Ultra-high Frequency Signals Research Articles

Optimization of wavelet and thresholding for partial discharge detection under HVDC

With the rapid development of HVDC technology, the detection and analysis of partial discharge (PD) under HVDC are new challenges to ensure reliable operation of the related power apparatus. The wavelet technique has been proposed for analyzing PD pulses under HVAC and ultra-high frequency signal, but its application for PD under HVDC has not been discussed. This paper dealt with the selection of the optimal wavelet and thresholding for PD pulses in order to apply the wavelet technique to PD detection under HVDC. Four electrode systems, namely protrusion on conductor, protrusion on enclosure, free particle, and crack inside spacer were fabricated to simulate typical defects in a gas insulated switchgear. The detected PD pulses were decomposed by multiresolution analysis. The correlation coefficient and dynamic time warping methods were used to select the optimal wavelet. The optimal threshold and thresholding function were chosen from various combinations with the simulated pulses. The results revealed that processing PD pulses with the mother wavelet of bior2.6, automatic threshold, and intermediate thresholding function presented the best performance.

Understanding the dielectric properties of pressboard material thermally aged in Dibenzyl Disulphide (DBDS) included transformer oil

Surface Discharge Inception Voltage (SDIV) of unaged and copper sulphide diffused Oil Impregnated Pressboard (OIP) material was studied by adopting Ultra High Frequency (UHF) technique. Copper sulphide diffusion into OIP material was carried out by thermal aging of pressboard material in Dibenzyl Disulphide (DBDS) added transformer oil. The SDIV reduces with increase of thermal aging temperature. The current pulse injected in positive and negative half cycle during the surface discharge of the AC shows the rise time of 0.8ns and 1ns respectively. The FFT analysis of UHF signal measured during surface discharge process has shown frequency content in the range 0.7–1.2 GHz with its dominant frequency at 0.9 GHz. The life of the charge presented on the pressboard increases with thermal aged specimen. Energy dispersive spectroscopy (EDS) analysis of OIP material indicates presence of copper and sulphur content. Frequency Domain Dielectric Spectroscopy (FDDS) studies indicate the permittivity(er), dissipation factor (tan(δ)), conductivity (σdc) increases and relaxation time (τ), coefficient (α) decrease with OIP material (in air / Nitrogen) aged at higher temperature in DBDS added transformer oil. Cole-Cole double relaxation model was used to understand the dielectric parameters of copper sulphide diffused OIP material. The DC conductivity of pressboard material aged in DBDS included transformer oil follows Arrhenius law and the activation energy decreases with increase in temperature of thermal aging.

Localization of multiple partial discharge sources in air-insulated substation using probability-based algorithm

Ultra-high-frequency (UHF) sensing technique has been introduced to detect and localize partial discharge (PD) sources in air-insulated substation (AIS). This paper presents a probability-based algorithm to localize multiple PD sources which may occur simultaneously in different power equipment. Assuming that the time difference of arrival (TDOA) between all pairs of antennas in a array are normally distributed, the probability density function (PDF) of PD source coordinates can be obtained by substituting the linearized form of time difference equations into PDFs of TDOAs. When large number of PD signals are recorded, the joint PDF (JPDF) can be calculated from the product of PDF of each TDOA. Then the PD coordinates to be solved are regarded as with highest probability, and can be solved by taking the derivative of JPDF. In the case of multiple PD sources, mixed UHF signals are separated by clustering the TDOA vectors with K Means clustering method. PD experiments are performed to test the presented algorithm, and the localization accuracy of proposed algorithm is compared with other typical methods such as Newton-Raphson, Particle Swarm Optimization and plane intersection method. The results indicate that the probability-based localization algorithm reasonably integrates the TDOAs of continuous signal sequence, which can effectively reduce the influence of TDOA estimation errors and improve the localization accuracy.

A Novel Method for Separating and Locating Multiple Partial Discharge Sources in a Substation.

