The Need for Field Testing of Transformers

By Liisa Colby, Doble Engineering

Field-testing of power transformers provides the maintenance engineer with a group of tools to assess the transformer’s condition and to identify problems before they become catastrophic. Consider field-testing as a collection of tools in a toolbox. An individual “tool”—or test—may not be sufficient to accurately diagnose transformer health. Selecting the appropriate tool, or series of tools, is needed to collect the right data to make an informed and knowledgeable decision.

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To be of value to the tester, the benefit for each test must be clear. Each test provides data about a different aspect of a transformer: dielectric, thermal or mechanical.

Field-testing takes time, effort and incurs a cost; so the need for testing must be balanced with the value of the results. It is up to the engineer to choose the appropriate tool for the job at hand: deciding what data is needed, and how it can be obtained most efficiently. Some testing is done while the transformer remains in service, other tests require the unit to be taken out of service for a brief period of time.

There are three major motivations for field testing transformers:

  1. To identify unsatisfactory conditions that may shorten the unit’s useful life or to confirm that a transformer is in suitable condition for service. This includes factory testing prior to transportation and installation or periodic maintenance.
  2. Field testing is also performed in response to an incident or an event that brings the transformer’s health into question. Doubts raised must be investigated and a decision made about transformer viability.
  3. To rank transformers by status for reasons of replacement or for asset management.

When and Why to Perform Field Tests

Field tests need to be performed at certain times in a transformer’s life to assure that key information on the unit’s condition is established. Here is a list of times that a transformer should be tested:

  • At acceptance,
  • When establishing a baseline,
  • When assessing condition after an electrical disturbance,
  • When assessing condition after relocation,
  • Routinely, to establish a condition trend,
  • To determine the dryness of insulation, and
  • When isolating a problem area.

The ultimate goal of testing is to obtain confidence in the transformer’s ability to continue functioning and to reduce the chance of catastrophic failure. Knowing the condition of a transformer fleet also helps with asset management. The test results can be used to rank and prioritize work on the transformers.

Some of the More Popular, Useful Field Tests

Numerous tests can be performed on a transformer both in the factory and the field. Some of the tests are practically limited to being performed in the factory due to the specialized test equipment and higher voltages needed to perform the tests.

Tests are not necessarily limited to electrical tests: oil analysis, infrared thermography and acoustic tests all add useful information when making a decision. Test choice depends on motivation: what caused the testing to be required in the first place. In response to a close in fault that trips the transformer, a thermographic scan of the coolers may not be the most appropriate first step, but may be justified if the fault was generated internally through overheating.

A number of tests require the transformer to remain in service. These include acoustic emission, radio frequency interference and infrared thermography. These are sometimes classed as inspection activities rather than testing activities, but are included in the table on page 59 for completeness.

At times, a simple insulation resistance test using standard commercially available equipment is all that is necessary to provide useful information on a transformer’s condition. This, however, is not usually the case, and more complex tests are needed to assess equipment condition. Following are brief explanations of some testing procedures.

Dissolved Gas Analysis

Dissolved gas analysis is the single most important test performed on oils from transformers. As the insulating materials in a transformer break down due to thermal and electrical stresses, gaseous byproducts are formed. The byproducts are characteristic of the type of incipient-fault condition.

Furfural Analysis

As the cellulosic insulation in a transformer ages, oil-soluble byproducts of the cellulose chain, called furanic compounds, are produced. High concentrations of 2-furfural, the predominant compound, are a clear indication of cellulose degradation, as this is the only type of material in transformers that yields this byproduct. When cellulosic materials are exposed to extreme temperatures, which results in charring, furanic compounds can be destroyed and the carbon oxides may be the only byproducts remaining in significant quantities.

Experience is required in evaluating the furanic compound data, as factors such as type of insulation preservation/oil expansion system, type of conductor wrapped insulation, family of transformer, and treatment of the oil or the transformer, can influence the interpretation. Tests for furanic compounds should be performed initially for all power transformers to have a baseline, for important or older transformers, when high carbon oxides are generated, for highly loaded transformers, and when other tests indicate accelerated aging.

Other important testing considerations include moisture-in-oil and oil quality.

