Surface Discharge Inception Voltage Research Articles

Impact of DBDS and Silver Sulfide on the Performance of Thermally Aged Mineral oil Impregnated Pressboard Material

The corrosive sulfur compounds present in the mineral oil causes the formation of dibenzyl disulfide (DBDS) complex and it can also erode the on-load tap changing selector contacts leading to generation of silver sulfide (AgS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ).The present work reports the effect of DBDS and Ag <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> S concentrations in mineral oil on thermally aged pressboard material. The results demonstrate that surface discharge inception voltage (SDIV) is higher for the aged pressboard specimen. The higher concentrations of DBDS showed a reduction of more than 50% in the magnitude of SDIV compared to its effect with the addition of Ag <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> S. The surface potential analysis indicates a higher initial potential for the aged specimens with an increased half lifetime. In addition, only deep traps observed on the aged specimens with a slight right shift in its trap energy level. The mechanical strength of aged pressboard specimens with DBDS and Ag <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> S is understood using tensile testing and simultaneous analysis is captured using Digital Image Correlation (DIC) technique to understand its strain percentage at different areas on the pressboard material during its elongation. The laser induced breakdown spectroscopy (LIBS) analysis is performed to identify the elements responsible for thermal ageing such as carbon, copper and sulfur on the surface of pressboard material. The plasma temperature calculated from sulfur peaks is higher for DBDS aged specimen compared to AgS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and a linear correlation is observed on the LIBS intensity with ageing duration. Further, the Principal Component Analysis (PCA) is used for its classification on the degradation of pressboard due to the diffusion of both copper and silver sulfide from the mineral oil. The two principal components (PC1 and PC2) provided a higher variance of 99.8% with a clear classification observed between the unaged and aged pressboard specimens due to the addition of both DBDS and Ag <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> S in mineral oil.

Fluorescent fiber-based identification of incipient discharges in liquid nitrogen

Identification of incipient discharges is a major challenge in liquid nitrogenbased super-conducting power apparatus. In the present study, a variety of fluorescent fibers with different absorption/emission characteristics were used for the identification of corona and surface discharges, and we find that a particular type of fluorescent fiber is more sensitive to identify discharges in liquid nitrogen insulation than the UHF technique. Enhancement in the magnitude of fluorescent signal/UHF signal is observed for an increase in applied voltage beyond the corona inception voltage (CIV). Also, it is observed that while reducing the applied voltage, the corona activity quenches at a much lower voltage than the inception voltage. No characteristic variation in rise time or the fall time of the fluorescent signal is observed on corona inception and due to discharges at higher voltages. The corona inception voltage is least for AC voltage conditions, and slightly higher inception voltage is observed for positive DC and negative DC voltage conditions. Different weight percentages of MgO doping in GFRP nanocomposites are prepared and used in the surface discharge studies. It is observed that the 3 wt% sample exhibits enhanced surface discharge inception voltage (SDIV) compared to samples with other doping concentrations and that the inception voltage is lower for AC voltage when compared with the DC voltage conditions. The characteristic spectral content of the fluorescent signal due to corona and surface discharge activity is observed up to 3 MHz. A phase-resolved partial discharge (PRPD) analysis was also carried out to identify corona and surface discharge activity using fluorescent fiber sensor and with a UHF sensor. From the PRPD studies, it is observed that the corona discharge occurs near the peak magnitude whereas surface discharge is observed at the rising part of AC voltage.

