Harmonic mitigation requirements for Variable Speed Drives are growing fast as standards are being enforced more diligently.  Many dynamic loads on one site can cause very high levels of power quality disturbance. If the grid is weak this can have severe effects on the availability and life span of equipment as well as causing small but measureable power losses. Most of all though, power quality is a matter of availability and productivity.  Power quality is a key driver for profitability in process industry.

Available Low Harmonic Drive Solutions

There are several solutions available in the market today. Here we focus on Low Harmonic Drives offering <5% THDt distortion focused on fulfilling IEEE-519 requirements. The solutions normally offered are passive multiband filters (PHF), phaseshifting 18-24 pulse drives (MPD), Serial Active Filters (AFE) and Parallel Active Harmonic Filters (AHF).

Passive Filters

Passive filters have often been used due to the low investment cost. If the cost of catastrophic risk and inability to cope with changes in load profile are included however, the passive solution can quickly become very expensive.  The high risk of operation, poor performance in the field and the issue of status monitoring make these solutions less desirable.

Active solutions are slightly more expensive but improve  the systems behaviour a lot. The active technologies can not be overloaded, offer monitoring and control and allow fast response in case of failure.

18-24 Pulse Systems

Phaseshifting 18-24 pulse systems are very sensitive to unbalances, which again reduce their effectiveness in the field. Foot print is also greater than that of a passive system.

Serial Active Filters (AFE)

Serial active filters or Active Front End (AFE), as they are commonly referred to, are a very common mitigation technique. The downside to these is that they are serial solutions that have to be sized at 100+% of the drive load. They are commonly comparatively inefficient, making them expensive in the long run. A serial solution also creates a far more vulnerable system a dito parallell solution.