Both wind power and hydropower systems contain a turbo machine interacting with a fluidum. Also, both applications are high torque and low speed in nature, and experience an extant axial force. As for capacity, the 10 MW design goal in the ALPHEUS project corresponds to the rated power of modern large wind turbines.
Furthermore, a Permanent Magnet Synchronous Machine (PMSM) with a fully rated power converter offers advantages in both research fields because of its high power density and efficiency under variable power operation. Therefore, we aim to apply advancements in wind power technology to the hydropower applications in ALPHEUS. This includes the torque and speed control of a PMSM, Maximum Power Point Tracking (MPPT) methods and Model Predictive Control (MPC) algorithms. Next to the machine control, innovations in the use and design of high density and high torque Axial Flux PMSMs (AF-PMSM) and bearing arrangements offer great possibilities for the proposed reversible pump-turbines (RPT).
Naturally, this knowledge is to be extended and adapted to successfully develop the PTO within the project. Next to the existing similarities, the contra-rotating RPTs (CR RPTs) have additional features that make for an exciting challenge in this project’s research. E.g. the CR RPT has two actuators, which interact with each other through the fluid. Furthermore, an inlet valve can be used to control the flow rate, resulting in a control system with three degrees of freedom.
What is unique and beneficial about the power train that you are developing for ALPHEUS?
Both actuators of the RPT are separately driven by two electrical machines. Therefore, their controllable speed ratio can be exploited to achieve a maximal dynamic response to grid changes, while maintaining optimal efficiency at different operating points. The AF-PMSMs with YASA topology have a high power and torque density and have no stator iron, which significantly reduces weight. Further efficiency improvements to the AF-PMSMs can be made by using segmented rotor magnets, concentrated pole magnets and grain-oriented material in the stator teeth. The ALPHEUS concept will contain a bulb in which the drivetrain is housed. Two coaxial shafts provide transmission between the turbo machines and the electrical machines. This drivetrain architecture retains hydraulic efficiency, while allowing optimal accessibility and reasonable bearing loads.
What challenges might you face scaling up the model motor for real-world application?
The biggest challenge in realizing a real-world application is the reliability. To support the grid, it is essential that the machine is available at all times. In a first stage, the AF-PMSMs will be developed for a small scale test setup. Here, the dynamic operation and reliability will be thoroughly investigated. Proper scaling laws will be used to ensure the reliability in real-world application. A significant advantage of using AF-PMSMs with a high pole number is the possibility of using a modular machine drive, which separately controls all stator windings. If a fault occurs in one of the windings, the drive can compensate for this with the other modules and the machine can resume operation with slightly reduced dynamics and efficiency until the problem is resolved.
How do you control the operation of the pump/turbine?
To control the machines’ rotational speeds and torques, Field Oriented Control (FOC) is used due to its fast dynamic torque response. The speed or torque setpoints result from a higher level control. In a first stage, an indirect MPPT method will be used. Here, the rotational speeds and inlet valve angle are tabulated for each measured head height between the reservoirs and power setpoint. This power setpoint results from the grid state. Once a detailed model of the RPT is attained, the high level control can expand to MPC, predicting the most optimal speed and angle setpoints to achieve the wanted power setpoint as fast and efficient as possible.
ABOUT THE AUTHORS
The role of Johan Abrahamsson – University of Uppsala, in the project is to work on the parts of the Power Take-Off system that deals with boundary conditions for the machine-side control, as well as to investigate different power-electronic architectures. He has a background in high-speed machines and advanced control of power-electronic systems.
Jeroen De Kooning coordinates the work on the ALPHEUS Power Take-Off (PTO). It is our aim to realize a cutting edge PTO hardware and control architecture that maximizes efficiency under variable power operation, while simultaneously ensuring the strong dynamics needed to provide ancillary services to the grid. For this, we will leverage our experience with the design and control of wind turbine drivetrains.
To take up the challenge of investigating the Power Take-Off system in the ALPHEUS project is in line with the motivation Daan Truijen has as a master of science in electrical engineering. That is, to contribute to the energy transition, in which hydropower has great potential. His aim within his PhD research is to realize the drivetrain and control system for the different reversible pump-turbine concepts in ALPHEUS. The PTO will maximize efficiency and dynamic response to increase the grid stability and provide ancillary services.