Abstract: Abstract:After the reformation of merging of induced draft fan and boost fan into one for the unit No.6 of a power plant, when the motor of the induced draft fan started directly, the voltage of 6kV bus lowered to 76% of the rated value, which can not meet the requirement of relevant regulations. A scheme of adding soft starter in primary circuit was adopted to lower the start current of the induced draft fan. It was shown by calculation and field test that when the limitation of overcurrent multiple of the soft starter is selected as 4.0, the voltage of 6kV bus will lowers to 84% of the rated value, which can meet the requirement of relevant regulations. At last, the influence of the soft starter in start process on the system and equipments and the matters needing attention in site use of the soft starter were pointed out.

Key words: Keywords:soft starter, merge induced draft fan and boost fan into one, voltage drop, limitation of overcurrent multiple, differential protection