Utility Voltage Unbalance

[rick hart]

I need some help sorting through this:
Four 100HP constant volume condensing water pump motors are tripping overloads. Motors are getting hot and trip in the hottest part of the day and randomly at night. Not good to lose AC in a Texas summer. Measurements at the motor show average current above the namplate and a voltage unbalance of approx 1% at each motor and a current unbalance of approx 8-12% on C phase. This voltage unbalance can be traced back to the service entrance. So far so good, I found the problem with the overheating and overload. Not!

The utility says they are allowed 3% UNLOADED at the meter. We are loaded to over 4.5 MW so I don't know what to make of this. The phase angle is good and right where it should be ( 0, 120, 240) but, the magnatude of A-B deviatrion from average is right at 1%, give or take a tenth depending on the time of day.

Most of what I can read about the issue says 1% is all NEMA will go for at the motor and the only thing I can find that says otherwise is a single IEEE whitepaper stating that, "Oh, 2-3% is not that big a deal" and a buch of calculus to demonstrate the opinion. The majority of resources says more than 1% reduces effeciency and heats the motor, including our Department of Energy. The motors are hot and overloaded -1% sounds like all I want to accept.

I know there are quite a few utility guys out there. What voltage unbalance are you allowed to get away with?
Also, what can I look for on my side that might be the cause? My current is about 4 amps out per phase max on a 222A average demand (13.2kV service) on two separate feeders, one has problems the other does not.
Thanks
Rick

 

[mpross]

Motor Problems?

Motor Problems?

I faintly recall a problem that we encountered with a motor, and your case seems quite similar.

We tested the windings of the motor, and other various items. We finally found a set of loose lugs in the freq. drive that were causing a high current condition on a couple of phases.

This would be the only idea that I have.

-Matt

 

[charlie tuna]

rick,
here in south florida the utility spec is + or - 10 per cent............. thats a big variance!!! on a 480 volt system it could be as high as 528 volts or as low as 432 volts. texas is like most of the rest of the country -- experiancing un-normal weather conditions and your a/c system is running at peak performance standards for longer than normal periods of time. when your system is at this peak -- even slight design issues can come into play. are all the pumps tripping? maybe the piping layout is allowing one or more of the pumps to pump more water. to really know, and stop guessing at what might or might not be happening -- you need to monitor the four pumps with data loggers for a period of "trips" --- then download the information and know whats happening.................. you will need this if you attempt to prove the utility is the cause -- they would never admit to anything!!! first thing i would do is make sure the correct and maximum size overloads are installed. then look into the data loggers --- find a contractor who has them or rent them..................... tuna

 

[rick hart]

Below are the measurements taken I refered to in the earlier post. The columns are for the individual pumps. Yes, Pump2, B current fluctuated on two different clamp-ons to the extereme. I think this motor is hosed...
Measurements were taken on the load side of motor starter. >4% VTHD (mostly 5th harmonic) was also measured, for what that may be worth.


total number of trip 31 66 51 40

AV- 277V nominal 261.8 261.8 262 262
BV- 277V nominal 261.5 261.6 261.6 262.3
CV- 277V nominal 265 265.1 265.1 265
V avg 262.7 262.8 262.9 263.2
Max V deviation 3.5 3.5 3.5 3.4
% voltage unbalance 1.3 1.33 1.33 1.29
AI (120.5 FLA) 120.2 96.4 125 111
BI (120.5 FLA) 127.1 109-128 132 120
CI (120.5 FLA) 131 114 134 121
average current 126.1 109.4 130.3 117.33
max current deviation 10.8A 17.6A 9A 10A
% current imbalance 8.5600 16.08 6.90 8.52
( I have attached a DOC. in case this table doesn't make sense after posting)

Charlie,
I'm not talking about undervoltage across all phases; I'm talking about ONE phase voltage different than the other two by 1%, regardless of what the average voltage is. Plus or minus 5% nominal, I'm OK with. As you can see, the average voltage is actually pretty good for heat of summer. That C phase voltage is what I am concerned about.

Pointy-head Engineer alert!
After a closer look at the phase angles, I came across something else I don't understand:

A Phase angle is 0 _ B phase is 120.2_ C phase is 239.3
add them together and I would think they should add up to 360 degree, Right?
359.5 is the sum I'm getting.

