Lilan Liu

Insulating characteristics of UHV resin impregnated paper bushing condenser's materials

Firstly, several sample bushings and epoxy resin samples were produced in laboratory. FDS were then performed on the bushings and epoxy resin samples with the built dielectric-response-measuring platform. HN model (short for Havriliak-Negami model) was used to fit the data. The satisfactory results illustrate the effectiveness of the HN model. Furthermore, the DC conductivity was extracted, which was proved to be temperature dependent. The change of DC conductivity is in accordance with the Arrhenius formula. In addition, the accelerated thermal aging experiment was done on the epoxy resin samples. After comparing the FDS data of samples with different aging hours, the results showed a change of dielectric constant and dielectric losses, which could be further applied to the measurement and evaluation of the insulating properties of bushings.

Calculation and experimental study on the distribution of stress and strain on UHV GIS spacer

UVH GIS (gas insulated switchgear) spacer is the key insulator in UVH transformer substation. Beside insulation, it may suffer the pressure from other structures and SF6 gas in the bus, which put forward a higher request on the mechanical properties of GIS spacer, the concentration of stress may cause tiny flaws which will lead to discharge on the spacer. In order to analyze the mechanical strength of UVH GIS spacer, the distribution of the stress under the pressure on both side of the spacer is calculated by ANSYS. At the same time, the fiber Bragg grating is arranged on the surface of the GIS spacer based on the calculation to measure the strain distribution during hydrostatic test. The calculation shows a similar result with experiment, the distribution of stress and strain in the GIS spacer is obtained. The hydrostatic test indicates that the interface between center conductor and epoxy insulator is the source of the damage, leading to the crack of the whole spacer.

Properties of interface between epoxy resin layers cured at different times

Interfacial materials have been used as transition layers between the conductor metal and the insulation composite in some high voltage power equipment. Epoxy resin composite is one of the most important materials used as the interfacial coating, then an interface between epoxy coating and epoxy insulation composite is formed. Therefore the influence of the epoxy-epoxy interface should be studied. The interface samples are formed by an epoxy resin layer cured on the epoxy resin which has cured with the same procedure. Compared with the sample without the interface, the existence of this interface makes the mechanical properties have different extent reduction. The tensile strength decreases 28.4%. The bending strength decreases 39.5%. The impact strength decreases 39.2%. The results of mechanical strength testes indicates that epoxy resin on both sides of the interface should link through chemical bonds, and the interface may form a “weak crosslinking” area. This interface has little influence on the thermal performance. And in the measurement range, the relative permittivity and dielectric loss show little change too. The existence of this interface formed by curing does not affect the application of the thermal and AC systems.

Calculation and optimization for UHV AC GIS spacer in electrical and mechanical fields

Epoxy spacers play supporting conductors, isolating chambers and electrical insulation roles in gas insulated switchgear (GIS), affecting the insulation level and equipment lifetime of GIS. Inappropriate configuration of GIS spacer will not only lead to flashover or partial discharge when imposed by high voltage but also bring about micro cracks during mechanical delivery test. UHV AC GIS spacers are demanded of higher electrical and mechanical performance for higher voltage and gas pressure. This paper is aimed to investigate the electrical and mechanical field distribution of UHV AC GIS spacer and to improve the performance of spacer. A basic calculation model of a section of UHV AC GIS was established in ANSYS, containing an epoxy spacer. Combining MATLAB with ANSYS, the shielding ball, center conductor and epoxy body structures are optimized by using the genetic algorithm. After optimization, electrical and mechanical properties of spacer were effectively improved.

Simulation of losses in DC-biased single phase three limb transformer

According to the characteristics of electrical connection in the single phase three limb power transformer model, an equivalent coupled electric and magnetic circuit was established to calculate the excitation current of DC-biased power transformer model and analyze the changes of excitation current with the DC bias voltage effectively. Based on the finite element theory, the internal magnetic field distribution of the transformer under DC bias condition was analyzed in detail by taking account of excitation current. Moreover, the thesis discussed the magnetic flux density distribution as well as changes of additional loss in transformer with increasing DC bias level. The results show that the core saturation becomes higher and the excitation current distortion is more serious with low harmonic sharply increasing when the DC current of the transformer increases. The simulation results of losses are in accordance with the theory, confirming the validity of the proposed losses calculation method for transformer under DC-biasing. The relevant conclusions provide a reference for the design and maintenance of the transformer considering DC bias.

Influence of moisture and temperature on the frequency domain spectroscopy characteristics of transformer oil

Transformer oil is an important liquid dielectric of oil-immersed transformers. Moisture and temperature bring a great harm on the transformer insulation. This research was carried out to find the influence of moisture and temperature on the frequency domain spectroscopy (FDS) characteristics of transformer oil. As a non-destructive diagnostic technique, FDS technique possesses characteristics of strong anti-interference ability and abundant information, which is very suitable for the measurement on spot. This paper focuses on the research of the relationship between moisture, temperature and dielectric properties of transformer oil with the help of IDAX300 measurement system. Oil samples of different moisture level, 8ppm, 20ppm, 27ppm and 42ppm, have been measured at different temperatures, 29°C, 40°C, 60°C and 80°C. Results shows that the relative permittivity, in low-frequency areas, increase rapidly with moisture content and temperature. The DC conductivity calculated from dissipation factor increases exponentially both with the increase of moisture content and temperature. This relationship can be described as a fitting formula. This finding can be used for the online diagnosis of transformer oil moisture content.

