Analytical Laboratory

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 12%	76	64	52	40	32
Requiring 14%	72	58	44	30	20
Maintenance 17%	66	49	32	15	20
Based on 20%	60	40	20
Condition Code 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.

For many years, dissolved gas analysis and fluid quality testing have been used as diagnostic tools for transformers. By far, the most widely accepted standards for transformer fluid testing and interpretation in the last decade have been the 1991 revision of the IEEE guides, C57.104 and C57.106. TJ/H2b Analytical Services has used the knowledge gained from the application of these guides as a foundation for the development of enhanced diagnostics. In particular, TJ/H2b has developed diagnostic enhancements that include:
 Expanding the number of fluid assessment tests.
 Updating pass/fail fluid assessment limits for newer maintenance philosophies.
 Trending the relationships for fluid data.
 Analyzing the relationships between fluid and gas data.
 Trending gas-in-oil data beyond two tests.
 Relating gas-in-oil data to phenomena in the transformer.
 Assessing solid insulation condition.
 Considering the relationship of these data to operation, system events, or maintenance activities.

In 1997, TJ|H2b began to look at fluid testing possibilities that would provide the capabilities required for upgraded transformer maintenance programs. The ability to use transformers up to their maximum safe loading capabilities is essential to optimizing their use. Necessarily, obtaining the optimal use of transformers changes the approach to maintenance. The major critical asset components of a transformer, the paper, windings and core were not designed to be maintained in the same manner as the components of tap changers or breakers. In a manner analogous to human health care, a combination of preventive practices, prudent operating practices, treatments and repairs are needed. Management of the asset requires the best use of these alternatives and this, in turn, requires the appropriate condition assessment tools.

Transformer Condition Assessment, TCA® has been developed to provide the information for management of the oil condition, management of the paper condition, and the identification of the problems such as core lamination problems, core ground problems, no load tap changer problems, terminal and connector problems.

A Transformer Case History
The case below demonstrates the capability to identify fault location
Dissolved Gas Analysis	Test 1	Test 2
Hygrogen	3246	4098
Methane	15941	21396
Ethane	4115	4860
Ethylene	27293	34664
Acetylene	168	247
Cardon Monoxide	187	251
Cardon Dioxide	6299	8158
Nitrogen	52053	59960
Oxygen	3927	1809

Particle Profile
5 to 15 um	9275	144800
15 to 25 um	1685	17450
25 to 50 um	780	3725
50 to 100um	140	170
> 100 um	15	10

TCA® gave a condition 4* and indicated severe heating, possible coking, and arcing at a connector or contact adjacent to but not directly in contact with paper. Internal inspection revealed a loose bolt on a load side bushing connector that had developed a large coke formation. This formation led to arching that had begun to deteriorate the connector.

The advantages of the Transformer Condition Assessment program can be summarized as follows:

 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.

The TCA® Diagnostic Service is available exclusively at TJ|H2b Analytical Services.