M. Ghaffarian Niasar

Change in partial discharge activity as related to degradation level in oil-impregnated paper insulation: effect of high voltage impulses

High voltage (HV) transients in electrical power systems are mainly caused by lightning strikes and switching operations. The dielectric properties of a particular electrical insulation at the instant of interaction with a high voltage transient would determine a level of the degradation of an electrical insulation. This work investigates how high voltage impulses would change Phase Resolved Partial Discharge (PRPD) patterns in test samples consisting of a cavity deliberately introduced between the sheets of oil-impregnated paper used in power transformer bushings. It also investigates how the change in PRPD patterns is related to the degradation level of oil-impregnated paper. In order to accomplish these objectives, the experiments were set in such a way that the effect of HV impulses and an early stage PD activity at an elevated AC stress, HV impulses in combination with a prolonged PD activity at an elevated AC stress, and a prolonged PD activity alone at an elevated AC stress could be investigated separately. The experimental results presented in this paper indicate that HV impulses below the impulse breakdown stress following an early stage AC PD activity would not cause a significant change in PRPD patterns, and would not damage oil-impregnated paper to a level which can be noticed by a visual observation. On the other hand, a prolonged AC PD activity alone can cause the change in PRPD patterns, but cannot quickly damage the oil-impregnated paper as it would do when it is combined with HV impulses. However, the combination of both, HV impulses and a prolonged AC PD activity caused a high drop in the PD parameters (total PD charge and a repetition rate). The study found that the decrease of the PD parameters can be considered as a sign of severe degradation of oil-impregnated paper.

Corona in oil as a function of geometry, temperature and humidity

Corona discharge in oil is one of a number of important defects that may exist in a power transformer. This paper investigates the corona in oil from a needle-plane geometry. Different aspect ratios of the geometry are investigated as well as the temperature dependence from 25 to 100 °C. The corona discharges are measured both in time domain with oscilloscopes and as phase resolved patterns. By using COMSOL, a simple simulation of the electric field in the needle-plane geometry is computed. Simulation of electrical field around the needle tip for different needle length and distance from the needle tip to plane is performed. An amplification factor of electrical field around the tip, according to these parameters is developed. By considering corona inception voltage as a function of electrical field around the tip, by using the same geometries for simulation and measurement, inception field for corona in oil as a function of electrical field is calculated. The results show the dependency of corona inception voltage, discharge magnitude and repetition rate on the geometrical parameters, temperature and humidity.

Dielectric response measurement of power transformer bushing by utilizing high voltage transients

Insulation has a significant role in most of power components failures. Therefore, proper diagnostics of insulation condition which can be done non-destructively and online in the field is of interest. In this paper, a new insulation diagnostic technique is proposed which utilizes natural transients in power system for online dielectric response measurement. In the proposed technique, lightning and switching impulses with wide range of frequency are used as stimuli for dielectric spectroscopy. Oil impregnated papers and condenser bushing are the objects under investigation and dielectric response of them is derived at those transients. Dielectric responses obtained by this method are verified by the responses measured by insulation diagnostic systems IDAX and LCR meter.

Comparison of phase resolved partial discharge patterns in small test samples, bushing specimen and aged transformer bushing

The transformer bushings are one of the key devices in electrical power networks. With time, the electrical insulations in the transformer bushings become aged. One effect of a long time ageing would be due to Partial Discharge (PD) defects. In this work, the phase resolved partial discharge (PRPD) pattern of the defects naturally present in an aged 36 kV Oil Impregnated Paper (OIP) bushing was compared with the PRPD patterns of defects deliberately introduced in simple test samples and in a bushing specimen. The results presented in this paper show that it might be possible to predict the type of the defects in aged transformer bushings through comparing measured PRPD patterns with PRPD patterns from defects deliberately introduced in simple test samples and in bushing specimens.

