4160 versus 13,800V motors - Pro's & Con's 2

[macmckim]

I have an end-user with number of 4P-5000HP, 4160V motors that are 25+ years old on large pumps that we have repaired or over-hauled number number times in past 10 years.

The end-user is looking at replacing the motors wih new machines in next couple of years. The idea the in-house engineer is considering is changing the motor voltage to 13,8kV. He says they have 13.8KV at site that goes through
transformers, stepping down to 4.16kV for the 5000HP mtrs.
The transformers are also 25 years old, but no problems as far as everyone knows.

My question - Are there any advantages or disadvantages to
use 13.8KV voltage motors?

The end-user engineer knows that new DOL starting equipment
will be required, new cabling (13.2kv type), 13.8kV mtrs will be more expensive than 4.16kV.

Any comments would be appreciated.

Mac

 

[rbulsara]

If 13.8 is available, it makes sense to use 13.8 kV motors, if getting replaced. It eliminates need to have the transformers and their accompanying losses,additional points of maintenance and failure.

If you were replacing 25 yr old motors, there is no wisdom in keeping its 25 yr old cables or transformers, they just become the weaker links in the new system and just a deferred expense.

I do not believe cost difference in 15kV cables and 4.16 kv (which will now use 8 kV cables, if replaced)would or should be deciding factor. Do a life cycle cost analysis of the entire new system alternatives, if applicable.



Rafiq Bulsara

http://www.srengineersct.com

 

[jraef]

I agree with Rafiq. Higher initial outlay because the equipment is more expensive, but lower risk of failure by having fewer components and lower operating costs because you eliminate the transformer losses.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[electricpete]

I only have experience from one plant.... our plant where we have 30 13.2kv motors and 60 or 80 4kv motors that I have watched over for 10 years. Based on this particular population, I would say that the reliability of 4kv motors is more than twice as high as 13.2kv. Some of these motors are in outdoor environments.


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(2B)+(2B)' ?

 

[rhatcher]

13.8kV pro: smaller conductors, no need for stepdown transformer
13.8kV con: more expensive motor, more expensive controller/starter, low availability/long lead time for replacement motors or motor repairs, more expensive repairs,

4.16kV pro: common availability of motor replacements or repairs, common less expensive controller/starters
4.16kV con: need for stepdown transformer, larger conductors

In my opinion 4.16kV is preferable. The difference in conductor size for motor leads is probably offset by the higher cost of 15kV conductors and the need for the stepdown transformer is probably offset by the higher cost of the motor and controller/starter. I would guess that the 4.16kV installation would be less expensive overall.

However, the biggest consideration for me would be the more common availability for replacement and repair of the 4.16kV motor and controller/starter. Also, I have developed the perception that the 13.8kV motor design is still not proven and/or is still be developed and refined so a higher probability exist for premature winding failure due to corona.

That's my opinion.

 

[jraef]

Interesting observation, I've often wondered about that. Any theories on why? Insulation issues perhaps?


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[ScottyUK]

The 13.8kV machine will suffer from PD, the chances are the 4.16kV machine will not. Detecting PD costs money. Fixing it early costs money. Fixing it later costs lots of money.

At 13.8kV you are probably looking at breakers, not contactors. Expensive, and maintenance-heavy.

At 13.8kV your choice of manufacturers shrinks signifcantly, and the price will rise steeply. Your choice of repair shop likely reduces too.

After all that I would still probably go with 13.8kV (11kV where I am) but it's not a clear cut decision at that rating.


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

 

[itsmoked]

The transformers also help with shielding the start-up currents from the rest of the network.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Gianoli]

Be careful with the larger motors feed at 5 kV. In order to realize the full rating of the motor you will likely be forced into using power circuit breakers rather than contactors. A typical 5000 HP motor at 4000 volts will draw somewhere between 585 - 640 Amps at full load. Depending on the motors service factor you may need a contactor or breaker potentially rated for 800 Amps (640 X 1.25 = 800 Amps). 5kV contactors are available in an 800 Amp rating however they are always de-rated to 720 in an enclosure. Then you can deal with the problem of finding a contactor fuse that has a continuous current rating of 720 Amps that will coordinate with upstream protective devices.

