Using fuses to protect power transformers!?
Using fuses to protect power transformers!?
(OP)
Our MainSub is fed from two 69kV lines. Each line feeds a 15/20mVA transformer. Each transformer is capable of carring the system load. No primary HV CBs or switchers were used to provide fault interruption with high speed differential relaying. Only primary fuses are being used. Some folks here are suggesting that a breaker and half scheme be used to protect the transformers using a differential scheme with overcurrent relays as a backup, because fuses are not completely adequate. The breaker & 1/2 solution can be very expensive... Please comment.






RE: Using fuses to protect power transformers!?
I'm gathering from your post that those suggesting the breaker and a half scheme are primarily interested in improving the reliability of your 69 kV supply, with the secondary interest of improving the transformer protection scheme. I don't know your station layout though, so that may be an incorrect assessment.
In my opinion, a breaker and a half scheme is overkill for a distribution station. I don't believe that it adds that much extra reliability to the station to make it worth the extra money.
At minimum for a two-transformer station, make sure you have one 69 kV breaker (between your transformers). This will allow 69 kV line faults to be cleared and only clear the feed to one transformer.
A better layout would be to have breakers on each of the 69 kV lines coming into your station with a 69 kV breaker separating your transformers. This will allow 69 kV line faults to be cleared without clearing the feed to both transformers. It also clears bus faults while only clearing the feed to one transformer. (This would probably be my choice to balance cost vs. reliability.) If you want to provide even more flexibility, a four position ring-bus is probably the way to go.
As for the transformer, high-side fusing is a pretty common practice on distribution transformers (with a low voltage main breaker). Yes, differential relaying with overcurrent backup would be better, but that's a judgement call on your part. Given that your transformers are of significant size, I'd personally want more than just high-side fusing (would probably go with a 69 kV circuit switcher, with the relaying you've described -- differential with overcurrent backup).
Hope this helps. Anyone else have comments?
RE: Using fuses to protect power transformers!?
RE: Using fuses to protect power transformers!?
But on the other hand, you have two transformers, so maybe you can live with the probable destruction of the transformer for any kind of internal fault.
I agree that circuit switchers on transformer primary would be a good alternative to circuit breakers, if the concern is strictly transformer protection.
If the 69 kV system is looped, or can be looped, then the breaker and a half (or ring bus) would provide you with additional operating flexibility.
You need to evaluate the impact of the loss of one transformer for several months and see what that is worth to your company.
Hope that helps -- good luck.
RE: Using fuses to protect power transformers!?
Harmonic-restrained differential protection is a strong driver. Miscellanous references…
http://standards.ieee.org/reading/ieee/std_public/description/dtransformers/C57.109-1993_desc.html
http://standards.ieee.org/reading/ieee/std_public/description/relaying/C37.91-1985_desc.html
See §8 C37.91-2000 is current
Possibly applicable to breaker-and-a-half scheme
http://standards.ieee.org/reading/ieee/std_public/description/relaying/C37.97-1979_desc.html
(R1990) See §3
RE: Using fuses to protect power transformers!?
Bung
RE: Using fuses to protect power transformers!?
S&C (http://www.sandc.com) makes the best circuit switchers.
We have 15 MVA transformers, and use circuit switchers on the
primary side, since they are cheaper than breakers, and
they can also be tripped by both protective relays and external alarm contacts (on your transformer).
Fuses work too, but cannot be tripped electrically (alarm contact and/or protective relay).