Transient Current Limiting Method of CHB-Based GFM Converters With Dual-Sequence Dynamic Reactive Power Support Under Voltage Sags

Cascaded H-bridge-based grid-forming converters (GFMCs) are capable of providing voltage and frequency support. However, under symmetrical and asymmetrical voltage sags, GFMCs tend to produce significant transient inrush currents that may exceed the converter design limit, posing risks to device reliability. Additionally, new grid codes (GCs) require GFMC to inject both positive- and negative-sequence reactive currents during asymmetric voltage sags. To address this issue while complying with the new GCs, a transient current limiting method with dynamic dual-sequence reactive power support is proposed. First, the mechanisms causing existing methods to fail in limiting transient inrush currents under symmetrical and asymmetrical voltage sags are analyzed. Neglecting the influence of negative-sequence currents is the culprit causing the unreliability of existing methods. Then, a flexible transient inrush current limiting method is proposed by considering the dynamic variation of the negative-sequence current. Compared to existing methods, the proposed method meets the new GC requirements for dual-sequence reactive power and can successfully limit transient inrush currents under symmetrical and asymmetrical voltage sags without relying on grid impedance or grid voltage information. The current limit can be set flexibly while maintaining the behavior of the voltage source and remaining effective during grid voltage restoration. Simulation and experimental results demonstrate the effectiveness of the proposed method.