Passive harmonic filter affecting generator

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joncole

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Hello guys and gals.

I have a pump station that is backed up by a 500kwatt generator. The 6 pumps are controlled by an 18 pulse Allen Bradley vfd system. Over the last year we have replaced 3 of the 18 pulse drive systems with Schneider 6 pulse vfd and a harmonic passive filter across the input of drive. Per the manufactor instructions, the passive filter is engaged 10 secs after the drive starts. When the filter engages, the voltage from the generator starts fluctuating so much that ab drives start faulting due too input phase loss and over voltage. Generator is fine without filters engaged. We installed these filters to help reduce harmonics affecting the regulator on the generator.

Why would the passive filter be affecting the generator this way?
 
Probably too much reactive load on the generator. If they are capacitive input filters, they can cause the power factor to go leading, which can cause problems.

What is the total load on the generator, and how much of it is the 6 pumps? Are the pumps lightly loaded?
 
steve66 said:
Probably too much reactive load on the generator. If they are capacitive input filters, they can cause the power factor to go leading, which can cause problems.

What is the total load on the generator, and how much of it is the 6 pumps? Are the pumps lightly loaded?
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I agree. Passive harmonic filters are capacitors and reactors, tuned to specific harmonics, typically the 5th and 7th (sometimes only the 5th). But they would be designed based on the system impedance when on utility power, which is the PURPOSE of the harmonic filters. When not on the utility power, the impedance is much much higher and it changes the way the passive filter interacts with the harmonics from the drive. That in turn can then affect the AVR circuit in the generator and the two issues begin a nice little switch dance.

Disable the circuit that engages the passive filters when on the backup generator, you don't need them. The purpose of harmonic mitigation is to avoid affecting the INPUT harmonic distortion at the PCC, so that it does not travel to your neighbor. When you are on a backup generator, you have no neighbor!
 
Thanks for the replies guys. The six pumps are really the only load on the generator. The motors are 125hp and the gen is 500k. So if all six pumps were running at full load than the generator could be loaded to near capacity. This should never happen. Usually 4 pumps are running at full speed would be the norm.

We put the filters in to mitigate harmonics affecting the generator. Do you guys think this is not necessary? Also when the filter is engaged it has a loud ac hum and the drive input voltage drops from about 450 to 400. Im assuming the voltage drop is due to the chokes that are installed as part of this filter assembly. Also the drives draw about 15-20 more amps than usual at full speed when the filter is engaged. Im assuming this is due to the voltage drop.

Would you recommend removing the filters? Do you think the generator will be fine without them?

Thanks.
 
joncole said:
Also when the filter is engaged it has a loud ac hum and the drive input voltage drops from about 450 to 400. Im assuming the voltage drop is due to the chokes that are installed as part of this filter assembly. Also the drives draw about 15-20 more amps than usual at full speed when the filter is engaged.
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That suggests that you're losing quite a bit in the chokes & wiring, if it were purely resistive, that would be over 2kw, and the filters would be plenty hot. Maybe you have a bad choke (loose windings?). I worry whenever there's a loud hum in something that shouldn't have any moving parts.
 
Jraef said:
I agree. Passive harmonic filters are capacitors and reactors, tuned to specific harmonics, typically the 5th and 7th (sometimes only the 5th). But they would be designed based on the system impedance when on utility power, which is the PURPOSE of the harmonic filters. When not on the utility power, the impedance is much much higher and it changes the way the passive filter interacts with the harmonics from the drive. That in turn can then affect the AVR circuit in the generator and the two issues begin a nice little switch dance.

Disable the circuit that engages the passive filters when on the backup generator, you don't need them. The purpose of harmonic mitigation is to avoid affecting the INPUT harmonic distortion at the PCC, so that it does not travel to your neighbor. When you are on a backup generator, you have no neighbor!
Click to expand...

What is PCC?
 
joncole said:
What is PCC?
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Point of Common Coupling, the juncture between the utility and the user. In IEEE519, there are limits of Voltage THD (Total Harmonic Distortion) set to what a utility must maintain AT the PCC, and limits of Current Total Demand Distortion (TDD) that must be met by the user, so as to not create the Voltage THD at the PCC and have it travel to the NEXT user on the same utility line. So the key to everything is that PCC. When you are on a generator, there is no PCC with the utility, only with your generator. So if the generator has issues with harmonics, then fine, but if not, there is no issue.

So who sold you on the idea of "put the filters in to mitigate harmonics affecting the generator"? Did you have a problem with the generator and the generator mfr told you to add passive harmonic filters? Or did a filter salesman sell you on the idea of prophylactic "mitigation" of harmonics to prevent some as yet unknown issue with the generator? Applying a passive filter to a generator based system takes some very careful engineering consideration. The added capacitors in the filters can interact with the caps in the drives under lightly loaded conditions, which can then affect the AVR (Automatic Voltage Regulator) in the generator to allow the voltage to swing drastically. I always tell people to let the generator mfr design the filter system for you, because THEY are the ones who will be affected by it.

If it was the generator mfr. in your case, did they give you a SPECIFIC solution to use, or did they just dump it all on you do find it on your own? If they dumped it on you, shame on them. If that were the case, I would have used one single ACTIVE harmonic filter at the PCC of the generator, sized fore the entire load and let it track and correct the harmonics in the entire system as they change with load.
 
Jraef said:
Point of Common Coupling, the juncture between the utility and the user. In IEEE519, there are limits of Voltage THD (Total Harmonic Distortion) set to what a utility must maintain AT the PCC, and limits of Current Total Demand Distortion (TDD) that must be met by the user, so as to not create the Voltage THD at the PCC and have it travel to the NEXT user on the same utility line. So the key to everything is that PCC. When you are on a generator, there is no PCC with the utility, only with your generator. So if the generator has issues with harmonics, then fine, but if not, there is no issue.

So who sold you on the idea of "put the filters in to mitigate harmonics affecting the generator"? Did you have a problem with the generator and the generator mfr told you to add passive harmonic filters? Or did a filter salesman sell you on the idea of prophylactic "mitigation" of harmonics to prevent some as yet unknown issue with the generator? Applying a passive filter to a generator based system takes some very careful engineering consideration. The added capacitors in the filters can interact with the caps in the drives under lightly loaded conditions, which can then affect the AVR (Automatic Voltage Regulator) in the generator to allow the voltage to swing drastically. I always tell people to let the generator mfr design the filter system for you, because THEY are the ones who will be affected by it.

If it was the generator mfr. in your case, did they give you a SPECIFIC solution to use, or did they just dump it all on you do find it on your own? If they dumped it on you, shame on them. If that were the case, I would have used one single ACTIVE harmonic filter at the PCC of the generator, sized fore the entire load and let it track and correct the harmonics in the entire system as they change with load.
Click to expand...

Well, my supervisor wanted to make sure we had something to mitigate harmonics because we were replacing the 18 pulse system with a 6 pulse. The passive filter is what Schneider drive people recommended. Obviouly that wasnt the way to go.

Thanks for your responses Jreaf. You have been a big help.
 
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