Oil-impregnated Pressboard Insulation Research Articles

Effect of Temperature and Electric Field Strength on Carrier Mobility of Oil-Impregnated Pressboard Under DC Voltage

The influence of carrier mobility on the space charge transport behavior inside the oil-impregnated pressboard insulation of converter transformers cannot be neglected. However, at present, current knowledge is usually derived from empirical or theoretical values, lacks experimental studies, and often ignores the effects of temperature and field strength under actual operating conditions. In this paper, based on the variable-temperature surface potential decay (SPD) method, a carrier mobility measurement platform for oil-impregnated pressboard is established, and the carrier mobility values for different combinations of oil and oil-impregnated pressboard are obtained experimentally to analyze and reveal the influence mechanisms of temperature and field strength on the carrier mobility. The results indicate the following: (1) The positive and negative carrier mobilities of oil-impregnated pressboard are in the range of 10−12–10−13 m2·V−1·s−1, and the negative carrier mobility is always higher than the positive carrier mobility. (2) The carrier mobility is positively correlated with the changes of temperature and field strength, and when the temperature increases from 20 °C to 80 °C, the positive and negative carrier mobilities increase by 4.01 times and 4.72 times, respectively; when the field strength increases from 1 kV/mm to 7 kV/mm, the positive and negative carrier mobility increases by 2.53 and 2.72 times, respectively. (3) The carrier mobility of the pressboard with a higher oil absorption rate changes more significantly with temperature; when the field strength is 7 kV/mm and the temperature increases from 20 °C to 80 °C, the positive polarity carrier mobility increases from 3.96 × 10−13 m2·V−1·s−1 to 2.64 × 10−11 m2·V−1·s−1, an increase of 66.67 times, while the increase in the carrier mobility of the pressboard with a lower oil absorption rate is only 1.59 times. (4) The carrier mobility of the naphthenic transformer oil-impregnated pressboard is higher than that of the paraffin-based transformer oil-impregnated pressboard, and the carrier mobility of two kinds of naphthenic transformer oil-impregnated pressboard is 3.16 times and 2.47 times higher than that of the paraffin-based transformer oil-impregnated pressboard, respectively, under the conditions of 60 °C and 7 kV/mm. (5) Utilizing the Darcy model and microscopic scanning results of the pressboard morphology, it was revealed that permeability and fiber structure are key factors influencing the variation in carrier mobility. The research results of this paper can provide theoretical basis for the calibration and optimization of the oil-pressboard insulation structure of converter transformers.

Health assessment of converter transformer pressboard insulation based on FDS and digital image processing

The stable operation of the converter transformer is an essential task for power system operation reliability and security. Nowadays, Frequency domain spectroscopy (FDS) technology is prominently utilized for assessing oil-impregnated pressboard insulation. The present study examines the effect of the pressboard insulation material as a function of frequency and elevated temperature. The experimental analysis on oil-impregnated pressboard insulation is carried out at temperatures from 30 °C to 130 °C with an incremental rise of 20 °C intervals with frequency variation from 1 Hz to 10 MHz. The frequency-dependent permittivity, conductivity and loss tangent angle studies also confirm the deterioration of oil-impregnated pressboard insulation. The surface morphological changes inside the pressboard insulation are recorded with the help of scanning electron microscopy (SEM). The synergistic effect generated on pressboard insulation is examined by fiber width changeand image processing approach by randomly selecting the average of three local areas of SEM image. A canny operator is selected to extract the exact boundaries of images and more change is recorded in the edge detection count after 90 °C.The porosity and pore size distribution can be increased with elevated temperatures. A single-phase, 315 MVA valve side star winding with 60 discs of single-phase converter transformers model is developed in MATLAB Simulink. An impulse of 100 kV, 1.2/50μsec is applied across the star winding to identify the pressboard insulation degradation derived from FDS data with the help of mathematical morphology and wavelet transform technique. The energy of the wavelet coefficient on the neutral current capture during the impulse test adds a significant contribution to analyzing pressboard insulation degradation. The results presented are in good agreement with the published work.

