Research On The Control Method Of Three-phase Inverter Under Unbalanced Loads

Electric power is an essential part of human production and life in the current, various devices can not work without power, and the quality power has been required more strictly. The voltage unbalance factor is an important one of power quality characteristics, which has a significant impact on the safety and stability of the power device, so the unbalance of the three phase voltage in the three phase inverters is studied.Generally, the traditional three-phase inverter are used widely with three phase balanced loads. But when the three phase loads are unbalanced or nonlinear, three phase voltage outputs can not be symmetrical. Carefully anatoming the reason that conventional three-phase inverter output voltage is bad for unbalanced and nonlinear loads, T equivalent circuit of each phase sequence is deduced by apply the superposition principle and symmetrical component method on the three phases invert with△/Y isolation transformer. The result that the zero-phase output is uncontrollable in case of three-phase invert with△/Y isolation transformer is deduce by the differential equation, which is verified by the simulation result.The performance of the inverter with unbalance load is related to the inverter’s topology and control strategy adopted. In this dissertation, the mathematic model of three-phase four-leg inverter is established in the a-b-c coordinate and is analyzed in the stable condition. SVPWM modulation algorithm in the αβγ coordinate is abandoned due to the characteristics of the three phases four-legs inverter and the large calculation, and the simple three-dimensional SVPWM modulation algorithm is used in the paper. The calculation in detail is demonstrated and the modulation algorithm is verified by using the DSP. Because in the a-b-c coordinate, the PI adjustor can not achieve isochronous control for AC component, the bisynchronous coordinate is built according to the characteristic of the positive sequence and negative sequence, which can effective restrain the second harmonious. But the zero sequence component is zero in the rotating coordinate, which result in the repeated control and complicated calculation, so the virtual αβ coordinate is built and used to coordinate transformation, which has the good performance.Finally, using the MATLAB softwave, the simulation of the three-phase four-leg invert with double closed loop control in different load validate consistency with the theory analysis, and the result is compared with the simulation of the traditional three-phase three-leg invert.