Non-linear model predictive control for variable speed hydropower.

Inverter Based Resources (IBR) are gaining more popularity in modern power systems, leading to decrease in grid inertia. Hydropower Plants with rotating synchronous machines have been excellent source of inertia in existing grids. Adding a frequency converter between the generator and the grid in these hydropower plant would enable to run the hydropower in variable speed leading to enhanced efficiency and more flexibility in operation. These Variable Speed Hydropower Plant (VSHP), having the capability of anciliary services for grid support could be a potential solution for providing synthetic inertia to low-inertia power grids. However, there is a little research done on control of VSHP for grid support. This paper aims at implementing Non-Linear Model Predictive Controller (NLMPC) algorithm for optimal control of VSHPs, and the results are compared with classical PID control method. Explicit comparison under realistic operating condition and analysis of constraint handling capabilities and robustness of MPC have been performed. Advanced model based NLMPC successfully coordinated the hydraulic and electric systems having different time constants by implementing multi-objective optimization and upstream constraints satisfaction. Furthermore, the NLMPC has shown relatively better performance than the classical controllers even during the occurrence of an unanticipated grid-side disturbance. This paper is claimed to be an important work in control domain since it develops the control system for VSHPs using time-domain models which can offer better insights over the nonlinearities that exist within hydropower systems. Apart from the computational complexity, NLMPC is found to be viable for the control of VSHP since it ensured stable operation, as can be seen from the analysis of different operational cases studied implemented in this paper. The key takeaway from this paper is the potential of utilizing advanced model based optimal control strategy for coordination among complex dynamic systems of VSHP for different loading patters and disturbances. Also, the MPC has a scope of providing more robust grid support by addressing overloads and uncertainties during operation.