Life Expectancy Estimation of Thermally Aged Cu Contaminant-Diffused Oil Impregnated Pressboard

Life expectancy of a thermally aged oil impregnated pressboard (OIP) in the presence of corrosive dibenzyl disulphide (DBDS) is studied in this work using a fast estimation method based on level of diffusion of Cu contaminant. Data from laser induced breakdown spectroscopy (LIBS) is used as the degradation parameter. Thermogravimetric analysis (TGA) is used for rapid evaluation of life expectancy at various temperatures. Aged OIP material is tested for its surface flashover breakdown strength and its thermal stability is studied by TGA and differential scanning calorimetry. Thermal activation energy is estimated using Flynn-Wall-Ozawa method and is agreement with isothermally calculated values. Aging law is derived and found to be nearly first order based on which temperature dependent life expectancy law is formulated. LIBS data of OIP collected from a failed transformer is considered to be end-of-life condition. Estimated life expectancy values using proposed fast method are compared with traditional long-aging evaluation and are in good agreement. Increase in corrosive DBDS concentration and change in aging medium using proposed method and are found to have a significant impact on life expectancy of the OIP insulation.

Effect of Al2O3 nanorods on the performance of oil-impregnated pressboard insulation

The mineral oil (MO) in conjunction with paper is the main insulation components in oil-immersed transformers; their insulation properties play a significant role in the safe and stable operation of power transformers. To strengthen the insulation level of ultra-high voltage transformer and to reduce its size and weight, it is imminent to enhance the insulating performance of transformer oil and oil-impregnated cellulose. Recently, a unique novel effort of suspension of nanorods (NRs) into MO has been carried out and the results have exhibited improved insulation characteristics of transformer oil. The Al2O3 NRs with favorable features were prepared in our laboratory to develop transformer oil/paper insulation system with better insulation performance. The transformer oil-based nanofluids (NFs) were prepared with nanorod shape, oleic acid surface modification and 0.8 g/L concentration of NRs. The impregnated pressboards were prepared by impregnating them into dried oil and NF under vacuum beneath 1 kPa at 80 °C for almost 48 h to obtain the oil-impregnated pressboard (OIP) and nanofluid-impregnated pressboard (NIP), respectively. The interface at solid/liquid is considered weak link and is the main reason of oil-filled transformer collapse. The effect of Al2O3 NRs on creeping discharge and flashover traits of oil/pressboard (OP) interface under AC and impulse voltages were studied. Partial discharge and creeping flashover test of OIPs and NIPs were conducted. TSDC and PEA tests were applied to examine the space charge properties of OP interface before and after suspension of Al2O3 NRs.

Effects of temperature on creepage discharge characteristics in oil-impregnated pressboard insulation under combined AC–DC voltage

Due to the complexity of the valve side winding voltage of the converter transformer, the insulation characteristics of the oil-impregnated pressboard (OIP) of the converter transformer are different from those of the traditional AC transformer. The study on effect of temperature on the creeping discharge characteristics of OIP under combined AC–DC voltage is seriously inadequate. Therefore, this paper investigates the characteristics of OIP creepage discharge under combined AC–DC voltage and discusses the influence of temperature on creepage discharge characteristics under different temperatures from 70 °C to 110 °C. The experimental results show that the partial discharge inception voltage and flashover voltage decrease with increasing temperature. The times of low amplitude discharge (LAD) decrease and amplitude of LAD increases. Simultaneously, the times of high amplitude discharge (HAD) gradually increase at each stage of creepage discharge with higher temperature. The analysis indicates that the charge carriers easily accumulate and quickly migrate directional movement along the electric field ahead of discharging. The residual charge carriers are more easily dissipated after discharging. The ‘hump’ region of LAD moves to the direction of higher discharge magnitude. The interval time between two continuous discharges is shortened obviously. The concentration of HAD accelerates the development of OIP insulation creepage discharge. The temperature had an accelerating effect on the development of discharge in the OIP under applying voltage.

The influence of polarity reversal time on interface charges in oil-impregnated pressboard insulation under non-uniform electric field

The polarity reversal test is one of the routine tests for the evaluation of electric insulation in converter transformers. The polarity reversal time (PRT) has significant effect on the accumulation of interface charges in oil-impregnated pressboard insulation. IEC standard recommends that polarity reversal should be performed within 2 min so as to match with the dispersion rate of interface charges in uniform filed, but not necessarily in the non-uniform field. In order to investigate the influence of PRT on the dissipation rate of interface charges in non-uniform electric field, the density of interface charges was measured by an electrostatic capacitance probe in the needle-plate non-uniform field at the PRT of 2 ms, 4 ms and 2 min respectively. The results showed that the polarity of interface charges did not change with the applied voltage jump at the PRT of 2 ms and 4 ms, while the polarity of interface charges almost synchronously changed with the polarity reversal of applied voltage at the PRT of 2 min. For the PRT of 2 ms and 4 ms, the electric field nearby the needle electrode would be enhanced with applied electric field of the same direction, whilst the total electric field of insulation system would be weakened at the PRT of 2 min. It was inferred that polarity of interface charge might have not enough time to change in polarity reversal test within the PRT of 4 ms under non-uniform electric field, which was more close to the severe condition of covert transformer. The paper suggest that the value of PRT be reconsidered when the polarity reversal test is performed in non-uniform electric field.

