Sub-Synchronous Damping by Battery Storage System in Grid Forming Control

Wind farms are susceptible to sub-synchronous oscillation (SSO) issues due to the interaction between the grid-side converter (GSC) and the weak-grid impedance. Integration of battery energy storage system (BESS) to damp SSO became popular due to its ability to also provide various grid services. Generally, BESS can be externally integrated at the PCC or internally integrated at the dc-link of the wind turbine (WT). In case of external BESS integration, grid-forming (GFM) control is usually applied to the BESS GSC. Proper increase of the GFM damping coefficient contributes to a better SSO damping, at the cost of a higher BESS power rating due to its droop meaning. Reconfiguring power loop of GFM to decouple the damping and the droop coefficient enables GSC to realize similar SSO damping performance while reducing the BESS power rating. In case of internal BESS integration, a virtual grid-forming control is proposed as an ancillary control of the WT GSC. This enables GSC to emulate the output behavior of two parallel converters, operated in grid-following and GFM mode respectively. Consequently, similar SSO damping performance can be realized without an external GFM converter. Simulation and experimental results are provided to verify the effectiveness of the proposed method.