P. Dyer

Accumulative effect on streamer propagation of lightning impulses on oil/pressboard interface

This paper reports the accumulative effect of multiple negative lightning impulse shots on streamer propagation along liquid/pressboard surface. Pressboards impregnated with the insulating liquids including a mineral oil, a natural ester and a synthetic ester were tested under a point-plane divergent field. Streamer propagation was recorded by a high speed intensified CCD camera. In addition, current and light signals were monitored by non-inductive current shunt and photo multiplier tube (PMT) respectively. It was found that with 60 s time interval between consecutive shots a streamer along mineral oil/pressboard interface advances itself following each shot and could eventually lead to flashover in a multiple-shot test, while streamers on ester/pressboard interface seem uninfluenced by the accumulative stresses.

Challenges in oil test database analysis for ageing assessment of in-service transformers

Transformer ageing assessment is an essential step towards facilitating asset management decisions and it involves the interpretation of oil test results normally recorded in a huge database for a large population of in-service transformers. In addition to the need for analysing individual transformer data, this study aims to demonstrate and illustrate some typical challenges that asset managers could face in analysing oil test databases pertaining to large transformer populations. Through analysing multiple oil test databases of in-service transformers, potential challenges are found to be the change in measurement procedure, oil treatment practice, oil temperature and oil contamination. It is hoped that the recognition and the consideration of these challenges could serve as a foothold towards more confident and reliable ageing assessment of in-service transformers for justifying asset management decisions.

An early degradation phenomenon identified through transformer oil database analysis

Ageing of large transformer fleets is a challenge for utilities. To assess the condition of existing transformer fleets, transformer oil is commonly tested for multiple parameters and the data are recorded in large databases for subsequent interpretation. Through analysing multiple databases including oil test results and individual transformer details pertaining to UK in-service transformers operating at primary voltage levels of 33, 132, 275 and 400 kV, population analyses revealed a generic early degradation phenomenon as indicated by an early peak in acidity and 2-FAL trends with in-service age. By exploring the phenomenon from manufacturer, loading and oil chemistry change perspectives, results suggested that the early degradation was most likely due to an oil chemistry change resulting from hydrotreatment oil refining method introduced in the late 1980s. Judging from the faster degradation trend of the affected transformers, a separate asset management strategy may be needed.

Numerical investigation of influences of coolant types on flow distribution and pressure drop in disc type transformer windings

In this paper dimensional analyses of flow distribution in and pressure drop over disc-type transformer windings in an oil forced and directed (OD) cooling mode are presented. Computational fluid dynamics (CFD) parametric sweeps were conducted to quantify the dimensionless relationships obtained from the dimensional analyses to form predictive correlation equation sets. With the help of the correlation equation sets, the influences of coolant types including a mineral oil, a gas-to-liquid (GTL) oil and a synthetic ester on flow distribution in and pressure drop over a winding model were investigated. It was found that the synthetic ester, which has a higher viscosity than the others, could be advantageous in terms of suppressing reverse flow and getting a relatively more uniform flow distribution at the cost of a higher static pressure drop.

Seasonal influence on moisture interpretation for transformer aging assessment

Aging of power transformers is one of the biggest challenges electrical utilities face. As transformers are commissioned, their insulation system composed of liquid (oil) and solid (paper and pressboard) insulations will deteriorate over time, jeopardizing longevity and reliability of transformers. As a means for optimizing capital expenditure without undermining the reliability of electricity supply, transformer aging or condition assessment has been a key practice for utilities. In most cases, it stems from interpreting the measurements of different parameters that can be tested from transformer oil. One such parameter is moisture in oil, which is one of the most commonly measured parameter by the UK utilities [1].

Generation of methanol and ethanol from inhibited mineral oil

Ageing assessment of transformer paper insulation is commonly conducted through measurement of different chemical indicators in oil such as carbon oxide gases and furanic compounds [1]. Apart from these conventional paper ageing indicators, recent research success has shown that methanol and ethanol in transformers can also be used to indicate the degradation of transformer paper insulation [2].