Prediction techniques compensating delay times caused by digital control of a three-phase buck-type PWM rectifier system

The digital control of a three-phase, three switch buck-type rectifier system is analyzed hi this paper. Three main sources of delay times in the control loop can be identified for the implementation on a digital signal processor, namely: the delay time due to the sampling of the control quantities; the one due to the calculation time of the DSP; and the one due to the sample-and-hold function of the PWM modulator. Their influence on the stability of the inner current control loop is discussed and two prediction methods for compensation, namely a linear prediction and the Smith prediction, are comparatively evaluated. The control performance and the effect of the delay times and the prediction methods are finally shown by simulation results and through and measurements on a 5 kW prototype.