Is there anyone out there willing to educate me on where I am going wrong?

 

[ramsy]

That *.doc attachment is a hopeless cause. There is no way Mikeholt.com lets me access this, no matter how many times I re-log on. Next time just surround your table in {code}table stuff {/code}" markup so we can read it, and of course replace "{}" with "[]"

Your C phase variation may also result from different impedences(Z) between phases, or additional loads on C phase, not related to the motor.

If you can't schedule a timely motor shut down, try another way to reveal if other loads are sharing the same C-phase feeder, inside the plant wiring. A different voltage drop in C phase could result from loading that phase unevenly while the motors are running.

To field check Z wiith a Volt Meter, Z=(Et-Er)/I, a shut down of each motor / phase in question is needed for the no-load portion of that formula (Et). If (I) fluctuates so does (Er) running volts, since Z remains constant as load varies. So, if you can pin down (Er) and (I) at the same moment, you'll have an accurate Z for each phase.

If this test proves impedance and loads are identical between motor phases, on the inside wiring, I believe evidence for a local PoCo xfrmr variation would be stronger.

 

[ramsy]

If the voltage variation is not responsible for all the motor current variation, I would fix the harmonics, isolate the neutral, or filter it, since harmoncs can create heating problems for motors. Harmonics also increases voltage drop, usually with line to neutral loads.

 

[Smart $]

My first thought is... what are the leg to leg voltages? Unless these motors are 3?, 4W (not likely in my experience), they could give a rat's patootey about leg to ground voltages. Leg to ground voltages serve only as a reference in locating the problem(s), nothing else.



Umm... Roger,

If the voltage imbalance is due to other loads contributing to voltage drop, it appears the loads would be on A and B phases, not C.

 

[charlie tuna]

a&m,
the above post is right on the money! the motors only see phase to phase voltage, in this case 480 volts nominal. individual phase voltage difference varies with single phase loading within the system. a 100 hp motor -- three phase --- isn't effected by such variations. where are these harmonics you mentioned coming from?? i have never heard of harmonics effecting the operation of a 100 hp - three phase motor. have you checked the overload sizes in the motor controllers. it might be one size off????

 

[mpross]

Symmetrical Components

Symmetrical Components

Pointy-head Engineer alert!
After a closer look at the phase angles, I came across something else I don't understand:

A Phase angle is 0 _ B phase is 120.2_ C phase is 239.3
add them together and I would think they should add up to 360 degree, Right?
359.5 is the sum I'm getting.

Is there anyone out there willing to educate me on where I am going wrong?

The phase angles will not always sum to zero, as in the case of an unbalanced load. There are ways of deducing "symmetrical components" by way of matrices. I think that topic has been discussed on this forum in the past month or so... ...maybe search that.

For a person in the field, these componenents are of no use. It has to do with analyzing a 3ph circuit as a balanced 1ph circuit, and is used often in the area of fault analysis.

-Matt

 

[hillbilly]

Are you sure that the motors are not simply overloaded? If these pumps are centrifugal (which I assume they are), any reduction in back pressure on the pump discharge (head) will result in increased flow and higher running amps on the motor. Has anyone adjusted any valves in the system to try and increase cooling output?
Do you have (or can you get) the pump curves for the pumps? They will show the HP required for the pumps when operating at your existing speed and system back (head) pressure. You may have to interpolate some values.
I'm not recommending it because I don't know the design specifics of your system or how critical it is...but...closing the discharge valve in small increments (on centrifugal pumps) will increase the head pressure and result in lower liquid flow and motor amperage. I have in the past monitored the motor amperage on centrifugal pumps while at the same time restricting the flow (discharge only) until the motor current fell to at or below the motor plate rating.
Do you have a engineer that you can consult on the piping system?
Are these pumps in parallel (common discharge)?, series (boost)? or individual pumping systems?
I'm not that familiar with chilled water cooling, but I've got a lot of experience in pumping systems.
I would evaluate the system as a whole, and not assume that the electrical service is at fault.
Hope this helps
steve

 

[rick hart]

First the easy stuff:
Overloads- the overloads installed are the largest available for a NEMA4 starter, onesize larger than recommended by SqD for shared ambient temp.

