Transformer Tier 1 Asset Health Index Scoring

Presented By:
Mark B. Goff, MSEE, PE
Tennessee Valley Authority

TechCon 2017


Transformers are one of the largest capital assets in power generation and transmission. The Tennessee Valley Authority is the largest public utility in the United States and has over 990 large power transformers. Understanding the health of these assets will help increase reliability and reduce cost.

This paper describes how to score key elements for individual transformers. After scoring the key elements, the transformer fleet can then be ranked. This process allows the asset managers or maintenance managers to be able to prioritize testing, maintenance or replacement. It also allows for the development of a more accurate business plan by understanding expenditures for the transformer fleet.


Subject Matter Experts (SMEs) develop the scoring criteria for all of TVA’s Tier 1 and Tier 2 assets. Technicians, Engineers and SMEs gather data throughout the year that populates the Asset Health Index (AHI) database. Assets’ scores are updated as maintenance, testing and inspections are performed. At a minimum each Tier 1 asset score is updated annually. The AHI scores are one of the major drivers for TVA’s 10 year Asset Management Plan for TVA’s Coal, Gas and Hydro generation fleet. TVA’s Fleet Asset Optimization (FAO) team has the primary objective to maintain TVA’s 10 year plan and cultivated a more efficient practice for capturing, evaluating, approving, and tracking all capital investment requests and operation and maintenance (O&M) investment requests. The goal is to improve fleet health through appropriate distribution of funds to the critical risk-bearing assets.

Tier 1 Assets



Fans, Centrifugal (FD, ID, etc)

Switchgear, Med. Voltage



DC Boards

Fire Protection

Switchgear, Low Voltage

Boiler Feed Pumps


Generators (Stators / Rotors)


AHI for Transformers

Purpose: This Scoring establishes the requirements, guidelines, and basis for the AHI score for Generator Step Up and Station Service Transformers for TVA Coal Operations (CO), Gas Operations (GO), and River Operations (RO) power production facilities.

Scope: This scoring system applies to all mineral oil-filled large power transformers in all CO, GO and RO facilities using a Tier 1 (fully automated) and Tier 2 (guideline SME based) approach.


  1. Pull transformer oil data from TVA’s Central Labs database program
    • Dissolved Gas-in-oil (DGA) – Sampled Quarterly
    • Oil Quality – Sampled Annually
  2. Store raw data into TVA’s work management system
  3. Run analytics for DGA and Oil Quality Scores based on the established TVA rule base
  4. Calculate Apparent Age Score based on Actual Age and Degradation Curve
  5. Scale the Tier 1score using guidelines set by the SME (Experiential Implementation)
  6. Populate TVA’s work management system with the AHI
  7. AHI used to feed TVA’s 10 year Business Plan

Fig. 1 Simplified AHI Scoring Map

Tier 1 Scoring (Dissolved Gas-in-Oil – 33.33% weighted score)

Dissolved Gas-in-Oil

  1. Hydrogen
  2. Methane
  3. Ethane
  4. Ethylene
  5. Acetylene
  6. Carbon Monoxide
  7. Carbon Dioxide
  8. Nitrogen
  9. Oxygen

Using TVA’s scoring system (based on a composite of IEEE C57.104-2008, IEC, Rogers Ratio [1], Dorenberg [2], Duval’s triangle [3]), a value of 0 to 3 is assigned. 0 – Bad, 1 – Deteriorated, 2 – Fair, 3 – Good.

Tier 1 Scoring (Oil Quality – 33.33% weighted score)

  1. Moisture – Test used for Scoring
  2. Dielectric – Test used for Scoring
  3. Power Factor – Test used for Scoring
  4. Interfacial Tension (IFT) – Test used for Scoring
  5. Total Acid Number (NN) – Test used for Scoring
  6. Color – Routine Test
  7. Inhibitor Content – Routine Test
  8. Relative Saturation – Routine Test
  9. Density of Oil Sample – Routine Test
  10. PCB – Special Test
  11. Corrosive Sulfur – Special Test
  12. Particle – Special Test
  13. Furan – Special Test

Using TVA’s scoring system (based on a composite of IEEE, IEC), a value of 0 to 3 is assigned. 0 – Bad, 1 – Deteriorated, 2 – Fair, 3 – Good. Tier 1 Scoring (Apparent Age and Degradation Curves – 33.33% weighted score)

Fig. 2 Transformer Degradation Curves

Tier 2 Scoring (Reduces the Tier 1 score by the following percentages )

Power Factor testing

  • Investigate Rating on High Side Bushing(s) C1 Power Factor – 50%
  • Investigate Rating on High Side Bushing(s) C2 Power Factor – 40%
  • Investigate Rating on Other Bushing(s) C1 Power Factor – 30%

Winding Resistance [4] (subtract from 100%)

  • Measured Resistance Greater than +/- 5% from Factory Test – 70%

Visual Inspection (subtract from 100%)

  • Pegged Low Oil Level Bushing(s) – 70%
  • Oil Leak on a Bushing(s) > 100kV – 50%
  • Low Oil Level High Side Bushing(s) – 30%

Infrared Inspection (subtract from 100%)

  • Bushing Cap Temperature Rise Over Ambient > 35C – 70%
  • Bushing Cap Temperature Rise Over Ambient > 10C – 20%

(AHI – Dissolved Gas-in-oil Scoring) * 1.111 = 0 to 3.333
(AHI – Oil Quality Scoring) * 1.111 = 0 to 3.333
(AHI – Apparent Age) * 1.111 = 0 to 3.333

(0 to 10) x SME Scoring = AHI

Fig. 3 Transformer Probability of Failure Curve

Benefits of Data Mining the TVA’s Oil Lab Database

  • Auto populates the oil test results for Transformer Tier 1 criteria
  • Replaces present oil database
  • Linking to In-house work management solution adds flexibility for future enhancements
  • Allows a quick turnaround and flexibility to update the rule base


By understanding the first principles of scoring the health of a transformer, the resources can be better applied to those assets that require a more involved analysis. Although many other algorithms can be used, the above method allows for the process to be fully automated for the Tier 1 criteria. DGA and Oil Quality tests have a huge advantage over off line electrical test because the equipment does not need to be out of service and oil testing is much more frequent.

An AHI scoring system allows asset managers and maintenance managers the ability to prioritize testing, maintenance or replacement. The AHI score can feed other systems such as business planning to compare various types of assets when competing for O&M or Capital dollars.


[1] R. R. Rogers, Doble publication 42AIC75, Sec. 10 – 201.

[2] E. Dornenburg, C. I. G. R. E. (1970), Paper 15 – 07.

[3] Delta-X Research, “The Duval Triangle DGA Diagnostic Method”, Internet: www.deltaxresearch.com/triangle.htm

[4] Biddle/Megger, “Instruction Manual AVTM830280 for Transformer Ohmmeter”. Internet: www.biddlemegger.com/biddle-ug/830280_UG.pdf

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