Inverters are widely applied in the field of grid-connected power generation of new energy, and an inverter is an apparatus that converts direct current power to alternating current power and supplies power to a power grid and a load. A typical inverter system is shown in FIG. 1. A direct current port of an inverter is connected to a direct current power supply and an alternating current port of the inverter is connected to a load, a grid-connected switch, and a power grid using a transformer.
An islanding effect of the inverter is when the grid-connected switch trips, the inverter fails to detect a power-off state in time and cuts itself off from the power grid, and finally a self-powered islanding power generation system including the inverter and the load is formed. The islanding effect brings the following harm to an electric power system and personnel concerned.
(1) The grid-connected switch and a related device may be damaged due to out-of-synchronization closing.
(2) Maintenance personnel who consider that a line is uncharged is subject to safety threats.
(3) The islanding effect may result in a fault removal failure and affect power grid recovery.
The inverter needs to have an islanding detection function. In other approaches, a common islanding detection method is based on a voltage total harmonic distortion (also referred to as THD), and the technical solution is mainly as follows.
(1) Detect an alternating voltage signal from an alternating current port of an inverter, and calculate a voltage THD using the following formula:
            U      ⁢                          ⁢      %        =                  1                  U          1                    ·                                    ∑                          k              =              2                        n                    ⁢                      U            k            2                                ,where U% indicates the voltage THD, U indicates a voltage fundamental wave, and Uk indicates a kth voltage harmonic.
(2) Perform averaging processing on U% to obtain an average value U0% of the voltage THD.
(3) When |U%−U0%| is less than or equal to a specified threshold, the inverter determines that islanding does not occur, and continues to supply power to a power grid.
(4) When |U%−U0%| is greater than the specified threshold, as shown in FIG. 2, the inverter determines that islanding occurs, and triggers anti-islanding protection.
In the voltage THD-based islanding detection method, content of total harmonics is reflected and changes in various harmonics cannot be reflected. When islanding occurs, various harmonics change, but the total harmonics do not change significantly, causing an islanding detection failure. For example, in FIG. 3A, when islanding occurs, a root mean square value of a fifth harmonic decreases while a root mean square value of a third harmonic increases, but a corresponding total harmonic distortion does not change significantly, as shown in FIG. 3B. Therefore, in the voltage THD-based islanding detection method, there is a relatively large detection dead zone and islanding detection reliability is reduced.