Motor overloaded- not likely since these are cooling towers that take as much water as they can get. They work independently of each other in separate systems. BUT, I will check to see if there is a restriction in all 4 lines or if the controls guys have monkeyed with a valve. The overload current seems too close together to be a mechanical origin but, I'll try anything to keep them running- 107F yesterday.
I have tried to find a cause other than electrical but keep coming back to the unbalance on C phase.

Line to line measurement from PowerLogic- A to B 468, B to C 468, C to A 463. Average 467
Largest V deviation 5V
5/466.3*100= 1.07% voltage unbalance over NEMA MG-12003, rev1 2004 recomendations (but not the utilities 3% unloaded mark) and consistant with the line to neutral measurements taken with handheld meters.

1% unbalance results in an 8 to 1 increase in curent drawn on the phase (C to A) out of balance the difference is heating of the motor.
So, I'm fairly certain that the problem is electrical but not nearly smart enough to know what to do about it.
I still would like to hear from a utility engineer about what they are allowed to provide unbalanced to thier customers.

I do appreciate all the comments so far, Thanks guys.

 

[Smart $]

With the information provided I'm inclined to believe the system is of borderline design, at least for this particular application of it (location, location, location... ohh! that's something else ). It doesn't appear to be any one factor contributing to the trip condition, but rather all in concert.

Overloads- the overloads installed are the largest available for a NEMA4 starter, onesize larger than recommended by SqD for shared ambient temp.

This statement gives me the impression all 4 starters/overloads share the same enclosure? Is this enclosure outside? Is it also exposed to direct sunlight during the hottest portion of the day? Is it ventilated in any manner? Can you measure the internal ambient temperature of the enclosure (enclosure closed, motors operating, hottest point of the day)? Even if separate enclosures, same follow up questions apply. Indirect heating of the overloads ambient temperature can be contributing to "nuisance" tripping. If you feel this is likely, do a simple experiment of leaving the enclosure(s) open while in operation (provided an adequate safety margin can be maintained). If this proves to be the case, consider an internal fan (perhaps thernostatically controlled).

Move on to the next most likely contributing factor, or the one having the easiest corrective measure.

 

[ramsy]

Agreed. The Zero sequence 5th harmonics reported would not be seen in line-to-line loads, and any related current increase or voltage-drops would be confined to the neutral.

However, motor control wiring, relays, detectors, and other ground-voltage-controlled components, regulating the motor, could be sensitive to neutral related harmonics.

 

[rick hart]

Gig 'em

Gig 'em

If 5th order harmonics can't be seen, how am I measuring them at the motor??? Not that I think this is my problem....

We did try and restrict the flow down where the current is below nameplate on at least two phases. This has kept them running overnight but the chiller guys are freaking out over the reduced flow. High and low side pressures are normal; condensing water is coming back 80 degree. So Far, so good.

Presently the readings are:
AI 116.4
BI 118.3
CI 125.7
AV 468
BV 468
CV 463
All 4 motors are reading pretty much the same.

The combination controllers are separate enclosures NEMA 3R, outside right next to the pumps- one controller for each motor.
Encosure inside temperatures are 102-104 outside air temp is 97. Motor temp is 197. The starters are in the shade right now but that will change around lunchtime.

Borderline design is right (government low bid) but it is a 12 year borderline that became an issue this summer.

I went back to see exactly where the voltage unbalance first appeared and there was a severe uptick (0.6 one day and 1.0 the next and holding) since June 6-right about the time we first started resetting overloads. From January til mid May we were at 0.3%.
Computers, you gotta love 'em!!

 

[Bob NH]

"Motor overloaded- not likely since these are cooling towers that take as much water as they can get. They work independently of each other in separate systems. BUT, I will check to see if there is a restriction in all 4 lines or if the controls guys have monkeyed with a valve. The overload current seems too close together to be a mechanical origin but, I'll try anything to keep them running- 107F yesterday."

There was an earlier post about overloading the motors.

Your comment "Motor overloaded- not likely since these are cooling towers that take as much water as they can get." suggests that maybe you overlooked the point about overloading. TOO MUCH flow causes an overload to the driver (motor) of a centrifugal pump. A restriction in the line will not overload the motor; it will have the opposite effect. NO RESTRICTION or LESS RESTRICTION THAN DESIGNED will overload the motor.

