Motor selection for large HP

[MEJatNDL]

I am considering using medium voltage 2300 volts for several 500 hp compressors now at 600 volts. Is 2300 volt systems more efficeint when considering new equipment.

 

[dpc]

It depends on a lot of site specific factors. In general, the 2400 V system will tend to be more efficient because the currents are lower (for the horsepower) so system losses can be less. But it will also depend on the number and size of the motors, motor type, speed, etc and the distances involved.

I'd suggest doing a search on this forum - there have been several discussions regarding low voltage versus medium-voltage motors.

 

[pooslinger]

I am assuming that 2300v is the service comming into your facility and you are modifying it to 600v to run your existing compressors. Consider the price of new controls to your purchase of the new units to the few kwhs your going to save with the higher potential.

 

[jraef]

Motor efficiency is probably comparable. But if as mentioned above you have 2300V incoming already, you eliminate the transformer losses and reduce distribution (I²R) loses by having lower current. If you have, for instance, 12.4kV and have a 600V transformer now but need to buy a 2300V transformer to accomplish this, the savings may be significantly lower and take a lot longer to pay back on the investment.

A major consideration in addition to the control equipment costs however might also be serviceability. Make sure your plant electricians are qualified to work on medium voltage. If they are, no problem. But if not, the training can be costly plus they will want more money from then on. Been there, done that, still have the scars from the bean counters!

 

[rcw retired EE]

If you don't already have 2300V in the plant, look at going to 4.16 kV. New NEC regulations require use of shielded cable at both voltage levels, so cable cost is about the same at either voltage.(Cheaper, non-shielded cable used to be acceptable for 2300V.)

Motors will cost more at 4.16 kV. Control gear costs are about the same at 2300V or 4.16 kV.

For 500 HP motors, the savings in going from 600V to 4.16 kV will be in reduced copper. If your runs are short, there may be no savings. Where it has paid out for us is if we have long runs underground. The higher voltages save on wire, conduit and labor costs.

 

[dpc]

I agree that there is probably little reason to install a new 2400 V system these days. If you starting from scratch, I'd go with 4160 V and 4000 V motors.

This analysis is also going to depend a lot on how many "several" is. If you have ten 500 hp motors, that's quite a bit different than if you have three or four.

 

[MEJatNDL]

Thanks for the input folks.

In this application there are more than 15, 500 HP motors used for refrigeration. These are currently installed at 600 volts. However for future installation, particularily for motors operated over longer periods of time and some distance away, I was thinking about medium voltage. Any experience on this?

How does the cost of the motor starters/controllers compare? It seems to me that the smaller currents involved at 4160 V should offset the higher costs for insulation.

Regards

 

[rcw retired EE]

I checked an old pricing guide for 500 HP motors. 4.16 kV motors were listed at 15-20% higher price than the 480V. Chekc with motor suppliers for up-to-date costs.

 

[jraef]

Although the motor starters would be rated for less current, medium voltage switchgear is considerably more expensive to begin with because of the higher voltage involved. So from that standpoint I agree that 4160V would be slightly better than 2300V since the major cost jump is just to get to MV in the first place.

You also will need to decide on what level of protection you want. Whereas with LV gear you just have to decide on breakers or fuses as SCPDs, in MV it isn't as simple. Here is a grossly simplistic primmer on what you are in for:

MV Circuit Breakers are typically only available in "draw-out" format, called Metal Clad switchgear, where each controller has a separate cubical that can be withdrawn and isolated from the bus bars when it's necessary to work on the equipment. The individual metal enclosed compartments also serve to contain any fault energy to a limited area thus reducing the likelihood of collateral damage. Most people consider this to be the safest way to use MV equipment. But Metal Clad gear is extremely expensive and very large. A basic single starter, regardless of how small, is likely to start at US$80,000. the cost per unit goes down a bit if you have multiple starters in one location.

An alternative is to use Fused Load Interrupter Switch (FLIS) style motor starters. They are less expensive, typically under US$15,000 to start with for an Across-the-Line starter. The drawback to them is that they use fuses, which cannot be "reset" after a fault and require having spares on hand (a very old argument), plus they don't utilize a draw-out mechanism to provide the isolation; they rely upon an air gap switch. Some people do not consider this to be as safe as draw-out when dealing with MV equipment.

The lowest cost MV control gear is now a hybrid of sorts; Fused motor controllers that rely upon a vacuum contactor to interrupt a load, backed up by fuses as SCPDs, but with a switch that is NOT rated to interrupt power if opened under load, especially if it is a fault load. The switch is often on a truck assembly that can be racked out for service isolation however. The big drawback to that style is in the fact that IF someone were to panic and try to open the disconnect under load, it just might blow up on them (obviously I'm not a fan of that design).

So you really have a lot more homework to do before making that decision. Going to MV means making a leap of technology, not just deciding on what will be more efficient.

By the way, do you typically buy your own transformers or do you expect the utility to do that for you? If you are expecting the utility to supply MV to you at your terminals, check with them before deciding anything. Some utilities will only allow certain voltages in certain areas, usually because of what they have available as spares etc. for ongoing service.

 

[alehman]

Another factor to consider is availability of replacement parts and cost of spares. What is the time to repair or replace a medium voltage motor vs. low voltage. If you have spare 600V motors and would need to add a spare medium voltage motor, that needs to be considered as well.

 

[MEJatNDL]

Thanks again folks for all your help. I've just discovered this forum and find its like having an engineering department on hand at all times.

I think I can conclude that upgrading to medium voltage even for a number of drives may be a long payback. A new plant however or a substantial expansion project may be an opportunity to release some of the low voltage system's capacity and for those loads with the most use through out the year, a medium voltage substation, MCC and drive system may be considered.

Regards,

Mike

 

[jraef]

Good conclusion.