Using fuses to protect power transformers!? 1

[hidalgoe]

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.

 

[jstickley]

hidalgoe --
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?

 

[stevenal]

We found the S&C transruptor to be a good compromise between fuses and circuit switchers.

 

[dpc]

As others have stated, this is more of an economic evaluation, than an engineering problem. Primary fuses on transformers of this size represent the extreme low end of protection, and are generally not considered acceptable for an industrial facility. Fuses do not provide anywhere near the protection of a differential relaying system. This is often considered acceptable for distribution substations where a fairly long outage is tolerable, and the utility has a mobile sub they can bring in to restore power while they are buying a new transformer.

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.

 

[busbar]

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

 

[Bung]

I suppose the main concern with fuses, apart from the protection coverage, is the effect on the system of operation of just one fuse (for whatever reason) and the resulting unaceptable voltages on the LV. Unless you are using some kind of ganged disconnect arrangement which knocks all 3 phases off if one phase operates. Also, I would be a little concerned about the possibility of ferroresonance if only one fuse is left in service. Admittedly the most fault likely fault scenarios don't lead to these situations, but they are possible.

Bung

 

[SphincterBoy]

Why don't you use Circuit Switchers instead of primary fuses?

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).