Oil-pressboard Insulation System Research Articles

Steady-state distribution characteristics of moisture in oil-pressboard insulation for converter transformer

The converter transformer is the core equipment of the UHVDC transmission project. Oil-pressboard insulation is the main form of transformer insulation. Moisture is a key factor affecting the performance of oil-pressboard insulation. At present, Oommen_'s curve is mainly used to calculate the moisture content of pressboard according to oil-pressboard moisture equilibrium curve and the moisture content of oil. Whether Oommen_'s curve is suitable for oil-pressboard insulation of converter transformer becomes a problem. KI50X-Weidmann insulation combination is selected as the research object. It can be found through experiments, the use of Oommen_'s curve to evaluate the moisture content of oil-pressboard insulation used in converter transformer will cause a large error. In response to this problem, this paper carried out an experimental study and the main work and main conclusions are as follows: Aiming at the combination of KI50X transformer oil-Weidmann pressboard, moisture equilibrium characteristics were obtained through experiments. The moisture equilibrium curve and moisture equilibrium relationship of KI50X-Weidmann oil-pressboard insulation system at 30°C, 60°C, 70°C and 85°C is obtained. This paper analyses the reasons for the difference between the measured value and estimated value, and puts forward the correction method. Oil-pressboard moisture equilibrium curve of is close to Oommen_'s curve and no correction is needed at 60°C and 70°C. At 30°C, the moisture content of pressboard needs to be corrected by 21.95% upward. The water saturation solubility KI50X insulating oil is 100.81mg/kg lower than that of the oil used by Oommen at 70°C, which verifies the influence of insulating material properties on the steady-state characteristics of moisture distribution.

AC Surface Flashover and Gas Generation Difference of the Cellulose Insulation Pressboard Immersed in Novel 3-Element Mixed Oil and Mineral Oil

Surface flashover fault is one of the most challenge issues in oil-cellulose insulation pressboard system used in power transformer. In this study, a comparative study of the AC surface flashover properties of the novel 3-element mixed oil-cellulose insulation pressboard (3EMO-IP) and mineral oil-cellulose insulation pressboard (MO-IP) was performed under needle-plate and finger-finger electrode, respectively, measurement including dielectric property, surface flashover voltage, damage of cellulose pressboard surface and the gas generation behaviors after multiple surface flashover. Results show that the cellulose insulation pressboard immersed in the novel 3-element mixed insulation oil (3EMO) has higher relative permittivity and dielectric loss factor at 50 Hz, and also has slightly lower surface resistivity. The AC surface flashover voltage of the 3EMO-IP is higher than that of MO-IP under needle-plate and finger-finger electrode (electrode distance 5 mm, 10 mm, 15 mm and 20 mm). Compared to MO-IP, the lower electric field intensity at the oil-pressboard interface, as well as more difficult for surface charge accumulation of 3EMO-IP and the higher breakdown voltage of 3EMO lead to the higher AC surface flashover voltage of 3EMO-IP. Moreover, the carbonization of fibers in 3EMO-IP is slightly less. After multiple surface flashover, C2H2 and total hydrocarbon gases are the main differences between 3EMO-IP and MO-IP, which is more marked with the increase of flashover times. This study offers a reference for improving the surface flashover property of oil-pressboard insulation system by using 3EMO.

From the editors' desk

The first article is "Transient and Steady-State DC Behavior of Oil-Impregnated Pressboard," by Fabian Schober, Stephan Harrer, and Andreas Kuchler of the University of Applied Sciences, Schweinfurt, Germany; Frank Berger of the Technische Universitat Ilmenau, Germany; and Wolfgang Exner and Christoph Krause of Weidmann Electrical Technology AG, Rapperswil, Switzerland. The article investigates the influence of pressboard density and oil conductivity on the conductivity of oil-impregnated pressboard, and both are shown to be important for achieving a desired conductivity of oil-impregnated pressboard for HVDC applications. Two pressboard types, low and high density, and two types of mineral oils, low conductive and high conductive, were used in the investigation. Both pressboard types exhibited similar apparent conductivities when impregnated with low-conductive oil, but when impregnated with high-conductive oil, an apparent conductivity 2.8 times higher was observed as a consequence of the larger oil volume. Yet, the numeric assessment of oil-impregnated pressboard conductivity by simple linear superposition of pressboard conductivity and oil conductivity does not provide the measured apparent conductivity. In addition, the influence of oil conductivity on the apparent oil-impregnated pressboard conductivity was evaluated. Pressboard impregnated with four types of mineral oil, spanning an apparent conductivity of 2.5 orders of magnitude, was found to be only in an apparent conductivity range of oil-impregnated pressboard of only 0.5 orders of magnitude. As an explanation for the observed phenomenon, it appears that new and modified charge transport paths in the oil volume altered the transport paths along pressboard fibers. Conduction processes therefore interact with each other, and this interaction opens a new and interesting field of further research. Selecting a suitable pressboard density and the right choice of mineral oil can be used to achieve the desired conductivity for the oil-pressboard insulation system, which may be an option for the optimization of HVDC insulation systems.

