In January 2026, a technical incident occurred in Estonia during testing of the new 100MW Hertz 1 (Kiisa) battery energy storage system (BESS). The event triggered protective relays, resulting in the emergency shutdown of over 1GW of HVDC capacity, specifically the EstLink 1 and EstLink 2 interconnectors. The root cause was an incorrect parameter configuration in the BESS Nidec Conversion grid-forming inverters, which induced low-frequency network oscillations. The feedback gains within the Virtual Synchronous Machine control algorithms were set with excessive sensitivity, effectively amplifying rather than damping the oscillations. I recall similar experiments in Matlab while modeling excitation controllers for synchronous machines during my diploma project. Back then, the physical inertia of real synchronous machines limited the visibility of such transients on a gigawatt scale. Now, with the rise of high-capacity inverter-based resources, these scenarios have become a physical reality. The issue has already been resolved by increasing response delays and tuning down the feedback gain coefficients. However, this case is the first practical demonstration I have witnessed of a well-known theoretical vulnerability: the susceptibility of inverter-dominated grids to cyber threats. While this specific incident was not a cyberattack but a standard test on high-power equipment, it highlights a critical risk. A simple modification of feedback gains in inverter control loops can lead to massive grid instability. BESS invertors requires the same level of protection and rigorous safety standards as nuclear or aviation control systems. And not only utility-scale BESS, but all grid-forming units connected to the entso-e grid. What was once a theoretical concern has now been proven in practice.
