Frequency regulation for parallel-connected UPS system under independent control

This paper proposes a frequency regulation technique for the stable operation of parallel connected UPS system under the independent control proposed by authors group. The features of independent control of parallel UPS are (1) stable operation is guaranteed by sensing the output current of each UPS, and no communication is required between the UPSpsilas, (2) The phase and amplitude of each UPS voltage are generated by a simple PI controller, (3) The lateral current between the UPSpsilas are compensated by the lateral current compensator, (4) the load sharing is autonomously achieved, (5) the system frequency changes depending on the control gains and load. The last disadvantage is now solved by adding a simple control loop in the independent control. The paper presents the theory, simulation and experiments.