Showing papers on "Pressboard published in 2013"

Surface tracking on pressboard in natural and synthetic transformer liquids under AC stress

Abstract: Experimental investigations were carried out to study the tracking process on pressboard in esters under ac voltages. The evolutions of creepage discharges were analyzed by correlating the visual records of surface tracking, the phase resolved PD patterns, the discharge current signals and the high speed shadowgraph images. It is found that the impregnated pressboard is susceptive to discharge erosion occurring on and inside the pressboard, which is characterized by "tree-shaped white and carbonized marks". The gaseous "white mark" channels would attract the subsequent discharges to follow the same routes, and the accumulative energy dissipation in these channels would then result in the carbonization of the channels and the emergence of a "carbonized mark". Once formed, the tree-shaped mark can continue to grow even under a reduced voltage or after a re-impregnation, until it bridges the gap and causes a final flashover. For a mineral-oilimpregnated pressboard, high moisture content (>3.5% by weight) is a necessity to initiate a "white mark"; whereas the "white mark" appears at a lower voltage and develops more easily on a dry ester-impregnated pressboard.

Creepage discharge on pressboards in synthetic and natural ester transformer liquids under ac stress

Abstract: This study reports experimental studies of creepage discharges on the pressboard (PB) surface in esters under ac divergent electric field. With the help of commercial partial discharges detector, wideband current measurement and high-speed image recorder, the differences of discharge patterns between open gap and on PB surface were studied in esters as well as in mineral oil. The results show that the presence of PB surface tends to promote discharges, especially negative discharges, and enables more discharges to occur at smaller phase angles. It is believed that the promotion effect of PB surface on discharges is caused by surface charges and bubble residences; because of more intense discharges and higher viscosity of esters, this effect is more significant in esters than in mineral oil.

A review of moisture diffusion coefficients in transformer solid insulation-part 1: coefficients for paper and pressboard

Abstract: The methodologies used by various researchers to determine the moisture diffusion coefficients in Kraft paper and pressboard transformer insulation are reviewed. Moisture concentration profiles during insulation drying, calculated using the various coefficients, are presented.

Investigation on creeping discharges propagating over pressboard immersed in mineral and vegetable oils under AC, DC and lightning impulse voltages

Abstract: This paper deals with the experimental characterization of discharges (namely the shape and stopping length) propagating over pressboard of different thicknesses immersed in mineral and vegetable oils, under AC, DC and lightning impulse voltages (1.2/50 μs) using a pointplane electrode arrangement; the point being perpendicular to pressboard. It's shown that the thickness of pressboard and the type of oil as well as the magnitude and polarity of voltage significantly influence the characteristics of discharges. For a given thickness, the stopping length of discharges Lf increases quasi-linearly with the voltage (U); and it is longer with a positive point than with a negative point whatever the type of voltage. Lf decreases when the thickness increases. The thinner the pressboard, the more branched are the discharge structures. The density of discharge branches increases when the pressboard thickness is reduced. On the other hand, for a given voltage and thickness of pressboard, Lf is longer under AC than under DC whatever the type of oil. Also, the characteristics Lf(U) in presence of processed vegetable oil and mineral oil are very close except for thin pressboard. The influence of electric field and capacitive effects on the propagation mechanism is also evidenced.

Fractal analysis of lightning impulse surface discharges propagating over pressboard immersed in mineral and vegetable oils

Abstract: This paper is aimed at the experimental study of discharges propagating over pressboard immersed in mineral and vegetable oils and the analysis and comparison of the fractal dimension of these discharges. The shape, stopping length (i.e., the maximum extension of creeping discharge) Lf, current and electrical charge of discharges are investigated versus the thickness (e) of pressboard and the type of oil in a point-plane electrode geometry stressed by a standard lightning impulse voltage. It is shown that Lf decreases when e increases and it is longer when the point is positive than with a negative point. The smaller e, the more branched the discharge structures are, indicating the important role of the electric field and capacitive effect in the propagation mechanism. Lf is generally greater of 60% in vegetable oils than in mineral oil when the point is positive and it is of 40% greater when the point is negative. By using the box counting method, we show that the observed discharge patterns present a fractal dimension D which depends on e and the kind of oil. D decreases when e increases. The value of the density of branches increases when the e decreases. D is generally higher in mineral oil than in vegetable oil. This dependency on the thickness of pressboard and the nature of oil reveals the existence of a relation between the fractal dimension and the physicochemical parameters of interface.