At the end of the day you will need to use a 1200 power circuit breaker or use a 800 Amp contactor which will limit how far into the service factor you can run your motor. Many of our 5000 HP, 5kV units are limited to 680 Amps because of the contactor issue.

Have you considered 6.9kV ? Contactors are available at this voltage level and you get away from some of the insulation you may experience with 13.2kV motors

 

[EEIre]

Also consider the implications of replacing the existing 4.16kV network with a 13.8kV network.

Is the existing 4.16kV network still sound? If so you should probaly just replace the 4.16kV motors

 

[macmckim]

Thankyou all for your comments. Opened my eyes on number of subjects that I did not think about or consider.

 

[edison123]

At that HP level, I would go with 13.8 KV and eliminate the trafos and multiples runs of cable. As for PD and other insulation related issues, a 13.8 KV motor is as reliable as 4.16 KV motor, if not better, if sourced from a good OEM. Breaking higher voltage is easier than breaking higher currents. So breakers' costs shouldn't vary that much.

Muthu

http://www.edison.co.in

 

[wolf39]

Mac:

You've overhauled your 4.16 kV motors a number of times the last 10 years. Their reliability therefore doesn't seem to be that good. I'd guess that a modern 13.8 kV motor supplied by an OEM with good reputation is more reliable than your old motors and also more reliable if you consider new 4.16 kV motors in combination with your 25 year old step-down transformers. The elimination of these transformers does increase the plant reliability and efficiency.

The loss evaluation is as follows (all technical data guessed):

Transformer rating 4,000 kVA
Transformer efficiency 98% = 80 kW loss total
Plant factor 80%
Cost per kWh 10 US cent

80 * 8760h * 0.8 * 0.10 = US$ 56,000 per annum or US$ 1,400,000 over a period of 25 years.

I'd suggest you to insert into the above formula your actual figures. And don't forget to consider the cost rise of each kWh over a 25 year period.

rhatcher:

A 13.8 kV motor has smaller conductors but the number of conductors are 3.3 times of a 4.16 kV unit.

Regards

Wolf

http://www.hydropower-consult.com

 

[rhatcher]

Wolf,

Good analysis of the efficiency cost of the transformer over 25 year of life. It is amazing how you can accrue 1.4 million dollars 10 cents at a time.

Although it my not have been clear, I was suggesting replacing the transformers as part of the project as well as the controllers/starters and the distribution conductors. It is the distribution conductors (transformer to starter, starter to motor) that I was describing as smaller or larger depending on the voltage.

The difference in the motor conductor size is irrelevant since you will end up with essentially the same amount of copper in the motor regardless of the voltage (low voltage = less turns of larger wire, high voltage = more turns of smaller wire, all proprtional). However, the insulation systems for 13.8kV is much different to allow for not only for the higher voltage but also for corona and partial discharge.

 

[potteryshard]

I asked the same question regarding a project I was involved in which used 2250HP motors; originally at 2400V, in the upgraded version at 4160V.

The answer I received was that the 13kV motors were possible, but because of the additional insulation needed for the stator windings, the slots would have to be more larger, which would probably entail a larger motor frame to keep core magnetic flux density within economic limits.

As I remember, the manufacturer also reccommended differential relaying on each motor if used at 13kV. A more limited pool of rewinders was also mentioned.

 

[wolf39]

Granted, a 13.8 kV motor is more expensive than a 4.16 kV unit. Still, we have to wait for Mac's loss evaluation. After multiplying his loss evaluation number with the number of pump motors installed, the plant owner may still decide to go for the 13.8 kV solution regardless of all sceptical comments. I would suggest, however, not to purchase a cheap no-name product but to order the 13.8 kV motors from an OEM with good reputation.

Wolf

http://www.hydropower-consult.com

 

[controlnovice]

Transformers could be in place to minimize the startup voltage drop on the main line. In this type setup, the transformer can be left in the line, or bypassed, once the motor is up and running.

You may not have the SSC available to start a 13.2kV motor DOL.

(Please feel free to correct me. It's been a while since I've done large motors)

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