The paper deals with optimal sizing of a gas turbine for repowering of cogeneration power plant Ljubljana considering possible plant operational strategy with respect to variations of electric and heat loads and energy costs. CHP plant is a main source for the Ljubljana town district heating system. Existing plant consists of two condensing steam turbines with steam extraction, back pressure turbine with steam extraction, auxiliary steam and hot water boilers for peak heat load production. This system delivers up to 111 MW into the power grid and up to 348 MW of heat. Repowering with gas turbine generator set with additionally fired heat recovery boiler is considered. For uncoupling heat and power generation a heat storage tank is assumed.

For sizing of new equipment and plant operational strategy a model based on mixed-integer linear programming was developed. Zero - one integer variables are adopted to indicate the on/off status of operation, continuous variables to indicate the operational level of each constituent equipment and an optimal solution is derived by branch and bound method.

Two prospective sizes of TG sets were tested for range of assumptions regarding power purchase tariff schedules. Different optimal operation policies resulted. The study provides background for contract negotiation and for investment decisions.

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