Comparison between SRF-PLL and DDSRF-PLL under unbalanced conditions

Abstract

This study aims to analyze and compare the performance of the Synchronous Reference Frame Phase-Locked Loop (SRF-PLL) and the Decoupled Double Synchronous Reference Frame Phase-Locked Loop (DDSRF-PLL) under unbalanced electrical grid conditions. Accurate grid synchronization is essential, especially in the presence of renewable energy sources, which often operate under asymmetric or distorted conditions. Simulations were carried out using the ATPDraw software to evaluate the behavior of both PLLs in two scenarios: balanced conditions and conditions with voltage unbalance and phase angle unbalance. The results show that the SRF-PLL presents significant limitations when subjected to disturbances, such as lack of stabilization and steady-state error. In contrast, the DDSRF-PLL proved to be more robust, efficiently isolating positive and negative sequence components, achieving faster stabilization of the control variable, and maintaining the accuracy of the estimated frequency even under severe unbalance conditions. The analysis of waveforms and transformed components confirms the superiority of the DDSRF-PLL in adverse scenarios, making it a recommended alternative for distributed generation systems and applications in electrical grids subject to unbalance.