What is the disadvantage of running a motor with a leading power factor? I've heard that in a sense you are giving reactive power back to the utility company, but wouldn't one advantage be that a leading power factor would try to improve the overall power factor of your facility / plant? In our case, this is an 8500 hp motor that's fed with 13.8kV. What if we ran this motor at a leading power factor? We don't get charged for poor power factor anyway.
Power Factor - Leading / Lagging
- Thread starter adamscb
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Some motors were. Synchronous condensers.
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- San Francisco Bay Area, CA, USA
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- Electrical Engineer
It's done all the time, it's a valid form of Power Factor Correction to run a synchronous motor on a large continuously running motor at a leading power factor in order to correct the PF of other inductive loads in a facility, referred to as being a "synchronous condenser". A very common occurrence of that is when a facility uses a lot of compressed air, the large air compressors will have Synchronous motors on them so that they can do this, because as long as the plant is running, it needs air and the compressors are going to be running, so they take advantage of that to correct their PF. I have even seen a few facilities that will run a synchronous motor as a synchronous condenser even though there is no load connected to the motor at all.
If you are charged a PF penalty, that penalty may be in effect whether the PF is leading or lagging, it often is. So over correcting is just as detrimental as not correcting. Even if you are not charged a penalty, correcting to .95 lagging or better can have other benefits, especially if your facility owns its transformers. But if not, there may be no benefit at all. Regardless of that, more than .95 leading bears a host of other risks and should be avoided.
The reason I'm asking is because there are power factor correcting capacitors on this motor, and one of them went bad. To get capacitors of the same value will take 7-8 weeks, but there are capacitors that are slightly larger, that can be at the plant in less than a week. I'm worrying that these new larger caps could over-correct, and thus cause a leading power factor. I've ran the numbers and it seems like they should work, but nevertheless I'm getting a second opinion.
Motor info: 8500hp, 13.8kV, FLA of 304A
used single-phase capacitors rated at 7960v, 375 kvar (x3)
new capacitors are 400 kvar at 8320v (x3)
motor is rated at 13.8kV, but it usually runs at 14.2kV
motor power factor is .91 lag at a point in which it is loaded more than usual (so this is "worst" case in my mind)
I ran the numbers (drew the power triangle and everything) and it seems like larger kvar value won't cause much of a change (because motor is so big), but like I said I'm getting a second opinion
Motor info: 8500hp, 13.8kV, FLA of 304A
used single-phase capacitors rated at 7960v, 375 kvar (x3)
new capacitors are 400 kvar at 8320v (x3)
motor is rated at 13.8kV, but it usually runs at 14.2kV
motor power factor is .91 lag at a point in which it is loaded more than usual (so this is "worst" case in my mind)
I ran the numbers (drew the power triangle and everything) and it seems like larger kvar value won't cause much of a change (because motor is so big), but like I said I'm getting a second opinion
I got 96 pf lag with the 375 kvar caps, and like 96.67 lag with the 400 kvar caps. I didn't do any per unit calculations, just used the power triangle
Very similar to what I got. I believe the first set of capacitors went bad (the 375 kvar caps) because they were being over-volted for years (designed for 7960v phase-to-ground, but they were receiving 14.2kV divided by sqrt(3))
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Yes, you should be fine from a power factor standpoint, but you may suffer from the "law of unintended consequences" by creating a new problem with switching in the larger caps.
By the way, your 14.2kV was the likely cause of the demise of those caps. The caps should be rated for the phase voltage (assuming the are Wye connected, which is likely). 13.8/1.732 = 7967V, which is within tolerance of your old caps. But by running at 14.2kV, the caps are seeing almost 8200V, which is stressing them, making the 8320V caps actually a good move in that aspect. So what's the 6 weeks of extra time worth to you? if it were me, I'd put these in, but order the correct ones, however NOT what was taken out! Order 375kVAR at 8320V and put them it at a more convenient time.
Agree about the over voltage decreasing service life
the 375-400 pf 0.96 to 0.965
I would leave the 400's in and save the $$$
I agree. It's a unique situation at our plant. Whoever bought this big 8500hp motor really screwed up, because our system voltage is 13.8kV. The motor should have been rated for 13.2kV, instead of 13.8kV. So what ends up happening is that the transformer that feeds this motor has an automatic tap changer, thus raising the bus voltage to 14.2kV
Bugman1400
Senior Member
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- Charlotte, NC
I didn't see anywhere in this thread where the higher voltage caps were de-rated for the lower operating voltage. It appears that the 400kVar caps will be operate at 366kVar which I think are a lower rating than the original. Also, most caps can operate until 1.1 pu (7960 x 1.1 = 8756) without an issue before some form of OV protection should take the caps out. I didn't see how old the caps were but, caps can fail for a variety of reasons. If you feel that the caps are failing prematurely, you may need to monitor the voltage with a datalogger. Many times spike on the system can poke holes or degrade the cap film. Some caps need to be switched in/out via reactor or resistor in order to reduce the typical switching transients. I think large, nearby motors may compound this issue.
Julius Right
Senior Member
- Occupation
- Electrical Engineer Power Station Physical Design Retired
The capacitors on electric motor are used to mitigate transitory overvoltage-mainly if vacuum circuit breaker is employed. The synchronous generator cannot be instead a capacitor for this job-in my opinion. See[for instance]:
file:///C:/Users/user/Downloads/2074-2503-1-SM.pdf
file:///C:/Users/user/Downloads/2074-2503-1-SM.pdf
- Location
- Placerville, CA, USA
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- Retired PV System Designer
A URL that points to a file on your local hard drive is not going to do the Forum members any good.
Either the whole file gets downloaded as an attachment (and only a limited number of file types are allowed) or you upload the file to a hosting site that others can reach (such as DropBox).
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Can't see your file, but it appears you are referring to surge capacitors. That's a different application than power factor correction capacitors.
In my experience, harmonics are quite often a contributing factor in PFC failures.
In one paper mill every element in a bank of automatic PFC units had failed. Paper machines use variable speed drives which have a high harmonic content. This was not taken into account and no mitigation measures were in place.
Julius Right
Senior Member
- Occupation
- Electrical Engineer Power Station Physical Design Retired
There are some disadvantages of Windows 10, one of them is this : it downloads the file without ask permission.
However, in order to copy link address it is another way. Sorry. This has to be the actual link:
http://cscanada.net/index.php/est/article/download/j.est.1923847920110202.121/2090
By the way I think the capacitor connected with synchronous motor job it is not to fix the power factor.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
I think you might be reading this wrong. The synchronous motor aspect was just a response to his first question as an example of how people legitimately run leading PF in one motor to correct for lagging in other, made in the absence of initial information. The OP never said it was synchronous. After our examples, he then added the issue of this being about larger PFC caps being added out of expediency. At that point I for one no longer entertained that he was using a synchronous motor because there would be no reason for there to have been PFC caps of it was. I have been under the impression that this is a large induction motor.
Bugman1400
Senior Member
- Location
- Charlotte, NC
Where did the 8200 come from? Isn't the nominal operating at 7967V?
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