Short fault of bridge arm in cycloconverter for fractional frequency transmission system

In a fractional frequency transmission system (FFTS), the fault characteristics are analyzed by switching property of thyristor when short-circuit fault occurs in one of the cycloconverter bridge arms. The different operation modes of the cycloconverter, i.e., the rectifier mode, the inverter mode and bridge-switching stage, are taken into consideration. The calculating method for voltage on the lower frequency side and currents on both lower frequency side and industrial frequency side are proposed, and the fault characteristics of the cycloconverter are thus quantitatively described. Analysis shows that in case of short-circuit fault, the bridge arms fail for commutating, the voltage on the low frequency side drops obviously, and the fault current on industrial frequency side gets very large. However the cycloconverter in FFTS usually works in the rectifier mode, the peak value of fault current on its industrial frequency side can be reduced by over 40%. Simulations verify the above analysis.