Investigation on thermal aging characteristics of vegetable oil-paper insulation with flowing dry air

Abstract: In order to investigate the thermal aging characteristics of oil-paper insulation in open-type transformers, a thermally accelerated aging experiment with flowing dry air is performed. The vegetable oil, NOMEX paper, pressboard, copper and silicon steel are sealed into the aging vessels and further aged, for 216 hours, by supply of flowing dry air at 170 °C. Several important parameters associated with the aging are observed and evaluated during the aging process. It is shown that the degree of polymerization (DP) of pressboard gradually decreases and fits well into the second-order kinematic model. Little deterioration of the NOMEX paper under 170 ° is confirmed by the tensile strength which stabilizes at a certain level after aging. In addition, the logarithm of the furfural concentration in oil is found to have a good linear relationship with DP of pressboard. Interestingly, when the aging time is about 150 hours and DP is 329, the aging process reaches an inflection point at which the DP approaches the leveling off degree of polymerization (LODP) value, the moisture content and the furfural content of oil begin to fall down while the acid number and the dissipation factor of oil increase with a faster rate. However, the viscosity of the fluid, one of the important parameters of transformer operation, does not change significantly. The obtained results demonstrate that exposure to air leads to serious oxidation of oil and rapid increase of acid and dissipation factor, and this way oxygen should be avoided for the open-type transformer filled with vegetable oil.

Bridging in contaminated transformer oil under DC and AC electric field

Abstract: There are two experimental set up have been established to investigate the bridging effect between two electrodes with different potentials under ac and dc voltages. This paper will explain the full details of the experimental setup and the results. These experiments carried out on pressboard particles under three different levels of ac and dc voltages. Several contamination levels have been investigated. Optical images of particle accumulation have been recorded along with associated conduction current measurement during experiments. At higher voltages the rate of bridge formation is increased along with an associated current increase. The major differences between ac and dc bridges are the shapes of the particle accumulation. The bridge formation rate for ac is slower than dc.

Partial discharge characteristics of surface tracking on oil-impregnated pressboard under AC voltages

Abstract: Creeping discharge at the oil-pressboard interface is a serious fault condition because it can lead to catastrophic failure under normal operating conditions of large transformers. Creeping discharge leads to the development of a conducting path that is generally accepted as white marks on a board surface due to localised heating. The visible white marks indicate a drying out process of the pressboard through moisture evaporation and the breaking of oil molecules to generate gases in the pressboard pores. The processes can continue from minutes to months or even years until failure. Characterising this type of faulty condition in a condition monitoring programme is important for transformer life management. This paper explains the behaviour of surface discharge observed during a surface discharge experiment at the oil-pressboard interface using a needle-bar electrode configuration. Correlation between partial discharge (PD) activities and measured data in terms of phase resolved partial discharge (PRPD) patterns have been undertaken. The experimental results also show a decreasing trend of the average discharge magnitude during the formation of the white marks that propagate towards the earth bar. This finding is qualitatively comparable to on-line problems reported previously, i.e. there was a falling trend and no obvious evidence of imminent failure from the monitoring data of PD activity of a large power transformer that eventually suffered from an unexpected fault.

Space charge behaviour in thick oil-impregnated pressboard under HVDC stresses

Abstract: As the main insulation applied in HVDC convertor transformer, the dielectric properties of oil-pressboard insulation system play an important role in the reliability of power system One of the key problems related to any HVDC insulation systems is the easy formation of space charge within the bulk and at the interfaces In this paper, space charge behaviour in the impregnated pressboard with 1mm in thickness is investigated using the pulsed electroacoustic (PEA) technique under different DC stresses at the room temperature To study the influence of transformer oil status, the 1mm thick dried pressboards are separately impregnated with fresh oil and serviced aged oil The PEA results show that homo-charge injection occurs for all the measurements Both applied stress and oil status can affect the space charge behaviour, however, it has been found the oil status is the dominant factor in the present study The electric field enhanced by the charges is about 45% in the aged oil samples and 15% in the fresh oil samples

Surface flashover characteristics of oil-paper insulation under combined AC-DC voltage

