Research On Power Balance Strategy Of Two-stage Cascaded H-bridge Photovoltaic Grid Connected Inverter

The increasingly serious environmental problems caused by the consumption of traditional fossil energy have brought new challenges to human survival and development.This challenge has forced global attention to turn to environmentally sustainable clean energy.At present,the use of clean energy represented by wind power,nuclear power,hydropower,and photovoltaic power generation has become an important part of energy generation around the world.And all of this is inseparable from the booming development of power electronics.Especially in the field of wind power and photovoltaic power generation,power electronics provides a stable and reliable interface for its grid connection.Innovations in power electronics technology directly promote the upgrade of the wind power and photovoltaic power generation industry.In this paper,a two-stage cascaded H-bridge multi-level photovoltaic grid-connected inverter is studied.Compared with a traditional two-level PV-connected grid-connected inverter,the two-stage cascaded H-bridge multi-level inverter combines the advantages of boost converter and cascaded H-bridge(CHB),which is featured with DC-side boost,modularity,strong scalability,and good grid-connected current quality and known as the next-generation photovoltaic grid-connected inverters.However,due to factors such as local shading,the output power of each first-stage photovoltaic unit is different.For a single-phase CHB structure,inter-bridge power imbalance will occur.In severe cases,over-modulation will occur,causing grid current distortion and making the system unstable.In the three-phase CHB structure,this kind of problem is more complicated,and there are both inter-phase imbalance and inter-bridge imbalance.Therefore,the research on the power balance control strategy of the photovoltaic grid-connected inverter in the CHB structure to make it more stable is important.This article focuses on the following aspects to study and analyze it.Firstly,the low-frequency mathematical models of single-phase and three-phase CHB multi-level photovoltaic grid connected inverters in different coordinate systems are established,which lays the foundation for the design of the whole controller.The Perturbation and Observation(P&O)method and the working principle of the first-stage boost converter are introduced.The Perturbation and Observation method is selected as the first-stage maximum power point tracking(MPPT)control strategy of the two-stage CHB multi-level photovoltaic grid connected inverter,whose effectiveness is verified based on Matlab/Simulink.Secondly,in order to solve the overmodulation problem caused by the different power of each photovoltaic unit in the two-stage cascaded H-bridge photovoltaic grid-connected inverter,this paper introduces a hybrid control strategy based on optimal third harmonic injection and power-limited MPPT Control method of two-stage single-phase grid-connected cascaded H-bridge photovoltaic inverter.The optimal third harmonic injection in this scheme can increase the modulation range.The power-limited MPPT algorithm can continue to meet the conditions by limiting the power of the photovoltaic module of the unit where the maximum modulation ratio is located beyond the third harmonic compensation range.The validity of the method is verified based on Matlab/Simulink.Finally,the inter-phase imbalance mechanism of the three-phase cascade H-bridge is analyzed,and a method of zero-sequence voltage injection is introduced to enable it to balance the three-phase grid-connected current.Combined with the method of optimal third-phase harmonic injection in inter-bridge,this paper proposes an algorithm of inter-phase imbalance and inter-bridge over-modulation,which can solve the problem of imbalance of grid-connected current under such operating conditions.A simulation model was established by Matlab/Simulink to verify the effectiveness of the proposed algorithm.