Enhancing Power System Stability with GFM-BESS-A Case Study on A Real Incident in Denmark

1 For citation: H. Gong, L. Lu, Y. Liao, J. B. Kwon, N. Qin, A. B. Montesinos, and A. G. Lameiras, “Enhancing power system stability with GFM-BESS: A case study on a real incident in Denmark,” in Proc. Wind & Solar Integration Workshop, Berlin, Germany, 2025. Offentlig/Public ENHANCING POWER SYSTEM STABILITY WITH GFM-BESS-A CASE STUDY ON A REAL INCIDENT IN DENMARK Hong Gong1*, Liang Lu1, Yicheng Liao1, Jun Bum Kwon1, Nan Qin1, Ana B Montesinos2, Ander G Lameiras2 1Energinet, Denmark 2Power Electronics, Spain *[email protected] Keywords: GFM-BESS, POWER SYSTEM STABILITY, SYSTEM-LEVEL EMT, REAL-WORLD EVENT, VOLTAGE SUPPORT Abstract This paper investigates the application of Grid-Forming Battery Energy Storage Systems (GFM-BESS) to enhance power system stability, using large-scale simulations of a real incident in Denmark’s eastern transmission system (DK2). On August 6, 2020, DK2 experienced a sequence of transmission line faults that drastically reduced system strength, disconnected key assets such as HVDC link and synchronous condenser (SynCon), and posed a risk of system collapse. The event highlighted the need for faster and more flexible grid support. To evaluate the potential of GFM-BESS, detailed electromagnetic transient simulations with vendor-specific models replicated system topology, operational scenarios, and fault sequences. Two configurations were compared: the original setup with SynCons and a modified system including GFM-BESS. Results show that GFM-BESS stabilizes voltage and improves fault recovery more effectively than SynCons, even at smaller capacities. Its near-instantaneous reactive power response enables rapid voltage restoration and mitigates active and reactive power oscillations. Performance depends on installation site, GFM-BESS capacity, and control tuning, emphasizing the importance of strategic deployment. Overall, GFM-BESS demonstrates superior adaptability and resilience, offering a robust solution for the secure transition to converter-dominated power systems. 1 Introduction The power system transition to high penetration level of renewable energy sources (RES), particularly wind and solar, is accelerating in Europe and globally [1]. In Denmark’s eastern transmission system (DK2), wind generation already makes up a large share of electricity production, with plans for further expansion to meet ambitious decarbonization targets. While this transition reduces greenhouse gas emissions, it also changes fundamental characteristics of the power system. The increasing share of inverter-based generation—predominantly grid-following (GFL) plants—lowers system strength, as these units depend on existing voltage and frequency references rather than establishing them. A further limitation is their inherently slower dynamic response during disturbances. Instead of reacting immediately to changes in system conditions, GFL units typically respond with a delay, which postpones the injection of reactive current and weakens their ability to support voltage recovery [2]. Together, reduced system strength and slower disturbance response make the grid more vulnerable to voltage fluctuations, low-frequency oscillations, and transient stability issues, particularly under faults or other disturbances. Traditionally, stability issues have been managed through centralized solutions owned by transmission system operators (TSOs), and most notably synchronous condensers (SynCons) [3]. SynCons provide short-circuit current, inertia, and reactive power, and continue to be valuable stability assets. However, they involve large investments, long lead times, and aging fleets that will eventually require replacement. At the same time, TSOs are exploring decentralized solutions enabled by advanced inverter controls. Among these, gridforming battery energy storage systems (GFM-BESS) are emerging as a promising option [4], [5]. Unlike conventional grid-following inverters, GFM-BESS can autonomously establish voltage and frequency references, deliver rapid active and reactive power, and provide synthetic inertia, offering a flexible alternative when stability provision is expected from developers rather than solely through central TSO-owned assets. To accelerate the deployment of GFM technologies and support the development of technical standards, Danish TSO, Energinet, launched the GFM Deployment Project [6]. The project involves close collaboration with OEMs and developers to collect detailed EMT models of GFM devices, including BESS, wind turbines, PV inverters, and STATCOMs. Within this framework, the