Design of controller for inverters with ultra-low THD: A repetitive and predictive-PID controller approach

This paper presents a design and an implementation of inverters with ultra-low total harmonic distortion (THD) under all kinds of loads. In this study, the ultra-low THD objective is achieved by combining the advantages of the repetitive controller, the predictive-PID controller and the reference signal feedforwarding method in a multi-loop structure. The high performance uninterruptible power supply (UPS) and the transformer test converter applications where the ultra-low THD is desired require the use of high performance controllers. This study successfully achieves the design of such a controller by means of computer simulation, which is based on the true model of the inverter system. The tuning of the controller and inverter parameters made through simulation has yielded a successful hardware design and a good performance during the experimental tests. The performance of controller is evaluated in terms of output voltage waveform distortion under actual linear and nonlinear loads. The experimental results that demonstrate the success of the controller are obtained from the voltage source inverters (VSI) built for a three-phase UPS application at 30 kVA and a transformer test converter application at 300 kVA. The maximum measured THD is less than 2.88% under the worst nonlinear load case. The converters employing the designed controller are currently in use with success in the field.