Research On Estimating Algorithms Of Power Quality Indices

With rapid development of economy and science technology, power quality (PQ) problems have increasingly become a substantial concern in recent years. Completely and accurately assessing power quality is the first step in solving and mitigating the problems, fast and advanced algorithms are the basis for effective measurement and evaluation.Due to the complexity of power system signal and the variety of power quality event, it is a challenge to detect power quality indices completely, accurately and promptly.In accordance with national and international power quality standard, a systematic and in-depth research about power quality major indices evaluation is carried out based on analyzing domestic and overseas research situations. By use of digital signal-processing methods as well as power system automation techniques, the research works pay more attention to the measurement theories and estimation algorithms of important indices. The main contributions of this dissertation are listed as follows:1)A new digital algorithm based on adaptive Short-Time Fourier Transform (STFT) is presented to estimate some basic power quality indices, such as voltage deviation, frequency deviation, voltage unbalance degree and power factor of fundamental component. The algorithm employs fixed sampling period although the frequency of the signal varies. No recursive calculation is needed to get evaluation values of above mentioned parameters. As a result, the algorithm is easy to implement with less computation burden and short response time. To restrain the spectrum leakage of harmonics, a novel kind of window called Rectangle Self-Convolution Window (RSCW) is adopted and the window length is adjusted according to the frequency variation of the signal. It is thus suitable for real-time measuring with high presicion.2)According to the spectrum characteristics of harmonics/inter-harmonics, an approach is proposed to compute harmonics/inter-harmonics in real time. Considering the characteristic of sine/cosine function, the harmonic component can be transformed into DC component, the rms value and initial phase angle of harmonic then can be got after low-pass filtering. As to inter-harmonics, it is generally considered to be a challenge because the inter-harmonics only have small ratio, it is hard to estimate them accurately. By subtracting fundamental and harmonic components from the original signal, maximum spectrum searching technique is used to compute frequency, rms value and initial phase angle of inter-harmonic. The proposed approach is easy to implement with high precision and good dynamic tracking feature, it suits to estimate harmonic/inter-harmonic parameters of voltage or current signals with discrete spectrum.3)It is known that flicker phenomena are closely related with harmonics and inter-harmonics, but at present, the detection methods of harmonics/inter-harmonics and flicker are completely different. Based on deducing the equivalent relation of inter-harmonic and amplitude modulation wave to flicker, the detection methods of inter-harmonic and flicker are combined. Therefore, according to national and International Electrotechnical Commission (IEC) standard, a new flicker evaluation algorithm is presented based on inter-harmonic estimation. The algorithm can not only compute instantaneous visual acuity, short-term flicker severity index and long-term flicker severity index, but also provide frequency and amplitude imformation of voltage fluctuation, thus, it benefits to analyze and manage voltage fluctuation and flicker. Furthermore, it provides a novel implemental model for flicker detection.4)To detect and locate non-steady-state disturbances, digital image processing theory and image edge morphology detecting method are introduced. The outstanding features of the method are easy implementation, less computing complexity, high time resolution and low demand of sampling frequency, therefore, it can be used to effectively detect common power disturbances such as voltage sag, swell, short-time interruption, short-time high frequency oscillatory, notching , etc.5)Deep analysis reveals that restrain background gradient is the key step for disturbing detection. Based on examining the origin of background gradient, two different methods for restrain background gradient are proposed. One uses Top-Hat morphology transform to restrain the background gradient produced by normal periodic alternation of analyzed signal, the other constructs suitable morphology filters to restrain the background gradient introduced by noises. Thus, the detecting sensitivity can be improved obviously.6)A new disturbing detection method based on order morphology and Top-Hat transform is then presented. It can not only restrain the background gradient produced by steady variation of analyzed signal, but also resist noises while keep the partial details of signal's abrupt change. Therefore, it ensures the sensitivity and time locating precision of distrurbing detection.7)An improved single phase voltage sag dececting algorithm is put forward. A new matrix, which is designed to decrease the phase delay that is unavoidable in traditional fictitious three phase systems, is constructed. The improved algorithm can almost instantaneously quantify the magnitude, duration and angel jump for voltage sag in single phase systems. Since'abnormal variation'phenomenon presents in the detecting process, a corresponding solving measure is carried out based on morphology calculation.8)As to three phase unbalanced voltage sag, an improved detection method is also presented. After carrying out positive and negative dq transform, morphology low pass filter which possesses the superiority of high accuracy and fast dynamic performance compared with traditional Butterworth filter is used to obtain positive-sequence fundamental component and negative-sequence fundamental component, and the zero-sequence component is obtained by using improved single phase voltage sag detection method, then, voltage sag characteristics can be got easily. The improved method can estimate the sag parameters with high accuracy in real time. Therefore, it is not only efficient for the real-time detection of voltage sag but also effective for the real-time voltage sag control and compensation in dynamic voltage restorers(DVRs).The purpose of the dissertation is to form an arithmetic system that related to some main power quality problems. It will provide effective theorem basis and algorithms for power quality on-line detection and off-line analysis. The validity of the proposed algorithms has verified by normorous simulations, and the algorithms have been successfully applied in the development of a practical device named'the VI-based Electric Parameter Measurement Device'.