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Aiming at a fault on service duty test of vacuum on-load tap changer (OLTC), the principle of the resonance test method is analyzed. Combined with the voltage and current waveform measured and the standard switching sequence, the cause and process of the test fault are qualitatively deduced. Based on the theoretical analysis, the simulation model of the test circuit and fault is built based on PSCAD. The voltage and current waveforms obtained by simulation are highly consistent with the test waveforms. The error of peak current between the simulation and the test waveform is 0.75%. The simulation results show the accuracy of the theoretical analysis and modeling calculation, they are verified by the follow-up inspection results of OLTC and vacuum tube solution. This paper provides a powerful technical means for fault analysis and fault mechanism research in the follow-up OLTC operation. It also has reference and guiding significance for the quality control, design, operation and maintenance, and monitoring of vacuum OLTC.

Insulation of valve-side windings in converter transformer withstands pulsating voltages, which will produce more serious insulation problems. In this paper, the electric breakdown experiments of oil-paper insulation specimens were executed at pulsating voltages and different temperatures. Experiment and analysis results showed that the breakdown voltage decreased with increasing temperature under pulsating voltage. The influence of temperature proves to be more significant once the temperature exceeds a limitation threshold. A fitting formula between breakdown voltage and the temperature was reported. Finally, in order to clearly understand the breakdown properties under pulsating voltage, the electric field distribution and space charge behavior under pulsating voltage at different temperature were discussed.

The oil-paper insulation of valve-side windings in a high voltage direct current (HVDC) converter transformer is subjected to both complicated voltage and severe thermal stresses. Partial discharge (PD) occurring in oil-paper bounded cavities is influenced by voltage stress type as well as temperature. This paper presents PD behaviors in an oil-paper bounded gas cavity under pulsating DC voltages. The voltage across the gas cavity and PD inception voltages at different test conditions are analyzed. Influences of temperature on statistical PD distributions and parameters are also presented. A numerical model is used to interpret PD behaviors under pulsating DC voltages and various temperatures. Experimental results and theoretical statements show that the temperature exhibits a great influence on the behaviors of cavity discharge under pulsating DC voltages. Volume resistivity, gas pressure, charge decay time constant, and electron generation rate are the primary parameters influencing PD behaviors at different temperatures.

This paper presents an experimental work investigating the failure data of oil-impregnated paper under pulsating DC voltage. The failure data of the oil-impregnated paper specimens were obtained through constant voltage stress tests; the Weibull distribution was used to estimate the statistical distribution of failure data. The exponential function (EF) and the inverse power function (IPF) were used to modify the Weibull distribution of failure data, so as to obtain two dual-variable Weibull failure distributions (WFDs), which were used to analyze failure data of oil-paper specimens obtained under pulsating DC and individual DC and AC voltages. Their adequacies were evaluated by using failure data obtained through the constant stress tests under various voltages. Both EF-modified and IPF-modified WFDs show more adequacy for failure data of oil-paper insulation under DC and pulsating DC voltages than AC voltages. The EF-modified WFD was more adequate than IPF-modified WFD in estimating the statistical distribution of failure data.

Oil-paper insulation of valve-side windings in converter transformers withstand electrical stresses combining with AC, DC and strong harmonic components. This paper presents the physical mechanisms and experimental researches on partial discharge (PD) of oil-paper insulation at pulsating DC voltage. Theoretical analysis showed that the phase-resolved distributions of PDs generated from different insulated models varied as the increase of the applied voltages following a certain rule. Four artificial insulation defect models were designed to generate PD signals at pulsating DC voltages. Theoretical statements and experimental results show that the PD pulses first appear at the maximum value of the applied pulsating DC voltage, and the resolved PD phase distribution became wider as the applied voltage increased. The PD phase-resolved distributions generated from the different discharge models are also different in the phase-resolved distributions and development progress. It implies that the theoretical analysis is suitable for interpretation of PD at pulsating DC voltage.

Converter transformers play important roles in high-voltage direct current transmission systems. This paper presents experimental and analysis results of the combined electrical and thermal aging of oil-impregnated paper at pulsating DC voltages. Breakdown voltages and time-to- breakdown of oil-paper specimens were measured by using short-time and constant-stress tests. The breakdown characteristics of combined electrical and thermal aging on insulation system were discussed. According to the relationship between failure time and aging temperature, the two- parameter Weibull model was improved. On the basis of the competing risk algorithm and the improved Weibull model, the two factors failure model was calculated. And the influence of temperature in the insulation system has been analyzed. This model performs better than the two- parameter Weibull model when both time and temperature are considered as variables in estimating the lifetime of oil-paper insulation.

