Research On Line Protection Principles Of Hybrid Bipolar HVDC Transmission System

Due to make full use of the advantages of traditional DC transmission and flexible DC transmission,a hybrid HVDC transmission system emerged.Most of the existing literature is to study the topology,mathematical model,control method and other aspects of the hybrid DC transmission system,but the line protection of the hybrid DC transmission system is rarely involved.Therefore,in order to ensure safe and reliable operation,it is necessary to study the line protection of the hybrid bipolar HVDC transmission system.The topology structure,mathematical model and control strategy of the DC system are analyzed,and a simulation model of ±500kV hybrid bipolar DC transmission system for relay protection is established,and builds a simulation model using PSCAD/EMTDC to verify the correctness of the model.A single-terminal electrical quantity protection method based on the ratio of transient components of boundary elements is presented in this paper.By analyzing the voltage characteristic signal at the protection installation,it is found that the transient high-frequency components of the voltage characteristic signal are obviously different when an internal or external fault occurs.According to the characteristic difference,the fault identification and fault pole selection can be realized,and its adaptability to hybrid bipolar DC transmission system is analyzed.Based on the research of the above-mentioned voltage characteristic signal transient energy protection method,the current information is added,the active power fault component is defined according to the voltage and current fault components generated when an internal or external fault occurs.A protection method for the transient energy ratio of the active power fault component is constructed by using the difference of the high and low frequency content of the line smoothing reactor when internal or external faults occurs.The active power fault component is decomposed to obtain the transient energy of each node.component is decomposed to obtain the transient energy of each node.Then,the value of the positive and negative energy ratios is used to distinguish between internal and external faults,and the single and bipolar faults are distinguished according to the difference between the positive and negative energy ratios.And the sum of part of the frequency band energy of the active power fault component on the positive and negative pole is used to implement the fault pole selection.