Nonlinear and Transient Stability Analysis of Phase-Locked Loops in Grid-Connected Converters

Abstract

The undesired nonlinear operation of phase-locked loops (PLL) is one of the main causes of transient instability in grid-connected converters. However, the stable operating region of PLL, knowing which is helpful for protection purposes, is hard to find due to the nonlinear characteristic of the transient process. In this article, the nonlinear characteristic of the PLL control loop relevant to the grid-connected converter operation is identified. The averaging method is applied to derive a time-domain expression for the PLL operation under disturbance. Based on the analytical expression, the transient response and related stability criterion are established, and the PLL design is improved while targeted to focus on various ac grid faults or disturbances. The results, applicable to transient stability enhancement, are verified by circuit simulations and experiments.