The article deduces analytic expression of voltage travelling wave for hybrid cascaded DC system and puts forward protection based on amplitude coefficients of line-mode fault-component voltage (LFCV). By analyzing time-domain expressions of fault voltage travelling waves

it is derived that under internal faults

for the head of line

the ratio of two amplitude coefficients of the LFCV is 1:2

while for the end of line

the ratio of two amplitude coefficients of the LFCV is 1:0. Then

it is further derived that under internal faults

for either side of line

the sum of two amplitude coefficients of the LFCV is far less than 0. However

under external faults

for the near faulty terminal of the line

the ratio of two amplitude coefficients of the LFCV is 1:–1; thus

the sum of two amplitude coefficients is 0. Therefore

the amplitude coefficients of LFCV can be utilized to distinguish the internal fault from the external fault. Simulation verifies that the method can reliably discriminate the internal and external faults even with high fault resistances and has high robust

ss confronting noise interference.