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This article reports an experimental investigation of the impact of magnetic field on corona discharge activity in mineral oil, using both ultrahigh-frequency (UHF) and fluorescence techniques under ac voltage. The results show that corona inception voltage (CIV) increased under ac voltage with varying harmonic frequencies without considering any phase shift and reduced marginally under magnetic field. In addition, the impact of the magnetic field shifted the dominating frequency of the UHF signal (0.9 GHz) obtained under ac voltage toward lower frequencies (0.3 and 0.6 GHz). Under high magnetic fields, the parameters such as rise time, fall time, and pulsewidth of the fluorescence signal increased in its magnitude compared to its effect without a magnetic field. Also, using both the UHF sensor and the fluorescence sensor, there was no clear difference in the phase-resolved partial discharge (PRPD) pattern caused by corona discharge (with and without magnetic field). Furthermore, the number of discharges is observed to be high under the impact of harmonic ac voltages with third- and seventh-order containing total harmonic distortions (THDs) of 4% and 40%. [ABSTRACT FROM AUTHOR]

Addition of alumina nano particles to silicone rubber (SR) has shown a significant improvement in electrical, mechanical and thermal properties of the SR nanocomposites. Static contact angle and water droplet-initiated corona inception voltage exhibited a marginal change on inclusion of nano alumina into SR matrix. Alumina filler improved the corona ageing resistance of the composites; however, a marginal reduction in the rate of hydrophobicity recovery is observed. Laser induced breakdown spectroscopy proved as an effective tool to rank the performance of the SR nano composites. Variation of crater depth with energy of laser pulse and number of laser pulses follows the exponential relation. Plasma temperature calculated by optical emission spectroscopy is in line with the amount of damage caused to the surface of the specimen by water droplet-initiated discharge. Dielectric response spectroscopic studies indicate that inclusion of nano fillers has resulted in lower interfacial polarization and lower imaginary permittivity. It is observed through dynamic mechanical analysis that the composites have higher storage modulus and the activation energy compared to base SR. [ABSTRACT FROM AUTHOR]

The study of epoxy nano micro composites clearly indicates that the material has better dielectric properties with a good discharge resistance and less surface charge accumulation on its surface. The impact of high-frequency AC voltage and the percentage of ripple content in DC voltage on corona inception caused by water droplets was analyzed. Under a high frequency, the variation in the shape of the water droplet is the same as that under 50 Hz AC voltage. The UHF signal radiated due to water droplet initiated discharges have signal bandwidth in the range of 300 MHz-3 GHz. The influence of corona discharge on surface properties of the material, its variation to contact angle and water droplet initiated corona activity under AC voltage was analyzed. The surface potential characteristics were altered with corona-aged specimen. The impact of an ion-trapping particle on nanocomposites clearly exhibits its characteristics through surface potential studies. Water diffusion into the insulating materials varies with the type of filler material. The temperatures measured during arcing, analyzed through laser-induced breakdown spectroscopy (LIBS), were correlated with the hardness of the material. In addition, the mechanical properties, especially storage modulus and tan δ, were analyzed through dynamic mechanical analysis (DMA) studies. [ABSTRACT FROM AUTHOR]

Synthetic ester oil (SEO) is becoming a replacement for mineral oil in transformers due to its biodegradability and for its better insulation properties. Silica nanoparticles dispersed in SEO can increase its corona inception voltage (CIV) and breakdown voltage compared to pure SEO. Physico-chemical studies were carried out to understand the characteristics of nanoparticles and nanofluids. Methodical experimental studies were carried out to understand the corona discharge activity in SEO based nanofluid. A ultra-high frequency (UHF) sensor was used to identify corona activity and it is concluded that the UHF signal radiated from corona activity in SEO and nanoparticle dispersed SEO have a bandwidth in the range of 700 MHz to 2 GHz. A comparative study on the CIV of SEO and nanofluid under harmonic AC voltages has been carried out, which indicates a reduction in the CIV as the total harmonic distortion of the supply voltage increases. Phase resolved partial discharge analysis indicates that the magnitude of discharges formed in nanofluid is much lower than in the SEO. Comparative statistical analysis are made with the breakdown voltages of the synthetic ester oil and nanofluid. A comparative analysis on the energy of the UHF signal due to the corona discharges is performed. [ABSTRACT FROM AUTHOR]

