Application of Supercapacitors in Smart Grid

Today's society has higher and higher requirements for the quality, safety and reliability of energy and power supply, and the traditional large-scale power grid power supply method can no longer meet this requirement due to its own defects. A new type of grid that can integrate distributed generation—microgrid emerges as the times require. It can save investment, reduce energy consumption, and improve system security and flexibility. It is the future development direction. Capacitors play a very important role as an indispensable energy storage system in microgrids. As a new type of energy storage device, supercapacitor has become one of the energy storage devices of microgrid due to its irreplaceable advantages. 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 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 a grid fault is detected or the power quality does not meet the requirements, the microgrid will be connected to the grid in time. 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 with conditions such as weather. 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 an excellent choice for handling peak loads, and the use of supercapacitors only needs to store as much energy as the peak loads .

3) Improve the power quality of the microgrid

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) Super capacitors are essential for smart distributed grid systems

From the perspective of the future development trend of the smart grid, the smart distributed grid system will be the mainstream of the future grid system. In order to realize the construction of a smart distributed grid system, it is necessary to have buffer equipment such as distributed energy storage devices and central energy storage devices. In cases where the energy generation process is unstable, a buffer is needed to store the energy. Energy storage devices are also required in situations where the energy generation process is stable and the demand is constantly changing. Unlike wind or solar energy, fuel cells can continuously output stable electricity as long as there is fuel. However, load demand varies considerably over time. Without an energy storage device, the fuel cell would have to be large enough to meet peak energy demands, and the cost would be prohibitive. By storing the excess energy in the energy storage device, the required peak energy can be supplied by the energy storage device in a short period of time.

In the distributed power grid system, the transient shock of the power system is inevitable, and the superior performance of the supercapacitor can reduce the impact of the transient shock on the performance of the entire system. Therefore, in the future smart distributed grid system, the supercapacitor bank energy storage system is indispensable.