Harmonics and VFDs part 2 5

[slavag]

Dear All.
I don't understand many technical points in original Marke's thread and I don't want add my Q's in this excellent discussion.
As subcontractor, I'm responsible for substation automation and protection ( design, test and commissioning) part of new big pump station.
My part is two transformer ( 161/11kV 65-80MVA, y/d) and 52bays on 11kV. My part clear for me 100%, we choiced protection, close all contracts, provide preleminary design, etc. After holidays I will start order eq. and design according to bays type. Part of 52 bays (16-18) bays
are MV VFDs for motors in range from 0.8 up to 1.5MW.
I read all topics on this issue and start worry. It's my first projects with this number of MV VFD's in one station, in past we provide protection for several Robicon's ( one per station), before two weeks provide some replacing of protection for MV VFD w/o any problem. I don't know what will possible influence of this level of THDV/I on protection. I remember as well recommendation of DPC and choice protection accordantly. My part only protection, but I prefer put attention also on MV VFD vendor's solution and all MV cables installation.
What do you recommend check and on what put attention?
What is recommended codes and standards for this type of pump station?
For example, before I decide add THDI monitoring on all MV VFD's bays and THDV/I monitors on infeed bays, is good idea or not? Possible connect alarms to station PLC's , etc.
Station also include 2 capacitor banks , as I remeber, 500kVAR. Maybe provide trip to this CB in case of high level of harmonics.
Thank you for any ideas and recommendations.
Best Regards.
Slava.
Dear Valvecrazy, is not my station and unpossible replace VFD's to valves.

 

[Valvecrazy]

Sorry slavag. If your designers had not been pressured to use VFD's because they would (supposedly?) save energy, you could have done the same thing with valves and saved yourself a lot of headaches.

I do have customers who use VFD's with valves. They use the valves for simple and reliable flow and pressure control. The VFD's are used as glorified soft starters. Sometimes they are also used to convert single phase to three phase. This at least takes the pressure transducer, speed control, and all the problems that go along with that, out of the picture. The Drive basically ramps up to full speed and the valve controls the system. This also keep the pump running at full speed at 60 hertz, which eliminates resonance frequencies, and also helps to keep odd number harmonics from being formed.

Many of these systems also have a transfer switch to an Across The Line starter. Then when the VFD craps out, a simple flip of the transfer switch, and the system is back up and running on the more reliable magnetic starter.

Usually the first time they have to run the system without the VFD, they realize the power consumption is basically the same, many technical problems go away, water delivery is more dependable, and they rarely fix the VFD

 

[waross]

valvecrazy
VFDs are a fact of life. Live with it. Learn a little more about VFDs. There are many reasons other than efficiency to use VFDs.
Yes I agree that virtually all pumping applications may be accomplished without VFDs as was done for decades.
Yes I agree that there are not always energy savings with VFDs.
However many engineers who are familiar with valve solutions, pump curves and VFDs have chosen to implement VFDs. Promised energy savings may not be the only factor influencing the choice.
The purpose of this forum is to provide solutions to problems, not to second guess designs.
Ranting against VFDs will not change the world.
There are sometimes problems with VFD applications.
There are sometimes problems with valve applications.
If I have a valve problem I hope that I may come to you for help.
PS. You have made your point: There is not always an energy saving with VFDs, and VFDs are sometimes used where they are not needed. Agreed. (And I am sure that even the VFD gurus will agree.)
respectfully

 

[davidbeach]

I will second what waross said, this forum is not a place for VFD vs. valve discussions. There is one thread that more than adequately addresses that issue; there are many applications of VFDs where valves won't do anything, think motion control, and there are other applications where VFDs are used for what ever reason and problems with the VFD installation need to be addressed and suggesting valves instead is just not helpful.

 

[Marke]

Hello slavag

The VFDs will produce harmonic current unless there are steps taken to reduce/eliminate the harmonics. This is a design issue.
The harmonic currents them selves do not necessarily cause a problem, and the effect of them can be anticipated with some engineering.
If the impedance of the bus that they are connected to is very low, there will be little harmonic voltage produced as a result of the harmonic currents, however, if the bus impedance is high, then the harmonic voltages developed by the harmonic currents will also be high.

The harmonic currents reduce the true power factor at the input to the drive. Most suppliers will tell you that the power factor is better than 0.95, but they are talking about displacement power factor. The true power factor may be closer to 0.8 at full load, but that is dependent on the design of the inverter front end.
You will need to oversize the supply components to accommodate this lower power factor.

If the harmonic voltages resulting form the VFDs are significant, you can cause interference with the operation of other equipment and cause increased currents in power factor correction capacitors and the like.

I would recommend that at this stage, someone does an investigation of what the expected level of harmonic voltages are an see if there is any requirement to make any additions to cover this.

Best regards,



Mark Empson

http://www.lmphotonics.com

 

[jraef]

All that said, one of the reasons why Robicon drives have been so successful in the large MV drive market is the fact that they go to great lengths to mitigate harmonics on the line side. Their "Perfect Harmony" series of drives are the gold standard of the industry when it comes to having low harmonic distortion. It's costly, but it is proven and independently documented.

Oh and as to valves vs VFDs, nothing is perfect but neither is anything perfectly flawed. I would never trust an installation that did not have valves in place, but I have seen plenty retrofit installations where the extra energy saved by the VFDs has paid for their installation many times over.

 

[slavag]

Marke.
Thanks a lot.
I understand you.
Best Regards.
Slava

 

[lansford]

Slavag,
Just add filters to the VFDs or order then on the largest drives?
Jim

 

[slavag]

Hi.
I call now with vendor of MV VFD.
He gave me lecture for about 20min.
He supply VFD with trafo 11/3.3 kV D/12 pulses. He told me: may be possible small level of 11th,13th harm on current (5th and 7th harm cancelled by Delta connection) on the 11kV side. About PF issue, it was exactly as Mark saied:
DPF will about 1, but lambda will about 0.7-0.8. I also would like ask him calculate harmonic level according to substation layout. Are possible?
Regards.
Slava

 

[Marke]

Hello slavag

Sounds like you are being supplied with 12 pulse input drives with a zig zag transformer. In this case, I believe that you do not need to worry about the harmonics. The 5th and 7th are virtually eliminated by the 12 pulse and zig zag transformer.
The only issue is the effective power factor at the input to the drive, you will need to oversize supply components accordingly.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[slavag]

Dear Mark.
Thanks again.
Happy New Year to all.
Best Regards.
Slava

 

[sibeen]

With a 12 pulse input configuration without any addittional filtering, the input current distortion (THDi) can expected to be in the order of 13%. What effect this has on the voltage waveform is going to depend upon the impedance of the supply.


The major harmonics will be the 11th and 13th and I would be checking to see that the Power factor correction capacitors are protected by an in-line choke to avoid overloading from the higher harmonics.

 

[slavag]

Hello Sibeen.
Thanks a lot for your reply.
Several comments to my previous posts:
1. I checked same type of VFD on another side:
DPF=0.98, lambda=0.85, THDI/V according to all standards.
VFD include all motor protection, DC link, etc...
2. According to last design request: Capacitor Bank outgoing
"must be include choke...".
Thanks to all again for your great help.
Best Regards.
Slava