Sine Wave Filters Have Limitations.

[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 11KV / 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.

[BigMax]

Hello Marke,

 

I've had some experience with a similar problem

 

I agree that a sinewave filter can do a good job on differential mode energy... keeps the motor happy and reduces the effect of Voltage magnification with long cables.

 

BUT, I also noted that common mode energy (coupled to earth via motor cable capacitance) can still be considerable when sinewave filters are used.

 

The approach I took was to consider where this common mode energy was going to... there must be a 'circuit' of course... and as you have experienced Voltage distortion at the distribution level (causing load failures) it seems clear that a lot of the common mode energy is returning at the 'point of common coupling'.. i.e at the MEN point ... then completing the circuit back to the VSD via its' mains supply.

 

What can be done?

 

I considered the common mode action of a typical EMC filter. To my way of thinking , the VSD input EMC filter (should) provide a low impedance return path for the common mode energy... with the bulk of this energy returning via the VSD local EMC filter rather than at the PCC / MEN point.

 

What I did simply was to take aditional (otherwise suitably rated) EMC filter(s) and added them to the input(s) of the of the VSDs (which had otherwise integral EMC Filters). Result? Much reduced THVD at the PCC

 

I hope this is helpful.

 

Cheers!

 

BigMax

 

 

[marke]

Hi BigMax

 

Thank you for your comments.

 

The problem I observed was due to the "earth" current returning down the neutral to the transformer and back on the phases. Because the neutral impedance at the high frequency is high, the resulting voltage drop is high.

 

I think that part of the problem is that the rate of rise of voltage of the common mode voltage is much lower than the output waveform of the drive due to the reactors in the filter. This concentrates the energy at lower frequencies than usual and hence the impedance of the return path to the DC bus is much higher. If we increased the value of the decoupling capacitors from the DC bus to earth, we should reduce the neutral current.

 

My other thought was to add capacitors from each output of the sinusoidal filter to the DC Bus. (6 capacitors in all)

 

Unfortunately, the installation is too far away to experiment!

 

Have a good day,

Best regards,

Mark.