| Distributed photovoltaic power generation-energy storage systems have been widely used in urban and rural building roofs and remote,non-electric areas in recent years because of high reliability of power supply,off-grid operation,and small impact on the power grid.With the decrease of battery voltages,the diversification of related power electronic converter topologies and the complexity of system control strategies,how to raise the bidirectional power conversion efficiency between the low-voltage battery and the high-voltage DC bus,improve the modeling accuracy of the power electronic converter,and ensure the system’s rapid response and stable operation in the transient process,has important research significance.Therefore,this article has carried out the research from the topology,the modeling and the control method all of three aspects,the main content includes:(1)A novel HVCR bidirectional CLTC resonant DC-DC converter circuit topology is proposed to improve bidirectional power conversion efficiency.The topology can achieve zero voltage switching(ZVS)and zero current switching(ZCS)over the full load range,enabling bidirectional wide voltage gain range from 48 V batteries to 400 V DC bus under a 5 k W load.Therefore,it is suitable for distributed energy storage applications.This paper designs the parameters based on the optimal efficiency at the main resonance point,studies the power loss distribution of the CLTC converter,and confirms the significant advantage of the CLTC in terms of efficiency through comparative experiments.(2)A modified fundamental wave equivalent analysis method is proposed to improve the calculation accuracy of voltage gain and voltage/current stress in resonant DC-DC converters.By solving the instantaneous value of the AC output voltage and current during the diode current discontinuous period,the AC resistance in traditional FHA method is revised and the accuracy of voltage gain is improved.In addition,based on the numerical relationship between the instantaneous values and the peak values of the fundamental waves,the calculation accuracy of voltage/current stress is improved.(3)A light load and dynamic control method for LLC resonant DC-DC converter in a distributed photovoltaic power generation-energy storage system is proposed,which solves the problem that the light-load DC bus voltage is uncontrollable and improves the response speed of the system.Under light load conditions,the full-bridge LLC is converted to a half-bridge LLC,which solves the problem of uncontrollable output voltage caused by voltage gain upturn.In addition,this method reduces the operating frequency of the LLC and thus improves the operating efficiency of the LLC under light load conditions.By reasonably selecting the operation mode of the LLC(half-bridge/full-bridge),the control method proposed in this paper can realize the voltage control of the two DC buses and the parallel current sharing control of the two LLCs within all load range.The system’s response speed is also improved the method.(4)An experimental platform is established,the previous theoretical analyses are verified by experiments on it.In this paper,a novel software synchronous rectification control strategy is proposed to improve bidirectional working efficiency of CLTC resonant DC-DC converter.The half-bridge mode is adopted to improve the light-load efficiency and is performed on a 5 k W CLTC prototype.Verification experiments are conducted in this paper.In addition,this paper uses the experimental platform to verify the proposed light load and dynamic control method for LLC resonant DC-DC converter in a distributed photovoltaic power generation-energy storage system.The effectiveness and feasibility of the proposed method is verified through grid-connected,off-grid and family load experiments with impact properties. |
