When the public grid is cut off or the distributed power generation (DG) system is disconnected from the public grid, the distributed power generation system continues to work and forms a self-sufficient island system with the surrounding loads.
As shown in Figure 1, a typical photovoltaic grid-connected power generation system includes a switching switch 1, a switching switch 2, one or more PV power sources, and grid loads and local loads. The grid load on the public grid, the local load on the photovoltaic power generation system, and the grid are connected at the PCC (Point of Common Coupling). If the switching switch 1 is disconnected, the photovoltaic power generation system, the grid load, and the local load together form an island power supply system. If the switching switch 2 is disconnected, the photovoltaic power generation system and the local load form an island power supply system. In the islanding system, if the photovoltaic power generation system is still working and provides electrical energy to the load, the islanding effect will occur.
The occurrence of islanding effect brings potential hazards to personnel and electrical equipment, mainly as follows:
➢ There is a certain safety hazard to the personnel who repair the public network lines, and the maintenance personnel may not be aware of the existence of distributed power sources.
➢Because the photovoltaic power generation system is usually connected to the grid with unit power factor, when it is disconnected from the large grid, the local load always has a demand for reactive power, which will make it impossible to meet the reactive power demand of the load during island operation. The frequency may be unstable and fluctuate or even collapse, which may cause damage to the load electrical equipment.
➢ When the power company resumes power supply, the island system will encounter problems when it is reconnected to the grid. The voltage phase of the system and the grid voltage phase are not synchronized, which causes a large current impact, causing problems such as damage to the PV power supply and local load.
➢ The lack of phase of the system’s three-phase load caused by single-phase power supply.
➢ The island power supply status is out of the monitoring of the power management department, and the system is uncontrollable, leading to high security risks.
Therefore, for the application of more and more photovoltaic grid-connected power generation systems, the probability of islanding is getting higher and higher, and this phenomenon must be protected to avoid islanding problems. This shows that it is particularly important to solve the island problem.
In April 2000, the 1EEE Standards Committee formulated a series of photovoltaic grid-connected requirements and technical standards. In 2004, the committee formulated the standard IEEE Std 1526TM-2003 for islanding detection. In 2005, China issued “GB/T 19939- 2005 Photovoltaic System Grid-connected Technical Requirements, on August 1, 2013, the National Energy Administration implemented the “NB/T32004-2013 Photovoltaic Power Grid-connected Inverter Technical Specification”, which clarified the relevant standards for island protection in China.
Tables 1 to 5 show China’s requirements for voltage protection, frequency fluctuations and current harmonics of photovoltaic grid-connected systems. Among them, Table 1 is the system protection response time requirements for abnormal voltage conditions except for high-power inverters. Table 2 shows the abnormal response time of the power station inverter, and the low voltage ride-through function has priority. Table 3 shows the response time required by the grid-connected system when the frequency fluctuates. When the inverter is running, the total distortion rate of the current injected into the grid is limited to 5%. Table 4 and Table 5 are the limits of the odd and even harmonic currents injected into the grid.
|Common coupling point voltage V||Maximum offline time/s|
|Common coupling point voltage V||Operational requirements|
|V<90%VN||Should meet low voltage ride through requirements|
|90%VN≤ V≤110%VN||normal operation|
|110%VN<V<120%VN||Should last at least 10s|
|120%VN≤ V≤ 130%VN||Should last at least 0.5s|
|Frequency ∫/Hz||Inverter response|
|∫≤48||Stop running within 0.2s|
|48<∫≤49.5||Stop running after 10min|
|50.2<∫≤50.5||Stop running after running for 2 minutes, the inverter that is out of service at this time must not be connected to the grid|
|∫＞50.5||Stop supplying power to the grid within 0.2s, and inverters that are out of service must not be connected to the grid at this time|
|Harmonic order h||3~9||11~15||17-21||23-33||35 or more|
|Harmonic order h||2~10||12-16||18~22||24-34||36 or more|