To enhance the succesful rate of runback(RB) for subcritical units is a difficult problem in the thermodynamic control.The reliability of RB functions for units administrated by Guohua Electric Power Company

and scattered in power grid of eight regions

all are somewhat lower

along with increasing the output capacity of units participated into peak-load regulation

the influence of un-scheduled unit-trip upon safe operation of the power grid is increased.For this

directing against the defects existing in RB control logic of feed-water pumps and primary air fans

optimization of control logic for RB target load calculation

RB speed rate calculation

RB's reset circuit

trip circuit of the coal pulverizer

and the automatic interlocking of feed-water pumps etc.have been carried out.After optimization

the application on one subcritical 600 MW unit shows that the control system can satisfy the requirements of RB for feed-water pump and primary air fan.In the RB process of primary air fan

the pressure in primary air manifold can be reduced to minimum 4.94 kPa in time duration of 3 s;in RB process of the feed-water pump

water-level in the drum is decreased from-40 mm to the minimum-187 mm.