Titania (TiO 2) nanoparticles with cetyl trimethyl ammonium bromide (CTAB) surfactant dispersed in transformer oil has higher corona inception voltage than the normal transformer oil, under AC, DC, and composite AC and DC voltages. It is observed that corona inception voltage is lower under composite voltages compared to AC and DC voltages. The corona activity radiates ultra high frequency (UHF) signals with its dominant frequency near 1 GHz. Operating a spectrum analyzer in zero span mode, the UHF signals generated from the corona activity, the number of discharges is observed to be more under composite voltages than under AC voltages, both at inception and at higher voltages. A reduction in interfacial tension and an improvement in flash point is observed in nano-titania with surfactant dispersed transformer oil. The addition of surfactant reduces the turbidity of the nanofluid. Phase Resolved Partial Discharge (PRPD) analysis with UHF signals measured, indicates that corona discharge activity occurs around the peak and the pre-peak rising portions of the supply voltage. It is observed that more discharges occur in the positive half cycle for AC superimposed with positive DC voltage and in the negative half cycle for AC superimposed with negative DC voltage. The magnitude of UHF signals formed due to corona activity is less with nanofluid, irrespective of voltage profiles. [ABSTRACT FROM AUTHOR]

The molecular level changes in natural ester oil due to thermal aging are studied. Accelerated thermal aging of natural ester oil at 180°C is performed and the subsequent effect on oil matrix is studied using corona inception voltage (CIV) variation under AC and DC voltage. Various physico-chemical diagnostic techniques such as UV-Vis absorbance, FT-IR, fluorescence microscope, DLS, and TEM were used to study the thermally aged natural ester oil. Thermal aging leads to the reduction in corona inception voltage as observed in the present work. The CIV is much higher under DC voltage than AC voltage but its variation is not much under positive/Negative DC voltage. Spectrum analyzer measurement indicates that burst type discharges occur under negative DC voltages. Thermal aging induces degradation of natural ester oil, resulting in the formation of carbonaceous particles. As these carbonaceous particles are fluorescent in nature, a facile fluorescence based analysis is developed to understand and monitor its degradation. 3D excitation emission matrix (EEM) fluorescence data are modeled with parallel factor analysis (PARAFAC). [ABSTRACT FROM AUTHOR]

Corona activity on the surface of silicone rubber has high impact on variation in hydrophobicity of the material, and on corona inception voltage (CIV) of water droplet sitting on the corona degraded surface. The CIV is high under negative DC as compared to positive DC/AC voltages. Increase in conductivity of water droplet reduces CIV. Surface charge accumulation studies indicate that the charge retention capability of the insulating material gets reduced drastically with silicone rubber damaged due to corona causing degradation of surface. Irrespective of the number of cycles of corona discharge activities carried out, the time constant of charge decay, after the removal of charge injection, remains the same. The water droplet initiated discharges under AC and DC voltages radiate UHF signal in the bandwidth 0.3?1.2 GHz. With single droplet sitting on top of the corona damaged silicone rubber, the amount of charge retained by the water droplet is low when the droplet is placed and charged immediately after corona discharge activity to surface of the insulating material. When the water droplet is placed after a certain time period, the charge accumulation characteristics of silicone rubber are regained. Leakage current measured during water droplet discharge studies through recurrent plot (RP) analysis reveals that, under AC voltage, it is possible to classify corona inception, arcing and bridging of water droplet between high voltage and ground electrode. The RP obtained for the current signals generated due to water droplet discharges under positive and negative DC voltages are different. The amount of damage caused to silicone rubber due to water droplet is high under negative DC voltage as compared to positive/AC voltages. While correlating the level of damage to the insulating material due to water droplet and the RQA parameters of RP, it is observed that higher the recurrence rate along with high determinism can cause severe damage to the insulating material. Structural analysis using ATR-FTIR and pyrolysis-GC/MS studies reveal significant functional group variation on surface of silicone rubber, especially when the number of cycles of corona ageing is more. [ABSTRACT FROM AUTHOR]

