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?

 

[stevenal]

Suggest looking at IEEE standards C57.91, C57.109, and C37.91

 

[Mbrooke]

Have a table or synopsis? I found a POCO report PDF that had tables of hot and cold overload capabilities but lost it after my desk top crashed. I'll link it if I find it in my history.

 

[stevenal]

I believe posting a table would violate copyright. Support standard development with a purchase. I keep all the Power and Energy standards handy.

 

[Mbrooke]

Bummer, I wash I could find that PDF link but I can't pin point it in my history. I have the IEC standards at hand though.

I simply don't have millions to purchase every ANSI/IEEE/IEC standard at hand. There are just to many. Aaron Swartz was right about all of it. UL and the NFPA lets you view most of theirs for free which I think is the right thing.

I'm seeing duration limits if 2x loading for 1,800 seconds, 3x for for 300 seconds, 4.75x for 60 seconds, 6.3x for 30 seconds, 11.3x for 10 seconds and 25x for 2 seconds based on page 8 of 13 and 9 of 14 in this pdf:

https://swp.energex.com.au/upload/technical_documents/20140917_065637_5377619.pdf

 

[Mbrooke]

My thought process regarding my scheme-

A 30/40/50MVA transformer (just as an example) has an ONAN FLA of 150 amps. 133% of 150 is 200 amps, so a 200 amp SMD-2B fuse to allow for inrush and secondary side device coordination. Clearing times for bus faults that do not result in trafo damage.

According to page 4 of this PDF a 200E 115kv fuse can carry 258 amps continuously:

https://www.sandc.com/globalassets/...on-bulletin-210-190.pdf?dt=637425487705431606

258x115x1.73=51.38 MVA, or 50MVA.

I'm thinking that under cold load restorations conditions, particularly where electric heat customers are in present on the feeders, I could load the unit to 50MVA for 1 hour.

Voltage drop without feeder regs or LTC would be fierce but it is doable in theory.

 

[Mbrooke]

In defense of Cranky and David- the trend is to ditch fuses for breakers and circuit switchers. I'll acknowledge that fact fare and square.

 

[cranky108]

Yes we use only three phase interrupting devices, if we can. A single phase device can leave low voltage on some customers, and you will wind up with damage clames and other legal issues.

That said, a single customer, who knows he could be single phased, if the fuse blows, maybe acceptable if he has a lower cost of service.

 

[Mbrooke]

Well, that would be forgetting about all the fused risers, laterals, padmounts, ect.

 

[stevenal]

Single open fuses on a wye distribution system have an entirely different effect from a single open fuse on the delta high side of a substation transformer. When the later occurs, there is no outage. Single phase customers on two of the phases will have half voltage, and three phase customers will see low voltage on two phases that are low and shifted in phase angle so they are in phase with each other and 180 out from the healthy phase.

 

[Mbrooke]

Is it really that bad though?

 

[stevenal]

I'd prefer an outage over 50%V.

 

[davidbeach]

The utility's claims processors would also prefer 0% over 50%.

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

 

[Mbrooke]

So how do POCOs in Cali get away with delta distribution?

 

[davidbeach]

Single phase loads connected phase-phase have zero volts if one low-side phase is missing. But the discussion was high-side fuses on delta-grounded wye transformers.

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

 

[Mbrooke]

Aren't almost all transformer's delta-wye grounded and fused in Cali? What happens when a 12kv lateral fuse blows and 2/3 of hundreds of single phase customers are subjected to 50% voltages and 3 phase customers to 50%-100%-50% voltage?

 

[davidbeach]

I doubt you’ll find very many that are fused on the low-side. Even if they were, the single phase customers would either get 0% or 100%. What a multiphase customer would see would depend on how they were connected and how any further transformation is connected.

That 100%-50%-50% is what the low-side sees if there’s an open phase on the high-side.

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

 

[cranky108]

What the three phase customer sees for a blown lateral fuse depends on the customer, and service type. Most of our fused lateral fuses are for single phase customers.
If you blow a transformer fuse, or blow a transformer, it would make no difference to that customer, as they would see the same thing. Can't change that.

What you find is that most areas are zoned for like type of loads. No random three phase customer in the middle of the block, unless the whole block is zoned for retail.

 

[stevenal]

See the SEL251 manual for a way to detect the high side open fuse condition from the low side.
While there are many good reasons for using three phase devices, the argument that really got us moving on the changeout was arc flash. Line to ground faults between the transformer and main breaker had very long clearing times when primary fuses were depended on. These same faults can be detected with a neutral CT or with zero sequence on the low side, and be set to clear much quicker with a six cycle or faster circuit switcher.

 

[Mbrooke]

Right, I'm talking about the high side, 12kv.

 

[davidbeach]

12kV is the low-side.

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