A medium-voltage drive utilizing series-cell multilevel topology for outstanding power quality

Early medium-voltage drive implementations suffered the serious drawbacks of high harmonic content of the input and output current, poor power factor, and high common mode voltage and dv/dt on the machine. Recently, there have been modifications applied to some of these old circuits to improve their performance. But these fixes require more switching devices and/or the addition of filters. A completely new method of medium-voltage power conversion based on the series connection of PWM-IGBT converter cells has overcome all of these difficulties without filters. In fact, the input current meets the most stringent requirements of IEEE-519 (1992), and no output current harmonic exceeds 1% of fundamental. Power factor above 95% at all load points and efficiency of 96% at full load are routinely attained. The distributed architecture of this new drive also provides fault tolerance and very high availability. The design and construction of this new drive is described, as well as the means by which it provides these outstanding performance characteristics. Field experience gathered from hundreds of operating units is also reviewed