Long-term Voltage Stability in Power Systems - Alleviating the Impact of Generator Current Limiters

The main issues in this dissertation are the behaviour of current limiters protecting synchronous generators, and the interactions that occur with other components in the power system during a voltage instability. The importance of these limiters for the voltage stability is shown both in the analysis of small models and in the simulations of larger networks. Voltage stability of a power system can be improved by a proper current limiting equipment for generators situated in load areas which have a deficit in power production. Two different remedial actions alleviating the impact of the current limiters are analysed. These are: *the optimum use of the generators field winding thermal capacity by including temperature measurements into the control. *the use of mechanical power production increases or decreases as a way to avoid a too high current in the generator. The system can then be supported locally until both field and armature currents are at their respective maximum steady-state levels, the maximum capability for the system is attained or constraints in the mechanical power production are reached. The dissertation analyses the consequences of active power rescheduling and its significance for field and armature current limiter operation. Aspects as the size of active power changes and its variation in time are discussed on the basis of simulation results. Long-term voltage stability is also decided by other interactions occurring in the power system. In particular the load behaviour and tap changer control are analysed. The importance of proper modelling of these components and using correct values are discussed