Large-capacity battery energy storage systems have been used in power systems for more than 20 years. In the early days, they were mainly used for frequency regulation, hot backup, voltage regulation and backup of isolated power grids. The application of battery energy storage systems in new energy grid integration has also been studied abroad.

　　In the late 1990s, Germany installed a battery energy storage system with a capacity of 1.2MWh at the Herne 1MW photovoltaic power station and the Bocholt 2MW wind farm respectively to provide peak shaving, uninterrupted power supply and improved power quality functions. Since 2003, Japan has installed a 6MW/6MWh all-vanadium redox flow battery (VRB) energy storage system at the Hokkaido 30.6MW wind farm to smooth output power fluctuations. On August 18, 2014, the 220 kV smart substation of the National Wind and Solar Storage and Transmission Demonstration Project was successfully launched.

　　As one of the first pilot projects of the State Grid Corporation of China to build a strong smart grid, the National Wind and Solar Storage and Transmission Demonstration Project is currently the largest grid-connected solar photovoltaic power station in the country, the wind farm with the largest single-machine capacity onshore in the country, and the largest chemical energy storage in the world. Power station is a new energy demonstration project with the highest level of intelligent operation and the most diverse operation methods.

　　The application purpose of energy storage power stations (systems) in the power grid mainly considers several functional applications such as "load regulation, cooperation with new energy access, compensation for line losses, power compensation, improvement of power quality, isolated grid operation, peak shaving and valley filling. For example : Peak shaving and valley filling, improving the operation curve of the power grid. To explain in a layman's terms, an energy storage power station is like a reservoir, which can store excess water during low periods of electricity consumption and use it again during peak periods of electricity consumption. In this way, It reduces the waste of electric energy; in addition, the energy storage power station can also reduce line losses and increase the service life of lines and equipment.

　　Since 2014, China has begun to develop energy Internet and energy storage systems on a large scale. This article mainly introduces the energy storage system briefly.

　　2. Off-grid energy storage system

　　Off-grid photovoltaic power generation system, also known as independent photovoltaic power generation system, mainly consists of PV components, DC/DC charging controller, off-grid inverter and load.

　　Off-grid systems consist of:

　　Battery components, photovoltaic charge and discharge controllers, battery packs, off-grid inverters, AC/DC loads.

　　The main function of the photovoltaic charge and discharge controller is to control the charge and discharge of the battery and protect the battery from overcharging and discharging. Off-grid inverter, the function of off-grid inverter is to convert DC power into AC power and provide it to the load.

　　Our common off-grid energy storage system is solar street lights. Photovoltaic modules, a controller the size of a cigarette box, an LED light with tens of watts, and one or more sets of batteries. It can provide night lighting.

　　The larger off-grid energy storage system is the "household system". When the author first entered the industry in 2006, the domestic photovoltaic industry was in its infancy. In order to solve the electricity problem of herdsmen in Qinghai and northwest Tibet, the country implemented several "Guangming Project" is to distribute a photovoltaic household system to each household.

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