A Simple DC Imbalance Rejection Technique for UPS Inverters

Based on an in-depth analysis of the DC imbalance problem of UPS inverters, an observation is made that in most cases where instantaneous voltage feedback control is applied, the control algorithm itself is the most significant cause of DC imbalance since it inevitably generates DC bias in the output PWM gating signals. This situation becomes even worse when some integral action is included in the control algorithm, such as a PI/PID controller, or a repetitive controller, which is integral in nature. With this understanding, a simple DC-bias rejection algorithm is proposed, which needs no extra sensors, and can be seamlessly integrated into a repetitive controller. Experimental results verified its effectiveness. An alternative form of the algorithm is also provided for non-repetitive-control cases