Titania (TiO2) nanoparticles dispersed transformer oil has high Corona Inception Voltage (CIV) and breakdown strength, and gets further enhanced on insertion of the barrier in the electrode gap. The volume of nanoparticles and the amount of surfactant have a strong influence on the dispersion of nanoparticles in transformer oil and were characterized through viscosity measurement, particle size analysis and by zeta potential measurements. It was also observed that with needle plane configuration, negative DC voltages have high breakdown voltage as compared to positive DC and AC voltages. Improvement in breakdown voltage with barrier can be obtained by placing the barrier at 20?80% of electrode gap from the tip of the high-voltage electrode. The breakdown voltage of needle plane electrode configuration was calculated based on normal distribution parameters. It was observed that the corona activity generates Ultra High Frequency (UHF) signal in the frequency range of 0.7?2 GHZ with its dominant frequency at 0.9 GHz and was characterized using a ternary plot. The discharge activity was studied by using a spectrum analyzer by operating it in zero span mode with 0.9 GHz as center frequency. It was observed that for a given voltage under AC and DC voltages, the magnitude of discharges is less with nanoparticles dispersed transformer oil. A pyrolysis study indicated no variation in the composition of the oil with the addition of titania. It was observed that the esters present in the oils were alkyl substituted oxalic acid and alkyl substituted sulfurous acid esters. [ABSTRACT FROM AUTHOR]

Epoxy nanocomposites with different percentages of nano clay filler are prepared using a shear mixer technique. Corona inception voltage (CIV) due to water droplets on the epoxy nanocomposite is high under negative direct voltage compared with positive polarity voltage. It was found that an increase in droplet volume and number of droplets within the gap reduces the CIV. The magnitude of discharge current flow is high under negative polarity. Surface charge accumulation studies indicate nonlinear variation in distribution of charges on the surface of the epoxy nanocomposite with water droplets. Optical Emission Spectroscopy (OES) results confirm high temperature plasma during water droplet initiated electrical discharges on the surface of epoxy resin. Laser Induced Breakdown Spectroscopy (LIBS) results indicate the presence of carbon on the degraded surface of the sample. The absorbed energy on the sample surface during arcing is measured through leakage current, and is observed to be higher for negative polarity. UHF signals were radiated due to corona discharges from water droplets on the epoxy nanocomposites under direct voltages and their bandwidth is in the range of 1–2 GHz. [ABSTRACT FROM PUBLISHER]

In liquid nitrogen insulation, the corona inception voltage is high with barrier in the electrode gap. The breakdown voltage of needle-plane gap has increased with barrier positioned near high voltage needle electrode. The current signal injected due to corona discharge in liquid nitrogen insulation, at the point of its inception and during streamer formation, have rise time, in negative and positive cycle of the AC voltage are about 2.9 ns and 2.5ns respectively and the characteristics has not altered due to barrier insulation. The UHF signal generated by corona discharges, in the liquid nitrogen electrode gap (with/without barrier) have frequency contents between 0.5-1.5 GHz. The magnitude of UHF signal formed due to corona activity (with barrier) on inception and at higher voltages (not at point of near breakdown) shows inverse relationship. Corona discharge followed with breakdown studies analyzed using zero span analysis. Phase resolved partial discharge signatures obtained due to corona discharges indicates that phase window at which corona discharge occurs become narrow with barrier in the electrode gap. [ABSTRACT FROM PUBLISHER]

Corona Inception voltage (CIV) / breakdown voltage increases when the barrier is placed anywhere from needle tip to the 20% of the needle-plane electrode gap above which a marginal reduction in breakdown voltage is observed but its value is much higher than the breakdown strength of the electrode gap without barrier. Thermally aged oil impregnated pressboard insulation, as barrier, have high impact on CIV and breakdown voltage. The injected current due to corona activity, its rise time, measured using HFCT, in the negative and positive half cycle of the AC voltage measured to be 600 ps and 500 ps respectively. The shape of the current pulse is the same with the aged pressboard insulation barrier in the electrode gap. The corona inception voltage was measured by using UHF sensor and the signal has frequency contents between 0.5 -2.2 GHz. The surface charge measurements indicates that the amount of charge accumulated is high with thermally aged oil impregnated pressboard (OIP) than the unused material. Polarity of surface charge acquired by OIP due to corona charging depends on the surface condition of the material. To understand the mechanical behavior of the aged specimen, tensile test and flexural test were carried out and could conclude that ageing of material reduces the fundamental mechanical properties of the material. SEM analysis indicates that scission of fibers occurs due to thermal ageing at high temperatures. Partial Discharge studies on corona activity indicate, on corona inception, discharges occur near peak of the applied AC voltage, irrespective of position of barrier. The phase width of the PD generated due to corona discharge activity is slightly high with thermally aged specimen compared with unused specimen. [ABSTRACT FROM AUTHOR]

