FILTER

Google harmonic filter design.

http://www.pqsoft.com/filter/index.htm is only one of many points of information.

Please respond with your comments.

harmonics is distortion in power of voltage or any signals.

IN the case of VFD's 6 stage of 12 stage active filter ing is done.. so 5th and 7th harmonics is nullified.

so the thing is like i too donn't know exactly what id done with this harmonics. I'm having a problem with some hospital appliences because of these Variable frequency drives.

Harmonic Filter.

1)First of all you must identify the bus where the harmonics current flow in the bus.

2) If the load that produces harmonics is identified , then therotically calculate the harmonics currents as per IEEE-519 or you can measure harmonics by connectiong Harmonics Analyser to the bus where these harmonics currents flow.

3)Compute magnitude of each individual characteridtics harmonics(5th,7th,11th ,13th harmonics etc) either by theoretical method or by the measurement out put of harmonic analyser.

4) Now we have fairly good idea of how much magnitude of each harmonics current produced by the load in the network.Add 10% for each harmonics for desgn factor.

5)The harmonics filter consists of reactor and capacitor which is tuned for each computed harmonics.ie at thae particular harmonic frquency say 5th harmonics , the reactive impedence due to inductor is equal to capacitive impedence.ie XL=XC at nth Harmonic tuned frequency. It means that the fiter is tuned to that particulsr harmonic frequency and gives minimum impedence at that frequency (Xl=XC).Hence the harmonics currents instead of flowing in the network is diverted to the respective harmonics tunedfilter.Hence network is free of harmonics of the frequency.

6)The reactor and capacitor which forms the filter circuit of tuned filter shall be sized to carry these harmonics currents in addition to the fundamental current it has to carry for power factor correction.

7)The harmonic filter act in two ways.One way is that at fundamental frequency (50/60HZ) the harmonic filter circuit acts as power factor improving device and at harmonic frequency it acts as filter to that particular frquency.

Hope i have made clear

V.Ambarani

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Here are a few general advises based on real experience.

If you never did this kind of work, start with a simple filter centered at 4.5X the line frequency. This will make it less aggressive to the 5th. Aggressivity in filters can be damaging.

Oversize your inductor current capacity by about 20%-30%. Harmonics currents produce more heating and the lower magnetic saturation will keep the filter more stable.

Use capacitors with higher voltage rating. The filter inductor will produce a voltage increase and as the capacitor ages, it is likely to loose some capacitance and change the tuning frequency. Higher voltage capacitors age slower because they are less stressed.

Make it out of two banks (same frequency) to add flexibility. You might not always need full filtering. Each bank need its own set of fuses and potentially a contactor to switch on when needed.

If the filter capacity is a good percentage of the supply feeder, you will need to take its impedance into account.

If other harmonic loads are present in the area, you might attract harmonics from other sources. The over-capacity of your filter might help your filter survive.

If you don't have a power meter available, begin with adding up all the loads current from drives in the plant. They can contribute up to 30% of 5th harmonics which is the major component. This will give you a rough estimate for the filter needed.

It is likely that you will have to tune up the design as time goes by and the plant loads are changed.

Good luck.