Grid-Following Energy Storage vs. Grid-Forming Energy Storage

      Grid-following energy storage systems operate as a current source, their operation dependent on the grid's voltage and frequency. In grid-following mode, the system's converter follows the grid's phase information, measuring the phase information at the grid connection point (PCC) via a phase-locked loop (PLL) to achieve synchronization with the grid. In this control mode, the energy storage system can’t independently provide voltage and frequency support, thus requiring the presence of a grid to function. Grid-following energy storage systems are typically used to supplement the grid's instantaneous power demand, improving grid stability and reliability.

      Grid-forming energy storage systems are essentially voltage sources, capable of internally setting voltage parameters to output stable voltage and frequency. They can operate both on-grid and off-grid, offering strong grid support capabilities. Due to their voltage source characteristics, grid-forming energy storage systems can effectively address issues such as short-circuit capacity and lack of rotational inertia in new power systems, thus holding significant application potential in these systems.

The differences between the two are as follows:

1. Technical differences:

      The grid-following type is essentially a current source, which can’t provide voltage and frequency support on its own and must rely on the grid voltage and frequency, thus it can’t support the system. The grid-forming type is essentially a voltage source, which internally sets voltage parameter signals to output voltage and frequency. It can operate both on-grid and off-grid, and has strong grid support capabilities. Furthermore, the grid-following type converter has stability issues in weak grids, while the grid-forming type converter can provide stable frequency support in weak grids, which helps improve grid stability. The grid-following type energy storage system is suitable for situations where the grid stability is good and no additional voltage and frequency support is required. The grid-forming type energy storage system is more suitable for new power systems, especially in areas with a high proportion of new energy sources and poor grid stability.

2. Differences in PCS:

      (1) The grid-following PCS mainly controls AC current measurement, tracks the existing grid voltage phase angle through a phase-locked loop, and then forms a control signal after coordinate transformation and PWM modulation, which is then fed back to the switching transistor. The grid-following PCS control structure is simple, and the phase-locked loop (PLL) technology is relatively mature at present. Therefore, the system can be controlled under the condition that the current and maximum power point of the system are determined. However, although the PLL technology on which the control relies is relatively mature, it still needs to passively obtain stable frequency and voltage reference values provided by the grid to work normally. Moreover, the stability of its own control loop is weaker than that of the grid-forming energy storage loop, and it can’t play the role of actively supporting the system.

      (2) Unlike the grid-forming control which uses a PLL to synchronize with the grid, the grid-forming PCS sets a voltage reference signal internally, and synchronizes with the rest of the grid after passing through the power calculation module and frequency droop control. Similar to synchronous generator control, it does not rely on an external voltage reference signal. The grid-forming PCS has the ability to adjust its own operation in real time without external power supply. It maintains the voltage output by adjusting the power output, forming a voltage source connected to the grid and maintaining system stability. In addition, it can form an independent grid in weak grids without a rigid voltage source. However, the overcurrent capacity of the grid-forming PCS is increased from 1.5 times to 3.0 times, so the cost is significantly increased compared to the grid-following type.