project enabled Energinet to evaluate a vendor’s GFM-BESS model under realistic operational conditions, applying the models in single machine infinite bus (SMIB) tests and system-level 2 For citation: H. Gong, L. Lu, Y. Liao, J. B. Kwon, N. Qin, A. B. Montesinos, and A. G. Lameiras, “Enhancing power system stability with GFM-BESS: A case study on a real incident in Denmark,” in Proc. Wind & Solar Integration Workshop, Berlin, Germany, 2025. Offentlig/Public evaluations supported by extensive technical dialogue with industry. Despite prior studies demonstrating that GFM-BESS can enhance voltage recovery, oscillation damping, and overall system resilience [7],[8], few analyses have applied detailed EMT simulations using vendor-specific GFM-BESS models in real incident scenarios to assess their effectiveness. The DK2 incident on August 6, 2020 [9] provided a unique opportunity to address this gap, exposing limitations of conventional stability measures and highlighting the need for faster, more flexible system responses. This paper presents EMT simulations of the DK2 2020 incident, comparing the existing SynCon-based configuration with a scenario where GFM-BESS replaces SynCons. The analysis examines the influence of BESS capacity, installation location, and control tuning. By combining real-world incident analysis with detailed simulations, this work provides practical insights for TSOs on balancing centralized and decentralized stability solutions, and on how GFM-BESS could complement or replace SynCons in future system development. 2. Incident overview-DK2, August 6, 2020 DK2 is the eastern Danish power system, interconnected with Sweden and the broader Nordic power system, and linked to DK1 (western Denmark) and Germany via two LCC-based HVDC links: Storebælt HVDC (SB-HVDC) and KONTEK HVDC (KO-HVDC). On August 6, 2020, DK2 experienced a sequence of severe transmission network disturbances while operating under low system strength. Prior to the incident, DK2 ran with a reduced number of large synchronous generators, two 400 kV lines out of service, and relatively low renewable generation. The system relied heavily on imports through HVDC links from DK1 and Germany, and AC interconnections from Sweden, resulting in a low short-circuit ratio (SCR) at critical substations and increased system vulnerability. As illustrated in Figure 1, the incident began with a fault on the 400 kV line between Bjæverskov and Ishøj (BJS_400_ISH). Ten minutes later, a second fault occurred on the 400 kV line between Bjæverskov and Hovergaard (BJS_400_HVE), further weakening system strength. These sequential outages increased the electrical distance between imported generation sources and local load centers, reduced voltage support, and diminished overall system robustness. The weakened conditions led to the sequential tripping of the SynCon at BJS due to reverse power protection and the SBHVDC link due to sub-synchronous oscillation (SSO) protection. As a result, an isolated 400 kV island formed, comprising Herslev (HKS), Bjæverskov (BJS), and Asnæskværket (ASV) substations. Due to the heavy reliance on imports from Sweden and limited local generation, the system was at high risk of collapse if additional contingencies had occurred. The combination of low system strength, sequential faults, and limited fast-acting stability resources meant the system could not recover quickly, highlighting the need for solutions capable of providing near-instantaneous active and reactive power support under weak-grid conditions. Because the incident involved multiple sequential faults in a low-SCR environment, it provides a realistic stress scenario for evaluating alternative stability solutions. In this study, the event is replicated in an EMT simulation to directly compare the performance of the existing system with a hypothetical GFM-BESS deployment under identical conditions. Figure 1: Overview of DK2 incident on August 6, 2020. 3. Methodology 3.1 Simulation environment The study was conducted using an EMT simulation platform developed by Energinet [9], which incorporates vendorspecific plant models to replicate the system’s dynamic behavior during faults and disturbances. The DK2 transmission system was represented in detail, including 400 kV and 132 kV transmission lines, transformers, interconnections, and major generation and load centers. Protection schemes, fault-clearing times, and switching actions were modeled based on operational records from the August 6, 2020, incident, ensuring an accurate representation of the disturbance sequence. Vendor-specific GFM-BESS models, including inverter and control dynamics coupled