1. Technical Field
The present disclosure relates to an apparatus for detecting islanding when power supply from a grid power source (commercial power source) is cut off.
2. Related Art
Recently, the development of a distributed generation system using solar cells or fuel cells which have a small influence on the environment has been actively conducted in terms of global environment protection. Such a distributed generation system converts DC power generated by the solar cells or the like into AC power with a commercial frequency through an inverter, and supplies the AC power to a system load in connection with a grid power source, while transmitting surplus power to the grid power source.
The distributed generation system must prevent reverse charge from the distributed generations to the grid power source when the power supply of the grid power source is stopped. Thus, when the power supply of the grid power source is stopped, the distributed generation system detects islanding of the distributed generations, isolates the distributed generations from the grid power source, and stops operation of the inverter.
The method for detecting islanding may include a passive method which detects a rapid change in a voltage waveform or phase when islanding is performed. However, when the balance between output power of the inverter and power consumption of the load is maintained, the passive method cannot detect islanding because no change occurs in the grid when the islanding is performed.
Furthermore, the method for detecting islanding may include an active method which slightly changes an output voltage or frequency of the inverter and detects that the change becomes prominent when islanding is performed.
In the active method, however, when a plurality of distributed power systems are connected in parallel to the grid, the change may be offset to degrade the islanding detection function.
Technology for solving such a problem has been disclosed in Japanese Patent Laid-open Publication No. 2012-085384.
The technology disclosed in the above-described patent relates to a method for detecting islanding of a distributed generation system which changes DC power of distributed generations into AC power through an inverter and supplies the AC power to a load in connection to a commercial power source of a power grid line. The method controls the inverter to overlap an output current of the inverter at a connection point with the commercial power source with a secondary harmonic current synchronized with a fundamental wave of the output current, and detects islanding based on an even harmonic wave of the fundamental wave included in the grid voltage at the connection point.
According to the method, since the output current of the inverter at the connection point, which is controlled in phase with the grid voltage, overlaps the secondary harmonic current synchronized with the fundamental wave of the output current, the harmonic current overlapped with the output current of the inverter in each of the distributed power systems may be synchronized with the phase of the grid voltage, when the distributed power systems are operated in a state where they are connected in parallel to the grid. Then, since the harmonic currents of the respective distributed power systems do not offset each other, the degradation of the islanding detection function may be prevented.
In the above-described technology, however, since both the upper and lower half cycles of the secondary harmonic wave are overlapped, the power quality is degraded.
Furthermore, while one distributed generation is installed and operated, another distributed generation might need to be additionally installed. In this case, as one of islanding detection methods, the injection directions of reactive power in the plurality of distributed generations may be synchronized through communication. However, when the inverters of the respective distributed generations are manufactured by different makers, the inverters are highly likely to have different control circuits or methods. Thus, the installation of communication lines may become difficult or complex.