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In this paper, five virtual inertia control structures are implemented and tested on a variable speed hydropower (VSHP) plant. The results show that all five can deliver fast power reserves to maintain grid stability after disturbances after a disturbance. The VSHP is well suited for the purposed since its output power can be changed almost instantaneously by utilizing the rotational energy of the turbine and generator. This will cause the turbine rotational speed to deviate from its optimal value temporarily. Then the governor control will regain the turbine rotational speed by controlling the guide vane opening and thereby the turbine flow and mechanical power. With that, the VSHP output power can be changed permanently to contribute with primarily frequency reserves. Dynamic and eigenvalue analyses are performed to compare five different versions of the basic VSG and VSM control structures; VSG, power-frequency PID-controller with permanent droop (VSG-PID), VSM, VSM with power-frequency PD-controller (VSM-PD), and 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) |
