Research of Variable Gain Nonlinear PI Controller Based Three-Phase Phase-Locked-Loop System

Three phase Phase-Locked-Loop is an important part of active power filter. In order to improve the behaviors of the conventional PLL system (CPLL), a novel control method of variable gain nonlinear PI controller based three phase PLL system (NLPLL) is proposed in this paper. As the proportional gain and integral gain of the nonlinear PI controller can adapt variety of the phase error, the schematic of the proposed is better than conventional PI controller in phase tracking velocity and phase tracking precision. The principle of three phase PLL system is introduced, and the design of the nonlinear PI controller is analyzed in detail. The results of simulation show that the proposed method can improve more effectively performance of the tracking speed and the tracking precision and meets the requests of the performance of high precision and real time for harmonics detection of active power filters.

I. Introduction

Nowadays, the widespread increase of nonlinear electric loads, such as three-phase rectifiers, uninterruptible power supplies, significant amounts of harmonic currents are being injected into power system. The presence of current and voltage harmonics may cause power quality degradation, additional losses in the transmission lines, over-voltages, over-heating, electro magnetic interference problems, and others undesirable effects. The traditional method of current harmonics reduction involves passive LC filters. However passive filters have several drawbacks such as large size, tuning and risk of resonance problems [1]. Therefore, many researches dedicate to find a much better solution than conventional approach. The approaches of active power filter have been proven to be very effective for its advantages such as current harmonics filtering, reactive power compensation, load current balancing, elimination of excessive neutral current and high performance under both dynamic and steady state operations [2]. But then, the exact and real-time phase locked loop is an important part of active power filters, it is more important while the mains voltage is unbalanced.