Calculation of VFDs in parallel current harmonics
Calculation of VFDs in parallel current harmonics
(OP)
Hello,
I am trying to calculate which would be the total injected current harmonics spectrum of some VFDs connected in paralell to the same MCC. I have the harmonics spectrum of the individual VFDs. My problem is when calculating the total harmonics in the MCC as there seem not to be the arithmetic sum of each of the harmonics generated by individual VFDS. The VFDs have 6-pulse rectifiers.
When using ABB Drivesize software and choosing a motor + ACS800-01 VFD an adding the same combination in parallel, Drivesize produces a report where I can see that each of the individual harmonics of the combination is not an arithmetic sum and also that the current harmonic ratio between 1 or 2 VFDs in parallel is different for each harmonic. I am trying to understand why and how is the calculation to be done.
I am trying to calculate which would be the total injected current harmonics spectrum of some VFDs connected in paralell to the same MCC. I have the harmonics spectrum of the individual VFDs. My problem is when calculating the total harmonics in the MCC as there seem not to be the arithmetic sum of each of the harmonics generated by individual VFDS. The VFDs have 6-pulse rectifiers.
When using ABB Drivesize software and choosing a motor + ACS800-01 VFD an adding the same combination in parallel, Drivesize produces a report where I can see that each of the individual harmonics of the combination is not an arithmetic sum and also that the current harmonic ratio between 1 or 2 VFDs in parallel is different for each harmonic. I am trying to understand why and how is the calculation to be done.





RE: Calculation of VFDs in parallel current harmonics
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(2B)+(2B)' ?
RE: Calculation of VFDs in parallel current harmonics
That is the first thing I though, but as both the motor + VFD are exactly the same (as well as cables and cable lengths) I think that vector addition is not the problem.
RE: Calculation of VFDs in parallel current harmonics
The only harmonic I can think of that will be in phase is the 3rd, which is due to the rectification. It will be in sync with the mains frequency, so ought to add arithmetically. All the others, not so much.
RE: Calculation of VFDs in parallel current harmonics
I didn't think 3rd harmonic would be seen on the input since it's zero sequence. I'm used to hearing about 5th and 7th, 11th 13th etc on input of standard 6-pulse rectifier.
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(2B)+(2B)' ?
RE: Calculation of VFDs in parallel current harmonics
Bill
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"Why not the best?"
Jimmy Carter
RE: Calculation of VFDs in parallel current harmonics
I need to determine the voltage THD in the MCC busbar where all VFD+motors are connected to check for a maximum allowable limit.
LiteYear: you are right, I use the term "injected" because I intend to model VFD harmonics spectra as current sources for each of the harmonic. The power system analysis software I use does not have an specific model for VFDs, but it allows to model harmonic current sources in loads, motors,... That software performs an vector addition of each of the harmonic currents, but as I said, the ABB proprietary software is not doing that and I would like to know how is it doing the calculations because results are very different.
RE: Calculation of VFDs in parallel current harmonics
RE: Calculation of VFDs in parallel current harmonics
If the VFDs are the same and the load on the motor is similar, I can't imagine the mains frequency harmonics differing much from VFD to VFD. In other words, my first guess would be that the harmonics add arithmetically. So I'm also curious, what is the ABB software doing? Perhaps ABB will give some insight?
RE: Calculation of VFDs in parallel current harmonics
The VFD switching will not be synchronised, they will drain some (small I assume) harmonic current from line side instead from DC bus cap.
in that case this (quite small) current harmonics will not be in sync and then will not add toghether between VFDs to the line...
RE: Calculation of VFDs in parallel current harmonics
And as you suggest, the load will take some of its current directly from the line and some from the DC bus cap. These components however, will be in the order of the switching frequency of the drive, which tends to be many times the main frequency so is not easily confused.
RE: Calculation of VFDs in parallel current harmonics
No. The harmonic order is n*p +-1. Where n = interger 1,2,3... and p = pulse number of the rectifier.
So, for a six pulse rectifier, we expect to see thee 5th, 7th, 11th, 13th...The magnitude of the harmonics also (hand waving) follow the order of the harmonics. The 5th harmonic will be 1/5 of the magnitude of the fundamental, the 7th - 1/7, the 11the -1/11 etc.
If I began to see a substantial magnitude of any even harmonic (asymmetrical current waveform) I'd begin to be a tad worried.
RE: Calculation of VFDs in parallel current harmonics
go to http://www.mirusinternational.com/ and download Solv and you will get all the calculations and modeling that you need to determine the voltage distortion, harmonics etc. for multiple VFDs.
The harmonic currents aare primarily the 5th, 7th, 11th, 13th etc and the weighting depends on the rectance of the supply and the reactance built into and/or added to the VFD.
Best regards,
Mark
Mark Empson
Advanced Motor Control Ltd
RE: Calculation of VFDs in parallel current harmonics
RE: Calculation of VFDs in parallel current harmonics
Marke, I will try that software. So the problem is how to calculate the weighting. I have the reactance of the supply so I would need the reactance of each VFD? Are there any technical papers that contain information about how to perform these calculations?
I was thinking that perhaps the weighting was some type of probability assumption because of the fire angles in the inverter side of the VFDs.
RE: Calculation of VFDs in parallel current harmonics
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Calculation of VFDs in parallel current harmonics
For 6-pulse rectification the main harmonics on the AC-side are 5th, 7th, 11th, 13th as already pointed out by Marke
On the DC-side the main harmonics are 6th, 12th, 18th.., but as the initial posters question relates to harmonics on the grid, this is out of scope here.
On the question of arithmetic or vector addition, I'd assume that for voltage source inverters the harmonics should sum up arithmetic. On reduction due to vector addition you can only hope for if there are current source inverters with phase angle control or phase shifting transformers between the sources and the point o common coupling (PCC).