Short circuit fault occurs frequently during the operation of power system. In case of short circuit fault in the power system

the shortcircuit current in the short circuited circuit can increase significantly and can

at the same time

lead to voltage reduction in the grid

even to the 70%～75% of the original one which

as consequence

keeps the power supply for some users damaged. The reason of the busbar bar voltage reduction is needed to be analyzed so to keep the system operate normally. After finding the reason

the corresponding measures are taken to reduce the voltage drop and let the busbar voltage recover to its normal level. Meanwhile

the short circuit calculation of the system shall be performed so to

in accordance with the short circuit calculation

make analysis on how to reduce voltage drop

calculate and select the most economic mode out of all schemes and

finally

design theprogram flow chart which can be really achieved by the system. In this paper

such four modes of synchronous machine excitation as constant excitation current

constant reactive power

constant power factor and constant active power are analyzed in detail. The constant active excitation mode is adopted to solveproblems encountered. Finally

the short circuit current rapid judgment is performed

thus it can be judged quickly if short circuit fault is happened ornotinaccordancewiththecurrentvariationofthesystem.