Three-phase short-circuit fault on the lower frequency bus of cycloconverter in FFTS

Fault current through the cycloconverter is analyzed by PSCAD/EMTDC, when the three-phase short-circuit fault occurs on bus of the lower frequency side (LFS) of the converter station in fractional frequency transmission system (FFTS). After introducing the simulation model of FFTS, some discussion is made for some phenomena which are different from the traditional transmission approach, such as the phenomenon that the impulse current on power frequency side (PFS) is larger than that on LFS at the same time, etc. Then, two methods are compared, which both could cut off the fault, including by cutting off the pulses of cycloconverter and by opening the breaker. The results show that the operation time of the method by cutting-off the pulse is much shorter than that by opening the breaker. Finally the impact of the parameters of the control system and the electric system on the impulse current is also mentioned.