Power Conversion Topology And Operation Strategy For DC PV Power Collection System

Nowadays,with continuous development of renewable generation,both the capacity of centralized PV power plants and the penetration of distributed PV generation have shown a rapid grouth.The inverter-based AC PV integration has been faced with multiple constraints from the aspects of stability,efficiency and power quality.As a result,the DC PV power collection system based on DC-DC converters has been regarded as a new technical approach with promising prospects and has attracted worldwide attention and R&D force.Due to the difference in power conversion mechanism and physical characteristics between AC and DC power,the DC PV power collection system faces unique issues and challenges in general configuration,converter topology,operation,control and protection.Focus on power conversion topology and operation strategy,a multi-angle research has been presented in this dissertation with the following work included:1)The power conversion topologies of DC PV power collection system have been investigated from the aspects of general configuration and converter topology to form a family tree.The operation characteristics and applicable conditions of various configuration in system level and converter modular composite level have been investigated to guide the selection of power conversion topology in DC PV power collection system.2)A novel input-independent and output-series(IIOS)type topology with internal power balancing units(PBUs)has been proposed for the MVDC integration of distributed PV.With an active internal power equalization capability,the steady-state input power and output voltage of the submodules can be decoupled to realize a full-working-range operation in the proposed topology.A detailed explaination of working mechanism and control strategy has been presented while the design method of topology and controllers has been proposed base on the operation boundary condition and small-signal model.The proposed topology has been compared with the existing topologies from the aspects of steady-state power efficiency,circuit complexity and the electrical stress of components to ientify its comparative advantages.The theoretical results have been validated by system simulation and down-scaled experiments.3)A new scheme of series-output power collection system with internal distributed hybrid energy storage system(DHESS)installed has been proposed to cope with the practical operation demand.A doubleobjective coordinated power control strategy between PV and storage has been proposed with the consideration of both internal power balancing and output power regulation.The state of charge(SOC)management strategy has also been proposed to match the proposed power control strategy.The theoretical results have been validated by system simulation.4)Focus on practical demand,a low cost fault isolation scheme without DC circuit breakers has been proposed along with the corresponding control & protection strategy based on zone division.Considering the unidirectional power flow and various working conditions in DC PV power collection system,the batch type,master-slave synchronous type and droop synchronous type coordinated start-up strategies between front and back stage converters have been proposed on the basis of LVDC bus voltage monitoring or real-time communication.The theoretical results have been validated by system simulation.The dissertation has provided novel solutions for the power mismatch issue in various capacity IIOS system as well as the protection and start-up control issue that is faced by specific project.The presented work and results will provide an effective surport for the further application of DC PV power collection technology in the scenarios of large-scale PV plants and high penetration distributed PV.