Analysis of common mode voltage-“neutral shift” inmedium voltage PWM adjustable speed drive (MV-ASD) systems

摘要

In this paper, an analysis of common mode voltage orn“neutral lift” in the new emerging voltage source inverterntype medium voltage adjustable speed drive (MV-ASD) systems isnpresented. Both cascaded multilevel (CML) inverter and 3-level NPCninverter topologies are analyzed. An equivalent circuit model tondetermine the common mode voltage stress is presented. Analysis andnsimulation results are discussed and worst case common mode voltagenexcursion is computed for an example 800 hp, 4160 V MV-ASD. It is shownnthat certain system components are excessively stressed and in thenMV-ASD system these data are particularly useful in specifying systemncomponents and for proper design of the system. Possible effects ofncommon mode voltage and its dv/dt on medium voltage motor bearings arendiscussed. A new multilevel PWM strategy is introduced which results innzero common-mode voltage. Simulation results are shown to illustrate theneffectiveness of these schemes. Finally, experimental results from a 800nhp, 4160 V, MV-ASD system are presented