A critical assessment of the continuous-system approximate methods for the stability analysis of a sampled data system

A critical assessment of the approximate methods for the stability analysis of a pulse-width modulated (PWM) static volt-ampere reactive (VAr) compensator is presented. Different continuous system approximate models for the stability analysis of the system are developed and their validity is investigated. In particular, it is shown that continuous system approximate models, in which only the principal component of the open-loop pulse transfer function is considered, cannot be used for pulsed stability analysis and closed-loop design of the system. Although these approximate analyses can be misleading and/or erroneous, a standard method such as z-domain analysis or pulse transfer function analysis in the s domain in which the sampling effect is not neglected yields useful results. A frequency domain method based on direct computation of a pulse transfer function in the s domain is used for pulsed stability analysis. The closed-loop operation of the system with a proportional-integral controller is investigated. The dynamic performance and the closed-loop design of the system are verified with the Electro-Magnetic Transients Program (EMTP)