The space charge behaviors of oil-paper insulation affect the stability and security of oil- filled converter transformers of traditional and new energies. This paper presents the results of the electrical aging of oil-impregnated paper under AC-DC combined voltages by the pulsed electro- acoustic technique. Data mining and feature extractions were performed on the influence of electrical aging on charge dynamics based on the experiment results in the first stage. Characteristic parameters such as total charge injection and apparent charge mobility were calculated. The influences of electrical aging on the trap energy distribution of an oil-paper insulation system were analyzed and discussed. Longer electrical aging time would increase the depth and energy density of charge trap, which decelerates the apparent charge mobility and increases the probability of hot electron formation. This mechanism would accelerate damage to the cellulose and the formation of discharge channels, enhance the acceleration of the electric field distortion, and shorten insulation lifetime under AC-DC combined voltages.

This communication proposed an ultra-high frequency (UHF) stacked Hilbert antenna array to broaden bandwidth and increase gain of Hilbert fractal antenna. Two parasitic curves were added to improve the third order Hilbert curve and stacked structure was applied to the new antenna element. The UHF stacked Hilbert antenna was designed as antenna array that involved 2 × 2 elements and fed by the Wilkinson's four-way power divider. Measurement results show that the UHF stacked Hilbert antenna array achieves a maximum gain of -1 dBi and exhibits a wide impedance bandwidth. Actual partial discharge (PD) experiments were carried out for typical artificial insulation defect models, while the proposed antenna array and an existing Hilbert fractal antenna were both used for PD measurements. The proposed antenna array was superior in this respect and effective for UHF online monitoring of PDs in electrical apparatus.

The oil-paper insulation in converter transformers is subjected to pulsating DC voltage, mainly consisting of AC and DC voltages. Surface partial discharges (PDs) occurred in converter transformer is significantly influenced by voltage type as well as moisture content in oil-impregnated paper insulation. The influence of moisture content on surface PD behaviors of oil-impregnated paper insulation is investigated at pulsating DC voltage in this work. Various changes of PD phase-resolved distributions, PD inception voltages and statistical feature parameters of PD at a range of moisture content will be presented. Experimental results make it clear that the moisture content shows a significant effect on the surface discharge behaviors of oil-impregnated paper insulation at pulsating DC voltage.

The oil impregnated paper is one of the typical composite structure containing a large number of cavities, which are filled with insulating oil through oil impregnation. In this paper, the oil-impregnated paper structure is simplified and an electrical breakdown model is developed to describe the breakdown progress in oil impregnated paper at pulsating DC voltage. Based on the traditional WZ model, the space discharge and physical time are introduced in the proposed model. The influence of temperature is also investigated. The skeleton types of discharge structure, discharge velocity, and charge movement during the breakdown are simulated. The breakdown simulation results show that the conductivity variation of insulation paper is an important factor which reduces the breakdown voltage under pulsating DC voltage at high temperature.

Oil-impregnated paper is widely used in power transmission equipment as a reliable insulation. However, copper sulphide deposition on oil-paper insulation can lead to insulation failures in power transformers. This paper presents the influences of copper sulfur corrosion and copper sulphide deposition on copper wires and oil-paper insulation in power transformers. Thermal aging tests of paper-wrapped copper wires and bare copper wires in insulating oil were carried out at 130 °C and 150 °C in laboratory. The corrosive characteristics of paper-wrapped copper wires and bare copper wires were analyzed. Dielectric properties of insulation paper and insulating oil were also analyzed at different stages of the thermal aging tests using a broadband dielectric spectrometer. Experiments and analysis results show that copper sulfide deposition on surfaces of copper wires and insulation paper changes the surface structures of copper wires and insulation paper. Copper sulfur corrosion changes the dielectric properties of oil-paper insulation, and the copper sulfide deposition greatly reduces the electrical breakdown strength of oil-paper insulation. Metal passivator is capable of preventing copper wires from sulfur corrosion. The experimental results are helpful for investigations for fault diagnosis of internal insulation in power transformers.

