Assessing Power Factor Distortion and Transient Current Response of Grid Converters During Fault Ride-Through with Extended PLL Models

Abstract

Phase-Locked Loops (PLLs) are widely used in power converters’ Voltage Oriented Control (VOC) to detect the phase angle of the grid voltage. PLLs’ steady-state characteristics and dynamics critically affect the VOC, particularly during severe grid faults. To enhance the control characteristics, PLLs are typically designed by evaluating the dynamics of the estimated phase angle and considering the trade-off between its distortion immunity and step response time. However, the estimated phase angle is only an auxiliary variable for the VOC, whereas the power factor distortion and current settling times are crucial VOC performance measures defined by grid standards. Consequently, extended large-signal and small-signal models of PLLs considering current control transfer characteristics are derived. The analysis shows that power factor distortions depend on the converter’s active and reactive power references, and the largest distortions occur during maximum reactive power injection. The derived model reveals a coupling between the current references in dq-frame with the error of the estimated phase angle. For the dynamics, this coupling is identified as primary reason that current settling times deteriorate during severe grid faults. Notably, this coupling mechanism dominate the current dynamics during grid faults even if large phase jumps and amplitude steps occur. These effects are analyzed with two PLL structures and verified in simulations and experiments.