A Grid Forming/Following Sequence Switching Control Strategy for Supporting Frequency Stability of Isolated Power Grids

Building isolated power grids with high penetration of renewable energy is an effective way to promote the development and utilization of renewable energy. For an isolated power grid, due to the high penetration of renewable energy, the system inertia is low, and frequency instability is prone to occur after the power grid is disturbed. Grid forming control strategy mimics the inertia response characteristics of synchronous machines, it can increase the equivalent inertia of systems and have the ability to actively support the system frequency. However, if renewable energy is always operated under a grid forming mode, it may increase the time for system frequency to unter stable condition. Accordingly, a grid forming and grid following sequence switching control strategy is proposed in this paper, which dynamically switches the two control strategies through different stages of the system frequency response for reducing the maximum frequency deviation and stabilization time of the system. This strategy acts on the system frequency inertia response and primary frequency regulation stage, and adjusts the frequency through control characteristics of the renewable energy grid-connected converters without additional power regulation instructions. The validity of the control strategy proposed in this paper is verified by building a simulation model of an practical isolated power grid in PSCAD/EMTDC.