Comparative harmonic analysis of power-quality indices in three-phase power systems

Abstract

Abstract- Based on Fast Fourier Transform (FFT), power quality indices (PQIs) definitions contained in the standards cannot be successfully applied for non-stationary signals analysis in unbalanced three-phase systems with non-sinusoidal conditions. Unlike FFT, Discrete Wavelet Transform (DWT) gives more accurate definitions preserving time-frequency information and reduces the computational effort. This paper proposes a comparative analysis of definitions employing these two instruments for unbalanced three-phase systems case in sinusoidal and nonsinusoidal situations using waveforms captured with data acquisition system which contains Hall voltage and current transducers. The experimental testing of harmonic distortion was made for a RL unbalanced three-phase load, a three-phase rectifier with diodes (TPRD) and a rectifier with near sinusoidal currents (RNSIC). I. I NTRODUCTION For the electric power quality monitoring, the IEEE working group has analysed the customers expectations concerning electric power delivery, the susceptibility limits of the electric and electronic equipments connected at the electric network and thus obtained the standards regarding the evaluation methods of electric power quality, the parameters that define this concept and the functioning classes of the measurement tools [1-3]. In the last years electric power quality became un important issue in the power systems domain because of the increased usage of nonlinear loads which are producing great harmonic distortion in power supply [4-12, 33-36]. Voltage and current quality can be determined based on the deviations between the real voltage/current and the ideal voltage/current waveforms [13]. These deviations are counted power quality disturbances and can be classified as stationary (with time unchangeable characteristics) or non-stationary (with time changeable characteristics) and each of them as sinusoidal or non-sinusoidal. The Fast Fourier Transform (FFT) approach in Power Quality Indices (PQIs) definition is replaced in this study with a time-frequency domain approach using Discrete Wavelet Transform (DWT). This method eliminates the FFT drawbacks as high computational effort, time related information loss, spectral leakage and provides representations for any non-stationary waveforms [14-17]. DWT is a useful tool in the waveforms analysis due to its accuracy and sensitivity at signal irregularities [5], [32]. The waveforms of voltage and current are obtained using a data acquisition system with linear/nonlinear loads composed of a Digital Signal Processor (DSP) board with high-speed 12 bit Analog to Digital Converters (ADC) coupled to a three-phase system using the Hall transducers. This paper presents a comparative analysis of DWT based measurements of power quality indices and the IEEE Standard 1459-2000 based values using unbalanced linear three-phase systems and nonlinear power loads. The power quality disturbances are calculated with a RL unbalanced three-phase load, a three-phase rectifier with diodes (TPRD) and a rectifier with near sinusoidal currents (RNSIC). II. T