Partial discharge analysis in a needle-plane gap with repetitive step voltage

Partial Discharge (PD) tests are carried out in a needle-plane gap where the plane is covered by an insulating material. The partial discharge activity is studied during application of a periodic negative step voltage, whose duration of the voltage period T1 and the pause time T2 between every two consecutive step voltage pulses could be varied separately to influence the decay of deposited surface charges. Compared with the PDs in the needle-plane setup without dielectric, the effect of dielectric placed on the top of ground electrode on the discharge activity is investigated. The results show for the case of the step voltage with the duration of 100 ms and the pause time of 10 s. The PD activity during the first voltage cycle is significantly different from the others. For a lower voltage, the pulse repetition in the virgin curve reaches a saturation level within the charging period; however, for the higher voltages it becomes more linear increasing.

Effect of temperature on surface discharge in oil

In this paper surface discharge along the oil-pressboard interface was investigated. Surface discharges were measured both in time domain by means of an oscilloscope and as phase resolve patterns. The effect of temperature from 20 °C to 90 °C on surface discharge inception, extinction and surface flashover voltage were investigated. Trend of number and maximum magnitude of PD was compared for 20 °C and 90 °C over time. The effect of voltage rise and surface flashover on number and maximum magnitude of PD were investigated. The results show that PD inception voltage is lower for higher temperature but surface flashover voltage is higher for higher temperature. However for 90 °C the final damage due to surface discharge is much severe compared to 20 °C.

Effect of high voltage impulses on partial discharge activity in a cavity embedded in paper insulation

High voltage impulses in electrical power systems are mainly caused by lightning strikes and switching operations. The dielectric properties of a particular insulation at the instant of interaction with a high voltage impulse would determine the level of the degradation of the electrical insulation caused by the high voltage impulse. In this work, the effect of high voltage impulses on the partial discharge (PD) parameters was investigated. The investigation was done in test samples consisting of a cavity deliberately introduced between oil-impregnated papers used in power transformer bushings. The measurement results presented in this paper show that high voltage impulses can have a large impact on the behavior of the PD parameters if the test sample is aged by partial discharges, but a smaller impact when the test sample is unaged.

The effect of partial discharge by-products on the dielectric frequency response of oil-paper insulation comprising of a small cavity

This work investigates the effect of alternating current partial discharge (AC PD) byproducts on the dielectric frequency response (DFR) of oil-paper insulation comprising of a small cavity, in the frequency range of 1.0 mHz to 1.0 kHz. The investigations were done by utilizing experimental results in combination with a Finite Element Method (FEM) model. The results presented in this paper show that ageing by-products in the cavity (PD stressed volume) alone will not result into the change in DFR of oil-paper insulation. According to simulation results, the significant change in the DFR results is mainly caused by PD by-products (positive and negative charges, and electrons) which tend to conduct or diffuse into the bulk of oil-paper insulation. The change in a dissipation factor curve due to PD by-products was observed to occur mainly in the middle and low frequency regions. The results indicate that the dissipation factor curves in the middle and low frequency regions depend on the level of PD by-products in oil and paper, respectively.

Dielectric frequency response of oil-impregnated paper: The effect of partial discharges compared to other influences

In this paper, the influence of partial discharges (PD) on the subsequent dielectric properties of oil-impregnated paper (OIP) is investigated. Dielectric frequency response (DFR) of OIP is investigated at varied levels of moisture, thermal aging, temperature, oil conductivity and electrode material, to allow comparison of these influences. It is shown that PD activity inside OIP can significantly change the real and imaginary parts of permittivity of the paper layers, especially in the frequency range below 100 Hz. It is shown that even if the PD activity is concentrated on a few percent of the test sample, the real part of permittivity measured on the entire sample after a few tens of hours of PD activity can increase up to 10 times at very low frequencies (~ 1 mHz) and the imaginary part of the permittivity can increase up to 100 times at low frequencies (~ 1 Hz). The change of the real and imaginary parts of permittivity due to PD activity is permanent. It is very similar to the change caused by thermal aging, and somewhat similar to the change caused by moisture and higher oil conductivity. It is also shown that the dissipation factor of OIP in contact with copper and brass increases, even in the absence of PD, but contact with aluminum and stainless steel has no influence on DFR of OIP. The results suggest that the influence of PD can be significant when estimating the moisture content or the level of thermal aging of OIP with DFR method.