Emergency Islanding Reconfiguration Strategy for Hybrid AC/DC Distribution Networks with Embedded Transient Security Constraints
Zhao Lufan, Cao Shankang, Yang Wenhao, Wei Fanrong, Lin Xiangning
State Key Laboratory of Advanced Electromagnetic Engineering and Technology School of Electrical and Electric Engineering Huazhong University of Science and Technology Wuhan 430074 China
Abstract：Extreme weather events such as typhoons and heavy rain can quickly cause sudden breaks between the main grid and distribution networks, leading to high risk of large-scale blackouts. Building islanded microgrids with distributed energy resources (DERs) to supply power in separate areas is an effective way to improve the resilience of distribution networks and reduce outage losses. Many current distribution networks use hybrid AC/DC layouts, where voltage source converters (VSCs) allow two-way energy exchange between AC and DC sides of various DERs, improving reconfiguration results. However, the different features of multiple sources, together with the coupling of different VSC control modes, can make transient instability worse during islanding. Most existing studies focus only on static power balance and do not include transient safety limits, so the reconfigured islanded grid may still fail due to dynamic instability. To address this, this paper proposes an emergency islanding reconfiguration strategy for hybrid AC/DC distribution networks with embedded transient security constraints. First, virtual nodes are introduced at both sides of the converter station to clearly simplify the complex power conversion problem that arises during the transient response of the VSC’s AC and DC sides, thereby effectively decoupling real-time power transfer between them. Secondly, transient frequency and voltage security constraints are embedded to establish a robust dual-dimensional framework that integrates static power flow analysis and transient stability assessment, solved efficiently using mixed-integer linear programming. Finally, detailed simulations based on a modified IEEE-33 bus test system verify that the proposed strategy can achieve fast, efficient, and orderly load restoration while ensuring strict transient frequency and voltage security throughout the islanding process. Simulation results show that without transient limits, grid-forming sources output too much power, causing load recovery to exceed safe margins. When renewable generation and loads change by 10%, the diesel generator’s reserve frequency control capacity is not enough. Once its output is limited, it cannot cover the remaining power gap, causing the frequency to fall. When the frequency drops to 47 Hz, the diesel generator trips offline due to under-frequency protection, leading to instability and collapse. In contrast, with the proposed method, grid-forming sources like diesel generators and energy storage adjust output dynamically through droop control when frequency or voltage changes. This keeps frequency and voltage deviations within about±0.7 Hz and±0.3 kV, while staying inside reserve limits. This approach demonstrates superior performance with 77.259% load restoration rate and 0.0085 p.u. voltage deviation. The following conclusions are drawn:(1) Disconnecting key tie-lines and tuning VSC control modes helps coordinate grid-forming and grid-following sources, improving adaptability to changes in generation and load.(2) The virtual node model keeps VSC dynamic behavior while allowing a unified view of AC-DC steady-state flows and separate analysis of transients, giving support for post-disaster recovery.(3)Adding both static and transient limits allows better balance between load recovery and stability while keeping frequency and voltage safe.
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