Optimized Fractional Order Based Droop Control with Improving the Flexibility of a Microgrid

This work presents an improved four-levels hierarchical control strategy for flexible microgrid based on three-phase voltage source inverters (VSIs) connected in parallel. In the proposed strategy, zero-level control is required to handle current and voltage control of VSIs. The primary-level that consists of a decentralized controller is based on a modified universal droop control by introducing a fractional-order derivative. It is used to enhance the power sharing quality during islanded mode and the desired power generation in grid-connected mode. In the secondary centralized control, the voltage and frequency deviations caused by the primary-control are restored. Also, this level includes the synchronization control loop that enables a seamless transition between both operation modes. In order to improve the flexibility of the microgrid, a sequential logic approach is proposed in the tertiary level. It is exploited to manage both the restoration and the synchronization loops at each transition, as well as the static switch (SS) which is used to connect/disconnect the microgrid to/from the main grid. The control parameters are optimized by using the self-learning particle swarm optimizer (SLPSO) algorithm. Simulations were performed to highlight the performances of the proposed hierarchical control approach compared with the well-known three-levels control scheme.