7. Disturbance Rejection

Digital current controllers have the crucial impact on performance of grid-side converters and ac drives. The tasks of the current controller include an error-free tracking of the input reference but also the suppression of the voltage disturbance. In ac drives, the voltage disturbances are the back-electromotive forces of ac machines. In grid-side inverters, the voltage disturbances are the line voltages. The voltage disturbances are commonly suppressed by enhancing the controller with an inner active resistance feedback, as described in Sect. 4.​5. In cases where the switching noise and parasitic oscillations introduce sampling errors, conventional sampling is replaced by the oversampling-based error-free feedback acquisition which derives the average of the measured currents over the past switching period. This one-PWM-period feedback averaging is introduced in Sect. 3.​3. The time delay introduced into the feedback path creates difficulties in designing the current controller with the active resistance. In this chapter, the possibility of applying the active resistance feedback in systems with the error-free sampling is introduced and discussed. The analysis is focused on studying the impact of transport delays, introduced by the feedback averaging on the range of the applicable active resistance gains. The internal model principle is applied in order to get the modified current controller where the active resistance gain does not affect the input step response. Disturbance rejection capability is tested analytically, by computer simulation and experimentally.