Non-digital Control Strategy Research And Realization Of Three-phase Four-leg Inverter

With the rapid development of technology, a variety of electrical equipments are widely usedin the production and the daily life of human beings, which led to the requirements of power qualityare increasing. Three-phase inverter is a common power conversion device which can convert directcurrent into three-phase alternating current to supply the load requirements. Three-phase full-bridgeinverter with asymmetric load or nonlinear load is difficult to produce symmetrical three-phase ACoutput steady. Three-phase full-bridge inverter is unable to meet the needs of production and thedaily life of human beings, thus a new type of three-phase inverter is required.First of all, this paper first introduces several three-phase inverter topologies, which has theability to bring unbalanced load. Also elaborated the development status and several controlstrategies of three-phase four-leg inverter. The system designed in the paper uses a triangular waveinjected PWM control strategy to improve the three-phase four-leg inverter DC voltage utilization.Voltage feedback dual closed-loop control method for three former legs. The former three phasemodulation signals processed by three-phase half-wave rectifier to generate the control signal forthe fourth leg. The structure of the system is simple and easy to achieve. Theoretical analysis,simulation results and experimental results demonstrate the feasibility of the control strategy.Secondly, system modeling for the three-phase four-leg inverter, design of voltage and currentdouble closed loop control network. Current loop is easy to implement, fast response, etc.; voltageloop to ensure that the output voltage has a good sinusoidal degree and a steady state characteristics.Finally, according to the performance requirements of the system, the main power circuit,control circuit, drive circuit, a protection circuit and a signal generator source is designed in detail.By using pure analog devices can avoid restriction of DSP processing ability and increase theswitch frequency of the system effectively. It has a fast dynamic response and low harmonic contentof output voltage waveform. The triangle wave injection can improve the utilization rate of DCvoltage; enhance the capacity of the system with unbalanced load. Simulation and experimentalverification for3KVA three-phase four-leg inverter has been made, the simulation and experimentalresults is given. The simulation results consistent with theoretical analysis, steady-state anddynamic conditions of the system is good both with balanced and unbalanced load.