Power balancing and dc fault ride through in DC grids with dc hubs and wind farms

Power balancing algorithm and dc fault ride through strategy in a DC grid with a DC hub are proposed in this article. Inductor-capacitor-inductor (LCL) dc hub is used in the DC grid to match different voltage levels and to prevent DC fault spreading. An automatic power balancing control is developed for the hub controller, and for DC grid terminals including wind farms and passive AC loads. Collectively these controls enable power balancing for any DC fault without the need for hardware braking systems. The challenges of connecting/disconnecting of DC grid segments and a DC grid black start with non-isolated dc hubs are resolved with new controls and specific operating sequence. A 3-terminal test dc grid is developed using a 3-port LCL dc hub where one terminal is connected to a wind farm, another terminal is connected to an ac grid and a third terminal to a passive ac load. Simulations on this difficult test system are performed for a range of DC faults, power steps and a black start using PSCAD/EMTDC and results verify conclusions.