The provision of traditional ancillary services is subject to change due to the decommissioning of large power plants. In this scenario, the use of small-scale flexibility installed in the active distribution networks offers a high potential in partially filling the gap that is emerging. However, as the flexibility increases, the loading capacity of the distribution networks reaches its limits. On the one hand, the available flexibility provided by the active distribution networks is not always available due to the network constraints. On the other hand, a guaranteed provision of the ancillary services is crucial for a secure system operation. This paper presents the integration of system serving flexibility provision into a predictive congestion management from the point of view of a distribution network operator. Therefore, a strategy for a two-dimensional, preventive curtailment (active and reactive power) of the flexible power units is developed and tested. Furthermore, the methodology is coupled with a probabilistic power flow to account for the uncertainty in the distribution network. The results show the effectiveness of the presented predictive congestion management by achieving both, securing a safe distribution network operation and guaranteeing the ancillary service provision.

The large dispersion of flexibility providing units among all voltage levels of the power system requires large, comprehensive grid models to assess the impact of flexibility usage during system operation. For this reason, aggregation methods are used to enable distributed concepts. This paper proposes a decentralized and hierarchical approach for the practical implementation of coordinated, vertical flexibility provision across multiple voltage levels, with the feasible operation region as an interface between adjacent grids. A top-down disaggregation process, based on a linear OPF model, is introduced in this paper. It is used during a flexibility provision request to distribute the requested operation point change to the underlying flexibility providing units. The objective function for this disaggregation process is chosen according to operational aspects. Both implemented methods for aggregation and disaggregation of flexibility consider grid constraints. The proposed concept is field tested during regular grid operation in a real distribution system. The results of the conducted studies show that accurate system operation in the more volatile distribution systems requires shorter time intervals compared to the transmission system. Good results with small control deviations were achieved by using time intervals of one minute for the MV and ten seconds for the LV level.

So far, several simulation methods for the aggregation of distributed flexibility providing units have been developed, which allows an estimation of the flexibility potential in distribution systems. However, the accuracy of the result in a real grid as well as the particularities of flexibility deployment as an ancillary service have yet to be demonstrated. Therefore, a holistic approach including the prediction, online aggregation and activation of flexibility was integrated into an existing grid monitoring system. Based on real grid measurements, the uncertainty of flexibility usage can be evaluated in a field test. The results prove, that a linearized OPF based aggregation method is suitable for use in grid operation. However, due to the stochastic behavior of LV-grids, a valid prediction of the flexibility potential cannot be guaranteed. Moreover, fluctuations in the flexibility provision may prevent a constant setpoint for a grid in a typical time interval of e.g. 15 minutes.

DOI: 10.1049/icp.2021.2085 ISBN: 978-1-83953-591-8 Location: Online Conference Conference date: 20-23 September 2021 Format: PDF XXXXX Inspec keywords: power grids; power system measurement; optimisation; power system state estimation; least squares approximations; load (electric); distribution networks Subjects: Optimisation techniques; Power system measurement and metering; Other topics in statistics; Distribution networks

High penetration of distributed energy resources (DER) in distribution grids poses new operational challenges as information flows increase as well. The development of new aggregation methods for information can help in enhancing the coordination among grid operators, and with DER operators, regarding the provision of ancillary services. This paper proposes a distributed bottom-up aggregation method to aggregate flexibility from DER and using this flexibility within a congestion management approach at the HV level. Based on linear optimization approaches, the computation time of the method is reduced, revealing the potential use of flexibility aggregation methods in everyday grid operation tasks. This approach is demonstrated on a grid model based on a real distribution grid.