Poor Power Factor Effects 2

[Matador]

I am a mechanical engineer with an electrical problem. The power calculation using amperage, voltage, motor power factor and efficiency when combined with pressure readings on the pump performance curve gives a flow whiich is 20% higher (5000 GPM) than other measurements indicate.

The overall power factor for the plant site is corrected to .99 using a couple of large 18,000 Hp motors designed for the purpose but the power factor may not be .99 where the 700 Hp motor is connected. In fact it could be as low as 0.80

Can my horsepower calculation be the apparent power rather than the real power which actually used by the pump? I measure 96 amps, use 4160 Volts and interpolate for motor pf & eff from the test report.

I'm using the formula hp=1.73*E*I*pf*eff/746. The motor test report indicated that at full load the values are E=4000V, I=100.7 amps, Eff=94.8, Pf=78.9, Syn speed=900 rpm, Slip @ fL=1.43.

How would a supply power factor of 0.80 affect the Hp calculation. It isn't accounted for anywhere?

Any help would be appreciated.

 

[redtrumpet]

Your meter readings appear out of whack. Are the phase sequences the same and the phases matched for the current and voltage transformers used with the meter? For example, is your phase vector rotation A-B-C for both the PTs and CTs, and is the A phase CT connected to the same phase as the A phase PT? It's easier to screw this up than it sounds. I had to throw away a week of metering data from a client because the site engineer hadn't verified the metering instrument transformer connections, which were wrong. In that case, the power factor read around .66 when it should have been around .9.

If the readings are correct, then you are operating at approximately 8% above rated voltage (average) with a 13% voltage unbalance. The overvoltage explains some of the drop in power factor, although the drop from .78 to .45 seems extreme, even considering one phase is 20.5% above rated. This overvoltage should also increase the slip by a noticeable amount - this can be verified by measuring shaft speed.

The 13% voltage unbalance, if correct, is mind-boggling on an MV system. Motor operation above 5% unbalance is definitely not recommended. However, I doubt this reading is correct because there should be a much greater current unbalance, whereas the measured phase currents are fairly balanced. A voltage unbalance of the level indicated by the measurements would cook this motor in short order under full load.

Check your meter and connections and post what you find.

 

[electricpete]

To keep it simple, start with the voltage and see if you have measured it correctly.

IF the reading are correct, then as redtrumpet said, the voltage unbalance and overvoltage is totally unacceptable. It would certainly cause a dramatic reduction in efficiency and power factor which could explain your initial symptoms. More importantly, your motor(s) are in grave danger of overheating and damage. For unbalanced voltage conditions, simple phase overcurrent protection does not protect the motor. (The effects of negative sequence currents on the rotor are much more severe than would be predicted by the magnitude of the current in any one phase.) The overvoltage condition cause extra heating in the core and also increases the stress on the insulation.

In summary, IF it's an actual condition it needs to be fixed pronto. IF not, then perhaps correcting the voltage connections will bring the other readings into line.

 

[jbartos]

Suggestion: As already mentioned in above postings, the osciloscope screening of voltage and current waveforms, phase shifts, and magnitudes should give a good and efficient start for further troubleshooting. It may also be a combination of several discrepancies, electrical and mechanical, which happen to add up.

 

[don01]

just a dumb idea or two.
With those sorts of numbers has the protection gear complained???
Are we sure we don't have some US gallons fighting imperial gallons??
If we use an accuratish meter test the terminals test the current with a tong do they verify (within a small order of accuracy) the super test rig?
I understand your confidence in your mass balance (and draw ups?) But we Know x amps y volts are going in. except for pf they have to go out and the pump is the place.
How is the mass balance calculated could there be the use of say dp flow meters or zero suppressed dp level meters. Are youre slurry lines purged? I don't mean to sound negative but I have a hunch that this is not just the motor. And I mean just a hunch.

Please keep us informed this IS really interesting
Don

 

[don01]

back again
Its nearly midnight here and I did it again. Tongs on a 4000v conductor - sorry.

New thought reading the early posts again. There is a 700 hp motor and a 900hp motor with same pump service. Yes the pump will only do the work it wants to do but the two motors ARE going to have different numbers. The pf of the 900 should fall.

