Method for flexible operation of a power plant assembly

The invention relates to a method for flexible operation of a power plant installation with a heat recovery steam generator having heat exchanger stages (10-15) for generating fresh steam (D) and/or reheater steam (HZU) for a steam turbine from an exhaust gas stream (S) of a gas turbine, wherein an auxiliary firing device (F) is arranged in a flue gas duct (R) of the heat recovery steam generator in the region of the heat exchanger stages (10-15). To control the fresh steam (D) and/or the reheater steam (HZU), at least one injection cooling device (20-23) is put into operation immediately upon activation of the auxiliary firing device (F).