Utility Transformer Sizing

[Al Pike]

I have a 13 kv line feeding 3, 25 kv xfmr's on a pole. The new install is to upgade the fire pump and associated equipment.

75hp,480v FP
1.5 hp jockey
FP controller,I dont know the control amps yet
15 kva 480/120 1 phase xfmr with 3 lights and a outlet or two maybe.This will be a seperate service tap.

How do I size the 13k/480 xfmr? The owner thinks they are too small.

125% of the FP and 100% of the other loads?And what about pri OCPD?

I've requested a engineer 1-line for this job,none yet. But Id like to learn something new today so any help is helping the cause.
Thanks.

 

[kingpb]

Depends on what you want to limit your voltage drop too.

For instance if you use a 225KVA Z=5% (3 x 75KVA) the 75Hp motor, when it starts will drop the motor on the 480V Bus approx 10%. That is also assuming that the utility has enough VAR support on the HV side of the transformers.

A 500KVA (3 x 167 KVA) Xfmr will allow a voltage drop of about 4.5%. These are rough numbers of course.

To get more detailed, it really is a nice design exercises, you can get the motor starting curves, and torque versus speed curves for the motor and load as well as motor parameters.

Using something like ETAP or SKM program, the static starting analysis shows you the initial drops, but assumes it does start. Once you see the thermal damage curves and do a dynamic analysis, that will show you if it really does start. Then you can fine tune your transformer size.

Bottom line, your gut feeling that 3 x 25KVA was too small is correct.

 

[bob]

I don't understand what your concern is. The 75 kva bank is quite adequate to run the fire pump. I assume these are utility owned transformers. Normally it is not your job to size the transformers. Why would the owner think they are too small?

 

[kingpb]

I don't understand what your concern is. The 75 kva bank is quite adequate to run the fire pump. I assume these are utility owned transformers. Normally it is not your job to size the transformers. Why would the owner think they are too small?

Running is one thing, what about starting?

ROM calc: is to multiply the transformer Z% by the amount of inrush in percent of the transformer current rating, I'll demonstrate-

75KVA @ 480V, rated current is 90.2A.

Assume XFMR Z=5%

Motor Inrush on starting (LRC) could be as high as 576A (6X), assuming voltage stays within motor limits.

(576/90.2 x 0.05)x 100 = 32% voltage drop on starting.

My experience says that the motor will never start, and this is a poor design.

I have verified this by use of ETAP model.

 

[bob]

A Google search indicated that a 25 KVA pole mtd transformer had a %Z of
1.5 to 2 %. Could you re-run the starting calculations using 2%?
Utilities I have dealt with would be happy to lease you the difference between
the 75 KVA and the 225 or 500 KVA. Very costly. They also might require some type of reduce starter depending on the stiffness of the system.

 

[kingpb]

A Google search indicated that a 25 KVA pole mtd transformer had a %Z of
1.5 to 2 %. Could you re-run the starting calculations using 2%?
Utilities I have dealt with would be happy to lease you the difference between
the 75 KVA and the 225 or 500 KVA. Very costly. They also might require some type of reduce starter depending on the stiffness of the system.

At 2% your still getting a 12% voltage drop. That in itslf is not out of the question, because you could always buy a motor that has a 80% starting terminal voltage capability. However, I must sress I am looking at static start model only.

To accurately model the real situation, you need motor and load speed vs torque curves. The issue becomes how long it sits at reduced voltage before the motor hits the thermal damage curve. With this being a fire pump, you'd want to make darn sure there won't be an issue.

 

[Al Pike]

Thanks Bob for your help,but the utility does not own the transformers the customer does.I think Im going to make them give me a engineered print to work off of,or Im bailing out of the job.

 

[mayanees]

Arrticle 695

Arrticle 695

NEC 695.7 defines voltage drop criteria for fire pumps at 15% during startup and 5% at 115% load.
My experience has been that if I stay within this criteria - no troubles.
John M