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Research area:

Improved Control Dynamics of Conventional Power Plants in the Context of Changing Energy Supply Structures

The share of renewable energies on the electric energy production in Germany has been increasing constantly because of the German Renewable Energy Sources Act (EEG). In 2015 a share of 30 % renewable energy supply was reached as can be seen in Figure 1. The renewable energy sources mostly consist of power stations with a highly volatile weather-dependant power infeed like photovoltaic arrays or wind power plants.

The remaining share of electric energy production is covered by conventionel power plants mostly consisting of large fossile fueled power stations. Because of the priotity of renewable power infeed as prescribed by §11 of the EEG the full load hours of the conventional power plants are reduced. Several power plants with higher marginal costs due to economic reasons already have been either disconnected from the power grid or pooled together as a power plant reserve. Correspondingly, the number of planned power plant new-builds is decreasing.

Renewable energy sources like wind power plants or photovoltaics do not contribute to primary frequency control and by now only marginally contribute to the dynamic voltage control. It is to be expected that a reduced number of conventional power plants in the future still has to carry out a substantial share of dynamic voltage and frequency control tasks.

Conventional power plants contribute to primary frequency control automatically through their turbine governors. A disturbance of the equilibrium of power production and consumption within the continental european power system results in a frequency gradient depending on the moment of inertia of the rotating machines in the system. The turbine governors act upon the emerging frequency deviation from nominal frequency and set back the power equilibrium forcing the frequency gradient to disappear. The control action takes place within 10-30 s after a disturbance and is highly affected by the number of power plants currently connected to the power system and their corresponding turbine dynamics.

Within this research project a simplified european power system model to study frequency dynamics is developed based upon european power plant lists and suitable turbine governor models as well as information from the ENTSO-E on load situations and transmission capacities between the different areas of the power system. Figure 2 shows a schematic diagramme of the simplified power system model. Simulations are performed regarding different european power plant compositions to investigate the impacts of large disturbances such as power plant outages on the future primary frequency control. Thus, recommendation for the future european power plant composition can be made to integrate high shares of renewable energy sources into a power system with a limited number of conventional power plants.

In contrary to the frequency control voltage control is only affecting local areas around a disturbance location. Dynamic voltage disturbances, e.g. short-circuit faults, are partially taken care of by the conventional power plants through their automatic voltage regulators (AVR). The AVR reacts upon a voltage difference measured at the terminal of the synchronous generator of a power plant and controls the excitation system to change the rotor's excitation voltage accordingly. The transient reaction of the voltage regulator on disturbances takes place within the first few seconds after a disturbance. To investigate the influences of different AVRs on transient voltage stability of a power system, generic power system models can be used. The AVRs are modeled according to the IEEE Standard 421.5-2006. Figure 3 shows an example model of an AVR with converter fed excitation.

Within this research project generic power system models together with standardized AVR models are subjected to different voltage disturbances. It is investigated how an AVR parameter optimization can improve transient voltage stability. This leads to recommendations on how to adjust the AVR parameters such that the transient voltage stability remains within acceptable limits in a future power system with a decreased number of conventional power plants and high shares of renewable energy supply.