Investigation on electrical, thermal and mechanical properties of thermally aged pressboard impregnated with mixed mineral oil and synthetic ester fluid

The proposed study compares the electrical, thermal and mechanical performances of thermally aged cellulose pressboard material impregnated in mineral oil, synthetic ester fluid and mixed oil. The electrical insulation characteristics of oil-impregnated pressboard (OIP) are studied by means of surface discharge inception voltage (SDIV) using an ultra-high frequency technique and surface potential measurement analysis. It indicates that OIP aged in mineral oil has higher surface potential due to higher charge trap density compared to that of the mixed oil and synthetic ester fluid. Mechanical and thermal characteristics of the aged OIP are studied by means of tensile strength measurement and thermo-gravimetric analysis, respectively. The change in the chemical composition of OIP is studied by means of laser-induced breakdown spectroscopy, which indicates lower plasma temperature with mineral oil-based OIP indicating higher degradation state. To further understand the microscopic structure of the thermally aged OIP in different oils, scanning electron microscopy and X-ray diffraction studies have been performed. The pressboard thermally aged in synthetic ester fluid and the mixed oil has better electrical, thermal and mechanical properties as compared with the pressboard aged in mineral oil.

Investigation on the performance of thermally aged natural ester fluid impregnated pressboard material

This paper reports the key findings from an experimental study carried out on thermally aged natural ester fluid impregnated pressboard (EOIP) material. The results show that the surface discharge inception voltage (SDIV) with thermally aged EOIP material is higher under negative DC voltage compared with positive DC and AC voltage. The SDIV increases with AC voltage frequency and a minimal reduction in voltage was observed under harmonic AC voltages with different total harmonic distortion (THD). The rate of change in voltage due to ripple content in DC voltage affects the SDIV of EOIP material, initiating discharges at lower voltages. The UHF signals radiated during the surface discharge process have a bandwidth in the range 0.9 to 2 GHz. Surface potential variation studies indicated the formation of high trap sites and trap density due to thermal aging. A reduction in the oxidation onset temperature and the activation energy of thermal decomposition were observed with thermally aged EOIP material. A pyrolysis study showed high composition of levoglucosan and acidic residue with the thermally aged specimen, which reduces its degree of polymerization. Impedance spectroscopic analysis indicates a non-Debye type of relaxation with thermally aged EOIP material. The mobility and charge transfer characteristics were also inferred from the modulus spectroscopy which showed less change for thermally aged pressboard material.

Investigation on Insulation Performance of Thermally Aged Natural Ester Oil Impregnated Pressboard

An attempt is made to understand the effect of thermal ageing on natural ester oil impregnated pressboard material. Surface discharge inception voltage of natural ester oil impregnated pressboard material is reduced drastically under AC, harmonic AC voltages and with DC voltages with high ripple content. Phase-resolved partial discharge studies show an increase in the number of discharges for natural ester oil impregnated pressboard (OIP) aged at higher temperature, under AC and harmonic AC voltages with different total harmonic distortions. Surface potential measurement study complies with results obtained by surface discharge inception voltage. It indicates the presence of high-density charge traps evolved as a function of ageing time and temperature. Laser-induced breakdown spectroscopy analysis is carried out to understand the characteristic variation with thermally aged OIP material. A decrease in plasma temperature is observed for sample aged at higher temperature. Differential scanning calorimetric studies indicate reduction in peak temperature with thermally aged specimens. Ageing observed to affect mechanical strength of the cellulosic material, as indicated by tensile test. Dielectric response spectroscopy study indicates that with an increase in thermal ageing, the permittivity, tan(δ), DC conductivity and relaxation time have increased. A reduction in activation energy of the aged material is observed, which indicates the status of the OIP material.

Understanding the electrical, thermal, and mechanical properties of epoxy magnesium oxide nanocomposites

Epoxy MgO nanocomposites exhibit high surface discharge inception voltage (SDIV) in SF6 gas ambience. Increase in SDIV is observed with 3wt% of MgO-added epoxy nanocomposites, especially when the SF6 ambient gas pressure is high. Impact of thermal ageing of epoxy nanocomposites on SDIV is minimum. Dielectric response studies were carried out experimentally and compared with modelling studies. The surface potential decay and charge trap characteristics of epoxy nanocomposites vary with percentage of nano filler. Thermal aging of epoxy nanocomposites has less influence on surface potential decay characteristics for 1 and 3 wt% MgO-added epoxy nanocomposites, indicating anti-thermal aging characteristics. Surface discharge activity generates UHF with its dominant frequency at around 1 GHz. The UHF signal magnitude formed due to SD activity is minimum with 3 wt% MgO-added epoxy nanocomposites. Laser-induced breakdown spectroscopy (LIBS) studies indicate that plasma temperature and threshold fluence increases with increase in wt% of MgO in epoxy resin. Tensile and flexural properties are greatly improved for nanocomposite compared to epoxy. The dynamic mechanical analysis (DMA) indicates increased storage modulus and reduction of tan δ with epoxy nanocomposites. The glass transition temperature (Tg) and activation energy increases with wt % of nano filler.

