Power Factor

[Histor]

Hi, We have a motor control center panel with power meter mounted on it. The power meter reading shows that the power factor of Bus A is 0.5Lag and Bus B is 0.3Lead. How dangerous is this? any Why the this occurred? What's the solution?

 

[che12345]

Mr. Histor (Electrical)(OP)28 Jul 23 21:33
"... We have a motor control center panel with power meter mounted on it. The power meter reading shows that the power factor of Bus A is 0.5Lag and Bus B is 0.3Lead. #1. How dangerous is this? any #2. why the this occurred? #3.What's the solution?"
I look at it as following for your consideration:
1. There is no "danger" , but consider the following:
a) For Bus A 0.5 lag. If you are billed by the Power utility for Pf below their requirement, improve it IMMEDIATELY ; as the charges are usually very high! . If not, forget it.
b) For Bus B 0.3 lead. Check whether are there any Pf improvement capacitor or others that result to leading. Switch them off. Check that CT connection, could be wrong polarity that result to leading. With leading Pf, the system voltage will rise with increase load, which is NOT favourable !.
2. a) For Bus A, many motors running idle or running at say half of the rated kW. Switch off the idle motors. Replace those motors running < half the kW rating by a smaller motor ASAP . Improve it by installing capacitor bank.
b) Switch off the capacitors or other devices that generating leading PF. Check CT polarity.
3. See above 2. a) and b).
Che Kuan Yau (Singapore)

 

[waross]

You have given no indication of the load in either KVA or in kW.
You may be having issues,
You may be wasting money,
or,
Everything may be rosy and your plant power-factor may be 100% with no penalties.
I can visualize a plant with a lot of lightly loaded motors on one bus, running at a low power factor.
There are a lot of cases where a motor must be running and must handle peak loads from time to time, but that often runs lightly loaded.
I can also visualize an oversized synchronous motor used as a synchronous condensor to correct the plant power factor.
Such a motor would be instrumented to the incoming mains rather than to the local bus.
You may have a power factor correction unit on bus B that is instrumented to the incoming mains.

You haven't given enough information to determine if there is a problem or not.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[waross]

A 1000 kW load at 80% PF may cost more in penalties than a 10 kW load at 10% PF.

We have no idea what PF you are being billed at.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[jraef]

Unless you have a bunch of capacitors connected to bus B, a leading PF is likely the result of an incorrectly installed CT on bus B.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[che12345]

@ Mr. waross (Electrical)29 Jul 23 05:57
"....You have given no indication of the load in either KVA or in kW...You may be having issues,.....You may be wasting money,.....or, Everything may be rosy and your plant power-factor may be 100% with no penalties...."
1. With due respect to your learned advice.
Please advise what kW of the loading has to do with the Pf ? . The Pf has nothing to do with the kW, e.g....the system/motor may be loaded 100kW at 0.5 lag while another motor loaded to same 100kW at 0.9 lag; but NOT lead.
2. In many locations, especially LV, the utility charge/billing is based on kWh only; irrespective of the Pf. But MV/HV , the utility charge/billing is often based on kWh and imposes very heavy penalty on lower than their Pf requirement. In this case immediate action should be taken. In many cases the investment can be recovered within a year or shorter, which would be a wise investment.
Che Kuan Yau (Singapore)

 

[waross]

I guess you have never been to North America,
Demand billing is very common here.
Power factor penalties are very common here.
Commercial and industrial, anything over 120/240 Volts will probably be billed on kW, KVA demand, and with PF penaltis.
For a given motor, the PF will vary as the loading (kW) varies.
PF penalties are typically a percentage of the kW billing, so as well as being related to kW loading, PF penalties are related to kW charges.
I have worked in several plants where capacitors were used to over-correct the PF of the larger motors, and the leading VARs used to offset the PF of smaller motors.
It becomes very expensive to try to correct the PF of a large number of smaller motors.
Much more economical to over-correct a few large motors so that the overall PF is good.
PF control panels are becoming more and more common as the methods of calculating PF penalties become more sophisticated.
The bus or MCC to which the PF correction is connected will typically run at a leading PF.
You can't make assumptions about plant PF based on the PF of individual MCCs unless you have kW or KVA information to evaluate the overall plant PF.

What does kW have to do with PF?
Anecdote.
A small machine shop came to me asking if I could do anything about his PF penalty.
He had two quotes to correct the PF: $1000 and $2000. The second guy was hoping that he hadn't contacted the first guy.
He probably would have subbed the job out to the first guy.
Penalty on the bill looked bad, but how much?
The actual dollar value was almost insignificant.
I connected a motor running capacitor across one phase and that was enough VARs to correct the PF and get rid of the penalties.
Beware of knee jerk reactions until you have the whole picture.
By the way, that was a 100 Amp, 120/240 Volt delta service with demand and PF monitoring.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[waross]

When I was designing lumber kilns, PF billing in a lot of places was based on monthly averages. That is monthly kKWHrs and monthly KVARHrs were used to calculate an average monthly pF.
The fans in a lumber kiln typically run 24/ 7 while the mill will run 10 x 8 hour shifts.
I would over-correct the PF of the kiln fans from 0.8 lag to 0.8 lead.
During the hours that the mill was not operating the kiln fans would be adding KVARHrs to improve the monthly average.
The kiln feeder always ran at a leading PF.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[waross]

Histor, the PF of individual MCCs is fairly meaning less.
You need only be concerned with the PF at the meters on the incoming supply.
If you can post a copy of a power bill, I can help you determine the overall PF of your plant.
Revenue bills seldom show PF directly, but show the values that are used to calculate the PF.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[edison123]

Under PF compensation in bus A and over compensation on bus B. Redistribute your PF correction caps. Or use an automatic PF control, which are dime a dozen.

Running with poor pf = poor efficiency of the system and most utilities penalize heavily for poor PF (lag or lead).

0.95 to 0.99 lag is the recommended range.

Muthu

http://www.edison.co.in

 

[waross]

Fellas, stop and think about it. As long as the conductors are adequate for the current, the PF of individual circuits is what it is.
Changing things around will not change the PF at the meters.
All that matters is the PF at the billing meters.
It is common for smaller motors to be grouped together in an MCC.
It is not unusual for one MCC to be populated with all small motors, and larger motors and PF correction to be fed from a second MCC.

Think about this:
I have a motor with capacitors, the motor PF is 0.5 lagging and the capacitor PF is 0.08 leading.
What does this tell me?
Well the motor is probably lightly loaded.
The capacitor bank is probably quite small for the discharge resistors to make a noticeable change from 0.0 leading.
I don't know anything else.
I don't know if the motor is properly PF corrected or not.
I don't know if the motor will be incurring any penalties or not.
I can only guess that the capacitor bank may be small (PF not reading as 0.0) and with a small capacitor bank, the motor may be small.
The first guess (the size of the capacitor bank) may be a WAG (Wild Assed Guess).
The second guess is a WAG based on the first WAG, WAG²
THE INFORMATION GIVEN TELLS US NOTHING USEFUL.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!