Yan-Hui Wei

Partial discharge signals separation using cumulative energy function and mathematical morphology gradient

Partial discharge (PD) measurement and interpretation have become a powerful tool for condition monitoring and failure risk assessment of high voltage power equipment insulation. The occurrence of multiple discharge sources affects interpretation accuracy. This paper presents a PD signal separation algorithm using cumulative energy (CE) function parameters clustering technique. The waveform of PD signals are acquired by digital detection instruments with high sampling rate. Cumulative energy functions in time domain (TCE) and frequency domain (FCE) are calculated from PD waveforms and their FFT spectrums, respectively. Mathematical morphology gradient (MMG) operation is applied to the TCE and FCE to describe their variation characteristics. The feature parameters including width, sharpness and gravity are extracted from CEs and MMGs in both time and frequency domain, and compose a six-dimension feature space. The improved density-based spatial clustering of applications with noise (IDBSCAN) clustering algorithm is adopted to discover clusters in the feature space. The proposed separation algorithm is examined with mixed current impulse signals acquired from PD experiments on artificial multi-defect models and an on-site transformer. The separation results indicate that the proposed algorithm is effective for separating mixed PD signals initiated from multiple sources.

Effect of space charge on breakdown characteristics of aged oil-paper insulation under DC voltage

Due to its excellent performance, oil-paper insulation has been widely used in many power apparatuses including converter transformer of high voltage direct current (HVDC) system. Space charge accumulation phenomena may significantly distort local electric field inside transformer and cause oil-paper insulation overstressing. As a result of long term operation, oil-paper gradually degrades. In most severe cases, the breakdown of insulation system may occur. In this work, the effect of space charge on breakdown characteristics of aged oil-paper insulation is investigated. Oil-paper samples with different aging states are obtained by thermally accelerated aging for various periods of time, categorized by the degree of polymerization (DP) of 1107 (new), 725, 504, 376 and 241, respectively. Experimental results show that the breakdown voltage of oil-paper samples without pre-stressing remains almost constant with thermal degradation. After the homo-polarity pre-stress, when the DP value is higher than ~300 (from 1107 to 376), the breakdown voltage has an increasing tendency with an increase in thermal degradation. On the contrary, the breakdown voltage presents a decreasing tendency after the hetero-polarity pre-stress. When the DP is lower than ~300 (DP241), the changes of breakdown voltage does not remain the same trend as previous samples after pre-stressing. Meanwhile, space charge distributions in different aged samples are measured by the pulsed electro-acoustic (PEA) method, and the trap parameters are correspondingly evaluated. It has been experimentally demonstrated that special attention should be paid to space charge accumulation and aging state of oil-paper insulation, which may lead to severe insulation accidents.

A study of oil-impregnated paper insulation aged with thermal-electrical stress: PD characteristics and trap parameters

The oil-paper insulation in a transformer gradually ages due to the combined effects of thermal and electrical stress that occur during its operation. In this paper, for investigating the changes that occur within the oil-paper during the aging process with different aging stresses, partial discharge (PD) characteristics and trap parameters are measured. An internal discharge cavity model is designed for the electrical stress, and the thermal stress is generated by a baking oven simultaneously. The experiments are designed such that the dominant factor on the aging process is either thermal, electrical, or a unity of both stress factors. The phase resolved partial discharge (PRPD) spectrograms are recorded at different aging time, and the trap parameters are extracted by thermally stimulated depolarization current (TSDC) method. Additionally, the surface morphologies of these samples are observed by using a scanning electron microscope (SEM). The results show that the trap energy presents a decreasing tendency when the electrical stress plays the dominant role, and the discharge energy becomes larger over the aging time until pre-breakdown. When the thermal stress dominates the aging process, the trap energy shows an increasing tendency over the aging time, and there exists a process of ‘material conditioning’ that occurs at the initial stage of the aging process. The change of trap parameters in aged oil-paper will further affect PD characteristics. Correlation analysis of PD and trap characteristics can promote more accurate condition estimation of aged oil-paper.

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

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.

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

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.

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

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 oil-paper 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.

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

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 140 oC 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 oils 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

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.

Study on hydrophobicity recovery and measurement of aged sillicone rubber material

Excellent hydrophobicity and hydrophobicity migration property of silicone rubber material guarantee outstanding pollution resistance and insulation performance of composite insulators in outdoor insulation. Under effect of multiple environmental factors, insulators get aged with increase of service duration. In this paper, hydrophobicity recovery characteristics are revealed by static contact angle method of different kinds of laboratory aged samples, that is, corona aged and salt contaminated HTV(high temperature vulcanized) SR(silicone rubber) samples. The experimental results indicate relationship between hydrophobicity change and ageing status of HTV silicone rubber. At last, influencing factors of static contact angle method and problems existing are discussed when the method is applied to estimate surface condition of salt contaminated samples.

Study on space charge characteristics of oil-paper insulation at breakdown moment

During the long-term operation of electrical equipment of HVDC system, accumulated space charge in oilpaper insulation can greatly distort the local electric field, which can cause the aging and breakdown of the equipment. Therefore, it is important to understand the influence of the space charge on breakdown characteristics of oil-paper insulation dielectrics under DC voltage. In this paper, four kinds of oil-paper samples with different aging status are obtained by accelerating thermal aging. At room temperature, each sample is applied with DC voltage till breakdown. During the processes of the breakdown, the space charge distribution is measured using pulsed electro-acoustic (PEA) method, and space charge distribution is analyzed at the breakdown moment. The experimental results show that positive space charge moved towards to opposite electrode under DC voltage and the more serious deterioration of the oil-paper insulation dielectrics, the deeper positive space charges moved. At breakdown moment, space charge and electric field polarity are reversed, and space charge density and electric field decrease with the delay time after breakdown.