To separate and locate multi-partial discharge (PD) sources in a substation, the use of spectrum differences of ultra-high frequency signals radiated from various sources as characteristic parameters has been previously reported. However, the separation success rate was poor when signal-to-noise ratio was low, and the localization result was a coordinate on two-dimensional plane. In this paper, a novel method is proposed to improve the separation rate and the localization accuracy. A directional measuring platform is built using two directional antennas. The time delay (TD) of the signals captured by the antennas is calculated, and TD sequences are obtained by rotating the platform at different angles. The sequences are separated with the TD distribution feature, and the directions of the multi-PD sources are calculated. The PD sources are located by directions using the error probability method. To verify the method, a simulated model with three PD sources was established by XFdtd. Simulation results show that the separation rate is increased from 71% to 95% compared with the previous method, and an accurate three-dimensional localization result was obtained. A field test with two PD sources was carried out, and the sources were separated and located accurately by the proposed method.

UHF Signal Processing and Pattern Recognition of Partial Discharge in Gas-Insulated Switchgear Using Chromatic Methodology.

The ultra-high frequency (UHF) method is widely used in insulation condition assessment. However, UHF signal processing algorithms are complicated and the size of the result is large, which hinders extracting features and recognizing partial discharge (PD) patterns. This article investigated the chromatic methodology that is novel in PD detection. The principle of chromatic methodologies in color science are introduced. The chromatic processing represents UHF signals sparsely. The UHF signals obtained from PD experiments were processed using chromatic methodology and characterized by three parameters in chromatic space (H, L, and S representing dominant wavelength, signal strength, and saturation, respectively). The features of the UHF signals were studied hierarchically. The results showed that the chromatic parameters were consistent with conventional frequency domain parameters. The global chromatic parameters can be used to distinguish UHF signals acquired by different sensors, and they reveal the propagation properties of the UHF signal in the L-shaped gas-insulated switchgear (GIS). Finally, typical PD defect patterns had been recognized by using novel chromatic parameters in an actual GIS tank and good performance of recognition was achieved.

Antipodal Vivaldi Antenna to Detect UHF Signals That Leaked Out of the Joint of a Transformer

In recent years, extensive research has been conducted on the ultrahigh-frequency (UHF) method. Considering that acquiring a partial discharge UHF signal is the first step in a series of tasks, such as fault diagnosis and defect location, the UHF sensor plays an important role in the UHF method. An antipodal Vivaldi antenna installed at a transformer’s joint is designed in this study according to the structural features of 750 kV transformers in China. Several effective structures, such as linear gradient microstrip and slot edge, are employed to improve the return loss and radiation characteristics. A metal box is designed after analyzing the influence of the metal around the antenna. The metal box can effectively shield against electromagnetic interference and does not deteriorate the performance of the antenna significantly. Experimental data show that this antenna is much more sensitive than the rectangular plane spiral antenna. The proposed antipodal Vivaldi antenna is suitable for detecting partial discharge in large transformers.

Understanding Corona discharge activity in titania nanoparticles dispersed in transformer oil under AC and DC voltages

Titania (TiO2) nanoparticles dispersed transformer oil has high Corona Inception Voltage (CIV) and breakdown strength, and gets further enhanced on insertion of the barrier in the electrode gap. The volume of nanoparticles and the amount of surfactant have a strong influence on the dispersion of nanoparticles in transformer oil and were characterized through viscosity measurement, particle size analysis and by zeta potential measurements. It was also observed that with needle plane configuration, negative DC voltages have high breakdown voltage as compared to positive DC and AC voltages. Improvement in breakdown voltage with barrier can be obtained by placing the barrier at 20–80% of electrode gap from the tip of the high-voltage electrode. The breakdown voltage of needle plane electrode configuration was calculated based on normal distribution parameters. It was observed that the corona activity generates Ultra High Frequency (UHF) signal in the frequency range of 0.7–2 GHZ with its dominant frequency at 0.9 GHz and was characterized using a ternary plot. The discharge activity was studied by using a spectrum analyzer by operating it in zero span mode with 0.9 GHz as center frequency. It was observed that for a given voltage under AC and DC voltages, the magnitude of discharges is less with nanoparticles dispersed transformer oil. A pyrolysis study indicated no variation in the composition of the oil with the addition of titania. It was observed that the esters present in the oils were alkyl substituted oxalic acid and alkyl substituted sulfurous acid esters.

