| Recent decades have witnessed the popularity of hybrid HVDC transmission in the field of DC transmission technology,owing to their significant economic advantages of conventional DC and technical advantages of flexible DC.In a certain hybrid HVDC system,appropriate control strategy and protection scheme have a crucial impact on the reliability and efficiency of its operation.Aiming at optimizing its operation,this thesis systematically investigates the research for high-capacity hybrid HVDC system from two aspects: fault ride-through control strategy and line protection configuration scheme.Accurate electromagnetic transient simulation is not only the cornerstone of various studies,but also a powerful tool to verify the theoretical analysis.Based on the actual parameters of Gezhouba-Shanghai Nanqiao HVDC reconstruction project and Baihetan-Jiangsu hybrid multi-terminal HVDC project,the electromagnetic transient simulation models of LCC-MMC(Line Commutation Converter-Modular Multilevel Converter)hybrid HVDC system and the receiving hybrid type MTDC system are established on PSCAD / EMTDC simulation platform.Through the simulation analysis of the two models under various operating characteristics,the correctness of these two models is verified,which lays a foundation for the subsequent study of control strategy and protection scheme.In terms of fault ride-through control,the reason why the LCC-MMC hybrid HVDC system can not ride through the AC fault at the sending end is firstly analyzed: The DC voltage at the sending decreases due to the connected AC fault,while the constant voltage control of MMC keeps the DC voltage at the receiving end unchanged,resulting in the rapid reduction of the DC current,or even intermittent phenomenon.With this cognition,the transient process of DC current when the sending end AC system fails is analyzed.Then the approximate analytical expression of DC current and its zero crossing time are obtained and verified by electromagnetic transient simulation.The change rule of DC current after the sending end fault is revealed deeply,which provides the theoretical basis for fault ride-through control.Based on the influence of the reactive power transmission between the MMC converter station and the AC system on the DC voltage of MMC,a current control strategy with sectional reactive power auxiliary is proposed.The drop range of DC voltage at the receiving end is effectively widened by making MMC station absorb reactive power from AC power grid.So the constant current control after fault can be realized.In order to consider the rapidity and stability of fault ride-through control,the control mode of subsection reactive power assistance is designed.The reactive power absorbed by MMC can be automatically converted from the instant of fault to the fault being traversed until the fault is removed.This control can not only effectively improve the fault ride-through ability,but also maintain the stable power transmission.The proposed control strategy creates an important engineering application value for demonstration of the conventional DC receiving terminal flexibility.In terms of line protection configuration,a DC line protection scheme suitable for the receiving hybrid type MTDC system is designed.The coupling effect of the high-pressure and low-pressure converter valves and the structure of the multi-drop point at the receiving end bring new challenges to the line protection of the system.Combined with the fault characteristics analysis of each DC line,the traditional DC main protection is used as the main protection of the DC line between the sending and receiving terminals.The traveling wave protection based on wavelet transform is introduced as the main protection of the line between the high and low voltage valve groups,which meets the requirements of the tree flexible DC line protection for the rapidity and reliability,and has an important guiding significance for the engineering practice of hybrid multi terminal DC transmission.The proposed protection scheme has important guiding significance for the engineering practice of hybrid MTDC transmission. |
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