Investigation of common mode current related dc-bus overvoltage in multiple converter systems

Active front end (AFE) converters have been extensively used in industrial motor-drive systems due to their energy regenerative capability and low harmonic distortions. However, the Pulse Width Modulation (PWM) switching operation in AFE converters generates higher common-mode voltage than their non-regenerative diode front-end (DFE) counterparts. Such significant common-mode voltage can be converted to common-mode current through common-mode capacitance in the system. This large common-mode current can cause overvoltage of the dc-bus of a DFE converter system where both AFE and DFE converter systems share the same ac source. This paper investigates such overvoltage phenomena occurring in a DFE dc-bus in a multiple converter system. Mechanism of such dc-bus overvoltage will be explained, and both simulation as well as experimental results are presented to confirm the theoretical analysis. Moreover, a dc-bus clamping circuit is recommended to clamp the DFE dc-bus voltage, thus any overvoltage that could trip the DFE drive can be avoided.