Electrochemical energy storage, as a key technology for balancing energy supply and demand and improving grid stability, is increasingly being applied in the grid. In practical operation, seamless off grid switching is an important link to ensure the reliable operation of electrochemical energy storage systems and improve power supply quality. Reasonable system configuration plays a decisive role in achieving seamless switching and ensuring efficient and stable operation of energy storage systems.
1 Seamless off grid switching method
1. Adopting advanced energy storage converter (PCS) control strategy: Energy storage converter is a key equipment for achieving seamless switching between grid and off grid. In grid connected mode, the energy storage converter operates in PQ mode, relying on the voltage and frequency support provided by the power grid to directly control the grid connected current; In off grid mode, it operates in VF mode to provide voltage and frequency references for other distributed power sources. Through advanced control algorithms such as virtual synchronous machine control and droop control, energy storage inverters can achieve smooth switching between two modes, ensuring that there are no voltage and frequency surges during the switching process.
2. Configure microgrid static transfer switch (STS): STS can quickly switch the energy storage system from grid connected mode to off grid mode or vice versa in case of grid failure or need to switch, thereby achieving seamless transition. Its switching time is usually less than 10ms, and can reach up to 4ms at the fastest, which can effectively avoid problems such as load power outage or equipment damage caused by long switching time.
Multi power management capability: STS can not only switch between energy storage systems and power grids, but also flexibly switch between multiple power sources, such as diesel generators, photovoltaic power sources, etc. This is of great significance for improving the power supply reliability and flexibility of microgrids. During the off grid switching process, STS can automatically select the optimal power combination based on the preset switching logic and power status, ensuring the stable operation of the system.
Bypass switch configuration: To avoid the risk of power outage after damage to the STS itself, a bypass switch can be added to the STS device. When STS malfunctions, the bypass switch can automatically activate to maintain normal power supply to the system. In addition, the STS device can integrate all distribution switches connected to PCS, load power supply, grid connection, and oil engine connection, and customize them into STS device cabinets to achieve centralized management and control of the entire microgrid power supply.
2 Power configuration requirements
1. Determine power level based on application scenarios: The power capacity of electrochemical energy storage systems needs to be determined based on specific application scenarios and system scale. For small-scale industrial and commercial energy storage systems, the power is usually below 250kW, mainly used to meet the peak shaving and valley filling and emergency backup power needs within the enterprise; The power of household energy storage systems is below 10kW, mainly used for backup of household electricity and partial power supply during certain periods. For medium and large-scale energy storage power plants, the power is usually greater than 10MW, such as supporting energy storage facilities for large wind farms and photovoltaic power plants, which are used to smooth the output power fluctuations of new energy, improve the stability and reliability of the power grid.
2. Consider system redundancy and overload capacity: When determining the power capacity of an energy storage system, it is also necessary to consider certain system redundancy and overload capacity. Redundant design can improve the reliability and availability of the system, allowing it to operate normally even in the event of component failures or maintenance. Overload capacity is used to cope with high power demands in sudden situations, such as emergency power support in case of grid failures or power compensation in case of sudden load increases. Generally speaking, the overload capacity of energy storage systems should meet the short-term overload requirements of the system, usually 1.2 to 1.5 times the rated power, and the duration depends on the specific application scenario and system design.
3 Optional isolation transformer and its function
1. Function of isolation transformer: The main function of isolation transformer is to electrically isolate the energy storage system from the power grid, prevent mutual interference and influence between high and low voltage, and improve the safety and reliability of the system. In some application scenarios, such as when an energy storage power station is connected to the grid through a 10 (6) kV voltage level, an isolation transformer is usually required to ensure that the output voltage of the energy storage system matches the grid voltage and provides electrical isolation protection.
2. Whether to configure: Whether to configure an isolation transformer depends on specific access requirements and application scenarios. For some small energy storage systems, such as household energy storage systems, it may not be necessary to configure isolation transformers; For medium and large-scale energy storage power stations or energy storage systems that require access to higher voltage level power grids, isolation transformers are usually required.