Rapid Impedance Estimation Algorithm for Mitigation of Synchronization Instability of Paralleled Converters under Grid Faults

Paralleled grid-connected converters operated as grid-following structures are vulnerable to transient synchronization instability during grid faults. This paper mathematically describes the instability phenomenon of paralleled converters and why it is more pronounced than for single-converter operation. Based on this model, instability can be averted by modifying each converter current reference depending on the external network impedance where asymptotic stability is proven. A rapid impedance estimation algorithm is presented, which can extract the network impedance based on the disturbance of the grid fault. This estimation is used to accurately adjust the converter current references in order to guarantee stability of all paralleled converters for any severity of the grid fault. The proposed control structure is verified in a detailed simulation study and through experimental tests, which demonstrate its potential and robustness.