Harmonic filter design question

[eeprom]

Hello,
I just bought a harmonic filter to mitigate the distortion produced by a 400 HP VFD. The filter consists of some capacitors and 2 line reactors. Here's how it is wired.

One line reactor is connected immediately downstream of the main breaker. From there it is connected to the VFD. Between the VFD and the line reactor, a second filter is connected phase to phase. The second reactor is wired in series with some capacitors, and that series combination is connected line to line. So, between each two phases, there is an LC circuit (not including the resistance of the reactor).

So, I have a couple of questions. The first is very basic.

1. How does a line reactor absorb high frequency current? I understand that the impedance varies with frequency. But in order to absorb the high frequency current, something has to absorb real (not reactive) power. There is a tiny DC resistance in the line reactor. Is this the load which absorbs the high frequency current? Where does the high frequency energy go?

Question 2. Why would the circuit be wired such as it is? If I had designed this, I would have put an LC shunt off of each phase, tying one end to ground. The filter I bough makes the connection line to line. And why the two line reactors?

thanks in advance

 

[ScottyUK]

Sounds like the series L-C is a resonant trap and will be tuned close to a frequency of interest, likely the 5th harmonic. The resistor will reduce the Q-factor of the tuned circuit and will limit current if the filter actually goes in to resonance under abnormal supply conditions. The series reactor is a low pass filter which presents a rising impedance as harmonic order increases and serves to decouple the drive and its filter from the network so you're filtering harmonics from the drive and not trying to flter harmonics from the whole network.


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If we learn from our mistakes I'm getting a great education!

 

[rockman7892]

ScottyUK

Are you saying that the 5th order harmonic is of interest assuming that the drive is a 6-pulse drive? What about the 7th harmonic in this case?

Can you explain how the series reactor acts as a low pass filter. I understand that the reactor impedence represented by XL = jwL will increase with increased frequencies as you mentioned, however what will this increased impedance do to this high frequency current?

 

[eeprom]

Thanks for your help. I spent all night studying that circuit and I had come to the same conclusion that the LC circuit is a bandpass with a very narrow band, assumed to be centered at 300Hz. I have the results from a harmonic distortion reading, and the 5th is by far the most prevalent harmonic.

 

[ScottyUK]

Rockman,

Yes, low order odd harmonics are of interest if it's a 6-pulse front end. If it's tuned to the 6th and the Q-factor isn't too high then it will trap 5th and 7th without being resonant at either. It's reasonable to expect that even order harmonics will be negligible in a properly behaving rectifier so the resonance at 6th isn't a problem.

Low pass filter was perhaps not the best description. It will serve to slow down commutation edges and reduce the production of high order harmonics inherent in fast switching edges.



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If we learn from our mistakes I'm getting a great education!

 

[VTer]

1. For a 6 pulse drive the filter is typically tuned for 5th order harmonic (typically the highest current and % fundamental magnitude) which implies that the circuit resonates at approximately 300Hz with such a filter. Note, however, that if you do design the filter for that frequency theoretically you can get very high resonant currents, therefore the filter is typically tuned at 4.7 rather than 5th order harmonic which means that the circuit is tuned at approx 282hz to avoid extremely high currents. Now at relatively high frequencies you can view inductance as “open circuit” and capacitance as a “short circuit”.Either way, Xl=2pifL indicates that effective reactance goes up with frequency therefore limiting current, but the opposite goes for a capacitor since Xc=1/(2pifC). With higher frequency, the Xc goes down reducing reactance. The basic theory behind tuning at a frequency is to match these two up per say (inductance with capacitance) and have them cancel each other out which in turn provides path of least resistance for currents at those frequencies .
2. The reason the circuit is being wired this way is to have these resonant frequency currents circulate inside the filter only, if you ground the filter you will provide a path back to the system canceling its very purpose. .


"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[ScottyUK]

Agree 5th harmonic trap is most common. Is this installed in the 50Hz world or the 60Hz world? We're making assumptions up to now. 300Hz is 6th harmonic where I work, but 5th harmonic in 60Hz world.


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If we learn from our mistakes I'm getting a great education!

 

[VTer]

I was referring to 60hz as fundamental, my apologies for not mentioning that.

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[ScottyUK]

If you're right - which the OP's noting of drive rating in HP rather than kW seems to suggest - then disregard my last post. I was trying to acount for reasons to tune a filter to 300Hz on a 50Hz system and missed the most obvious reason. Ah well...

Came across this while looking for something else on CDA's website: CDA filter link . I haven't read it properly but might be of interest.


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If we learn from our mistakes I'm getting a great education!

 

[rockman7892]

VTer

If I understand you correctly the LC circuit is wired inbetween phases so that the resonant harmonic currents will be trapped and fluculate between the reactor and the cap. It sounds like you are saying that this LC circuit is designed with values such that at the 5th harmonic there would be a resonant frequency between the cap and the reactor and thus set up a resonant current trapped inbetween these two devices? Do I understnad this correctly?

