Estimation and prediction of the hydrogenerator rotor thermal capability and influence on the reactive power capabilities

The ongoing energy transition implies increasing penetration of renewable energy sources into the power system, which significantly affects the manner of exploitation of conventional generating units. The consequence of the intermittent production cycling of renewable energy sources is the need for increased flexibility of conventional generating units in order to preserve system stability. In conditions of long-term voltage instability, the available capability for voltage regulation, which is available to conventional generators, depends on the temperature rise of the generator parts. The temperature rise determines the maximum output power (rated value) of the generator. When the hydrogenerator operates at a power lower than the rated electrical output power, there is an additional short-term thermal capability available. Knowing of the hydrogenerator rotor thermal capability is of key importance for the possibility of the generator short-term operation outside the permitted operating area defined by the static operating diagram and the use of the remaining short-term thermal capability. This dissertation is aimed at improving the estimation and prediction of the hydrogenerator rotor thermal capability and its influence on the reactive power capabilities. For this purpose, a combined coupled thermal-electrical model of the hydrogenerator field winding was developed based on the finite difference method, which allows insight into the complete thermal image of the field winding. An algorithm was developed for determining the temperature of the hydrogenerator field winding by solving a combined coupled thermalelectrical model. For the purposes of verifying the average field winding temperature of the hydrogenerator, which is obtained using the developed coupled model, two estimator models were developed to determine the average field winding temperature of the hydrogenerator based on the measurement of electrical quantities and temperatures in the power plant. Experimental measurements were carried out during the heat run test on a real hydrogenerator. After that, the verification of the developed combined thermal-electrical model of the field winding and estimator models for determining the average field winding temperature was done using experimental measurements. A model for estimation of the field winding hot spot temperature was developed based on field current measurement and an average field winding temperature. As part of the analysis of the influence of the field winding thermal capability on the limit values of reactive power, an algorithm was developed for adaptive adjustment of the overexcitation limiter. The algorithm is based on the use of the entire available rotor thermal capability in order to increase the flexibility of operation in overexcited operating conditions. This dissertation provides contributions related to the condition monitoring, planning of the overhaul inspections and exploitation of hydrogenerator.