Model predictive control with a fixed switching frequency for an AC-to-AC converter

This paper discusses the control of an AC-to-AC converter that forms part of an 11 kV electronic voltage regulator. A novel finite-set model predictive control technique is presented to allow fixed frequency switching. A fixed switching period is defined and divided into smaller evaluation steps. The switching behaviour is obtained by using the analogy of PWM with a triangular carrier wave. During the first half of the switching period, the system may only switch from the on-state to the off-state and during the second half, only from the off-state to the on-state. To account for the repetitive nature of the switching waveform, an average receding horizon prediction method is introduced. An anticipated prediction is made by calculating the average value for one switching period ahead, according to the position and state in the current switching period. The method presented good practical results and proved to be effective in maintaining a fixed switching frequency.