How to Extract the Harmonics in Three Phase Unbalanced Load

Use tuned devices to capture these harmonics. This principle is ued in Tuned Harmonic Filters.

Use an FFT algorithm to compute the averaged frequency domain. Then extract the values of the harmonics of interest.

The best accuracy would be obtained if you derive the sampling frequency for the FFT from the fundamental frequency. That is, if 60 cycles is the fundamental, use a multiplier to get 600 cycles for your sampling rate.

Thanks.

I heard that there is other method available for based on dqo transformation based. Can you tell me about that method. I heard that this method is very efficient from computation point of view.

Thanks once again

The DQO transform is a trigonometric conversion of three phases to a single phase, assuming the angles are constant. If you do not need the phase shift of each leg independently, then you can use DQO. Extraction of the values for each harmonic would be the same as for a single channel.

If you need each phase independently for the IGBT circuits, you will need three FFT channels (many 4-channel analyzers are available). Extracting the M and Ph of each harmonic can be done with software provided by most FFT analyzers.

Thanks for the reply.

I want to design active harmonic filter with 3 phase 4 wire system. This is based on the IGBT circuit. The IGBT need to control in pair so that i can reduce the harmonics in the line.

I think doing the harmonic analysis with FFT with controller will consume lot of computational power & will not be able to extract higher harmonics such as 11th, 13th, 17th, & 19th.

Did you use FFT for this application? If yes then how is the computational efficiency? Any specical contoller need to use?

Thanks in advance for your support.

Paras

I used multi-channel FFT analyzers for dozens of applications over a 13 year period. They are easy to use, easy to program, and very fast. Capturing three channels with a bandwidth of 2,000 Hz (enough to accurately measure the 19th harmonic of 60 cycles) is fast. Then programming it to extract the amplitude and phase of each harmonic is a small task for anyone with programming experience (Basic, for instance). The results would be available to your IGBT devices in seconds, and can be repeated in near real-time if needed.

Try visiting the Spectral Dynamics web site, and search for the Bobcat analyzer series for an example of this type of analyzer. It uses a standard PC for the computational engine.