Study On Fault Ride-through And Transient Energy Transfer&Dissipation Of HVDC Wind Power Integration System

The high-voltage direct-current(HVDC)transmission technology based on the modular multilevel converter(MMC)can effectively solve the technical limitations of conventional AC and DC transmission technology for wind power and photovoltaic integration.With the build of the clean and low-carbon energy system,large-scale & long-distance renewable energy integrating with HVDC technology has been rapidly developed.Taking China as an example,driven by the clean energy goal of “carbon peak and carbon neutrality”,HVDC projects across over regions have been put into production and power transmission in the past three years(±500k V Zhangbei MTDC power grid in 2020,and ±800k V Kunliulong three-terminal hybrid DC transmission project in 2020).Nowadays,China is at the forefront of the world in HVDC transmission technology.With the increasing voltage level and transmission capacity of HVDC transmission projects,the problems of system AC and DC short-circuit faults become more and more serious.Due to the fast-rising DC fault current and the fragile power electronic equipment of the HVDC grid,to protect the safe operation of the HVDC wind power integration system,the system needs to have nonblocking AC and DC fault ride-through capability to avoid damage to the equipment.This paper focuses on how to ride through AC and DC faults of HVDC wind power integration systems.Starting from the two practical engineering topologies of half-bridge and hybrid MMC and researching from dual-terminal to multi-terminal,this paper analyzes the AC and DC fault characteristics of HVDC wind power integration systems under different MMC topologies.Then,the fault transient energy transfer rules are systematically summarized,and the corresponding AC and DC fault ride-through and energy dissipation strategies are proposed.The main work of this paper and the results obtained are as follows:1.Analyzed the basic operation principle of the HVDC wind power integration system,deduced its control strategy,and introduced two typical HVDC wind power integration systems.Finally,simulation under PSCAD/EMTDC verifies the steady-state power tracking capability of the HVDC wind power integration system based on the HB MMC and the low DC voltage operation capability of the HVDC wind power integration system based on the hybrid MMC.2.For two kinds of typical double-terminal HVDC wind power integration system based on either HB-MMC or hybrid MMC,the equivalent circuits and transient characteristics of AC and DC faults are studied,respectively.Furthermore,the transient energy transfer characteristics of the two typical wind power integration systems are contrastively analyzed.The interaction mechanism of “wind farm-converter station-HVDC system” and the system transient energy transfer mechanism during the fault period are firstly revealed,which provides a theoretical basis for the AC & DC fault ride through and transient energy dissipation strategy of the system.3.Aiming at the double-terminal HVDC wind power integration system,the typical topology and operating principle of energy dissipation devices(EDD)applied to the system to ride-through AC/DC faults are analyzed,and different EDDs are classified.Based on summarizing conventional EDDs,a flexible EDD topology(FB-DBS)for MMC-HVDC wind power integration system is proposed.Based on the analysis of the working principle of the FB-DBS,its modulation algorithm,parameter designs and transient energy absorption control strategies are systematically designed.Besides,the coordination method of FB-DBS and hybrid DCCB is proposed and the AC and DC fault ride-through control strategy is put forward.The FB-DBS needs no communication system,achieves flexible access and controllable dissipation energy,and the current and voltage change rate are small during the input procedure.Therefore,it effectively reduces the electromagnetic interference to the environment,achieves smooth access during the fault and ensures the safety operation of the converter.4.Aiming at the DC fault problem of the four-terminal MTDC wind power integration system based on hybrid MMC,the evolution mechanism of the DC port voltage,current and transient energy of the hybrid MMC are studied.Then,the key factors affecting DC fault current and transient energy are discovered.An automatic current limiting control with dissipation resistor(ACLC-BR)strategy is proposed,which automatically suppresses the discharge of the sub-module capacitor by quickly reducing the DC modulation ratio,thereby limiting the development of fault current and greatly reducing storage energy of DC reactors during the fault.Besides,the law of transient energy transfer after applying current-limiting dissipation measures is analyzed.Finally,the electromagnetic transient simulation is carried out under the four-terminal MTDC wind power integration grid with ACLC-BR.5.Aiming at the AC fault problem of the four-terminal wind power integration system,the change characteristics of the DC voltage and the sub-module capacitor voltage of the DC voltage control MMC station under different locations and types of AC faults are analyzed.The transient power flow changes of the AC faults at different locations are summarized.According to the law,the maximum allowable response time for AC faults is obtained.On this basis,the power flow transfer entropy is defined.Based on the sensitivity of power flow transfer entropy to different AC faults,a remote AC fault location and property identification method based on power flow transfer entropy is proposed,and a coordinated strategy for transient energy dissipation and wind farm power control is designed.The proposed control strategy enables the system to automatically tackle any instantaneous or permanent AC fault at the receiving end,and achieve continuous operation without blocking MMC during faults.