Research Of The Four-Leg Voltage Source Inverters Based On Space Vector Modulation

The four leg inverter with a neutral leg can control the zero sequence voltages and currents, suitable for the unbalanced loads, widely used in applications require a precise neutral current control, such as UPS, aviation Power, et al.There are many control strategies for three phase four leg inverter, such as SPWM, hysteresis current control and space vector modulation. The SPWM control has a good waveform but the DC BUS voltage utilization is low. People can easily design the hysteresis current control hardware circuit, but the switching frequency is variable which makes us difficult the design the output filter, moreover the output waveform is not good. The space vector modulation techniques, by selecting the zero vector properly, it can reduce the harmonic content of the output voltage, so it has become a popular control method. This paper researches the four leg inverter based on SVM modulation in detail.The principle of the SVM for four leg inverter is introduced in detail; the control of the inverter mainly contains two parts: calculation of the instruction voltage and SVM algorithm. A novel space vector modulation is applicated to four-leg inverter in this paper which uses abc coordinates avoiding theαβγtransformation, which makes the selection of the switching vectors and duty cycles easier, drastically reducing the complexity of modulation algorithm. There are two methods to calculate the instruction voltage: offline and online.Offline control strategy refers to work under known load condition, according to the load condition calculate the instruction voltage, then store them in the datasheet, each time read the instruction voltage from the datasheet, when input DC BUS voltage changes, A PI controller is used to adjust the instruction voltage thereby fix the output voltage rms. As a fast algorithm, the whole control system can be realized based on a DSP, but this control strategy can't work under unknown load condition. According to the offline control strategy, a prototype is made in this paper. The simulation and experimental results illustrate the good static and dynamic...