Stability of rectifiers with voltage-controlled oscillator firing systems

A new linearised discrete model for a rectifier developed recently has been used successfully in stability studies of convertors with conventional individual-phase firing systems. The model is modified to simulate rectifiers incorporating voltage-controlled oscillator firing systems that possess an integral characteristic. This modified model is used in the stability study of a rectifier operating under constant-current control into an inductive load. Standard sampled-data system theory is used in the analysis. To achieve adequate transient response, it is shown that compensation is necessary; the effect of this compensation on the stability boundary of the system is then investigated. The static error in direct current caused by a.c.-system frequency variations is also studied, and possible solutions to annul this error are discussed. One such solution involves proportional-integral control which results in conditional stability; however, it is shown that adequate stability can be achieved if suitable gain and time constants are chosen. Experimental results obtained on an h.v.d.c. simulator are in good agreement with predictions from theory. It is shown that a firing system can be devised incorporating a voltage-controlled oscillator but possessing a proportional rather than an integral characteristic.