Large motor inrush vs transformer size

[greenspark1]

Hi,
We are planning to put 4x 300HP 480V motors on a new 2000 kVA utility oil-filled transformer. At 2400A rated output, the transformer can handle the full load amps (365A/ea), but the starting current could be an issue. The motors have soft starters from the factory but I don't have specifics of the magnitude of the inrush. They are each getting a 600A fuse (from the manufacturer) and a 700A circuit breaker upstream.
I am trying to convince myself that inrush won't cause a voltage dip that will affect other systems on the transformer. Any resources/calcs others have used to determine the right xfmr size in similar applications? Thanks!

 

[Jraef]

Hi,
We are planning to put 4x 300HP 480V motors on a new 2000 kVA utility oil-filled transformer. At 2400A rated output, the transformer can handle the full load amps (365A/ea), but the starting current could be an issue. The motors have soft starters from the factory but I don't have specifics of the magnitude of the inrush. They are each getting a 600A fuse (from the manufacturer) and a 700A circuit breaker upstream.
I am trying to convince myself that inrush won't cause a voltage dip that will affect other systems on the transformer. Any resources/calcs others have used to determine the right xfmr size in similar applications? Thanks!

You will need to know if there is a possibility of having all three restart at the same time, like after a power failure, or if there is a strictly controlled staggered restart process. If they are ever going to be expected to restart all at the same time, even with soft starters the quick and dirty rule of thumb is HP x 2 = kVA of the transformer. That would make it a 2400kVA transformer. If staggered start is guaranteed, the rule becomes largest motor HP x 2, + total kVA on line when it starts. Assuming no other loads, that makes it around 1350 kVA minimum, you should be fine.

That rule of thumb by the way, assumes a 10% voltage drop is acceptable on the low voltage side of the transformer. If you have other more sensitive loads on the same transformer, that may be too much.

 

[greenspark1]

Thanks for the response. I suspect they will always be staggered start but need to confirm for sure. 10% Vdrop sure sounds like a lot, not sure how plug loads, lights, and some computers might respond to that. Any formulas, spreadsheets, or software that will help calculate this?

 

[Jraef]

Thanks for the response. I suspect they will always be staggered start but need to confirm for sure. 10% Vdrop sure sounds like a lot, not sure how plug loads, lights, and some computers might respond to that. Any formulas, spreadsheets, or software that will help calculate this?

You might be able to keep the VD lower by increasing the transformer sizing factor.

The software to do this correctly is expensive (probably $4-5k minimum), you need something like SKM or ETAP.

There is a really old spreadsheet that GE made years ago for their Reduced Voltage Autotransformer Starters that can be somewhat useful, but it has not been updated since 1999, and it cannot be effectively used on soft starters as an exact measurement unless you know how to tweak it, because of the difference in the way a soft starter and an autotransformer starter draw current from the line. But I use it on occasion just to confirm myself. The difference is, in an RVAT starter, the motor current is different from the Line current by the transformer ratio but in a a soft start, the motor current and line current are the same. So if you use the GE spreadsheet, you have to adjust for that.

http://electrical-engineering-portal.com/download-center/electrical-software/motor-voltage-drop

 

[Besoeker]

Hi,
We are planning to put 4x 300HP 480V motors on a new 2000 kVA utility oil-filled transformer. At 2400A rated output, the transformer can handle the full load amps (365A/ea), but the starting current could be an issue. The motors have soft starters from the factory but I don't have specifics of the magnitude of the inrush. They are each getting a 600A fuse (from the manufacturer) and a 700A circuit breaker upstream.
I am trying to convince myself that inrush won't cause a voltage dip that will affect other systems on the transformer. Any resources/calcs others have used to determine the right xfmr size in similar applications? Thanks!

For a direct on line start for one motor assuming six times FLC you might expect about a 4% drop.

Transformer Regulation

Transformer
Rating (kVA) 2000
Voltage (V) 480
Impedance (%) 5.00
Resistance (W) 1.38E-03
Reactance (W) 5.59E-03

Nominal Load
kW (kW) 225
pf pu 0.12
kVA (kVA) 1875
Current (A) 2255
phi (rad) 1.4505
Z (W) 0.12288
R (W) 0.0147456
X (W) 0.121992056

Total
R (W) 1.61E-02
X (W) 1.28E-01
Z (W) 0.12859463
Actual current (A) 2155
pc FLC (%) 90%
Actual kVA (kVA) 1792

Output
V 265
Regulation 4.44%


However, with a current limited soft start you ought to see a rather lower number.

 

[kingpb]

If they are on soft starters, you shouldn't have an issue. But with that said, a study should be done. Who selected the soft starters? Did someone confirm that they will be able to start the load on reduced voltage? No mention of load type, so somebody better check starting torque. Soft starters do not work for every application.