In order to optimize the off-design performance of the gas-steam combined cycle system

a new type of variable back pressure modification method is proposed based on the conventional gas-steam combined cycle system.The reforming system has a draught fan in the tail flue of the heat recover steam generator of the conventional combined cycle unit.Then the back pressure of the gas turbine under some operating conditions of the unit can be reduced by the fan.Taking the gas-steam combined cycle S109 FA as the reference unit

the reference unit and the reforming unit are modeled

and then the off-design performances of these two units are analyzed and compared through adopting the same turbine inlet temperature and the fuel adjusting operation strategy.The results show that

the efficiencies of the topping cycles and bottoming cycles under the partial load of the reforming unit are improved compared with the reference unit.At the combined cycle of 84%~35% load

the efficiency of topping cycle increases by 0~2.7%

the efficiency of bottoming cycle increases by 0~3.0% and the efficiency of combined cycle increases by 0~2.5%.The variable back pressure modification system can use IGV to control flow and improve the gas initial parameters of the corresponding flow

which provides technical reference for efficient transformation and operation of gas-steam combined cycle power plant.