Variable Speed Hydropower operated with dynamic droop for grid-frequency support

With the reduction in grid inertia due to penetration of non-rotating generation sources in modern power grids, flexible operation of hydropower plants has become necessary. Variable Speed Hydropower Plants (VSHP) have emerged as an alternative to this. For quick frequency support in the grid post-disturbance, droop of the generation source plays major role as it determines the ramping of generation. However, there lies a gap on studies involving variable droop implemented in VSHPs, and its capability of frequency support. Also, the variable droop during operation demands very fast guide vanes operation, which could lead to violation on hydraulic pressures and flows limits. This paper implements a Non-linear Model Predictive Control (NLMPC) algorithm for operating a VSHP in varying droop scenario, offering a dynamic droop enabling the VSHP to pick up more load when needed. The NLMPC imposes hydraulic limits as constraints on the optimization problem whose main objective is to provide fast power response to balance the grid frequency. The controller also simultaneously ensures stable operation of hydraulic system of VSHP which is decoupled from the grid. The simulation is run on IEEE 9 bus power system injected by a VSHP and is compared with base case of same test system for different magnitudes of step load increment and contingency. The results yield a maximum of 3.5% reduction in frequency nadir and significant reduction in Rate of Change of Frequency (RoCoF) in the grid. This reduction in system frequency nadir and RoCoF can be claimed as significant contribution in grid support. In addition to this, capability of NLMPC to control the guide vanes leading to hydraulic control simultaneously when the VSHP is slowing down to provide optimal frequency support to the grid highlights its suitability in hydropower control applications.