to a DC battery source, were integrated into the platform and initially scaled to match the capacities of SynCons at the BJS and HKS stations, enabling direct comparison of their performance under identical system conditions. This high-fidelity simulation environment supports systematic studies of advanced grid-forming technologies in DK2, ASV HKS BJS ISH HVE SB-HVDC KO-HVDC sc HKS_G sc BJS_G 400 kV HVDC sc SynCon DK1 German GØR Sweden Decoupled in 400 kV lines 220kV 132 kV GLN 1st fault 2nd fault 3 For citation: H. Gong, L. Lu, Y. Liao, J. B. Kwon, N. Qin, A. B. Montesinos, and A. G. Lameiras, “Enhancing power system stability with GFM-BESS: A case study on a real incident in Denmark,” in Proc. Wind & Solar Integration Workshop, Berlin, Germany, 2025. Offentlig/Public allowing evaluation of fast voltage and frequency support, active/reactive power response, and synthetic inertia provision during faults and disturbances. 3.2 System configurations As shown in Figure 2, two primary configurations were evaluated: • Base Case – SynCon Configuration: The operational setup recorded during the August 6, 2020, incident, including two SynCons located at HKS and BJS. This configuration serves as the reference scenario for evaluating system stability under weakgrid conditions. • Test Case – GFM-BESS Configuration: In this scenario, one of the SynCons was replaced with a vendor-specific GFM-BESS, while the other SynCon remained in service. This setup allows assessment of how GFM-BESS can complement existing stability assets and provide fast voltage during disturbances. (a) (b) (c) Figure 2: System configurations evaluated: (a) Base Case with two SynCons; (b) Test Case with GFM-BESS at BJS; (c) Test Case with GFM-BESS at HKS. 3.3 Parameter variations (further study cases) To evaluate the robustness of GFM-BESS performance, a series of further study cases were defined based on modifications to the GFM-BESS configuration described in Section 3.2: • Test Case 2 – Installation Location: GFM-BESS installed at BJS or HKS, as shown in Figure 2 (b) and Figure 2 (c), to assess location-dependent performance variations. • Test Case 3 – Capacity Scaling: Small (150 MW), medium (200 MW), and large (250 MW) GFM-BESS ratings, to examine trade-offs between GFM-BESS capacity and performance effectiveness, and to compare with synchronous condenser capacities. • Test Case 4 – Control Parameter Tuning: Variations in inertia constant and voltage controller parameters, to investigate trade-offs between system response and overall stability margins. These further study cases allow systematic assessment of how BESS-GFM performance depends on location, capacity, and control design, providing insights into optimal deployment strategies under different system conditions. 3.4 Performance metrics System performance was evaluated against the following operational stability criteria: • Stable HVDC Operation: KO-HVDC and SBHVDC restored pre-fault conditions without consecutive commutation failures. • No SynCons Trips: All synchronous condensers remained in service during and after fault events. • Voltage and Frequency Stability: Voltages and frequency maintained within acceptable operational limits throughout disturbance and recovery. ASV HKS BJS ISH HVE SB-HVDC KO-HVDC sc HKS_G sc BJS_G 400 kV HVDC sc SynCon 220kV 132 kV ASV HKS BJS ISH HVE SB-HVDC KO-HVDC sc HKS_G 400 kV HVDC sc SynCon 220kV 132 kV GFM-BESS ASV HKS BJS ISH HVE SB-HVDC KO-HVDC sc BJS_G 400 kV HVDC sc SynCon 220kV 132 kV GFM-BESS 4 For citation: H. Gong, L. Lu, Y. Liao, J. B. Kwon, N. Qin, A. B. Montesinos, and A. G. Lameiras, “Enhancing power system stability with GFM-BESS: A case study on a real incident in Denmark,” in Proc. Wind & Solar Integration Workshop, Berlin, Germany, 2025. Offentlig/Public 4 Results 4.1 Base case – SynCon configuration (a) (b) (c) Figure 3: Simulation results: (a) Voltage at BJS and HKS; (b) Active power of HVDCs (SB-HVDC and KO-HVDC); (c) Reactive power of SynCons at BJS and HKS. Figure 3 presents the simulation results of voltages at HKS (blue) and BJS (red), active power at KO-HVDC (blue) and SB-HVDC (red), and reactive power of the SynCons at BJS (blue) and HKS (red) during the disturbance sequence. At t ≈ 12 s, a single-phase fault occurs on the BJS_400_ISH line and is cleared by disconnecting the faulted phase. At t ≈ 13 s, the fault is reapplied to emulate the first auto-reclosure attempt but persists, resulting in the tripping of line BJS_400_ISH. At t ≈ 15 s, a second fault occurs, followed by the tripping of the BJS_400_HVE line at t ≈ 16.5 s, which significantly reduces voltage support in the affected area. In the simulation