Pump systems are typically designed with some flow and pressure margin. Pipes are designed with a little extra capacity. Put those together and you have a basis for overloading the motor. The system is usually taken care of with a control valve, perhaps operating on water temperature control or condenser pressure. If the A/C is having trouble condensing, it will say "give me more cooling water". Opening that control all the way can overload the pump and motor.

Someone needs to find the characteristics of the pump and see if there is an operating point that exceeds the power capability of the motor.

If there is a pressure gauge at the discharge of the pump, then a lower pressure than usual is a sign that the pump/motor may be overloaded because lower pressure corresponds to higher flow.

 

[ramsy]

If 5th order harmonics can't be seen, how am I measuring them at the motor??? Not that I think this is my problem....

Oops not zero sequence, it looks like 5th order harmonics are negative sequence. These are found in phases, but are expected to cancel out with the positive sequences harmonics.

The negative sequenced harmonics are the 5th, 11th, 17th, etc. They present a force that opposes the motor rotation and tries to make the motor rotate in the opposite direction. The force of these harmonics acting upon each other creates heat which leads to premature failure. Google temp. cache of complete link Google flushes its cache periodically.

I thought motor control systems were more sensitive to harmonics than the motor itself, but then again I also thought motor harmonics on phases only occured with variable speed drives VSD's. So much for what I thought I know.

I would demand more evidence to verify this 5th order harmonic as cause for failure. The above link is the only data I kept in reference to this issue. 2nd opinions, on the effects of predominant negative sequences on motor phases, may clarify the source of the above reference's conflict of interest, --motive to sell power quality services--.

 

[rick hart]

All's well that ends well

All's well that ends well

Bob NH,
I knew that the pumps could be overloaded with too much flow but that was pretty confusing the way I wrote that,huh?

What I should have said is the flow was properly set this last Spring, before the problems cropped up, and that was why overloading was unlikely. Talking about the flow here did give me a way to work around the immediate problem though.

We backed the flow down until two phases were below namplate; that kept the towers pumps working, even with one phase above motor nameplate current. The chiller guys were a little anxious about starving the condenser water but no harm done....

The voltage unbalance has since been corrected. It turns out that the utility had an air switch burn out opn our feeder. They had us redirected to a different substation farther away, unbeknownst to the desk-bound utility engineer I was having to deal with. After the switch was repaired, the utility had gotten busy with other jobs and left us re-directed; it has been hot around here. The field guy saw the e-mail string to our service rep and engineer and followed up to make sure the redirect was put back; it was not. After they put us back where we belonged, the voltage stabilzed, we put the water flow back to where it should be and the load is fine.

Just goes to show you, sometimes you need a pointy head engineer and somtimes you need a guy with a worn hard hat.

 

[robbietan]

I know of utilities not having a phase loss detection, as it would be too expensive and would compromise the supply of single phase customers.

better have a phase loss detector installed wherever you operate large three phase motors...

 

[charlie tuna]

when you reduced the load to below "nameplate rating" by reducing the flow tells me you are trying to get something out of the motor that it isn't designed to produce. i see utility voltage readings that are out of balance all the time -- eight volts is nothing!!! you have the mechanical guys doing anything to keep their chillers "on line" during a very hot time of the year and maybe the chillers aren't producing efficently as they should because of their age,etc.. a pump motor is going to turn(or try to turn) a basic speed that is a function of the utility's frequency. the motor is designed by the size of the pump. the pump is designed for the pressure of the system and the gallons of fluid it must move. if you install a 2hp motor on a 5hp rated pump it will work if you control the amount of work the motor can do by restricting the number of gallons the pump moves -- this can be done by an orfice in the line. the orfice is set. sometimes it's done with a valve -- which of course is not set". let those mechanical guy start messing with that valve and the overload problem arises. oh ya, the chillers are running because the extra flow is there --- but they are running the pump motors beyound their design "nameplate rating"........... and many times this can be done successfully --- sounds in this case you just are too tight in design "overage" to be successful..................... my $.02 tuna

 

[beanland]

Voltage Unbalance

Voltage Unbalance

There are motor derating curves for voltage unbalance. 3% causes a 90% derating. 4.5% causes an 80% derating factor. If voltage is low, current increases, causing added heat. 5% low voltage causes 5% increase in current or a 10% increase in stator heating.