A cardboard factory requires steam and electricity, which are produced in its own powerplant. Conventional cogeneration systems cannot cope with the large fluctuations in steam demand, inherent to the cardboard production process, while power demand remains almost constant. For this reason, two additional cogeneration systems, based on steam-injected gas turbines, were installed. Steam injection allows for a more flexible steam-production to power-production ratio. The effect of steam injection in these so-called Cheng Cycle systems is analysed by means of a steady-state model, which is based on construction drawings and available simulation data. Validation of the model with measurements shows good agreement of model behaviour with actual behaviour. With the model, the flexibility of the Cheng Cycle system is investigated and steam injection is analysed. It shows that steam injection should only be applied when excess steam has to be produced to meet the electricity demand. Although the analysis is based on an existing situation, the developed models and calculation methods may easily be extended to other systems.