Warning! Harmonic distortion! Effects, measurements, analysis
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What are the harmonic frequencies? The distortion of the power signal in the power network from the shape of the ideal sinusoid indicates a certain harmonic content in this signal. Figure 1 shows three waveforms from a three-phase network with a high degree of harmonic content. The shape of the waveform of each phase is far from the ideal sinusoid. Theory says that each distorted signal can be decomposed into a set of harmonics being a multiples of the fundamental frequency. For a power network with fundamental frequency of f1 50 Hz harmonics are all component signals with frequencies being multiples of 50 Hz, such as f2 = 100 Hz, f3 = 150 Hz, etc. Figure 1. Chart from the Sonel Analysis program. Three current waveforms in a three-phase network, recorded with the PQM-702 analyzer.
Four examples of the negative impact of harmonics 1. Temperature of the conductor outside the limit The skin effect is the cause of excessive heating of the conductor wire with high harmonic content in the current signal. For the frequency of the 50 Hz, the skin effect is small, but above 250 Hz, which corresponds to the 5th harmonic, the effect is significant. The third current harmonic and its multiples does not cancel in a three-phase four-wire network. The lack of the third harmonic cancellation effect results in the addition of each phase harmonic currents in the neutral conductor. In this case, a current which will flow can significantly exceed the thermal limit of the neutral conductor. This can lead to overheating and, as a result, melting or even igniting the insulation of the cable and flammable materials in the vicinity.
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