As mentioned above, electrical power systems are undergoing important transformations with the increased adoption of decentralized energy resources. This changes introduce, among others, major sources of uncertainties in the management and operation of future electrical grids. Consequently, one research question is how to systematically test controller interactions to determine if there is a negative behavior that can hinder the operation of distribution grids. With this in mind, OFFIS has been focused on addressing this question in the project EWE Netz Dynamische Simulation.
In the context of this research project several control are used for either voltage or reactive power support within the grid. They are on-load tap changer of the main transformer and inverter based control for renewable generators (cosφ(P) and Q(U) characteristics) as well as reactive power control for wind farms [Q(Q) characteristic].
In the following, one of the most prominent interaction is presented. In this particular case three control strategies are operating at the same time. First is the tap changer control for the main transformer with a dynamic set-point adjustment (this configuration is defined to adjust the voltage increase that ensues after a sudden influx of active power through the transformer). Second is the reactive power control for two wind farms connected at the main busbar i.e. Q(Q) control. The last one is cosφ(P)-characteristic for the remaining wind farm located far away from the main busbar. In the Fig. 1 the main results are presented.
As mentioned before the use case for this project is voltage and reactive power control. In the figure, the medium voltage at the main busbar at 20 kV is shown. Additionally it is shown the reactive power provision from the onshore wind farms. The closest one to the main busbar is providing reactive power for attempting to fulfill the grid operator requirement of Q = 0 at the point of common coupling (interconnection with the external grid). Furthermore, the remaining wind farms within the distribution grid are set to control the voltage via providing or absorbing reactive power in the so called cosφ(P) control for decentralized energy resources.
The interaction of the three control strategies shown in Fig. 1 is considered an operational inefficiency because while the medium voltage is on the rise due to a sudden feed-in of active power from the PV units in the systems the wind farm within the grid should absorb reactive power in order to provide voltage support thus decreasing its magnitude. However, around the same time there is a requirement to keep the reactive power balance at the point of common coupling which result in a reactive power provision coming from the closest wind farm connected to the 20 kV busbar. Consequently, both the absorption and production of reactive power from two different wind farms are canceling each other which results in an uncontrolled rise of the voltage. This is not considered a critical problem because the tap changer control of the transformer is able to act thus bringing the voltage value back to the defined deadband. Important to realize however is that the voltage is adjusted after three tap action in a time frame of less than 2 min, this is also undesired when considering the wear and tear of the mechanical parts of the tap changer of the main transformer.
In essence, the aforementioned project is relevant to the current research because it addresses both the requirement engineering process (points 1 through 3 in the Fig. 2) and the dynamic testing of controller interactions (point 4). However, there are still open points to deal in the context of this research. Firstly is that there has been no requirement for the use of a formal use case elicitation (chiefly the use of the IEC 62559–2) and its mapping onto the SGAM as opposed to this research project. This entails that it is important to describe the main use case derived from this project according to the presented artifact. Secondly, it is necessary to use, during the testing phase, a more general design to increase the robustness the proposed approach (e.g. developing a set of synthetic grid models that consider different topologies). Additionally, another important aspect is the definition of an appropriate evaluation method for the optimization of the simulation analysis based on modern design of experiments techniques allowing a comprehensive interaction analysis considering a complex combinatorial problem with increased time constraints taking into account that this method should be able to perform as close as possible to the operation of the power system.