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There is a relative lack of research on the development of creepage discharge in aged oil-paper insulation subjected to AC-DC combined voltage in converter transformers. In this study, based on the physical and chemical properties of oil-paper, we propose a sphere-plate electrode model to study the creepage discharge parameters and characteristic spectra of aged oil-paper subjected to AC-DC combined voltage. The results show that the creepage discharge inception voltage of aged oil-paper is related to the AC voltage component of the AC-DC combined voltage. However, when the DC voltage component is 60 kV, the flashover phenomenon was found to occur in one of the severely aged samples before the AC voltage was applied. Despite being subjected to the same DC voltage, the flashover phenomenon did not occur in three other samples aged to different degrees. Large-amplitude discharges were found to appear earlier in the severely aged samples. At the same creepage discharge stage, as the aging time of the oil-paper increased, the number of large-amplitude discharges was found to rapidly increase, and the time interval between adjacent discharges was found to gradually decrease. At each stage of the creepage discharge, the small-amplitude discharge has a “hump” shaped distribution. The larger the DC voltage component, the shorter the endurance time of aged oil-paper, and the more severe the damage to the oil-paper. The DC voltage component was found to have an accelerating effect on the development of creepage discharge in aged oil-paper insulation.

UHV DC transmission lines have large transmission capacity and have networking functions. Large-scale use of insulators in DC UHV lines is an significant part of ensuring the safe and stable operation of transmission lines. Once a pollution flashover occurs, the losses will be huge. The northwestern region has the characteristics of frequent sand and dust weather and high wind speed. In the sand and dust weather, pollution flashovers and other accidents may occur, causing large-scale blackouts and train delays. In response to the above problems, with the sand and dust environment in Xinjiang Uygur Autonomous Region and Qinghai province as the background, the impact of sand and dust on the electric field distribution of insulators under DC field was studied, and a numerical simulation model of insulator operating characteristics under wind and sand environment was established to analyze wind speed, particle size and concentration of sand dust. The influence of peripheral flow field on the pressure and electric field distribution of composite insulators. The study found that wind and sand have a significant impact on the electric field and potential distribution along the insulator. In a dusty environment, the electric field along the shed at both ends of the insulator is still the largest, and the particle charging makes the electric field at both ends increase, and the field strength distortion intensity near the middle shed Compared with the same position in a clean environment, the field strength value becomes lower; the arc is prone to occur at the high voltage end during sandstorm, and the increase of dust particles can directly distort the field strength along the surface; the effect of wind speed on the surface pressure of the insulator is obvious. With the increase of particle size and concentration, its effect on surface pressure tends to be stable. Severe electric field distortion may cause the insulation performance of insulators to fail. Therefore, it is necessary to pay attention to the influence of wind and sand on the electric field changes of insulators during power grid operation to prevent pollution flashover caused by this.

With the rapid development of ultra-high voltage (UHV) transmission technology in China, insulation diagnosis for power equipment has been applied widely to improve the reliability of the giant power system. Partial discharge (PD) detection, with the advantages of high precision and convenient deployment, plays an important role in defect detection, location, and classification. However, the low signal to noise ratio (SNR) of PD signals and poor electromagnetic environment make it difficult to extract useful features from PD signals. To distinguish the PD signals from noise, a noise reduction method based on deep learning is proposed in this paper. With short-time Fourier transform (STFT) technology, time-domain PD signals with various noise were first transformed into time-frequency domain patterns. After preprocessing the raw pattern, a deep convolutional neural network (CNN) was built, trained, and tuned for noise reduction of PD signals. Finally, the noise reduction model was verified and compared with the traditional method based on wavelet analysis. The results demonstrate that noise reduction mothed based on deep learning can restore the PD signals with more than 90% noise reduced. Given the advantage that the convolution kernel could sense both the time-related information and the frequency-related information in a spectrum simultaneously, the DnCNN model performs better than wavelet analysis. The noise reduction model proposed in this paper is based on new generation artificial intelligence, providing a new approach for PD signal restoration, which promotes the reliability of the power system.

The recognition of the development stage of creepage discharge of oil-paper insulation under AC-DC combined voltage is the basis of fault monitoring and diagnosis of converter transformers, and there are few related studies. In this paper, the AC-DC combined voltage with a ratio of 1:1 was used to study the development stage recognition method of creepage discharge of oil-paper insulation under the cylinder-plate electrode structure. Firstly, the pulse current method was used to collect the discharge signals in the process of creepage discharge development. Finally, based on the online sequential extreme learning machine (OS-ELM), the above characteristic parameters were used to recognize the development stage of oilpaper insulation creepage discharge. The research results show that when the size of the sample training set in the OS-ELM algorithm is closed to the number of hidden layer neurons, higher recognition accuracy can be obtained, and the type of activation function has less influence on it. Based on the OS-ELM algorithm, the developmental stage of the creepage discharge is recognized. The development process of the creepage discharge is recognized as four stages, which is the same as the trend of the characteristic parameters of the whole creepage discharge development process, and the recognition accuracy is 91.4%. The algorithm has fast rate, high accuracy and batch training data characteristics, which can be widely used in the field of online monitoring and evaluation of electrical equipment status.

Dielectric barrier discharge (DBD) excitated by pulsed power is a promising method for producing nonthermal plasma at atmospheric pressure. Discharge characteristic in a DBD with salt water as electrodes by a home-made unipolar nanosecond-pulse power source is presented in this paper. The generator is capable of providing repetitive pulses with the voltage up to 30 kV and duration of 70 ns at a 300 Ω resistive load. Applied voltage and discharge current are measured under various experimental conditions. The DBD created between two liquid electrodes shows that the discharge is homogeneous and diffuse in the whole discharge regime. Spectra diagnosis is conducted by an optical emission spectroscopy. The air plasma has strong emission from nitrogen species below 400 nm, notably the nitrogen second positive system.

