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 (similar to 1 mHz) and the imaginary part of the permittivity can increase up to 100 times at low frequencies (similar to 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.

In this paper, we investigate the effect of partial discharges (PDs) on the thermal breakdown voltage of oil-impregnated paper (OIP). A disk-shaped cavity in a stack of paper sheets formed the PD source. The paper sample was exposed to PDs for 18 h. Dielectric spectroscopy measurements were performed on the samples before and after exposure to PD activity. The dielectric spectroscopy results showed a big change in the real and imaginary part of the permittivity. These results were used to calculate the activation energy and heat losses inside the insulation. In order to investigate the possibility of thermal breakdown in the system, the OIP system was simulated by using the FEM software COMSOL Multiphysics. The heat produced by PD pulses and dielectric losses was considered in the simulation. The results show that for the OIP insulation, PD activity can reduce the thermal breakdown voltage by a factor up to 4 times.

In this paper the impact of thermal and electrical aging on AC and impulse breakdown strength of oil-impregnated paper was investigated. Sheets of paper were thermally aged at 140 °C for 30 days. A comparison between AC and impulse breakdown strength of unaged paper layers, 10, 20 and 30 days thermally aged paper was performed. In order to investigate the effect of electrical aging, first the paper layers were exposed to partial discharges for 6 hours at accelerated electric stress. The partial discharge was created inside paper samples by introducing a disc shaped cavity on the center of the paper samples. The effect of partial discharges was then investigated on the impulse breakdown strength of oil-impregnated paper. The study shows that thermal aging has very small effect on AC breakdown strength. However, impulse breakdown strength of oil-impregnated paper may reduce up to 40% due to exposure to PD.

In this thesis, partial discharge (PD) phenomenon in oil-impregnated paper (OIP) is investigated under accelerated electrical stress. The thesis is mainly focused on the characteristic of PD activity and the influence it has on the insulation properties of OIP. PD source was created by introducing an air filled cavity embedded between layers of OIP. PD activity is investigated from the initiation up to final puncture breakdown of the OIP. The time-evolution of number, maximum magnitude and average magnitude of PD is investigated for cavities with different diameter and height. It was found that time to breakdown is shorter if the cavity diameter is larger and cavities with higher depth produce larger PDs. Comparison between PD activity in three cases, i.e. unaged OIP, thermally aged OIP and OIP samples with higher moisture content is performed. In general, it is found that for all cases the number and the maximum magnitude of PD follows a similar trend versus ageing time. During the very beginning of the experiment large discharges occur and they disappear after a short ageing time. Number and maximum magnitude of PD increase with time until reaching a peak value. Finally both parameters decrease with time and puncture breakdown occurs in the sample. Even though PD activity in thermally aged OIP is higher compared to the unaged OIP samples, the time to breakdown for new and thermally aged OIP samples is similar while it is shorter for OIP samples with higher moisture content.

In this thesis, partial discharge (PD) phenomenon in oil-impregnated paper (OIP) is investigated under accelerated electrical stress. The thesis is mainly focused on the characteristic of PD activity and the influence it has on the insulation properties of OIP. PD source was created by introducing an air filled cavity embedded between layers of OIP. PD activity is investigated from the initiation up to final puncture breakdown of the OIP. The time-evolution of number, maximum magnitude and average magnitude of PD is investigated for cavities with different diameter and height. It was found that time to breakdown is shorter if the cavity diameter is larger and cavities with higher depth produce larger PDs. Comparison between PD activity in three cases, i.e. unaged OIP, thermally aged OIP and OIP samples with higher moisture content is performed. In general, it is found that for all cases the number and the maximum magnitude of PD follows a similar trend versus ageing time. During the very beginning of the experiment large discharges occur and they disappear after a short ageing time. Number and maximum magnitude of PD increase with time until reaching a peak value. Finally both parameters decrease with time and puncture breakdown occurs in the sample. Even though PD activity in thermally aged OIP is higher compared to the unaged OIP samples, the time to breakdown for new and thermally aged OIP samples is similar while it is shorter for OIP samples with higher moisture content.

Breakdown strength of OIP samples is measured before and after ageing with PDs. It is found that the breakdown strength of OIP samples decreases by around 40% after the sample is exposed to accelerated electrical ageing. Furthermore a thermal model was developed to investigate the possible transition of breakdown mechanism from erosion to thermal breakdown in OIP dielectrics. It was found that PD activity can lower the thermal breakdown voltage of OIP up to four times.

