Gordon Wilson

Experimental studies of influence of DC and AC electric fields on bridging in contaminated transformer oil

Analysis of real operating condition revealed that HVDC transformers experience combined effect of DC biased AC electric field. The dynamics of pressboard particle in contaminated transformer oil under the influence of DC, AC and DC biased AC electric field has been investigated in this paper. Different levels of particle concentrations are tested at different applied voltages. Optical images of the particles accumulation together with conduction current have been recorded during the experiments. A complete bridge between the electrodes of cellulose particles were observed for all the tests carried out under DC and DC biased AC electric field. Opposite to that, for AC experiments, pressboard particles accumulated on surfaces of both electrodes but did not create a full bridge between the electrodes. It is concluded that a combination of DC and AC voltages in a HVDC transformer could lead to a bridge formation within the equipment which could cause failure.

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.

Bridging phenomenon in contaminated transformer oil

The effect of cellulose particles contamination on transformer oil has been investigated. Different sizes and concentration levels of cellulose fibres were used to contaminate the transformer oil for the experiments. The conduction current was measured using electrodes of 10 mm diameter with a 10 mm gap and the optical microscopic images of bridge creation between the electrodes were also recorded. The bridge was thickened with increasing concentration level and the bridging time was shortened as the voltage increased. The conduction current increased with concentration level and voltage. Smaller size particles had stronger bond while creating the bridge and higher conduction current.

Quantitative immunoassay for determining polyaromatic hydrocarbons in electrical insulating oils

Abstract The development and application of a combined sample extraction and immunoassay protocol for the quantification of polyaromatic hydrocarbons (PAHs) in transformer oils is reported. Tests were performed on 12 different used transformer oils from three major manufacturers. The removal of matrix interferents was achieved by loading oil fractions onto silica solid phase extraction cartridges and eluting with non-polar solvent prior to evaporation and reconstitution in a more polar medium. Extracts were immunoassayed using two commercially available PAH test kits either having broad specificity towards priority PAHs or enhanced binding specificity toward more carcinogenic PAHs. The total and carcinogenic PAH test kits yielded PAH levels in the oil extracts 5.86-fold and 126-fold lower than the industry-standard IP346 method. The latter method, widely used by the industry, since it correlates with biological carcinogenicity tests, grossly over-estimates PAH levels in oils since it is a non-specific gravimetric solvent extraction approach. The assay was found to be unaffected by the extract sample matrix and was capable of determining PAHs at the nanogram per millilitre level. The assay protocol was simple, low-cost and rapid (

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.

Bridging in contaminated transformer oil under AC, DC and DC biased AC electric field

Cellulose particle accumulation under AC, DC and DC biased AC electric field in transformer oil has been investigated in this paper. Different levels of particle concentrations tested with spherical electrode system. Optical microscopic images and conduction current of the particle accumulation process have been recorded during the experiments. A complete bridge of cellulose particles only observed under DC and DC biased AC electric field.

Bridging in contaminated transformer oil under DC and AC electric field.

There are two experimental set up have been established to investigate the bridging effect between two electrodes with different potentials under ac and dc voltages. This paper will explain the full details of the experimental setup and the results. These experiments carried out on pressboard particles under three different levels of ac and dc voltages. Several contamination levels have been investigated. Optical images of particle accumulation have been recorded along with associated conduction current measurement during experiments. At higher voltages the rate of bridge formation is increased along with an associated current increase. The major differences between ac and dc bridges are the shapes of the particle accumulation. The bridge formation rate for ac is slower than dc.

Numerical simulations of bridging phenomena in contaminated transformer oil

Experiments were carried out on transformer oil contaminated by cellulose fibre. After the high voltage was applied to the electrodes, the pressboard particles accumulated in the highest electric field region and eventually formed a bridge. We have developed a mathematical model to simulate the bridging dynamics. The dielectrophoretic and drag forces are considered for simulation of the particles motion and accumulation. The predicted bridge formation rate and the conduction current between the electrodes showed similar trend to the experimental results. The model is a successful means of predicting bridge formation phenomenon compared with the experimental data.

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.