Transformer Sizing

[casey1234]

I have calculated this, but don?t have confidence in the results. Any help would be appreciated.
Transformer will be 19.9 primary x 4160 secondary

I have 3 ? 4160 users
1 - 800hp motor Air Compressor
1 - 450hp motor Vacuum Pump
1 ? 500KVA 4160 x 480 transformer

What size transformer (KVA) would be required for this? I come up with a 2000 KVA.

 

[bob]

Casey
Before I attempt to answer your question I would like to know the following:
Transformer %Z
Does the 800 and 450 hp start across the line or by starter?
What kind of load is on the 500 kva transformer?
What about future loads?

 

[kingpb]

I have calculated this, but don’t have confidence in the results. Any help would be appreciated.
Transformer will be 19.9 primary x 4160 secondary

I have 3 – 4160 users
1 - 800hp motor Air Compressor
1 - 450hp motor Vacuum Pump
1 – 500KVA 4160 x 480 transformer

What size transformer (KVA) would be required for this? I come up with a 2000 KVA.

Assumptions:

Transformer is standard Z and X/R
800Hp motor starts across the line with all other load running
500kVA Xfmr is fully loaded
Cable impedance is ignored
It is acceptable to use transformer taps
Bus voltage on start can dip to nearly 80%

If the above assumptions are acceptable, then 2000KVA could be used. However, this is based on static starting. Unless the compressor motor and load inertia is known, it is not possible to ascertain if the compressor will actually start at that low of a bus voltage prior to hitting it's thermal damage curve.

If the compressor is on a reduced voltage starter, then 2000KVA should be an acceptable size.

Using a 2500KVA and across the line starting, the 4.16kV bus voltage will probably drop to around 85% on start-up, this is normally considered acceptable. But to be sure, a dynamic motor starting analysis should be done to verify that the compressor will start prior to hitting it's thermal damage curve.

Without a bunch of other details this is all I can offer.

 

[casey1234]

The two motors start by a starter.
The transformer is about half loaded.
There will be no future 4160 loads, however additional 480v users may be added to the transformer.
I am being told that the 800hp motor has a locked rotor current of 700A.

 

[kingpb]

The two motors start by a starter.
The transformer is about half loaded.
There will be no future 4160 loads, however additional 480v users may be added to the transformer.
I am being told that the 800hp motor has a locked rotor current of 700A.

I assume that is a MV across the line (full voltage) starter?

You should plan on the 500KVA being fully loaded. (future)

IMO: plan on using nothing smaller then 2500KVA.