This paper studies the functioning of a thermal stress limiter for a 13K215 steam turbine in Połaniec Power Plant, which is a typical example of a 200 MW steam turbine retrofit. The share of renewable energy sources in the energy market is growing, leading to an increased demand for flexibility in conventional units, i.e., fast loading, fast unloading and fast start up etc. Thermal stress in steam turbine thick-walled elements, the steam turbine rotor in particular, is a major limit on the flexible operation of steam turbines. Therefore, steam turbine are usually controlled and protected by on-line stress control, i.e., a thermal stress limiter. In steam turbine retrofits in Połaniec Power Plant, each steam turbine was delivered by Alstom together with a thermal stress limiter implemented on a stand-alone PLC (Programmable Logic Controller). The thermal stress limiter for 13K215 steam turbine retrofits protects HP and IP rotors as well as HP and IP valve chests. The thermal stress limiter ensures safe operation of the steam turbine for the operating period required by the steam turbine owner. The thermal stress limiter also ensures the shortest possible steam turbine start up time for a guaranteed number of startups.