Study on control strategy of parallel processing UPS systems

Compared with conventional two-stage conversion UPS systems, 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 the 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. Simulation results conducted with PSCAD/EMTDC are given to verify the validity of the method.