Abstract: The composition, the principle and existing problems of the boiler air preheater control system in the 300MW unit of a power plant were introduced. Before the reformation, when the power supply or the control mode of the control system was switched over, the driving oil pump often tripped; and the tripping caused that the condition of the standby motor of the air preheater interlocking start was false, the main motor and standby motor were stopped all, and this caused the “Run Back” operation. The defects were discussed emphatically. The technical reformation scheme for solving the problems was put forward. After the reformation, when the control power supply was switched over, the tripped driving oil pump will start up automatically, and the interlocking starting condition of the standby motor of air preheater was ensured. 
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Key words: boiler, air preheater, control system, power switchover, interlocking start