Understanding the electrical, thermal and mechanical properties of LDPE‐clay nanocomposites

The performance of low-density polyethylene (LDPE) clay nanocomposites was analysed. The inclusion of nano montmorillonite (MMT) clay in LDPE material has significantly increased the contact angle, corona ageing resistance, water droplet initiated corona inception voltage and surface discharge inception voltage of the composites. The surface charge decay rate of the samples significantly reduced on the inclusion of clay indicating modified trap distribution characteristics due to the inclusion of the filler. Dynamical mechanical analysis indicates increased storage modulus and reduced tan (δ) due to nanofillers inclusion. Laser-induced breakdown spectroscopy indicates that on inclusion of nanofillers the plasma temperature increases and crater depth decreases. In particular, increased discharge resistance, improved thermomechanical properties are observed with LDPE–MMT clay composites compared to pure LDPE.

The role of time-lag in the surface discharge inception under positive repetitive pulse voltage

Polyetheretherketone is used in the packaging of high voltage power electronics as a frame material, and the surface discharge behaviors of polyetheretherketone are the main concerns for the packaging design. Pulse width modulation voltage, which shows a variable duty cycle, is applied to polyetheretherketone at working conditions. The surface discharge behaviors of polyetheretherketone under a positive repetitive pulse voltage of 50 Hz with different duty cycles were investigated in this paper. It is generally considered that the surface discharge inception is mainly related to electric field distribution; however, the pulse width of voltage could significantly affect the surface discharge inception characteristics. In this study, the surface discharge inception voltage, time-integrated streamer images, and cumulative surface discharge patterns under a positive repetitive pulse voltage of 50 Hz were obtained. It was found that the surface discharge inception voltage decreased with the increase in the duty cycle. This relationship was explained by the role of time-lag. Time-lag distribution analysis was performed, and the time-lag distribution was mainly determined by stochastic time-lag. The mean stochastic time-lag was obtained by the Laue plot, and the relationship between the mean stochastic time-lag and applied voltage could be well fitted by the inverse power law. Furthermore, the relationship between the surface discharge inception voltage and duty cycle could be exactly fitted by the inverse power law, which was derived from the relationship between the stochastic mean time-lag and applied voltage. The closely related relationships demonstrate that time-lag plays a critical role in the surface discharge inception at different duty cycles.

Understanding the fundamental properties of epoxy molybdenum disulfide nanocomposites

The surface discharge inception voltage (SDIV) of epoxy MoS2 nanocomposite is high under DC voltage compared to AC voltages. It is also observed that SDIV increases up to 0.5 wt% of MoS2 added epoxy resin and above which a marginal reduction in SDIV is observed. Surface charge accumulation is less with epoxy nanocomposite compared with epoxy resin. With increase in percentage of MoS2, complete reduction in charge retention time is observed. Surface roughness measurement with the surface discharge caused damage zone with epoxy nanocomposites indicate that surface roughness is less when the percentage of MoS2 is increased and above certain weight percentage, the surface roughness is almost same. Epoxy nanocomposites show a marginal increase in contact angle with increase in weight percentage of molybdenum disulfide. LIBS studies indicate that the crater depth and the plasma temperarure measured is low for epoxy nanocomposites with 0.5 wt% MoS2. POLYM. COMPOS., 40:1556–1563, 2019. © 2018 Society of Plastics Engineers

Thermal aging of cellulosic pressboard material and its surface discharge and chemical characterization

