A Diagnostic Service That Can Reduce LTC Maintenance and Failures. 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.

 

TASA® Case History
Load Tap Changer
Key Gases
ppm
Hygrogen (H2)
82
Methane (CH4)
56
Ethane (C2H6)
48
Ethylene (C2H4)
148
Acetylene (C2H2)
2
Carbon Monoxide (CO)
494
Cardon Dioxide (CO2)
4260

Despite the very low levels of gas in this unit, these data produced a Condition code 4 (heating problem) using TASA®

The advantages of the Tapchanger Activity Signature Analysis program are:
  • Units that require maintenance are identified.
  • Units that do not currently require maintenance are identified
  • Maintenance activities can be better focused.
  • Operators and planners can assess loading capabilities.
  • Failures and collateral damage to transformers can be reduced.
  • Reliability is enhanced.
  • Costs are reduced.
  • Safety is not compromised.

Percent Savings in Maintenance
Fixed - Year Maintenance Interval (Years)
2
3
4
5
6
% of Units Requiring Maintenance Based on Condition Code
12%
76
64
52
40
32
14%
72
58
44
30
20
17%
66
49
32
15
20
20%
60
40
20
25%
50
25

Percent savings are based on a comparison of the percentage of units requiring maintenance as given by the condition code versus the number that would be maintained on a fixed-time interval.

The TASA® Diagnostic Service is available exclusively at TJ/H2b Analytical Services