Abstract: The valve side winding of the converter transformers withstood several types of voltage. This paper investigated the effects of voltage component on the surface flashover characteristics of oil-paper insulation under combined AC-DC voltage in different ratio. The whole flashover process of surface flashover of oil-paper insulation under combined AC-DC voltage was studied by using two types of electrodes. The experimental conclusion was shown that the flashover voltage increased with the DC ratio in the combined AC-DC voltage component. Flashover voltage decreased significantly when the radial electric field increases in the electric field component. Furthermore, electric field distributions versus combined AC-DC voltage in different ratio were calculated by Finite Element Analysis (FEA). Due to the different distribution characteristics of DC electric field and AC electric field, when the DC ratio was higher, the surface flashover location tended to pressboard surface. Conversely, when the AC ratio was higher, it tended to the oil gap between the electrodes. Moreover, based on high-speed camera, it was used to capture emitted light produced by surface flashover, record flashover initiation and propagation process in small oil gap between the electrodes.

Characterization and Modelling of the Mechanical Properties of Pressboard

Abstract: Cellulose-based components constitute the bulk of the current insulation for transformers. Cellulose is an organic polymer material which combines excellent electrical properties and good mechanical performance. As a polymeric material, cellulose is very sensitive to moisture and temperature. These factors can influence the electrical and mechanical performance of a transformer throughout its lifetime. In order to ensure the quality of the product during transformer manufacturing, as well as during transformer life-time services, adequate models for predicting the physical properties of its constituents are needed. The present investigation tackles the mechanical description of pressboard. For this purpose, a three dimensional mechanical model is developed for simulating the in-plane and out-of-plane behavior of the pressboard material. The model is based on an anisotropic viscoelastic-viscoplastic constitutive law, which includes features that are particular for cellulose-based materials, e.g. the peculiar double nature of fiber-network-based and porous material. The material is orthotropic by nature, i.e. the in-plane mechanical properties markedly differ from the out-of-plane ones. Particular regard is taken when considering the effect of out-of-plane stresses which both cause viscous deformation and permanent compaction. The analyses on the mechanical behavior of pressboard are performed by comparing the experimental data on pressboard and the results of model simulations.

Relay protection device pressboard outlet logic detection apparatus

Abstract: The invention relates to a relay protection device pressboard outlet logic detection apparatus. The logic detection apparatus comprises a relay protection device outlet pressboard ms-level instantaneous voltage impulse detection circuit, a relay protection test-stand dry contact state detection circuit, a microprocessor, a drive, timing display and sound-light alarm circuit and a power indication and grounding disconnection indication circuit, wherein the output terminals of the relay protection device outlet pressboard ms-level instantaneous voltage impulse detection circuit and the relay protection test-stand dry contact state detection circuit are connected with corresponding input terminals of the microprocessor respectively; the output terminal of the microprocessor is connected with the input terminal of the drive, timing display and sound-light alarm circuit; the input terminal of the power indication and grounding disconnection indication circuit is connected with the output terminal of the drive, timing display and sound-light alarm circuit. The advantages of the logic detection apparatus are that: detection accuracy and sensitivity are high, and wiring and operation are simple.

Influence of temperature on developing process of surface flashover in oil-paper insulation under combined AC-DC voltage

Abstract: In order to study the influence of temperature on surface flashover behavior with oil-paper interface under combined AC-DC voltage, a discharge test assembly was established to investigate the influence of temperature on developing process of surface flashover of oil-paper interface from 25 °C to 90 °C. Based on impulse current method, the process of surface flashover of oil-paper interface under combined AC-DC voltage was studied by electrodes configurations that simulated the tangential electric field and radial electric field of surface of pressboard, in which the voltage was increased step-by-step. Meanwhile the partial discharge (PD) spectrums at whole flashover stages were studied. Moreover, high speed camera was used to record the whole process of surface flashover which was divided into 4 stages: (1)Inception stage, (2)Streamer stage, (3)Intermittent breakdown stage, (4) Surface flashover stage. The experiments results shown that flashover voltage and PD inception voltage at high temperature (HT) was lower than the voltage at low temperature (LT). The average amplitudes of PD at LT were lower than at HT. The growth rate and repetition rate of PD at HT was faster than at LT. It was easier to develop the flashover at oil-paper interface under combined AC and DC voltage at HT. So the flashover voltage was lower at HT. the surface of pressboard was destructed severely at HT, the track of surface discharge was obvious at HT.

