Study on control strategy of parallel processing UPS systems

Compared with the conventional two-stage conversion UPS systems, the parallel processing UPS systems have several advantages such as compact structure, lower losses and improved power factor. But there is a significant problem of un-damped (or damped slowly) dc transient components of supply currents due to the system structure with only a link reactor between source and load. Based on analysis of the mechanism of the problem, this paper proposes an improved control method to solve the problem. With compensating the pre-calculated steady state supply currents, rather than the transient ones, the dc transient components of supply currents can be eliminated quickly. The simulation results conducted with PSCAD/EMTDC are given to verify the validity of the method.