Cellulosic pressboard is a key insulation material in oil filled transformers. Surface discharge or partial discharge degrades the insulation material and reduces the life time of the transformer. This study is aimed at understanding the effect of thermal aging of pressboard on the variation of its surface discharge and chemical characteristics. Thermal degradation of pressboard was carried out at 180 °C for 500 h in presence of ester oil under simulated aging conditions. The variation of salient electrical characteristics like surface discharge inception voltage (SDIV), charge measurement and optical emission spectra of the discharges were assessed at different aging periods. A significant reduction in SDIV was observed due to thermal aging. The charge measurement indicated that total charge deposition on pressboard increased with aging duration. Optical emission spectroscopy yielded valuable information regarding the elements evolved during discharge activity. Chemical characterization of the virgin and aged pressboard samples was carried out using analytical pyrolysis coupled with gas chromatograph-mass spectrometric technique (Py-GC/MS). The production of anhydrosugars, furan derivatives and oxygenates from the pressboard increased with aging time, while the esters in the oil were degraded to carboxylic acids with aging. Owing to the weakening of the hydrogen bonding network in oil impregnated pressboard with thermal ageing, high yield of end-chain depropagation products like anhydrosugars was observed during pyrolysis. The weak sites in cellulosic pressboard are expected to act as charge deposition sites.

Understanding the physico‐chemical and surface discharge properties of epoxy silicon carbide nanocomposites

This study focuses on characterization of silicon carbide (SiC)‐epoxy nanocomposites with different mass loading of SiC using different techniques to evaluate the physicochemical, thermo‐physical and surface discharge activity of the insulating materials. A marginal increase in surface discharge inception voltage (SDIV) was observed with increase in wt% loading of SiC in the epoxy but not much variation is observed with increase in supply frequency. Surface discharge process radiates electromagnetic waves in the UHF signal frequency range of 0.5–1.5 GHz. The dielectric properties of the material get altered with the percentage of SiC material in epoxy resin. TEM analysis indicates uniformity of dispersion of nano particle in epoxy resin. Charge retention capability of the material is high when the deposited charges are unipolar. Optical emission studies indicate the presence of silicon peaks with SiC‐epoxy nanocomposites in wavelength range of 350‐400 nm. Oxygen peaks were observed for all samples. Using the spectral peak positions and intensities of the peaks obtained, the plasma temperature for SiC‐epoxy nanocomposites was estimated to be c.a. 2240 K low when compared to that for pure epoxy. LIBS spectra clearly indicate an increase in carbon intensity with corona damaged specimen. Also for the same level of fluency, the amount of damage is less with higher percentage of SiC included epoxy nanocomposites. Pyrolysis GC/MS study indicates that the presence of SiC in the epoxy matrix enhances the timescale of primary pyrolysis vapors such as bisphenol components to form more monomeric phenols via secondary cracking reactions. POLYM. COMPOS., 39:3268–3279, 2018. © 2017 Society of Plastics Engineers

Understanding surface discharge activity with epoxy silicon carbide nanocomposites

Surface discharge inception voltage (SDIV) with epoxy silicon carbide nanocomposite material in LN2 medium is about three times higher than that in air. Under AC voltage that SDIV increases with increase in wt% of silicon carbide (SIC) material in epoxy nanocomposites. Dielectric measurements indicates an increase in temperature of the specimen and the weight percentage of SIC filler in epoxy resin increases the permittivity of the material. Also, irrespective of percentage of SiC in epoxy resin, the tan(δ) reduces with increase in frequency and the converse at room temperature. The UHF signal measured during surface discharge activity in air/LN2 medium has frequency content in the range of 0.5–1.5 GHz. The charge decay characteristics of epoxy SiC nanocomposites indicate the deposited charge is less and the charge decay time reduces with increase in wt% of SiC in epoxy resin. The surface roughness caused due to surface discharge activity is high under negative DC voltage compared with positive DC/AC voltage. The phase resolved partial discharge (PRPD) analysis indicates discharges occur at the near peak of the AC voltage on inception of surface discharge and at higher stress it appears at the rising portion of AC voltage. POLYM. ENG. SCI., 57:1349–1355, 2017. © 2017 Society of Plastics Engineers

