Research On Power Emergency Coordination Control Strategy Of Modular Multilevel Converter Based HVDC Grids

Economic development must proceed in parallel with the protection of ecological environment.Under this principle,China has gone on large-scale wind/solar clean energy deployment,leading the world renewable energy installation capacity.Consuming variable renewable energy becomes the outstanding issue today.MMC-HVDC grid technologies,based on modular multi-level converter(MMC),have unique advantages over renewable energy consumption,is the star technology and gained popularity in the power industry,At present,continental MMC-HVDC grids utilize overhead transmission lines,and line faults are recognized the most frequent and common faults.Fast and reliable fault isolation plays a significant role to promote this new technology,as well to ensure safe and reliable operation of the over-all MMC-HVDC grid.This paper investigates the coordinated emergency control strategies for the MMC-HVDC overheard grid,the contents of the Theses include:(1)Research onto the MMC mathematical models,topologies,control strategies and operational processes,as well MMC-HVDC grid arrangements.Based on these studies,a bi-polar MMC-HVDC overheard grid is constructed using power system transient simulation platform PSCAD/EMTDC.(2)Investigates polar-to-ground,polar-to-polar faults and their transients,derives current changing formulas for pre-fault and post-faults conditions for the faulty pole,healthy pole,and the fault-affected lines.Fault affected lines are proposed in the paper to facilitate coordinated emergency control strategies over the MMC-HVDC grid.(3)Analyses the state-of-the-art fault isolation technologies utilized in MMC-HVDC grid.Among the three technologies,fault isolation via hybrid HVDC breaker is deeply studied including the HVDC breaker topologies and principles,as well traditional fault isolation time sequences utilizing the hybrid HVDC breakers.A new coordinated fault isolation sequence is proposed to improve isolation speed.The proposed new sequence allows the hybrid HVDC breaker to start after the relay starts but before it triggers.Both traditional and methods are simulated and studied in PSCAD/EMTDC,the proposed coordinated control sequence is proven to be faster.(4)Studies the MMC-HVDC grid control strategies,proposes coordinated control strategies to solve the over-loading and/or over-voltage issues that may occur on the grid during and/or after a fault.A PSCAD/EMTDC coordinated control strategy model is established,and simulation results show the effectiveness.