4160 versus 13,800V motors - Pro's & Con's
4160 versus 13,800V motors - Pro's & Con's
(OP)
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
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





RE: 4160 versus 13,800V motors - Pro's & Con's
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
RE: 4160 versus 13,800V motors - Pro's & Con's
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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RE: 4160 versus 13,800V motors - Pro's & Con's
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(2B)+(2B)' ?
RE: 4160 versus 13,800V motors - Pro's & Con's
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.
RE: 4160 versus 13,800V motors - Pro's & Con's
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: 4160 versus 13,800V motors - Pro's & Con's
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!
RE: 4160 versus 13,800V motors - Pro's & Con's
Keith Cress
kcress - http://www.flaminsystems.com
RE: 4160 versus 13,800V motors - Pro's & Con's
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
RE: 4160 versus 13,800V motors - Pro's & Con's
Is the existing 4.16kV network still sound? If so you should probaly just replace the 4.16kV motors
RE: 4160 versus 13,800V motors - Pro's & Con's
RE: 4160 versus 13,800V motors - Pro's & Con's
Muthu
www.edison.co.in
RE: 4160 versus 13,800V motors - Pro's & Con's
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
www.hydropower-consult.com
RE: 4160 versus 13,800V motors - Pro's & Con's
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.
RE: 4160 versus 13,800V motors - Pro's & Con's
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.
RE: 4160 versus 13,800V motors - Pro's & Con's
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
www.hydropower-consult.com
RE: 4160 versus 13,800V motors - Pro's & Con's
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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