Study On Parallel And Switching Technique Of UPS

With the development of science and technology and the improved requirement of the client, Uninterruptible Power Supply (UPS) system, as an important power electronics device, is widely used for supplying critical loads which can not afford utility power failure.Modularization and redundancy is the development trend of the future UPS,but the technology of parallel and switching strategy is the difficult problem in redundant designing modularized UPS, mathematical models on inverters parallel system, the circulating current characteristics, parallel control, Digital Phase Lock Loop and switching strategy are systematical and deep researched in this dissertation.when a single phase of Diode-clamped three phase three-level inverter is controlled alone,it's models is equivalent to the models of single phase half-bridge inverter,so models of single-phase inverter are established to easy analyze, the expression of it's output impedance is deduced. Mathematical models on two inverters parallel system are established, the circulating current is defined, the circulating current characteristics is analyzed,According to the characteristics of output impedance ,a relationship of between active circulating current or reactive circulating current and amplitude or phase of output voltage on two inverters parallel system is labored,the principle of parallel control is brought forward.Carrier disposition PWM which is applied to achieve the multi-system function of UPS being researched in this paper is advanced by comparing and analyzing many controlled strategy.the merit of PID controller is analyzed, The design method of inverter instantaneous voltage PID controller based on pole-assignment is proposed.Mean voltage PI controller outer loop is added to instantaneous voltage PID controller inner loop to achieve steady-state error free.Parallel operation modulate amplitude and phase of the inverter output voltage in accord with the magnitude of active circulating current and reactive circulating current which should match the proportion of resistance and reactance of the inverter output impedance. the results of the simulation and the experiments tested the decoupling control can reject circulating current well. The origin and rejection scheme for DC circulating current are also discussed.In this paper, a digital phase–locked loop (DPLL) are adopted to keep the frequency and phase of UPS output voltage identical with utility source, and a re-modulation technique is employed to improve the precision of PLL.At the same time ,a method of quick detecting AC voltage amplitude is proposed,the celerity is attested by the results of the simulation and the experiments. Based on the demand of the practical application,the process of designing focus bypass'hardware and software is narrated,the celerity and reliable of swithing are tested by the results of the experiments also.