: Fluid Testing Packages for Electrical Asset Management

The Life of a Transformer

Introduction

Maintaining transformers is more important now than ever before. Globally, demand for new transformers has lead times beyond three years. Unless there are enough spare transformers in inventory, it only makes sense to have a robust maintenance program. The three components that determine the lifespan of a transformer are: heat, moisture and oxygen.

The financial pressure of the industry has also impacted transformer manufacturers to reduce costs. Unlike transformers manufactured fifty years ago, new transformers are computer designed with minimal engineering margins. As a result, new transformers are designed with tight operating clearances. Any prolonged overloading or overheating conditions can dramatically reduce the lifespan of the transformer. Thus, it is important for transformer owners to monitor and test transformer oil on a regular basis. The testing frequency should be based on the oil status, voltage class and type of transformer. The question is, are you testing your transformer oil enough, and more importantly, what are you doing to prolong the life of your existing transformers?

Mineral Oil

Mineral oil is the lifeblood of a transformer and is by far, the dominant fluid used in transformers. There are many reasons why mineral oil is used as an insulating fluid in transformers. One, it is an excellent dielectric medium for insulating the components within the transformer and two; it is a good heat transferring agent to dissipate the heat away from the windings to the tank walls and radiators. The radiators are designed to maximize the surface area to effectively cool the oil. The circulation of cool oil back through the windings is a continuous process. And third, it is still the cheapest fluid available for transformer applications.

But transformers will self-destruct over time. Heat and moisture combined with Oxygen will cause a transformer to fail. For every 10°C increase in operating temperature, the oxidation byproducts that attack the cellulose paper double. If the transformer is properly maintained, the destruction process can take many years. But if the transformer is neglected, the transformer aging process can accelerate, and the lifespan can be quickly shortened. This is because of the insulation system within the transformer. The insulation system of a transformer consists of two components: oil and cellulose paper. Although the combination of oil and paper offers an excellent dielectric medium for the transformer, it is the interaction of the oil and the paper that will ultimately destroy the transformer. The hydrocarbons in the oil will form oxidation byproducts that decompose the cellulose insulation system. These byproducts will increase the moisture in the transformer as well as create an environment that will overheat the transformer. In time, the formation of these byproducts can cause the failure of the transformer.

In order to treat the “problem” within a transformer, it is important to understand how the “problem” comes to existence. Moisture can either enter the oil externally through condensation or internally through a chemical reaction.

Given a choice between air or transformer oil, oxygen will leave the air and dissolve into the transformer oil. Thus, a free breathing transformer will begin to transfer oxygen from the air to the oil. Oxygen is also a problem for sealed transformer tanks that underwent a vacuum-filling procedure. A transformer that is vacuum-filled will still contain small quantities of oxygen within the oil. This is a problem because oxygen, in the presence of a catalyst, such as copper or water, will chemically react with the hydrocarbons within the transformer oil. This chemical reaction forms byproducts that attack the paper insulation system of the transformer. The decomposition of the paper will release water and peroxide into the oil. Peroxide is the first oil decaying byproduct formed. Just as oxygen has a preference for oil, water and peroxide will leave the oil and adsorb into the paper. Thus, the cellulose paper has an affinity for the very byproducts that contribute towards its destruction. In addition, the adsorption of water and peroxide into the paper further contributes to the release of even more oxygen into the oil. The problem becomes compounded and in time, creates other decaying byproducts such as acids, alcohols, and sludges. Initially the paper will adsorb these decaying byproducts and in effect, act as a filter. Although the oil testing results will indicate no breakdown is occurring, in reality, the deterioration process has already begun—it’s just not detectable. Once the paper is saturated with acids, sludges and other harmful byproducts, these byproducts will precipitate out and contaminate the oil. In addition, the higher the operating temperature, the faster the oxidative breakdown occurs. So it is also important to limit or avoid overloading transformers for long periods of time—especially transformers with known poor oil qualities.

Unfortunately, these byproducts will destroy the insulation system and lead to the failure of the transformer. Thus, it is a cyclical “problem” that can never be stopped; it can only be slowed down. By treating the symptoms of the “problem”, you can prolong the lifespan of the transformer.

So, knowing that we can never eliminate the “problem” attacking the insulation system within the transformer, the only thing to do is to monitor the symptoms. This can be accomplished by analyzing the oil. By testing the oil, the results can give you a strong indication of what is occurring inside the substation transformer.

Oil Testing

Before you begin oil testing, it is important to identify which transformers need tested, how often, and which tests are necessary for analyzing the oil. It is recommended that all substation class transformers have the following tests performed at least annually. Listed below is a summary of recommended oil tests that should be performed as well as a description of the test1. This list is by no means a complete list of all the tests that can be performed. Rather, it is a recommended list.

The decision is up to each customer to determine which tests should be conducted and analyzed. It is important that when sampling, the individual verifies that there is positive pressure on the pressure gauge so that no air bubbles enter the transformer. Otherwise, serious injury and failure of equipment may result.

Taking an oil sample for transformer oil quality testing

MVA employee drawing an oil sample for Oil Quality Testing.

The intention of this paper is not to provide an in depth chemical analysis on how each test can measure the chemical changes taking place within the oil and the insulation. For the purposes of this paper, the information provided about each test is to serve as a guide for the decision maker.

Summary of Oil Tests and the Test Objective

ASTM Test Method D791
Interfacial Tension Test

The IFT test measures the presence of soluble contaminants and oxidation products. A decreasing value indicates an increase in contaminants and or oxidation products within the oil.

ASTM Test Method D974
Acid Number Test

The acid neutralization number is a measure of the amount of acid materials present in the oil. As the transformer ages, the oil will oxidize and

increase in acidity. The acid value can also increase from contamination of other foreign material such as paint, varnish, etc.

ASTM Test Method D1533
Oil Moisture Test

The Karl Fischer Moisture Test measures water ppm within the transformer oil. Note, it does not measure the water content in the paper insulation.

ASTM Test Method D924
Power Factor Test

The power factor test measures the dielectric losses of the oil, or energy that is dissipated as heat. A low value indicates low losses. It is a useful test for measuring changes within the oil resulting from contamination or deterioration.

ASTM Test Methods D-877 and D-1816
Dialectric Breakdown Voltage

The dialectric test measures the ability of the oil to withstand electric stress without failure. The higher the value, the lower the presence of contaminants such as water, dirt, or other conductive particles.

ASTM Test Method D-3612
Dissolved Gas Analysis Test

The DGA test measures various gas ppm levels that are present. Different gases will dissolve in the oil that indicate various types of thermal and electrical stress occurring within the transformer.

ASTM Test Method D-1500
Oil Color Test

The color test is a simple test that indicates oil quality. The higher the color number is, the higher the probability of contamination or deterioration of the oil.

Test Method EPA 8082
PCB Test

Test the PCB ppm level of oil.

Oil Analysis

Although it is important to test the oil in transformers, it is more important to know how to interpret the data from the results. No one test can be used independently to determine the oil condition.

Rather, all of the results should be reviewed simultaneously to give a full understanding about what is occurring in both the oil and the transformer. This will allow the customer to review the options and make a decision as to how to treat the oil.

In terms of what is actually occurring in the transformer, listed below is a simple table that provides a general summary of what’s happening in the transformer based on the neutralization number and the interfacial tension value.

Transformer oil results analysis

Although the acid test determines conditions under which sludge may form, it does not necessarily indicate that sludging conditions exist. The IFT test is a good indicator of the sludging characteristics of transformer oil because it correlates to the concentration of polar molecules in suspension and in the solution in the oil. Thus, the IFT test serves as an early warning to the beginning stages of deterioration.

It is important to not just consider these two tests as indicators as to whether or not oil treatment is necessary. Other oil quality results should be reviewed as well as dissolved gas analysis (DGA) results should also be considered.