Some people in the installation of photovoltaic systems, will hold a "even if the grid blackout, if there is sun, their homes can also use electricity" mentality, the reality is that when the grid blackout, their home photovoltaic system will only sun, will also stop running, the same can not use electricity.
The reason for this phenomenon is the grid-connected inverter, which must be configured with an anti-islanding device that will stop working when the grid voltage is zero. Anti-islanding device is a necessary device for all grid-connected inverters, the reason for this is mainly for the safety of the grid, when the grid blackout, maintenance personnel ready to overhaul the circuit, such as photovoltaic systems are still constantly uploading power, it is easy to cause safety accidents. Therefore, the national standard stipulates that the PV grid-connected inverter must have the detection and control function of the islanding effect.
The detection method of islanding effect has passive detection and active detection, passive detection method detects the amplitude of voltage and current at the output of the grid-connected inverter, the inverter does not add interference signals to the grid, and determines whether the grid is blacked out by detecting whether the parameters such as current phase shift and frequency exceed the specified value; this way is not for causing grid pollution, and there is no energy loss; and active detection means that the grid-connected inverter actively and regularly apply some disturbance signals to the grid, such as frequency shift and phase shift. Since the grid can be seen as an infinite voltage source, these disturbance signals will be absorbed by the grid when there is a grid, and if a grid outage occurs, these disturbance signals will form a positive feedback, which will eventually form a frequency or voltage overrun, from which we can judge whether an islanding effect has occurred.
At present, the technology of grid-connected inverter anti-islanding function is fully mature, so in the household grid-connected project, it is not necessary to add special anti-islanding device, because some places are not only PV grid-connected inverter access to the grid, there may be wind power, biomass power, energy storage systems and other distributed power, the National Grid Corporation stipulates that when the access capacity to the grid exceeds 25% of the rated capacity of the distribution substation, the distribution substation The low-voltage side knife fusion switch should be transformed into a low-voltage switch, and an anti-islanding device should be installed at the low-voltage busbar of the distribution substation; the low-voltage switch should have an operational blocking function with the anti-islanding device, and when there is a contact between the busbars, the contact switch should also have an operational blocking function with the anti-islanding device.
