The Single-end Protection Principles Of HVDC Transmission Lines

Compared with AC power system,HVDC power system has many advantages such as large capacity,long transmission distance,low line loss and access to new energy.The DC transmission line is the channel to transmit power and the performance of the line protection is important to safe operation of the line.HVDC line protections can be divided into double-end protection and single-end protection.Since HVDC lines usually transmit power over long distance and the environment along the line is serious,thus the communication channels in the double-end line protection will greatly reduce reliability of the protection.The single-end protection of the HVDC line does not require the exchange of measurements between the rectifier side and the inverter side,so it has higher reliability and economic benefits.Research and improvement of single-end DC line protection is important to HVDC systems.The working principles of several common HVDC single-end protections are studied and their defects are also respectively pointed out.Next,the temporal and spatial variation of voltage and current along the HVDC transmission line is analyzed.And it is concluded that the low frequency voltage signal of the DC line is approximately linearly distributed along the line.On this basis,the principle of mho relay in AC line protection is improved,and a new type of distance protection for HVDC transmission lines is proposed.The ratio of the compensation voltage to the measurement voltage is adopted as the action criterion.The Yun-Guang ±800kV HVDC system is established in PSCAD/EMTDC and the performance of the new protection is verified by simulations.Compared with other single-end protections of HVDC lines,the new protection has lower sampling rate,smaller calculation amount,faster operation speed,clear operation criteria and is not affected by the system operation mode.