In this paper, the rapid aging due to Partial Discharge (PD) activity was investigated in unaged and thermally aged oil-impregnated paper with a disk-shaped cavity between the sheets of paper. The PD inception voltage and the voltage for instantaneous breakdown were measured, and the time to breakdown was measured by applying voltages less than the instantaneous breakdown voltage. A comparison between time to breakdown of unaged and thermally aged paper shows no significant difference between thermally aged and unaged paper at sustaining PD activity. The time to breakdown as a function of applied electric field was plotted and compared for both cases showing points scattered around a line in log-log scale. Changes of the PD parameters, such as PD magnitude and PD repetition rate were analyzed from the beginning of PD activity up to the moment of breakdown. The results show that the number and magnitude of PD increase at the beginning of aging until they reach to a maximum value, then both quantities decrease slowly over time until the final puncture breakdown occurs. The results emphasize the importance of PD monitoring on real equipment with oil-impregnated paper as insulation system, such as power transformers, since a focus on the number and magnitude of PD at just the present time may mislead the interpretation. Dielectric spectroscopy measurements performed on unaged and thermally aged paper showed an increasing trend of epsilon '' after the sheets of paper were exposed to thermal aging for longer time. Dielectric spectroscopy measurements performed on samples before and after exposure to PD activity showed a big change of epsilon '', This change can be attributed to byproducts and ions produced by PD activity.

This work investigates the effect of alternating current partial discharge (AC PD) byproducts on the dielectric frequency response (DFR) of oil-paper insulation 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 AC PD by-products in the oil-paper insulation can result in a dissipation factor curve whose magnitudes increase with increasing volume of the aged zone. The change in a dissipation factor curve due to PD byproducts 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.

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.

In this paper the effect of partial discharges on the thermal breakdown voltage of oil-impregnated paper is investigated. A disc shaped cavity in a stack of paper sheets formed a PD source. The paper sample was exposed to partial discharges for 18 hours. Dielectric spectroscopy measurements were performed on the samples before and after exposure to PD activity. The dielectric spectroscopy results showed a big change in the real and imaginary part of the permittivity. These results were used to calculate activation energy and heat losses inside the insulation. In order to investigate the possibility of thermal breakdown in the system, the oil-impregnated paper system was simulated by using the FEM software Comsol Multiphysics. The heat produced by PD pulses and dielectric losses was considered in the simulation. The results show that for the oil-impregnated paper insulation, the PD activity can reduce the thermal breakdown voltage by a factor up to 4 times.

Partial Discharge (PD) tests were performed in a metal-dielectric air gap between a spherical electrode and the surface of machine insulation, which consists of mica, epoxy resin and glass-fiber. The PD activities were investigated at three types of applied voltages: triangular, square and trapezoidal waveform. In the trapezoidal waveform, the duration of the linearly rising part of the applied voltage was varied. The effect of the time derivative of the applied voltage dU/dt on the PD activities was studied. The PD activities were studied at a power frequency (50 Hz) and at a lower frequency (10 Hz) of the applied voltage. The results show that the time derivative of the applied voltage has a significant effect on the PD behavior. In each waveform, most of the PD pulses occur during the linearly rising period, but never occur during the linearly falling period. At lower frequency, compared with the positive pulses, the number of negative ones decreases dramatically with the increasing rising time, and PD activities even disappear at the triangular waveform.

Air filled cavity may appear inside solid insulation either due to bad manufacturing or as a result of insulation ageing. A cavity is a weak point in insulation and if the electric stress is high enough; partial discharge can be initiated inside the cavity. In this paper changing of PD parameters (number of PD, maximum magnitude of PD and average magnitude of PD) was investigated over time on oil impregnated paper with an artificial cavity in between the sheets of paper. PD parameters were recorded from the inception of PD until final puncture breakdown occurs in the sample. Experiment was performed on sample with different thickness. Specific pattern on changing of PD number and maximum magnitude of PD was observed for most of the experiments. The pattern includes three stages, big discharges appear at inception of PD and over a very short time they disappear. At stage two, the number and magnitude of PD increase rapidly from small values until they reach to a maximum value. At stage three, the number and magnitude of PD decreases and reach to a constant value. The PD parameter keep constant until the time that final breakdown occurs. The dielectric spectroscopy performed on the sample before and after exposure to PD shows that PD activity causes a permanent shift on ε" which can be explained by PD by-products.

This work investigates the effect of different dielectric barriers on coronadischarge in the needle-plane geometry, where a layer of dielectric material was placedover the surface of the plane electrode. Different dielectric materials used werepolytetrafluoroethylene (PTFE), polyethylene (PE), polycarbonate (PC), polyvinylchloride(PVC), epoxy and pressboard. The discharges were generated by the application ofperiodic negative step voltage pulses. The results show that the evolution of coronapulses over time depends on the dielectric properties of the barriers. Unlike pressboardwith a higher conductivity, the dielectric materials with a lower conductivity such as PTFEand PE would have more surface charges deposited on its surface during the firstcharging period of 100 ms, and the charges would not disappear during the relaxationperiod of 10 s, therefore, the discharge activities can be reduced significantly after thefirst charging period.

