Grid-side energy storage field mode

In the grid-connected state, voltage and frequency are determined by the grid, and any excess generated power is fed into the power grid. In the stand-alone state, any imbalance between generated power and load demand is typically addressed locally using battery energy storage systems (BESSs) .

In the grid-connected state, when the frequency is at its nominal value, the GSC does not exchange power with the grid and only manages the battery SOC. In this study, the initial value of the battery's SOC is considered to be around the 60 %.

In the grid-connected state, the battery SOC is managed by the GSC, and for SOC < 90 %, the battery is charged by the GSC according to the SOC level, and for SOC > 95 %, the GSC limits the SOC below the upper limit.

Compared with other large-scale ESSs such as pumped storage and compressed air storage, the battery energy storage system (BESS) has the most promising application in the power system owing to its high energy efficiency and simple requirements for geographical conditions .

In this paper, by modifying control structures of the rotor-side and grid-side converters (RSC and GSC), the RSC is controlled in the grid-following (GFL) mode and the GSC in the grid-forming mode using the virtual synchronous generator technique.

At t = 19 s, the synchronization unit starts synchronizing the DFIG with the grid, and thus, at t = 20.5 s, the circuit breaker connects the DFIG to the grid. Next, at t = 21 s, the synchronization unit becomes inactive. Again, at t = 23 s, the DFIG disconnects from the grid, and the system switches to the stand-alone state.