Microgrid system is a kind of mini power supply network composed of distributed power supply, energy storage device and electric load, with two modes of grid-connected and independent operation, which is a new means of accessing distributed power supply to the grid.
The grid-connected microgrid system can operate either in parallel with the external grid or off-grid independently; the off-grid microgrid is not connected to the external grid and the power is self-balancing. The operation mode and characteristics of microgrid system are very different from traditional grid, which puts new requirements on the planning, construction, operation and maintenance of distribution grid.
In most cases, grid-connected microgrids operate in grid-connected operation mode, and in less time in islanded operation mode. The microgrid always monitors the electrical quantities such as voltage and frequency of the grid in real time, and will transition from normal grid-connected operation mode to islanding operation mode in case of abnormal conditions such as grid outage or power quality exceeds the standard; when the grid returns to normal, it will switch from islanding operation mode back to grid-connected operation mode automatically or according to the instructions of grid dispatching department.
Microgrid switching from grid-connected to islanded operation may be the result of a planned event or caused by a sudden (unplanned) event. Planned switchover means that the start of the microgrid mode switchover is predetermined and follows a predetermined process; unplanned switchover is mostly caused by a large grid fault, when the microgrid may automatically switch from grid-connected mode to islanded mode through a protection device. The success rate of microgrid switching in this mode is often less than 100%, which requires the microgrid to have black start capability, that is to say, the microgrid can be black started after a fault or unsuccessful mode switching causes a total shutdown of the microgrid.
In islanding mode, the fluctuation of load and distributed power supply in the microgrid will cause voltage and frequency changes in the network, which requires the microgrid to have the ability to control the voltage and frequency in the mainland of the microgrid within the specified range.
In order to maintain the normal operation of the microgrid and keep track of its operational status, the microgrid needs to be equipped with an advanced monitoring system. If there are multiple distributed power sources in the microgrid, the power output of each distributed power source needs to be effectively coordinated to ensure the stable operation of the microgrid.
In order to realize the transition from islanding mode to grid-connected mode, the monitoring system of the microgrid should be able to obtain the voltage and frequency information of the grid side of the grid. When the voltage and frequency of the distribution network meet the range of the relevant requirements of the national standard, the microgrid will adjust its own voltage and phase to meet the conditions of the same period, and then restore the microgrid from islanding mode to grid-connected mode operation.
The development of microgrid has four basic characteristics.
Mainly reflected in the low voltage level, the voltage level is generally 35kV and below; system scale is small, the battery storage capacity of microgrid is not more than 20MW, usually megawatt level and below.
The power supply is mainly renewable energy, or the power generation type aiming at the comprehensive utilization of energy such as natural gas multiple supply; the ratio of installed capacity of renewable energy to the maximum load of grid-connected microgrid is above 50%, or the comprehensive utilization efficiency of energy is above 70%.
The microgrid basically achieves self-balancing of power supply and demand within the microgrid. The annual exchange of electricity between grid-connected microgrid and external grid generally does not exceed 50% of annual electricity consumption, and can guarantee continuous power supply for important loads within a period of time when operating independently; independent microgrid should have the ability of black start.
It can reduce the impact of large-scale distributed power supply access to the grid. The grid-connected microgrid has controllable power exchange with external grid and time period, through the cooperative control of power supply, load and battery storage system to realize the power exchange with the grid.
The electricity inside the microgrid system comes from distributed energy sources such as natural gas, photovoltaic and wind power. In the northwest and other places with sufficient scenery resources, the construction of large wind farms, photovoltaic power plants, users (industrial parks, commercial areas, schools, hospitals and even large real estate projects) in access to small wind turbines, photovoltaic, microgrid battery storage, gas turbines and other power equipment, so that the power can be consumed nearby, eliminating the loss of transmission in the grid and improving the efficiency of energy use.
The electric energy exchange between microgrid system and grid system is connected through the common connection point of microgrid and grid system, avoiding multiple distributed power sources to connect directly with grid system. The microgrid system is mainly used for power supply within the region, and does not transmit or transmit very small power to the outside, which has negligible impact on the grid system.
Microgrid system adopts advanced control methods and a large number of power electronic devices to connect distributed power sources, battery storage devices and controllable loads together, making it a controllable load for the grid system, and it can implement both grid-connected and independent operation, fully maintaining the safe and stable operation of the microgrid system and the large grid.
As the world's leading microgrid company, BSLBATT provides scale microgrid solutions for PV EPC companies, we can provide turnkey microgrid systems including: Microgrid Battery System, BESS Controller, Hybrid Power Conversion System (PCS).
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