Field winding temperature of the synchronous generator is one of limiting factors shaping generator PQ capability curve. Therefore of great operational importance is to have information about this temperature estimated with the utmost accuracy. It can be measured directly using built-in temperature sensors or determined indirectly. With the increasing penetration of renewable energy sources into the high-voltage transmission system, the fundamental changes happen in the way conventional generating units work, including hydrogenerators. They are facing the new operating modes characterized by rapid load changes and an increased number of start-stop cycles. These modes of operation lead to additional thermal stress of the hydrogenerator parts, including the rotor. In order to be able to respond to new demands that they are faced with, it is necessary to estimate the actual thermal capacity of the rotor and its influence on the dynamic hydrogenerator PQ capability curve.