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Supply Transformer Saturation

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#1 Colin


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Posted 31 March 2007 - 05:45 AM


I am refurbishing a pumping station which is equipped with two 315 kW centrifugal pumps. The control panel is supplied from 11000/415V 1250 kVA standard distributon transformer with 5% regualtion.
One pump is equipped with an autotransformer starter and the other with a soft starter. The FLC with both pumps running is 840A. There are no problems starting the pumps.

I am uprating the pumps to 450kW both with new soft starters and am concerned that the distribution transformer may not be able to cope if a second pump is started with one pump already at full load.

I estimate that with one pump already on line that the inrush will approach 3600A momentarily assuming the soft start is set up to 400% FLC. (the transformer FLC is 1666A)

I am fairly confident that the transformer will be OK as the voltage is unlikely to fall below 390V but am struggling with the maths. Also, the transformer manufacturer has been no help.

Any ideas ?

#2 mariomaggi


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Posted 31 March 2007 - 07:32 AM

Dear Colin,
considering also the possibility to save energy, using a variable frequency inverter you will solve this problem and you will have the minimum starting current. You can also use only one inverter to start one pump each time. Depending form the type of application, it should be possible to install an undersized inverter, using it for example at 150% load for 60 seconds, to reach for example 90% or 95% of speed, after that you could switch to DOL.

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#3 marke


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Posted 31 March 2007 - 09:06 AM

Hello Colin

Welcome to the forum.

Firstly, there is no problem with the supply transformer saturating. The transformer will only saturate if you increase the flux too high. Changing the load on the output of the transformer will not change the flux. This is a function of the input voltage, not the output current, so you have no problem there.

Overloading the transformer will cause an increase in the voltage drop and the losses in the transformer.
If the transformer is rated at say 1000A with an impedance of 5%, than at 1000A, you will get a 5% voltage drop, and at 2000A, you will get a 10% voltage drop.

In your case, provided that the full load current of both pumps running together is less than the rating of the transformer, the only issue that you will have during start, is an increase in the voltage drop and possibly, you may have an issue with the transformer protection operating on the overload during start.

If the transformer has a rating of 1250KVA at 415 volts, then the rated current is 1739 Amps.

Assuming that the rated current of the 450KW motors is in the order of 800Amps, and the start current is 400%, we will have a short term overload of 800 plus 4 x 800 (= 4000) Amps when the second motor is started.
The transformer has an impedance of 5%, so the voltage drop during the start of the second motor will be 4000/1739 x 5% = 11.5%

If you carefully select the new motors, you could start with a start current lower than 400%. You need to look at the start current and start torque curves for the motors and select motors that have a high Locked Rotor Torque and a low Locked Rotor Current in order to get the minimum start current.

Best regards,

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