Table 7 shows that in the event of a single pole fault occurring on the DC line, the corresponding fault pole protection will operate and remove the fault, while the sound pole protection will operate correctly and it can still operate; in the event of a double pole fault occurring on the DC line, both pole protections will operate; in the event of a fault occurring outside the DC line, the protection will operate correctly.
When a fault occurs, the current fault component changes quickly, and the protection can be started at about 0.5ms. Current fault waveform collection and comprehensive distance calculation can be completed in 1ms. With a certain margin, the protection scheme proposed in this paper can operate reliably within 2ms, and the reliability and rapidity are further improved with a high transition resistance.
Conclusions
This paper analyzes the current fault characteristics of multi-terminal flexible DC power grid, introduces the comprehensive distance of current fault waveform, and proposes a line protection scheme based on the similarity of current fault waveform. By combining Euclidean DTW distance and entropy weight method to deal with current fault component waveform signal, the comprehensive distance of current fault component waveform of the DC line is calculated, and the similarity protection criterion is constructed. The calculated comprehensive distance is used to realize internal and external faults identification, and the ratio of positive and negative comprehensive distance and pole comprehensive distance is used to realize fault pole identification. PSCAD is used to build a flexible DC grid model to simulate different fault conditions and transition resistances, so as to output fault data under different fault conditions. MATLAB is used to analyze and process fault data to verify the protection algorithm. A large number of simulation results show that when a fault occurs, the fault pole protection does not refuse to operate, the non-fault pole protection does operate correctly, and the protection action time is very short. Therefore, the protection scheme proposed has high reliability, its rapidity and adaptability are further improved. And it has a certain ability to resist transition resistance. The research results will provide a theoretical basis for the safe and reliable operation of flexible DC power grid.
Acknowledgments
This work is supported by the National Natural Science Foundation of China (No. 51777166), Key support projects of the National Natural Science Foundation (No. U1766209).