Intentional Islanding

The microgrid controller must execute a break-before-make or make-before-break transfer depending on the disturbance type. During a seamless reconnection, the controller synchronizes voltage magnitude, frequency, and phase angle to within tight tolerances (typically ±5% voltage, ±0.1 Hz frequency, ±5° phase) before closing the point of common coupling (PCC) breaker.

• Black Start Capability: Essential if the islanded grid fully collapses; requires a grid-forming inverter or diesel generator to establish the initial voltage reference.
• Static Transfer Switch: Solid-state devices can achieve sub-cycle (< 4ms) transfers, preventing sensitive loads from resetting.

Stable intentional islanding fundamentally requires at least one grid-forming inverter to act as a voltage source. Unlike grid-following inverters that act as current sources and rely on an external stiff grid for reference, grid-forming units establish and maintain the local voltage and frequency setpoints.

• Droop Control: Grid-forming inverters typically use P-f and Q-V droop curves to autonomously share real and reactive power among multiple parallel sources without requiring high-speed communication.
• Virtual Synchronous Machine: Advanced grid-forming algorithms emulate the inertial response of a physical synchronous generator, slowing the frequency nadir during load steps.

Upon disconnection, the microgrid must instantaneously match local generation to local load. Any mismatch manifests as a frequency deviation. Load shedding relays are pre-armed to rapidly discard non-critical loads if frequency drops below a defined threshold (e.g., 59.5 Hz).

• State of Charge Management: Battery energy storage systems (BESS) must enter island mode with sufficient energy reserves to cover the critical load for the expected outage duration.
• Resilience Metric: The success of an islanding event is measured by the SAIDI (System Average Interruption Duration Index) avoided for customers inside the island boundary.

Fault current levels drop significantly when transitioning from a high-fault-current grid connection to a low-inertia island powered by inverters. Adaptive protection schemes must instantly switch relay settings groups to ensure fault detection sensitivity remains adequate.

• IEC 61850 GOOSE: High-speed peer-to-peer messaging allows protection relays to share topology changes and block tripping or adjust pickup values within milliseconds.
• Fault Ride-Through: Inverters must be programmed to ride through transient voltage sags during the islanding transition without tripping offline, per IEEE 1547 ride-through requirements.

Intentional islanding is a planned, controlled operational mode, distinct from unintentional islanding, which is a safety hazard. Islanding detection schemes (e.g., Sandia Frequency Shift, Rate of Change of Frequency) are designed to detect and shut down generation during unintentional events, but must be selectively disabled or overridden by the microgrid controller during a planned transition.

• Transfer Trip: A direct communication signal from the utility breaker to the microgrid controller confirms an upstream disconnection, enabling a reliable transition without relying solely on passive detection.

Intentional islanding relies on a hierarchical control architecture. Primary control (droop) handles millisecond-level load sharing. Secondary control restores frequency and voltage to nominal setpoints after the initial droop deviation. Tertiary control optimizes economic dispatch of storage and generation assets for the duration of the islanded period.

• Model Predictive Control: Tertiary controllers often use MPC to forecast load and solar generation, pre-emptively managing battery energy storage system charge/discharge schedules to maximize survival time.