Understanding the dielectric properties of pressboard material thermally aged in Dibenzyl Disulphide (DBDS) included transformer oil

Surface Discharge Inception Voltage (SDIV) of unaged and copper sulphide diffused Oil Impregnated Pressboard (OIP) material was studied by adopting Ultra High Frequency (UHF) technique. Copper sulphide diffusion into OIP material was carried out by thermal aging of pressboard material in Dibenzyl Disulphide (DBDS) added transformer oil. The SDIV reduces with increase of thermal aging temperature. The current pulse injected in positive and negative half cycle during the surface discharge of the AC shows the rise time of 0.8ns and 1ns respectively. The FFT analysis of UHF signal measured during surface discharge process has shown frequency content in the range 0.7–1.2 GHz with its dominant frequency at 0.9 GHz. The life of the charge presented on the pressboard increases with thermal aged specimen. Energy dispersive spectroscopy (EDS) analysis of OIP material indicates presence of copper and sulphur content. Frequency Domain Dielectric Spectroscopy (FDDS) studies indicate the permittivity(er), dissipation factor (tan(δ)), conductivity (σdc) increases and relaxation time (τ), coefficient (α) decrease with OIP material (in air / Nitrogen) aged at higher temperature in DBDS added transformer oil. Cole-Cole double relaxation model was used to understand the dielectric parameters of copper sulphide diffused OIP material. The DC conductivity of pressboard material aged in DBDS included transformer oil follows Arrhenius law and the activation energy decreases with increase in temperature of thermal aging.

Understanding the surface discharge characteristics of thermally aged copper sulphide diffused oil impregnated pressboard material

An attempt has been made to understand the impact of thermal aging with copper sulphide diffused oil impregnated pressboard (OIP) material for surface discharge inception voltage (SDIV) variation. The SDIV of Copper sulphide diffused OIP material reduces with increase in thermal aging. The UHF signals generated due to surface discharges with copper sulphide diffused OIP at the point of inception have frequency content in the range 0.7-1.5 GHz with its dominant frequency at 0.9GHz. HFCT measurement indicated that the rise time of injected current pulse due to surface discharge activity in the negative and positive half cycle of the AC voltage are about 0.8ns and 1ns respectively. It is also found that increase in thermal aging of copper sulphide diffused OIP, the amplitude and width of the current pulse increases. The magnitude of surface charge and its mean life is high with thermal aging temperature of copper sulphide diffused OIP material. Polarity of surface charge acquired by OIP due to AC corona charging depends on the surface condition of the pressboard material and its charge decay rate is lower than under DC voltage. Atomic Absorption Spectroscopy (AAS) and Energy Dispersive Spectrometry (EDS) study confirms the presence of copper sulphide on the surface of OIP and is found to be increased for copper sulfide diffused OIP material compare to unused OIP material. Scanning Electron Microscope (SEM) analysis indicates formation of cracks with thermally aged OIP material. Phase Resolved Partial Discharge (PRPD) study indicates that the surface discharge activity occurs near the peaks of the applied AC voltage at inception and at the rising portion of the applied AC voltage at higher voltages. It is found that the tensile and flexural strength reduces drastically for thermally aged copper sulphide diffused OIP material compare to unused specimen.

