At present, many different islanding detection methods have been proposed. Generally speaking, islanding detection methods can be divided into two categories: remote technology and local technology.
①Remote technology
The remote technology is to complete the detection through the communication between the power grid and the distributed power sources. It refers to the communication between the power grid and distributed power sources, and the islanding detection achieved has high reliability. However, because of the need for communication equipment, its cost is usually higher than that of local technology. Remote technologies include power line carrier (PLC, Power Line Communication) method, transmission circuit breaker trip method and SCADA (Supervisory Control and Data Acquisition) technology. This type of method is suitable for high-power distributed power grid-connected power stations.
The power line carrier method uses a signal generator connected to the secondary side of the substation bus to continuously send signals to all distribution lines. Each distributed power source has its own signal receiver, which is judged by judging the continuity of the received signal In the occurrence of islanding, the communication line is the power grid line. The advantage of this method is high reliability, and at the same time, it does not need to consider changes in the topology of the distribution network. The disadvantage is that additional signal transmitters and receiver devices in the distributed power supply are required. The signal generators are usually medium-voltage equipment, which requires step-down transformer equipment. If there are only a few distributed power sources, it will greatly increase the cost of the system and will also interfere with other power line carrier communication signals.
The tripping method of the transmission circuit breaker is carried out by detecting the status of all the circuit breakers and the automatic reclosing between each distributed power source and the grid. This method requires wireless communication to transmit status information. The disadvantage is that the installation of detection and radio transmission devices on the circuit breaker needs to be approved by the electric power department. At the same time, there are also safety and interference problems of the high voltage and arcing on the circuit breaker to the above devices.
Monitoring and data acquisition SCADA technology is to perform islanding detection from the perspective of the system. It is completed by a main control unit communicating with the remote units RTUs that come with multiple distributed power sources. The speed of islanding detection and protection action is greatly affected by the communication mode.
②Local technology
The local technology is that each distributed power source judges whether the islanding occurs by detecting the voltage information at the point of common coupling PCC.
Local technologies are divided into two categories: passive detection methods and active detection methods.
The passive detection method only recognizes islanding by detecting whether the voltage parameter at the common coupling point PCC exceeds the threshold in real time without injecting any disturbance signals into the power grid. Therefore, the passive detection method has no effect on the quality of power supply. Since the passive islanding detection method has a large detection blind zone, in order to meet the requirements of the system safety standard, the active detection method must be added to the inverter control strategy. Therefore, islanding detection often uses a combination of passive and active methods. The active detection method generally achieves the disturbance effect on the power grid by changing the output signal regularly. According to the inverter output current formula Iinv=Imsin(2πf+θ), it can be known that the amplitude (Im), frequency (f) or initial phase (θ) is disturbed to promote the effective value of the voltage at the PCC of the system in the islanding state , Frequency or harmonic content is beyond the normal range, so as to achieve the purpose of detecting islanding effect. Figure 1 is a schematic diagram of the power flow of a single photovoltaic grid-connected power generation system.
