Reseach On Parallel Operation Control Of UPS Inverters Based On Circulating Impedance

With users'increasing demand of high power quality and fast development of distributed power supply, UPS paralleling system is getting more and more widely used. Based on the research and design on parallel-connected modularization UPS, several contents about inverter are studied in detail, such as the parallel model, circulating impedance and circulating current characteristics of the paralleling operation system based on instantaneous voltage PID feedback control,parallel sharing current technology based on current decomposition method and the parallel sharing current technology based on instantaneous circulating current feed back and current decomposition, etc.With the load current treated as a measurable disturbance input, a linear mathematical model is established for a PWM inverter and a waveform control method about instantaneous voltage PID feedback is introduced. A electric model with equivalent output impedance for the PWM inverter is presented and a parallel equivalent electric model with output impedance is established. The circulating impedance in parallel multi-inverter system and the relationship between circulating impedance and circulating current characteristics are analyzed. An inverter used closed-control or not, the circulating impedance is different. The parallel system is connected to each other with inductance or not, the circulating impedance is different too. These differences are analyzed.According to two main control variables, active power and reactive power, the formula of average power is introduced and simplified as current decomposition method. The active current and the reactive current are controlled separately to adjust phase and amplitude of the voltage. Using the current decomposition method, the experiment has been carried out and the results are presented.A electric model with instantaneous circulating current feedback for the parallel inverter system is presented, the characteristics of circulating impedance and harmonic circulating current are analyzed. The disadvantages of the scheme using instantaneous circulating current feed back are discussed, a new parallel sharing current technology based on instantaneous circulating current feedback and current decomposition is presented. The influence on the circulating current by various controller parameters is discussed. Experiment results indicate that the method which based on instantaneous circulating current feed back and current decomposition is feasible.Because of the characteristic of parallel-connected modularization UPS, the focus bypass is designed. The information is transmitted through control signal line between inverter and bypass. Experimental results show that the control method meets the control requirement of parallel-connected modularization UPS.