Research And Design On The Three-phase Four-leg Inverter

Source crisis nowadays makes people pay attention to exploit the renewable resourcesuch like sun, wind. However the electric energy which is put out from these sourcescannot be used directly, we must need the inverter device. Conventional three-phaseinverter adopts three-leg topology. But when the loads are unsymmetric, undesiredcomponents of negative sequence and zero sequence which come from three-phase outputvoltages will cause unsymmetric output in system. For the sake of suppressing theasymmetry, a three-phase four-leg inverter which is proposed in this paper can ensure thethree-phase symmetrical outputs when the loads are unsymmetric, in addition it has theadvantage with smaller capacity and lower cost.This paper firstly introduces and compares some three-phase four-wire inverters, andthe three-phase four-leg inverter turns out to be the best topology among them. Then itgives a3D-SVPWM algorithm in a-b-c coordinates which includes the selection oftetrahedrons and calculations of duty ratio. Because of using a-b-c coordinate，thisalgorithm is much simpler than inα-β-γcoordinates. Secondly a new control scheme calleda zero-sequence component injected PWM method which using2D-SVPWM instead of3D-SVPWM is proposed for the three-phase four-leg inverter. It is abbreviated asAZSPWM. The proposed algorithm is simpler than conventional3D-SVPWM and easier todigital implement. Furthermore, NZPWM method and NSPWM method involved inAZSPWM, which can be adopted in this algorithm eliminate the zero switch so as toreduce switching losses and suppress common mode voltage.ASZPWM is proved to beequivalent to3D-SVPWM algorithm by analysing. Thirdly, it makes the simulative test to3D-SVPWM and ASZPWM algorithm on three-phase four-leg inverter system. The resultsindicate that the consistency of simulation results and theory analysis were confirmed.Finally the hardware system design of the three-phase four-leg inverter has been put out.This paper uses DSP TMS320F2812produced by TI as main control chip. Hardware designmainly includes the main circuit, filter circuit and control circuit.All in all, theoretical analysis and simulation results show that3D-SVPWM andAZSPWM schemes suggested in this paper can both output the good quality three-phase output AC voltages when the loads are unsymmetric, By comparison, the AZSPWM has thebetter outstanding advantage in controlling the three-phase four-leg inverter.