The Research On Space Vector Modulation Technology Of Multi-level Inverter

In multi-level inverters, the voltage stress of each power device is relatively lower, that is suitable to realize the high voltage and large capacity inverters with the low voltage resistant power devices. And it is possible to enhance the quality of inverter's output voltage obviously by using multi-level inverters. Therefore, with the development of power devices, the multi-level converter technology has been an effective approach for inverters to achieve high voltage and large power.But, to multi-level inverters, the complexity of controlling is increased remarkably by the increase of voltage levels. This paper researched on the improvement of space vector modulation technology for multi-level inverters, that to low down the complexity of controlling, the common mode voltage of inverters'output voltage, and the switching times of power devices.(1) Firstly, this paper introduced the main circuit topology and the modulation scheme of multi-level inverters, and reviewed the present research situation of SVPWM controlling scheme. This paper divided the research direction into two areas, the controlling of two-phase voltage space and the controlling of expected performance. And the research in this paper is covered the two areas.(2) Based on the rule that all the coordinate value of the basic vector is integral inα′β′coordinate plane, this paper put forward a way to position the reference vector's sector S ( k ) by the parameter V0 ( k) and z ( k ). And the formula which is used to calculate the dwelling time of each vector is given out. Hereby, an algorithm for approaching the reference vector inα′β′coordinate plane was drawn out, which is easy to be realized by arithmetic and rounding operation. A demand of synchronized sampling was presented to the reference vector. And the concept of convergent modulation algorithm inα′β′coordinate plane and abc coordinate plane was put forward, which can decrease the sub harmonic component of multi-level inverter's output voltage.(3) This paper presented a concept of basic switching that changes only one phase's output voltage with only one level. With the basic switching, a modulation scheme which can drop down the switching times of the power devices was drawn out. Based on the concept, switching route is used to describe the switching progress of the inverter's switching status. And the concept of"closed route"is given out, that only the closed route can guarantee the modulation algorithm to be converged. A conception of characteristics quadrangle and characteristics network is presented, and it is proved that it can traverse the vertex of the sector only switching along the side of characteristics quadrangle and the characteristics network with basic switching. This paper found that it can be divided into two kinds of characteristics networks with the radius range of reference vector's locus. Characteristics quadrangle is used to analyze the switching route which has the minimum switching times in characteristics networks. At last, a switching frequency optimized SVPWM algorithm is put forward, which is useful to depressing the switching times of power devices effectively. Compared to the phase disposition sinusoidal modulation (PD SPWM) method, the total switching times among one reference voltage period of the frequency optimized SVPWM is only a half of which of PD SPWM.(4) This paper brought out a conception of minimum implementation, and with which it can restrict the zero order voltage component of inverter's output into one third of voltage level. The minimum implementation means that the implementation can make the absolute value of zero order coordinate of basic vector to be less than 1. The research points out that in the infinite level inverters all the basic vectors have a minimum implementation's existence. While in the n cell l level inverters, the basic vector which has the minimum complementation is limited into an area. Even to limit the zero voltage amplitude into one third of one unit voltage level in the system which's n≤8, the loss of the output voltage amplitude is less than ten percent. Therefore, this paper draw out a five segment modulation method which can limit the zero order voltage component of inverter's output into one third of voltage level, and given out the steps to realize the zero order voltage optimized SVPWM algorithm. The result of simulations and experiments have corroborated the theoretical predict.(5) At last, an experiment platform is set up with 3 cells 7 levels inverters. The zero order voltage optimized SVPWM algorithm is realized on the experiment platform. The real time calculation performance and the capability in depressing the zero order voltage is tested in the experiment. And the correctness and the effectivity of the modulation algorithms are proved.