Analysis Of Power Quality Disturbances Using S-transform

Power Quality disturbances cause several problems, such as overheating, failure of motors, disoperation of protective equipment and inaccurate metering. As a consequence, more concern about different issues that relate to the power quality has been raised in recent years. It is failed to accomplish analysis of all the types of power quality disturbances utilizing single time or frequency means because of the diversity of power quality disturbances. Therefore, the time-frequency analysis methods become the hot tools. The S-transform can be seen either as an extension of ideas of wavelet or short-time Fourier transform, and it has characteristics superior to them because it has no use for the choice of base function. S-transform is more robust to noise than wavelet and it is essentially a short-time Fourier-transform whose window width varies inversely with the frequency. In accordance with national and international power quality standard and research situations, systematic and in-depth researches about power quality disturbances analysis approaches are carried out. The main contributions of this dissertation are listed as follows:A modified S-transform based method for noise level estimation of power quality signal is proposed. According to the time-frequency property of power quality in modified S-transform, the concept of energy congregation level is defined. The time-frequency resolution is determined by making the value of energy congregation level minimum, so there has the minimal aliasing area to facilitate the separation of disturbances signal and noise. By use of white noise distribution characteristic the area of power quality signal's energy and noise in modified S-transform plane is detected. Based on the time-frequency plane only contains the power spectrum in noise area the estimation of noise variance is implemented. Simulation results show that the estimation results of noise variance are of high degree of confidence while the sampling rate achieves a certain extent.Two denoising approaches of power quality signals are proposed based on concept of energy congregation level and method of noise estimation respectively. One of the ways of denoising is realized just using iterative algorithm. The rectified S-transform is obtained by computing the energy congregation level and hard threshold denoising algorithm is carried out repeatedly. The quadratic change rate of energy congregation level is defined to find the optimal iterations number used in iterating algorithm to obtain satisfactory denoising results. Then the inverse rectified S-transform is performed to get the denoised power quality disturbances signals. The other way of denoising is realized by utilizing the power quality region of the time-frequency plane which is determined in the process of noise estimation and the hard threshold algorithm is used to verify the value of the modified S-transform matrix elements. The denoising results compared with wavelet-based method illustrate the advantage of the proposed approaches.A classification method of power quality disturbances is proposed using modified S-transform and decision tree. The approach is realized by extracting 4 features from modified S-transform module matrix and employing binary classification tree. 9 types of disturbances are recognized effectively using the proposed method. The simulation results show that the proposed classification methods are feasible for real applications for their significant accuracy and immune to noise.In order to overcome the defect of large amount calculation of S-transform, the definition of incomplete S-transform is proposed and applied to power quality disturbances detection. The detection methods of power quality based on S-transform usually use the major vectors of S-transform module time-frequency matrix instead of all of the vectors. If just the major vectors corresponding disturbances major frequency fractional are computed, much computation time will be saved. Dynamics is utilized to describe the envelope of power spectrum so as to detect the valid frequency samples of FFT. The simulation results illustrate that the proposed method increases speed of algorithm and has no loss of detection precision.A new method of parameters estimation of power quality disturbances is proposed based on modified incomplete S-transform, which combine the modified S-transform with incomplete S-transform that have complementary advantages in time frequency resolutions and computation speed. The optimal windowing coefficients in connection with different disturbances are adaptively selected, and then the major vectors obtained using the modified incomplete S-transform and these vectors are utilized to estimate the parameters. Simulation results verified that the proposed method has characteristics of lower complexity, less computation and high precision.On the basis of phase property of S-transform that it provides multi-resolution analysis while retaining the absolute phase of each frequency, a cross S-transform method for locating capacitor switching disturbances is proposed. The disturbance frequency vectors of S-transform of transient voltage and current, which are obtained by performing incomplete S-transform, are employed to perform cross S-transform and the phase difference spectrum is obtained. The phase difference of transient voltage and current is determined by extracting the value at disturbance start point from the phase spectrum. The relative location of switching capacitor is determined by the polarity of the value and the exact location can be obtained by combining with detection results of multiple monitoring stations. The proposed method has been verified by simulation experiments.A comprehensive power quality analysis system is designed based on the series algorithm described above. The architecture is designed witch consists of pre- processing unit, power parameter and energy measurement unit, disturbance triggering unit, power quality analysis unit and assessment system. Some key algorithms are proposed in the process that elaborates the function units, including dynamic-based frequency measurement method, triggering signal generating approach and PC-based power quality analysis expert system. The series S-transform-based power quality analysis method are effectively integrated in the system.The series S-transform-based power quality disturbances analysis methods and comprehensive analysis system expounded in the dissertation aim to provide effective theorem basis and algorithms for power quality on-line detection and off-line analysis, and may be used in engineering practice.