Integration of Non-synchronous Generation - Frequency Dynamics

Abstract: Traditionally the predominant generation has been large synchronous generators providing the system inertia. Nowadays when wind power and other non-synchronously connected units displaces this synchronous generation, the system inertia is reduced. Hence higher rate of change of frequency and larger frequency deviations are expected. This thesis deals with the impact on rate of change of frequency and frequency deviation in the Nordic power system with decreasing system inertia as well as with the introduction of synthetic inertia and frequency support from wind turbines. Increasing wind power is a question of system inertia, governor frequency response, load frequency response and magnitude of disturbance rather than the amount of wind power in the system. As long as the system inertia is maintained at an acceptable level, the wind power production can be arbitrarily selected. A synthetic inertia controller acting on df/dt calculated from a slight filtered frequency gives a frequency response similar to that for a synchronous generator. Further, it can damp the electromechanical oscillations between (groups of) generators in the system, provided that the turbine can handle a few periods of oscillations which are reflected in the output power. If measured frequency is filtered heavier prior to calculating df/dt the controller does not act on the oscillations. The wind turbine response is also delayed which is preferable as the hydropower reacts faster and the frequency deviation is reduced. A frequency support controller acting on Δf is best used in combination with curtailed wind turbine output power. A predictable frequency response is obtained which can be handled similar to frequency response from traditional generation. The Δf-controller is also possible to use without curtailment but the tuning of it is very critical, especially in a system with high level of wind power. If this controller is not handled with great caution it may be devastating to the frequency dynamics. The entire power system has to be considered to verify wind turbine behavior in combination with already existing turbine governors. Otherwise an attempt to improve frequency dynamics can end up with worse frequency dynamics than without any control on wind power. The amount of induction generators in the system is also likely to increase. It is shown that the synchronous and induction generator capability of delivering power to the grid during a frequency disturbance is almost solely determined by the generator and turbine mechanical system, i.e. the amount of inertia. Introducing new types of production with induction generators may affect the frequency dynamics due to a change in inertia but this is not directly related to the type of generator.

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