Transformer ONAN Primary Fusing

[Mbrooke]

What fuse size is typically selected for transformers installed without fans? 100%, 125%, 133% or 167% of the primary FLA ONAN rating?

The sizes I have in mind are between 5/7.5MVA to 30/40/50MVA.

When a unit of this size is energized from a cold start how long can it remain overload above its ONAN rating? How long can these units sustain 133% of 167% loading without fans?

 

[davidbeach]

What ever size it already had? So many better options than fuses for new installations today.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[Mbrooke]

Lets say there were no fuses prior.

 

[davidbeach]

Relays and interrupting devices are more appropriate than fuses. Internal failures of fuse protected transformers are far messier than of relay protected transformers. Lots of oil outside the tank in the first case, interrupted within cycles of the onset of a turn-turn fault in the second with all the oil remaining in the tank.

Also, I’ve yet to see an event report from a fuse.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[Mbrooke]

Right, but there are times when fuse protection is a lot more economical or practical such as a mobile (vagabond) unit or temp substation.

Fuses do not require batteries, CTs, control house, wiring, testing, ect.

 

[davidbeach]

That's a false dilemma. We have seven mobiles, all of which have a high-side circuit switcher and low-side circuit breakers. As far as I can recall, all of our temps also have interrupting devices rather than fuses.

Mobiles are about the worst place to put fuses. The typical mobile is an attempt to cram the most transformer into the least amount of space and they require the pumps and oil coolers to be running any time the transformer is energized. With fuses there's no way for the mobile to trip itself on overtemp or loss of cooling. In fact, the last fuse protected transformer that I'm aware of us losing in service was a mobile and it died on overload.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[Mbrooke]

True, but remember, all of that adds cost and complexity. Mobile or site built. A 34.5kv or 115kv gang switch rated for magnetizing current, SMD fuse, 10 MVA transformer loaded to ONAN, 15kv load break switch, SM-5 fuse holder (or a Cooper VW Hydraulic Recloser) and LAs are as simple and easy as it gets. 32 step regulator if required.


Another scenario are 4.16kv legacy systems that are going to be converted out in less than 30 years but the substation is well past its in service life. In that case you either set down a pad mount transformer with fused risers a pole mount recloser or build a "temp" substation within the existing foot print.

 

[dpc]

Re loss of life from overloading. If you do a search you'll find lots of papers and calculation methods for predicting loss of life from overloading. Just one example:

https://www.usbr.gov/power/data/fist/fist1_5/vol1-5.pdf

Plenty of small rural substations still in service in Oregon/Washington with only primary fuses. I'd check with S&C on sizing recommendations. Something like 125 to 150% of FLA in most cases, depending on a lot of variables.

For anything new, put in a breaker or circuit switcher - not even worth evaluating fuses, IMO.

 

[davidbeach]

There's many more things that can be done than I would recommend doing. I wish some of my predecessors hadn't been so focused on first cost and had payed more attention to the future implications of their tightwaddedness. Some projects today a significantly more difficult and significantly more expensive than they would be if things had be built for the future. Do what you will, but make sure to make clear who did it so the future you knows who to mutter nasty things about.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[Mbrooke]

And Pennsylvania. Tons of primary fusing on several Eastern Penn utilities.

@DPC: I'm aware of loss of life, but if the oil is cold I'm thinking a transformer of that size can tolerate 167% overloading for 1 hour without going over 65*C.

@David: to be fair: the past did not have circuit switchers, micro processor equipment, same material or labor costs, idea of load growth or even the same understanding of electrical theory or economics. Heck in the 1970s most electricians thought a ground rod threw a breaker as being taught this.

I like passion, don't get me wrong, but I can't see what is so bad in regards to primary fusing.

The way I see it is that transformer faults are rare, and those that do happen tend to come from the tap changer. I'd argue the risk of spilling oil is less of an issue than a sustained secondary fault that isn't cleared by relayed devices which can and does happen in the real world.

Maybe I'm biased, but in the Eastern US and Russia DTT of transfomers was and still is common in addition to interconnecting auto transformers coming directly off busbars.

 

[davidbeach]

Not necessarily rural or small. More like substations time has forgotten. Fuses were the thing, once. The thing to the point that we have very few distribution transformers with EM relays and a much long list with Basler Solid State relays. So pretty much anything that hasn't been modified since sometime after the Baslers first became available still has fuses. From small to not so small, both rural/remote and in long established urban areas.

The rural ones are actually turning into a problem faster than the urban ones. Feeders accumulate PV installations and eventually start turning negative. One feeder per transformer or a couple of feeders that go negative and that fuse protected transformer wants to go negative as well. At that point the fuses have to be replaced and a circuit switcher installed along with proper relaying.

So, yes, fully familiar with fuse protected transformers, but I wouldn't purposefully do that to us again.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[Mbrooke]

Didn't breakers (in some part of the system) come first and then high voltage fuses? I always thought early HV fuses were considered impractical or not economically feasible. But my knowledge on history is indeed limited I will admit.

I can understand the issue of customer generation, but outside of that have fuses really been that much of a disaster? How many transformers have failed because of fuses? How many have failed throwing oil or catching fire?

I'm just not seeing the limitations past what I already know to not be applicable or of low probability in my case.

I guess I'm saying me ears are still open

 

[cranky108]

The problem was that first cost was a concern, and when it was time to upgrade to a larger transformer, or better protection, the lack of a control house, or space to add one becomes a problem.

That said, there are many old small subs in rural areas, that still exist, and like a company I once worked for, the rates were calculated to lose one or two 10MVA transformers a year, so fuses were not a problem.

I do see that distributed generation does cause a need for better protection, but in some areas the old design of a fused transformer works, but you also need enough space to add a future control building.

I also recall seeing a few small towns, where it was not economical to upgrade the 4160 distribution to 15kV class, and likely the old fused transformer should remain.

 

[Mbrooke]

Or you can add a another fused trafo?

I understand what you guys are saying going by your needs, but at the same time not everyone wants to spend more money on a substation they know will be relatively temporary.


@Cranky108: Do you remember how fuses were sized for this units?

 

[cranky108]

We used the S&C guide for 125% of highest capacity, but coordinated the mech. damage curve for the base rating.
Yes, they did overload some of the transformers during the summer.
I recall we used SMD-20s.

The biggest issue was if the transformer had fans. And not the single 8" fan someone found, but the real number and type of fans recommended by the manufacturer.
Me asking if the transformer has fans, and answer, yes it has a fan.

 

[Mbrooke]

125% of the FA/FA rating/ 166% of the ONAN FLA? That sounds high.

 

[dpc]

This is a good resource if you haven't seen it:

https://www.sandc.com/globalassets/...on-bulletin-210-110.pdf?dt=637423666529154625

 

[OffTap100]

125%

We have many(5) 66/15kv 15MVA+ single transformer substations that feed only one industrial customer. Fuses only on high side. Never had an issue, but I will say in most every case, the transformers are oversized a bit, for the current load.

Rest of our subs/switch yards are conventional.

There is nothing wrong with fusing if applied correctly.

 

[Mbrooke]

@DPC: I've seen it, but I was wondering if anyone had any real world experience. The selection guide seems to show a rather broad spectrum. It also doesn't say how long a transformer can remain overload at 135 and 166% without loss of life ie cold energization, cold load pickup.

@Offtap100: This is the answer I was looking for! I agree, there is nothing wrong with fusing when done right.

 

[Mbrooke]

Before I part way, anyone know why there are asterisks for delta-wye units? Is this because a L-L fault on the secondary may not blow the primary fuse?