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How to measure the effectiveness of harmonics filter


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#1 jonleelk

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Posted 13 October 2005 - 04:40 PM

A simplified illustration of the hardware:

3 phase delta-wye transformer -> AC reactor -> shunt LC harmonic filter with capacitor arranged in delta connection -> 3 phase AC-DC converters -> 3 phase DC motors.

With the aging of the capacitors in the harmonic filter, I need to measure the effectiveness of the harmonic filter. Other than using RCL meter to measure the value of the LC in the shunt harmonic filter, at which location should I conduct the THD measurement? Should I be measuring current or voltage THD?

Based on my understanding, I should be measuring current THD at the location between the secondary side of the transformer and the AC reactor (since the harmonic current will be circulating between the LC harmonic filter and the DC motors).

Is this correct? What instrunment should I be using? ;p;

Appreciate the assistance of the experts.

#2 marke

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Posted 13 October 2005 - 05:52 PM

Hello jonleelk

Welcome to the forum.

There is no absolute right or wrong to this, it depends on what your particular issues are.
Generally, the reason for the filter is to reduce the effect of the harmonic currents on the supply voltage. It is the voltage disturbance that causes interference with other equipment.

My approach would be to look at the voltage disturbance at the output of the transformer. If this is within acceptable limits, I would do no more.

Many modern oscilloscopes and meters have an FFT function which is able to give you a spectral output of you applied voltage. I would beg, borrow, hire such equipment if you do not already have it and make a quick test.

Best regards,

#3 jonleelk

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Posted 14 October 2005 - 04:58 AM

Thank you Mark for the reply. When u mention looking at the voltage disturbance at the output of the transformer, are u referring to taking the measurement at the primary or the secondary? My guess u are referring to the secondary, before the AC reactor. Correct?

Base on my understanfing, as the harmonics current is circulating between the harmonics filter and the load, voltage wise, it should be on the same common bus after the AC reactor, whether I am taking measurement before the shunt LC filter or after the shunt LC filter. However, if I measure the voltage disturbance before the AC reactor, it should reflect the effectiveness of the harmonic filter as most of the harmonics current will be reduced by the series AC reactor at the transformer output.

I also have some other equipment where there is no AC reactor between the transformer output and the shunt LC harmonics filter. For those equipment, am I able to determine the effectiveness of the harmonics filter if I am still taking a voltage measurement at the transformer output? This was why I was thinking of measuring the current distortion.

Reason for the measurement is that recently, we are encountering supply voltage fluctuation to more than 110%. Based on consultant's opinion, it is due to harmonics resonance. As such, I need to conduct a measurement on the effectiveness of the harmonics filter installed on my machines to determine which might be the likely cause to the resonance.

#4 marke

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Posted 14 October 2005 - 09:18 AM

Hello jonleelk

Yes, you should measure on the secondary of the transformer. If you have significant voltage haromincs at this point, then your filter should be improved.

If you are haveing supply resonance problems, I would look at the power factor correction capacitors nerby and ensure that they have detuning reactors fitted. Most capacitors are installed without detuning reactors and this is a major source of problems.

Best regards,




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