The external detection method of the inverter is a method to control the inverter to stop the grid-connected operation through the control or communication of the grid to the inverter when islanding occurs. cost, the implementation process is more complicated. According to the different control methods of the grid on the inverter, there are mainly the following two methods.
(1) Impedance insertion method
The impedance insertion method is shown in Figure 1. Connect a low-impedance load at point b. The normally open switch S3 is linked with the circuit breaker s. After S1 is disconnected for a period of time, S3 is closed again. When an island occurs, that is, the S1 disconnection delay After a period of time, S3 is closed, and the low-impedance load is connected to the system at this time, and the balance between the inverter and the load is broken, so that the voltage frequency or amplitude changes to achieve the purpose of island detection. It is worth noting that there must be a certain delay between the trips of S3 and S1, and when the impedance is added, it may happen that the total load just matches the output power of the inverter, so that the islanding state cannot be detected.
(2) Power line carrier communication method
Power line carrier communication is a special communication method that uses power line as an information media medium, and uses high-speed transmission technology to transmit analog or digital signals through carrier waves. Its biggest feature is that it does not need to re-set up the network, as long as there are wires, data transmission can be carried out.
The power line carrier communication system consists of a power line carrier machine, a power line and a coupling device. The function of the carrier controller is to modulate and mediate the original signal of the user, and to meet the requirements of communication quality. The coupling capacitor and the combined filter form a band-pass filter. Its function is to pass the high-frequency carrier signal and prevent the power frequency high voltage and power frequency current on the power line from entering the carrier equipment to ensure the safety of personnel and equipment. The function of the wave blocking inductance is to pass The power current prevents the high-frequency carrier signal from leaking into the power equipment, so as to reduce the intervention attenuation of high-frequency signals by substations or branch lines, and the mutual interference between high-frequency channels on different power lines on the same bus.
Figure 2 is a schematic diagram including detection of power line carrier communication. The signal information is equipped with a receiver R at the user end to detect whether there is a signal sent by the transmitter on the power grid. If the signal is not detected, the power grid has been disconnected. The detection efficiency is not affected even when the inverter is connected. However, in order to ensure the high efficiency of detection, there are strict requirements on the power line carrier communication signal, and a transmitter T is required, which greatly increases the cost and leads to limited application.