The Control And Realization Of Four-Leg Three-phase Inverter

The three-phase four-wire inverter with four legs can deal with an arbitrary load: unbalanced and/or balanced load;linear and/or nonlinear load.Mathematics model of four-leg three-phase inverter is the base of theoretic analysis, the relationship between variables, such as output voltage and inductance current, can be established by the model.The pulse-width modulation (PWM) is introduced since it is the most widely used method. The characteristics of the modulation signals in three-leg three-phase inverter and four-leg three-phase inverter are analyzed. In order to get a high utilization of dc bus voltage, the distorted modulation waveforms can be used, such waveforms contain 3-th harmonic in addition to the fundamental and can change the neutral point voltage of the output. The performance of the output voltage feedback control system is analyzed, the parameters of the controller can be gained by computer simulation. The PWM control strategy with 3-th harmonic in addition is presented for four-leg three-phase inverter, the inverter can deal with unbalanced load and has a high utilization of dc bus voltage.Space-vector modulation (SVM) techniques can be used in four-leg three-phase inverter. The two-dimensional SVM control strategy can still be available in four-leg three phase inverter, the first three legs can be controlled by conventional SVM method, and the fourth leg can be controlled by the principle that the mean values of neutral point voltage of the output in three-leg three-phase inverter and four-leg three-phase inverter are equal in one switch period.Some applications of three-phase inverter has a control structure comprising an internal current feedback loop. Consequently, the performance of the converter system largely depends on the quality of the applied current control strategy. The main task of the control scheme in a current control inverter is to force the currents in a three-phase ac load to follow the reference signals. By comparing the command and measured instantaneous values of the phase currents, the current controller generates the switching states for the inverter power devices which decrease the current errors. Hence, in general, the current controller implements two tasks: error compensation and modulation. The property of internal current feedback loop has been analyzed. Two methods to generate the switching states for the fourth leg are proposed, and the previously presented control strategy of the other three legs can still be available. By using current control, a lot of advantages can be achieved, such as control of instantaneous current waveform and high accuracy, peak current protection, extremely good dynamics, etc.