N. Haque

Assessment of interfacial charge accumulation in oil-paper interface in transformer insulation from polarization-depolarization current measurements

Accumulation of interfacial space charge in oil-paper interface is a critical issue in insulation diagnostics of transformers. This interfacial charge mainly accumulates due to the conductivity difference of oil and paper. Accumulation of interfacial charge leads to localized field enhancement, which further leads to partial discharges and acceleration in the aging of insulation. Therefore, from the point of view of transformer insulation diagnostics, assessment of interfacial charge is very important. However, it is not easy to estimate interfacial space charge behavior from the transformer diagnostics methods currently in use. In case of Polarization-Depolarization Current (PDC) measurement, a well known method for transformer condition monitoring, the effect of interfacial charge is reflected in the non-linearity of current response during polarization and de-polarization. During de-polarization process, a part of the interfacial charge accumulated during polarization period is absorbed by the electrodes producing a current, which is difficult to separate using conventional linear dielectric theory. In this paper, an attempt has been made to separate this current component from de-polarization current through considering charge de-trapping mechanism. Terming this current component as de-trapping current, its relationship with other parameters of transformer insulation is discussed. The developed methodology has been applied on several practical transformers. It was observed that the time constant of de-trapping current is related to the paper conductivity, oil conductivity, dissipation factor and age of the insulation.

Investigations on charge trapping and de-trapping properties of polymeric insulators through discharge current measurements

It is generally accepted that formation and accumulation of space charge in polymeric insulation is a critical issue in the design of high voltage electric systems, as it leads to insulation degradation and premature failure under high voltage surges and polarity reversals. Even at service stress level, accumulation of space charge may cause electrothermal aging. The insulation degradation and aging is reflected in the trapping and detrapping characteristics of the insulation, making it a useful aging marker. Space charge measurement methods are frequently used for trapping and de-trapping analysis. However, estimation of charge trapping and de-trapping behavior is very difficult due to the involvement of material properties and also partly due to the fact that proper measurement of space charge is itself a very complicated issue, requiring very sophisticated experimental arrangement and signal processing techniques. Moreover, application of these methods is limited to polymeric insulations of certain geometries (planar or radial). In this paper, the charge trapping and de-trapping behavior of low density polyethylene (LDPE), a widely used polymeric material for electrical insulation purpose, have been investigated employing a direct method based on the discharge characteristics obtained after the sample was stressed for short durations. A relationship is established between the de-trapped charge and the extracted charge measured from the discharge current. From this relationship a relative distribution of trapped charges across different trap depths was obtained. It was noticed that the trapped charge distribution is of exponential nature in the trap depth range of 0.93–1.11 eV. The presented method facilitates direct estimation of trap distribution independent of the insulation geometry. It was also observed that the trapped charge is highly dependent on the applied field.

Studies of the effect of temperature on the charge trapping and de-trapping processes of polymeric insulators through depolarization current measurements

Formation of space charge in polymeric insulation is a critical issue in the design of high voltage electric systems. Accumulation of space charge accelerates the material aging and may lead to premature breakdown of the insulation. Therefore, it becomes increasingly important to understand the charge injection, trapping and de-trapping processes in dielectric under application of high voltage. However analysis of charge trapping and de-trapping is very difficult due to material properties and also the fact that the measurement of space charge itself is a very complicated issue. Most of the trapping and de-trapping investigation in literature has been performed through indirect means, by measuring the space charge distribution in a dielectric specimen under charging and discharging conditions. The effect of temperature on the trapping and de-trapping properties of polymeric insulating materials has also received limited attention. In this paper, the charge trapping and de-trapping behavior of low density polyethylene (LDPE), a widely used polymeric material for electrical insulation purpose, have been investigated using a direct method based on the depolarization characteristics of a stressed sample. A model has been developed to obtain the trapped charge distribution according to their energy level in the sample directly from the depolarization current. The measurements were performed at different temperature to study the effect of temperature. It was observed that temperature plays a critical role in charge injection, trap formation, trapping and de-trapping.

Investigation of charge trapping behavior of LDPE using de-trapping characteristics

Formation of space charge in polymeric insulation is a critical issue in the design of high voltage electric systems. Accumulation of space charge accelerates the material aging and may lead to premature breakdown of the insulation. Therefore, it becomes increasingly important to understand the charge injection, trapping and de-trapping processes in dielectric under application of high voltage. In this paper, the charge trapping and de-trapping behavior of low density polyethylene have been investigated. A new experimental set-up has been developed to obtain the de-trapping characteristics of a charged dielectric sample. From the obtained response, the trapping parameters i.e. trap depth, trapped charge etc is evaluated. It was observed, that the trapped charges in LDPE specimen is mainly concentrated to two trap levels, one shallow trap level (0.85 eV) and one deep trap level (1.01 eV).

Simulation of pressure wave propagation method for space charge measurement in dielectrics

Pressure Wave Propagation (PWP) method is one of the most significant techniques used in the field of nondestructive investigation on space charge. Even after more than three decades from its invention, several issues related to the experimental arrangement, resolution of the measurement and overall measurement accuracy remain unaddressed. In this paper, to highlight the aforesaid issues, an overview of PWP method has been presented with simulated results. It is believed that the simulation would be useful for understanding the basic concepts of PWP method as well for designing improved diagnostic equipment.

Space charge measurement in dielectrics using Pressure Wave Propagation method

This paper describes the principle of Pressure Wave Propagation (PWP) method, which is an important non destructive measurement technique for space charge study in dielectric specimens. An experimental set-up incorporating a piezoelectric transducer, high voltage dc source and amplifier is developed to carry out space charge measurements on charged thin dielectric samples. Preliminary results have been obtained on LDPE samples of 65 μm and 210 μm stressed under different voltage levels. It is believed that with the help of sophisticated data processing system, the developed set-up would be useful as diagnostic equipment for space charge detection and measurement.