Harmonics and technical barriers in adjustable speed drives

The author reviews the characteristics of harmonics produced by an adjustable-speed drive system as well as methods of reducing them, and presents installation examples. He analyzes the presence of harmonics in both six-pulse and twelve-pulse converter systems and highlights the differences between three-phase and six-phase machines in suppressing these harmonics. The effect of using harmonic filters and a special transformer winding arrangement to reduce harmonics is discussed. Harmonics measurements for two actual installations are presented and analyzed. It is concluded that the order and magnitude of the harmonics generated greatly depend on the drive configuration and system impedance. A three-phase machine connected to a six-pulse system will generate more harmonics than a six-phase machine connected to a twelve-pulse system. Harmonic losses in the stator and rotor must carefully be taken into account during design to keep motor temperature rise within acceptable limits. Harmonics fed back to the power system are reduced by filters connected to the incoming power supply. These filters are relatively large and occupy substantial space in the substation yard.<>