The Research On Three Phase Four Legs Inverter With Unbalanced Loads

The balance of voltage is a the very important of power quality, which affects the normal operation of the power grid and the safe of the power electric equipments. As the core component of three-phase power supply, the three phase inverter is widely used in some important sectors, so reaserching on high performance of three phase four brige inverter with unbalanced power supply to improve power quality has very important practical significance.For three leg and four leg circuit system model carry out the detailed theoretical derivation,known for the unbalanced load and nonlinear load, three-leg inverter cannot make the output voltage meets the requirements of national standards,but four leg inverter be able to mintain the symmetry of the three-phase voltage and well regulate between the three-phase to neutral point potential, then unbalanced current is provided to the output channel by the more leg. In order to realize the high symmetry of output voltage and the modulation of each phase, This paper chooses four leg circuit and the classical double PI closed-loop control. the simplify control strategy is proposed, namely, using the real-time method to decompose the unbalanced three-phase voltage to the positive sequence, negative sequence and zero sequence, through the positive and negative sequence Park conversion get DQ axis voltage which are coupled to each other. In order to realize the decoupling of the components in the DQ coordinate system, the current feed-forward decoupling regulator is added, the output of the D axis and the Q axis component are converted to independent,which is convenient for the control method to realisng high accuracy and low steady state error. the zero sequence component is zero in the DQ rotating coordinate system, and a little error PI regulator is adopted to eliminate the effect of zero sequence component.addition, through the comparison with the two-dimensional in three leg pulse width modulation method, launched four leg switch control in ABC coordinates, the three dimensional space pulse width modulation make a detailed theoretical research, and the realization steps of the simplified scheme. the DSP processor are used to complete the software program,the hardware system of three-phase four bridge inverter is designed in detail.Closed loop simulation of several kinds of methods, such as resistive load and asymmertric resistive load,nonlinear asymmetric load and lose phase running, The simulation data verified that the design can effectively eliminate the influence of unbalanced load, the asymmetric rate of three phase voltage is lower than 5%. Control strategy is feasible and effective.