A Coplanar Vivaldi-Style Launcher for Goubau Single-Wire Transmission Lines

In this letter, we present a coplanar Vivaldi-style launcher for single-wire transmission lines (SWTLs), sometimes called Goubau lines. The launcher converts a transverse electromagnetic mode (TEM) mode from a coaxial feed port to the transverse magnetic (TM01) surface-wave mode supported by the SWTL. The original Goubau design for SWTL launchers uses large horns to enforce the mode conversion. In contrast, the proposed coplanar Vivaldi-type launcher comprises a planar structure with dimensions of 100 mm × 150 mm, which is more convenient to install on existing infrastructure, such as power lines, communication wires, structural steel cables, or pipes. The proposed launcher can be used to provide a broadband communication channel for ultrahigh frequency and microwave radio-frequency signals from 0.81 to 2.29 GHz. It has a measured 10 dB return loss bandwidth of 1.39 GHz. The measured line loss with an 8 mm diameter SWTL conductor is approximately 0.19 dB/m at 1.5 GHz, which is comparable to high-quality coaxial cables. We have also shown that multiple launcher pairs can be used back-to-back on the same SWTL conductor to provide isolated communication links. With a launcher separation distance of 250 mm, the measured back-to-back isolation exceeds 30 dB.

U-Shaped Ultrahigh Frequency Atmospheric Pressure Plasma Jet With Magnetic Loop Antenna

An atmospheric pressure argon plasma was generated using a magnetic loop antenna with a pulse modulated ultrahigh frequency signal of 850 MHz. Finite-element simulations were performed to study the patterns of gas flow and electromagnetic waves. Fast photography of the plasma jet was used to study temporal behavior of the plasma jet. Emission and mass spectrometry were used to identify active species in the plasma jet. The detected dominant species included excited argon atoms and singly charged ions of argon, nitrogen, oxygen, nitrogen oxide, and water. To study the effect of the plasma jet on biological samples, a thin onion epidermal membrane was treated with the plasma jet. The hydrophobic layer displayed hydrophilic characteristics after treatment for 1 min.

Propogation characteristics of PD‐induced UHF signal in 126 kV GIS with three‐phase construction based on time–frequency analysis

Partial discharge (PD) detection of the gas-insulated switchgear (GIS) is an effective method for substation security. The GIS with three-phase construction is widely used for substations. The propagation characteristics of PD-induced ultra-high-frequency (UHF) signal in three-phase GIS is investigated in this study by time–frequency analysis method. The variation of peak-to-peak value (V pp) and cumulative energy, and especially the energy density in the time–frequency plane are used to represent the feature of the UHF signal. Furthermore, the different situation of the insulation defects and the detection point positions are compared, and the reason for the difference is explained as well. As a result, this study identified a novel method to determine the PD source position in the GIS with three-phase construction.

Classification and separation of partial discharge ultra‐high‐frequency signals in a 252 kV gas insulated substation by using cumulative energy technique

Ultra-high-frequency (UHF) method is regarded as an effective approach for partial discharge (PD) detection in gas insulated substation (GIS). The feature parameters representing UHF signal characteristics can be extracted from its waveform, and applied to PD source classification and mixed signals separation. In this study, an UHF signal feature extraction algorithm based on cumulative energy (CE) technique is proposed. The CE functions in time domain (TCE) and frequency domain (FCE) are calculated from UHF waveforms and their fast Fourier transform spectrums, respectively. The mathematical morphology gradient (MMG) operation is applied to TCE and FCE to characterise their rising behavior. The feature parameters including width, area and sharpness are extracted from CEs and MMGs in both time and frequency domain. Moreover, the extracted features are applied to UHF signals classification and mixed signals separation. First, experiments on four kinds of typical defects in a 252 kV GIS are performed, and the classification performance of the extracted features is examined with the acquired signals. Second, by clustering the extracted features with fuzzy maximum-likelihood algorithm, mixed UHF signals originated from multiple PD sources are successfully separated. The results indicate that the proposed features are effective for representing UHF signal characteristics.

Structure and spectroscopic analysis of the graphene monolayer film directly grown on the quartz substrate via the HF-CVD technique

Direct growth of a monolayer graphene film on a quartz substrate by a hot filament chemical vapor deposition technique is reported. The monolayer graphene film prepared was characterized by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), selected area electron diffraction (SAED), and atomic force microscopy (AFM). The optical properties were studied by spectroscopic elliposmetry. The experimental data were fitted by the Forouhi-Bloomer model to estimate the extinction coefficient and the refractive index of the monolayer graphene film. The refractive index spectrum in the visible region was studied based on the harmonic oscillator model. The lattice dielectric constant, real and imaginary dielectric constants and the ratio of the charge carrier number to the effective mass were determined. The surface and volume energy loss parameters were also found and showed that the value of the surface energy loss is greater than the volume energy loss. The determination of these optical constants will open new avenue for novel applications of graphene films in the field of wave plates, light modulators, ultrahigh-frequency signal processing and LCDs.

