Development of a new current controlled pulse width modulator for voltage source inverters

Abstract

This thesis is concerned with the improvement of current controllers for use with voltage source inverters. Analysis of hysteresis current controller (HCC) is presented. The common problems of non-uniform and burst switching of scalar type controllers (hysteresis current controller and ramp comparison controller (!tCC)) are addressed. An improved model of HCC, referred to as PRC is proposed for uniform switching frequency operation. Theoretical basis is established where, an add-on function of programmed amplitude and slope can keep the switching frequency of HCC to a predefined value. In addition to uniform switching frequency operation, the proposed model has significant improvement on the total harmonic distortion coefficient. Analysis of the predictive controller is also made in this research. The shoot through current behavior of predictive controller is improved by the use of a voltage vector limiter. A generalized approach is proposed for the design of the proposed predictive controller with inductive load. Separate schemes are proposed for implementation of PRC and predictive controllers based on general purpose PC architecture. Incorporating the merits of both PRC and predictive controller, a new and novel current controller (referred to as NEW-CC) is developed in this work. This controller operates on regular sampled basis and predicts pulse widths of individual phases once on each carrier period. The load parameters are tracked on each fundamental as well as carrier cycles. Uniform switching as well as shoot through current limiting is achieved in this new controller. The performance of this new controller is compared with the other current controllers and is found to give better performance. The new current controller has lower total harmonic distortion than other current controllers. This controller do not have sub-harmonics in the load current spectrum as is normally encountered in scalar type current controllers at large bands. The new current controller has been experimentally tested. Its performance is in good agreement with the theoretical results.