Minimizing Electromechanical Transients in Hydropower SEIGs: A Simple Approach for Smooth Islanded to Grid Synchronization

This paper addresses the management of electromechanical transients during disconnection and reconnection to the grid of turbine-driven induction generators (IGs) in hydraulic plants. Sudden loss of resistance torque after grid faults can lead to overspeed, mechanical stress, and potential water hammer effects. Conventional solutions rely on electric brakes and bypass valves to stop the IG and avoid damaging the whole mechanical and hydraulic system. When the grid is restored, to avoid severe torque and current transients, the IG must be reconnected as a motor in no-load conditions, the water flow is restored, and the machine starts operating as a generator. Such disconnection/reconnection transients result in long downtime even for brief faults. This paper proposes a low-cost technique to avoid such complex and time-consuming procedures. The IG is not shut down but rapidly disconnected from the grid and operated as a self-excited induction generator (SEIG) using a capacitor bank while supplying a local resistive load. A steady-state analysis was performed to properly size passive components, ensuring the islanded SEIG operates near its grid-connected working point to enable smooth reconnection without expensive power converters. The proposed technique was tested in numerical simulation with a suitably devised space-vector dynamic SEIG model that is valid for both grid-connected and islanded operation, as well as experimentally using a dedicated test setup.