Utility Scale Interface for Renewables and Storage Using a Power Electronic Transformer with Back-to-Back MMCs: A Novel Proposal and Control Scheme for Virtual Inertia

The increasing penetration of renewable energy sources into the grid has brought upon the need for synthetic inertia. This is to be meted out by the corresponding power electronic interfaces integrating these Distributed Energy Sources into the grid. In particular, a novel interface scheme using back-to-back MMCs to integrate wind turbines to the collection grid at voltages of 34.5kV has been shown to possess substantial potential for virtual inertia. The large stored potential energy in the virtual dc link between the MMCs is used to emulate the characteristics of a synchronous generator in case of step load changes. Inertial parameters of the back-to-back MMC has been derived enabling comparison with a synchronous generator connected to the grid. A control scheme to provide additional power to emulate the inertial characteristics of a synchronous generator has been developed. The corresponding over-ratings of currents and voltages to support this function have been quantitatively described. Frequency response characteristics due to step load changes in a weak grid has been considerably improved with the proposed interface scheme as verified by simulation results.