Shekhar Mahmud

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.

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.

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.

Experimental studies of influence of different electrodes on bridging in contaminated transformer oil

Contaminated transformer oil has been tested under non uniform electric fields and the effect of different electrode systems presented in this paper. Three different electric fields were examined i.e. DC, AC and DC biased AC. These experiments revealed that with all the different electrodes arrangements, contaminated particles always formed bridges between electrodes under DC electric field. The bridges were thicker and more particles were attracted with more uniform electric field (spherical electrode) than with a divergent electric field (needle-plane).

Effect of Kraft Paper Barriers on Bridging in Contaminated Transformer Oil

The paper investigates effects of Kraft Paper to prevent bridging in pressboard fibre contaminated transformer oil. There are three different configuration of Kraft paper tested. These are 1) covered electrode with loose bond, 2) tight bond and 3) bare electrode with a paper barrier in the middle. There were no bridges formed with loosely bonded tests with DC electric fields. For the other two cases, there was always a complete bridge formed.

Effect of Different Shapes of Electrodes on Bridging in Contaminated Transformer Oil

Contaminated transformer oil has been tested under non uniform electric fields and the effect of different electrode systems presented in this paper. Three different electric fields were examined i.e. DC, AC and DC biased AC. These experiments revealed that with all the different electrodes arrangements, contaminated particles were always formed bridges between electrodes under DC electric field. AC field does not induce any bridging. Combination of AC and DC enhances the bridging dynamics. The bridges were thicker or more particles attracted with more uniform electric field (spherical electrode) than diverse electric field (needle-plane).