Analysis and design of a series voltage unbalance compensator based on a three-phase VSI operating with unbalanced switching functions

Voltage unbalance in AC supply systems is typically corrected by means of a shunt connected thyristor-controlled static VAr compensator. This approach has the disadvantage of slow response, harmonic injection into the AC system, and the requirement for large passive components. The proposed system consists of a three-phase PWM voltage source inverter, connected in series with the line through a three-phase transformer. The unbalance compensation is achieved by canceling the negative sequence component of the line-to-line voltages of the source. It is also shown that by having the inverter operate with unbalanced switching functions, it is possible to balance the load voltage and to control the amplitude of the positive sequence component in order to perform load voltage regulation. A complete mathematical description of the method is presented, demonstrating that the compensation can be achieved with low kVA inverters and low harmonic injection. Implementation procedures, design equations, and a design example are also included in order to illustrate the proposed method. Experimental results of a 1.5 kVA laboratory prototype system confirm the feasibility of the technique