Tham khảo tài liệu 'power quality monitoring analysis and enhancement part 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 112 Power Quality - Monitoring Analysis and Enhancement S jT J NT N-1 ĨH m 0 m n NT g- n m2Ị-n2 ei2nj 18 where n 0 and H n NT is the Fourier transform of time series. The STFT transform limitation is the fixed window width chosen a priori for analysis of nonstationary signals containing low-frequency and high-frequency components. Consequently the frequency-time resolution is fixed too and is difficult to analyze a sinusoidal signal of low frequecy for instance the signal from power supply network affected by a high frequency disturbance for instance a transient phenomenon . The wavelet transform limitations are low resolution for low-frequency components the decomposition frequency bands are fixed noise sensitivity. The S transform is an extension of the wavelet transform resulted using a phase correction which provides superior resolution. Applying DST on a signal the result is a matrix within the rows are frequences and the columns are time values. Using the S-transform To ilustrate the ability of the ST to detect localize and quantify power quality disturbances are considered two signals. First signal is afected by a voltage swell low-frequency components and the second by an impulsive transient high-frequency components . Figure 11 shows the 3D ST plots for both signals. From the 3D plots can be observed the amplitude variations of the frequency spectral components in the signals. Fig. 11. S-transform 3D representation Methodes of Power Quality Analysis 113 Figure 12 presents the ST of a clean signal and six types of power quality disturbances voltage sag voltage swell voltage interruption voltage harmonics impulsive transient and oscillatory transient a the signal b the normalized time-frequency contour of ST c the maximum of amplitude-time characteristic of S transform and d the maximum of amplitude-frequency characteristic of of S transform. From visual inspection of these plots are obtained amplitude frequency and time information in order to detect .
