Anti-islanding effect is an important aspect of PV systems
1. Overview of the islanding effect
The islanding phenomenon refers to the fact that when the power grid is interrupted due to electrical faults or natural factors, the PV grid-connected power system still supplies power to the surrounding loads, thus forming an island of self-sufficient power supply that is beyond the control of the power company.
As the PV power system works in parallel with the grid, the grid will stop working due to faulty equipment maintenance or operation errors, which means that the islanding effect is a common problem in the grid-connected PV power system, so accurate and timely detection of the islanding effect is a key issue in the design of the grid-connected PV power system.
When the islanding effect occurs, it will cause the following hazards.
① The grid cannot control the voltage and frequency in the islanding, which may cause damage to the user's equipment if the voltage and frequency exceed the allowed range.
② If the load capacity is larger than the inverter power supply capacity inverter, the power supply overload operation, easy to be burned.
③ the line connected to the inverter power supply is still charged, causing hazards to the maintenance personnel and reducing the safety of the grid.
④ reclosing the island will cause the line to trip again, and there is also the possibility of damaging the inverter power supply and other equipment.
2. The detection method of islanding effect
When islanding occurs, the grid-connected inverter must be cut out quickly and accurately, so the islanding effect should be detected and controlled.
The islanding detection methods are mainly divided into two types: passive and active.
The passive islanding detection method determines whether the islanding effect occurs by detecting whether the inverter output deviates from the range specified by the grid-connected standard, such as voltage, frequency or phase. The working principle of this method is simple and easy to implement, but the occurrence of the islanding effect cannot be detected when the inverter output power is balanced with the local load power.
The active islanding detection method refers to controlling the inverter so that there is a certain disturbance in its output power frequency and phase. When the grid is working normally, these perturbations are not detected due to the balancing effect of the grid. Once the grid fails the disturbances in the inverter output will rapidly accumulate and exceed the range allowed by the grid connection standard, thus triggering the protection circuit for the islanding effect. This method has high detection accuracy and small detection blind area, but the control is more complex and reduces the quality of the inverter output power.
Simulation diagram of anti-islanding detection
(1) Passive method
The working principle of the passive islanding effect detection method is to determine whether the islanding effect occurs based on the change of inverter output voltage and frequency when the power grid is disconnected. When a fault occurs in the grid, the phase harmonics of the output voltage of the inverter will change in addition to the output voltage and output frequency.
Therefore, the passive islanding effect detection method can detect the above output changes of the inverter to determine whether a fault occurs in the grid, but if the PV system output power is balanced with the local load power, the passive detection method will lose its detection capability.
①Voltage, frequency detection
The voltage and frequency detection method is a detection method that stops the inverter from running on the grid when the voltage amplitude and frequency at the common coupling point exceed the normal range. In the process of grid-connected operation of PV grid-connected power generation system, in addition to preventing the occurrence of islanding effect, it is also necessary to ensure that the inverter output voltage is synchronized with the grid. Therefore, the grid voltage amplitude and frequency should be constantly tested to prevent faults such as over-voltage or under-voltage, over-frequency or under-frequency.
Therefore, the passive islanding detection method, which detects voltage and frequency, only uses the existing detection parameters to make judgments without adding detection circuits. The biggest drawback of this method is that when the inverter output power is balanced with the load power, the voltage and frequency at the inverter output remain unchanged after the grid is disconnected, which results in the missed judgment of islanding detection.
②Phase detection
The principle of inverter output voltage phase detection method is similar to voltage and frequency detection methods. When there is a fault in the grid, the load impedance carried by the inverter of the PV power generation system will change, resulting in a change in the phase of the inverter output voltage and output current after the grid fault, and the system can determine whether there is a fault in the grid based on the change in phase.
Since inductive loads are more common in the grid, this method is more effective than voltage and frequency detection methods in detecting the islanding effect. However, when the load is resistive or the grid is disconnected, the load impedance characteristics remain unchanged, and the method loses its islanding detection capability.
③Harmonic detection
The harmonic detection method refers to the loss of the balance of the grid when the grid fails to work, and the output current of the PV power generation system will generate a large number of harmonics when it passes through the transformer and other non-linear equipment. According to the change of harmonics, we can determine whether the grid is in a fault state.
Experimental research and practical application show that this method has good detection effect, but it is difficult to determine a unified harmonic standard for islanding effect detection because of the complex harmonic variations of a large number of nonlinear devices in the current grid.
The above three methods are the more commonly used passive islanding detection methods, which are all used in the actual PV system, but there is a large detection blind spot because the passive islanding detection method has high requirements for whether the inverter output power matches the load power.
