even when the LV breaker is open.

REF protection is only any good if it trips the TX HV Breaker, and it should also trip the LV breaker (if there is one). I have seen a few REF schemes fitted to TXs which were arranged to trip the LV breaker only.............useless!

You could argue that for an internal TX fault, the faulty unit would have to be replaced, so why bother with extra protection, but you need to clear earth faults in the system as quickly as possible and try to prevent them developing into arcing or phase faults, not just to limit damage to equipment, but also to protect people and / or the surroundings / environment.