The effect of moisture and temperature on the oil-immersed-paper used in EHV converter transformer

The insulation demand of converter transformer is higher than that of ordinary transformer, due to the different functions and operating environment. In order to assess the insulation properties of converter transformer, the study of the temperature and moisture effect on the oil-immersed-paper is conducted. The research objects are weidmann T4 kraft paper (made in Taizhou, China) and Kelamayi transformer oil KI50AX (made in Xingjiang, China), both used in EHV converter transformer. The polarization-depolarization current (PDC) and frequency domain spectra (FDS) tests are performed on the oil-impregnated pressboard samples of diverse water content under different temperatures. The results show that the dielectric properties and resistivity of oil-impregnated-paper are remarkably affected by moisture content and temperature, the real part of relative permittivity and dielectric dissipation factor change significantly with temperature and moisture at frequency range of 10-3~10-1Hz; the natural logarithm of samples resistivity changes linearly with both the moisture content and the temperature. The influence of temperature should be considered when diagnosing moisture content of transformer through PDC and FDS.

Study on electric properties of epoxy with high mass fraction of alumina fillers

In order to improve the mechanical and thermal properties of the epoxy resin, high mass fraction of micro-alumina has been applied in the epoxy resin which is also considered to be a kind of extender in manufacture. However, the introduction of alumina fillers may cause deterioration of the electric property. In this paper, epoxy with different amounts of the micro alumina fillers ranging from 0 to 300 phr (parts per hundreds of weight of resin) were prepared to investigate their permittivity, dielectric loss, volume resistivity and dielectric strength. The results indicates that the addition of alumina can not only produces interface between matrix and fillers, but also brings impurities into the epoxy, as well as introduces dipole and interfacial polarization to the system. Therefore, the permittivity and dielectric loss of the epoxy system increase and the conductivity of the sample decrease with the addition of alumina fillers. The increase of thermal conductivity and the decrease of the cross linking degree were appeared with the addition of the fillers, along with the defect, leading to the decline of the AC dielectric strength.

Thermal and dielectric properties of the epoxy resin toughened by different kinds of liquid rubbers

Epoxy resin is widely used in the electrical equipment, such as dry-type bushing and transformer, and epoxy resin is friable material which needs to be toughened. Adding liquid rubbers is one of the most mature and effective ways to toughen matrix. Four kinds of liquid rubbers (HTPB, CTBN, HTBN and EHTPB) are adopted to toughen epoxy resin respectively, and then the thermal and electrical properties are measured and compared. Phase separation occurs between all these four liquid rubbers and epoxy resin during epoxy resin curing, and the dispersed rubber particles play an important role in toughening. For HTBN and CTBN, the dispersion phase is more evenly. All kinds of liquid rubbers are resulted in a slight decrease in the glass transition temperature, and have little effect on the thermal conductivity and diffusion coefficient of epoxy resin matrix. DC resistivity has a different extent of decline after the addition of liquid rubbers. In dielectric spectroscopy, all the kinds appear a relaxation behavior which should be caused by the interfacial polarization between rubber particles and epoxy resin. The time and strength of these relaxations are related to the kind of liquid rubber. When using the liquid rubber to toughen epoxy resin, the kind and content of liquid rubbers should be considered based on the product requirements.

Study on the dielectric properties of the epoxy resin toughened with different molecular weight HTBN

Epoxy resin (EP) is one of the most popular insulation materials used in the electrical equipment. Adding different liquid rubbers into EP is the common way to increase the toughness, but the dielectric properties of the EP change while the toughness increasing. In this paper, the hydroxyl-terminated liquid nitrile rubbers (HTBN) with different molecular weight were used as fillers to toughen the EP. Through comparing and analyzing the changes of the glass transition temperature and the permittivity of the EP specimens with different kinds and same concentration of HTBNs, the influence of the HTBNs molecular weight to the properties was gained. The glass transition temperatures of the two kinds of specimens are very close to each other, so the glass transition temperature is mainly affected by the adding amount of HTBN. Addition of HTBN brings a new polarization into the composite. This polarization makes a mutation step in the real part and a relaxation peak in the imaginary part of permittivity curve. For the relaxation time of the new polarization, the composite with bigger molecular weight of HTBN is less than another HTBN. For the intensity of the new polarization, the composite with bigger molecular weight of HTBN is greater than another HTBN. The research result shows that the molecular weight of the filler is an important factor to the dielectric properties of the EP toughed with HTBN.