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 PWM (pulse width modulation) static VAr (volt-ampere reactive) compensator, which is a typical sampled data power converter system. is presented. The open-loop transfer function of the system is derived and verified experimentally. The closed-loop operation of the system with a PI (proportional-integral) controller is investigated. 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, by considering only the principal strip model and by approximating the dead time of the zero-order hold unit, approximate models are obtained which cannot accurately predict the stability region of the system. Therefore, these approximate models are not generally reliable for design purposes. A modified frequency-domain model is used to predict the stability region accurately. The dynamic performance and the closed-loop design of this sampled data system are verified with the Electromagnetic Transients Program. All the predicted results are experimentally verified on a laboratory-scale prototype PWM static VAr compensator.<
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