Electrostatic Hazard in High-Power Transformers: Results of Ten Years of Experience With ${P}^{\prime}$ Capacitive Sensor

Abstract: In a high-power transformer, oil flowing on pressboard surface is suspected to be responsible of electrostatic hazards and failures. Different methods of risk assessment have been proposed to understand and prevent it: ministatic tester in the Westinghouse protocol, ministatic tester in the spinning disk measurement, monitoring of tangent delta, dissolved gas measurement, etc. At P' Institute of Poitiers, an original sensor was developed and used for quantification of the electric charge generated and of accumulated charge for an oil flow onto the surface of a transformer pressboard insulated from ground. Operational for ten years, this bench has been used to study over a hundred couples of oil/pressboard, pairs of new oil and pressboard, pairs of aged oil and pressboard, pairs of suspicious oil and pressboards, etc. This paper presents a comparative analysis of these ten years of experience. This analysis provides, among other results, a critical electrostatic hazard assessment in transformers and an attempt of discrimination tentative of a suspected transformer.

Study of surface discharge behaviour at the oil-pressboard interface

Abstract: This thesis is concerned with the surface discharge behaviour at the oil-pressboard interface. For large transformers this is classified as a serious failure mode because it can lead to catastrophic failure under normal AC voltage operating conditions. To increase understanding on this failure mode, a surface discharge experiment at the oil-pressboard interface has been conducted on different moisture levels in pressboard by applying a long period of AC voltage stress. The processes in the surface discharge at the oil-pressboard interface until the appearance of a first full discharge have been recognised and correlated with the measured data. The results show that the different moisture levels within the pressboard play an important role on the partial discharge (PD) activity of certain processes. The decreasing trend in the PD data during the surface discharges cannot be treated as a reliable condition monitoring measure of health because it is the key indicator of white marks propagation toward the earth point. The characteristics of full discharge events have been analysed to develop knowledge for condition monitoring of surface discharge at the oil-pressboard interface. Full discharges are corona-like events in which their random occurrences are dominated by accumulated charges on the pressboard surface along the white marks rather than the polarity of applied AC voltage. A 2-D axial symmetry surface discharge model has also been developed using COMSOL Multiphysics, a finite element analysis (FEA) software package. The model considers the pressboard region near the interface (a transition region) as porous, whilst in the bulk region of pressboard as a perfect insulator. The model is developed using continuity equations and coupled with the Poisson’s equation to study the problem in terms of charge transport mechanisms and electric field distributions. The thermal conduction equation is included to study the thermal effects of surface discharge activity at the oil-pressboard interface. The behaviour of surface discharge is studied by validating the simulated surface discharge current pulse with the measured current. The simulation results show that a field dependent molecular ionisation mechanism plays an important role in the streamer propagation during the period of the rising front of the current pulse, whilst during the period of decaying tail of the current pulse, the contribution of an electron attachment process is dominant. The modelling results suggest that degradation marks (white and black marks) are due to high energy over long periods of partial discharge events that lead to thermal degradation at the oil-pressboard interface.

Oil conductivity – an important quantity for the design and the condition assessment of HVDC insulation systems

Abstract: HVDC (high voltage direct current) transmission is a very effective way of power transmission and it is necessary to supply the worlds growing amount of energy. In HVDC equipment the major insulating materials are oil and pressboard. Oil conductivity is an important quantity for the design, the diagnosis and the condition assessment of HVDC insulation systems. Unfortunately, insulating oil has a strongly nonlinear conductivity which depends on various parameters (field strength, temperature, water content and ageing) and is time-dependent because of charge carrier drift processes. Measuring methods are required to provide the information necessary for the condition assessment of HVDC systems. Therefore, measurements of new and aged oils are compared and taken into account for a diagnosis of oil-filled HVDC insulation systems. Furthermore, today’s standards are analyzed and compared to the so called PDC method. Relations between initial and steady-state conductivities at different temperatures, field strengths and for various oil types are presented and consequences for the condition assessment of insulation systems are discussed.

Comparison of partial discharge inception voltages for ester and mineral oils under divergent fields

Abstract: Ester oils are making their way into the transformer market, but still some doubts exist about their ability to be successful fluids for high and extra-high voltage transformers. In this paper, we compare the PDIV of two commercially available fluids: a natural ester and mineral oil. Two different setups were considered: Point to plane in oil and point to plane on a pressboard sheet immersed in oil. Fluids and pressboard sheets were fully conditioned prior testing. Results suggest that ester oils perform slightly better with simple point-to-plane electrodes, independently of the gap length. When testing oil/plane electrode configurations with pressboard, ester oils perform much better at short gaps. This behavior has been explained considering the hydrophilicity of ester oils, which helps in further drying the pressboard prior testing. Therefore, ester oils could prove superior to mineral oils in suppressing creeping discharges.

