Research On The Topology And Control Strategy Of DC Grid-connected Photovoltaic Generation Systems

As a new type of green renewable energy,solar energy is one of the new energy which human will attach importance to development and utilization in the future.Over the past few decades,the development of solar photovoltaic power generation live through from the initial "roof plan" to today’s large-scale grid,which is the main direction development of photovoltaic power generation in future.At present,the photovoltaic grid is mainly by the DC power that the PV module generated through the inverter into AC power into the grid.As the representative of the renewable energy,The future of wind power and photovoltaic power generation will get a large-scale promotion and application due to the national policy.Local consumption will become an important feature of the future distribution network.In the future,the new type of load such as variable frequency electric appliance,electric vehicle charging station,data center and new electronic terminal,are emerging,and the load type tends to be diversified.Therefore,the photovoltaic power generation boost accessing to the DC power grid is an important direction in future.Based on the demonstration project of Zhang-bei flexible substation,the paper research on topology and its control strategy for large-scale photovoltaic power generation DC grid connection.First,the paper draws lessons from the current PV grid-connected topology and the latest research progress on the DC grid topology of domestic and foreign experts and scholars,combing with the Zhang-bei flexible substation demonstration project requirements,chooses input-parallel output-series DC-DC converter as PV power DC grid-connected topology,and selects the phase-shifted full-bridge circuit as the basic DC-DC converter module unit.Then,the paper introduces the basic principle of phase-shifted full-bridge circuit,points out the existence of secondary side occupancy loss and so on.According to the engineering requirements,the parameters of phase-shifted full-bridge circuit are optimized.Then,the paper analyzes the input-parallel output-series DC-DC converter composed of the basic phase-shifted full-bridge circuit,and it also analyzes the relationship between the output voltage sharing and the input current sharing,and it establishes the system small signal model.Last,the factors that affect the output voltage equalization between the modular converters are simulated and verified by simulation.Finally,a simplified control strategy is proposed for the phase-shifted full-bridge circuit.The simulation results by PSCAD/EMTDC platform show that the output voltage can be quickly stabilized around the reference,whether the input voltage is changed or the load is changed verified.For the combined DC-DC converter,a staggered control strategy which can reduce the output voltage ripple is proposed and maintaining the output voltage equalization control strategy is also proposed.The simulation results show that the output voltage ripple is becoming smaller with the number of modules increasing in the combinatorial converter with the interleaving control strategy;and output voltage sharing control strategy can eliminate the difference between the parameters of the module to maintain the balance between the output voltage balance between modules.