Combined with the fault identification criteria (13) and (14), Table 4 shows that: when a fault occurs on the positions f1, f2 , f3 and f4 , the values of D1 of them are less than the setting value of Dset2, which is equals to 1.1, indicating that the fault is an internal fault; when a fault occurs on the positions f5, f6 , f7 and f8, the values of D1 of them are greater than the setting value of Dset2, which is equals to 1.1, indicating that the fault is an external fault. To sum up, when the metallic short circuit fault occurs, the proposed protection criterion can reliably distinguish whether a fault occurs on or beyond the DC line.
6.2.2 High transition resistance simulation results : When a short-circuit fault occurs on a DC line, the transition resistance value can affect the reliability and sensitivity of the protection.
Due to its limiting effect, the comprehensive distance of current fault waveform fluctuates to a certain extent, so it is necessary to conduct simulation verification for different transition resistances.
Take fault f1 as an example, set the transition resistance as 500 Ω, and keep the other parameters unchanged. Fig.8 shows the simulation results.