Virtual Inertia from Renewable Energy Sources: Mathematical Representation and Control Strategy

Renewable Energy Sources (RES) make the frequency events worse due to lack of rotating inertia. Commonly RES are connected with the power grid as a current source and work in Maximum Power Point Tracking (MPPT) mode. Therefore, RES do not participate in grid frequency regulation. The inertia towards grid frequency control is contributed solely by conventional power plants. With the increase in RES, the mechanical inertia per unit operating electrical power is getting reduced and making the electrical grid less stable towards any frequency event. This paper presents an analytical method to calculate the equivalent inertia constant (H _e ) in terms of MJ per MVA for converter-based RES which use a dc-link capacitor as the energy storage element for providing synthetic or virtual inertia towards the grid frequency regulation. The mathematical analysis is useful for designing the synthetic inertia system and to calculate the maximum possible equivalent inertia constant (H _emax ), which an existing renewable energy source can achieve. A control strategy is developed, based on swing equation with the help of the mathematical analysis, presented in this paper. A realtime simulation study, using OPAL-RT (OP4510) on a power system, with thirty percent penetration of RES, validates the mathematical analysis, presented in this paper. It is noticed that the value of the dc-link capacitor of RES needs to be incredibly large to provide inertia which is equivalent to the mechanical inertia of the synchronous machine.