The large number of grid-connected applications of the photovoltaic power generation systems makes it a key issue to ensure the safety and stability of the system. Among them, the detection and protection of islanding effect is one of the technical problems to be solved in the photovoltaic grid-connected systems. It introduces the concept, harm and the major detection methods of island effect, discusses how to guarantee safety in terms of the grid-connection by the use of island detection technologies.
Island effect: The photovoltaic power generation system is connected to the power grid, and for some reason, the power grid is interrupted, but the photovoltaic system fails to detect this power failure and still supplies power to the local load.
Islanding can cause the following problems:
Islanding detection methods can be divided into two broad categories: passive detection and active detection .
Passive detection: It detects the island state by monitoring changes in electrical parameters of the power grid, such as voltage, frequency, and phase change detection. The photovoltaic inverter monitors the real-time voltage and frequency of the grid; once these parameters are out of the set range, the system can identify the island condition.
Advantages: The approach is easy to implement, and there are no perturbations to the grid.
Disadvantage: insensitive to island status of low power mismatch.
Harmonic Analysis:
Island states are detected by monitoring the variation of harmonic components in isolated islands.
Advantage: it is suitable for the detection of islanding phenomenon caused by nonlinear load.
Disadvantages: sensitive to noise interference, limited detection accuracy.
The principle of active detection is to inject signals or disturbances into the power grid and detect the isolated island state by monitoring the change of response.
Perturbation injection method :
The photovoltaic inverter injects frequency, phase or voltage disturbances into the grid and determines whether there is an island according to the feedback signal.
Advantages: Effectively working for low power mismatch island state
Disadvantages: the impact on power quality of the grid is slight.
Active Frequency Drift Method (AFD) :
In operation, the inverter deliberately introduces a small frequency offset. If there is a power grid, the frequency is adjusted by the network side to return to normal; if it is in an island state, the frequency shift continues to increase.
Advantage: high detection sensitivity.
Disadvantages: The control accuracy of the inverter is high.
This hybrid method has the advantages of both active and passive detection, and most active detections are usually confirmed afterwards after passive detection has triggered a suspected signal of the island. Advantages: The high sensitivity of detection has little interference to the power grid. Disadvantages: The implementation complexity is very high.
Although it is comparatively mature in researching island detection technologies, it still exists in some ways to affect its practical applications:
Directions for the future are as follows:
Detection with AI: The use of machine learning algorithms in order to come up with more accurate results regarding island detection.
Distributed detection technology: improved response to the isolated island state by detection through multi-inverter collaboration; Energy storage system detection strategy: photovoltaic-energy storage hybrid system island detection method design
The islanding effect is one of the possible risks for PV-connected systems. These should therefore be deterred through practical methods of detection. The merits and disadvantages exist for both the former one and the latter one. While the hybrid detection method is the more advanced of all these solutions. On actual operation projects, selection for an appropriate scale of a system, environment of a power grid, and requirements for a relevant regulation should be considered continuously by using technological progresses comprehensively, to ensure the grid-connection operation safety and stability of the photovoltaic system.
