Mechanism and Hazard Analysis of Islanding Effect in Grid-Connected Photovoltaic Power Generation Systems

With the transformation of the global energy structure towards cleanliness and low carbonization, photovoltaic power generation, as a core component of distributed power sources, has been widely integrated into the public power grid system, providing important support for energy supply. However, during the operation of grid-connected photovoltaic power generation systems, the islanding effect, as a typical abnormal operation phenomenon, has become a key hidden danger affecting the safe and stable operation of the power grid, equipment life, and personnel safety due to its strong concealment and wide hazard range. In-depth analysis of the generation mechanism of the islanding effect and clarification of its specific hazards are of great engineering practical significance for the design optimization, safety control, and standard improvement of grid-connected photovoltaic systems.

1. Definition and Generation Mechanism of Islanding Effect

The islanding effect is a unique electrical abnormal phenomenon in grid-connected photovoltaic power generation systems. Specifically, when the public power grid stops supplying power due to electrical faults, equipment maintenance, operational errors, or natural factors (such as typhoons, heavy rains, earthquakes, etc.), the grid-connected photovoltaic power generation system fails to detect the grid voltage loss in a timely manner and still continues to supply power to the surrounding local loads, thereby forming a self-sufficient power supply "island" that is completely isolated from the main grid and cannot be monitored or controlled by the power company.

From the perspective of operation logic, the grid-connected photovoltaic power generation system operates in parallel with the public power grid. Under normal operation, the power grid, as the core of power balance, plays a stable regulatory role in the output voltage and frequency of the photovoltaic system. When the power grid stops supplying power due to various sudden situations, if the islanding detection device of the photovoltaic system does not respond in a timely manner, its inverter power supply will continue to supply power to local loads. At this time, the local power supply area is completely disconnected from the main grid, forming an independent power supply unit—namely, an "island". It is worth noting that the islanding effect is not an accidental phenomenon but a common potential problem in grid-connected photovoltaic systems. Its occurrence probability is closely related to the stability of the power grid operation, the detection accuracy of the photovoltaic system, and the load matching degree. Therefore, accurately and timely detecting and cutting off the island has become a key technical problem in the design and operation of grid-connected photovoltaic power generation systems.

According to the relevant standards and industry norms of the National Energy Administration, grid-connected photovoltaic systems must be equipped with complete anti-islanding protection devices to ensure that the connection with the power grid is quickly cut off after grid power failure, thereby curbing the generation of the islanding effect from the source. This is also one of the core technical requirements for grid-connected photovoltaic systems to connect to the public power grid.

2. Main Hazards of Islanding Effect

The occurrence of the islanding effect not only affects the normal operation of the grid-connected photovoltaic system itself but also causes multi-dimensional hazards to the public power grid safety, electrical equipment, maintenance personnel, and power system recovery. Specifically, it can be divided into the following four aspects, and its hazard level covers personal safety, equipment safety, and power grid safety, which need to be focused on prevention and control.

(1) Damaging the Operational Stability of Electrical Equipment and Causing Equipment Damage

Under normal operation, the public power grid stabilizes the power supply voltage and frequency within the specified range through its own regulatory mechanism (China's power frequency grid frequency is 50Hz, and the voltage deviation must comply with the requirements of the "Technical Regulations for Grid Connection of Photovoltaic Power Generation Systems"), providing a stable power supply environment for electrical equipment. When the islanding effect occurs, the power supply in the island area is completely supported by the photovoltaic inverter power supply, losing the regulatory role of the main grid, leading to the voltage and frequency in the island being prone to deviate from the allowable range.

On the one hand, excessively high or low voltage will directly affect the normal operation of user-side electrical equipment, such as burning out the circuit components of household appliances and office equipment, shortening the service life of the equipment, and even causing permanent damage to the equipment. On the other hand, frequency deviation will affect the speed of motor-type equipment (such as water pumps, fans, etc.), leading to abnormal operation and reduced efficiency of the equipment, and may even cause mechanical failures of the equipment in severe cases. In addition, if the load capacity in the island is greater than the rated capacity of the photovoltaic inverter power supply, the inverter power supply will operate in an overload state for a long time, which is prone to burning internal components due to overheating and overcurrent, resulting in damage to the photovoltaic system's own equipment and increasing maintenance costs.

