Active Harmonic Mitigation: What the Manufacturers Don't Tell You

Despite their high costs, active harmonic mitigation solutions such as parallel active power filters (APFs) and active front end (AFE) drives are growing in popularity. As the newest technology, they are being touted as a better choice than the various forms of passive harmonic mitigation solutions that are presently available. Is this actually the case? Active solutions incorporate switching strategies using insulated-gate bipolar transistors (IGBTs) to make the current drawn by the adjustable-speed drive (ASD), or another nonlinear load, more sinusoidal. You will rarely hear from manufacturers that this switching introduces higherfrequency harmonics, normally above the 50th. When measurements are taken up to the 50th harmonic, the current total harmonic distortion (ITHD) is often quite low. However, when measured up to the 100th harmonic or higher, the ITHDs almost always exceed their claimed performance levels, which consider only harmonics up to the 50th. This is certainly a concern because higher-frequency harmonics are more likely to cause power system problems and issues with other connected loads than the lower-frequency harmonics that they are designed to reduce.