Managing power transformers in service: The most important economic aspects

Maintenance costs vs benefits

Utilities are under intense pressure to maximize the use of their current transformer asset base in order to keep rates down while yet providing excellent reliability. In order to achieve higher loading during both regular and emergency operations, long-standing loading techniques are being evaluated. Important factors to consider while raising loading limits include insulation life, transformer condition, and evaluation of associated failure risk.

Customers’ supply could be compromised in the case of a transformer breakdown in mature utilities due to aging transformer populations and reduced overall contingency margins. The low maintenance to capital cost ratio is a defining feature of transformers.

The overall maintenance expense, however, might be somewhat expensive when applied to a large population of transformers. The conventional time-based maintenance method is being weakened by constrained resources and the necessity to concentrate on prioritized demands.

This technical article includes sample flowcharts for decision processes and cost-benefit analyses.

The majority of the financial considerations for overseeing operational transformers that are evaluated as suitable for service by their manager are addressed in this technical article.

Table of Contents:

1. Should you keep the transformer operational?
2. Transformer Operation
   1. Load Analysis – Are There Any Surprises?
      1. Connection Between Transformer Loading and Loss-of-Life?
      2. Assessing Transformer Condition
   2. Transformer Availability and Handling Outages
   3. Voltage Regulation and Power Quality
3. Transformer Maintenance (Management Not Related To Operations)
   1. How Much Does Transformer Maintenance Cost?
   2. Enough Spare Units and Spare Parts?
   3. On-Line Monitoring and Predicting Future
      1. Example №1
      2. Example №2
   4. Periodic On-Line Condition Assessments
   5. Off-Line Condition Assessment
   6. Databases for Transformer Management and Condition Assessment
   7. Miscellaneous
4. BONUS (PDF) 🔗 Download Distribution Transformers Under Harmonic Distortion: an Economic Study

1. Should you keep the transformer operational?

When looking at power system transformers through the lens of economic management, there are a few key differences from other network assets:

1. The capital cost to operating and maintenance cost ratio is excessive.
2. Difficulties with the available data on the subject of failure rate as a function of unit age.
3. Failures are often of a random character.
4. There is no commonly accepted criterion for technical end-of-life.
5. Transformer normal load is typically much lower than the peak load planned for.
6. Additionally, unit failure is often deemed “acceptable” according to the (n-1) criterion.

This is why, according to conventional economic understanding, the best course of action for transformer management is to put off capital expenditure until later, meaning to keep the current transformers running. Environmental, public relations, cultural customs, and perceived risk reduction are some of the non-economic issues that often impact decisions in managing transformer populations.

In this circumstance, the repair of a transformer could be prompted by economic considerations when a unit is decidedly removed from operation due to an unacceptable danger of failure or an actual breakdown.

Figure 1 – Transformer rust spots and corrosion – not maintained well?

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2. Transformer Operation

2.1 Load Analysis – Are There Any Surprises?

The determination of the permitted loading on the unit is one of the most significant economic decisions that a management will face, second only in importance to the decision to keep a transformer in service. Under particular presumptions, a transformer is defined.

These circumstances might alter at some point in the future, necessitating a reevaluation of the transformer’s load.

In conclusion, the following must be considered while making policy decisions regarding transformer loading:

1. Details on the transformer
2. The state of the transformer, considering its age
3. The surrounding environment
4. Electrical operating conditions
5. Type of loading
6. Acceptable loss-of-life
7. Risk of major/minor failure including financial loss & insurance impact
8. Financial benefits of loading policy

Figure 2 depicts a typical decision-making model that may be employed when deciding policy on transformer loading.

Figure 2 – Decision-making model

Let’s elaborate the points in above Figure 2:

1. The nature of the load (cyclical, emergency 1 hour, etc.) is as significant as the magnitude of the load.
2. Evaluating the transformer’s condition can range from an oil screening/DGA test to a comprehensive array of condition assessment tests, including insulation tan-delta, winding mechanical integrity, and OLTC condition.
3. Was the transformer initially designated to operate beyond its rated capacity?
4. Exceeding the nameplate load capacity may or may not result in accelerated aging, depending on factors such as reduced ambient temperatures or the duration of the additional load.
   As a result, the transformer’s lifespan may be diminished below the anticipated or requisite value!
5. Is the transformer usually considered completely operational?
6. It is necessary to assess the danger of transformer failure under the new load conditions.
   Aproximately 10% of transformer failures result from exceeding nameplate loading!
7. Is the newly identified risk of failure acceptable?
8. Estimate the present value of future transformer-year losses.
9. Assess the financial benefit to the company of postponing the investment.
10. Does the advantage of postponing the investment surpass the expense of loss of life?

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2.1.1 Connection Between Transformer Loading and Loss-of-Life?

Loading a transformer over its nameplate capacity not only increases the danger of failure but also results in rapid insulation aging, leading to a consequential loss of life. This is a multifaceted subject that has been examined extensively over the years.

Two valuable resources for the transformer manager to assess the consequence loss of life resulting from a specific loading pattern are IEEE’s Guide C57.91 and IEC’s Standard 60354, both of which provide comprehensive coverage and models for calculating purposes.