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?

 

[Mbrooke]

You'd still have 2/3 of single phase customers at 50% voltage for a blown fuse on a 3 phase lateral.

 

[Palletjack]

You'd still have 2/3 of single phase customers at 50% voltage for a blown fuse on a 3 phase lateral.

Can you show me your calculations for that?

 

[Mbrooke]

A, B, C tapping off the main line, 140K fuse in each phase.

1/3 of single phase pole pigs are connected A-B, 1/3 B-C, 1/3 A-C.

B phase fuse blows

A-C pole pigs see 12kv- 120/240 on the secondary.

All A-B and B-C are now in series with each other. Assuming equal restive load on each 120/240 secondary, the voltage will settle to 60/120 volts.

Yes I know in reality the division typically won't be 50/50 and it may bob up and down.

But thats the point I'm trying to make. POCOs routinely place customers at latent risk for open phasing, low voltage, fluctuating voltage, double phasing and bonkers phase displacements as normal practice.

 

[Palletjack]

A, B, C tapping off the main line, 140K fuse in each phase.

1/3 of single phase pole pigs are connected A-B, 1/3 B-C, 1/3 A-C.

B phase fuse blows

A-C pole pigs see 12kv- 120/240 on the secondary.

All A-B and B-C are now in series with each other. Assuming equal restive load on each 120/240 secondary, the voltage will settle to 60/120 volts.

Yes I know in reality the division typically won't be 50/50 and it may bob up and down.

But thats the point I'm trying to make. POCOs routinely place customers at latent risk for open phasing, low voltage, fluctuating voltage, double phasing and bonkers phase displacements as normal practice.

There’s not a single calculation in there.
Try again

I’m trying to figure out how you figure the high side of a single phase distribution transformer circuit will be completed with a fuse blown.

 

[Mbrooke]

500kw per 150 homes- 450 homes total.

500,000 watts- V2/P=R, so 240 volts gives us 0.1152 ohms


A-C, 0.1152 ohms, V2/R= 500,000 watts

A-B and B-C in series, 0.1152 x 2 = 0.2304 V2/R = 250,000 watts.

....................

Forget what I'm saying:

Note that if we use two resistors of equal value, that is R1 = R2, then the voltage dropped across each resistor would be exactly half the supply voltage for two resistances in series as the voltage divider ratio would equal 50%.

https://www.electronics-tutorials.ws/dccircuits/voltage-divider.html

Plug the numbers in and see what you get.

 

[Mbrooke]

I’m trying to figure out how you figure the high side of a single phase distribution transformer circuit will be completed with a fuse blown.

Ok, this clears your side up.


I'm not talking about the trafo fuse itself, but the 140K, 100K or 65K lateral fuse.

A-C phases will still have 12kv potential across them.

Which means A phase will push voltage through each A-B connected pole pig, up into B phase, then into each B-C phase pole pig going back onto phase C. B phase is just a bridge so to speak. All A-B pole pigs are electrical in series with all B-C pole pigs.

 

[Mbrooke]

Vout = V (240) x (r2/r2+r1)

V= 240 x (0.1152/0.2304)
V= 240 x 0.5
V= 120

12,000/240= 50

The 50:1 ratio still holds for each individual pole pig despite being in series. So we can ignore that from our equation.

 

[stevenal]

To be clear, I was speaking above of a 4 wire wye distribution system where single phase distribution transformers are connected phase to neutral.

I can see this gets problematic when single phase transformers are connected line to line. Do the Cal delta utilities fuse the laterals as you say and pay the claims?

 

[Mbrooke]

Some fuse, some don't fuse. If or how they pay claims I have no idea to be honest.

 

[davidbeach]

Just can't run three-phase fused taps. Fuse the single phase taps and use three-phase trip reclosers on the three-phase taps.

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

 

[Mbrooke]

David, there are POCOs who fuse 3 phase taps with delta primary transformers or L-L connected pole pigs down stream. And yes they are fuse tubes. Not solid blade disconnects.

You'd need a whole lot of reclosers for all those 3 phase taps and spurs. Reclosers that can be used to segment the trunk line into 500-250 customer.

Also fusing single phase taps doesn't protect you from under voltage.

If the 12kv source is solidly grounded you can still have 7kv going to ground after the lateral fuse has blown.

If the 12kv source is ungrounded, a cross country fault can cause substantial current flow to ground after the lateral fuse has blown.

 

[Palletjack]

See what I mean...

 

[Mbrooke]

Again, we're talking Line-Line, not Line-Neutral connections.