Space charge behavior in oil gap and impregnated pressboard combined system under HVDC stresses

The reliability of oil-pressboard insulation system used in high voltage direct current (HVDC) convertor transformers can be affected by the presence of space charge. In this paper, the space charge behaviors in a 0.5 mm thick oil film combined with a 1mm thick impregnated pressboard have been investigated by the pulsed electroacoustic (PEA) technique under 12 kV/mm and 20 kV/mm at room temperature for both short term and long time tests. Two types of oil with different aging status were used for comparison. The results show that a charge peak is quickly formed at the interface between oil and pressboard with the same polarity as the electrode at the oil film side. However, the dynamics of the interfacial charges are very different for fresh oil and aged oil samples. The maximum electric field occurs in the middle of the pressboard, which is significantly enhanced in the aged oil samples. Compared with the Maxwell-Wagner (MW) polarization, more space charges are formed in the insulation system. It needs much longer time to reach steady state, depending on the status of the oil. The interfacial and bulk effects result in a significant electric field enhancement in the middle of pressboard, especially in aged oil samples.

Influence of natural ester on frequency dielectric response of impregnated insulation pressboard

It is important to investigate the dielectric behaviour of natural ester-paper insulation, as the number of transformers choosing natural ester as their insulation oil is increasing. A comparative study of the pressboard with different conditions immersed in natural ester and mineral oil was conducted. X–Y model simulation was used to indicate the difference between the frequency-domain spectroscopy characteristics of transformer main insulation system using natural ester-pressboard insulation (NEPI) system and mineral oil-pressboard insulation (MOPI) system, which have the same moisture content or experience the same ageing period. Compared with the MOPI with low moisture content, the NEPI with the same low moisture content has higher ɛ′r and tanδ values in the lower frequency region at various temperatures; although no significant differences exist in the ɛ′r and tanδ curves of these two kinds of oil-impregnated pressboard, when its moisture content is high. The X–Y model simulation results also show that the dielectric loss difference between the NEPI system and the MOPI system with low moisture content is more obvious at higher temperature. Besides, the pressboard aged in natural ester has higher ɛ′r and tanδ values than that aged in mineral oil after ageing for the same time. X–Y model simulation results show that the tanδ values in the lower and middle frequency regions could be used as the ‘fingerprint’ of the ageing condition of the NEPI system and the mineral-oil pressboard insulation system.

Identification of two simultaneous partial discharge sources in an oil-pressboard insulation system using acoustic emission techniques

Insulation failure is one of the major causes of catastrophic failure of transformers. It is established that partial discharge (PD) causes insulation degradation and premature failure of insulation. In power apparatus, more than one PD source may be active simultaneously. The nature of insulation degradation for multiple PD sources is different from that due to single PD source. Therefore, it will be helpful for severity assessment of insulation degradation, if the number of active PD sources are identified and classified. This paper presents a method for identification and classification of two simultaneously active PD sources using acoustic emission techniques. The acoustic emission (AE) signals are measured for laboratory simulated PD in an oil-pressboard insulation system for three different electrode systems. The measurements of partial discharge acoustic emission (PDAE) signals are carried out for single PD source and for two simultaneous PD sources. The measured signals are analyzed using discrete wavelet transform (DWT), box counting fractal dimension and lacunarity. Box counting fractal dimension and lacunarity are calculated for DWT decomposed signal of major frequency band. Energy distribution in different frequency bands of DWT decomposed signal along with box counting fractal dimension and lacunarity is used for classification of two simultaneous PD sources.

Dielectric Breakdown Characteristics of Oil-pressboard Insulation System against AC/DC Superposed Voltage

This paper describes breakdown characteristics of an oil-pressboard insulation system to a superposition voltage of AC and DC voltages. Although AC electric field is decided by the ratio of the relative permittivity of a pressboard and insulating oil, DC electric field is decided by ratio α of volume resistivities. From the measurement in this study, 13—78 and 1.8—5.7 are obtained as the volume resistivity ratios α at temperature of 30°C and 80°C, respectively. The breakdown voltages against AC, DC, and those superposition voltages are surveyed to insulation models. In normal temperature, the breakdown voltage to the superposition voltage of AC and DC is determined by AC electric field applied to the oil duct. Since the α becomes as low as 2-3 at temperature of 80°C, AC and DC voltages almost equally contribute to the electric field of the oil duct as a result. That is, it became clear that superposed DC voltage boosts the electric field across oil ducts at operating high temperature.

油入複合絶縁における直流電界分布と沿面絶縁破壊特性

油入複合絶縁における直流電界分布と沿面絶縁破壊特性