Investigation of partial discharge between moving charged metal particles and electrodes in insulating oil under flow state and AC condition

Theoretical analysis and experimental study were conducted to clear the partial discharge (PD) between moving charged metal particles and electrodes in insulation oil under flow state and AC condition. The rise time of the PD pulse current is tens of nanoseconds, which is not affected by velocity and voltage. The dominant frequency component of the ultra-high-frequency (UHF) signal due to charged metal particle movement is near 0.39 GHz, which is independent from velocity and voltage. The magnitudes of PD UHF signals are discontinuous and the time of occurrence of PD depends on the trajectories of charged metal particles. PD &-U-N maps clearly indicate that the phase information of the PD UHF signal on the maps is similar regardless of flow velocity and AC condition, but the voltage intensifies the PD and the flow velocity weakens the PD. The trajectories of charged metal particles in the horizontal transformer oil tract were investigated through stress analysis of charged metal particles under flow state and AC voltage. When charged metal particles move along the oil tract under the oil flow, reciprocating oscillation simultaneously occurs in the vertical direction.

Application of SA-SVM Incremental Algorithm in GIS PD Pattern Recognition

With changes in insulated defects, the environment, and so on, new partial discharge (PD) data are highly different from the original samples. It leads to a decrease in on-line recognition rate. The UHF signal and pulse current signal of four kinds of typical artificial defect models in gas insulated switchgear (GIS) are obtained simultaneously by experiment. The relationship map of ultrahigh frequency (UHF) cumulative energy and its corresponding apparent discharge of four kinds of typical artificial defect models are plotted. UHF cumulative energy and its corresponding apparent discharge are used as inputs. The support vector machine (SVM) incremental method is constructed. Examples show that the PD SVM incremental method based on simulated annealing (SA) effectively speeds up the data update rate and improves the adaptability of the classifier compared with the original method, in that the total sample is constituted by the old and new data. The PD SVM incremental method is a better pattern recognition technology for PD on-line monitoring.

Combined Diagnosis of PD Based on the Multidimensional Parameters

This paper presents a comprehensive multiparameter diagnosis method based on multiple partial discharge (PD) signals which include high-frequency current (HFC), ultrasound, and ultrahigh frequency (UHF). The HFC, ultrasound, and UHF PD are calculated under different types of faults. Therefor the characteristic values, as nine basic characteristic parameters, eight phase characteristic parameters, and the like are calculated. Diagnose signals are found with the method based on information fusion and semisupervised learning for HFC PD, adaptive mutation parameters of particle entropy for ultrasonic signals, and IIA-ART2A neural network for UHF signals. In addition, integrate the diagnostic results, which are the probability of fault of various defects and matrix, of different PD diagnosis signals, and analysis with Sugeno fuzzy integral to get the final diagnosis.

Electrical treeing in XLPE cable insulation under harmonic AC voltages

The shape of electrical trees gets altered due to applied voltage profile. Bush type electrical trees forms under harmonic voltages with lower THDs. With increase in harmonic level with increased THDs fibrillar type electrical trees were formed. Weibull distribution studies indicate that increase in harmonic level (odd/even) with higher THDs accelerates the tree growth in cable insulation causing early failure of cable insulation. The frequency content of the Ultra High Frequency (UHF) signals radiated due to tree formation lies in the frequency band of 0.5-2 GHz. Adoption of spectral power ratios to UHF signal can classify the shape of electrical trees that has incepted and propagating in the insulating material under different voltages. Phase Resolved Partial Discharge (PRPD) analysis using UHF signal indicates that discharges occurs under harmonic voltages where the rate of rise of voltage is high thereby accelerating the tree growth. During tree growth discharges occur in the entire phase of the triangular voltage. Partial discharges occur at the peak with the low frequency AC voltage and at the raising portion when the supply voltage is increased.

