PURPOSE:To enable to perform sufficiently lowered excitation at load breaking time by a method wherein the limit value of the under-excitation limiter is lowered according to the trip condition of a load-break switch. CONSTITUTION:An automatic voltage regulator 1 outputs a deviation signal between the terminal voltage et of a synchronous machine 2 and a reference value, and the under-excitation limiter 5 outputs a magnetizing signal when the relation between active power and reactive power of the synchronous machine 2 exceeds the limit value to come on the under-excitation side. A high value priority circuit 7 supplies to a field winding 4 the signal having the higher value out of the output signals of the automatic voltage regulator 1 and the under-excitation limiter 5. The limit value of the under-excitation limiter 5 is lowered forcibly by a trip signal of the load-break switch. Accordingly sufficiently lowered excitation can be attained at load breaking time.
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