Simplifying the design of parallel-connected UPS inverters by means of the per unit system

Parallel UPS inverters provide redundancy and scalability to energy backup systems and the main design challenge is to guarantee that each unit will supply the same power to a common load. Therefore, the key of a good design is the study of the active and reactive powers unbalance. In this paper, the unbalance of a drooping parallelism method is analyzed and practical simplifications are applied in the exact equations. These equations are then rewritten in the per unit system, making the study even more intuitive and generalizing the design. The design method is first tested with a research association of two analog based UPS inverters and then with an entire line of commercial DSP based UPS inverters.