(2) Threatening the Personal Safety of Maintenance Personnel and Reducing Power Grid Safety

After the power grid stops supplying power, power maintenance personnel usually conduct inspection and maintenance on the faulty lines. At this time, the maintenance personnel assume that the lines are in a power-off state and carry out maintenance operations. However, when the islanding effect occurs, the faulty lines connected to the photovoltaic inverter power supply are still in a live state, and this live state is highly concealed—maintenance personnel cannot quickly detect the existence of the island through conventional detection methods, and are likely to misjudge the live state of the lines, thereby causing electric shock accidents and seriously threatening the personal safety of maintenance personnel.

At the same time, the live lines in the island area will become potential hidden dangers for the safe operation of the power grid. Once maintenance personnel or other personnel come into contact with the live lines, it will not only cause personal injury but also may trigger secondary disasters such as short circuits and fires, further expanding the impact of the accident and significantly reducing the safety and reliability of the entire power grid system. This is also one of the most serious hazards of the islanding effect. Therefore, one of the core goals of the rapid response of anti-islanding protection is to ensure the safety of personnel operations.

(3) Interfering with Power Grid Reclosing Operation and Aggravating Equipment Damage

When the power grid stops supplying power due to a fault, the power system usually starts the reclosing operation to try to restore line power supply, which is a conventional means of power grid fault recovery. However, in the presence of the islanding effect, the reclosing operation will face serious safety risks: due to the inconsistency between the power supply frequency and voltage phase of the island area and the main grid, when the reclosing action occurs, the main grid power supply and the island power supply will have a severe phase conflict, generating a huge inrush current, leading to the line tripping again and failing to restore power supply normally.

More seriously, this inrush current will cause a strong impact on power equipment such as photovoltaic inverter power supplies, power grid switchgear, and transformers, which may lead to damage to equipment insulation and burnout of core components, further expanding the scope of equipment damage, increasing the difficulty and cost of power grid fault recovery, and even may trigger chain reactions, affecting the power supply stability of surrounding areas.

(4) Affecting the Overall Operational Order of the Power Grid and Increasing Maintenance Difficulty

The independent power supply area formed by the islanding effect is outside the monitoring scope of the power company. The power dispatching department cannot grasp the power supply status, load changes, and equipment operation status in the island, resulting in the loss of integrity and controllability of power grid operation dispatching. On the one hand, the power imbalance in the island may cause continuous fluctuations in voltage and frequency. If the island is accidentally reconnected to the main grid, it will impact the operational stability of the main grid and affect the frequency regulation and voltage control of the power grid. On the other hand, the concealment of the islanding effect greatly increases the difficulty of fault detection and handling. Maintenance personnel need to spend a lot of time locating the island position and troubleshooting the fault causes, extending the power grid fault recovery time, affecting the normal power consumption of users, and increasing the workload and cost of power grid maintenance.

3. Conclusion

In summary, the islanding effect is an unavoidable safety hazard during the operation of grid-connected photovoltaic power generation systems. Its generation is closely related to the power grid operation status and the detection capability of the photovoltaic system, and it will cause various serious hazards to personal safety, electrical equipment, power grid operation, and fault recovery. With the continuous expansion of photovoltaic power generation scale and the popularization of distributed photovoltaic systems, preventing and controlling the islanding effect has become a key link to ensure the safe, stable, and efficient operation of grid-connected photovoltaic systems.

At present, China has established a relevant standard system covering anti-islanding detection, protection device installation, and system operation management for grid-connected photovoltaics, clearly requiring that grid-connected photovoltaic systems must be equipped with qualified anti-islanding protection devices to ensure that the grid connection is quickly cut off after grid power failure. In the future, with the continuous upgrading of detection technology, the application of new detection methods such as frequency change rate and phase disturbance will further improve the accuracy and speed of islanding detection, effectively curbing the occurrence of the islanding effect. At the same time, strengthening the operation and maintenance management of photovoltaic systems and strictly implementing equipment detection and certification requirements can also reduce the hazards of the islanding effect from the source, providing strong guarantee for the healthy development of the photovoltaic power generation industry and the safe and stable operation of the power grid.