VFD Harmonics

[PSU Electrical]

I'm currently working on a water pumping station project and was hoping to get some thoughts or opinions on some VFD options I'm faced with. The station itself is pretty simple, (2) 500hp, 460VAC Centrifugal Pumps, (2) 50hp Booster Pumps down by the river and about another 100kW or so in misc. lighting, HVAC, etc. We have a 1500kVA transformer from the utility (480VAC, 1800A service, 2500A Switchgear lineup). The bulk of the load is obviously the (2) 500hp pumps which will be operated with variable speed drives. As the A/E involved, we are working with a pump OEM and of course the end user to help assist in procuring the VFD's to drive these pumps. Originally, we were proceeding with Rockwell PF700, 18-pulse drives but the pump OEM then decided to introduce a "cost effective" alternative by quoting ABB ACS800 drives. Of course Rockwell then said, well why don't we quote you PF755 6-pulse with harmonic input filters. I've been told from some people that in water treatment plants, pump stations, etc. 18-pulse drives are the norm these days for meeting IEEE519. I was just curious if anyone has any opinions or thoughts on the above mentioned drives or perhaps some suggestions that I need to confirm or question to these vendors or utility. I spoke with the utility and they told me they will meter at the secondary bushings of the transformer and typically look for less then 10% THD. This seemed pretty high to me. I'd appreciate any thoughts or suggestions.

I'd also like to just say thank you to all who are involved on this site; I have absorbed a great deal reading these boards every day at lunch.

 

[ghostbuster]

Please be cautious.Drives can also produce rf interference as well as harmonics that can couple into nearby PLC,s etc. unless certain precautions are instituted in the design.We have had to sort out several of these messes after the equipment has been insalled.

 

[Jraef]

My 2 cents...

Most likely the ABB offering is their "Clean Power" version of the ACS800 which is an Active Front End (AFE) drive. I've used it, I like it a lot, great drive, grat drive mfr in my opinion (although I don't like many other ABB products). AFE technology is essentially two back-to-back drives, the front end is a PMW unit as well as the normal back end. So what it does is monitor the effects of the load in terms of the harmonics it creates on the line and creates "counter harmonics" (my words, not theirs) to neutralize them. I liken it to "noise canceling" headphones and if you have ever tried them, you know they work. What A-B is proposing is a separate stand-alone Active Harmonic Filter in front of the drive, it would essentially do the same thing. Hard to imagine that the installed cost for them would be lower however because the two separate units have to be mounted and wired. If you are going to allow it, make sure the bidding contractors understand that they will have added handling and labor costs and that you will not allow a change order to cover them (saw that happen once) if it is their choice to use that package.

Compared to 18 pulse VFDs, from projects I have been involved in both approaches tend to work in that they satisfy the IEEE requirements. The advantage of the AFE or Active Filter approach is that they can also help a little to mitigate harmonics NOT CREATED by the VFD they are associated with, such as the lighting or the booster pumps (if they have VFDs, you didn;t say). That is especially true of the Active Filter version, but that requires making sure the filter is a little over sized in terms of capacity.

The 18 pulse front end versions use extra magnetics (phase shifting transformers) to make a lot of the harmonics neutralize themselves passively. So there are technically fewer "active" elements involved that can become points of failure down the road a few years, hence the general reluctance for the Consulting Engineering community to move away from specifying them. But it was only a few years ago that CEs were reluctant to specify VFDs at all! Times and attitudes change.

BTW, that 10% spec was probably Current THD, not voltage.

 

[jdsmith]

If the utility wants to see 10% or less current THD then you need something more than a 6 pulse drive with a line reactor. You could use an 18 pulse drive, an active front end drive, or a 6 pulse drive with an active harmonic filter. Passive harmonic filters are also available, but for a pumping station with loads that will vary substantially you would have to pick a certain operating point, say 400 HP total output for both 500 HP drives, and size the passive filter to that operating point. Any operating point above or below that would have somewhat higher harmonics.

I am not a fan of 18 pulse drives because of the 18 pulse transformer - it's big and it has to conduct the full 500 HP rating of each drive so it will create a lot of heat. I'm considering an active harmonic filter on a couple projects now but I don't have much experience with them yet.

