Harmonic filters – where size matters
Frank Griffith, a consultant engineer with ABB in the UK, looks at some of the factors that affect the sizing of harmonic filters for drives. Griffith has extensive experience of designing drive systems with several pioneering designs to his name. He is an MIET and and a lecturer on drives topics
Effective control of harmonics often requires a much larger filter than was initially thought necessary.
Harmonic distortion affects the shape of the sine wave of the network voltage. This shape is influenced by the background distortion as well as by the distortion from the connected load, such as a drive or a rectifier. When filters are installed and begin to clean up the harmonics, less harmonics are injected on the network. The voltage shape improves and the waveform gets closer to a clean sine wave.
But the proportion of harmonic components changes with the shape of the voltage waveform, generally increasing in magnitude as the voltage shape improves. The fifth harmonic component tends to make a flat voltage peak, which becomes more rounded, due to reduction of the harmonic supply current, which can increase the fifth harmonic component so much that the capacity of the filter is exceeded.
Harmonic filters can be sized using dedicated software or with measurements on site. However, measurements can be misleading, as it is difficult to predict just how much the fifth harmonic will change when the filters are operating. As a rule of thumb, filter ratings should be oversized by 50%, relative to the accumulated load, to ensure there is sufficient capacity to deal with all parts of the harmonic spectrum effectively