replication, protections are disabled to observe the full consequences of the disturbances and provide a consistent baseline for evaluating alternative solutions, such as replacing a SynCon with a GFM-BESS. The sequential line outages reduce system strength, resulting in oscillatory active power at both SB-HVDC and KO-HVDC. Although the existing SynCons at BJS and HKS inject reactive power and fault current, their electromechanical response is insufficient to restore voltages promptly. These results reflect the behavior observed during the real incident, illustrating how sequential line outages can drive the system toward instability and highlighting the critical need for near-instantaneous reactive power support under weak-grid conditions, which conventional SynCons alone may not reliably provide. 4.2 Test case – GFM-BESS configuration (a) (b) (c) Figure 4: Simulation results: (a) Voltage at BJS and HKS; (b) Active power of HVDCs (SB-HVDC and KO-HVDC); (c) Reactive power of SynCons at BJS and HKS. Figure 4 presents the simulation results for the test case in which the SynCon at BJS is replaced by a 250 MW GFMBESS, while the SynCon at HKS remains. Voltage at HKS Voltage at BJS Active power of KO Active power of SB Reactive power of SynCon at BJS Reactive power of SynCon at HKS Voltage at HKS Voltage at BJS Active power of KO Active power of SB Reactive power of GFM-BESS at BJS Reactive power of SynCon at HKS 5 For citation: H. Gong, L. Lu, Y. Liao, J. B. Kwon, N. Qin, A. B. Montesinos, and A. G. Lameiras, “Enhancing power system stability with GFM-BESS: A case study on a real incident in Denmark,” in Proc. Wind & Solar Integration Workshop, Berlin, Germany, 2025. Offentlig/Public The GFM-BESS at BJS provides nearly instantaneous voltage support and strengthens the local grid, stabilizing AC voltages at both HKS and BJS. Consequently, the SB-HVDC link restores pre-fault conditions without consecutive commutation failures, and oscillations in active power at both SB and KO are mitigated. Importantly, the SynCon at HKS remains in service because its reverse active power protection is not triggered, while system voltages and frequency are maintained within acceptable operational limits throughout the disturbance and recovery. These results demonstrate that replacing the BJS SynCon with a GFM-BESS enhances system resilience and helps prevent HVDC instability, without reproducing the previously observed fault sequence. 4.3 Test case 2 – Sensitivity to installation location Figure 5 compares the dynamic performance of the system when a 250 MW GFM-BESS is installed at HKS versus at BJS, with the SynCon retained at the alternate location. Although the SB-HVDC converter is located at HKS, its commutation failures during the BJS_400_ISH line trip are mainly driven by the voltage decrease at BJS, where the fault occurs. When the GFM-BESS is installed at BJS, its nearly instantaneous reactive current injection directly supports the local voltage, helping stabilize the commutation process in the KO-HVDC link and indirectly limiting adverse power oscillations that propagate to SB-HVDC. By contrast, when the GFM-BESS is installed at HKS, its reactive injection is less effective in mitigating the voltage dip at the faulted BJS bus. The weaker system strength at HKS and network impedance (electrical distance) between HKS and BJS reduce the immediacy of its voltage support at the disturbance location, making both KO-HVDC and SB-HVDC more vulnerable to commutation failures. Increasing the GFMBESS rating at HKS can partially compensate for this limitation by injecting sufficient reactive current to offset the reduction in effectiveness caused by the network impedance, thereby ensuring that adequate voltage support reaches BJS and enabling faster voltage restoration. These results demonstrate that instantaneous voltage support at the disturbed node is the critical factor for preventing HVDC commutation failures, and that the effectiveness of a GFM-BESS strongly depends on its location. (a) (b) (c) Figure 5: Simulation results with GFM-BESS integrated at HKS and BJS, respectively: (a) Voltage at HKS; (b) Active power of SB; (c) Reactive power of GFM-BESS. 4.4 Test case 3 – Sensitivity to capacity scaling Figure 6 presents simulation results for cases in which the SynCon at BJS is replaced by GFM-BESS units of different ratings: 150 MW, 200 MW, and 250 MW, respectively, while the SynCon at HKS remains. (a) (b) Voltage at HKS with GFM-BESS integrated at HKS Voltage at HKS with GFM-BESS integrated at BJS Active power of SB with GFM-BESS integrated at HKS Active power of SB with GFM-BESS integrated at BJS Reactive