Analysis of common mode Voltage - "Neutral shift" in medium voltage PWM adjustable speed drive (MV-ASD) systems

In this paper analysis of common mode voltage or "neutral shift" in the new emerging voltage source inverter type medium voltage adjustable speed driv e (MV-ASD) systems is presented. Both cascaded multilevel (CML) inverter an d 3-level NPC inverter topologies are analyzed. An equivalent circuit model to determine the common mode voltage stress is presented. Analysis and sim ulation results are discussed and worst case common mode voltage excursion is computed for an example 800 Hp, 4160 V MV-ASD. It is shown that certain system components are excessively stressed and in the MV-ASD system these d ata are particularly useful in specifying system components and for proper design of the system. Possible effects of common mode voltage and its dv/dt on medium voltage motor bearings are discussed. A new multilevel PWM strat egy is introduced which results in zero common-mode voltage. Simulation res ults are shown to illustrate the effectiveness of these schemes. Finally, e xperimental results from a 800 hp, 4160 V,MV-ASD system are presented.