LDPE is one of the main power cable insulation materials, the temperature and voltage duration are the key factors to impact the lifetime of LDPE. Today study on the DC cable failure model is the key point. This paper adopts step-stress test to the LDPE samples in different temperatures. The voltage durations applied to the samples are 20s, 45s, 120s, 300s. The test results show the withstand voltage time of the LDPE samples during different temperatures and voltage durations. According to the test results, the parameters are calculated by using least square method. The Weibull distribution affected by temperatures, and the Weibull distribution affected by voltage durations are established respectively. On this basis, combining with the competing risk model, this paper establishes a DC cable failure model affected by temperatures and voltage durations. The failure model can effectively assess the insulation conditions of a DC cable.

The development of high voltage DC power cable is far lagging behind that of AC power cable partly due to the breakdown properties of DC cable insulation under the influence of multiple factors such as space charge and temperature. This paper presents an experimental research on electrical breakdown properties of LDPE at various temperatures to investigate the properties of DC cable insulation. A thermal aging chamber was designed for the breakdown experiment of LDPE at different levels of temperature. The breakdown experiments of LDPE were carried out and thus the influence of temperature and voltage type on the breakdown properties of LDPE was obtained. The results show that temperature has significant influence on the breakdown field strength of LDPE under DC voltage. The dispersion of breakdown field strength becomes bigger with the increase of temperature. In contrast, the influence of temperature on breakdown strength of LDPE insulation under AC voltage is relatively small.

This paper presented the improved failure model by experimental and analysis results on electrical breakdown characteristics of oil-paper insulation at pulsating DC voltages. The time-to-breakdown of oil-paper specimens were measured by using constant-stress tests. The improved failure model was established and calculated by the parameters of Weibull model and the improved competing risk algorithm. It optimizes the faults of traditional two-parameter Weibull distribution when both time and voltage are considered as variables in estimating the lifetime of oil-impregnated paper.

The converter transformer is one of the most critical equipment in high-voltage direct current (HVDC) transmission systems. Valve-side windings withstand complex stresses combined with AC, DC, and pulse voltages. Oil-paper insulation is used within most of HVDC converter transformers. Partial discharge (PD) in oil-paper insulation is a major reason for the development of internal faults in HVDC converter transformer. The current paper presents the development process for the PD of oil-paper insulation under pulsating DC voltage combined with AC and DC voltages at different temperature. In the experiments, the artificial insulation defect models were designed to generate PD signals under pulsating DC voltage detected by a Rogowski coil sensor. The Statistical histograms of φ-q-n based on these PD signals were constructed. The variations in initial discharge voltages and φ-q-n of the PDs with pulsating DC voltages generated from different temperature were analyzed. Experimental results showed that the initial discharge voltages and PD phase-resolved distributions of different temperature were different from one another. The findings of the current study are useful in further research on the mechanism and pattern recognition of PDs in converter transformers.

Atmospheric turbulence causes random fluctuation of terahertz beam, therefore results in bit error for data transmission. This paper studies the influence of turbulence scintillation on the Bit-Error-Rate (BER) of THz wireless communication system, gives the relationship between effective transmission distance and frequency under different BER performance. Research shows that the BER of spherical wave is less than that of plane wave in the same conditions; frequency and transmission distance causes important influence on the perfomance of BER; the effective transmission distance decreases along with frequency increasing in the case of a certain BER, and the effective transmission distance of spherical wave is longer than the effective transmission distance of plane wave under the same conditions.

Considering the two main factors which affect the terahertz band atmospheric refractive index structure parameter: temperature and humidity, this paper studies the influence on the refractive index structure parameter from temperature structure constant, humidity structure constant and joint structure constant of temperature and humidity by using the actual measured values in the infrared band, and shows the difference between the atmospheric refractive index structure parameter of THz band and that of optical band. Relationship between scintillation index and various parameters in the condition of zero-inner-scale or nonzero-inner-scale for plane wave and sphere wave is analyzed and discussed. Research shows that the THz scintillation effect is strong, and the scintillation index of plane wave is larger than the spherical wave; the inner scales of turbulence, frequency and transmission distance produces important influences on the scintillation index.

Valve windings of converter transformer withstand pulsating voltages. In order to reveal the relationship between the properties of space charge and electrical-thermal aging of oil-paper insulation in converter transformer, this paper did the electrical-thermal aging test at 70 °C under 7.6kV pulsating DC voltage and obtained the samples of different aging time. The internal space charge injection and accumulation properties of aged samples under 20kV/mm and 30kV/mm DC electric field was measured and analyzed by the Pulsed Electro-Acoustic (PEA) method. The results showed that homo-charges injection still occurs at both electrodes in the electrical-thermal aging samples. Aged oil-paper insulation was dominated by negative charge injection as the stressing time increases, which directly cause hetero-charge accumulation in the vicinity of the anode, and homo-charge accumulation near the cathode. It could help explain the reason of the electrical-thermal aging lifetime of oil-paper in sulation is obviously shorter than the electrical or thermal aging. In addition, the longer the aging time, the more the total charge accumulated in the samples.