High voltage transients in electrical power systems are mainly caused by lightning strikes and switching operations. The high voltage transients above the basic insulation level (BIL) would degrade the electrical insulation of the equipment. In this work, the effect of high voltage impulses on the trend of the surface discharge at the oil-paper interface was investigated. The partial discharge patterns were recorded by the Insulation Condition Monitoring (ICM) system. The HV impulses were generated by a standard 1.2/50 μs HV impulse generator. The measurement results presented in this paper seem to show that surface discharge would cause complete breakdown (continuous flashovers) at the oil-paper interface much faster when the test sample is first aged by HV impulses compared to when it is unaged.

Transients of varied spectral content, such as lightning and switching impulses, are inevitable phenomena that happen in power systems. These natural transients can be used as the stimuli for online dielectric spectroscopy for monitoring power components. The wide range of frequency in those impulses makes it possible to evaluate the dielectric response of their insulation. This paper presents a new technique for online diagnostic of oil-impregnated paper by utilizing those natural transients. Paper with different moisture levels was used to investigate the effectiveness of the proposed technique.The presence of water in the impregnated paper can be seen within those range of frequencies existed in high voltage transients. However, the validity of the results depends on the bandwidth of the applied impulse. Change in frequency response due to different moisture content in oil impregnated paper can be detected by this method. The results have been verified with results from low-voltage frequency-domain dielectric spectroscopy (FDS).

Oil is the main insulation in power transformers and over long time of ageing its insulation properties can change. In this paper ageing of oil due to the exposure to electric discharges was investigated. The effect of high energy discharges (complete arc) and low energy discharges (partial discharges) on oil properties such as breakdown strength and oil conductivity was investigated. An experimental setup consisting of two spherical electrodes was designed. The adjustable distance between the two electrodes made it possible to have PD with different magnitude. The oil conductivity and breakdown strength was measured for three sets of experiments. The first group of experiments was performed on new oil in order to have a reference for comparison. In the second group of experiments the new oil samples was exposed to 1000 and 3000 lightning impulses. In the third group of experiments new oil samples was exposed to partial discharge for different duration of time. Oil conductivity and breakdown strength of these aged samples were compared with new oil. The results show that after exposure to lightning impulse oil conductivity increases and breakdown strength decreases, However PD activity for short time does not change the oil conductivity but it reduces the breakdown strength.

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.

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.

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.

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.

To locate accurately the occurrence of a partial discharge in a power transformer winding, a wide frequency band model of the winding is required. In this paper a wide frequency band Multi Conductor Transmission Line model (MTL) is employed to simulate the propagation of PD signal in transformer windings. The MTL model is briefly reviewed and the related equations of the model are reformulated to easily simulate occurrence of a PD signal along the winding. Software is developed in Matlab to calculate the windings resonance frequencies and the magnitudes of over-voltages occurring between different disks along the winding. Then propagation of PD signal in a power transformer (900MVA, 525kV), is simulated using the (MTL) model with frequency dependent parameters. Also this method is employed to model the laboratory transformer winding.

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.

Partial discharge measurement is a common method for monitoring and diagnostics of power transformers, and can detect insulation malfunctions before they lead to failure. Different parameters extracted from the measured PD activity can be correlated to the PD source, and as a result it is possible to identify the PD source by analyzing the PD activity.

In this thesis, possible defects that could cause harmful PDs in transformers were investigated. These defects include corona in oil, a void in pressboard, a metal object at floating potential, surface discharge in oil, a free bubble in oil and small free metallic particles in oil. The characteristics of disturbing discharge sources were analyzed, like corona in air, surface discharge in air, and discharge from an unearthed object near to the test setup.

The PD activity was recorded both in the time domain and phase domain, and possible characteristics for each PD pattern and waveform were extracted in order to find the best characteristic for the purpose of classification.

The results show that in the phase domain parameters such as phase of occurrence, repetition rate and shape of PD Pattern are most suitable for classification while magnitude of discharge can only be useful in specific cases. The results show that the PD waveforms correlated to different defects are similar; however the time domain data include all the information from the phase domain, and also has the power to identify the number of PD sources.

 The PD dependency on temperature was investigated on the four test objects including surface discharges in oil, corona in oil, bubble discharges in oil, and metal object at floating potential. The effect of humidity was investigated for corona in oil. The results show that at higher temperature the corona activity in oil and PD activity due to a metal object at floating potential in oil decrease. However, for a bubble in oil and for surface discharge in oil the PD activity increases with the increase of the oil temperature. It was shown that the amount of moisture in oil has a strong impact on number of corona pulses in oil.

The last part focused on ageing of oil-impregnated paper due to PD activity. Investigation was made of the behavior of PD activity and its corresponding parameters such as PD repetition rate and magnitude, from inception until complete puncture breakdown. The results show that both the number and magnitude of PD increase over time until they reach to a peak value. After this point over time both curves decrease slowly, and eventually full breakdown occurs.

The effect of thermal ageing of oil impregnated paper on time to breakdown and PD parameters was investigated. The results show that thermal aging of oil-impregnated paper increases the number and magnitude of PD. Dielectric spectroscopy was performed on the samples before and after PD ageing and the result was used in order to explain the behavior of PD over time.

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.