Abstract: In order to determine the minimum capacity of an active compensation device, compensation current, compensation capacity and active filter capacity were analyzed. The research result shows that the capacity of an active compensation device has no matter with the load access port. To change the load access port can only influence the distribution of three-phase compensation currents, but not the capacity of the active compensation device. The capacity relates to the load capacity and power factor angle, and it ranges from 1.0 to 1.2 times the load capacity under full compensation when the power factor angle changes in (0, π). The capacity has no relation with the transformer connection mode, so to adopt a special transformer can not make any sense to the economic performance of an active compensation system.