MV vs. LV Motors

[nhee]

A new facility will include 5 - 600 hp motors, along with roughly 200 KVA of 480 VAC load. It would seem to be an application requiring MV distribution and motors, but a cost analysis is pointing toward LV (480 VAC) -

- The motors are part of packaged compressor units which are standard with 480 VAC motors and pre-mounted RV starters.
- There is a significant (20%) cost adder to go with MV motors.
- MV motors would require starters to ship "loose", requiring add'l field labor/space. The utility requires RV starters.
- All loads are in close proximity (less than 100') to the proposed switchboard location and service entrance, reducing the conductor/labor costs associated with the LV installation.
- Significant add'l cost to go with the MV switchgear.

Are there other factors I should be considering in the MV-LV decision? Although at first glance it looks like it should be MV, the costs seem to point to a 480 VAC system.

 

[Blackcountryman]

nhee,

You also need to consider the skills and competancies of the maintenance staff. While they may be familiar with and understand working on LV to work on MV may need additional training

John

 

[BJC]

Something don't sound right.
Are you considering the cost of the starters? If the starters are furnished with the compressors and your not considering them in the evaluation it will not give you the true cost.
As a rule motors over 250 HP are more economical at MV.
Evaluate buying the compressors without starters. Most manufacturers of swithgear build lineups of starters ( or even indivual starters) for most compressor units.
I have had Cutler Hammer buid starter lineups for several types of compressors. They know the requirements and controls of the units your going to use. Other swithgear mfgs can do it as well.
Try pricing something like a Cutler Hammer Ampguard with autotransformer starters. Use the price of compressors without starters.
Some other things to consider:
At 480 volts a size 7 reduced voltage starter is not cheap and are they are maintenance intensive.
5 600 Hp motors are going to max out a 2,500 Kva transformer. That is the largest "standard" size you can get from a utility. You'll need an engineered set up or more than one.

 

[nhee]

Thanks for the comments.

BJC - the 480 VAC packaged units come pre-assembled with the RV starters (wye-delta). The 4160 VAC units require separate starters.

In my analysis for the 480 system, a switchboard would feed the packaged units. In the 4160 option (units supplied without starters), starters would be integrated into the switchgear.

Regarding maintenance issues, another factor is that the systems will likely be run only 200 hours per year. Agree that it is usually more cost effective to run systems this size at 4160, but this seems to be the exception.

 

[BJC]

For 200 Hrs a year renting diesel powered units on trailers may be the most economical solution.

 

[jraef]

nhee,
I am assuming that you already have 4160V available to you from the utility since you are considering it. In your cost analysis, make sure you also consider the cost of transformers for those 460V motors. 3000HP is a considerable connected load, and in order to avoid a signiicant voltage drop the transformer(s) will need to be very big.

Even if the utility is going to provide you with a 480V service, they tend to want to supply as little transformer as they can get away with (and/or what they normally keep in stock), so they may have more stringent restrictions on starting kVA at 480V than they would at 4160V. Check that out first. In other words, if the utility says that you must limit the 480V voltage drop to a really low value, and the transformers they give you are barely adequate, you may not be able to start the last of those chillers in the sequence unless you use a VFD, and that will represent a big increase in cost.

BTW, 200hrs is awfully low for a permanent installation, BJC has a point. Here in California even with our ultra-strict air polution regulations, you can run a diesel engine up to 1000 hours/year without having to do a full environmental impact study. It might be worth checking into.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[nhee]

Good suggestions - we had made the same suggestion re: rental units but it is not an option in this case.

 

[BJC]

while your at it consider the fault current rating required. Whatever transformer source current you have will be supplimented by the motors. Equipment to handle that fault at 480 will be spendy.
MV switchgear that can handle 100mva of fault current is not extraordinary.