Power of a sewage pump instalation 1

[martindavila]

Hello (anf sorry for my english)

Well, I have a problem measuring the Transformers of an instalation of sewage pumping.

It consist in 8 pumps of 560 kW of power, two of them must have a PWM to control the speed. They work in 690 volts (50 Hz) and have a cos(phi)=0.85. The company supply in 15.000 volts.

We were thinking in using 4 transformers of 2.500 kVA and to separate the pumps in 2 groups of 4 pumps each group and using 2 transformers in pararell for each group.

The pumps have a start up current of 5 times the basic one and they stop and start may be 3 times per hour each one. They don´t have to start up at the same time.

There are some doubts about if the transformers are enough to hold up the heat cause of the many start ups.


Can anybody help me to obtain a solution??

Thanks

Martín Davila
sammy@talgo.uvigo.es

 

[rbulsara]

Transformers are more fogiving than motors, I believe..so if your motors can handle it..surely the tranformers can..

Also consider forced fan cooling on transforemrs, controlled by temprature sensors in the transformers.

 

[gordonl]

Transformers have very long thermal time constants, so the thermal affects of motor starting aren't a consideration. (If the motors are VFD controled than there is no large starting current)

10MVA of transformers is definetly enough for 5.25MVA of motors. If your aim is redundancy you may want to consider 2.5MVA with forced cooling for times when only one of the two parallel units is in service.

If the motors are VFD controlled than you may want to check your harmonics and get a K Rated transformer which has extra margin for transformer heating due to harmonics.

 

[dpc]

I agree with comments above, and I would be **much** more concerned about the motors than the transformers. I'm not quite clear, but if you are proposing to start some of these motors using full-voltage starting up to 3 times per hour, this does not seem like a good idea. If you are only starting and stopping the motors using the AFD, then it may not be as much of a problem.

For sewage pumping, reliability is a key concern - you should suggest that additional pumps be equipped with adjustable speed drives to improve reliability. Just two may not be enough because of the limited flow range that the pumps can operate over. You will probably need the two pumps with AFDs running all the time in order to control the wet well level.

Anyway, you don't want to be starting and stopping 560 kW motors 3 times an hour if you can possibly avoid it.

Transformers will be fine.