⑴Islanding phenomenon and protection standards
When the public grid is cut off or the distributed power generation (DG) system is disconnected from the public grid, the distributed power generation system continues to work and forms a self-sufficient island system with the surrounding loads.
As shown in Figure 1, a typical photovoltaic grid-connected power generation system includes a switching switch 1, a switching switch 2, one or more PV power sources, and grid loads and local loads. The grid load on the public grid, the local load on the photovoltaic power generation system and the grid are connected at the PCC (Point of Common Coupling). If the switching switch 1 is disconnected, the photovoltaic power generation system, the grid load, and the local load together form an island power supply system. If the switching switch 2 is disconnected, the photovoltaic power generation system and the local load form an island power supply system. In the islanding system, if the photovoltaic power generation system is still working and provides electrical energy to the load, the islanding effect will occur.
The occurrence of islanding effect brings potential hazards to personnel and electrical equipment, mainly as follows:
➢ There is a certain safety hazard to the personnel who repair the public network lines, and the maintenance personnel may not be aware of the existence of distributed power sources.
➢Because the photovoltaic power generation system is usually connected to the grid with unit power factor, when it is disconnected from the large grid, the local load always has a demand for reactive power, which will make it impossible to meet the reactive power demand of the load during island operation. The frequency may be unstable and fluctuate or even collapse, which may cause damage to the load electrical equipment.
➢ When the power company resumes power supply, the island system will encounter problems when it is reconnected to the grid. The voltage phase of the system and the grid voltage phase are not synchronized, which causes a large current impact, causing problems such as damage to the PV power supply and local load. The lack of phase of the system’s three-phase load caused by single-phase power supply.
➢ The island power supply status is out of the monitoring of the power management department, and the system is uncontrollable, leading to high security risks.
Therefore, for the application of more and more photovoltaic grid-connected power generation systems, the probability of islanding is getting higher and higher, and this phenomenon must be protected to avoid islanding problems. This shows that it is particularly important to solve the island problem.
⑵Island detection method
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.
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 conduct islanding detection from the perspective of the system. It is completed by a main control unit communicating with the remote unit RTU that comes with multiple distributed power sources. The speed of islanding detection and protection action is greatly affected by the communication method.
The local technology is that each distributed power source itself judges whether islanding occurs by detecting the voltage information at the common coupling point PCC. Local technologies are divided into two categories: passive detection and active detection.
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 standards, it must be
Active detection method is added to the converter 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 1mv =I sin(2rf+0), it can be known that the amplitude (Im), frequency (f) or initial phase (@) 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 2 is a schematic diagram of the power flow of a single photovoltaic grid-connected power generation system.