Transformer Condition Assessment

In today’s power Generation, Transmission, Distribution and Industrial electricity systems there is a growing need to determine the real world condition of the critical apparatus.

Some reasons for this growing emphasis on transformer condition assessment are the general aging of the equipment (reference Newsletter No. 7), increased loading and decreasing resources to monitor and maintain the assets.

There have been a number of emerging technologies targeting transformers that are at varying stages of effectiveness. The cost and relative experience of these evaluation processes are factors when considering application, whether for a single transformer or an entire fleet. There are well-established, mature testing/evaluation methods available that when evaluated together, can provide an effective determination of transformer health.

Evaluation Parameters

  • Dissolved Gas in Oil Analysis (DGA)
  • Standard Oil Chemical Analysis
  • Power Factor/Dissipation Factor
  • Furan Analysis
  • Chronological Age
  • Loading Factors
  • Visual Inspection, Leaking, Damage
  • Cooling Type, Radiators, Fans, Pumps

There are of course other factors that can be used based on an organization’s monitoring standards. The more data the better, providing you have a process to organize and evaluate this data.

Establishing Thresholds – What is Normal?

Hopefully without sounding too much like a Consultant, it depends. We wouldn’t expect a Transmission transformer that has been at half load for most of its life to have the same characteristics as a Generator transformer operating at full load or an Industrial transformer application operating well above full load capability. In general you can establish a set of criteria for these broad groups of transformer applications and then be prepared to adjust the norms for families of transformers within these groups.

There are a number of Industry established thresholds. For DGA, IEEE has defined some levels (reference Newsletter No. 5). Oil testing labs will also offer some general guidelines. Threshold values are based on the population of transformers evaluated and the risk level an organization chooses to accept.

It is important to adjust threshold values for age where applicable, and for voltage class, as we expect more stringent guidelines as the voltage level of the transformer increases. Don’t ignore your own history. As an example, the “rule of thumb” for the Power/Dissipation norm of 0.5 % does not apply to many older transformers.

The best analytical comparisons come from transformers of similar size and voltage class, exposed to similar loading and stress conditions.

Establishing a Transformer Rating

With the threshold or normal levels established, a rating system on deteriorating levels can be developed to group your equipment into classes of concern, e.g.:

Code
Description
Potential Action
1
Test result/condition normal
Maintain standard test cycle
2
Test result/condition slightly outside of the norm
Maintain standard test cycle, review other data, monitor
3
Test result/condition, one or more parameters moderately outside of the norm
Retest on shortened cycle, review other data, review “sister” equipment data
4
Test result/condition, one or more parameters significantly outside the norm, definite degradation from previous results
  • Retest ASAP
  • Review complete history
  • Monitor very closely
  • Remove from Service to investigate

To effectively evaluate and Rate the results, some of the tests must be dissected and individual components screened. As an example, DGA typically tests for 9 dissolved gases. If the Acetylene level was 5 times the norm, I would be much more concerned than if the Hydrogen was 5 times the norm. A representative Rating for DGA can be established and compared with the other parameters to develop an overall Rating for each transformer.

An example of Rating versus dissolved gas level for Acetylene, C2H2.

Code
1
2
3
4
ppm
<= 3
4 to 10
11 to 25
>25

Once your transformers have been rated, you can concentrate on those that are showing stress. A typical Transmission transformer fleet has 85 to 90 % coded as 1 or 2.

Establishing a Transformer Ranking

The Ranking system is an extension of the Rating system that applies a greater degree of granularity to the transformers on watch (Ratings 3 and 4). By extrapolating the evaluation, a severity level can be obtained. Individual tests within a test can be weighted and then combined into an over all algorithm which takes into account the available test data and the relative value/reliability. The Ranking System is a finer screening process that will assist in determining courses of action and priorities.

Obviously care must be taken in the application of any ranking/weighted evaluation algorithm, particularly if missing data is significant. The relevance of the output is directly proportional to the completeness/accuracy of the data. The use of a “rules based” evaluation program can only be effective under the guidance of experienced personnel.

Keys to Effect Transformer Condition Assessment

  • Having sufficient data
  • Organizing the data in a way that it can be evaluated effectively
  • Actually evaluating the data, and comparing results against valid threshold levels
  • Having a well established set of “next step” scenarios
  • Getting expert advice for unusual situations (self serving advice)
  • Acting on the results of your analysis

In the end, there is no magic box or formula that will predict end of equipment life but with a structured process, some practical guidelines and common sense, many uncontrolled outages can be avoided and your transformer assets can be effectively managed. Turning data into usable information brings real value to your organization.

van Kooy Transformer Consulting Services Inc. has evaluated thousands of transformers for Utility and Industrial transformer owners throughout North America.