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Main insulation of windings connected with valves in high voltage direct current (HVDC) convert transformers withstands pulsating DC voltages. This paper presents breakdown properties of oil-impregnated paper under pulsating DC voltages at different temperatures. Progressive stress tests were carried out to obtain breakdown voltages of oil-impregnated paper with thickness of 0.2 mm under pulsating DC voltages at different temperatures. Constant stress tests were executed to measure time to breakdown of the oil-paper specimens. Both types of the experiments were also undertaken under either AC or DC voltages. An electrical aging lifetime model was proposed for the oil-impregnated paper, and parameters of the electrical aging lifetime model were presented. Experiments and analysis results show that the breakdown voltages and time to breakdown of oil-paper insulation are influenced by temperature significantly.

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.

Converter transformers are the key and the most important components in high voltage direct current (HVDC) power transmission systems. Statistics show that the failure rate of HVDC converter transformers is approximately twice of that of transformers in AC power systems. This paper presents an approach integrated with a two-dimensional cloud model and an entropy-based weight model to evaluate the condition of HVDC converter transformers. The integrated approach can describe the complexity of HVDC converter transformers and achieve an effective assessment of their condition. Data from electrical testing, DGA, oil testing, and visual inspection were chosen to form the double-level assessment index system. Analysis results show that the integrated approach is capable of providing a relevant and effective assessment which in turn, provides valuable information for the maintenance of HVDC converter transformers.

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.

This paper presents simulation and experiments of UHF detection for partial discharge (PD) in power transformers. Simulations of PD electromagnetic waves propagating in simulated transformer tank, by finite difference time domain (FDTD) approach, were implemented to examine the effects of electromagnetic wave refraction and reflection by transformer components. Actual PD experiments in simulation transformer tank have been carried out for two typically artificial insulation defect models while the existing Hilbert fractal antenna was used for the PD measurement. Results show that UHF PD signals are mildly influenced by reflection from transformer components, while they are strongly susceptible to refraction by transformer components.

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.

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.

Ultra-high-frequency (UHF) approaches, when used to detect partial discharge in converter transformers, can avoid the impulse interference generated by converter valve, thus contributing to high sensitivity. The electromagnetic waves (EM-waves) propagation in transformer could be affected by transformer components and other factors. In this paper, a simulation model of converter transformer was built up based on a reasonable simplification of transformer components. And then the EM-waves propagation characteristics in transformer was simulated and studied with Finite Difference Time Domain (FDTD) method. The simulation results explained the propagation characteristics of EM-waves between the core and windings, the attenuation and distortion of EM-waves caused by the core, windings and bushings.

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.

This paper presents an approach for calibration of ultra-high-frequency (UHF) partial discharge (PD) signals in high voltage direct current (HVDC) transformer. A linear relationship between the amplitude of the UHF PD signal and PD apparent charge was obtained through the theoretical analysis. The finite-difference time-domain (FDTD) model of the simulated converter transformer showed that the UHF PD electromagnetic waves in converter transformer can propagate outside by the transformer bushing. The UHF PD signals propagated from the transformer bushing were obtained by the FDTD calculation. The simulation results showed that the amplitudes of the UHF PD signals were related to the transformer bushing and the detection position. Artificial cavity insulation model were designed to generate UHF PD signals, which were detected by a Peano fractal antenna in a series of experiments. The experimental results show that the antenna can detect the UHF PD signals inside of the transformer from the bushing and the PD apparent charge can be calibrated by the proposed approximate equations.

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.