Development rules of inter-turn partial discharge defects on oil-paper insulation

Abstract: In power transformers, the main reason why insulation faults occur is that partial discharge (PD) produces insulation deterioration at the weakest point of the insulation structure and ultimately lead to insulation breakdown. The insulation defects exist in the form of electrical trees or carbon channels reduce the effective insulating distance. Take the opaqueness of pressboard into consideration, the carbon channels generated by PD can't be easily observed. Therefore, in this paper, tests based on inter-turn partial discharge model are conducted to study the development rules of oil-paper insulation material defects. Constant voltage application was adopted in this series of tests. High voltage of 40 kV is applied on the inter-turn electrode specimens for different time in different groups of tests. After test, insulation tapes are stripped from inter-turn electrodes layer by layer so as to observe the carbon channels clearly. Results indicate that the development process of the carbon channel can be roughly divided into three stages. Analysis of discharge feature reveals two different shapes of q-φ scatter diagrams, the shape of seeds and the shape of inverted tacks. The shape shown in scatter diagram could be used as effective criteria to diagnose the inter-turn insulation defects produced by partial discharge at later stage.

Condition assessment of instrument transformers using dielectric response analysis

Abstract: Dielectric response analysis is an advanced technique, providing information about the insulation condition of the measured test object. It is typically used for power transformers, where the water content in the solid cellulosic insulation can be assessed. Water in oil-paper/pressboard insulations is decreasing the dielectric strength, accelerating the ageing and causing the bubble effect at high temperatures. Also instrument transformers are oil-paper insulated and therefore exposed to the harming effects of water. The condition assessment of the insulation system of instrument transformers has not played an important role up to now since typical measurement methods which might be used for power transformers are not applicable here. For example oil sampling is often avoided due to the small oil volume. However, the increasing age is leading to a higher risk of explosions, which can damage surrounding parts and cause high follow-up costs. This paper introduces a new approach to use the dielectric response analysis for condition assessment of instrument transformers. The measurement of the dielectric response in a wide frequency range (e.g. 100 μHz up to 5 kHz) provides information about the insulation condition and especially for oil-paper/pressboard insulations about the water content in the solid insulation. The measured curve is formed by the superposition of the dielectric response of the cellulosic material and the oil. It is reflecting the conductivity and polarization effects of each material. Due to the high content of cellulosic material, the oil has a minor impact on the dielectric response of an instrument transformer and the curve is highly determined by the behavior of the cellulosic material. The resulting curve can be used for the assessment of the insulation condition in various ways. Qualitative comparisons, e.g. between instrument transformers of the same type and age, might give general information which of these is in better condition. The dissipation factor and the capacitance trend can also deliver valuable data. Furthermore, the water content in the solid insulation can be assessed. An automatic analysis algorithm compares the measured dielectric properties of the actual instrument transformer with modeled dielectric properties to calculate the water content in the solid insulation. This paper describes the modeling in concerns of water content, oil conductivity and geometry. Furthermore, the influences of temperature and ageing products are explained. To confirm the validity of this assessment, measurements on several instrument transformers in service were performed. Dielectric response curves and the trend of the capacitances for new and aged instrument transformers with different water contents are discussed. Selected examples presented in this paper show, that the dielectric response is well-modeled and the water content can be derived. Also ageing effects can be observed. To avoid explosions of their instrument transformers, this method is now regularly used by TNB.

Study on the influence of gases on white marks and carbonized tracks generated by partial discharge in transformer oil-paper insulation

Abstract: Partial discharge (PD) in oil-paper insulation of transformer is considered to be a serious fault condition. The movement of excessive charge over pressboard may result in mechanical and chemical damage to material through thermal and electronic action. With the development of PD, gases, white marks and tracks appeared. To research the relationship among the gases white marks and carbonized tracks, a needle-plate electrode configuration was set in laboratory. Pressboards with grooves of varying depths were made to release the gases. And all the pressboards were dissected layer by layer so as to observe carbonized tracks inside the pressboard. Besides, the phenomena of gases, white marks and tracks were recorded by taking photos. The experiment showed that: with the development of PD, carbonized tracks appeared on the pressboards. And as the decomposition of the insulation oil and pressboards, a large amount of gases were generated and overflowed through the pressboard pores, then white marks were formed; Moreover, due to the existence of residual gases, the surface of pressboard was raised where PD happened; As the gases loosed and even broke the pressboards' structure, PD took place more easily. As a result, carbonized tracks grew to be more.

