DC characteristic analysis of three-phase LC filter- uncontrollable rectifier using circuit DQ transformation

The three-phase uncontrollable rectifiers are still widely applied, at present, in kinds of household appliances, because of its low cost, simple structure and excellent applicability. There are no LC filters in some simplified schematics. Sometimes, filter inductors are connected between rectifiers and power grids. For some higher requirements the three-phase passive power factor correction techniques with input LC filter are used in getting a higher power factor, just as the high power inverter air conditioners. Since the three-phase LC filter uncontrollable rectifier circuit is a strong coupling nonlinear system, it is difficult to analyse, model and select the parameters of LC filters. In this paper, circuit DQ transformation method is adopted to analyse the circuit's working principle better. It can convert the nonlinear circuit into linear system, then propose the novel concepts of the equivalent switching function, the control angle and the overlapping angle of uncontrollable rectifier. From this way, it makes the system controllable and discovers the controllability in three-phase LC filter uncontrollable rectifier systems. Through the analysis of the system DC characteristics, the expressions of output voltage, active power, passive power and power factor are calculated, which are identical to those of other controlled rectifiers. The relationship between the control angle and the load resistance are obtained through the simulation analysis. Experimental results on the rated output power 7.5kW three-phase rectifying system has proven the above theoretical analysis and circuit DQ transformation method. It has also proven the proposed concepts, which is practical to analyze the performances of the uncontrollable rectifier systems.