Miao Hao

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

The space charge accumulation can affect the dielectric performance of oil-impregnated pressboard in convertor transformers. In this paper, the space charge behaviour in 1mm thick impregnated pressboards is investigated by the means of pulsed electroacoustic method. The dried pressboards are separately impregnated in fresh oil and service aged oil as comparison. The space charge distributions are investigated under ±15 kV/mm and ±25 kV/mm at room temperature. The homocharge injection can be observed for all the samples, which is much more significant in the aged oil impregnated samples. The amount of injected charges and the depth of the injected charges accumulation in the pressboard bulk are closely related to the oil properties, amplitude and polarity of the applied voltages. The service aged oil shows the dominant effects on the local electric field distortion, about 48% enhanced in the aged oil impregnated samples while about 15% enhanced in the fresh oil impregnated samples. The estimations of the apparent charge mobility and trap depth also indicate the great impacts caused by the service aged oil.

Study of the dielectric response in mineral oil using frequency-domain measurement

Dielectric spectroscopy is a powerful tool to study dipole relaxation, electrical conduction, and structure of molecules. Electrode polarization, as a parasitic effect due to the blocking of charge carriers in the vicinity of an electrode, can make the frequency response at low frequency difficult to understand. Since charge carriers in mineral oil are not only generated from dissociation but also from injection at electrodes, current induced by motion of injected charge carriers should also be taken into consideration. The polarization caused by the injection current has been studied in this paper. When the electric field is not intense, the injection current is proportional to the field and only contributes to the imaginary part of the complex permittivity. A new model has been proposed with this injection current being involved. The frequency responses of three different kinds of mineral oils have been measured and this new polarization model has been used to fit the experimental data. According to the...

Space charge behaviour in oil and impregnated pressboard combined insulation system

Dielectric performance 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 0.5mm thick oil film combined with a 1mm thick impregnated pressboard have been investigated by the pulsed electroacoustic (PEA) technique under 12kV/mm and 20kV/mm at room temperature. 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. Difference in the electric field between those obtained from measurement results and those calculated from Maxwell-Wagner theory has been discussed.

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.

Space charge characteristics in oil and oil-impregnated pressboard and electric field distortion after polarity reversal

The formation of space charge in the oil/paper insulation under HVDC condition could influence the electric field distribution. In the case of HVDC converter transformer, the distortion of the electric field may affect its performance. It is, therefore, important to analyze factors that can affect space charge formation and dissipation characteristics such as temperature, moisture, electric stress and ageing process. This paper mainly focuses on the effect of electric stress and oil ageing on space charge characteristics in oil/paper insulation system. The pulsed electroacoustic technique (PEA) method was used to monitor space charge dynamics. This data was further incorporated into a model established using COMSOL software to estimate space charge impact on electric field distribution, with an emphasis on the electric field after polarity reversal operation. The results show that the electric field arising from the space charge formation is much higher compared to the average electric field after the polarity reversal operation. Moreover, the electric field caused by space charge decreases faster for the aged oil-impregnated pressboard compared to the fresh one, which could be related to different space charge dissipation characteristic related to the condition of the oil.

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

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.

Study of the charge dynamics in mineral oil under a non-homogeneous field

The presence of mobile charge carriers can affect the frequency response of complex permittivity of insulating liquid when the frequency is low. It has been found that the experimental results of the complex permittivity of mineral oils are in a good agreement with Coelhos frequency response theory when an extra contribution to the complex permittivity arising from DC conduction is taken into account. It has been reported that small quantity of high mobility charge carriers are responsible for this DC conduction. In our previous work, we have found that if the charge carriers are fast enough so that they can reach the opposite electrode in a cycle, the motion of these fast charge carriers can only contribute to the imaginary part of the complex permittivity under a homogeneous field. In this paper, the polarization induced by the motion of those fast charge carriers under a non- homogeneous field has been studied. It has been found when the field is not high, the total current caused by the drift and diffusion of those fast charge carriers is proportional to the external voltage. A modified space charge polarization theory has been proposed. The frequency dependence of the complex permittivity obtained experimentally has been fitted using this modified Coelho model and compared with our previous simulation. This modified model enables one to gain a better understanding of the frequency response in mineral oil in low frequency regions.

Space charge polarization in insulating mineral oil

Dielectric spectroscopy is a powerful tool to study dipole relaxation, electrical conduction and structure of materials. Space charge polarization, as a parasitic effect due to the blocking of charge carriers in the vicinity of electrode, can mask dielectric response of the sample at low frequencies. In this paper, the frequency response of three different kinds of mineral oils have been measured and compared with the simulation based on a classic space charge polarization model. Since the creation of charge carriers in mineral oil is not only from dissociation but also injection from electrode, a modified model with injected part could describe the measurement result more accurately. The dielectric response due to electrode polarization has been simulated considering both the movement of internal charge carriers and injected charge carriers. According to the simulation results, the frequency-dependent curve of complex dielectric permittivity calculated from the polarization model could fit the experimental data well. The amount of the injected charge carriers increases with the aging time. A better fit can be obtained when high mobility charge carriers are taken into account. The present model of space charge polarization enables one to obtain a better understanding of electrical conduction in mineral oil.

The effect of composition and processing on electric characteristics of XLPE in HVDC cable applications

Polyethylene exhibits many key characteristics including low dielectric loss, high breakdown strength and good processability. Most modern extruded high voltage cables employ cross-linked polyethylene (XLPE) as the insulation material. The main advantage of XLPE is its excellent thermo-mechanical properties; it is relatively cheap and has low dielectric loss and low conductivity making it an ideal material for this application. Crosslinking enhances a number of thermo-mechanical properties such as deformation resistance at higher temperatures, tensile strength and creep properties. In comparison with lov density polyethylene (LDPE), the heat deformation characteristics of XLPE are superior and, for this reason, XLPE is currently the most common insulation material for power cables ranging from low to high voltages. This paper reports on an investigation into the development of a new XLPE formulation for use in high voltage direct current (HVDC) cable applications. Specifically, the electrical performance of two novel LDPE resins are compared with an industrial standard (reference) LDPE material. For crosslinking, dicumyl peroxide (DCP) was selected, as the decomposition temperature is high enough to prevent pre-curing during processing and to allow an efficient and rapid crosslinking at moderate temperatures. Moreover, the behavior of various systems is compared in terms of electrical breakdown performance and the influence of material composition and processing on these parameters is described.

Quantitative study of electric conduction in mineral oil by time domain and frequency domain measurement

Polarization and depolarization current measurement is a non-destructive testing method to study the dielectric properties of insulating materials. Two types of mineral oils with different aging times have been studied using both time domain and frequency domain methods separately. In this paper, a novel approach to understanding the correlation between the electric conduction in time domain and frequency domain has been proposed. When the charge carriers drift to the electrodes, they may be blocked and accumulated near the electrode. The amount of charge in the vicinity of electrode has been estimated. In the depolarization period, these charge layers will return to the bulk resulting in a depolarization current. In this paper, an analytical solution to the time dependence of the depolarization current has been developed and used to fit the experimental results. The field and temperature dependence of the polarization and depolarization current have been used to study the characteristics of charge carriers in mineral oil. The calculated conductivity has been used to simulate the frequency response in mineral oil and the simulating results have been compared with the experimental data.