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# 6.9 kV or 13.8 kV to feed big synchronous motors?

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## 6.9 kV or 13.8 kV to feed big synchronous motors?

(OP)
Hi folks!

We are preparing a bid to a big water pump project that includes 6 synchronous motors from 5 MW to 13 MW.
I wonder if somebody could give me some informations about the advantage/disadvantage in choosing the voltages to feed the motors: 6.9 kV or 13.8 kV?

Cheers,

BrasÃ­lia, Brazil
https://www.researchgate.net/profile/Herivelto_Bro...

Replies continue below

### RE: 6.9 kV or 13.8 kV to feed big synchronous motors?

13 MW = 17.3 kHP (17300 HP)
Rule of thumb: HP should not exceed Voltage.
For one 13 MW motor you could probably go either way.
For more than one 13 MW motor it may be well to go for 13.8 kV.
Feeder conductors; For long runs there is a considerable saving in copper at the higher voltage.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

### RE: 6.9 kV or 13.8 kV to feed big synchronous motors?

"..13 MW = 17.3 HP
Rule of thumb: HP should not exceed Voltage...."

There is a typo error .
Che Kuan Yau (Singapore)

### RE: 6.9 kV or 13.8 kV to feed big synchronous motors?

Got it. Thanks for the heads up.
(Need more coffee. grin)

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

### RE: 6.9 kV or 13.8 kV to feed big synchronous motors?

We had a transformer failure in a station using 6.9 kV. Search for a possible replacement or temporary rental until the original unit was repaired quickly confirmed what I suspected - 6.9 kV is NOT common. We found ONE rental, and it was less than 2/3 of the capacity we needed. Of course we took it and limped along for the year it took for repairs to be made.

Serious consideration should be given to using unusual voltages in view of possible failures.

Incidentally, we now own a fully rated spare transformer.

old field guy

### RE: 6.9 kV or 13.8 kV to feed big synchronous motors?

13,800 Pros:
- 50% reduction in line current
- May allow for simpler feeder circuit configuration due to lower currents.
13.800 Cons
- Depending on the operating environment,
- motor insulation systems will be more complex and potentially less reliable especially if the operating environment is poor
6,900 Pros:
- May be able to use contractors rather than breakers for motor circuit control for the smaller motors.
- More VFD options available.

If you intend to start the motors at full voltage, and not require speed control, and if the operating environment is decent I believe a simpler system can be designed at 13,800. Conversely, if you need speed control or the operating environment is poor 6,900 volts may be a better option.

### RE: 6.9 kV or 13.8 kV to feed big synchronous motors?

Higher voltage = less current. Less current translates to: 1) less metal in a conductor, 2) a requirement for better (i.e. thicker and possibly less pliable) insulation, 3) greater spacing between conductors, and 4) larger construction dimensions (minimum bend radii for example).

Higher voltage ALSO has increased tendency for other effects such as corona and/or partial discharge (PD) . . . both of which will negatively impact the life of the equipment. The environmental conditions (humidity, dust/debris, temperature, and altitude) will play a role in the decision - the higher the condition, the more likely corona and/or PD will occur.

For rotating machines, higher voltage will require larger clearance and creepage distances - which may result in a physically larger machine. It may also result in a more complicated winding configuration (turns per coil, coils per pole, parallel circuits, etc.).

Lastly - availability of ancillary equipment (transformers, breakers, surge protection, etc.) may preclude choosing one or the other. In general, 13.8 kV is more likely to be available than 6.9 kV since most distribution networks (on the utility side) operate at the higher voltage.

Converting energy to motion for more than half a century

### RE: 6.9 kV or 13.8 kV to feed big synchronous motors?

Both are standard industrial voltages, but some equipment may be more available for one over the other. true the higher voltage will need a smaller conductor, with a higher insulation.
They make larger motors and generators for both voltages, but think about the auxiliary equipment, that might need a step down transformer. That should also be in your thoughts. Also the size of your switching equipment.

### RE: 6.9 kV or 13.8 kV to feed big synchronous motors?

Are you intending to do speed control, i.e. variable speed drives? That can make a huge difference in the decision.

" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

### RE: 6.9 kV or 13.8 kV to feed big synchronous motors?

As an example, we have a 2.6 MW ball mill that is supplied with 13.8 kV from our on-site generator. The difference I think is ours is a WRIM, not synchronous motors. The motor is also controlled using liquid resistor on starting.

### RE: 6.9 kV or 13.8 kV to feed big synchronous motors?

At these power levels, 13.8 KV is preferred. 13.8 KV motors are as reliable and time tested as 6.9 KV since they are custom designed and built for 13.8 KV.

Handling large currents is more difficult than large voltages and hence the HV/EHV transmission systems.

Muthu
www.edison.co.in

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