Silicone rubber (SIR) widely used in the insulation of the prefabricated power cable accessories is actually a kind of polymer composite which is usually composited by neat SIR and several weight percentages of silica (SiO 2 ) particles. The aim of dispersing SiO 2 powders into the SIR is mainly to enhance the mechanic properties of the polymer. Some research indicates that the particles may also affect the electrical treeing progress of the polymer composite. Electrical treeing is one of the main causes of degradation and break down of solid insulating material such as SIR. In this article, a serial of SIR samples with different weight percentages of SiO 2 particles were made, a classic needle-plate electrode system was used to test their electrical treeing initiation performance and an optical microscope system was founded to observe the electrical treeing progress. In the experiment, the AC voltage applied on the electrodes was raised at a certain rate and when the electrical trees initiated in the SIR samples, the voltages and the photos of the trees were recorded. By comparing the results of experiment, it was found that when the weight percentages were under a threshold value, the SiO 2 particles hindered the electrical tree growth in the polymer composites. This work afforded experimental evidence to the research on the electrical treeing in polymers.

It is well known that the excellent pollution performance of silicone rubber (SR) insulators originates from its hydrophobicity and hydrophobicity transfer properties. With the sustainable growth of composite insulator usage in China, a convenient method should be given to judge the hydrophobicity properties and to help the utilities choose proper products. In this paper, a four-parameter method used in the evaluation of hydrophobicity status was introduced. The four parameters are hydrophobicity on clean surface, hydrophobicity loss property, hydrophobicity recovery property and hydrophobicity transfer property named H/sub c,/ H/sub l,/ H/sub r/ and H/sub t,/ respectively. Relative values of the four parameters, which were based on the research work of hydrophobicity, were also given. This method has already been used in some parts of China for its easy operation and effectiveness regarded. In addition, the mechanism of hydrophobicity transfer property was further investigated and the difference of the results between contact angle and hydrophobicity classification was also discussed in detail.

In this paper, model of composite insulators were designed to study distortion, movement of the water droplets when discharging occurs on the shed surface of composite insulators applied with uniformity electric field. The distortion and movements of single and multiple water droplets on the surface of insulator in ac and dc electric fields were studied. The results show that the water droplets gradually increase in length in the action of electric field, and the distortion considerably increases with the increase of the electric fields. From these results, the effect of water droplets on flashover voltage is analyzed.

As a simple aging test method for composite insulators, the tracking wheel test has been studied for many years, it is easy to be repeatable due to its simple operation, reproducible results and good equivalence. Recently the tracking wheel test is considered to be published as an IEC standard. More and more composite insulators have been used in HVDC transmission line, and thus their long-term properties are cared. As an aging test method, how about the properties of the tracking wheel test method being used under DC voltage are not clear. In this paper, different voltages such as positive DC voltage, negative DC voltage are applied to the sample to study the DC tracking wheel test. Several kinds of samples are also used in the tests. Comparing with AC tracking wheel test, some differences are shown in the tests, and different samples also show some interesting differences in the test.

Publisher Summary This chapter presents the influences of annealing method on the space charge properties in low-density polyethylene (LDPE). LDPE films are prepared by three kinds of annealing methods, which are different in cooling velocity. The crystallinity of the LDPE samples are measured and compared. Surfaces of samples are observed. Higher cooling velocity results in lower crystallinity. High DC negative voltage is applied on the samples and then samples are open-circuit. Space charge formed in samples is measured during the above procedure. The space charge properties based on crystallinity and morphology are discussed.

The usage of composite insulators in various contaminated areas is increasing prominently in recent years. Most of composite insulators are operating in heavy or medium pollution environment in China. As the exposure time in the field prolonging, the change of hydrophobicity and the build-up of pollution layer directly affect pollution performance of the composite insulators. This paper is based on the work of investigation on the pollution performance and aging of nearly 40 composite insulators operated in Jiamusi city, the north east area of China. These composite insulators were removed from 110 kV lines, and they are all made of HTV silicone rubber and mostly are in service for nine years. Four aspects of hydrophobicity of these insulators are studied after the measurement of pollution status.

Long term leakage currents in BaTiO/sub 3/ based multilayer ceramic capacitors (MLCs) with and without polarity reversal at room temperature and 225/spl deg/C were measured, and comparison was made between these conduction results. It is shown that charges are injected and accumulated near the electrode/dielectric interface of MLC at room temperature, while ions are transported and accumulated near the cathode at elevated temperature. Also, the DC conduction current can be dominated by different conduction processes due to the change of migration ion density in bulk which changes with temperature. It is considered that the relationship between the space charge accumulation and conduction process discussed here will be useful to study the electrical properties of dielectric materials.

Polyethylene (PE) is a major kind of internal insulating material. With great progresses of space charge measurement technologies in the last three decades, lots of researches are focused on space charge in PE. The heat pressing and annealing condition of polyethylene affect its morphology obviously. During the heat pressing, the surface of PE forms different surface topographies because of different substrate materials. Surface topography has great relation to the epitaxial crystallization layer and influences the space charge characteristic of PE dramatically. This paper studied the formation process of different surface topographies and their micrographic characters in low density polyethylene (LDPE). pulsed electro-acoustic (PEA) method was used to measure the space charge distribution of samples with different surface topographies and morphologies in LDPE. The effect of surface topography and morphology to space charge packet were studied. The surface topography has great influence on space charge packet polarity and morphology has influence on both movement speed rate and polarity of space charge packet.