Research On Fault Protection Of Modular Multilevel Converter Based HVDC Grids

With the rapid growth of power demand and the development of power electronic technology,the high voltage direct current(HVDC)grid based on modular multilevel converter(MMC)topology has attracted widespread attention.Fault protection is one of the key technical difficulties of the MMC-based HVDC grids due to the characteristics including the high-speed rising of the fault current and the lack of natural zero crossing point.In addition,the sub-module(SM)fault is also one of the main risks in MMC.All these difficulties limit the development of the HVDC grids.This thesis focuses on key issues of the fault protection of MMC-based HVDC grids,and it is organized as follows:(1)Considering the synthetical effects of the ac system and the dc system,an optimized control strategy based on dynamic redundancy is proposed.One of the main innovations is that the reference value of the capacitor voltage is derived according to the maximum output voltage of each converter arm and the safety margin which can be adjusted artificially.The redundancy can be adjusted dynamically,and the utilization ratio of the sub-modules can be effectively improved.In addition,the capacitor voltage and the inner stress are reduced,and the fault ride-through capability of the system can be enhanced.The comprehensive effect of redundant SMs and the bridge-arm asymmetric operation are also considered.The mathematical model is rebuilt,and the switch function and circular current are analysed in detail.On this basis,an optimized redundant protection strategy is proposed.Unlike previous strategies,the SMs in healthy arms need not be bypassed when SM faults occur and the asymmetry degree will be reduced by regulating the base voltage of SM.(2)A cross-connected SM(CCSM)is proposed and discussed.The CCSM contains 6 Integrated Gate Commutated Thyristors,6 anti-parallel diodes and 4 capacitors,and the efficiency of the SM is as high as that of clamp-double SMs.The modulation and voltage balancing strategy are also discussed.A model of two-terminal MMC-HVDC system is built in PSCAD/EMTDC,and the simulation result proves the validity and the feasibility of the proposed CCSM.(3)The fault protection principle for HVDC grids with hybrid HVDC breakers is studied.The main difficulties to realize the fault protection of HVDC grid are presented and discussed.Then strategy based on local detection and local action is proposed.A 4-terminal DC grid is constructed as the test system.Based on the test system,the protection performances by the ordinary relay protection strategy and the proposed local detection and local action strategy are systematically compared.And the selectivity of the proposed strategy is thoroughly studied through complete line fault scanning of the test system.The results show that the proposed strategy has strong advantages in protection selectivity and protection speed.Furthermore,the strategy can greatly reduce the interrupted current level of the HVDC breaker,and can greatly reduce the cost of the DC breaker.(4)An assembly HVDC breaker and the corresponding control strategy are proposed and discussed.The assembly HVDC breaker consists of an active short-circuit breaker,a disconnecting circuit breaker,an ultra-fast mechanical disconnector,a load commutation switch and an accessory discharging switch.The proposed breaker can handle the dc-side fault with competitively low cost,and the operating speed is fast enough.A model of a four-terminal HVDC grid is developed in PSCAD/EMTDC,and the simulation result proves the validity and the feasibility of the proposed solution.Compared with the two existing solutions,the proposed solution is more suitable for the meshed HVDC grid with overhead lines(5)An enhanced assembly HVDC breaker with the ability of self-energizing and power flow control is proposed.The load commutation switch is configured by a series connection of full bridge sub-modules instead of IGBTs.The discharge protection is also added to prevent the capacitor from overvoltage.Through the proposed strategy,the capacitor voltage will maintain at an appropriate value during the normal operation.With more flexible controlling strategy,the breaker has variable operation modes and the function is extended.Compared with the traditional assembly breaker,the proposed topology can realize self-energizing so that the problem of the power supply system in insulation and voltage resistance can be avoided.In addition,the function of DC power flow controller is integrated,which greatly improves the feasibility of the proposed breaker in HVDC grids applications.