The process of increasing the licensed power level of a commercial Nuclear Power Generating Station (NPGS) is called a power uprate. Power uprates have been implemented at a number of NPGSs in many countries. Such uprates are an economical way of producing more electricity at an NPGS and have attracted interest due to increased electricity prices. The increase in the electricity produced at an NPGS can be achieved in two ways: increase the thermal power in the reactor and improve the thermal conversion efficiency in the secondary side of the power plant by refurbishing or replacing the high pressure or low pressure turbine units, by feedwater heater refurbishment/replacement, or by a combination of these actions. Hence, their synchronous generator has to be repowered or replaced at some point of the NPGS life time. Obviously, the step-up transformer should be also replaced. This paper addresses the impact of turbogenerator uprating of a 1092 Mw Spanish NPGS in its transient response in case of grid events, namely faults. Repowering has mainly comprised machine stator rewinding. The main purpose of machine rewinding was to increase the life time of the machine after more than 30 years of successful operation. It has affected not only the steady-state characteristics (namely reactive power capability diagram) but the transient and subtransient reactances and time constants have changed as well. Moreover, the rotor inertia in the machine MVA base has changed. The impact of machine rewinding on its transient response will be evaluated using a detailed model of synchronous generator and its excitation and turbine-governor models. Grid Codes have been developed by Transmission System Operators (TSOs) to formalize their obligations and to establish the framework of their technical relationships with different actors of electric power systems (generators, distributors, consumers, etc.). Grid codes have legal liability in the country they apply. Grid codes have added new requirements with respect to IEC and IEEE standards. ENTSO-E Network Code on Requirements for Generators has been developed to harmonize Grid Code requirements through Europe. Hence, the fault ride capability of synchronous generators as formulated by the ENTSO-E Network Code on Requirements for Generators will be discussed in this test case.
Keywords: Turbogenerator uprating, turbogenerator rewinding, transient response.
Published: August 2016.