Grid-Aware Islanding and Resynchronisation of AC/DC Microgrids

TL;DR

This work addresses reliable islanding and resynchronisation of hybrid AC/DC microgrids by introducing a grid-aware, SC-based OPF that incorporates grid-forming interfacing converters (ICs). It extends a unified load-flow model to handle grid-forming IC operation and losses, enabling a linear, fast optimization that preserves grid constraints during transitions. The approach uses a four-state state machine (Grid-connected, Prepare-for-island, Island, Resynchronisation) and a centralized control architecture with real-time state estimation and a synchro-check relay, validated on a 27-node lab-scale hybrid grid. Experimental results show stable islanding with voltage deviations under a few percent, precise phasor alignment during resynchronisation, and sub-second OPF computation, demonstrating practical viability for fast, reliable islanding in high DER penetration scenarios.

Abstract

This paper proposes an optimal, grid-aware control framework for the islanding, island-operation and resynchronisation of hybrid AC/DC microgrids. The optimal control framework is based on a formally derived linearized load-flow model for multiterminal hybrid AC/DC networks. The load flow model integrates the AC grid, DC grid, and interfacing converters (IC) into a unified representation. This work extends an existing load flow model to include the ICs' grid-forming operation. In traditional islanding control frameworks, the grid-forming converter is typically interfaced with an energy storage system that can provide bidirectional power to maintain the power balance. The proposed framework, however, allows the ICs to operate as the grid-forming unit while being connected to a DC grid rather than a single resource. This configuration allows for a wider operating range and, thus, a more flexible control. Furthermore, the optimal grid-aware control framework can steer the system to ensure a feasible operation without any grid constraint violations before, during, and after the islanding manoeuvre. The framework also guarantees smooth transitions, i.e., without any significant transient behaviour, when transitioning between grid-connected and islanding operations. The optimal control framework is experimentally validated on a 27-bus hybrid AC/DC network consisting of 3 ICs that interface the AC and DC networks. The hybrid grid hosts various controllable and stochastic resources.

Figures (12)

• Figure 1: The generic hybrid AC/DC network. For simplicity, only one AC-DC converter is illustrated. The Grid Connection Point (GCP) and the main breaker are indicated.
• Figure 2: The state machine architecture. The state transition conditions are indicated by the curly braces brackets
• Figure 3: Experimental setup: (a) AC grid and DERs ($B01 - B21$), (b) DC measurement units (DMU$1 - 8$), (c) Interfacing converters (IC$1-4$), and (d) DC grid ($B19 - B26$).
• Figure 4: The hybrid AC/DC low-voltage grid. The resources and their power ratings are indicated. The table provides a summary of the node types.
• Figure 5: Architecture of the islanding and resynchronization control framework.