Detection and Mitigation of Islanding in Distributed Power Systems

Definition

Islanding occurs when the utility grid supply is interrupted due to faults, operator error, or maintenance, while distributed renewable generation systems remain connected and continue supplying local loads. This creates a self-sustained, islanded section that the utility cannot control.

Consequences of islanding

1. The utility cannot regulate voltage and frequency within the island. If voltage or frequency move outside allowable limits, connected equipment may be damaged.
2. If the load exceeds the inverter capacity, the inverter may be overloaded and damaged.
3. If a reclosing operation is attempted while the island still exists, it can cause the line to trip again and may damage inverters or other equipment.
4. Lines connected to inverters may remain energized, creating a safety hazard for maintenance personnel and reducing overall grid safety.

Detection methods

Common methods to detect islanding include:

1. Frequency drift detection: In an islanded condition, the microgrid frequency often deviates from the main grid frequency. Monitoring frequency variations can indicate islanding. Frequency monitoring is typically performed by dedicated frequency meters or monitoring systems.
2. Reactive power change detection from generators: In islanded operation, reactive power support from the main grid is unavailable, so generator reactive power behavior correlates with load changes. Monitoring generators' reactive power output or power factor can help identify islanding.
3. Voltage anomaly detection: Voltage fluctuations in an islanded microgrid often differ from those on the main grid. Monitoring voltage variations with voltage monitoring equipment or a supervisory system can detect islanding.
4. Frequency-voltage relationship analysis: The relationship between frequency and voltage in an islanded microgrid may differ from that in the main grid. Analyzing the relationship between frequency and voltage variations can help identify islanding.
5. Reverse power flow detection: Distributed generators feeding local loads create reverse power flow when the grid is disconnected. Monitoring power flow direction in the power system can reveal islanding. This detection typically uses power flow analysis equipment or monitoring systems.

Different microgrid configurations may require one or a combination of these methods. Selecting appropriate detection equipment, calibrating it, and performing regular maintenance are important to ensure reliable and accurate detection.

Mitigation and prevention methods

Common measures to prevent or mitigate islanding include:

1. Centralized monitoring and control: Implement a centralized monitoring and control system to monitor microgrid and main grid status in real time and ensure consistency of operating parameters. If islanding is detected, the system can isolate the islanded section promptly.
2. Reliable interlock control: Use robust interlock and transfer-control strategies to ensure correct switching between the microgrid and the main grid. Define appropriate transfer conditions and logic to allow reconnection only after the main grid has stably recovered.
3. Intelligent protection devices: Deploy protection devices that detect faults and islanding and take appropriate isolation and protection actions. These devices monitor parameters such as voltage and frequency and act according to predefined rules.
4. Programmable controllers: Use programmable logic controllers (PLC) or equivalent control equipment to implement automated control and management. Well-designed control logic can automate connection and disconnection between the microgrid and the main grid.
5. Intelligent load management: Implement load management to distribute and control loads sensibly, avoiding excessive or unbalanced loading during islanded operation. This improves microgrid stability and reliability.
6. Compliance testing and oversight: Follow applicable standards and perform regular compliance testing and oversight to ensure microgrid design and operation meet safety and regulatory requirements, reducing the risk of islanding-related hazards.