Zihao Guo

Effects of silane coupling agents on the electrical properties of silica/epoxy nanocomposites

Effects of surface modification agents on the properties of silica (SiO2) /epoxy resin (EP) nanocomposites were studied for the significant impacts of the interface structure between the fillers and resin matrix. The surface modification agent of KH550, KH560 and KH570 were grafted to the surfaces of SiO2 fillers. The original and modified SiO2 fillers were characterized by FTIR and TG. Nanocomposite samples were prepared by solution blended process, using 0.3 wt% SiO2 as filler and epoxy resin as polymer matrix respectively. DSC analysis was performed to investigate the glass transition temperature (Tg). Dielectric spectroscopy and breakdown tests were conducted to measure the dielectric permittivity, dielectric loss and breakdown strength. The results show that SiO2 can be grafted with KH550, KH560 and KH570 effectively. Compared with neat epoxy, the Tg and breakdown strength of nanocomposites increased. Influences on the dielectric permittivity were found, depending on the interface between SiO2 fillers and resin matrix. In terms of Tg, breakdown strength and dielectric loss, the KH550 and KH560 modified nanocomposites performed better than the KH570 modified nanocomposites, which is due to the amino and epoxy group on the molecular structure of KH550 and KH560 respectively. These chemical groups played positive roles in cross-linking reaction and building better interfaces for nanocomposites.

Experiment study on arc ablation resistance performance of epoxy with alumina fillers

GIS spacer plays an important role in the bus of the gas insulated switchgear in UHV substation. However, partial discharge and flashover may take place on the surface of the GIS spacer which may cause severe damage to the stability and safety of the whole system in long-term operation. In order to improve the arc ablation resistance performance of GIS spacer, micron alumina fillers are added to the epoxy system. In this paper, composites with different alumina contents ranging from 0 to 300% (alumina/epoxy in weight) was prepared to investigate their performance of resistance to arc ablation. Results showed that the addition of alumina has succeeded in improving the performance of their resistance to arc ablation, while the failure time increased from 90s to 220s. The curve was obtained between different filler content and the sample performance of resistance to arc ablation. Meanwhile, the thermal conductivity, glass transition temperature and thermal decomposition temperature of different alumina-epoxy system were measured. The conclusion indicates that the system with higher thermal conductivity performed better on the ability of resistance to arc ablation. This research helped to optimize the epoxy-alumina formulation system and to improve the electric properties of the GIS spacer.

Influences of semi-conductive coatings on the electric field distribution of GIS spacer interface

Ultra high voltage (UHV) AC Gas Insulated Switchgear (GIS) spacer is one of the most important components in GIS. In the process of long-term operation, partial discharge often occurs at the interface between the center conductor and epoxy resin (EP) basin body if this interface has some defects, and the discharge sometimes even leads to the crack of basin. It has become a bottleneck for the safe and stable operation of GIS equipment. In this paper, the interface material with EP, hydroxy-terminated butadiene nitrile liquid rubber (HTBN) and conductive carbon black was created, and its parameters were measured. With three-dimensional finite element method, the interfacial structure with semi-conductive coatings were designed, the calculation models of GIS spacer which had interfacial layer structure were built. The influences of semi-conductive coatings on the electric field distribution of the interface between center conductor and basin body were studied. To different defects on the conductor surface, the improvement of the semi-conductive coatings to the electric field distribution was analyzed. Compared with the interfacial layer without coatings, there is an 80 percent decrease of electric field intensity of surface defects than that without coatings, which proves its significant shielding effect. The rationality of GIS spacers performance of material and structure of semi-conductive coatings are verified, which is helpful to improve the uneven distribution of electric field at the interface of UHV electrical equipment.

