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Power Distribution: Motor Starting

Power Distribution: Motor Starting

Power Distribution: Motor Starting

(OP)
I need some help to settle an argument...

I have a power distribution network with secondary selective switchgear (i.e. two incoming feeders to the switchgear with an open bus tie breaker).

The the main distribution switchgear is fed by generators synchronized with a feed from the grid and there are some large motors attached to one bus of the secondary selective switchgear a couple of levels below.

My question is, is it likely to be more difficult to start the motor when the switchgear it is fed by has both incomers connected and the bus tie breaker open or with a single incomer connected and the bus tie breaker closed (assuming all other factors stay the same)?

Let me know if you need more info. I'll be grateful if anyone can help me to settle the argument.
 

RE: Power Distribution: Motor Starting

If your system can handle it, paralleling the two incoming sources should lower the effective impedance and make motor starting easier.  However, this does give you more fault current and you need to see if that's OK.

If the two sources are from different busses, you will get circulating current when you close the tie breaker and this should be looked at too.  If the effects are relatively minor, then my initial comments of easier motor starting with the 2 paralleled sources still apply.

RE: Power Distribution: Motor Starting

The sources aren't paralleled according to the OP.
  

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If we learn from our mistakes I'm getting a great education!
 

RE: Power Distribution: Motor Starting

Good catch, Scotty.  Looks like it doesn't make a difference if you have one incoming or two if the bus tie stays open.

Unless of course you pick the bus with the generation paralleled with one incoming feeder.  This would be a better choice.

RE: Power Distribution: Motor Starting

(OP)
I think you've given me the answer I expected.

The impedance is the same regardless of whether the motor switchgear is fed with both incomers connected and the bus tie breaker open or with a single incomer connected and the bus tie breaker closed.

Thank you.
 

RE: Power Distribution: Motor Starting

You might argue that the extra loads on the system with the bus tie closed would cause a higher volt=drop at the bus which would extend the starting time. Of course if you had capacitance on the bar then the reverse might occur. Difficult to tell.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: Power Distribution: Motor Starting

There really is not enough info. As a guess though, running on one feeder with the tie closed will be the worse system because there is a larger base load on the system before the motor is started. The larger base load means the voltage may drop more when the motor is started. Another consideration is that each feeder might not be the same and there might be one with a lower impedance than the other.

However, the above does not mean the motor is more difficult to start. If it gets enough voltage applied then it will still start just fine.

But then, maybe I don't understand how you are describing your system...
 

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