Showing papers by "Junbo Deng published in 2015"

Aging characterization of high temperature vulcanized silicone rubber housing material used for outdoor insulation

Abstract: During the field service of composite insulators, high temperature vulcanized (HTV) silicone rubber (SIR) material used for insulator’s sheath and sheds is gradually aging, and thus how to effectively evaluate its aging state has become an inevitable issue. In this paper, artificially corona-aged and naturally site-aged SIR materials are employed as samples, and their properties such as hydrophobicity, leakage current, trap density/energy level, surface microstructure and chemical composition are investigated for comparative study. With the increasing of corona aging intensity, the contact angle of SIR samples drops gradually and recovers more slowly. The hydrophobicity of site-aged insulators also declines, from HC1 for 2-year service to HC5 for 15-year service. The leakage current of corona-aged and site-aged samples both increase with aging, which indicates that aging induces the increase of surface conductivity of SIR material. The peak trap density of corona-aged and site-aged samples increases with the aging duration or service duration remarkably. The changes of these properties are attributed to the changes of micro-structures and compositions in the surface layer of SIR. The scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) analysis results reflect that a hardened inorganic silica-like (SiO2) layer with many polar chemical groups and distributed micro-pores is formed on the surface of SIR material after corona aging. Because of this silica-like layer, the hydrophobicity decreases while surface conductivity and trap density of SIR material increases. Besides the traditional properties like hydrophobicity and surface conductivity, the trap density is expected to be a novel parameter for effective evaluation of aging state of HTV silicone rubber material.

Numerical optimization and 3D-printing fabrication concept of high voltage FGM insulator

Abstract: The application of Functionally Graded Material (FGM) in the solid insulation of high voltage apparatus is discussed. Firstly, the concept of FGM and its effect on electric field (E-field) optimization is introduced. Secondly, optimization of permittivity FGM (e-FGM) insulator is studied. A numerical technique for the optimization of permittivity distribution in e- FGM spacer is proposed and simulated on three typical spacer models, i.e. cone type, disk type and basin type. It is confirmed that the FGM application could significantly improve the E-field distribution. Moreover, the effect of the shrinking coefficient in the algorithm is discussed. Finally, concept of a novel fabrication method for FGM spacers is proposed based on the rapid- developing 3D printing technology. The process and advantage of this ‘bottom to up’ method is discussed.

Partial discharge characteristics and trap parameters of aged oil-impregnated paper

Abstract: The degradation of oil-impregnated paper induced by partial discharge (PD) seriously affects the life of power equipment. In order to investigate the relationship between partial discharge and the degradation of oil-impregnated paper, PD characteristics and trap parameters of the impregnated paper are measured. In this work, a cavity discharge model is adopted and samples with different degrees of polymerization (DP) are fabricated using thermally accelerated aging to simulate transformers with different lifetimes. For each sample, phase resolved partial discharge (PRPD) spectrograms are recorded for various partial discharge times. The characteristics of partial discharge are analyzed, and the trap parameters of specimens are obtained by the thermally stimulated depolarization current (TSDC) method. Following this, the microstructures of the samples are observed using a scanning electron microscope (SEM). It is found that the damaging breakdown process of oil-immersed paper can be divided into three stages: initial stage, developing stage and pre-breakdown stage. There also exist two levels of trap on the surface of oil-impregnated paper: shallow (0.10-0.59 eV) and relatively deep (0.60-0.90 eV) energy. Importantly, the thermal aging makes the trap energy deeper while electrical aging makes the trap energy shallower.

Insulation status diagnosis of XLPE cable based on polarization and depolarization current (PDC)

Abstract: Cross-linked polyethylene (XLPE) insulation cable is widely used in distribution power system, so it is important to reasonably estimate cable insulation status. Cable insulation is mainly affected by high temperature and water tree. Therefore, in this paper, we use PDC method to evaluate cable insulation, which is non-destructive due to its low voltage. In order to find out the relationship between PDC curve and aging status, we put 5 single-core cable samples in oven and set aging temperature as 140C. Then, we divide these samples into different aging time (new, 10 days, 20 days, 30 days, 40 days) and measure the PDC curve. The result shows that with the increase of aging time, the PDC curve decreases quicker and the absolute value of steady current becomes higher. PDC will be a very practical way to judge cable insulation in the future.

Influence of thermal degradation on surface discharge characteristics for oil-polypropylene insulation

Abstract: Oil-polypropylene (PP) composite insulation has been widely used in power capacitors, and also being extended to other equipment such as specific transformers, etc. As discharge can readily creep along PP surface, the interface between oil and PP is a weak point in the insulation structure. The oil-PP insulation degrades under the impact of thermal and electrical stress during normal operation. In this study, the influence of thermal degradation on surface discharge characteristics for oil-PP insulation is investigated. Degraded samples are obtained by thermally accelerated aging of virgin oil-PP samples. The microscopic morphology of PP samples are observed with SEM, and the physical and electrical properties of degraded oil are measured. In addition, the isothermal surface potential decay (ISPD) measurement is employed to investigate the charge keeping capability of PP samples. After that, surface discharge detection experiments under AC voltage are performed for the virgin and degraded samples. The ISPD results indicate that charge decay rate of degraded PP sample is faster than virgin sample, which imply that space charge memory effect of virgin PP samples is more remarkable. For this reason, the PD inception phase of virgin sample is smaller than those of degraded samples. It is found that thermal degradation of oil-PP insulation enhances the surface discharge activity. It is considered that rough surface profile and higher bubble conservation capability of degraded PP sample, and impurities in degraded oil are responsible for the enhancement effect of thermal degradation on surface discharge.

