What is the islanding effect? Why does the islanding effect occur?

I. What is the islanding effect?

Under normal grid-connected conditions, photovoltaic inverters operate according to the voltage and frequency benchmarks of the public power grid. Power generation is prioritized for local loads, with surplus power fed into the grid and used to supplement power during power shortages. The entire process is under the control and dispatch of the power grid, ensuring stable and controllable operation.

The islanding effect, however, specifically refers to unplanned grid disconnection: When the main power grid experiences a power outage due to faults, maintenance, lightning strikes, line tripping, etc., and the photovoltaic system fails to detect the grid disconnection signal in time, it continues to supply power to isolated local lines and loads, forming a self-sufficient, uncontrolled "power island" detached from the main power grid.

II. Why does the islanding effect occur?

The islanding effect does not occur randomly; it requires the simultaneous fulfillment of four conditions: power supply, circuit, detection, and external triggering. The core causes are clear and technical:

1. Precise matching of power supply and demand (core internal factor)

This is the most critical condition for island formation. At the instant the main grid disconnects, if the real-time photovoltaic power generation is roughly equal to the local load power consumption, the system voltage and frequency fluctuations are extremely weak. The inverter cannot detect the grid disconnection and will continue to generate electricity.** Scenarios with full photovoltaic power generation at midday and all residential or commercial loads running are high-risk scenarios for this problem.

2. LC Resonant Circuit Energy Storage and Voltage Stabilization

Distribution lines and electrical equipment have built-in inductor and capacitor energy storage components. At the instant the grid disconnects, these energy storage components generate reactive power compensation, briefly stabilizing the line's electrical parameters, masking the power outage characteristics, and misleading the inverter into believing the grid is operating normally, thus failing to trigger shutdown protection.

3. Inherent Blind Spot in Islanding Detection

Inverters are equipped with both passive and active dual detection mechanisms, but passive detection only monitors voltage, frequency, and phase changes. In power matching scenarios, the detection accuracy is insufficient, resulting in an inherent blind spot. Furthermore, equipment aging, sensor accuracy degradation, improper parameter tuning, or unauthorized disabling of the anti-islanding function can all directly induce islanding faults.

4. External Triggers of Power Grid Abnormalities

Lightning strikes, short-circuit faults, malfunctions of transformer switches, planned power outages for maintenance, and external damage to power lines can cause physical disconnection at the grid connection point, which is a direct external cause of the islanding effect.