The flexible generation of electrical energy is heavily dependent on hydropower. Advanced control systems may offer an enhanced operational security. During power system disturbances, a generator may need to operate beyond its limits to maintain a stable operation. This increased performance can be obtained if the total thermal capacity of the generator is exploited by applying optimal control theory in the automatic voltage regulator (AVR). A low order model is required for online implementation and prediction of critical thermal stresses in the machine. Periodic extension of the generator capability diagram has been historically overlooked. In this contribution, a thermal network for air-cooled hydrogenerators for use in real-time monitoring and optimal control is proposed and validated. The thermal model is extended with a heat exchanger model. The dynamic model is developed from an energy balance of the system and verified through FEM simulations and temperature measurements in a case study. Transient simulation results of the online low order model are in good agreement with measurements. Finally, a satisfactory simulator speed is obtained which confirms the feasibility of this approach in real-world applications.

The ever-increasing penetration of intermittent energy sources introduces new demanding operating regimes for the bulk power generation in the power grid. During a worst-case power system disturbance scenario, the generator needs to operate beyond its limits to maintain stable operation. Therefore, a new online low-order thermal model of a hydrogenerator has been recently proposed, where the periodic extension of the long-forgotten capability diagram of the machine was in-depth investigated. An increased performance can be obtained if the total thermal capacity of the generator is exploited by applying optimal control theory in the Automatic Voltage Regulator (AVR). This paper proposes a Non-linear Model Predictive Controller (NMPC) combined with an Unscented Kalman Filter (UKF) with a modeling framework geared for use in a supervisory control structure for the conventional control system. The method provides maximum utilization of the machine's thermal capacity by providing the controller with an Enhanced Capability Diagram (ECD). Case studies on a single-machine system and a 16-bus multi-machine system were investigated. The results show that a satisfying controller action during different long-term voltage instability scenarios is realized.

The efficacy of flexible reservoir-based hydropower for renewable integration is evident throughout the world in places like Norway, Canada, Brazil, India, and China, to name a few. The flexibility of hydropower, both at the reservoir and generation side, could enable higher shares of intermittent renewable sources in regions where hydropower is readily available. In Norway, hydropower is the backbone, enabling extraordinary cheap electricity from new renewables. Compared to Germany, where coal power plants are the main backup producers for weather-dependent renewables, Norwegian hydropower is much more flexible to dispatch when the load changes. Although we are still waiting for the missing link to back up intermittent renewables everywhere, pursuing the flexibility of hydropower can be a transformative solution in places where it applies. This solution is especially true for the Nordic power grid, where Norway alone hosts half of Europe’s entire hydropower storage capacity.

This review article covers the multidisciplinary challenges shaping hydropower development within a broader energy transition. Classical hydropower generation facilities are moving toward flexible operations to adapt to and facilitate the integration of variable renewable energy sources (VRES). This review article explores the paradigm shift in terms of machine design considerations, power plant operation, and grid regulator implications. Machine designers, consider magnetic saturation, energy efficiency estimation, and mechanical wear and tear. This could involve, high flux densities in certain core regions, lower weighted average efficiencies, and higher machine failures owing to insulation degradation. Moreover, power plant operators must consider the effects of the flexible power production caused by VRES. In some cases, hydropower ramping ranges from 1 to 5 megawatts (MW) per second for large hydropower plants, which pose technical challenges for operators. In contrast, grid regulators must strengthen reactive power mandates and system-bearing services to ensure stability. In certain regions, there could be longer periods of 30 to 40 percent reduction in physical inertia owing to the lower share of traditional power plants in a VRES-dominated power system.

The future power grid will integrate large shares of variable renewables, such as solar and wind, through inverter-based solutions. These developments change the power grid’s characteristics owing to lower shares of classical synchronous generators and result in weaker, low-inertia power grids that challenge their secure and robust operation. To deal with this unsolved challenge, there is a potential for small modular reactors (SMRs) to provide system-bearing ancillary services, such as uncontrolled physical inertia and short circuit capacity, that can enable large-scale penetration of renewables. This article shows that SMRs can avoid the need for synchronous condensers to run renewable-rich power grids, thereby reducing ancillary service costs by approximately $20/MWh and simultaneously ensuring higher grid strength and inertia. Consequently, running renewable-rich power grids with SMRs as a backbone can be a competitive grid-forming solution in the journey toward a deeply decarbonized power system.