Pressboard

About: Pressboard is a research topic. Over the lifetime, 1123 publications have been published within this topic receiving 9089 citations.

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.

Effect of temperature on space charge characteristics of oil-paper insulation and its numerical simulation

Abstract: In this study, the space charge in oil-impregnated thick pressboard and Kraft paper were measured by the pulsed electroacoustic (PEA) method from -20 to +60 °C. A bipolar charge transport model was used to simulate the transport processes of homocharge and heterocharge, and the acoustic attenuation process in the samples. The test results show that space charge accumulates in the thick pressboard and Kraft paper increases first and then decreases with temperature, and its maximum appears at 40°C, while little space charge accumulates at -20°C. The space charge near the lower electrode shrinks in the oil-impregnated thick pressboard at high temperature and the shrinkage is larger at 60°C. In the oil-impregnated Kraft paper, the accumulation and charge density of positive charge is different from negative charge. The ion transport process model simulates the charge shrinkage near the electrode, and disappears at the upper electrode after the acoustic attenuation simulation. The model established in this paper can well simulate the space charge distribution of oil-impregnated thick pressboard and Kraft paper under different temperatures.

Modeling and Measurements of Electric Fields in Composite Oil/Cellulose Insulation

Abstract: In this paper we report on basic studies of the electric fields under dc stress in a composite insulation consisting of transformer oil and oil-impregnated pressboard, typical for HVDC converter transformers. The time dependent electrical fields in our model geometry are measured using electro-optic Kerr method, and theoretically studied with an ion drift-diffusion conduction model. The measured and calculated electrical fields are compared and analyzed.

Effect of Dielectric and Magnetic Material Characteristics on Frequency Response of Power Transformers

Abstract: Frequency response analysis (FRA) technique has become increasingly popular for the assessment of mechanical integrity in power transformers. This method is based on the fact that the shape of the frequency response at higher frequencies is uniquely determined by the geometrical construction of the transformer. However, characteristics of the insulation system (pressboard, paper and oil) and the laminated ferromagnetic core may influence the distributed inductances and capacitances, resulting in deviations of the frequency response. The main goal of this thesis is to investigate the influence of properties of the core and insulation materials on the frequency response of power transformers. To be able to realize this goal, a small-signal high frequency model based on lumped circuit approach, of which all the circuit parameters are calculated by considering the frequency dependent phenomena related to transformer insulation and core materials, is developed. Capacitances and conductances are calculated based on analytical expressions derived from approximated geometrical parameters and measured frequency dependent permittivities of paper and pressboard. Hierarchical multi-scale modelling approach is adapted for the purpose of calculating inductances and resistances, including anisotropic properties of the core and diamagnetic properties of the windings. Complex permeability of silicon steel laminations, which is a vital input parameter in inductance calculations, is estimated using an optimization algorithm as well as by means of a single sheet tester. The developed circuit model is implemented in MATLAB as a state space model, which is used to quantify the influence of insulation condition on the frequency response and to explore a possibility of using FRA technique for characterization of the transformer insulation. According to the model prediction, the moisture content in transformer insulation can influence the response most, since it modifies both the real part of dielectric permittivity and the losses. Measurements were performed on two transformers at different magnetization levels to show effects of remanent magnetization in the core due to removal of 3-phase supply, relaxation demagnetization and DC flux on the frequency response of winding impedance. It was found that the low frequency response (<10 kHz) is affected by magnetic viscosity yielding relaxation of permeability over a long period of time. Therefore, to avoid the effects of core magnetization, diagnostic FRA measurements on power transformers should not be performed directly after disconnection from the network.

Aging Assessment Model of Transformer Insulation Based on Furfural Indicator under Different Oil/Pressboard Ratios and Oil Change

Abstract: Oil change, as an effective method, could improve the performance of liquid insulation. However, due to the furfural loss in oil after the oil change, the aging assessment results of cellulose insulation based on the traditional furfural-degree of polymerization (DP) model is not accurate. Besides, the different oil/pressboard mass ratio of the transformer also has a significant impact on the aging assessment results. Current research only focuses on the aging assessment based on furfural analysis under the single oil/paper mass ratio and oil change conditions. In view of this, the aging assessment model for transformer insulation based on furfural indicator under different oil/pressboard ratios and oil change conditions was studied. First, oil/pressboard insulation systems with different oil/pressboard mass ratios were performed for total oil changes. Then, the initial furfural-DP equation and the furfural corrected equation were obtained. Finally, the modified furfural-DP model extended to different oil/pressboard mass ratios under oil change was established. The proposed model is expected to promote the accuracy of the aging assessment of transformer cellulose insulation under different oil/pressboard mass ratios and oil change conditions.