With the improvement of voltage level of HVDC transmission, the insulation of converter transformer has become increasingly prominent. Different from the general power transformer, valve side windings on converter transformer withstand pulsating voltages, which makes insulation problem get more serious. In this paper, experiments were carried out under pulsating voltage over a wide range of the pulsating coefficients defined as p=Eac/Edc, which can obtain the relationship between p and breakdown voltage of oil-paper insulation. Experiment results showed that the breakdown voltages were regularly decreased by the increases of p, and the breakdown voltage value of pre-stressed DC-voltage is higher than that of pre-stressed AC-voltage, also the scatter of the pre-stressed DC-voltage is better. Moreover, the short-time breakdown tests of oil-paper insulation specimens were also carried out at different levels of temperature. The results showed that when the temperature exceeds a certain threshold, the temperature will show a significant impact on the breakdown strength. A great influence of temperature on the DC component of the breakdown voltage was observed, while the effect on AC component is relatively small. At last, this paper presents an expression which relates the breakdown strength to the temperature.

One of the major insulations of high voltage direct current (HVDC) converter transformers is oil-paper insulation. Valve-side windings withstand complex stresses combined with AC, DC, and pulse voltages. Partial discharge in oil-paper insulation is a major incipient fault of an HVDC converter transformer inside insulation. There are few studies on the development characteristics of partial discharge in oil-paper insulation and dissolved gases in oils of an HVDC converter transformer. Based on artificial insulation defect models of oil-paper insulation, the present study investigates the variation characteristics of dissolved gases in the discharge development process. Experiment and analysis results show that the characteristics of dissolved gases in oils under AC-DC combined voltages are appreciably different with those under an AC voltage. These results provide theoretical support for further research on partial discharge and on dissolved gases of other internal insulating defects and the fault diagnosis of HVDC converter transformers.

The insulation of windings connected with converter valves have to withstand AC, DC and repetitive impulse combined voltages in converter transformers. This paper presented the failure evaluation model by experimental research on electrical breakdown characteristics of oil-paper insulation under AC, DC and AC-DC combined voltages. Both short-time tests and constant-stress tests were carried out when the insulation system was subjected to different applied voltages. The breakdown strength and times-to-breakdown of oil-paper specimens have been investigated by tests. The inverse power model was presented by the weibull parameters which were estimated by the times-to-breakdown of specimens. Using the parameters of weibull model and inverse power model, the functions of failure evaluation model were calculated. It could establish the relationship between times, voltages and failure probability of insulation. And final evaluation results showed it was more effective than tradition failure model and weibull model. The DC component behavior in the insulation system has been analyzed and the influence of AC-DC combined voltages on insulation system was discussed.

Smart grid is a very important concept that optimizes and improves the reliability of power transmission systems. The smart grid is required to be more aware of its operative conditions and should have at least some degree of self healing capability. This paper discusses the condition monitoring technologies of HV equipment in smart grid. The development status of condition monitoring technologies in substation, conversion substation, and transmission line are introduced. To solve the problems of condition monitoring systems of power systems, and develop the technologies of condition monitoring in smart grid, the intelligent sensor networks, communication network with IEC 61850 protocol for condition monitoring of HV equipments in smart grid are propoed. At last, the intelligent sensors, intelligent electronic device, the non-contact energy storage and power management module are introduced.

Ultra-high-frequency (UHF) approaches have caught increasing attention recently and have been considered as a promising technology for online monitoring partial discharge (PD) signals. This paper presents a Peano fractal antenna for UHF PD online monitoring of transformer with small size and multiband. The approximate formula for calculating the first resonant frequency of the Peano fractal antenna is presented. The results show that the first resonant frequency of the Peano fractal antenna is smaller than the Hilbert fractal antenna when the outer dimensions are equivalent approximately. The optimal geometric parameters of the antenna were obtained through simulation. Actual PD experiments had been carried out for two typically artificial insulation defect models, while the proposed antenna and the existing Hilbert antenna were both used for the PD measurement. The experimental results show that Peano fractal antenna is qualified for PD online UHF monitoring and a little more suitable than the Hilbert fractal antenna for pattern recognition by analyzing the waveforms of detected UHF PD signals.