Li Junhao

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.

Study of time delay of UHF signal arrival in location partial discharge

In the study of location partial discharge in transformer based on the UHF method, in order to accurately locate partial discharge source, precisely achieving time delay of different signals which are received by different UHF sensors and radiated from the same partial discharge source is essential. Three methods of time delay estimation are introduced and compared, including the initial peak, the inflection point of cumulative energy curve, cross correlation, and the measures of signal de-noising based on the wavelet transform and cubic splines interpolation are introduced to optimize the methods mentioned above. Then the pin-to-flat electrode model and the wedge model are adopted respectively to experiment these methods. Results demonstrate that absolute error between the time delay estimation through optimized cross correlation and the time delay in theory, which is less than 30 ps, is the least among the errors through other methods mentioned above, and the optimized cross correlation analysis can achieve the highest precision among other methods and can be used to accurately locate partial discharge source.

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.

Severity diagnosis and assessment of the partial discharge provoked by protrusion in GIS

It is a common partial discharge phenomenon that is provoked by metal spikes on the surface of the high voltage bus bar of the GIS (Gas Insulated Switchgear). To measure the partial discharge based on UHF(Ultra High Frequency) under frequency voltage is also a very common field test project. In order to diagnose and assess the severity levels of the PD (partial discharge) provoked by metal spikes on the surface of the high voltage bus bar of the GIS, with the application of gradually increased voltage, long-term tests were conducted on a well-established 363 kV GIS experiment platform to observe the entire evolution process of PD from its very initiation till breakdown. And this paper also captures the PRPD and PRPS spectrograms during the tests from initial stage to breakdown stage. The results of this paper provide an effective rule to diagnose and assess the severity levels of the PD provoked by metal spikes on the surface of the high voltage bus bar of the GIS.

Research on partial discharge characteristics of protrusion defect in GIS under oscillating lightning impulse voltage

In order to ensure the safe operation of the gas insulated switchgear (GIS), on-site impulse withstand voltage test is becoming more and more important. Oscillating impulse voltage is very suitable for the field test due to its high generating efficiency. By detecting the partial discharge when conducting on-site impulse withstand voltage test, the insulation state of GIS can be overall known. The protrusion is a typical aPublication Event Topic :nd harmful defect in GIS, so it is very significant to research the partial discharge characteristics of protrusion in GIS under oscillating lightning impulse voltage. In this paper, we applied oscillating lightning impulse voltage on a 330kV GIS which was set a protrusion at the conductor and we measured the partial discharge by the inner UHF sensor at the same time. We analyze the variation of discharge position, maximum discharge amplitude and discharge number with the increasing of voltage and compare the results of different frequencies voltage. Our studies show that the partial discharges mainly occur at the peak of the oscillating lightning impulse voltage. With the increasing of the voltage amplitude, the maximum discharge amplitude and discharge number also increase. And with the increasing of the voltage frequency, the partial discharge inception voltage increases. The results of this paper are helpful for the on-site impulse withstand voltage test.

Simulation study of E-M wave propagation characteristics in GIS model considering isolating switch

In order to make more profound comprehension of propagation characteristics for E-M signals from partial discharge in GIS, the GIS models with isolating switch are creation to analysis the E-M wave propagation characteristics using FDTD method. The influences of the isolating switch and difference receive degree on the propagation characteristics of E-M wave with comparison of isolating switch close and cut are examined in this paper. As a result, this study identified that the crest value of received signal is decrease but the whole energy is little change as the excite source becomes more distant. The energy reflection and dispersion in isolating switch fracture resulted in a decrease in the crest value and energy.

Measurement of partial discharge pattern on typical defects in GIS

Gas insulated switchgear (GIS) is the most important equipment in the power grid. Especially, for insulation failure, the monitoring, analysis and pattern recognition technology of partial discharge in GIS is the most important. Thus a 220 kV GIS bus bar cavity was taken as an object to design three PD physical models. The PD signals on different defects were detected with the IEC60270 method. The effects of the applied voltage on the center-phase distribution and average amplitude of the discharge pulses were investigated. The experimental results indicated that the phase-resolved PD (PRPD) patterns, average amplitude and central phase changing of PD signals had very similar distribution characteristics and those of different defect had different ones. The results provide a test basis for the recognition of PD types and a deeper study of the mechanism of PD.

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.