Influence of harmonic AC voltage on surface discharge formation in transformer insulation

The Ultra High Frequency (UHF) signal radiated during the surface discharge process under harmonic AC voltages with different Total Harmonic Distortion (THD) shows a dominant frequency at 0.8 GHz. The surface discharge inception voltage decreases with an increase in harmonics and THD as observed in the present work. The peak factor of the harmonic voltage and the surface discharge inception factor show inverse relationship. The rise time of injected current pulse due to surface discharges measured using High Frequency Current Transformer (HFCT) is about 600 ps and 900 ps for the positive and negative current signal, respectively. Cumulative energy spectra and Ternary plot confirms that UHF signal generated due to surface discharge process does not significantly vary under harmonic AC voltage with different THD's. The magnitude of UHF signal formed due to surface discharge under 50 Hz AC voltage is higher than that formed under the harmonic AC voltage. Surface charge measurement after corona charging process with the oil impregnated pressboard material indicates that the amount of charge deposited is high with the specimen damaged under harmonic AC voltage. Degraded zone of oil impregnated pressboard insulation due to surface discharges was analyzed using Attenuated Total Reflectance Fourier Transform Infra-red (ATR-FTIR) spectroscopy. Phase resolved partial discharge (PRPD) studies indicate that surface discharge activity predominates when the rate of rise of voltage is high.

Understanding surface discharge activity in copper sulphide diffused oil impregnated pressboard under AC voltages

The surface discharge injected current were measured by using High Frequency Current Transformer (HFCT). It is observed that the rise time of the current pulse is about 800ps. In addition, the rise time of the current pulses has not altered in copper sulphide (CuS) diffused Oil Impregnated Pressboard (OIP) material, but the width of the current signal has increased with the increase in the amount of copper sulphide diffusion into the pressboard. It is confirmed that the injected current pulse radiates Ultra High Frequency (UHF) signal and the dominant frequency of the UHF signal formed due to the surface discharge injected current is about 0.9 GHz. The characteristic frequency contents of the UHF signal formed due to surface discharge activity is the same with the virgin and copper sulphide diffused pressboard material. The surface discharge inception voltage is high with the virgin specimen compared with copper sulphide diffused oil impregnated paper insulating material and is the same irrespective of the amount of diffusion of copper sulphide to it. The Phase Resolved Partial Discharge (PRPD) analysis indicates that at the point of surface discharge inception, the discharge occurs at the peak of the applied AC voltage and when the applied voltage magnitude is increased, the discharge occurs at the rising portion of the applied AC voltage. The characteristics of the surface discharges formed are the same with the copper sulphide diffused oil impregnated pressboard material. It is noticed that with increase in ageing time, a reduction in partial discharge magnitude is observed. Attenuated total reflectance Fourier transform infra-red spectroscopy (ATR-FTIR) analysis was carried out to understand the characteristic changes that occur in the degraded zone formed due to surface discharges. The measurement of surface charge indicates that ageing of oil impregnated paper material increases the surface charge accumulation. Energy Dispersive Spectroscopy (EDS) confirms the presence of copper and sulphur in the aged Oil Impregnated Pressboard material. The Atomic Absorption Spectroscopy (AAS) results indicate that the amount of copper content in pressboard material have increased with ageing time.

Analysis of surface discharge activity in epoxy nanocomposites in liquid nitrogen under AC voltage

A marginal increase in surface discharge inception voltage is observed with increase in percentage of clay in epoxy nanocomposites in liquid nitrogen medium. The flashover voltage of epoxy nanocomposites increases with increase in percentage of clay in epoxy resin up to 3 wt% and above which a slight reduction is observed but much higher than the pure epoxy resin.The intercalated and exfoliated properties were analyzed through TEM studies.Contact angle marginally vary with percentage of clay in epoxy resin. The surface variations of epoxy nanocomposites analysed through surface roughness measurement and by attenuated total reflectance Fourier transform infra-red spectroscopy (ATR-FTIR).The rise time of injected current pulse due to surface discharge process is measured to be 800 ps. The UHF signal generated due to surface discharge (at the point of inception and propagation) formation on epoxy resin with different percentage of clay in liquid nitrogen medium have dominant frequency near 1 GHz. The PRPD pattern obtained at the point of inception of surface discharges, the magnitude of discharges is nearly same in both half cycle of the applied AC voltage. Surface discharge occurs in the rising part of the applied AC voltage. When the applied voltage magnitude is increased, the higher magnitude discharges occurs at the positive half cycle of the applied AC voltage.