In the present work, epoxy nano composite samples of different clay content are subjected to water droplet initiated discharge tests using controlled harmonic content high voltage supply. The effect of a clay additive up to 1% by weight is shown to increase droplet contact angle by 8% with a subsequent reduction beyond this content. Using a UHF technique to identify the corona inception voltage, it is difficult to identify correlation with harmonic content. However, it was found that the corona inception factor (CIF) and the peak factor of the applied AC/harmonic AC voltages showed an inverse relationship. The rise time of the current pulse due to water droplet initiated corona discharge under AC/harmonic AC voltage was found to be about 700 ps and from FFT analysis, the corresponding UHF generated signal has a dominant frequency of about 1 GHz. The characteristics of the captured UHF signals are similar for all investigated clay contents and harmonic AC voltages. It was found that the discharges caused damage near the high voltage electrode and that the surface carbonization is related to the accumulated dissipated energy during the discharge process. Analysis of the voltage, current signals and phase resolved partial discharge (PRPD) studies indicate that discharges occur around the voltage peak and also when the rate of rise of voltage is high. The number of discharges is higher in the positive half cycle compared with the negative half cycle. [ABSTRACT FROM AUTHOR]

Ultra-high-frequency signals are generated due to particle movement, floating conductors, corona, and surface discharges in gas-insulated switchgear. The ultra-high-frequency signal generated due to particle movement is independent of operating pressure and applied voltage. The bandwidth of the ultra-high-frequency signal formed due to corona and surface discharges vary with applied voltage. In a ternary plot, each type of discharge has a unique location. If a variety of discharge occurs simultaneously, it is difficult to classify the type of discharges through the ternary plot. It is demonstrated that the type of discharge can be classified efficiently by adopting a multi-class support vector machine. [ABSTRACT FROM PUBLISHER]

The major cause of failure of GIS is the presence of foreign particles causing partial discharges/corona in the insulation structure. In the present work, partial discharge activity in GIS under AC, DC, and the composite voltage formed by AC and DC voltages, was studied using the acoustic emission technique. It is observed that the magnitude of the acoustic emission (AE) signal produced due to partial discharges under positive DC is high compared to negative DC and AC voltage. Under the composite voltages, it is observed that superimposing AC voltage over positive DC enhances the partial discharge activity, and superimposing the AC voltage over the negative DC voltage could limit the partial discharge activity. The rate of rise and the duration of the AE signal indicates that partial discharge activity under positive DC voltage is severe compared to AC and negative DC voltages. The measured counts and energy content of the AE signal produced due to partial discharges under positive DC is high compared to the negative and AC voltages. It is also realized, based on a partial power analysis, that the maximum energy content of the AE signal lies in the range 125–250 kHz, for the signals generated under AC/DC/composite voltage formed by AC and DC voltage. [ABSTRACT FROM AUTHOR]

Incipient discharges formed due to particle movement, corona and surface discharges in liquid nitrogen under ac voltages radiates UHF signals. The frequency content of the UHF signal generated due to particle movement at the point of inception lies in the range 0.5 - 2 GHz. Corona and surface discharge radiated UHF signal which vary with applied voltage. At higher applied voltage to the electrode gap, the UHF signal generated due to surface discharges and corona, have increased energy content of the signal in the frequency range 2 - 3 GHz. It is observed that irrespective of type of incipient discharge formation, multiple discharge action occurs, which could be observed through zero span analysis. Ternary diagram can be used as a condition monitoring tool and could help one to classify the type of discharges (partial discharge/corona/surface discharge) that occurs, for any remedial action, during operation. [ABSTRACT FROM AUTHOR]