DGA has been applied successfully for many years to non-current switching oil-filled power equipment. The application of dissolved gas analysis to load tap changers (LTC’s) has both similarities and differences to the use of DGA in other oil-filled power equipment. It is similar in that the same processes produce the same gases. However, in terms of gas production, LTC’s are far more complex than transformers. LTC’s may or may not produce all of the so-called ‘fault gasses’ in normal operation and the gases that are produced may or may not be lost through venting.
The first applications of DGA to evaluate load tap changer condition were based on experiences with transformers. Threshold limits were developed for the gasses produced by overheating both individually and in combination. Many factors such as design, operations, ventilation, and on-line filtration affect gas levels. Consequently, this gas threshold approach offered limited success but proved the potential usefulness of fluid testing for LTC condition assessment. Since gas data alone cannot provide sufficient information to fully assess LTC condition, new approaches were required for LTC evaluations. The search for this new approach led to the development of Tap Changer Activity Signature Analysis or TASA®, which provides a condition assessment of the load path components.
In addition to providing useful information for the maintenance of insulating fluid, fluid assessment tests are used in conjunction with LTC gas data to provide diagnostic information about the condition of load tap changers. Keeping the oil free of water, arc decomposition products and other contaminants is essential for proper operation of the load tap changer. Particle profiling provides important information about the deterioration of materials that result in particle production. This includes information about in-service processes such as fluid degradation, contact deterioration and mechanical wear of moving parts and rust formation. Two of the most important fluid degradation processes to be evaluated are charring of the oil and coke formation.