Grid Forming Control of THSSC Under Unbalanced Grid Condition

Static synchronous compensators (STATCOM) are utilized in power systems to improve power quality, such as voltage support, reactive power support, harmonic mitigation, and flicker mitigation. Compared to the conventionally used hybrid series converter (HSC) based STATCOM, the transformer-less hybrid series static compensator (THSSC) has distinct advantages such as lower dv/dt stress, decreased energy storage requirements, fewer capacitors needed for submodules (SMs), fewer switching devices, and transformer-less functionality. However, its control system uses a phase-locked loop (PLL) for grid angle detection, which can be disrupted by grid disturbances. To address this issue, this paper implements a grid forming control (GFM) for THSSC. GFM is a control strategy that allows the THSSC to actively regulate the grid voltage and frequency, thereby maintaining grid stability even under unbalanced grid conditions. Key challenges while implementing the GFM strategy, such as capacitor voltage of modules, dynamic stability, and fast response, are investigated. This paper shows the impact of GFM on the performance of THSCC under different test cases. Finally, hardware results are presented under a balanced and unbalanced grid to verify the proposed GFM control.