Calculation of VFDs in parallel current harmonics 2

[radug]

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.

 

[electricpete]

Is it possible there is a phase difference among vfd's for a given harmonic ? Then vector addition would be required.

=====================================
(2B)+(2B)' ?

 

[radug]

Electricpete,

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.

 

[LiteYear]

I'm not sure what you mean by "injected currents". Are you measuring harmonics on the load or the line side? In any case (but more so on the load side) the harmonics and side bands of the switching frequency are a function of the switching behaviour - even two identical VFDs will never be perfectly in sync. They may start at different times and their control algorithms may react differently to noisy signals. So the harmonics are unlikely to have the same phase.

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.

 

[electricpete]

I assume "injection" is reference to typical modeling of harmonic loads as harmonic current sources and would therefore would refer to vfd input.

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.

=====================================
(2B)+(2B)' ?

 

[waross]

A possible cause may be the hysteresis of the transformer core. There may be a non-linear effect at low current levels that will be seen once with either one or two VFDs online.

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

 

[radug]

Hello again,

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.

 

[LionelHutz]

The current harmonics of two 6-pulse drives in parallel should just add-up. I'm not sure what the ABB software is doing.

 

[LiteYear]

Ah, okay, so you're interested in the line side of the VFDs then. In that case, as electricpete suggests, there's no 3rd harmonic or DC bus to worry about. All the harmonic "injection" comes from the non-linear conduction of the rectification stage. This conduction profile is a function mostly of any DC bus reactance, any line reactance and of the load on the motor.

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?

 

[xj25]

With 6pulse rectifiers harmonic ripple will be about 300Hz (50Hz line) if I remember well, and in sync between VFDs.

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...

 

[LiteYear]

That's right xj25, the first major harmonic is the 6th due to the six rectification pulses per cycle. Though note that the waveform is far from sinusoidal, so it also has 7th, 11th and 13th, 17th and 19th, etc. harmonics in it in decreasing magnitude. Ignoring variations in component values, these should be the same for parallel, identical VFDs.

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.

 

[sibeen]

the first major harmonic is the 6th due to the six rectification pulses per cycle

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.

 

[Marke]

Hello radug

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

 

[LiteYear]

Oops, quite right sibeen, Definitely no 6th. Mea culpa. Since the negative half cycle is the same as the positive half cycle, there are no even harmonics. Instead we get sidelobes of each of the 6n harmonics - 5 and 7, 11 and 13, etc.

 

[radug]

Thank you all for all your replies.

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.

 

[waross]

Is the software result higher or lower than the arithmetic result and by how much??

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

 

[electricuwe]

To solve the contradiction between the different statements:

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).