Research On The Key Technologies Of PV Grid-connected Inverters Based On Cascaded H-bridge Topology And Their Applications

Cascaded H-bridge(CHB)multilevel inverter has good market application potential and development prospect in photovoltaic(PV)grid-connected power generation due to its superior comprehensive performance.On the one hand,it has significant advantages in small and medium-sized high-efficiency household PV power generation because of its advantages of component level shutdown,component level MPPT and multi-level output voltage;On the other hand,its modular multi-level structure can not only expand the system to a higher voltage and power level,but also realize the high-efficiency and high-power direct connection without line-frequency step-up transformer,especially suitable for the PV power generation system with a single-unit of more than 20 MW.However,power imbalance is one of the key problems of CHB PV inverter,and the existing research still needs to be further improved.In addition,the development of direct-linked PV inverter based on CHB is in the initial stage,in order to give full play to the advantages of the topology in engineering applications,it is necessary to conduct in-depth research on topology,operation modes and control strategies.This paper focuses on the above key issues and conducts a systematic study.The research work and innovation are as follows:(1)Aiming at the problem of H-bridge(HB)over-modulation caused by power imbalance of single-phase CHB PV inverter,two power imbalance control strategies based on modified hybrid modulation technology and reactive power compensation are proposed respectively,and then their advantages and disadvantages are compared and analyzed;(2)Aiming at solving the problems existing in the modified hybrid modulation technology and reactive power compensation strategy,an optimized third harmonic compensation strategy and a multiple harmonic compensation strategy are proposed respectively,among which the multiple harmonic compensation strategy has the better comprehensive performance compared with other control methods;(3)Aiming at the problem that resonant current of LLC converter in the module of direct-linked PV inverter based on three-phase CHB has envelope ripple,this paper analyzes the mechanism of envelope ripple,expounds the influence of envelope ripple on the whole system,and proposes a suppression strategy of resonant current envelope ripple based on quasi PR regulator;(4)In order to solve the problem of unbalanced power transmission between modules in the fault redundancy condition,a power balance control strategy based on zero-sequence voltage compensation is proposed,and the influence of zero sequence voltage compensation on the whole system is analyzed;(5)Aiming at the problem of active power return in the process of low voltage ride through of direct-linked PV inverter based on three-phase CHB,two control strategies based on active current injection and zero-sequence voltage compensation are proposed respectively,among which the zero sequence voltage compensation control strategy has stronger applicability.