power of GFM-BESS at BJS Reactive power of GFM-BESS at HKS Voltage at HKS with 150MW GFM-BESS Voltage at HKS with 200MW GFM-BESS Voltage at HKS with 250MW GFM-BESS Active power of SB with 150MW GFM-BESS Active power of SB with 200MW GFM-BESS Active power of SB with 250MW GFM-BESS 6 For citation: H. Gong, L. Lu, Y. Liao, J. B. Kwon, N. Qin, A. B. Montesinos, and A. G. Lameiras, “Enhancing power system stability with GFM-BESS: A case study on a real incident in Denmark,” in Proc. Wind & Solar Integration Workshop, Berlin, Germany, 2025. Offentlig/Public (c) Figure 6: Simulation results with different sizes of GFM-BESS: (a) Voltage at HKS; (b) Active power of SB; (c) Reactive power of GFM-BESS. The 150 MW GFM-BESS does not provide sufficient reactive current capability to restore the commutation voltage, leading to HVDC commutation failures and active power oscillations at both SB-HVDC and KO-HVDC. In contrast, both the 200 MW and 250 MW cases maintain stable HVDC operation, with the higher rating achieving faster voltage recovery and more secure stabilization of active power flows at SB-HVDC and KO-HVDC. Larger GFM-BESS units deliver stronger instantaneous reactive power support, thereby reinforcing system strength and stabilizing AC voltages at both BJS and HKS. These results confirm that adequate GFM-BESS sizing at electrically strong nodes such as BJS is critical for preventing HVDC commutation failures and for maintaining voltage and frequency within acceptable limits, ensuring overall system resilience during disturbances. 4.5 Test case 4 – Sensitivity to control parameter tuning Table 1: Cases of GFM-BESS control parameters. Case Inertia Constant Voltage Controller Bandwidth A Low Low B High Low C High High Table 1 presents three GFM-BESS control cases with different combinations of inertia constant and voltage controller parameters: Case A has low inertia and low bandwidth, Case B has high inertia and low bandwidth, and Case C has high inertia and high bandwidth. Figure 7 shows system responses under different GFM-BESS controller parameters settings when the SynCon at BJS is replaced by a 200 MW GFM-BESS, while the SynCon at HKS remains. Cases A and B exhibit slow reactive power response, resulting in delayed voltage recovery at the faulted node, HVDC commutation failures, and large oscillations in active power at (a) (b) (c) Figure 7: Simulation results with different controller parameters of GFMBESS: (a) Voltage at HKS; (b) Active power of SB; (c) Reactive power of GFM-BESS. SB-HVDC. In contrast, Case C achieves rapid voltage recovery and stable power transfer thanks to high voltagecontroller bandwidth, which enables fast reactive power injection, and higher inertia constant, which resists rapid changes in active power, slowing the initial voltage drop during disturbances and providing additional time for the system to respond effectively. These results confirm that appropriate GFM-BESS controller tuning, including both sufficient voltage-controller bandwidth and an appropriately chosen inertia constant, is critical for preventing HVDC commutation failures and maintaining voltage and frequency within operational limits, ensuring robust system resilience during disturbances. 5 Discussion 5.1 Benefits of GFM-BESS Reactive power of 150MW GFM-BESS Reactive power of 200MW GFM-BESS Reactive power of 250MW GFM-BESS Voltage at HKS: Case A Voltage at HKS: Case B Voltage at HKS: Case C Active power of SB: Case A Active power of SB: Case B Active power of SB: Case C Reactive power of GFM-BESS: Case A Reactive power of GFM-BESS: Case B Reactive power of GFM-BESS: Case C 7 For citation: H. Gong, L. Lu, Y. Liao, J. B. Kwon, N. Qin, A. B. Montesinos, and A. G. Lameiras, “Enhancing power system stability with GFM-BESS: A case study on a real incident in Denmark,” in Proc. Wind & Solar Integration Workshop, Berlin, Germany, 2025. Offentlig/Public The simulations demonstrate several key benefits of GFMBESS, primarily due to their fast-acting, flexibly tuneable power electronic controls. GFM-BESS can: • Establish and maintain a voltage reference independently of local system strength, enabling stable operation even in weak-grid conditions. • Deliver nearly instantaneous reactive power support to arrest voltage dips, helping prevent HVDC commutation failures. • Buffer system variations through an adjustable inertia constant, which slows the rate of voltage and frequency changes during disturbances, providing additional time for fast voltage-controller action. Together, high inertia and fast voltage control ensure rapid stabilization of voltages and active/reactive power flows, as demonstrated in Case C. The simulation results also highlight the role of controller tunability: Cases A and B, with lower or narrower settings, exhibit large active power oscillations at both SB-HVDC and KO-HVDC, while Case C stabilizes the system rapidly. These results emphasize that the tunable controls of GFM-BESS enable robust voltage and power stabilization, providing significant operational flexibility in weak-grid, highrenewable scenarios. 