Research And Realization Of SVPWM Algorithm For Cascade Multilevel Inverter

The common two-level and three-level inverters technique is usually not applied in high-voltage and high-power situations, due to the restriction of rated parameter of switch. Multi-level inverters technique can realize high-voltage and high-power output by using low-voltage full controlled power electronic devices. Comparing with the series-parallel connection of power electronic devices, multi-level inverters technique can effectively reduce the higher harmonics generated in PWM control and also du/dt, di/dt, and switching frequency. But the more level, the more complexity of the inverter control circuit, and it is much more difficulty to realize. This thesis deeply studies cascaded multi-level inverters in-depth, especially the application of space vector modulation technique presented here in detail:firstly, the space vector optimized algorithm reduces the difficulty of increasing level; secondly, decreases switching loss by reducing switching frequency, restrains zero order voltage of multi-level inverters by choosing suitable realization.1. On the α’-β’ plane, the basic vectors are composed of the sector triangle vertices vectors in time-shared. The seven-segment and five-segment modulation algorithms are widely adopted to switch among the three vertex vectors. Inverter switching frequency is found to be higher in analysis for seven-segment and five-segment modulation algorithms. To reduce the switching frequency and keep the closeness of the switching path, in the section3adopts characteristic quadrangle made of two sector triangles is proposed. Traversing over the three vertices of the sector by two basic switchings is possible only when along four sides of the characteristic quadrangle. Simulation and experimental results are consistent with the theoretical analysis that the switching frequency of the proposed scheme is about1/3of the seven-segment modulation algorithm and2/3of the five-segment modulation algorithm.2. In order to determine three categories characteristic of quadrangles formed by two kinds triangles, a method is presented that using the slope of the sampling point tangent line and the segment intersection of the adjacent vectors to decide the quadrangle type. This method makes the characteristic quadrangle come true.3. There are redundancy realizations corresponding to the basic vectors of multilevel inverters. The line voltage of the redundancy realization is fixed, but the phase voltage and the zero order voltage changes. The method of optimized zero order voltage proposed in section4is to find the realization which zero order voltage is the minimum absolute value. The realization found will be taken as the effective phased signals.4. In the zero order voltage optimized control algorithm, this thesis proposes a modulation algorithm adopting the traversal over triangle vertices along the characteristic quadrangle network formed by two adjacent sector triangles. Very small number of switchings can be obtained if switching between characteristic quadrangles. If characteristic quadrangles can not be formed by two adjacent sector triangles indicating the isolated sector triangle formed, then we can adopt a five-segment modulation algorithm to fulfill the switching among the three sector triangle vertices.5. In section4, the zero order voltage is reduced to some extent in the zero order voltage optimized SVPWM modulation algorithm, but with the modulation coefficient increasing, the variation range of zero order voltage increases. For a three-cell seven-level inverter, in any sector triangle. It is always able to find a vertex vector corresponding realization meeting the conditions of the zero order voltage N∈{0,-1,+1}. Using this characteristic of the three-cell seven-level inverter, the section5gives the mixed SVPWM algorithm and makes the zero order voltage N∈{0,-1,+1}. On the other hand, the mixed modulation algorithm uses the five-segment modulation algorithm and the characteristic quadrangle to approximate the reference vector which can reduce the switching frequency.Finally, a three-cell seven-level inverters experiment platform is set up. The switching frequency optimized modulation algorithm, zero order voltage optimized SVPWM modulation algorithm and mixed algorithm are realized on the experiment platform. Simulation and experimental results are consistent with the theoretical predictions, suggesting the correctness and effectiveness of the proposed algorithm.