Note that the AB Powerflex 755 drives above 350 HP were just released in July of this year - buying a new product may not be a concern for your customer, but some people aren't comfortable with something that's too new. That being said, I have standardized my plant on AB PF 755 drives for applications up to 300 HP and we've been impressed with them so far.

If you can, buy the VFD directly from the manufacturer or appropriate distributor, not from a pump manufacturer. They typically have kickbacks from selling motors and VFDs and they may want to sell you the VFD with the best kickback, not the best one for your application.

 

[jdsmith]

Please be cautious.Drives can also produce rf interference as well as harmonics that can couple into nearby PLC,s etc. unless certain precautions are instituted in the design.We have had to sort out several of these messes after the equipment has been insalled.

The interference with other equipment is due to noise radiated from the motor leads - this is one reason that shielded motor leads should be used even if the distance from the drive to motor is short. It is important to note that the frequency of the radiated electromagnetic energy is not the switching frequency that can be set in the drive, but the frequency related to the rise time of the PWM pulses.

 

[Mike01]

Active Harmonic Filters

Active Harmonic Filters

Just curious if anyone else has had issues with active harmonic filters, I have seen on some projects that attempted to use them to offset the cost and maintenance of an 18-pulse drive where there was a lot drives were installed and harmonics were a potential issue. However once installed and the project was complete the active filters kept tripping their OCPD, and did not perform properly, manufacturers got involved and some now just sit there disconnected. Some CT's were wired incorreclty, and from what I now under stand they do not like to be installed on 4-wire systems, or where there are other cap's involved, there are alot of critera to be met on the system as a whole for proper operation. Just curious how they work for everyone esle.

 

[Jraef]

Just curious if anyone else has had issues with active harmonic filters, I have seen on some projects that attempted to use them to offset the cost and maintenance of an 18-pulse drive where there was a lot drives were installed and harmonics were a potential issue. However once installed and the project was complete the active filters kept tripping their OCPD, and did not perform properly, manufacturers got involved and some now just sit there disconnected. Some CT's were wired incorreclty, and from what I now under stand they do not like to be installed on 4-wire systems, or where there are other cap's involved, there are alot of critera to be met on the system as a whole for proper operation. Just curious how they work for everyone esle.

Like anything else, there are good products, "minimally acceptable" products, and there is crap. Sounds like you got hit with some minimally acceptable product that turned to crap.

Not for 4-wire systems? Someone is blowing smoke up your you-know-what. Probably more than 90% of new services are 4-wire, they would have a very tiny available market. But what they may have meant (and didn't know how to articulate) is that Active Filters can have troubles with unbalanced line voltages, as can AFE drives and in fact, 18 pulse drives too. It's a problem that these things don't fix.

Problems with caps are real however, especially bulk PFC caps on the same line as an Active Filter. But if most of the loads are VFDs being fed by the Active Filter, were the PFC caps still necessary? Someone should have looked into that.

 

[Mike01]

Active Filter / Front End

Active Filter / Front End

Like anything else, there are good products, "minimally acceptable" products, and there is crap. Sounds like you got hit with some minimally acceptable product that turned to crap.

Not for 4-wire systems? Someone is blowing smoke up your you-know-what. Probably more than 90% of new services are 4-wire, they would have a very tiny available market. But what they may have meant (and didn't know how to articulate) is that Active Filters can have troubles with unbalanced line voltages, as can AFE drives and in fact, 18 pulse drives too. It's a problem that these things don't fix.

Problems with caps are real however, especially bulk PFC caps on the same line as an Active Filter. But if most of the loads are VFDs being fed by the Active Filter, were the PFC caps still necessary? Someone should have looked into that.

Jraef,
As I was not directly involved with these projects it was just discussed within our department recently, and the manufacturer (this was a reputable manufacturer in the power industry) when brought in to help resolve the installation I have some additional information but am not in the office and will follow up and would be intrested in you opinion. We did recently have a conservation with our local ABB rep. regarding the clean power drive with Active front, however do not have any personal experience with them it sound like you have just cuirous what you think of them. Thanks..