Input DC Voltages Balancing Strategy For Two-stage Cascaded Multilevel Photovoltaic Grid-connected Inverter

The grid-connected photovoltaic(PV) system, which first stage is DC/DC converter and second stage is cascaded multilevel inverter, has the advantages of easy modularization, high efficiency. It is suitable for large-scale photovoltaic power plants and other applications. The PV panels for each cascaded cells are independent of each other. Affected by the environment temperature, the output power of PV panels are fluctuations, and the maximum power point will be different. If no measures are taken, the input DC voltage of H bridges will appear uneven phenomenon, resulting in the increase of the inverter output voltage harmonic level. The control strategy to realize voltage balance of the H bridges is studied in this paper, the main research contents are as follows:This paper analysis the basic principle of the carrier phase shift control(CPS) and phase opposition disposition control(POD), points out that both of two control strategies can not adapt to the fluctuation of output power of the PV panels, and the input DC voltage of H bridges can not be balanced. Briefly introduce the voltage balancing method under the CPS, points out that the output of the inverter multilevel voltage harmonic will increase. Propose the voltage balancing idea and control target under the POD.According to voltage balancing idea and control target under the POD, the voltage balancing strategy for low frequency distributing switch states with four times the grid frequency(4fo) is proposed. The position of the switch state distribution is determined through the output signal of PLL. The scope of voltage balancing strategy is analysised, the method expanding the scope to realize a PV panel power output of 0 is proposed.According to voltage balancing idea and control target under the POD, the voltage balancing strategy for high frequency distributing switch states with two times the carrier frequency is proposed which the values of the switch state at the time of distribution are the same. And the method to expand the scope of the low frequency distribution method is extended to high frequency distribution method, and the concrete realization method is gived.The control parameters and the main circuit parameters of a 3kW cascaded seven level photovoltaic grid-connected inverter prototype are designed. The voltage balancing strategies and control target in this paper are verified by experiments under the prototype.