An energy storage inverter is a power electronic device that utilizes energy storage units (such as batteries, supercapacitors, etc.) to achieve energy storage and release. Its primary function is to convert electrical energy into chemical energy for storage and to convert the stored energy back into electrical energy for release when needed.

Energy storage inverters (or inverters) can operate in two main modes: grid-following and grid-forming. The principles of these two operating modes are compared in the following figure, where (a) represents the grid-following mode, and (b) represents the grid-forming mode. (Source: Grid Forming Inverter Modeling, Control, and Applications, IEEE Access)

As shown in figure (a), the behavior of an inverter operating in grid-following mode can be approximately equivalent to controlling a current source parallel to a large resistor, meaning its external characteristics are similar to a current source, providing a constant current within a certain range. Grid-following inverters measure the voltage Vpcc at the point of common coupling and calculate the voltage phase using a Phase-Locked Loop (PLL). Then, by changing the terminal voltage, the desired current components (Id, Iq) are obtained, and by separately controlling the injection of Id and Iq, the output of active and reactive power is achieved. In this mode, the grid only provides a reference voltage. The output voltage waveform of the inverter remains synchronized with the grid, relying on the grid to provide stable voltage and frequency to feed electrical energy into the grid.

As shown in figure (b), the behavior of an inverter operating in grid-forming mode can be approximately equivalent to controlling a voltage source in series with a small resistor, meaning its external characteristics are similar to a voltage source, providing a constant voltage within a certain range. Unlike the grid-following mode, the grid-forming mode does not measure Vpcc to synchronize with the grid but directly constructs Vpcc to adjust the output power. It can establish its own reference voltage and frequency, without needing to anchor to the grid's voltage and frequency, and thus can operate in an isolated (islanded) state from the main grid.

Currently, most inverters in the power system operate in grid-following mode, tracking and measuring the grid's voltage and frequency, and feeding a certain amount of power into the grid. Its advantage is that it can feed in the maximum active power, achieving the maximum energy yield. However, as the number of grid-following inverters connected to the grid increases, even minor load fluctuations can trigger an excessive response from the inverters, leading to widespread disconnection of the grid-connected energy storage system and even triggering a chain reaction, causing the power system to collapse. Grid-forming control mode inverters aim to maintain a constant AC voltage output, which can help stabilize the grid. Moreover, grid-forming inverters can also provide fault ride-through, black start, and active-reactive power stability functions, ultimately achieving 100% renewable energy supply. However, grid-forming inverters must compromise between stability and maximizing energy yield.