Research On High-frequency UPS Parallel System

With users'increasing demand of high power quality and fast development of distributed power supply, UPS paralleling system is getting more and more widely use. High-frequency UPS has an excellent input characteristic, high power density and low cost, so it was popular. But because of the low output impedance, the parallel control is more difficult. This paper focuses on the parallel system of High-frequency UPS, including analyzing of the circulating current characteristics, parallel control, bypass switching strategy, protection and monitor. Models of single-phase inverter are established, and the causes of waveform distortion are analyzed. The parallel model according with the fact is proposed and the affect factors of the equivalent output impedance are discussed. These factors are control system, output transformer impedance and circuit impedance. Based on the parallel model, the current characteristics of the parallel system are analyzed, and the simulator result approves it. The origin and rejection scheme for harmonic circulating current are also discussed. Based on the analysis of the circulating current characteristics, the parallel control for high-frequency UPS using dual-loop control is put in practice. The methods for rejecting the saturation of the load-sharing control loop, rapid circulating current rejection and synchronous bypass switching for parallel modules are also suggested, as well as the acquisition of the power of circulating current by hardware. The feasibility and the validity of the theory analysis are attested by the results of the simulation and the experiments. The reliability of the parallel redundant system is lower than the result of theory analysis in fact. After analyzing the causes this paper from these aspects to assure the parallel system's reliably running: firstly, the bypass switching side, including the control of the bypass switching both only a single module and several modules are running. The regulations of single module's launch into and quit from the parallel system are also discussed. Secondly, the protect measures side, including the faults construe and it's aftermath, the method of fault detecting, fault managing in parallel system are narrated. In the end, monitor software is design for the UPS power system, involving the hardware interface, communication protocol and data transfer flow.