Research On Improved SVM Algorithm Of Three-level PV Grid-connected Inverter

Three-level photovoltaic grid-connected inverter with a LCL filter device has beenwidely used for increasing the output of the electrical performance. but it faceschallenges in academic research and engineering applications for its special topology, itwill bring so many control and efficiency issues,such as the problems of the midpointpotential、the common-mode voltage and system efficiency.In this paper, we have changed those optimization methods through in the controlloop, and by improving the modulation loop algorithm to balance the midpoint potential,suppress common-mode voltage and improve system efficiency. This paper hasconsidered the actual topology parameters setting; set the voltage threshold through thederivation of the midpoint current transient expression, to ensure the system securityand stability. On the basis of the voltage threshold, we have analyzed the impact ofthree-level space vector of the midpoint potential and the common-mode voltage.Considered that it exist some mutually exclusive between the midpoint potential withthe common mode voltage in the control strategy, we have propose an improved spacevector modulation algorithm based on the threshold control. Outside the scope of themidpoint potential threshold, the algorithm uses the midpoint potential greatlyconserved seven-segment modulation method to balance the midpoint potential in asingle control cycle, so this method has high accuracy control midpoint potential, andfast system response features; Within the threshold of the midpoint potential, we haveused the five-segment modulation method of a constraint vector to reject thecommon-mode voltage.by using the five-segment modulation method, it’s not only canreduce the amplitude of the common-mode voltage to50%, and the switching frequencycan be reduced by1/3.So it can achieve the reduced switching losses and improve thesystem efficiency.Beside， It is worth mentioning that when the system isrunning,switching the modulation scheme determined by the controller according to theperformance requirements of the system,so it comparing with other man-switchingmode method，this improved algorithm has more flexibility. And most implementationsof the improved algorithm can obtain a relatively simple expression by offline methodIn this paper, we have doing some the appropriate verification about theoreticalanalysis and mathematical operation of the improved algorithm. Simulation andexperimental results are well demonstrated the improved SVM modulation algorithmdesigned to control the midpoint potential, suppress common-mode voltage, the goodperformance of the system to improve efficiency. Therefore, in practice application itwill not increase the complexity of system design and force the burden on the processor,it has strong practicability.