Line-frequency rectifier dc-bus voltage instability analysis and mitigation

Use of small dc bus capacitors is desirable in variable-speed drives and distributed power systems as it can reduce the size and improve reliability of the system. However, small dc bus capacitors may lead to instability of the dc bus voltage. Such voltage stability problem can occur when the bus is fed from a PWM rectifier or a line-frequency rectifier. This paper considers the case when a line-frequency rectifier is used and presents a method for the analysis of dc bus voltage instability in such systems. Small-signal input impedance of multipulse line-frequency rectifiers is used in conjunction with the ac source output impedance to detect possible instability on the dc bus. Stabilization of the dc bus voltage by a passive damping is presented and compared to other stabilization methods such as increasing dc bus capacitance or placing the damper at the ac input of the rectifier. Experimental results from a 12-pulse rectifier feeding an electronic load that is programmed to emulate constant-power load behavior typical of DPS and variable-speed drives are presented to validate the analysis and design approach.