Sinusoidal filter on VFD does not cure all 4

[Marke]

Recently, I was asked to have a look at an installation with a VFD where there were major problems with equipment failure when the VFD was running.

The VFD is a well recognized brand on a 400V MEN system. The main switchboard is about 250M from the supply transformer which supplies this switchboard only.

The supply transformer is about 30KVA, delta / star 11KW / 400V neutral earthed.

The main load on the switchboard is a 15KW VFD with in built EMC filtering and DC bus chokes. Other load on the switchboard comprises local lighting and control equipment.

The output of the VFD is connected to a sinusoidal filter which in turn is connected to a submersible pump by a 300M length of neutral screened cable.

When the drive is operated, the carrier frequency is superimposed on the supply voltage at the input to the switchboard. The level of "noise" voltage on the input is in the order of 150 volts relative to neutral.
Most single phase electronic equipment connected phase - neutral has failed due to the high noise voltage.

If we disconnect the output from the VFD, there is no problem. If we connect the sinusoidal filter only, there is no problem.If we connect the motor via the screened cable with the screen disconnected at both ends (not legal!) the noise voltage on the input is reduced to about 10 - 20 volts superimposed on the phase - neutral voltage.

Part of the problem appears to the the sine wave filter which comprises a three phase reactor with three capacitors on the output connected between the three phases.

The output waveform phase to phase is a good sinewave, but the output voltage between an output and earth, is a reduced PWM waveform. This PWM waveform is applied to the cable capacitance core to the screen and results in a high earth current.

In addition to the sine wave filter curently installed, it is obvious that we need additional filtering to remove the PWM waveform to earth.
I am tempted to try additional capacitors from the outputs of the sine wave filter to the positive and negative rails on the DC bus to provide a return path for the HF signals.
Any comments?

Best regards,
Mark.

Mark Empson
L M Photonics Ltd

 

[DickDV]

Marke, whenever I stumble over problem systems like you are describing, my first reaction is to install an isolation transformer with electrostatic shielding. It's not inexpensive but it immediately minimizes so many potential problem sources.

In addition to being a reasonably good input harmonic filter, it essentially isolates ground problems to the drive system alone. In the case you describe, I would expect that the ground isolation feature alone would provide most of the relief you are looking for.

If it doesn't do the whole job, at least you have the benefit of knowing that your system (including its ground network) is pretty much isolated and your investigative work can be contained within the drive/motor-lead/motor system.

 

[sibeen]

Silly question time.

If you decide to use an isolation transformer on the output of a drive, what frequency do you design the transformer to operate at?

If the answer is the nominal frequency of 50 or 60 Hz, do you then need to oversize the transformer to take into account the times when it will operate at frequencies below the nominal?

Also, do you need to provide some form of mitigation of the inrush current to the transformer or can this be accomplished by varying the V/Hz?

 

[itsmoked]

sibeen; The transformer is for the constant freq supply not the output.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[sibeen]

That was quick

So it's for the input to the drive?

 

[itsmoked]

Yes.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Marke]

Not necessarily, one of the options that I referred to above, was to put a delta primary star secondary transfomer on the output of the drive. (refer to my posting on 3 Jan)
In terms of transformer design, it should not be an issue as the transformer is like a motor and provided that the flux is kept constant, there will be no issues. The VFD will reduce the voltage as the frequency is reduced and so the flux will remain with in spec.
The advantage of the transformer on the output is that the drive is designed to produce a sinusoidal flux in the gap of the motor, so it will produce a sinusoidal flux in the core of the transformer, and so the secondary voltage from the transformer should be largely sinusoidal.

Best regards,
Mark.

Mark Empson
L M Photonics Ltd

 

[LionelHutz]

Mark;

I'm thinking you've got a good handle on the basic problem. I'd agree that a transformer on the output should work. I experimented with this once with a standard 600V:4160V transformer and the output voltage on the transformer was close to a sinewave. To me, this seems to address the root problem.

I'm guessing a transformer on the input would work, but just because it would isolate the VFD from the rest of the power system. You'd still get the common mode PWM coupling back into this new transformer. This wouldn't really address the root problem but rather be a patch.