Now we have pf correction upstream of the motors and a number of transformers in between. My factory has a pf of.98 ~.98 at 415V (1000amp) using a cap bank and controller. this fed from the util transformer 20 meters away.
BUT I'll bet that the power factor in the street upstream of that trani aint .98

I could be wrong but its a thought
regards Don


Ps I have never had any luck getting an accurate indication of amps onto a DCS. Always the transducer or ct or calibration or somebloody thing aint quite right.
This is in petro chem and slurry type system with some pretty expensive gear.

 

[Matador]

We tried the meter on the 900 Hp motor and the same voltage unbalance showed up. The power factors were also very low.

I called the company where we rented the meter to have them check the calibration when they get it back in the office. The plan is to rent another meter and take another set of measurements. If the voltage unbalance really exists we have some serious problems.

I'll update things again when I have more information from the new test data.


Thanks everyone for your input.

 

[electricpete]

Thanks for keeping us posted Matador. It sounds like you are on the right track to pursue the voltage imbalance. You really don't need any sophisticated equipment to do this... almost any multimeter will have the capabilitiy to read the nominal voltage magnitude of 115 or 120vac you'll see on the secondary side of your potential transformers. And even if it hasn't been calibrated, it should be sufficient to check the balance because the same error would normally show up on all three phases. (unless you suspect a repeatability error, which can be ruled out thru repeated measurements). A few measurements with the multimeter should be enough to confirm/prove an actual imbalance (although you might want to more sophisticated equipment capable of measuring relative phases when it comes to troubleshooting).

There are a few other things interesting about this problem. Hopefully I won't detract from the straightforward plan identified above if I go off on a few tangents.

I've never worked on a motor that had current/voltage imbalance before and I havent read a whole lot about it.

One question that is often considered is whether a system voltage balance is causing the motor to draw unbalanced currents (supply problem), or whether imbalanced motor currents are creating a voltage imbalance (motor problem).

In your case the voltage imbalance magnitude far exceeds any current imbalance, which seems to suggest a system problem vs motor problem. Likewise you see similar voltage imbalance regadless of which motor is running... once again points to supply problem instead of motor problem.

As Redtrumpet noted, it seems surprising that a 13% voltage imbalance gives rise to only 1.5% current imbalance. I was always under the impression that the current imbalance should be several times as large of the voltage imbalance. I base this on the fact that motor negative sequence impedance is often assumed equal to starting impedance (for fault calculations... per IEEE Brown book) which is 5-7 times lower that positive sequence impedance. This would seem to imply the current imbalance should be perhaps 6 times as much as the voltage imbalance. You've got it going the other way... current imbalance on the order of 1/10 of voltage imbalance.

I'm assuming that it's easy to make errors in measuring phase and absolute magnitude, but it's very unlikely to make errors that change the computed value of voltage or current imbalance... (as discussed above this is simple comparison of three magnitudes measured in similar manner with presumed similar errors).

I'm sure that there may be a large component of the current which is excitation current due to the overvoltage. That contributes a component of current that becomes larger as voltage increases.

Also interesting to see that the lowest current occurs at the terminal with highest voltage (to ground).

Does anyone know in general whether the phase with highest voltage sees the highest current when problem is caused by supply voltage imbalance? (I do believe that the phase with the highest voltage has the lowest current when problem is due to motor... but in that case no reason for voltage to ever increase above nominal).

All of this is to say that the motor current magnitudes dont' seem to fit the simple explanation of a supply system voltage imbalance (can anyone explain them?)... although I'll admit I've never seen a supply imbalance and don't know exactly what it should do to the currents. Nevertheless the imbalanced supply system voltage measurements clearly need to be first line of investigation. Just try to keep an open mind along the way.

 

[jbartos]

Suggestion: Lighting and receptacle 3-phase panels are very often checked for the proper load balance to implement a tolerable voltage imbalance.
The medium voltage and attached larger loads do not have this problem so frequent since the large three phase loads are manufactured to be fairly accurately balanced in load to avoid very costly consequences.