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Voltage Imbalance

Voltage Imbalance

Voltage Imbalance

(OP)
I've been having problems with voltage imbalance damaging motors at a remote facility where the utility power quality is very poor.  We have two 30hp 600V motors that we run on soft starts until they started tripping on voltage imbalance too often.  They were then bypassed, and the motors failed within a year or so.  Loads are centrifugal pumps and the actual load is approximately 25hp.

The utility logged the voltage imbalance at the facility for two weeks.  It ranges from 0.5% to 2.5% during a typical day.  Half of the days it spikes up to 3.5% (for a short duration).  Twice it reached almost 6% (again for a short duration).

I'm considering either using oversized (50-60hp) VFD's with the current 30hp motors (non-inverter duty rated),  40hp motors on 40hp soft starts with imbalance protection disabled, or 40hp motors on conventional starters (client would prefer this).

I've found this derating chart from a NEMA standard, but it only goes up to 5%.

http://www.engineeringtoolbox.com/electrical-motor-voltage-imbalance-d_648.html

Do you think installing a 40hp motor on conventional starters would be reasonable?

RE: Voltage Imbalance

With VFDs, the motor should be insulated, so to speak, from the line voltage imbalance, provided the VFD doesn't trip itself off.  

I don't think oversizing the motors is going to be a solution for that magnitude of voltage imbalance.  

Is any of this due to the plant load itself being imbalanced?

I'd be leaning a little more on the utility.  Unless the plant load itself is extremely unbalanced, the power coming from the utility does not appear to be meeting ANSI guidelines, assuming you are in ANSI-land.  

 

David Castor
www.cvoes.com

RE: Voltage Imbalance

(OP)
dpc,

I've used VFD's to solve this problem in the past, but the client has had problems with these in the past (surges? undersized?) and prefers a more simple and robust solution given the remote location.

The voltage imbalance is definitely coming from the utility.  It is a problem with other facilities in this area.  The utility is in northern Canada.  Although they've been helpful in diagnosing the problem, their power quality standards do not guarantee low voltage imbalance in remote areas.

Oversizing the motors for 5% imbalance would theoretically require a 33hp motor.  I'm thinking that a 40hp motor would be satisfactory for the occasional dip to 6%, although the NEMA chart may only go up to 5% for a reason.

RE: Voltage Imbalance

Any chance of working with the utility on this? If they are willing to float a large wye:delta transformer bank with the primary neutral connected, on the system near your plant, it will do a dramatic job of balancing voltages and phase angles. Secondary voltage on the delta is unimportant, Sizing takes a little art, or a lot of calculation.
In the old days of "U" frame motors we regularly started 250 HP motors across the line, two 250 HP motors started eight times a day across the line. If "U" frame motors are available they may be worth the extra cost. They are much more rugged than "T" frame motors.
 

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Voltage Imbalance

You can probably use the 5% derating factor (75%) as long as the periods of imbalance greater than 5% are small.  

NEMA MG-1 does say that operation of motors with greater than 5% imbalance is not recommended.  



 

David Castor
www.cvoes.com

RE: Voltage Imbalance

I suggest simple automatic voltage regultator with continously variable auto transformer (variac) having individual phase corrections. Simple. Robust. Not much of electronics (except for AVR's).  

Muthu
www.edison.co.in

RE: Voltage Imbalance

This is a now common occurrence in some areas of California as well. Residential encroachment into traditional farming areas leave the 3 phase irrigation pumps with wide swings of voltage imbalance, especially in the summer when residential AC systems come on and off during the day (when the pumps are needed most). Utilities can't adequately keep up with the rapid rate of change and variability in 1 phase load distribution, so voltage imbalances on the 3 phase systems result and it changes with regards to which phase is low throughout the day. Some farmers I've talked to about it are dealing with it by over sizing the motors as you planned, but some are then also using a Current Monitor to watch for current imbalances in excess of 30% for a period of time that would damage the motor, usually about 10 minutes. But you want that to auto-reset as well.


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