Autonomous and Distributed Synchronization and Restoration of an Islanded Network of Microgrids

The transition towards clean energy and the introduction of Inverter-Based Resources (IBRs) are leading to the formation of Microgrids (MGs) and Networks of MGs (NMGs). MGs and NMGs can operate autonomously in islanded mode, which requires Grid-Forming (GFM) IBRs that can perform black start, synchronization, restoration and regulation. However, such IBRs can face synchronization instability issues, which might be worsened by inadequate secondary level frequency and voltage regulation. Accordingly, we propose an autonomous and distributed synchronization and restoration scheme using Distributed-Averaging Proportional-Integral (DAPI) control. To validate the proposed method, we model and simulate a high-fidelity islanded and modified IEEE 123 bus system, modeled as an NMG consisting of 7 MGs. The MATLAB/Simulink simulation results demonstrate an effective autonomous soft-start, synchronization, connection and regulation procedure using DAPI control and distributed breaker operation logic.

💡 Research Summary

The paper addresses the challenge of autonomously restoring and synchronizing an islanded network of microgrids (NMG) that rely on inverter‑based resources (IBRs) operating in grid‑forming (GFM) mode. Traditional droop‑controlled IBRs can initiate a black start, but without additional coordination they may suffer from synchronization instability caused by small differences in voltage, frequency, and phase angle when multiple microgrids are re‑connected. To overcome this, the authors propose a fully distributed, leader‑less control architecture that combines primary droop control with a secondary Distributed‑Averaging Proportional‑Integral (DAPI) consensus algorithm and a dedicated phase‑consensus dynamics for initial angle alignment.

The primary level uses conventional active‑power‑frequency and reactive‑power‑voltage droop equations, providing a proportional relationship between power output and the local electrical quantities. Because droop control alone leaves steady‑state errors, the secondary DAPI layer introduces integral action that is shared among neighboring IBRs through a fixed communication graph. Each IBR exchanges its frequency and voltage deviation signals with its peers, and the consensus dynamics (equations 6 and 7) drive the collective error to zero while preserving proportional power sharing.

A novel contribution is the phase‑consensus law (equation 8), which adds a term proportional to the difference between the local phase angle and those of adjacent microgrids. This term, weighted by communication gains, forces all IBRs to converge to a common angle before any physical interconnection occurs. Once the phase, frequency, and voltage differences fall within strict tolerances (the authors adopt limits tighter than IEEE 1547‑2018: Δf < 0.01 Hz, ΔV < 1 %, Δθ < 2.5°), each IBR generates a local synchronization flag (IBRSYNC_LOCAL). A second logical AND combines the local flag with the flags received from all neighboring IBRs, producing a global synchronization signal (IBRSYNC). Only when every IBR in the pair of neighboring microgrids reports IBRSYNC = 1 does the associated breaker relay close, ensuring simultaneous and safe reconnection across the entire NMG.

The methodology is validated on a high‑fidelity, unbalanced IEEE 123‑bus distribution feeder that has been modified to host seven microgrids. Each microgrid contains a single GFM IBR equipped with a battery energy storage system, an LCL filter, and detailed inner voltage and current control loops. Communication links between IBRs are represented by dashed lines in the model, forming a connected graph required for the consensus algorithms.

Simulation results demonstrate the full restoration sequence: at t = 0 s each microgrid performs a soft‑start, ramping its reference voltage from 0 to the nominal 480 V (line‑to‑line) within 0.5 s. Decentralized DAPI regulation quickly brings frequency and voltage to their setpoints, while the phase‑consensus dynamics reduce angle differences. By t = 3 s all synchronization conditions are satisfied, the breaker logic triggers, and the seven microgrids merge into a single network. After reconnection, active power is equally shared among the IBRs, frequency stabilizes at 60 Hz, and voltage remains within ±5 % of the nominal value. The parameter ξ = 0.05 illustrates the trade‑off between voltage regulation and reactive‑power sharing; without DAPI, uncontrolled reactive‑power circulation appears, confirming the necessity of the secondary layer.

The paper’s contributions are threefold: (1) a distributed, leader‑less synchronization and restoration scheme that integrates phase consensus with DAPI, (2) a complete soft‑start‑to‑reconnection workflow that autonomously handles black‑start, frequency/voltage regulation, power sharing, and breaker operation, and (3) a realistic large‑scale NMG testbed that validates the approach under stringent IEEE 1547‑2018 criteria. The results suggest that the proposed framework can serve as a foundation for future standards and practical deployments of resilient, self‑healing microgrid networks.