How to Detect and Prevent Islanding in Solar Grid Systems

Oliver Watts

Field: Inspection and testing

China

Definition of Islanding Effect

When the utility grid power supply is interrupted due to faults, operational errors, or scheduled maintenance outages, distributed renewable energy generation systems may continue operating and supplying power to local loads, forming a self-sustained “island” that is beyond the control of the utility company.

Hazards Caused by Islanding Effect

Loss of Voltage and Frequency Control: The utility cannot regulate voltage and frequency within the islanded section. If these parameters deviate beyond allowable limits, connected user equipment may be damaged.
Overload Risk: If the load demand exceeds the inverter’s rated capacity, the power source may become overloaded and suffer thermal damage or failure.
Reclosing Damage: Automatic reclosing of circuit breakers onto an islanded section can cause immediate re-tripping and potentially damage inverters or other equipment.
Safety Hazard to Personnel: Lines connected to the inverter remain energized during outages, posing serious electrocution risks to maintenance crews and compromising overall grid safety.

Detection Methods for Islanding Effect

Several primary methods are used to detect islanding:

Frequency Drift Detection: In an islanded microgrid, system frequency typically deviates from the main grid’s nominal value. Monitoring frequency variations helps identify islanding conditions. This can be implemented using dedicated frequency monitoring devices or SCADA systems.
Reactive Power Variation Detection: Without access to the main grid’s reactive power support, the relationship between a generator’s reactive power output and load changes becomes distinctive in island mode. Monitoring reactive power or power factor enables islanding detection.
Voltage Abnormality Detection: Voltage fluctuations in an islanded microgrid often differ significantly from those of the main grid. Detecting such anomalies via voltage monitoring equipment can signal islanding.
Frequency-Voltage Correlation Analysis: The dynamic relationship between frequency and voltage in an islanded system may differ from that in grid-connected mode. Analyzing this correlation helps distinguish islanding events.
Reverse Power Flow Detection: During islanding, distributed generators may feed power back toward what should be a de-energized line. Monitoring power flow direction using power analyzers or protection relays can indicate islanding.

Note: Depending on the specific microgrid configuration and operational context, a single method may be insufficient. Often, a combination of passive and active detection techniques is employed. Additionally, proper selection, calibration, and maintenance of monitoring equipment are essential to ensure reliable and accurate detection.

Prevention and Mitigation Strategies for Islanding Effect

To effectively prevent or mitigate islanding, the following measures are commonly adopted:

Centralized Monitoring and Control: Implement a centralized system to continuously monitor the interconnection status and operational parameters of both the microgrid and the main grid. Upon islanding detection, the system should automatically disconnect the islanded section.
Reliable Anti-Islanding Coordination Logic: Employ robust switching logic that ensures reconnection to the main grid only occurs after stable grid conditions are confirmed, preventing unsafe reclosing.
Intelligent Protection Devices: Deploy smart protective relays capable of real-time monitoring of voltage, frequency, and other critical parameters. These devices can autonomously trip inverters or disconnect circuits when islanding is detected.
Programmable Logic Controllers (PLCs): Use PLCs or advanced controllers to automate disconnection and reconnection procedures based on predefined safety rules and grid conditions.
Smart Load Management: Integrate intelligent load control systems to dynamically balance or shed loads during islanded operation, preventing overloads and enhancing system stability.
Compliance Testing and Regulatory Oversight: Adhere to relevant standards (e.g., IEEE 1547, IEC 62109) and conduct regular compliance testing to ensure anti-islanding functions meet safety and performance requirements, thereby minimizing risks to both the grid and end-users.

Reference Standards