Partial discharge activities of pressboard/oil insulation under AC plus DC voltages

Abstract: In this paper, focus is made on partial discharge (PD) activity in oil-impregnated pressboard used in converter transformers. Voltages over the main insulation of the valve-side windings of the converter transformer were analyzed using the PSIM software. Next, transformer defects were simulated by insulation models to generate PD signals. The dependence of PD inception voltage (PDIV) on the AC plus DC voltages with and without pre-charging is reported along with PD patterns. Experimental results show that: a) using AC plus DC voltages, PDIV values need pre-charging to be estimated accurately, b) activities can be more energetic than those under sinusoidal AC voltages, although repetition rates are generally lower.

Deterioration of solid insulation for thermal degradation of transformer oil

Abstract: Power transformers are the most vital components of power system. Insulation used in power transformers are of insulating fluids and solid insulation system like pressboard, Kraft paper. In this work an effort has been made to investigate the detoriation of solid insulation systems for thermal degradation of transformer oil. The aging degree of solid insulation system is examined through Atomic force Microscope (AFM), Scanning Electron Microscope (SEM) and X-ray diffraction (XRD) techniques.

Weibull Statistics in Dielectric Strength of Oil-impregnated Pressboard under Ramped AC and DC Voltages

Abstract: AC and DC breakdown tests were performed on large populations of oil-impregnated pressboard samples. The effect of voltage ramp rate on dielectric strength has been investigated. A statistical analysis of breakdown data was carried out using the Weibull distribution. The 90% confidence intervals of Weibull graphs were calculated. The study shows that dielectric strength and shape parameter change versus ramp rate. The variations are attributed to the size and number of defects. Discharges occur from the oil to the oil-pressboard interface and lead to breakdown. DC dielectric strength is larger than that corresponding to AC voltage. This is ascribed to the dissipated energy difference under the two types of field and the fatigue produced by the alternating voltage. This phenomenon is related to space charge. Under DC stress, dielectric strength is higher under negative polarity. It is assigned to the different quantities of space charge accumulated under the two polarities.

Conductivity of Pressboard for HVDC Insulation Systems

Abstract: The cellulosic material pressboard is a key component in insulation systems for HVDC (high-voltage direct current) equipment. Pressboard compounds are considered from a chemical point of view in order to link it in a subsequent study with electric conduction processes in pressboard. For a better understanding of pressboard, its manufacturing is described and fiber geometry is evaluated by SEM (scanning electron microscopy) analyses. Especially for HVDC insulation, pressboard conductivity is the most important property. For an air-impregnated pressboard sample, the dependence of conductivity on time, temperature and water content is presented. For this purpose, step response measurements in time domain are performed with a sensitive current device. Furthermore, the influence of oil-impregnation on pressboard conductivity is evaluated. The conductivity of oil-impregnated pressboard is up to 13 times higher than the conductivity of unimpregnated pressboard for the given conditions.

Dielectric properties of coconut oil impregnated pressboard samples

Abstract: Coconut oil has been reported as a potential alternative for mineral oil when used as the power transformer liquid insulation. However, the information on dielectric properties of coconut oil impregnated transformer solid insulation i.e. paper and pressboard are still lacking. This paper presents variations of capacitance and dielectric losses of coconut oil and mineral oil impregnated pressboard samples during oven heating. First 24 pressboard samples having either 2% or 4% moisture contents were prepared. Then the samples were impregnated by coconut and mineral oil (half each) and the samples were dried inside two ovens over 800 hours at two different temperature (110 °C and 130 °C) levels. The dielectric properties were obtained by Frequency Domain Dielectric Spectroscopy (FDS) measurements. The results showed that in both coconut oil and mineral oil impregnated pressboard samples, capacitance and dielectric losses increased with heating time, temperature level and initial moisture content. Further the losses were relatively higher for coconut oil impregnated pressboard samples in comparison to mineral oil.