Estimation of time difference between partial discharge UHF signals using differential energy method

Abstract: Partial discharge (PD) site in high voltage power equipment can be located by solving the time difference equations. Estimating the time difference between PD signals with high accuracy is of great importance for accurate locating. This paper presents a time difference determining algorithm using cumulative energy function and differential energy method. The differential energy characterize the increase of cumulative energy, and the first significant peak of the differential energy is recognized as the arrival time. After that, a Monte Carlo statistics procedure is adopted to analyze the influence of noise level on the accuracy of time difference. The proposed estimation algorithm is examined with UHF signals acquired from four antennas in laboratory. The results indicate that the algorithm is effective to determine the time difference between UHF signals.

Dielectric performance of silicone oil-paper composite insulation system under thermal aging

Abstract: Silicone oil is a promising alternative insulating liquid especially for the sites where fireproofing and environment protection must be considered. In this paper, the chemical and dielectric performance of silicone oil-paper composite insulation system under thermal aging is discussed. Two kinds of oil-paper systems are tested: mineral oil and silicone oil (Shin-Etsu Chemical KF-96-20 Lower viscosity fluid). In this study, mineral oil is used as the comparison sample. The accelerated aging process is at 140C in nitrogen. Aging samples with the aging duration of 250 hours, 500 hours and 750 hours and 1000 hours are tested. Experimental results indicate that the silicone oil has much lower acidity content and dielectric loss compared to the mineral oil. New mineral has a little higher AC breakdown voltage compared to silicone oil. For both silicone oil and mineral oil, with the increase of impurities such as water and solid particles during the aging process, the AC breakdown voltage decreases slightly. Acidic materials generated during aging process have little influence on the AC breakdown voltage. Different from the mineral oil whose impulse breakdown voltage decreases significantly with the increase of aging time, however, the silicone oil's negative impulse breakdown voltage of different aging time almost keeps the same.

Partial discharge characteristics and trap parameters analysis of oil-paper after thermal aging

Abstract: The degradation of oil-paper caused by thermal aging seriously affects the lifetime of power equipment such as transformers. In order to investigate the relationship between partial discharge (PD) and degradation of oil-paper insulation, PD characteristics and trap parameters of the paper are measured. In this work, we fabricate oil-paper model with cavity discharge, and the adopted oil-immersed paper specimens with different degree of polymerization (DP) are achieved by thermally accelerated aging to simulate transformers with different operation time. For each sample, the phase resolved partial discharge (PRPD) spectrogram is recorded. The trap parameters such as density and energy of different aged paper specimen are obtained by thermally stimulated depolarization current (TSDC) method. Furthermore, the microstructure of each paper specimen is observed by scanning electron microscope (SEM). Experimental results show two levels of trap existing on the surface of oil-paper: shallow (0.10–0.59 eV) and relatively deep (0.60–0.90 eV) energy level. Further analysis illustrates that thermally aging makes the trap energy deeper. It is considered that the change of PD and trap characteristics is of great significance for insulation condition assessment, and the trap parameter is promising to be new characteristic for aged paper insulation.

Two-dimension simulation of small scale dielectric barrier discharge in argon

Abstract: Low temperature plasma generated by dielectric barrier discharge (DBD) has been paid more and more attention in recent years for its wide range of potential applications. In this paper, atmospheric argon DBD is ignited in a small gas gap of 0.1mm between two parallel glass plates covered by circular copper electrodes with a radius of 0.7 mm. A two-dimension axisymmetric numeric model, coupled with fluid and Poisson equations, is built to clarify the discharge evolution. The basic process of discharge, mainly controlled by electron energy and electric field, is analyzed, which is divided into three stages including Townsend-like discharge, streamer development and discharge extinction. Through the simulation, the spatial-temporal distribution of electron, space charge, surface charge and electric field are investigated, which indicates that the space charge plays a key role in the development of discharge. When the DBD is driven by applied voltage, five discharge channels can be found at the end of the discharge current pulse. The electric field distorted by space charge is expected to be responsible for movement of ionization wave. The surface charge is believed to be the main factor affecting the distribution of five discharge channels.

Partial discharge features and parameters characterization under typical defect geometries in oil-paper insulation: Considering the aging states

Abstract: The oil-paper insulation has been widely used in large power equipment, and thermal and electrical stresses are the two main origins causing the deterioration and failure of oilpaper insulation system. In this paper, accelerated aging experiments of oil impregnated paper are conducted and partial discharge (PD) measurement are performed under typical defect models, i.e. protrusion, surface discharge and cavity inside the oil-immersed paper. The PD experimental results of three typical defects indicate that discharges initiated from a protrusion in oil have the similarity with that in air but with a limited apparent charge. The surface discharge are much more disruptive for oil-paper insulation. The intervals between consecutive discharges has the good tendency of decrease in the whole discharge events.