The Research On Protection Of MTDC

Compared with the double-ended HVDC systems, multi-terminal DC system has advantages in large-capacity and long-distance transmission of power and work in a more flexible and economical way. For DC line, fast and reliable operation of relay protection is critical to maintain stability of HVDC system. Currently the principle of travelling wave protection provided by the ABB company and the SIEMENS company is used as the main protection. And differential protection, under-voltage differential protection, low voltage protection are used as the backup protection. Due to the theoretical defects of various protection principles at present, it is indispensable to study new protection methods to accomplish the line protection in a better way.A parallel type of four multi-terminal DC system is built in PSCAD based on the analysis of the control mode, principle and topological structure of parallel multi-terminal DC system in this paper. Considering the requirement of real-time of line protection algorithm and the detection of fault transient signal effectively, an improved recursive wavelet algorithm is used to extract the quantity of fault transient signals under specific frequency, which only uses historical data to calculate with the advantages of a small amount of calculation and fast speed.Failure criterion is constructed according to the difference of fault signal high frequency component based on the filtering unit when external or internal fault occurs. And fault pole identification is implemented according to the difference changing direction of current when external or internal fault occurs. When the two conditions above meets at the same time, line protection acts. The validation of the algorithm is carried on using MATLAB.Simulation results show that the protection with the IRWT algorithm, which is applied in parallel four-terminal HVDC transmission line protection system, can operate reliably when a fault occurs on the line and does not operate incorrectly when a fault occurs beyond the line.Research results of this paper have great theoretical and practical value in engineering.