Nowadays, the demand of energy and power supply quality and safety and reliability is higher and higher. Because of its defects, the traditional power supply mode of large grid can not meet this demand. A new grid which is capable of integrating distributed generation—microgrid came into being. It can save cost, reduce energy consumption, improve system security and flexibility. So, it will be more useful in future.As an essential energy storage system in microgrid, capacitor plays a very important role. As a new type of energy storage device, super capacitor has become one of the preferred devices for microgrid energy storage because of its irreplaceable advantages. Microgrid consists of micro power supply, load, energy storage, energy manager and other parts.
The forms of energy storage in microgrid are: connected to the DC bus of micro power supply, the feeder including important load or the AC bus of microgrid. Among them, the first two can be called distributed energy storage, and the last one is called central energy storage. When the grid connected operation, the power fluctuation in the microgrid is balanced by the large grid, and the energy storage is in the state of charging and standby. When the microgrid is switched from grid connected operation to isolated operation, the central energy storage is started immediately to make up for the power shortage. The load fluctuation or micro power supply fluctuation can be balanced by central energy storage or distributed energy storage. Among them, there are two ways to balance the power fluctuation of micro power supply. One is to connect the distributed energy storage and the micro power supply which needs energy storage on a feeder, or to connect the energy storage directly to the DC bus of the micro power supply.
1.Short time power supply
There are two typical operation modes in microgrid: under normal circumstances, microgrid and conventional distribution network are connected to the grid for operation, which is called grid connected operation mode. When grid fail or power quality is detected to meet the requirements, microgrid will be disconnected from the grid in time to operate independently, which is called isolated network operation mode. Microgrid often needs to absorb part of the active power from the conventional distribution network, so when the microgrid is converted from grid mode to isolated network mode, there will be power shortage, and the installation of energy storage equipment is conducive to the smooth transition of the two modes.
2.Used as energy buffer
Because of the small scale of microgrid and the small inertia of the system, the fluctuation of network and load is very serious, which affects the stable operation of the whole microgrid. We always expect efficient generators (such as fuel cells) in microgrid to work at its rated capacity all the time. However, the load of microgrid is not constant all day, on the contrary, it will fluctuate with weather changes and other situations. In order to meet the peak load power supply, it is necessary to use fuel oil and gas peaking power plants for peak load adjustment. Because of the high price of fuel, the operation cost of this way is too expensive. The super capacitor energy storage system can effectively solve this problem. It can store the extra energy of the power supply when the load is low, and feed back to the microgrid when the load is peak to adjust the power demand. The high power density and energy density of super capacitor make it the best choice to deal with the peak load, and the super capacitor only needs to store the energy equivalent to the peak load.
3.Improe the power quality of microgrid
Energy storage system plays an important role in improving the power quality of microgrid. Through the inverter control unit, the reactive and active power provided by the super capacitor energy storage system to users and network can be adjusted, so as to improve the power quality. Because super capacitor can quickly absorb and release high-power power energy, it is very suitable to apply it to the power quality control device of microgrid to solve some transient problems in the system, such as instantaneous power failure, voltage surge, voltage sag and other problems caused by system fault. At this time, super capacitor is used to provide fast power buffer, absorb or supplement power, and provide Active or reactive power compensation is carried out by power support to stabilize and smooth the fluctuation of grid voltage.
4.Super capacitor is necessary in intelligent distributed grid system
From the perspective of the future development trend of smart grid, smart distributed grid system will be the mainstream of the future grid system. In order to realize the construction of intelligent distributed grid system, it is necessary to have distributed energy storage device, central energy storage device and other buffer devices. In the case of unstable energy generation process, a buffer is needed to store energy. When the process of energy generation is stable and the demand is constantly changing, energy storage devices are also needed. Fuel cell is different from wind energy or solar energy. As long as there is fuel, it can continuously output stable electric energy. However, load demand varies greatly with time. If there is no energy storage device, the cost of fuel cell is too high. By storing the excess energy in the energy storage device, the peak energy needed can be provided through the energy storage device in a short time.
In the distributed power system, the transient impact of power system is inevitable, and the superior performance of super capacitor can reduce the impact of transient impact on the performance of the whole system. Therefore, in the future intelligent distributed grid system, super capacitor bank energy storage system is essential.
