Virtual Inertia Implementation in Variable Speed Hydropower Plant

This paper investigates how a variable speed hydropower (VSHP) plant with virtual inertia (VI) control can deliver fast power reserves to compensate for variable production of renewables and maintain grid stability after disturbances. The advantage of the VSHP is that it is able to change the electric power output significantly within a few seconds and maintaining it for a longer period. During the first seconds, the energy is taken from the rotating masses in the generator and the turbine. Thereafter, the water flow and thereby the mechanical power are adjusted by governor action to return the turbine to its optimal speed. Two VI control structures are implemented and tested on a VSHP plant simulation model. The virtual synchronous generator (VSG) and the virtual synchronous machine (VSM) are further developed to fulfil the main objective of the control; to maximize the grid support from the VSHP by utilizing the turbine and generator rotational energy. At the same time, the impact on the hydraulic system must be considered in order to fulfil the objectives for internal control of the plant; i.e. to optimize the turbine rotational speed, minimize water hammering and mass oscillations, minimize guide vane servo operation and minimize hydraulic and electric losses. Dynamic analyses are performed to compare the control structures; the power-frequency PID controller with permanent droop (VSG-PID) based on the VSG and the VSM with power-frequency PID controller and permanent droop (VSM-PID). They are evaluated by two main criteria; their ability to deliver instantaneous power (inertia) to reduce the rate of change of frequency (ROCOF) and their contribution to frequency containment control (steady state frequency droop response).