Research On Protection Strategy Of Multi-terminal HVDC Grid Based On Fault Characteristics

With the increasing demand for new energy consumption and the continuous development of power electronics technology,multi-terminal high voltage DC(MT-HVDC)transmission has become an important development trend of energy Internet in the world.In the field of power supply to remote areas,access to new energy sources,and interconnection of non-synchronous power grids,MT-HVDC power grids are showing good development momentum.Presently,the key technologies affecting the development of MT-HVDC systems include fault identification,location and rapid removal of direct current line faults.In this context,this thesis focuses on the four-terminal HVDC transmission system from the aspects of fault characteristics analysis of DC transmission line,line relay protection principle,fault current limiting and isolation measures.The main work and achievements of this thesis are described below:(1)Based on the understanding of the structure and working principle of the MT-HVDC system,this thesis studies the HVDC grid based on the typical two-level voltage source converter(VSC)and the modular multi-level converter(MMC)respectively,and analyzes the fault transient characteristics of the two after the DC side faults.By equivalence of fault process of transmission line under short-circuit fault,the mathematical analysis method of short circuit current after fault of two poles of the system is put forward,and the approximate expression of short circuit current in HVDC transmission system with long transmission distance and large transmission capacity is deduced.(2)The fault characteristics of HVDC system in and outside DC line are extracted and analyzed,and a fault pole selection criterion based on transient voltage amplitude ratio is proposed.Traditional transmission line fault detection relies on line communication means,which cannot guarantee the rapidity and reliability of protection and may lead to device damage.Based on this problem,a fault identification scheme based on single-terminal transient of transmission line is introduced in this thesis.In this scheme,the line inductance is calculated by the current rising rate at the initial stage of the fault,and the fault is determined after comparing with the set value,which lays a foundation for the distance protection calculation of the position of the fault point,and the fault location is realized by using the single terminal measuring line wave characteristics.With the help of PSCAD/EMTDC platform,a four-terminal HVDC system model was built to complete the DC fault simulation and verification.Simulation results show that the new protection principle is effective.(3)This thesis makes a comparative analysis on the existing fault isolation schemes of HVDC transmission lines,and introduces a new scheme that combines inductive current limiting device with DC circuit breaker(DCCB).In this thesis,the working mechanism of the circuit breaker is analyzed,the calculation method of several key parameters of the device is given,and the four-terminal HVDC grid model is built on the PSCAD/EMTDC platform,and the fault simulation is completed.The simulation results show that,compared with the traditional circuit breaker,the proposed DCCB topology can effectively limit the rise speed of fault current and the maximum current value,and quickly remove the fault point when the system fails.The device can satisfy the requirement of fault current suppression for HVDC system.