Application of Supercapacitor (Farad Capacitor) in Smart Grid

The microgrid is composed of microsources, loads, energy storage, and energy managers. The forms of energy storage in the microgrid are: connected to the DC bus of the micropower supply, the feeder containing important loads, or the AC bus of the microgrid. Among them, the first two can be called distributed energy storage, and the latter one is called central energy storage.

　　During grid-connected operation, the power fluctuations in the microgrid are balanced by the large grid, and the energy storage is in a charging standby state at this time. When the microgrid is switched from grid-connected operation to isolated grid operation, the central energy storage will start immediately to make up for the power shortage. The fluctuation of load or the fluctuation of micro power supply can be balanced by central energy storage or distributed energy storage during the operation of isolated grid of micro grid. Among them, there are two ways of balancing the power fluctuation of the micro-power supply. The distributed energy storage and the micro-power supply requiring energy storage are connected to a feeder in parallel, or the energy storage is directly connected to the DC bus of the micro-power supply.

　　1. Provide short-term power supply

　　There are two typical operation modes of the microgrid: under normal circumstances, the microgrid and the conventional distribution grid are connected to the grid, which is called the grid-connected operation mode; when the grid failure or the power quality does not meet the requirements, the microgrid will promptly connect to the grid. The grid is disconnected to operate independently, which is called the isolated grid operation mode. Microgrids often need to absorb part of the active power from the conventional distribution network. Therefore, when the microgrid is converted from the grid-connected mode to the isolated grid mode, there will be a power shortage. The installation of energy storage equipment helps the smooth transition of the two modes.

　　2. Used as an energy buffer device

　　Due to the small scale of the microgrid and the small inertia of the system, the frequent fluctuation of the network and load is very serious, which affects the stable operation of the entire microgrid. We always expect a high-efficiency generator (such as a fuel cell) in a microgrid to always operate at its rated capacity. But the load on the microgrid doesn't stay the same throughout the day; instead, it fluctuates as the weather changes, for example. In order to meet the peak load power supply, it is necessary to use oil and gas peak shaving power plants to adjust the peak load. Due to the high fuel price, the operation cost of this method is too expensive. The supercapacitor energy storage system can effectively solve this problem. It can store the excess energy of the power supply when the load is low, and feed it back to the microgrid to adjust the power demand when the load is peak. The high power density and high energy density of supercapacitors make them the most suitable choice for handling peak loads, and the use of supercapacitors only needs to store energy equivalent to the peak load.

　　3. Improve the power quality of microgrids

　　The energy storage system plays a very important role in improving the power quality of the microgrid. Through the inverter control unit, the reactive and active power provided by the supercapacitor energy storage system to the user and the network can be adjusted, so as to achieve the purpose of improving the power quality. Because supercapacitors can quickly absorb and release high-power electrical energy, it is very suitable to be applied to the power quality adjustment device of the microgrid to solve some transient problems in the system, such as instantaneous power failure and voltage swell caused by system faults. At this time, supercapacitors are used to provide fast power buffer, absorb or supplement electric energy, and provide active power support for active or reactive power compensation to stabilize and smooth the fluctuation of grid voltage.

　　4. Optimize the operation of the micro power supply

　　Green energy, such as solar energy and wind energy, is often non-uniform, and the power output is prone to change. This requires the use of a buffer to store energy. Since the power output generated by these sources may not be able to meet the peak power demand of the microgrid, energy storage devices can be used to provide the required peak power for a short period of time until the power generation increases and the demand decreases. An appropriate amount of energy storage can play a transitional role in the event that the DG unit cannot operate normally. For example, at night when solar power is used, when there is no wind for wind power generation, or when other types of DG units are under maintenance, the energy storage in the system can play a transitional role.