Si Wenrong

Digital detection, grouping and classification of partial discharge signals at DC voltage

In this paper, a digital dc PD pulse detection system with bandwidth of 10 kHz - 40 MHz is introduced, which was developed using some artificial intelligence methodologies. Focus is made on digital detection, grouping and classification of random pulse signals generated by PD phenomena at dc voltage. Digital detection is developed only resorting to a band-pass filter, a high-speed digitizer (100 MS/s) and a PC with data processing software. Grouping is realized with feature extraction of pulse waveshapes using equivalent time-frequency method (ETFM), making the 2D parameters plane or 3D parameters space, then using the unsupervised clustering Fuzzy C-Means (FCM) method to achieve fast separation for pulse sequence. And classification resorts to least square support vector machine (LS-SVM) based on a fingerprint, which is derivate form 2D histograms of basic parameters, the discharge magnitude q and the time between discharges Deltat of each sub-group. Field application is made for typical defects of oil-paper insulation under dc voltage. At last, several methods to improve separability of the grouping technique are also given for some special cases, including threshold value grouping, marginal coordinates grouping based on 2D parameters plane and grouping using ETFM preprocessed by wavelet denosing. Experimental results show that the dc PD detection system developed with artificial intelligence methodologies is practical and effective.

PDC and PD Pulse Burst Characteristics in differently aged Transformer Oils

PDC (polarization/depolarization) and PD (partial discharge) characteristics of three typical transformer oils, having differently aged level, were examined using PDC test and PD test under AC conditions. PDC test shows that with the aging level increased that impurity density and conductance of oil is increased, the bound charge in oil wills more and more. Partial discharges, which take place in insulating oils for electric field concentration, appear in the form of pulse bursts that consist of a series of discrete current pulses. The first pulse of pulse burst is due to the initial streamer in the liquid phase, and all subsequent pulses will be the PD pulse responses in the growing cavity. The time interval between the first and second pulses is the form time of cavity. The number of pulse within burst as function of voltage and time interval between first and second discrete pulses within the PD pulse burst as a function of voltage are studied in this paper.

Study on time-frequency characteristic of PD Pulse using Wigner-Ville Distributions

According to the non-stationary of PD pulse signal, the Joint Time-Frequency Analysis (JTFA) is put forward to process the signal to obtain their characteristics in time-frequency domain. At present, the Wigner-Ville Distribution (WVD) which belongs to the Cohen class Time-Frequency Energy Distribution is popular in processing the non-stationary signals. So the simulation PD pulse is chosen as the subject investigated to study on the original WVD, Smoothing-Pseudo WVD (SPWVD) and Rearranged-Smoothing-Pseudo WVD (RSPWVD) in this paper. The results show that WVD provides a high sensitivity to variation in short duration of non-stationary PD signal, which is available to extract the time-varying information. And the SPWVD can remove the interference cross-terms with reducing the time-frequency resolution, which is caused by bilinear function. For having good time-frequency concentration, the RSPWVD provides a powerful tool to evaluate parameters for single PD pulse shape.

Research on Field Test of Impulse Grounding Impedance for Small Substation

In this paper, application of a portable instrument to obtain impulse grounding impedance (IGI) characteristics for grounding grid of small substation is introduced. The test currents of 8/20 µs, 10/350 µs and 30/80 µs were chosen as the injection signal sources to the grounding grid respectively with simultaneous measurement of the response transient voltage waveshapes in time domain. The portable instrument and test object for experimental tests are simply described. Field tests were carried out to the grounding grid in a 35 kV substation for 3 consecutive times with different impulse currents, while the corresponding IGI calculated with processing of the impulse current and voltage waveshapes are also given. All of that provides a reference for basic research of lightning protection for the small substation and a better understanding of transient characteristics of grounding grid under the lightning stroke.

A Subnanosecond High-voltage Pulse Generator used in UHF PD Experiment for Power Transformer and GIS

Subnanosecond pulse generators, which provide kilovolt pulse, are required for applications such as pulsed lasers, electro-optical devices, electron heating of plasmas, and bioelectric. A compact high-voltage subnanosecond generator is described in this paper. The generator includes a high-voltage subnanosecond discharge tube. It can generate electrical pulse of less than 5 ns duration; rise time is less than 500 ps and amplitude is higher than 4 kV. The low cost of the generator components and the compact size make these devices easy to build and convenient to use in subnanosecond pulsed power experiments. It also can use as UHF (ultra high frequency) source in the UHF detection in GIS and power transformer testing.

A New Approach to Extract PD Pulse using Independent Component Analysis

Assumption, ambiguities, and solution of the basic independent component analysis (ICA) model are briefly introduced in this paper. According to the leptokurtic distribution of PD pulse magnitude and other practical conditions, it indicates that there is a feasibility for the basic ICA model used to extract PD pulse signal under strong white noise. Simulation tests for PD pulse extraction using UWB detection technology with two channels have been done. And how to eliminate the ambiguities for the basic ICA model to extract PD pulse signal under DC voltage is also presented. The results show PD pulse can be exactly extracted from the strong white noise with keeping PD pulse amplitude of the relative relationship and the maximum amplitude corresponding to the time point, and other important information of the PD pulse sequence.