Simulation study of electric influence caused by defects on UHV AC GIS spacer

UHV AC GIS spacer is the critical component of gas insulated switchgear (GIS) in UHV substation. It plays a supporting conductor, isolating chamber and electrical insulation role. Defects of GIS spacer like delamination, metallic particles, protrusions will be introduced during manufacturing, installation, operating process, etc. These defects will worsen electrical field distribution and degrade insulation performance of GIS spacer, then safety and stabilization of power system is threatened. In this paper, a basic calculation model of a part of GIS was established using ANSYS. GIS spacer whose main insulation is epoxy was included. Several kinds of defects that may occur on spacer were studied. Interface gaps with different shapes were simulated. The law of electric field distribution with length and width of gaps was summarized. Surface defects like protrusions or pits were modeled on spacer surface with different positions and sizes to see how they influence electric field distribution. Results showed that a longer and narrower gap would lead to a higher electric field. Because of the existence of protrusions or pits, electric field would be enhanced in the near part but depressed in the middle part. The enhancement degrees were nearly the same regardless of the sizes and positions of surface defects.

Transient electric field calculation of UHV GIS spacer under lightning impulse

Transient process takes place when a rapidly changing excitation is imposed on a insulation system, like lightening impulse voltage, polarity reversal voltage, et al. In previous studies transient electric field is calculated considering the relative permittivity as a constant. However, a fast changing excitation contains many high frequency harmonic components under which the permittivity may become quite different. In this paper, a frequency division calculation (FDC) method for transient electric field is proposed. For a fast changing excitation applied to a system, its waveform in frequency domain contains many different frequency harmonic components and its response is the sum of the response of each harmonic component. Different frequency component is calculated with corresponding permittivity, therefore the variation of permittivity with frequency is taken into consideration. FDC method is applied to compute the transient electric field (TEF) distribution of a whole ultra-high voltage (UHV) 1100kV GIS spacer under lightning impulse voltage with finite element software ANSYS. Material parameters for calculation are acquired from experiments. Influences of the frequency properties of dielectric medium on FDC method results are also investigated.

Influence of key structure parameters on electrical field distribution of tri-post insulator used in UHV

Gas insulated metal enclosed line (GIL) has been widely applied in power transmission system due to its high safety and reliability level. Because of its cost saving production and excellent mechanical property, tri-post insulator is the key structure of insulation in GIL. Therefore, the safe operation of tri-post insulator determines the insulation strength level of GIL. Tri-post insulators have the characteristic structure with post zone and sphere zone. The thickness and width of sphere zone, conical degree and radius of post zone, depth and radius of metal insert have great influence on electrical field strength. High electrical field are mainly distributed on surfaces of insulator and metal inserts. Over-high partial electrical field may cause the partial discharge or insulation failure. This paper has researched how these parameters impact the field strength, and adjusted the parameters listed above to lower the maximum field value, which is vital for insulation structure design and production.

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.

Influence of grafting modification on dielectric properties of epoxy-grafted SiO 2 /Epoxy composites

In order to study the effects of grafting modification on the dispersion and dielectric properties of SiO2/epoxy nanocomposites, in toluene solvent, epoxy molecules were grafted to the silane coupling agent (KH550) which were pre-grafted on the surface of nano-SiO2 fillers previously. Original SiO2 fillers and epoxy-grafted SiO2 fillers were used for preparing two kinds of SiO2/epoxy nanocomposites by solution blended process respectively. Results of Fourier Transform Infrared spectrometer (FT-IR), thermogravimetric analyzer (TGA) and transmission electron microscopy (TEM) show that epoxy molecules were grafted on the surfaces of SiO2 fillers successfully by KH550, and the grafted SiO2 fillers have better dispersion in organic phase. Broadband dielectric spectroscopy and electrical breakdown tester were performed to measure the complex permittivity and AC breakdown strength of composites. Compared with neat epoxy, the addition of grafted SiO2 fillers improves the breakdown strength and makes lower dielectric constant at low temperature and high frequency, while the addition of original SiO2 fillers makes bigger dielectric constant and dielectric loss than neat epoxy and grafted SiO2/epoxy nanocomposite especially at high temperature and low frequency. The grafted SiO2 fillers are helpful to build better interfaces between fillers and resin matrix and to accelerate the cross-linking reaction, but meanwhile introduce new branched segments to the resin matrix.

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.

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.