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OILWATCH Transformer Laboratories is helping project developers and independent power producers improve the reliability of their wind farm transformers. These expensive and critical equipment items bolster turbine generator output from a few hundred volts to medium voltage distribution levels.
Pad mount liquid-insulated transformers are typically used on wind farms. However, they are usually rated as distribution transformers and not generator step-up units and this has contributed to their failure in these applications. In extenuating circumstances, a failure can result in explosions or fire that could damage property and the environment, while also placing lives at risk. Moreover, load may be interrupted for protracted periods, depending on the extent of the damage which may involve either undertaking major repairs or replacing the entire transformer.
Considering the failure rate of these transformers, many project developers and owners are now insisting on more durable units that have been designed specifically for these applications. Among the factors that need to be considered in their design include transformer loading and sizing; harmonics and sinusoidal loads; voltage variations; and other unique requirements that contribute to the failure of this equipment.
“The role of the transformer in these applications needs to be more carefully analysed and evaluated. It is imperative that developers and owners base their decisions on total cost of ownership; network stability; and uptime. This is opposed to the lowest initial cost of the transformers without considering the negative impacts of extended downtime and on revenue due to having to repair and replace them prematurely,” Shiven Brijlal, Managing Director of Oilwatch Transformer Laboratories, tells Cape Business News. Oilwatch Transformer Laboratories has an impressive track record helping asset managers achieve the required levels of safety and reliability from their fleet of transformers at a minimum cost. This is by providing them with a sound understanding of the changes in the internal condition of the transformer. In doing so, asset managers are better able to determine the minimum required budgets for maintenance and replacement of their power transformers. This information also enables them to identify the most effective and targeted manner to spend this budget, which also usually needs to be justified to stakeholders and regulators in an increasingly competitive environment.
A critical first step in any power transformer analysis entails undertaking an insulating oil investigation. Analysing the dielectric liquid inside the transformer provides important information on its internal condition. This includes behavioural patterns of the liquid itself and the various components that are immersed in it. In addition to being used to assess the condition of the transformer, these tests inform predictive maintenance planning to prevent unexpected outages.
This investigation includes a Dissolved Gas Analysis (DGA). A DGA is a powerful diagnostic technique that is used to analyse dissolved gasses that are generated during the dielectric fluid and solid insulation decomposition process. The main gas signatures identified in a DGA include hydrogen and carbon oxides, in addition to sparking and arcing gasses. Hydrogen is the key gas for partial discharges in oil or gas and methane will also typically be present. Carbon oxides are generated by the breakdown of the solid cellulose material and hydrocarbon by the partial discharges of the sparking type. High levels of acetylene, hydrocarbon gasses and hydrogen are as a result of a complete breakdown of the insulation, usually due to winding to winding or winding to ground. As part of the analysis, the laboratory also considers “stray gassing of oil”, which is a non-damage fault. This occurs in transformers at temperatures from 105°C. Hydrogen, methane and ethane may form in all equipment at these temperatures or because of oil oxidation, depending upon the oil chemical structure.
A highly specialised and multidisciplinary field, DGA is a tried-and-tested technique that has been successfully deployed since the 1960s to help prevent catastrophic failures of power transformers. It has since become the most informative test available to asset managers to assess the condition of their power transformers. Laboratory DGAs remain popular because they are more cost-effective, efficient and accurate than online and portable alternatives.
Brijlal says that to achieve a greater than 90% Confidence Normality in a DGA interpretation, a combination of the relevant standards and methods is required, with the rates of gas rise of overriding importance.
“Access to reliable data is just as critical in reliability engineering. Inaccuracies can quickly aggregate and escalate from a minor objection into something much larger that can, inevitably, compromise all the efforts previously invested in better understanding the condition of a transformer. It is, thus, critical to first obtain a reliable sample when undertaking this type of analysis,” he says.
Oilwatch Transformer Laboratories is one of only a few transformer oil testing laboratories that is accredited to the ISO/IEC 17025 standard. This global standard recognises the technical competence of a laboratory to perform specific tests. It, therefore, transcends the requirements of the ISO/IEC 9001 which only relates to the correct calibration of laboratory equipment. The updated standard also takes into consideration the importance of impartiality. Only authorised personnel may, therefore, release opinions and interpretations based on relevant standards.