High Performance Current Regulation for Low Pulse Ratio Inverters

AC current regulation for a low pulse ratio inverter is particularly challenging because the controller bandwidth is often comparable to the target fundamental frequency, which can lead to a poor transient response. This paper presents an improved current regulation strategy to overcome this issue, using state feedback concepts to deterministically compensate for PWM transport delay so that the linear controller gains can be significantly increased. Design principles for the controller are presented using discrete Linear Quadratic Regulator theory, including strategies to make the controller robust to plant parameter variations and PWM saturation. Experimental results for both a 50 Hz and 400 Hz system confirm the theoretical concepts and demonstrate the superior transient performance of the proposed strategy.