• Zhejiang Zhilu Transmission and Distribution Equipment Co., Ltd

Working Mechanism of Anti - Islanding Protection Devices in Photovoltaic Grid - Connection Cabinets

Abstract

1. Introduction

• Electrical safety hazards for maintenance personnel;

• Damage to grid equipment due to unbalanced power flows;

• Interference with grid re - synchronization and voltage/frequency stability.
  Anti - islanding protection devices in PV grid - connection cabinets are designed to detect islanding conditions and promptly disconnect the PV system from the grid, ensuring compliance with international standards (e.g., IEEE 1547, UL 1741, IEC 62116).

2. Fundamental Principles of Anti - Islanding Protection

2.1 Islanding Condition Triggers

1. Grid disconnection: The main utility grid is interrupted (e.g., due to faults, maintenance).

2. Power balance: The PV system's output matches the local load demand, maintaining voltage and frequency stability in the isolated grid segment.

2.2 Protection Objectives

• Rapid detection: Identify islanding within milliseconds to minimize hazard duration.

• Reliable disconnection: Trigger the circuit breaker in the grid - connection cabinet to isolate the PV system.

• False alarm prevention: Avoid unnecessary tripping under normal grid conditions (e.g., transient voltage fluctuations).

3. Detection Methods of Anti - Islanding Devices

3.1 Passive Detection Methods

3.1.1 Voltage and Frequency Monitoring

• Principle: When islanding occurs, the PV system's power mismatch with the local load causes voltage magnitude, frequency, or phase angle to deviate from grid standards.

• Threshold setting:

• Voltage deviation: ±5%–10% of nominal voltage (e.g., >242V or <218V for 230V systems).

• Frequency deviation: ±0.5Hz–1Hz of nominal frequency (e.g., >50.5Hz or <49.5Hz for 50Hz grids).

• Limitation: Slow response under near - balanced power conditions (e.g., when PV output ≈ local load).

3.1.2 Voltage Harmonic Distortion Detection

• Principle: Islanding may alter the harmonic profile of the local grid due to changes in load characteristics.

• Implementation: Analyze total harmonic distortion (THD) of the output voltage; a sudden increase indicates potential islanding.

3.2 Active Detection Methods

3.2.1 Frequency Shift Methods

• Frequency 扰动法 (Frequency Shift Method):

• Working mechanism: The inverter slightly perturbs the output frequency (e.g., by ±0.05Hz) during normal operation. When grid - connected, the grid "pulls" the frequency back to nominal. During islanding, the frequency drifts beyond the threshold, triggering detection.

• Key parameter: Frequency drift rate (e.g., >0.5Hz/s) and timeout (e.g., <2s).

3.2.2 Active Power Variation (APV) Method

• Principle: Periodically adjust the PV system's active power output. In grid - connected mode, the grid absorbs the variation; during islanding, the power mismatch causes voltage/frequency changes detectable by the device.

3.2.3 Impedance Measurement

• Implementation: Inject a small current perturbation and measure the system's impedance response. Islanding changes the impedance characteristics, allowing detection.

4. Protection Device Working Logic and Action Process

4.1 Detection - Judgment - Action Flowchart

1. Continuous monitoring: The anti - islanding device continuously samples voltage, current, frequency, and harmonic data from the grid - connection point.

2. Signal processing: Digital signal processing (DSP) algorithms analyze real - time data against preset thresholds.

3. Islanding judgment: If parameters exceed thresholds (e.g., frequency >50.5Hz for >200ms), the device triggers an alarm.

4. Protection action:

• Send a trip signal to the circuit breaker in the grid - connection cabinet;

• Disconnect the PV system from the grid within 2 seconds (as required by IEEE 1547).

4.2 Coordination with Inverter Control

• Integrated protection: Modern anti - islanding devices often coordinate with the inverter's control system:

• The device sends islanding signals to the inverter, which reduces power output or introduces larger frequency shifts to accelerate detection;

• Inverter - based anti - islanding (IB - AIP) algorithms complement hardware - based protection for faster response.

5. Technical Standards and Performance Requirements

5.1 Key International Standards

5.2 Performance Indicators

• Detection accuracy: >99.9% for typical islanding scenarios;

• False trip rate: <0.01% under normal grid conditions;

• Response time: ≤500ms for critical cases (e.g., severe frequency/voltage deviations).

6. Practical Applications and Device Types

6.1 Device Configurations

1. Inverter - integrated protection: Common in small - scale PV systems (e.g., residential rooftops), where anti - islanding algorithms are embedded in the inverter.

2. Independent anti - islanding relays: Used in medium to large systems, installed in the grid - connection cabinet as a separate device (e.g., SEL - 712, OMICRON CPC100).

3. Smart grid - connection cabinets: Integrate protection functions with digital monitoring, enabling remote islanding detection via SCADA systems.

6.2 Case Study: Large - Scale PV Power Plant

• Scenario: A 50MW PV power plant uses a combination of passive (voltage/frequency monitoring) and active (frequency shift) detection in its grid - connection cabinets.

• Protection logic:

1. Passive detection monitors real - time voltage/frequency;

2. If no deviation is detected within 1.5s, the device activates active frequency shift to induce a detectable drift;

3. Circuit breaker trips within 2s if islanding is confirmed.

7. Challenges and Future Trends

• Challenge 1: Balancing detection speed and false alarms in low - power mismatch scenarios (e.g., PV output ≈ 50% of local load).

• Challenge 2: Adapting to complex grid topologies (e.g., microgrids with multiple distributed energy resources).

• Future trends:

• Machine learning - based islanding detection using historical data to improve accuracy;

• Integration with smart grid communication protocols (e.g., IEC 61850) for coordinated protection across the grid;

• Development of anti - islanding devices compatible with energy storage systems in hybrid PV - storage grids.

8. Conclusion