Research On Control Technology Of Three Phase Four Legs Inverters

The voltage unbalance factor is an important one of power quality characteristics, which has a significant impact on the normal working of electrical equipment and electrical safety. Three-phase inverter is the core of three-phase UPS and high-quality power supply, which are populor in the key departments of national production and life. And the study on the three-phase inverter with unbalance load is of great significance.The performance of the inverter with unbalance load is related to the inverter's topology and control strategy adopted. The study on unbalance mechanism will be helpful to topology choosing and control strategy design. Asymmetric system parameters and asymmetrical load are two elements to cause imbalance.In this thesis, the topology of three-phase three-legs and three-phase four-legs inverter are analyzed. It is found that the zero-phase output is uncontrollable in case of three-phase three-legs inverter with ?/Y isolation transformer. Current loop is provided for zero-phase current in three-phase four-legs inverter, each phase can be controlled independently. The three-phase transformer with an additional coupled winding on the secondary side of each phase can distribute the load of every phase properly with the coupled windings properly connected in series.A control strategy involving symmetrical component method, positive and negative PARK transformation, DQ axis current decoupling & feedforward and PI controler for zero-sequence is proposed here. The simulation is taken and verifies the validity of the strategy. Furthermore, three-dimensional pulse-width modulation is derived in detail, the physical meaning of the modulation scheme is pointed out, and a concrete implementation steps is given. It is revealed that the understanding on the essence of modulation scheme is the key to innovation.