A novel instantaneous power control strategy for a PWM rectifier under unbalanced input voltage conditions

Instantaneous power regulation is an effective way to improve the performance of a PWM rectifier operating under unbalanced input voltage. By properly presetting current commands, this approach can maintain the desired performance features normally achieved only under ideal balanced operating conditions. These desirable performance features include sinusoidal input current at a near unity power factor, nearly constant DC output voltage with a small output capacitor and also fast dynamic response. However, the relationship between power and current in an AC to DC PWM rectifier is highly nonlinear under unbalanced operating conditions and it is not normally clear that a solution to the nonlinear system fulfilling all of the desirable requirements exists. The uniqueness and existence of a solution for such a nonlinear system is difficult to demonstrate. This paper presents a novel instantaneous power control strategy in which current commands are well regulated so as to distribute input power instantaneously and appropriately to the DC load and to the synchronous inductances while maintaining a constant DC voltage and sinusoidal input currents. A good feature of the proposed method is that the current commands are given by a set of simple equations, which can be easily implemented. Also, the simplicity of the equations avoids complex mathematical arguments for demonstrating uniqueness and existence of the solutions. It is shown also that near unity power factor operation can be achieved with the proposed scheme. Experimental results demonstrate that the scheme fulfils all the objectives of a high performance controller.