An adaptive online monitoring system using NFSOGI-PLL for three-phase voltage unbalance in grids

•First present and analyze the design of NFSOGI-PLL synchronization methods.•The filtering and tracking capabilities of the NFSOGI-PLL have been experimentally verified.•The application of NFSOGI-PLL for adaptive online monitoring of three-phase voltage unbalance is demonstrated.•The performance of the proposed online monitoring system for three-phase voltage unbalance has been rigorously tested in both healthy and unhealthy grid conditions. The extensive utilization of unbalanced and nonlinear loads has significantly impacted power quality in modern grids. Among various metrics, the three-phase voltage unbalance factor (TPVUF) stands out as a crucial indicator. To mitigate the challenges posed by harmonics and DC components that interfere with accurate TPVUF measurements, this paper introduces the Notch Filter-based Second-Order Generalized Integrator Phase-Locked Loop (NFSOGI-PLL) for the first time and designs an adaptive online monitoring system for TPVUF based on NFSOGI-PLL. To validate its performance, the hardware design and rapid software development of the NFSOGI-PLL-based TPVUF online monitoring system are thoroughly implemented, and the system is tested under various conditions. The results indicate that the proposed system not only provides real-time performance and high accuracy, complying with the rigorous standards set by GB/T 15543-2008 and IEC 61000-4-27, but also demonstrates remarkable stability under diverse influences. These findings underscore the exceptional filtering and tracking abilities of the NFSOGI-PLL in power systems, as well as the accuracy and anti-interference capabilities of the NFSOGI-based TPVUF online monitoring system, which holds great potential for widespread application in power system control and research.