The ongoing energy transition to cleaner energy involves three main changes: using less energy (energy savings on the demand side), making energy production more efficient, and using renewable and low-carbon sources instead of fossil fuels. However, relying more on intermittent renewable energy sources means we need to balance them with conventional sources for a stable electricity supply. Hydrogenerators can provide this stability, but also flexibility, by quickly increasing power when needed. They are designed for a daily average number of start-stop cycles equal to twice per day. On the other hand, they will face new challenges, as they were not designed for frequent and large load changes, which will put additional stress to the hydrogenerator parts. The continuous, safe, and reliable operation of the hydrogenator is determined by the boundaries of the capability curve (active-reactive power PQ diagram) provided by the manufacturer. Most of the limits given in the PQ diagram are isotherms indicating permitted temperatures of certain generator parts. In the inductive region, the predominant limitation is on the rotor current. If we wish to use the hydrogenerator as a flexible power source and maximize its available capacities, it is crucial to know the rotor temperature. Unfortunately, temperature sensors are not typically installed on the rotor due to its rotation and various associated issues, such as problems with the proper installation of temperature sensors because of large centrifugal forces and strong electromagnetic fields that affect them, issues with the power supply of the measuring system, and difficulties with data transmission from transmitters mounted on the rotating rotor. To ensure the safe operation of the hydrogenerator, we need to monitor the temperature of the rotor winding. The rotor winding temperature can be determined either indirectly or by direct measurements. The indirect method is widely used and is based on measurements of the rotor winding resistance, as specified in relevant standards. It is relatively easy to apply, but the following should be kept in mind: it requires precise measurements of the rotor voltage and current, which can be challenging, and provides only information about the average temperature of the rotor winding. On the other hand, the direct method requires installation of temperature sensors on rotor parts and provides information about the local temperature of rotor part on which the sensor is mounted. The accuracy of the measurement is highly dependent on the way the temperature sensor is mounted and its position. Specifically, the sensor should be mounted in such a way that it is completely isolated from the cooling medium and at the same time has good thermal contact with the part of the generator whose temperature is being measured. This paper presents a comparison of two independent systems for hydrogenerator rotor thermal supervision, along with their respective advantages and disadvantages. The results of measuring the rotor temperature (both indirect and direct) during the heat run test of a hydrogenator at the hydro power plant "Pirot" are also given. Models for comparing the two rotor winding temperature measuring systems are presented with the aim of enhancing the reliability of hydrogenator rotor thermal supervision. These models can be used for hydrogenerator asset management, planning of near-term and long-term outage activities, improved rotor thermal supervision, and more.