I hope others can give you some good ideas, but here are my thoughts about it:
If you don't have any value for steam/heat in your model, I don't think it makes any difference whether you model the CHP plants as pure power plants or CHP. In both cases, you need the efficiency, availability, and cost data. If you have the data for these parameters, you can model the technology equally well as a pure power plant, or a CHP plant with NCAP_CHPR(FX)=0. The results would be identical. And as your steam has no value, you could even add some realistic NCAP_CHPR(FX)=X, and you would still get identical results in terms of fuel consumption and electricity generation.
But because you say that cogeneration represents 15% of total installed electricity capacity, it seems cogeneration already has an important role in your system. If you are going to model options for new investments into power plants, it would thus seem resonable to include CHP as an option also for the future. But to do that, you would need to model the demand for the process steam / heat, which the CHP plants can deliver, so that you would also have a value for the steam/heat. Otherwise you will not be able to model the competitiveness of CHP in your system in a realistic way.