Stability analysis and performance characteristics of an open-loop PWM VAr compensator

An open-loop, three-phase, transistorized volt-ampere reactive (VAr) compensator with a single pulse-width modulation (PWM) pattern is analyzed. The VAr generation is controlled by shifting the fixed PWM pattern with respect to the AC-mains voltage. The system is naturally stable, thereby circumventing the need to control the capacitor DC voltage, which requires a stable voltage for each power factor operation condition. This particular behaviour is studied, and a complete stability analysis based on the d-q frame is developed. From this analysis is obtained the circle diagram of the VAr compensator, which gives all the operating characteristics related to the DC voltage the power angle operation, and the amount of reactive power generated. The sensitivity of the system depends mainly on the X/R ratio, and the dynamic response depends on the size of the DC capacitor. Simulations and experiments confirm the results obtained in the theoretical analysis.<<ETX>>