Distorted Currents Detection Based On Adaptive Method And Its Improvement

In APF (Active Power Filter), it is necessary to get distorted current components or harmonics in electric distribution network for suppressing distorted currents or harmonics, and real-time and accurate detection of currents can improve APF's quality. So real-time and accurate distorted current detection is an important problem in electric power quality control.This dissertation made a selection which employs least mean square (LMS) adaptive algorithm to distorted current detection system as the key, and discussed theories and algorithms of the adaptive system. In theory, research included some characters of linearity and time-varying of the adaptive system based on LMS algorithm, and instantaneous characters of adaptive distorted current detection system. For the key facter of adaptive algorithm, this dissertation proved principle of selecting step-size to accelerate convergence based on LMS algorithm. This dissertation discussed errors and drawbacks of adaptive distorted current detection system based on LMS, and proposed methods to improve performance of distorted current detection in power system. These methods included:1) Improved adaptive distorted current detection method based on embedded low pass filter. By filtering fluctuation of weights and errors in LMS algorithm, the adaptive system can detect distorted currents timely and accurately. This dissertation analyzed stability of an LMS algorithm which adopts an average filter to eliminate fluctuation (aLPF-LMS) in harmonic detection.2) Optimal variable step-size LMS algorithm (OVS-LMS) and recursion optimal variable step-size LMS algorithm (ROVS-LMS).The algorithm can improve adaptive convergence speed reduce errors of steady state in distorted current and harmonic detection.3) Combined LMS which improves precision of distorted current detection. When keeping dynamics of detecting distorted current, the combination can reduce errors of steady state with twin-LMS.4) Employing band pass filter (BPF) to pretreate input of adaptive system based on LMS algorithm. The LMS algorithm can reduce the errors of steady state of harmonic current detection and improve convergence speed of system. This dissertation also discussed adaptive harmonic detection methods based on improved LMS algorithm.This dissertation discussed adaptive distorted current detection methods of3-phase circuits based on LMS algorithm, and discussed distorted current detection of3-phase balance system and3-phase unbalance system. For three phase symmetrical system, this dissertation discussed harmonic detection methods based on the instantaneous reactive power theory and adaptive LMS algorithm. These methods include adaptive3-phase harmonic detection method based on improved LMS algorithm and resampling LMS algorithm, overcoming the drawback of harmonic detection based on the instantaneous reactive power theory, improving dynamic performance and reducing the steady state errors in3-phase harmonic detection.In these studies, simulations of adaptive distorted current detection or harmonic detection methods based on modified LMS algorithms have been done by using MATLAB, dynamics and steady state errors of these algorithms have been analyzed, including3-phase circuits.Finally, modified algorithms including LPFw-LMS, LPFd-LMS and LPFwd-LMS have been tested and verified by experiments. According to experimental results, these algorithms are proved to detect distorted current efficiently.