POWER
FINDING THE RIGHT MEASURES FOR TRANSFORMER PROTECTION
by a number of factors including design, materials, insulation system contaminants, and operating conditions. The combination of water, oxygen, and acids within the solid insulation speeds up the ageing process and degrades cellulose molecules, resulting in diminished mechanical strength and a shorter operational lifespan for the transformer. The challenge for transformer managers is to be able to reliably detect the early indicators of premature ageing so that timely action can be taken. Under normal operating temperatures and in a water and oxygen-free environment, ageing processes would be almost non-existent. However, the creation of such conditions is not practical or economically viable. It has been reported, for example, that if oxygen levels in the oil remain below 2,000 ppm, the insulation system ages more than five times slower than in a free-breathing transformer. However, this
T
ypically, the anticipated lifetime for transformers with cellulose based solid insulation can be several decades when operating under nominal load and specified ambient temperature conditions. However,
transformer ageing is affected
ageing is not limited to just the cellulose; insulation oil also undergoes ageing, mainly through oxidation, which forms carboxylic acids that can accelerate paper ageing.
OXYGEN AND MOISTURE CHALLENGES Ambient air is the sole source of oxygen in transformers and the primary source for moisture. Transformers are therefore fitted with preservation systems such as rubber bags or nitrogen blanketing, which create a protective barrier between the insulation oil and the surrounding environment. Modern power transformers are usually fully sealed, and older units are frequently upgraded with sealing systems to prevent the ingress of moisture and oxygen. Air leaks are generally caused by gasket or rubber bag deterioration, particularly when transformers operate at higher loads. The increasing use of sealed designs means that asset managers need to ensure that transformer tanks remain airtight.
THE LIMITATIONS OF OLDER MONITORING METHODS Traditional methods for the detection of air leaks involve the measurement of oxygen and nitrogen in standard DGA oil samples. However, the risk
of sample contamination by air has been a major challenge because samples can pick up oxygen as they are handled and transported, resulting in incorrect data interpretation. In recent decades, technology has been developed for the online measurement of oxygen concentration in oil to confirm air entry. However, this method may not detect slow air leaks when active oxidation processes are simultaneously consuming oxygen. Over six decades have passed since the industry started using dissolved gas analysis, but there is still no recognised standard practice for the interpretation of oxygen concentration, so an experienced DGA specialist is usually required for the interpretation of oxygen measurements. Some DGA monitors calculate nitrogen levels from integrated oxygen measurements, but these calculations assume constant oxygen/nitrogen ratios in air-saturated oil, making them unsuitable in that respect for sealed transformers.
WHAT REALLY MATTERS FOR ASSET MAINTENANCE TEAMS?
Figure 1: Total gas pressure (TGP) and calculated partial pressures of oxygen and nitrogen at 50°C with linear fitting
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In order to fully understand the ageing process, it is necessary to measure oil quality parameters, such as acidity, and ageing
Autumn 2024 UKManufacturing
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