The DC breakdown of mineral insulating oil and oil-pressboard

Abstract: AC-DC converter transformer is one of the most important equipments in the high-voltage D.C. transmission system. And the mineral insulating oil and pressboard immersing in oil are important insulation dielectrics used in AC-DC converter transformers. Thus, it is very significant to understand the dielectric properties of the mineral insulating oil and oil-pressboard. In this paper, the DC breakdown of two new mineral insulating oils and a pressboard immersing in these oils under different temperature has been systematically investigated. The oil and oil-pressboard was degassed and dehydrated before the test. In the testing progress, the test device of breakdown strength was protected by nitrogen and isolated from outside. The breakdown strength of two kinds of mineral insulating oil were tested at five temperatures with the range of 25-80°C, and the breakdown strength of pressboard immersed in both oil were also tested at three temperature with the same range. From the results, it can be obtained that the breakdown strength of oil increased with the increase of temperature, whereas the oil-pressboard showed opposite tendency. This paper enhanced the understanding of the breakdown strength for mineral insulating oil and pressboard immersed in oil, provided the theoretical and test basis to expand the engineering application of this mineral insulating oil and pressboard.

Hot pressboard for hot press

Abstract: The utility model discloses a hot pressboard for a hot press. The hot pressboard for the hot press comprises a hot pressboard body (1) and a thermal cycle pipeline (2) which is arranged in the hot pressboard body (1) and used for heating and drying board materials and is characterized in that a stainless steel layer (3) is covered on the surface of the hot pressboard body (1). By means of the hot pressboard for the hot press, the problem that scratches can be left on the surface of a decorating board easily by a traditional hot pressboard and the problem that the decorating board is prone to breakage caused by high temperature on a part due to the fact that a traditional heat transfer board transfers heat unevenly when the decorating board is machined through a traditional hot press are effectively solved.

The DC electric field analysis of valve side outlet device in converter transformer considering nonlinear and anisotropic characteristics

Abstract: Valve side outlet device, which consists of bushing terminal and surrounding barrier system, is the insulation structure located in the connection part between valve winding lead and bushing. The insulation structure of outlet device is made up of oil and paperboard. It is the most complicated and the most important insulation part in converter transformer, and it not only withstands AC voltage, but also withstands DC voltage in operation stage. The electric field distribution is determined by the resistivity of each material under DC steady state voltage. Resistivity of oil and pressboard are affected by the electric field strength applied to them, so the resistivity of both insulation materials is nonlinear. Moreover, the resistivity of pressboard is anisotropic. There is a great difference value between the direction of through and along the paper layer. In this paper, the nonlinear and anisotropic DC electric field distribution of valve outlet device was calculated, and the result was compared with the results which not considering anisotropism or nonlinearity of material. The comparison revealed that anisotropic and nonlinear characteristics must be taken into account in the research and design process.

Investigations of thermal degradation and spectral response of transformer oil - doi: 10.4025/actascitechnol.v35i2.15190

Abstract: In order to maintain continuous power supply in the electrical system, condition monitoring of the sensitivity equipments in power system is very essential. Among those equipment in which more concentration required is Power transformer. To determine the performance of transformer, it is necessary to have investigations on ageing of transformer oil due to thermal degradation. In this work an effort has been made to analyze various critical parameters like acidity, Breakdown Voltage (BDV), viscosity, volume resistivity, loss tangent, dielectric constant and spectral response for different range of thermal degradation of transformer oil with various compositions of materials like pressboard and Kraft paper. As initiative an exclusive Thermal-UltraViolet (UV) analysis has been carried out to investigate the spectral response of transformer oil. By estimating the condition of oil, the unexpected failure of transformer and power outages can be prevented.

Characteristics of surface charge on oil impregnated pressboard under non-uniform DC stress

Abstract: It is very important of the surface charge distributions on the oil impregnated pressboard under nonuniform DC electric field, for the design and application issue of a converter transformer, which is not reported in the literature. In order to have a better understanding of the characteristics of the surface charges on oil impregnated pressboard produced by the non-uniform DC electric field, we studied surface charge distribution on oil impregnated pressboard produced by the needle plate electrode based on an electrostatic capacitive probe measuring system. It is found that under the highly non-uniform electric field, the surface charge polarity on oil impregnated pressboard mainly depend on the polarity of applied voltage and the accumulation and dissipation characteristics of the surface charges under the positive stress are different from that under the negative stress.