Power Factor Correction Design 4

[Ashdgee]

Good day fellow engineers

Okay so I am designing a new MCC (Motor Control Center) for a mine.It has 6 x 500kW VSDs and 4 x 7.5kW Direct Online Pumps and non-motor load.WE have done a load demand power calculation and we know our reactive power requirements.I have allowed some space for the power factor correction cubicle and we are going to have a 6 step PF correction.

Here is my problem.I need to work out the size of the de-tuning reactors for each of the capacitor steps. But that is not possible unless you have details of the ACTUAL harmonic spectrum (voltage and current distortion)generated by the new loads.The only way is to install the MCC panel without the PFC and then connect a power analyzer to study the harmonic content.

But the client needs us to get the panel fully assembled with the PFC before delivering to site.How is it possible to specify your PFC without having analysed the actual harmonic content ? Because we need the PFC to be detuned.

Is there any way to design it using 'typical harmonic spectrum'.

Your assistance will be very helpful

 

[dpc]

I'm not an expert, but pretty sure the harmonic spectrum of these PWM drives will vary with load. I'd talk with your AFD supplier for guidance on designing a cap bank filter.

BUT, are you sure you really need power factor correction? The drives should have a good displacement power factor I would think. The four 7.5 kW motors running across-the-line are pretty insignificant. On the surface, it doesn't seem worth the bother unless there are loads requiring a lot of reactive power.

Cheers,

Dave

 

[Ashdgee]

Thanks for your response Dave.

On the other non motor loads, there are supplies to fluorescent lights (we told the client to go with LEDs but did not agree) as well as welding machines.On my other project , we had some issued with harmonics generated from 450kW VFDs but we did an analysis and specified an Active Harmonic Filter.That was fairly easy as it was an existing installation.

The problem with this on is that its a new installation and there is no way one can know the harmonic behavior prior to installation.From my previous project experience ,VSDs generate significant harmonics.

I will see if I can contact the VSD supplier

Regards

Ashford

 

[cuky2000]

Since this is an existing installation, consider using a PQ analyzer to measures at the MCC TDH, the level of each harmonic frequency, as well as other issues related to power quality that is associated with system disturbances or potential causes of malfunction (trips or alarm) of control circuits such as swells, dips, etc.

If the field measurement is followed by analysis and detail study this will be ideal to minimize potential misapplication.

Below is a general recommendation from Eaton to select PFC in the presence of nonlinear loads is the ratio of the total nonlinear kVA to the service transformer kVA capacity rating:

• If the total 3Φ nonlinear load kVAtot[sub]nl[/sub]> 25% of the main transformer capacity, harmonic filters will almost always be required for PF correction.
• If the plant’s total nonlinear load is between 15%< kVAtot[sub]nl[/sub]<25%, other factors should be considered.

 

[Ashdgee]

@cuky2000

No.It is a new installation.(I meant to say I had a similar project thta dealt with an existing installation but this one is a new MCC)

 

[LionelHutz]

If the loads are VFD then you need to correct the VFD harmonics. Filter the VFD harmonics and the power factor will be corrected.

 

[mpparent]

Often-times if you ask, the manufacturer's have the harmonic spectrum for their drives. Can you model the system using SKM, Easypower, etc. and see what the overall harmonic spectrum is? That should get you fairly close. Also, IEEE 519, IMHO, is often misapplied. Furthermore, I have seen current distortion above 30% with no real ill effect. As long as voltage distortion is relatively low then harmonics related problems are generally not an issue. This is where I will quote IEEE 519 5% voltage THD as a good metric.

Mike

Mike

 

[waross]

Displacement power factor may be corrected by capacitors.
Consider capacitors directly connected to or switched with large motors.
Distortion power factor may be corrected or mitigated by filters.
Consider filters on feeders to MCCs containing a large number of VFDs.
Better;
Consider one feeder and MCC for DOL motors. Correct the displacement PF with capacitors.
Consider a second feeder and MCC for VFD drives. Mitigate the distortion PF with filters.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

With 6 x 500kW drives, I would either use Low Harmonic Active Front End (AFE) drives, or use Active Harmonic Filters (AHF) ahead of them on any common bus(es).

If you use the AFE drives, besides using Selective Harmonic Elimination to get the I-THD <5% and being able to operate at a 1.0 displacement power factor, many of them are now also capable of operating at a LEADING power factor if you have other inductive loads in the facility that need correction.

Some of the better AHF units also come with displacement PF correction and although they function differently, the net result will be the same. The AHFs are sold by sizes of "corrective current", i.e. 100A, 200A, 500A, etc. so the rough guestimate on sizing them is to assume 30% I-THD from standard 6 pulse inverters (with 5% Line Reactors). So if we assume 400V motors at 890A each, each VFD will represent around 270A of corrective current. 6 drives would then require 1620A of corrective current. If these are all fed from the same bus, that would be 3 x 500A and 1 x 200A units (they can be used cumulatively). All in all, it would likely be less expensive and simpler to use AFE drives and be done with it.


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

 

[GroovyGuy]

Howdy Ashdgee,
I don't believe that you will have any issues with PF correction with the bulk of your load consisting of ASDs. Check with you ASD OEM, but I believe that the input PF of a 6-pulse type of ASD is near unity. However, you will need to consider the harmonics of your system. I can visualize several Square D type of active harmonic filters (is Accusine or equal) connected to your MCC bus.
Regards,
GG

ps Did you consider using (say) a 18 or 24 pulse type of MV ASD? I know that they are more expensive than a LV ASD drive, but they don't have the harmonic issues of a
LV 6-pulse drive, that together with the increased copper costs of a LV drive solution, can make the MV ASD a tad more attractive.

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[Ashdgee]

Thank you all for your response.

We ended up proposing an active harmonic filter for the MCC.We modelled the single line diagram in ETAP and ran a harmonic analysis.

We modeled using typical spectrums for 6 pulse VFDs and the Total Voltage Distortion was about 9.5%.We have proposed an active harmonic filter for this.

 

[electricuwe]

Do your 500 kW drives have DC-link inductors included or AC inductors added in their AC-connection ? This is a very cost effective lever to reduce the harmonic content of the input current. What is the available short circuit power ? 9.5 % voltage THD to me indicate that you have drives without any inductors on a quite weak bus.

A smart solution to you problem would be to use VFDs also for the four 7.5 kW motors. Then you can operate your complete LV distribution with high level of harmonics (input rectifiers of drives tolerate a much higher harmonic level as DOL motors) and you only have to comply at the medium voltage level.