A microgrid consists of micro power sources, loads, energy storage, and energy managers. The forms of energy storage in microgrids include: connected to the DC bus of the microgrid, on the feeder line containing important loads, or on the AC bus of the microgrid. Among them, the first two can be called distributed energy storage, and the last one is called central energy storage.
When connected to the grid, the power fluctuations within the microgrid are balanced by the large grid, and the energy storage is in a charging standby state. When the microgrid switches from grid connected operation to isolated operation, the central energy storage system immediately starts to compensate for the power shortage. The fluctuations in load or micro power sources during the operation of a microgrid in isolation can be balanced by central or distributed energy storage. Among them, there are two ways to balance the power fluctuations of micro power sources: connecting distributed energy storage and micro power sources that require energy storage in parallel on a certain feeder line, or directly connecting the energy storage to the DC bus of the micro power source.
1. Provide short-term power supply
There are two typical operating modes for microgrids: under normal circumstances, microgrids operate in parallel with conventional distribution networks, known as grid connected operation mode; When a power grid fault or insufficient power quality is detected, the microgrid will promptly disconnect from the grid and operate independently, known as the isolated grid operation mode. Microgrids often need to absorb some active power from conventional distribution networks, so there may be power shortages when transitioning from grid connected mode to isolated mode. Installing energy storage devices can help facilitate a smooth transition between the two modes.
2. Used as an energy buffering device
Due to the small scale and low system inertia of microgrids, frequent fluctuations in network and load become very serious, which affects the stable operation of the entire microgrid. We always expect efficient generators (such as fuel cells) in microgrids to operate at their rated capacity. However, the load of microgrids does not remain constant throughout the day. On the contrary, it fluctuates with changes in weather and other conditions. In order to meet peak load power supply, it is necessary to use fuel and gas peaking power plants for peak load adjustment. Due to the high fuel prices, the operating costs of this method are too expensive. The supercapacitor energy storage system can effectively solve this problem by storing excess electricity from the power source during low load periods and feeding it back to the microgrid to adjust power demand during peak load periods. The high power density and energy density of supercapacitors make them the best choice for handling peak loads, and using supercapacitors only requires storing energy equivalent to that of peak loads.
3. Improve the power quality of microgrids
The energy storage system plays a crucial role in improving the power quality of microgrids. Through the inverter control unit, the reactive and active power provided by the supercapacitor energy storage system to users and the network can be adjusted, thereby achieving the goal of improving power quality. Due to their ability to quickly absorb and release high-power electrical energy, supercapacitors are highly suitable for application in power quality control devices of microgrids to solve transient problems in the system, such as instantaneous power outages, voltage surges, and voltage drops caused by system faults. At this time, supercapacitors are used to provide fast power buffering, absorb or supplement electrical energy, and provide active power support for active or reactive power compensation, in order to stabilize and smooth the fluctuations of grid voltage.
4. Optimize the operation of the micro power supply
Green energy sources such as solar and wind energy often have non-uniformity, and the output of electricity is prone to change. This requires the use of a buffer to store energy. Due to the fact that the electricity output generated by these energy sources may not meet the peak electricity demand of the microgrid, energy storage devices can be used to provide the required peak electricity in a short period of time until the power generation increases and the demand decreases. Moderate energy storage can serve as a transitional measure when the DG unit is unable to operate normally. If solar power is used for night time, wind power is used in the absence of wind, or other types of DG units are under maintenance, then the energy storage in the system can play a transitional role.
In situations where energy generation is stable and demand is constantly changing, energy storage devices are also necessary. By storing excess energy in energy storage devices, the required peak energy can be provided in a short period of time through the energy storage devices.
