Combination voltage-controlled and current-controlled PWM inverters for parallel operation of UPS

In this paper the scheme of combination voltage-controlled and current-controlled PWM inverters for parallel operation of single-phase UPS is proposed. The voltage-controlled PWM inverter (VCPI) unit as a master is developed to keep a constant sinusoidal wave output voltage. The current-controlled PWM inverter (CCPI) units as a slave are controlled to track the distributive current. The power distribution center performs the function of distributing the output current of each active unit. In this proposed scheme of parallel operation, each of the units can be designed nearly independently, and the CCPI units do not need a PLL circuit for synchronization. As a result, the parallel operation of the UPS is easy to implement and to expand system capacity. For the purpose of illustration, the system, including three single-phase units which are operated in parallel, is analyzed and experimentally verified