Location of partial discharges sources by means of blind source separation of UHF signals

Partial discharges (PD) detection is a widely extended technique for the diagnosis of electrical equipment. Ultra-high frequency (UHF) detection techniques appear as the best choice if the goal is to detect PD online and to locate devices with insulation problems in substations and overhead lines. The location of PD is based on the determination of the difference of the time of arrival of electromagnetic pulses radiated by a source of PD to an array of antennas distributed around the monitored area. However, when measuring electromagnetic pulses radiated by PD activity many interfering signals, such as those coming from television (TV), global positioning system (GPS), wireless communication signals and others coming from electrical equipment distort the waveform detected by the sensors. Under these circumstances, the application of traditional techniques to estimate the time differences may fail. In this paper, the use Blind Source Separation (BSS) techniques applied to pairs of UHF sensors is proposed to extract the information of the difference of the time of arrival of the electromagnetic pulses radiated by a source of PD. The paper is focused on the application of the algorithm and the description of an experimental setup for controlled generation and detection of PD to verify the performance of the proposed technique.

Electrical treeing in XLPE cable insulation at cryogenic temperature under harmonic AC voltages

The shape of electrical trees formed in XLPE cable insulation at low temperature under AC and harmonic voltages are fibrillar type. Life of cable insulation failure due to electrical treeing was analysed through Weibull distribution studies. It is observed that the voltage wave shape and peak factor exhibit high influence on the life of cable insulation due to electrical treeing. Harmonics with higher THD’s and the failure rate of cable insulation due to treeing are the same. The presence of 4th and 5th harmonics in supply voltage could cause early failure of cable insulation. Ultra High Frequency (UHF) signal radiated during tree growth process, under harmonic voltages lies in the range 0.5–1.5GHz. Phase Resolved Partial Discharge (PRPD) analysis using spectrum analyser has revealed that under 50Hz, discharges occur near zero crossing and in the presence of harmonics, it occurs when the rate of voltage rise is high. The discharge occurs at its peak under low frequency AC voltage of 1Hz.

Investigation of frequency characteristics of typical PD and the propagation properties in GIS

The secure operation of Gas Insulated Switchgear (GIS) is crucial for the bulk power systems, since the failure of GIS apparatus would cause severe interruption of power transforming and power supply at substations. Therefore, the monitoring and maintenance schemes of GIS devices have gradually taken the reliability-based indices into account, of which Partial Discharge (PD) levels in online monitoring are of substantial significance because PD serves as a major cause of insulation break-down. Ultra High Frequency (UHF) method is becoming widely adopted as the major inspection approach for PD in GIS. This paper presents the experiment-based results of the frequency characteristics of UHF signals caused by PD in typical defects using entire GIS operating system platforms. Specifically, the results covered the propagation attenuation of the UHF signals with peak to peak value and cumulative energy through typical structures, including basin-type insulators, L-shaped branch, isolating switches, current transformers, etc. The results serve as substantial criteria for optimal band width selection for high frequency detection, and as notable concerns to improve the sensitivity of UHF detection and the efficiency of online monitoring by effectively locating the external sensors for PD UHF detection.

A Novel Algorithm for Separating Multiple PD Sources in a Substation Based on Spectrum Reconstruction of UHF Signals

Field noise interference suppression and effective extraction of signal characteristics are two keys to partial-discharge (PD) signal detection and analysis. In this paper, the autoregressive moving average (ARMA) process is utilized to model ultra-high-frequency (UHF) signals radiated by PDs. The estimation, which is based on high-order cumulants, of the ARMA orders and parameters is given theoretical analysis and implementation. The spectra of the detected signals are reconstructed with the model estimated. Then, characteristic frequencies are selected based on the reconstructed spectra and Fisher-like class separation measures. The radial basis function neural network is trained for the separation of the observed signals. Using the proposed method, UHF signals generated by electromagnetic simulation software are efficaciously modeled, reconstructed, and separated from mixing Gaussian white noises of varying signal-to-noise ratios and fixed-frequency signals. Finally, the step to obtain the number of PD sources in the assumed substation is proposed. UHF signals collected in a substation are processed by the proposed procedure, and PD sources are separated. This separation result is compared with PD sources localization results calculated by the time delay sequence, and the effectiveness of the method in the substation field interference circumstances is verified.