Zhou Yuanxiang

Investigation on composite insulators in contaminated areas

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.

Effect of interface on space charge behavior in multi-layer oil-paper insulation

Oil-paper is used as the main insulation materials in the convert transformer, one of the vital equipment in HVDC power system. The characteristics of space charge in oil-paper insulation are complex, so the research on space charge phenomenon in converter transformers is instructive on insulation design. This paper reports an investigation into the space charge accumulation and dissipation in a two-layer oil-paper insulation system under negative and positive voltages. Planar samples have been subjected to DC electric stresses for extended periods of time and the space charge distribution has been measured by the pulsed electroacoustic (PEA) technique. The influence of the interface between oil-paper layers on space charge dynamics has been analyzed, so does the influence of the polarities of the applied voltage. The test results showed that both the interface and the polarity affected the behavior of space charge. The interface between two oil-paper layers acted as a barrier to both negative and positive charge, as a result charge accumulated adjacent to the interface. The polarity of applied voltage played an important role in determining the interface charge accumulation, since the charge accumulated at the interface was in accordance with applied voltage, namely negative voltage for negative charge and positive voltage for positive charge. Because of the interface, the space charge dynamics in two-layer samples were slower than single layer paper and the interface trapped charge made charge around it dissipated slowly.

Generation of Atmospheric Pressure Plasma by Repetitive Nanosecond Pulses in Air Using Water Electrodes

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.

Study of water droplet discharge by electric field computation and high-speed video

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.

Study of tracking wheel test method under DC voltage

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.

Effect of silica particles on electrical treeing initiation in silicone rubber

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 (SiO2) particles. The aim of dispersing SiO2 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 breakdown of solid insulating material such as SIR. In this article, a serial of SIR samples with different weight percentages of SiO2 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 SiO2 particles hindered the electrical tree growth in the polymer composites. This work afforded experimental evidence to the research on the electrical treeing in polymers.

Four-parameter method for hydrophobicity judgement and mechanism of hydrophobicity transfer property

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.

Morphology Effects on Space Charge Characteristics of Low Density Polyethylene

Low density polyethylene (LDPE) film samples with different morphology were prepared by three kinds of annealing methods which were different in cooling rates in this study. A pulsed electro-acoustic (PEA) space charge measurement system was improved to solve the surface discharge problems for small samples applied with a high voltage. Negative direct current (DC) fields from 50 to above 220 kV/mm were applied to the samples. The influences of morphologies on space charge and space charge packet characteristics were measured by the improved high voltage withstand (HVW) PEA system. Mobility and trap depth of released charges were calculated by space charge decay. It was found that there is a different probability of space charge packet initiation under applied field from -60 to -100 kV/mm. Average velocity and mobility of the space charge packets were calculated by space charge packet dynamics. It was found that the lower cooling rate samples have higher crystallinity, more homo-charge accumulation, lower mobility and deeper trap depth. The mechanism of morphological effects on space charge phenomena have been presumed to give a plausible explanation for their inherent relationships. The morphology in the metal–dielectric interface and in the bulk is convincingly suggested to be responsible for the injection and propagation processes of space charge. A model of positive space charge initiation in LDPE samples was also suggested and analyzed. The mechanism of morphological effects and the charge injection model are well fit with the injection and propagation processes of space charge. The different effects of morphology in the metal–dielectric interface and in the bulk of polymers are stressed.

Nanosecond Repetitively Pulsed Dielectric Barrier Discharge in Air at Atmospheric Pressure

Dielectric barrier discharge (DBD) between two cylindrical glass containers with salt water generated by a nanosecond repetitively pulsed power generator is reported. The electrical parameters, luminous images and spectrum diagnosis are presented. It is shown that the DBD possesses a large discharge current and an intense optical emission from the nitrogen second positive system below 400 nm. The gas temperature remains very close to room temperature regardless of pulse polarity. Luminous photographs with a short exposure time down to 2 ns indicate that no filament is observed and the discharge is homogeneous.

Effect of surface topography and morphology on space charge packets in polyethylene

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.