Feasibility of a universal approach for temperature correction in frequency domain spectroscopy of transformer insulation

Over the past few years, frequency domain spectroscopy (FDS) has been widely utilized to assess transformer insulation. Many investigations have revealed that the FDS measurement has been observed to be dependent on service conditions (aging, moisture, geometry, and temperature). During the onsite FDS measurement, the inner insulation temperature of the de-energized transformer gradually decreases, which affects the FDS results due to moisture migration from oil to the cellulose insulation. This process gives rise to misinterpreting of the condition of the transformer insulation. Presently, the master curve technique reported by several researchers is utilized for temperature correction in FDS measurements. In fact, various researchers have obtained different master curves, so making a universal approach to temperature correction has become an important topic. In the present investigations, a series of experiments was performed on oil-impregnated pressboard insulation under laboratory conditions which was followed by FDS measurements at lower than operating temperature and a universal master curve for temperature correction was studied. Activation energies from oil-impregnated pressboard tests support the effectiveness of the proposed universal approach for temperature correction.

Analysis of copper contamination in transformer insulating material with nanosecond- and femtosecond-laser-induced breakdown spectroscopy

This work examines the oil-impregnated pressboard insulation of high-voltage power transformers, for the determination of copper contamination. Nanosecond- and femtosecond-laser-induced breakdown spectroscopy revealed atomic copper lines and molecular copper monoxide bands due to copper sulphide diffusion. X-ray diffraction studies also indicated the presence of CuO emission. Elemental and molecular mapping compared transformer insulating material ageing in different media—air, N2, He and vacuum.

Effect of Surface Discharges on Lightning Impulse Breakdown Voltage of Oil-Impregnated Pressboard in Power Transformers

Power transformers inevitably experience impulse over-voltage stresses such as from lightning and/or switching events despite various protection systems that may be in place. At locations along oil/pressboard interfaces in the transformer, surface discharges may initiate due to various reasons. The surface discharges may or may not eventually lead to voltage flashover faults. This paper presents a study of how relatively small surface discharges affect the lightning voltage impulse (LI) strength of oil-impregnated pressboard insulation. It is found that surface discharges of magnitudes in the order of some hundreds of pC can reduce the LI withstand voltage of the oil-impregnated pressboard by up to 15%. Although the negative polarity LI withstand voltage of surface discharge-aged pressboard is higher than that of positive polarity, the reduction in the LI voltage strength is more pronounced for the negative LI impulse voltage. The findings suggest further scrutiny of the standardised acceptable levels of PD in power transformers.

Effects of DC prestressing on partial discharge in oil-impregnated pressboard insulation

The AC (50 Hz) partial discharge (PD) characteristics of oil-impregnated pressboard (OIP) insulation are severely influenced by the DC voltage component in the converter transformer. Therefore, this study investigates the effect of different DC prestressing voltage (DCPV) amplitudes, prestressing time, and repetitions times of the applied AC voltage on the OIP insulation AC PD characteristics. Meanwhile, the space charge characteristics of distribution and dissipation are measured by pulsed electro-acoustic method. The experimental results show that the PD inception voltage (PDIV) and PD extinction voltage (PDEV) after DCPV were lower than that under AC voltage without DCPV. The PDIV decrease with higher DCPV at prestressing time from 30 to 60 min. But it increase with higher DCPV at prestressing time from 90 to 120 min. However, PDIV initially increase then decrease with the increasing repetition times of the applied AC voltage. The space charge distribution in 1 mm OIP insulation takes 60 min to reach a balanced condition. The residual space charge is relatively higher at a longer prestressing voltage time and higher DC voltage when the DC voltage was turn off. The PDIV and PDEV after DCPV is improved by Joule heating with longer prestressing time and higher DCPV. The space charge had negative effect on them.

Effects of the Insulation Quality on the Frequency Response of Power Transformers

This paper presents results of frequency domain spectroscopy (FDS) measurements on oil-impregnated pressboard insulation, their analyses and use of the data for modeling high frequency response (FRA) of transformers. The dielectric responses were measured in a broad frequency range, i.e. from 0.1 mHz to 1 MHz, on model samples containing different amount of moisture. The responses were parameterized with terms representing dc conductivity, low frequency dispersion and Cole-Cole polarization mechanisms and they were thereafter used to model the FRA response of a three-phase transformer.