Comparative analysis of power quality using wavelets for real time implementation

With increasing demand for electric power globally, alternative energy sources such as wind and solar are integrated into the power supply grid. Integration technologies use power electronic converters and controllers which introduce harmonics and other power quality issues. In addition, growing usage of BLDC motors, UPS, LED lights, inverters, momentary usage of heavy machinery by domestic consumers has caused the harmonics and other power quality issues such as flicker, voltage sag and swell to penetrate into the power supply network. Real Time detection and mitigation of such events is therefore necessary. A framework for Real Time Power Quality Monitoring is discussed. Precise detection and localization of sag or swell events using Wavelet Packet Transform is carried out for a typical power system. Supply voltage signal fed to power electronic converter supplying a BLDC motor is compressed using Discrete Wavelet Transform (DWT) and Discrete Cosine Transform (DCT). For a compression ratio of 19.6923, DWT provide lesser Total Harmonic Distortion (THD) as compared to DCT. High frequency white noise present in the current signal supplying the BLDC motor converter is filtered using Wavelet Packet Transform.

I. Introduction

With the increasing global needs, the demand for electrical utility is also increasing. This increased requirement is met by usage of alternative sources of electrical energy such as wind and solar energy conversion systems. The DC output provided by Solar PV panels is converted to AC using inverters. Also wind energy conversion systems require power electronics converters to provide constant voltage constant frequency supply from variable speed drive. Efficient usage of electric energy is required so as to reduce greenhouse gas emissions. Reduction in power consumption is achieved by using energy efficient LED lighting. This gives rise to need for solid state conversion systems using switches such as IGBT's and MOSFET's. This introduces harmonics into the power system network thereby affecting quality of power rendered to other consumers connected to the distribution grid. Earlier the side effects of harmonics were localized only to industrial consumers due to usage of nonlinear load such as arc furnaces, large variable frequency drives, heavy rectifiers, etc. But recently this problem has percolated to commercial users due to rapidly growing usage of inverters, Switch Mode Power Supplies (SMPS), Uninterruptible power supplies (UPS) and LED lights. Harmonics lead to shortened transformer life due to overheating [1], high neutral current, high voltage and current distortion, high neutral-to-ground voltage and Poor power factor conditions [2]. As Per IEEE 519 for systems of voltage rating less than 69 kV individual frequency components should have less than 3 percent harmonic distortion while Total Harmonic Distortion should be less than 5%.