If you ground this LC circuit would the harmonic resonant current not still flow back and forth between the cap and reactor or would it flow to ground and back to the source?

 

[ScottyUK]

The harmonic trap presents a low impedance near resonance so current at 5th harmonic (60Hz) preferentially flows down that path rather than into the source.


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If we learn from our mistakes I'm getting a great education!

 

[VTer]

rockman,

“It sounds like you are saying that this LC circuit is designed with values such that at the 5th harmonic there would be a resonant frequency”

Not exactly to resonate at 5th for the reasons mentioned above, but close to it, and yes that is my understanding of it.
And as far as grounding the filter,
Yes the current will flow back and forth through your system, but the questions is do you really want this current in your grounding system? Note that you cannot completely eliminate these harmonic currents rather just reduce the THD.

Refer the attachment for a diagram that shows a phase to phase connection.


"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[Skogsgurra]

It is unfortunate that this thread is centered on low order harmonics. That usually is not a problem with VFDs. True, they are there, but it is the HF that is the big problem in VFDs and that is mostly why filters are used between grid and VFD.

If you look at the components, they are probably not tuned to a frequency close to 250 or 350 Hz (in 50 Hz grids) or 300 or 420 Hz (in 60 Hz grids), but a frequency that is a lot higher.

The filter simply shorts out frequencies starting at around 50 - 100 kHz (150 kHz is where the European standards atart).

Also, to answer the OPs first question; they do not absorb much of the energy. They just short the frequency components so they cannot propagate out into he grid.

Another point to consider is that real good filters also have capacitors going to ground in order to keep common mode interference low - not just normal mode interference.

The resistors seen in some filters are mostly there to discharge the capacitors so the filters don't carry lethal voltages after disconnection from grid.

It is only in some motor reactor applications (other side of the inverter) that you find damping resistors parallel to the coils. They are there to avoid ringing between reactor and cable capacitance.

There is a lot of confusion about these filters. Manufacturers, salespersons, users are among the confused. Not saying that all are confused, but a large percentatge.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[rockman7892]

VTer

Thanks for the circuit diagram. I'm trying to understand how the harmonic current propogates in this circuit. I'm assuming Ih in this circuit is the total harmonic current and If is the current that is passed through the filter, while It is the current that goes back to the transformer.

Is the Ih current the current consisting of the 5th and 7th harmonic?

Why does only the harmonic currents flow as If and not back through It. In other words how does the LC combination divert and trap the harmonic current. When we talked above about the resonant current going back and forth between the cap and reactor, does it travel back and forth on the short path on the branch that they are on, or does it travel back and forth back through the transformer.

 

[VTer]

rockman,

Ih Current is the total current drawn by the vfd which includes the fundamental and harmonic currents .Now by getting the different values for Xt, Xl and Xc at different frequencies (5th, 7th, 11th, 13th…), you can calculate approximately how much current will be “diverted” by the filter using the equation provided in previous attachment. For example, lets assume 1MVA xfmr 5%Z, 60Hz, 4160V pri/480V sec. feeding a VFD driving a 350hp motor. 6 Pulse drive.

Ifla=1200A
Im=414A
Calculate Zeff = %Z(Im/Ifla) = 1.75%

From Table (Prediction of Harmonic Spectrum based upon Effective Source Impedance) I get
(at Zeff=1.75, Individual Harmonic magnitudes (% fundamental) gives 1st at 100%, 5th at 48%, 7th at 27%, 11th at 10%, 13th at 6%, and THD at 59%.)

Now these are your expected harmonics before the filter is applied.

Next step then would be to design the filter.
First thing to do is to convert the hp to approx. Kvars.
Kvar=hp*0.30 = 105
Then calculate C
Cf=Var/(2pi60V^2) = 1209 uF
Xc then would be approx 2.195ohms in our setup.

Now having calculated the C you can calculate L and design it to resonate at 282Hz (4.7*fund.)
Lh=1/[Cf(2pif)^2] = 263uH
Xl=.099

Those are your values for the filter.
Now to check the harmonic distortion after the filter has been applied. Refer to diagram previously attached.
Now next thing to do is to find Xt.(from previous diagram)
Xt=(kV^2/MVA)*Zpu
Xt=0.01152ohms at fundamental

Now all of the values are in respect to the fundamental.
Calculate Xt, Xl and Xc for 5th harmonic by multiplying fundamental values by 5.
You get Xt= 0.0576, Xl=0.495, and Xc=.438
Do the same for 7th, 11th, 13th….

Now again refer to the diagram and equation attached. Using the equation you can calculate It current, and from this example I get
5th at 23%, 7th at 20%, 11th at 8%, 13th at 5% and total with 33%.

So as you can see you reduced but not completely eliminated the current distortion.

FYI:I also get that the total voltage distortion with at approx. 3.98% which meets IEEE-519 voltage distortion standard, I believe.

I hope this helps.



"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla