Risk & Reliability of Transformers

BY Alan Ross
Articles & Papers

Getting experts to agree on most things is challenging at best, and often, downright impossible. Getting transformer experts to agree might even be more challenging. Despite the fact that transformers have been around for nearly a century with varying opinions on transformer design and maintenance, most experts agree that not all transformer failures are alike. And also, that the life of the insulation or paper is the primary limiting factor towards the life of the transformer. Since most industrial companies use power to produce something, the risks associated with loss of that power have been misunderstood and all too often - ignored. But risk comes in many different forms. Risk of failure can be quantified by the Insurance Industry as the cost to replace the transformer. But the loss of productive capacity - Business Interruption Loss - is usually much higher than the cost of transformer replacement.

Additionally, what is the risk of failure when the condition of the unit has deteriorated and routine preventative maintenance ignored? When asked “Why do you test the oil in your transformers?” the most common response is, “Our insurance company requires it.” The need for a reliable system must become the reason why we test transformers, since every reliable system starts with reliable, uninterrupted power.

Condition Based Risks

Why do transformers fail in the first place? Many failures happen after an incident, such as lightening strike or a failure down line from the transformer. These anomalies can be characterized as electrical, mechanical or thermal, but for the most part they are separate from the condition of the unit. These may be root cause failures but they 2neglect a significant contributing factor, the condition of the unit. An overloaded transformer with well maintained dielectric fluid, which is not already weakened by poor maintenance, is much less likely to fail than an overloaded transformer that has not been maintained.

Can transformer failures be avoided? Can life extension mitigate this risk? I am tremendously biased when it comes to maximizing the life of a transformer since my relationship with S. D. Myers. I have seen first-hand how effective it is to develop standards, rigorously test and track the condition of the fleet of transformers and maintain that fleet to those standards. Quite simply, the easiest risk to mitigate is the Condition Based risk.

We maintain our productive assets with increasing rigor, yet too often we confuse chemical or electrical testing of a transformer with the maintenance of a transformer. In one illustrative case, we were asked to develop a standard for testing and maintenance for a customer with multiple transformers and multiple sites. There was a great deal of commonality of the units and we had done chemical testing for over seven years tracking the condition in the oil of the units. When we asked for the electrical test data to begin to address the complete condition of each unit, we were told, we only test with you.

I think it was that moment that changed my perspective on the whole issue of risk and reliability of electrical systems, with particular emphasis on the heart of the system, the transformer. There is so much more to a complete Condition Assessment than trended oil tests. While that is a great and important first step, the next steps are as equally critical. Is recommended maintenance on oil processing standard throughout the company, or is it left up to each individual responsible for maintaining one plant? Not all oil processing is alike. While some contractors clean the oil, others may process on the unit until they address issues in the paper and the oil.

What criteria should be used for electrical testing to create a “Best Practices” testing and maintenance protocol? What data is available from the manufacturer at assembly and installation to determine changes over time? A simple SFRA test can serve as the baseline for future condition assessment, yet too often we do not have that data. It is a matter of developing a reliable system that can address the condition of the unit without a lot of added costs or downtime, during the life of that unit, not at the end of life. It is all about assessment, planning and systems.

The Weakest Link-Paper

While there are cases when transformers fail without warning, most failures are ones that we can see coming. Deteriorating paper and oil emit gases, which can lead to a better diagnosis of the condition of the unit. Moisture content has become a tremendously valuable predictor of failure. Sadly, gassing and high moisture content is not perceived as critically important as it should be. Accelerated deterioration of the paper insulation comes about from moisture, acid, overheating or oxidation, and it weakens the ability of the unit to perform.

One of the underlying causes of this lack of preventative maintenance when it comes to transformers is the fact they have been too trustworthy for too long. Transformers have been overbuilt in the past and many have gone beyond their useful life, but they continue to hum along behind the substation fence. With the onset of computer modeling for transformer design, the specifications began to be met without overbuilding the transformer. In short, we have been spoiled and have become complacent.

Transformers built today are built with much less margin for error than those of the past. If we take the same approach to reliability based on our past experiences, we are most likely going to see more unplanned outages and downtime from transformer failures than ever before.

Paper Aging

The best measure of paper aging is Furan analysis. As paper ages, it release Furan gases that are incredibly good indicators of the aging of the paper. The Department of Defense, Reliability Information Analysis Center (RiAC) states the following:

“Furans are a family of organic compounds which are formed by degradation of paper insulation. Overheating, oxidation and degradation by high moisture content contribute to the destruction of insulation and form furanic compounds. Changes in furans between dissolve(d) gas analysis tests are more important than individual numbers.”

The importance of regular Dissolved Gas Analysis (DGA) is most evident by the fact it is the delta, the changing levels of furans that are important. However DGA will tell us much more that the aging of the paper, it will also tell us what is happening within the unit.

Excuse Me, I’ve Got Gas

Gases are dissolved in the transformer oil and are formed by both normal operation and aging and by anomalies, or abnormal events. By analyzing the volume, types, proportions, and rate of production of dissolved gases, we can get a very good picture of what has happened or what is happening inside the unit.. Because these gases can reveal the faults of a transformer, they are known as "fault gases". Gases are produced by oxidation, vaporization, insulation decomposition, oil breakdown and electrolytic action.

While it is becoming increasingly common for primary units within a substation or, in the case of steel production, the furnace units, to have DGA Monitors installed on the units, the monitor will likely only detect that a gas or gases are present. An oil sample with DGA analysis can determine the amount of gas and in the case of frequent testing, the rate of change in that gas. From that data, knowledge is developed, meaning the cause of the increased gas is determined. In extreme cases, the analysis will indicate a potential catastrophic failure. In most cases it will indicate a weakening of the overall condition of the unit and the need to avoid overheating are higher loading.

End of Life Risks

Recently, we were asked to consider the End of Life risks for a metal processing facility. While the greatest risk was the application risk, the most costly risk was for several rectifiers that were built in the 90’s. These three rectifiers were not part of the normal inventory or production schedule of the transformer manufacturer. These units were, in essence, custom units with a 26-week lead-time. If there is no planned back up, no spare and no way to operate without one of the rectifiers then the End of Life risk increases.

Transformer manufacturing and installation peaked in the 1960’s and 1970’s in America, with the rapid expansion of industry. Many of those units are no longer in production, have very tight physical footprints due to the building of productive infrastructure around them and may not even be able to travel the roads and rails that were available years ago. In one instance the old rail spur ended seven miles from the plant so getting the unit out and back in again was more costly than the cost of rewinding the unit. It also took special transportation and permitting requirements, a competency the company did not have.

When the End of Life risk and the Application risk were combined, we could monetize the cost of developing a Reaction plan. That cost will likely move any decision on planning up the corporate ladder to a much higher level. Is it better to plan ahead by developing a Reaction plan; develop budgetary costs for unit rebuild or replacement; understand and budget for transportation and contractor access; or is it better to wait until something happens? Obviously, in this instance, good Reaction plans will save tremendous amounts of time and money while allowing corporate management to develop capital budget plans over years rather than within days of a failure.

Life Extension

Obviously the first and most important take-away from this risk analysis must be that we are doing everything possible and economically feasible to extend the life of these critical assets. Even a company with only one unit is at risk of they happen to run their data center of that one unit. How long can most of us go if our data and/or ERP systems shut down for a couple of weeks? Consider the call from the CEO on that one.

The subject of transformer maintenance and Life Extension has been a term that is defined in many different ways and in many different markets. Today, the perspective on transformer maintenance and life extension is becoming a universal concern. The motivating factors behind the unification of strategy on transformer maintenance and life extension have been driven by some common factors that include the aging population of transformers and a higher than expected failure rate from newer replacement units.

The general aging population of the electrical power equipment, given that the infrastructure building peaked in the 60’s and 70’s as pointed out earlier, means a great potential for failure over the next decade. Prior to having to develop Impact assessments, Reaction Plans and Condition Assessments, we should first consider a Life Extension system to develop and maintain the necessary testing and preventive maintenance practices as a priority.

Developing a Reliable System

So where does that leave us? What should any company dependent on the safe, reliable and uninterrupted use of electrical power do?

  1. Understand the short and long term, direct and indirect impact and costs associated with unplanned power outages from the loss of a transformer
  2. Develop and implement a set of standards for determining the operating condition of your transformer(s) through the use of intrusive and non-intrusive testing
  3. Develop a Preventative Maintenance plan to implement company-wide giving reliability professionals the tools they need to monitor and maintain this critical and often overlooked asset class
  4. Develop a Reaction plan for every critical transformer in your area of responsibility/control


Preventative Maintenance Plans

Obviously any good PM Plan will start with an assessment of the condition of the unit(s). However, while most transformer oil testing labs in the U.S. are high quality and dependable, the weakest link in that testing process is often the sampling itself. Contaminated sampling leads to invalid test results or worse, false negatives. When the sampling process is coupled with a reliable Field Inspection, the knowledge gleaned from that sample is more predictive of potential failure. It also leads to a clear maintenance profile for the unit.

Too often, transformer maintenance companies are focused on cleaning the oil, based on good inspection and sampling information. But cleaning the oil may not clean the paper. Moisture is a perfect example. There is usually much more moisture in the paper than in the oil. processing on the oil to remove the moisture is a temporary fix. Within days or weeks moisture will leach out of the paper and into the oil, leading to a high Karl Fisher (the proscribed test for moisture) reading. The same principle is true for acid build up in the oil and paper.

A “Best Practices” for transformer maintenance will be the result of a program for:

  1. Oil Testing on a regular basis with corresponding Maintenance Assessment reporting. Remember, if we test because the insurance company requires it, we are missing the point of developing a reliable electrical system.
  2. Electrical Testing - For more critical units - and in the event of an oil testing result that indicates a need for additional testing - standardized testing will be required.
  3. Mechanical Inspections - Checking the level, proper functioning gages, bushings, fans, radiators and connections.
  4. Infra Red (IR) - While most companies provide an annual IR scan of their entire system, there is a great advantage to conducting an annual routine IR at the time of the oil test. The combined information can lead to specifics about faults, loose connections or potential bushing failure.
  5. Monitoring for gases. Fault gases can lead to catastrophic failure which goes well beyond a simple unplanned outage. Monitoring for these gases creates and early warning system. Since Hydrogen is usually always present with gases, a cost-effective monitoring solution may well be a single gas monitor with a robust follow up oil testing program.
  6. Why do we test? Amazingly, one of the most frequent responses we hear is, “Because our insurance company requires it.” Really? That is why you should perform frequent transformer oil testing? The reason we test is to determine the condition of the unit so we can perform PREVENTIVE maintenance. A well maintained and service transformer can last over half a century. We test that we might gain knowledge about the condition of our equipment and that knowledge leads to wisdom to provide a low cost, reliable electric system.

While we may not get every transformer expert to agree on everything, there is universal agreement that a healthy and robust electrical system starts with the heart of that system - the transformer.



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