 

[Ares]

I belive the impedance and capacitance to ground of the long line results in some sort of resonance with output filter.

Isolation transformers "a delta primary star secondary transfomer on the output of the drive" should solve the issue.

I would connect the shield of cable to that transformer X0 but also add a ground rod at the pump and connect the shield there for safety. 300m is significant distance and your ground at the pump may not be 0V

 

[sibeen]

Did you get this resolved, Mark? I'd like to hear how it ended up.

 

[Marke]

No, not yet. I will let you know the outcome when we have a working solution.

Best regards,
Mark.

Mark Empson
L M Photonics Ltd

 

[burnt2x]

Not necessarily, one of the options that I referred to above, was to put a delta primary star secondary transfomer on the output of the drive. (refer to my posting on 3 Jan)
In terms of transformer design, it should not be an issue as the transformer is like a motor and provided that the flux is kept constant, there will be no issues. The VFD will reduce the voltage as the frequency is reduced and so the flux will remain with in spec.
The advantage of the transformer on the output is that the drive is designed to produce a sinusoidal flux in the gap of the motor, so it will produce a sinusoidal flux in the core of the transformer, and so the secondary voltage from the transformer should be largely sinusoidal.

From my own experience, we inserted the 1:1 isolating transformer before the VFD, as DickDV suggested. I cannot really compare your case with mine because of the size difference (Ours is a 607kW positive suction pump supplied by a 1000 kVA isolation trafo). We've been running for about six years now without glitches.

 

[DMattReyn]

There is a solution for exactly this problem from EPCOS called a SineFormer. Also reduces motor bearing current to extend motor life. Check it out.

 

[dpc]

DMattReyn,

The entire point of this thread is that sine filters alone don't always solve all problems.

Are you connected with EPCOS?

"An 'expert' is someone who has made every possible mistake in a very narrow field of study." -- Edward Teller

 

[Marke]

Hello DMattReyn

This filter is not quite the same as the one that was used, but I do not believe that it will cure the problems.
The filter used had only one reactor per phase and three capacitors tying all three phases together at the higher frequency. (the first stage of this one is the same)
This filter includes a second reactor per phase and a capacitor to earth.
This would work provided that the second reactor was large enough to filter out all the common mode PWM waveform, but the remaining common mode PWM current is still routed to earth via the output capacitors which is essentially the problem that we have now except that the output capacitance is in the cable, not lumped C inside the filter.
I believe that the output capacitors need to go back to the DC bus on the drive, or to earth with the bypass capacitors on the drive considerably increased in size to make them work at the carrier frequency rather than the RF frequencies.

Thank you and best regards,

Mark Empson
L M Photonics Ltd

 

[DMattReyn]

The second stage of the EPCOS SineFormer is an engineered EMC filter, so the second reactor is a CMC and the capacitor to earth is specially designed to be a lower impedance path to ground for the common mode current than the parasitic capacitance of the cable.

This product was specifically designed for this problem and is the proper solution. These SineFormers are available through the industrial distributor Galco in Detroit, MI. Are you familiar with them?

You can see some more detail about this in the product brief from the EPCOS website at the link here.

 

[Marke]

Hello DMattReyn

Yes, I understand this, but the problem that we have is not an EMC issue in that it is not very high frequency components that we are seeing, they are lower frequency components and the "EMC filter" will not solve this.
If we divert the common mode HF signals to earth, it still needs to get back to the inverter output. This is done via he decoupling capacitors inside the inverter that decouple the DC bus to the chassis. Your filter takes some/all of the common mode signal and shorts it to earth, but it still only takes it to earth which is what we currently have. To get back to the inverter output, the signals pass via the earth and then the earth - neutral connection, down the neutral to the transformer and back on the three phases to the input of the VFD.
The impedance from the VFD to the transformer is high at these frequencies and so there is a high voltage drop on the neutral circuit causing the phase neutral voltage to be heavily modulated.
I do not see that taking this information is going to help the problem as the problem that we have at present, is too much HF signal on the earth and neutral conductors.

Best regards,

Mark Empson
L M Photonics Ltd

 

[sibeen]

Mark, was there ever any resolution on this project?