5.2 Operational implications for weak grids Replacing a SynCon with a well-sized and strategically located GFM-BESS can significantly enhance stability margins in low-SCR environments. The active power oscillations observed at SB-HVDC and KO-HVDC originate from weak system strength, where delayed voltage recovery and commutation failures lead to unstable active power oscillations. By providing fast reactive support and rapid voltage stabilization at the critical nodes, the GFM-BESS reduces the severity of these oscillations and supports stable HVDC power transfer. Key operational benefits include: • Fast voltage recovery and stable HVDC operation, lowering the risk of commutation failures during sequential line trips. • Mitigation of active power oscillations at SB-HVDC and KO-HVDC, thereby relieving stress on interconnections and decreasing the likelihood of unnecessary protection actions. • Robust reactive power support, showing that adequate GFM-BESS capacity combined with proper controller tuning is essential to stabilize weak nodes and maintain overall system reliability. 5.3 Deployment and siting considerations Simulation results highlight that location and capacity are both critical for effective GFM-BESS deployment. Installing a GFM-BESS at electrically strong nodes close to disturbance areas (e.g., BJS) allows fast and effective reactive power injection, leading to quicker stabilization of AC voltages at both BJS and HKS (Figure 5). Placement at weaker or more distant nodes (e.g., HKS) slows voltage recovery and reduces the effectiveness of system stabilization. Proper siting ensures that GFM-BESS delivers maximum support without unnecessary oversizing. 5.4 Control tuning trade-offs These results illustrate the trade-off between response speed and stability: aggressive tuning improves fast voltage support but may interact with other devices or cause overshoot, while conservative tuning ensures stability but may fail to stabilize fast disturbances. For TSOs, this emphasizes the need for sitespecific commissioning and tuning tests, with parameters adapted to local grid characteristics such as short-circuit ratio, renewable penetration, and fault-clearing practices, to achieve robust voltage support and secure HVDC operation. 6 Conclusions This study evaluated the role of Grid-Forming Battery Energy Storage Systems (GFM-BESS) in enhancing stability of the DK2 transmission system during the August 6, 2020, disturbance. EMT simulations demonstrate that GFM-BESS delivers distinct stability benefits under weak-grid conditions: • Rapid fault recovery: Near-instantaneous reactive current injection restores voltages within the first cycles after a fault, suppressing power oscillations and preventing instability in HVDC operation. • Dynamic resilience through controller design: An appropriate inertia constant moderates the rate of active power change during disturbances, reducing the initial voltage drop and allowing more time for control action. Meanwhile, a high voltage-controller bandwidth ensures fast and effective reactive power injection, accelerating system recovery. • Scalable and adaptive deployment: Effective performance is achieved even with moderate capacities, but siting near disturbance-prone nodes and adequate sizing are essential for maximizing impact. Overall, GFM-BESS provides rapid, flexible, and siteadaptive support that strengthens voltage stability, secures HVDC operation, and enhances overall system resilience. These findings highlight the value of system-specific studies to guide optimal sizing, placement, and tuning of GFM-BESS as power systems transition toward high shares of inverterbased resources. 7 Acknowledgement The authors sincerely acknowledge the support and collaboration of Nick Morley, Nicky Ferguson, Nathan Murray from Eku Energy for their valuable technical insights and expertise, which greatly contributed to the interpretation 8 For citation: H. Gong, L. Lu, Y. Liao, J. B. Kwon, N. Qin, A. B. Montesinos, and A. G. Lameiras, “Enhancing power system stability with GFM-BESS: A case study on a real incident in Denmark,” in Proc. Wind & Solar Integration Workshop, Berlin, Germany, 2025. 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