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).