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.
After performing a TASA® analysis on your tapchanger, you will receive a condition rating. The recommended sampling interval ranges from ‘immediately’ to ‘annually’, depending on the condition of your unit. Please refer to the scoring methodology or reach out to TJH2b for further explanation on recommended intervals.
TASA™ condition codes, in general, indicate the following+:
+Some variations of the recommended mitigations or retest intervals may occur at the discretion of the laboratory based on information provided. With tapchangers, we are dealing with a complex subject involving a number of variables. As such, general “rules of thumb” with regard to sampling intervals cannot be applied in every case. A number of other factors must be taken into consideration, such as:
As such, the appropriate action is up to the discretion of the Asset Manager who is best placed to take other influencing factors into account according to established internal procedures. Our codes and recommendations, while based on a vast amount of experience as well as guidelines published by IEEE and other standards bodies, are to be taken only as useful guides.
A Tapchanger Activity Signature Assessment (TASA®) includes a condition rating, diagnostics, and potential remediation actions where applicable. Upon receipt of the report from TJH2b, we recommend that results and diagnostics are reviewed promptly, following internal procedures. If further support is needed, TJH2b offers free, 24/7 consultation services to help you understand reports and provide guidance.
TJH2b recommends following the latest revision of ASTM D923 (Standard Practices for Sampling Electrical Insulating Liquids) or an equivalent standard in order to ensure consistent, high-quality sampling practices. TJH2b has developed a tool to help you meet the standard and take a proper, representative sample every time: Turbulent Flush Sampling System.