A Study on the Wireless Parallel Operation of a Single-phase UPS Inverter using Single-loop Robust Voltage Controller Dae-kwan Ku Department of Electrical Communication System Engineering Graduate School of Soonchunhyang University South Korea

A Study on the Wireless Parallel Operation of a Single-phase UPS Inverter using Single-loop Robust Voltage Controller Dae-kwan Ku Department of Electrical Communication System Engineering 
Graduate School of Soonchunhyang University Asan, South Korea
 (Supervised by Professor Jun - Keun Ji, Guee - Soo Cha) 

ABSTRACT
This paper proposes a wireless parallel operation method of single-phase UPS inverter using single-loop robust voltage controller and resistive droop control. Conventional parallel operation methods of UPS inverter using communication line are that such as centralized control, master-slave(MS), average current sharing(ACS), and circular chain control(3C). Although these methods achieve both good THD of output voltage and equal current sharing, but they need communication line. Therefore could reduce the system reliabilty and expandability. The wireless parallel operation such as droop control is suitable method to overcome for demerits of parallel operation methods using communication line. The droop control method don't need communication line because it uses only local power measurements. Thus droop control method is proper method for parallel operation of modular UPS. Conventional P-ω, Q-E inductive droop control method is derived from the assumption that UPS inverter output impedance is pure inductive. Therefore it's not proper method for low voltage grid such as parallel UPS inverter system. Hence, this paper uses P-E, Q-ω resistive droop control method. This method is derived from the assumption that UPS inverter output impedance is pure resistive. In addition, a single-loop robust voltage controller is used for the UPS inverter. The single-loop voltage controller can reduce material cost owing to the decrease of current sensor in the UPS system. Then simulations and experiments for the two-parallel system of single-phase UPS inverters with resistive and nonlinear loads demonstrate the feasibility of proposed parallel operation method providing improved performance - good output voltage control and equal power sharing. Key words : UPS inverter, parallel operation, robust voltage control, droop control.