Repetitive Fast Voltage Stresses-Causes and Effects

The possibility of synthesizing power waveforms with power electronics has introduced an element of freedom that enables both energy conservation and new applications. The first applications were for low-voltage motors, especially fan motors, where great energy savings can be obtained by controlling the motor speed instead of choking the air flow. As time has passed, more motor applications have been developed, progressing to higher power and higher voltage. There are also other applications which use the benefits of a synthesized power waveform. For power transmission purposes, HVDC transmission is becoming an attractive alternative to AC transmission not only for sub-sea applications, but also because of the complicated legal procedure to build new overhead lines. Equally important is the use of power electronics in reactive power compensation, where the controllability enables substantially more power to flow through existing power lines than without power compensation. There are not many great improvements that do not also give rise to new problems, and synthesized waveforms are no exception. One area of concern is that the electrical insulation system is now exposed to wave shapes other than the traditional 50- or 60-Hz sinusoidal power frequency. To be able to synthesize the power frequency, the power electronic devices must operate much faster, usually in the kHz range. Further, the devices should have a fast transition time to reduce losses and this implies that very fast voltage transients are created. Taken together, this is a new area for electrical insulation. Applications with higher frequency have had a special niche for some time, particularly in isolated electrical systems like those in ships, airplanes, and spacecraft. Fast voltage transients are created by network switching and lightning. Continuous fast voltage transients with a high repetition rate has, however, not been a concern before the advent of power electronics and the effect on common insulation systems is to a large extent unknown. Moreover, even known effects have not yet progressed all the way from science to standard and there is thus presently both a lack of understanding and a lack of consensus in designing insulation systems for repetitive fast voltage transients. © 2009 IEEE.