There are two typical operating modes of microgrids: Under normal circumstances, the microgrid operates in parallel with the conventional distribution network, which is called the grid-connected mode; when a grid failure is detected or the power quality does not meet the requirements, the microgrid will be disconnected from the grid in time. Running independently is called island mode.
Switching between the two must be smooth and fast. Compared with the external network/grid, the microgrid appears as a single controlled unit and can simultaneously meet the user’s requirements for power quality and power supply security. The power supply inside the microgrid is mainly responsible for energy conversion and providing necessary control by power electronic devices.
(1) Grid-connected operation
Grid-connected operation means that the microgrid is connected to the public grid, and the microgrid circuit breaker is closed to exchange power with the main grid distribution system. Photovoltaic systems are connected to the grid to generate electricity. The energy storage system can perform charging and discharging operations in grid-connected mode. During grid-connected operation, it can be converted to off-grid operation mode through the control device.
(2) Off-grid operation
Off-grid operation, also known as island operation, refers to an operation mode that is disconnected from the main grid distribution system when the power grid fails or is required by plan, and consists of DG (distributedgenerator), energy storage devices and loads. The energy storage converter PCS works in off-grid operation mode to continue supplying power to the microgrid load. The photovoltaic system continues to generate power due to the restoration of power supply to the busbar. The energy storage system usually only supplies power to the load.
In terms of application scenarios, grid-connected microgrids are mainly concentrated in urban power grids. Off-grid microgrids are mainly concentrated in areas with relatively harsh environments and inconvenient power conditions, such as islands, remote areas, and mountain villages.
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