A Novel Repetitive Controller Assisted Phase-Locked Loop with Self-Learning Disturbance Rejection Capability for Three-Phase Grids

Mi Tang,Stefano Bifaretti,Sabino Pipolo,Pericle Zanchetta,Shafiq Odhano

doi:10.1109/jestpe.2019.2941835

Abstract

The synchronization between the power grid and distributed power sources is a crucial issue in the concept of smart grids. For tracking the real-time frequency and phase of three-phase grids, phase-locked loop (PLL) technology is commonly used. Many existing PLLs with enhanced disturbance/harmonic rejection capabilities, either fail to maintain fast response or are not adaptive to grid frequency variations or have high computational complexity. This article, therefore, proposes a low computational burden repetitive controller (RC) assisted PLL (RCA-PLL) that is not only effective on harmonic rejection but also has remarkable steady-state performance while maintaining fast dynamic. Moreover, the proposed PLL is adaptive to variable frequency conditions and can self-learn the harmonics to be canceled. The disturbance/harmonic rejection capabilities together with dynamic and steady-state performances of the RCA-PLL have been highlighted in this article. The proposed approach is also experimentally compared to the synchronous rotation frame PLL (SRF-PLL) and the steady-state linear Kalman filter PLL (SSLKF-PLL), considering the effect of harmonics from the grid-connected converters, unbalances, sensor scaling errors, dc offsets, grid frequency variations, and phase jumps. The computational burden of the RCA-PLL is also minimized, achieving an experimental execution time of only $12~\mu \text{s}$ .

Highlights

Accurate real-time phase tracking of power grids is required for the synchronization of distributed power sources and their integration in the modern concept of smart grids
With more and more power electronics interfaced to the power networks, issues such as harmonics introduced by power converters, load unbalances, as well as measurement scaling error and d.c. offsets, produce periodic disturbances and degrade the accuracy of the phase tracking using a traditional phase-locked loop (PLL)
The results indicate that, when the grid voltage is distorted, ripple may appear in the frequency and phase identified by the synchronous rotation frame PLL (SRF-PLL) in Fig. 1, the mean value of the frequency still remains accurate since the ProportionalIntegrator (PI) controller will bring the d.c. value of Uq to zero

Summary

Introduction

Accurate real-time phase tracking of power grids is required for the synchronization of distributed power sources and their integration in the modern concept of smart grids. We have PLLs in natural abc coordinates (such as the zero crossing method[2]), in stationary αβ coordinates (such as the second order generalized integrator (SOGI) PLL[3]), and in rotating dq coordinates (as the synchronous rotation frame (SRF) PLL[4], the dq frame filter based PLL[5]). Authors in [6] have compared the SRF-PLL and some PLLs in the other two categories under conditions such as harmonics, voltage dips, and grid frequency variation; the SRFPLL results as the simplest method that can sufficiently perform in all the test conditions. It needs to be dynamically slow to properly attenuate the harmonics and it provides poor performance on phase jumps [7]

Objectives

Results

Conclusion