Design-oriented analysis of DC bus dynamics in adjustable speed drives under input voltage unbalance and sag conditions

Utility power quality problems like input voltage unbalance and sag conditions severely stress diode bridge rectifiers used in adjustable-speed drive (ASD) equipment. The key phenomenon that causes these problems is the transition of the rectifier from three-phase to single-phase operation. This is accompanied by increased harmonic currents that shorten the dc-bus capacitor life span. In addition, the bus ripple voltage causes undesired harmonic phase currents in Volts-per-Hertz ac drives, leading to elevated machine heating and the generation of pulsating torque at the second line harmonic (i.e., 120 Hz). This paper develops closed-form expressions that lay a solid foundation to analyze and quantify these phenomena, making it much easier to investigate alternative mitigation strategies. The paper identifies the boundary conditions that determine the entry of the rectifier stage into single-phase operation under unbalanced voltage sag conditions. Closed-form equations for the dc bus voltage and current are derived valid for finite line impedance, finite bus capacitance and varying loads. These expressions make it possible to conveniently conduct parametric studies of the system behavior under wide ranges of operating conditions without resorting to time-consuming time-domain simulations. Simulation and experimental tests are used to verify the accuracy and effectiveness of the analysis by comparing key predicted power quality performance indices (e.g., input power factor, current crest factor, THD) to simulated and measured values using a 5 hp ASD system.