Fig.9 Simulation results for internal and external faults identification
As can be seen from Fig.9, the proposed protection criteria can accurately identify internal and external faults. When a fault with high transition resistance (500 Ω) occurs at the end of the line, the protection can also act reliably and quickly remove the fault. And when a fault occurs outside the DC line, the protection will operate correctly.
6.3 Fault pole identification simulation results
After determining the internal and external faults, fault pole identification should be carried out for the fault occurring at the points of f1, f2, f3 and f4. f1 and f2 simulate single pole earth faults, which are positive and negative earth faults respectively; f3 and f4 simulate double pole faults, which are double pole crossover faults and double pole short circuit earth faults respectively. The ratio K2 of the positive and negative current fault waveform distances is calculated and the resulting values of K2 are shown in Table 6.
Table 6 The value of K2 of fault pole identification with different transition resistance