Active Harmonic Mitigation - What the Manufacturers don't tell you: Copyright Material IEEE, Paper No. PCIC-2018-43

Despite their high cost, active harmonic mitigation solutions, such as parallel Active Power Filters (APFs) and Active Front End Drives (AFEs), are growing in popularity. As the latest technology, they are being touted as a better choice than the various forms of passive harmonic mitigation solutions presently available. Is this actually the case? Active solutions incorporate switching strategies using IGBTs in order to make the current drawn by the adjustable speed drive, or other nonlinear load, more sinusoidal. What you will rarely hear from manufacturers is that this switching introduces higher frequency harmonics, normally above the 50
^th
. When measurements are taken up to the 50
^th
. current total harmonic distortion (ITHD) is often quite low but when measured up to the 100
^th
or higher, ITHD almost always exceeds their claimed performance levels which consider only harmonics up to the 50
^th
. 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. Although there are IEEE and IEC industry standards that restrict levels of harmonics in the low frequency and very high frequency ranges, there are no standards presently that address the range between 2 kHz and 150 kHz. Manufacturers therefore, often design active harmonic mitigation equipment that generates relatively high levels of these mid-range frequencies, particularly since switching frequencies of IGBTs typically fall precisely within this range.