Island operation mode
Under normal operation, the main power supply system and DG jointly supply power to the surrounding loads, and when the power supply system loses pressure, the DG independently continues to supply power to a certain part of the user’s power grid. DG and local loads form a small isolated grid called an island. In the island operation mode, the capacity of the power supply and the load in the island must be balanced. If the power (active and reactive) is unbalanced, the voltage and frequency in the island will not be able to maintain stability, and it will not be able to operate continuously. From the operating mode, silos are divided into planned and unplanned silos.
(1) Planned islanding: In order to maintain the stable operation of the islanding system, a reasonable islanding area and control strategy should be determined in advance according to the capacity of the distributed power supply and the size of the local load. After being isolated from the main system, the power balance and voltage and frequency stability in the island can be maintained without major adjustment. This pre-defined island area is called a plan island. Generally speaking, planned islanding is a favorable supplement of DG to the large power grid, and can be used as an emergency power supply means for important users.
(2) Unplanned islanding: refers to the unplanned and uncontrolled islanding phenomenon. It mainly refers to the case of tripping due to faults on the main distribution system side and DG running with unmatched loads. After being separated from the main system, the power in the unplanned island is unbalanced. If it runs for a long time, it will inevitably lead to a serious deviation of the voltage and frequency in the island system, causing serious damage to the DG and its surrounding load electrical equipment. In addition, after a fault on the main power distribution system side and the protection device on the power distribution system side trips, it is still possible for the DG in the unplanned islanding system to continue to provide short-circuit current to the fault point, so that the fault can be maintained and the insulation cannot be restored, which will cause the system to fail to operate correctly, such as system-side closing, backup self-switching, or distribution network reconstruction after the fault. Therefore, it is necessary to configure island protection to control the DG to exit operation when the island is unplanned.
Island operation control method
The control of intermittent power supply to form islands can be controlled manually or automatically.
(1) Manual control mode means that after manually disconnecting the incoming switch and each load switch of the substation system, restart the DG and connect it with no load, and then turn on the corresponding load in turn according to the operating experience.
(2) Automatic control is to use the automatic system, after judging that the incoming switch of the substation system and each load switch are disconnected, the DG is merged into the busbar with no load, and then, according to the load power value of each line at the busbar before the fault, and according to the requirement that the island can operate stably, run with load one by one. If the formed island contains multiple distributed power sources, the control system should incorporate the DGs into the busbar in turn according to the size of the DG capacity. After the system failure is restored, the DG should be connected to the grid again, regardless of the manual or automatic control method, the access switch of the DG should be disconnected first, and the access switch of the system tie line can be turned on after confirming that the busbar is free of voltage, and the DG can be reconnected to the grid after the substation is in normal operation.
The main reasons for the formation of distributed photovoltaic islands in the power grid are line faults in the upper power grid, frequency, voltage over-limit and oscillation out-of-step. When the grid failure cannot be recovered in time, distributed photovoltaics should be able to bring as much load as possible. In addition, the fewer the number of distributed photovoltaic islands, the easier it is to operate and recover from faults.
In a distributed photovoltaic system, to achieve island operation, two necessary conditions must first be met: ①the power balance constraint, that is, the basic balance of power in the island; ②The transmission line safety constraint, that is, the load of each line and equipment in the island should be within the steady-state safety constraint.
The key to islanding control is the grid-connected inverter to achieve high-performance output voltage control. After the grid-connected inverter is in the islanding state, once it is detected that the load voltage exceeds the normal range, the working mode of the grid-connected inverter changes from output current control to output voltage control, maintaining the voltage amplitude and frequency to meet the local load operation requirements. Even if the grid-connected inverter is in the islanding state, if the output voltage of the grid-connected inverter meets the normal operation requirements of the load, the grid-connected inverter can still be in current control mode, and if the output voltage exceeds the normal range, the grid-connected inverter switches to voltage control mode to maintain stable load voltage and frequency. The stability of the load terminal voltage in the island in the voltage control mode requires the power adjustment of the distributed photovoltaic system.