II kV Motor Size 1

[BRIS]

I am costing a project including a pump station. The project will take power from a 220 kV supply through a 220/11kv sub-station. The pump station will have an 11 kV supply.

Pumps are 2 duty one standby each rated at 650 kW. The Preliminary design shows 11 kV motors with DOL. Pumps are fixed speed.

My rule of thumb is 3.3kV motors up to about 1.3 MW.

1) is it practicle, feasable and economic to use 11 kV motors. Is it more likely that the design/build contractor will transform down to 3.3 kV and use 3.3 kV motors.

I could change the specification from 2 duty 1 standby each rated at 650 kw to 1 duty one standby each rated at 1.3 MW.

Obviosly what I ned to do is a propoer costed study but I don't have time. Any good rules of thumb?

 

[Rodmcm]

650KW at 3.3k is normal. At 1.3MW I would be thinking 11kV

 

[BRIS]

Sorry my post might not be clear: I have 11kV for a 650 kW motor.

 

[motorspert]

BRIS
No problem - 11kV at 650kW will cost a bit more, but will be cheaper than a stepdown transformer plus a 3.3kV motor. You dont say what speed, but i assume 4pole is required.

 

[BRIS]

motorspert

Thanks for the reply. We will stick with 11 kV. The design/build contractor can offer 3.3 kV as an alternative if he wishes. (the costs are insignificant within the overall project).

 

[ScottyUK]

Is this the only 11kV board on the site? The ongoing maintenance costs will be higher for 11kV, plus PD starts to become a problem at 11kV where it is virtually non-existant at 3.3kV. Jointing requires is more complex at 11kV and skilled jointers are becoming extinct. Does the client understand these factors?

How do costs stack up against a 3.3kV system if you factor in a spare 11kV motor? The 3.3kV machine should be a more-or-less standard design, while the 11kV one is definitely a 'special'. If the plant is critical then a motor available off the shelf is a big bonus against waiting a couple of weeks for a stator rewind.


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Sometimes I only open my mouth to swap feet...

 

[BRIS]

Scotty - thanks - more helpful comments.

This is a complete new irrigation development. The only significant electrical load is the pumps 2 by 650 kW. Subject to approval of the local power authority we will tap into either a nearby 220 kV or 66 kV transmission line (probably the 66 kV) . The length of our overhead line from the new substation to the pump station will be 750 m.

I understand that it would have been much more sensible and appropriate to use 3.3 kV (i.e a 66/3.3 kV substation and supply at 3.3 kV). But 11 kV is possible even if not sensible?

I have to decide whether to pull and revise the bid documents already submitted to the client or let them stand. I have added a rider to the effect that the design/build contractor shall review and offer alternatives - not ideal!

 

[motorspert]

BRIS
Yes, you are right - possible but not ideal. We supply many motors at that voltage down to 100kW - I think your approach is ok.
Scotty is right, there are good reasons why 3.3kv is better and in an ideal world etc.