Dielectric Power Factor and Capacitance

The standard “Doble Test” is a broadband test in that it is sensitive to a number of faults relating to dielectric and mechanical failure, making it ideal for routine testing. Power-factor measurements give indication on the material condition of the insulation, while capacitance measurements help verify the insulation’s mechanical integrity.

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The test technique makes it possible to segregate the specimen into major components for more effective test result analysis.

Recovery Voltage Measurements

This is a simple measurement that tries to measure a transformer winding’s moisture content. It is not very common. The measurement has been subject to much debate concerning its accuracy, with the effects of moisture transfer between oil and paper surfaces and oil quality sometimes having substantial effect on the result.

The measurement is in fact a number of measurements, each of which records the voltage which is recovered after a charge-discharge cycle on a winding. A DC voltage is applied to one transformer winding while grounding the others. The voltage is applied for a length of time and then allowed to discharge for half that time. The recovered voltage is then recorded and plotted as a function of charge time.

Related measurements include polarization-depolarization plots and frequency domain spectroscopy which both aim to identify moisture levels in solid insulation.

Exciting Current and Loss

The exciting current test is a single-phase test that was introduced in North America as a diagnostic tool in 1967 and today is part of standard electrical tests in the field. It is primarily a test of a transformer’s magnetic circuit. Its diagnostic capabilities include detection of defects in the magnetic core structure, such as shorted laminations or fundamental changes in the iron characteristics, failures in the turn-to-turn insulation, or problems in the tap-changing device. These conditions result in a change of the effective reluctance of the magnetic circuit, which consequently affects the required current necessary to force a given flux through the core, or, in other words, the exciting current measurement.

The exciting current test consists of a simple measurement of the single-phase current and watts loss, typically on the high-voltage side of the transformer, with the terminals of the other windings left floating (with the exception of a grounded neutral). The field measurements are performed at rated frequency and usually made at voltages up to 10 kV. Three-phase transformers are tested by applying a single-phase test voltage to one phase at a time.

DC Winding Resistance

Transformer winding resistances are measured in the field to check for abnormalities due to loose connections, broken strands and high contact resistance in tap changers. The inductance of the winding will impede the change in current during the test. The instrument that is reading the resistance must be applied long enough for the current to reach a steady value. Taking the reading too soon will provide a resistance value that is incorrect and too high.

Interpretation of results is usually based on a comparison of measurements made separately on each phase or with original data that was measured in the factory. Test results are generally considered acceptable if there is agreement to within 5 percent for any of the above comparisons.

Transformer Winding Turns Ratio

The transformer turns ratio test determines the ratio of the number of the turns in the primary winding in relation to the number of turns in the secondary winding. This test is often used as a factory acceptance test to make sure that the transformer has been designed and built correctly.

Typically, two test methods are used for determining a transformer’s turns ratio. The traditional way is to apply a voltage to the higher-voltage winding and then measure the primary and secondary voltages to obtain the ratio. Some test instruments are designed to directly read the ratio by applying a relatively low voltage to the primary winding and then scaling the measurement of the secondary voltage.

Another way to measure the ratio, which is relatively new, is to use a capacitor with an instrument that can read capacitance.

Leakage Impedance Test (Short Circuit Impedance/Leakage Reactance)

When a transformer’s low-voltage terminals are short-circuited, leakage impedance can be measured. The measurement of leakage reactance, as with capacitance and frequency response analysis tests, is relied upon for detection of winding deformation.

In the factory, the leakage impedance test is performed at the rated current of the windings because a power supply is available to provide the unit’s impedance voltage. In the field, the voltage required to energize the transformer to be tested is usually not available. Portable test instruments have been designed to successfully measure the leakage reactance using a lower current.

Sweep Frequency Response Analysis

The Sweep Frequency Response Analysis (SFRA) is a testing method to measure the frequency response of passive electrical elements for power apparatus. The result is a transfer function, which produces a fingerprint related to the transformer’s mechanical geometry.

SFRA is a simple test to perform, requiring the transformer be in the same condition as for a standard Doble Power Factor test. The test requires good connections and good grounding.

The fingerprint features a number of resonances between 20 Hz and 20 MHz, each strongly related to transformer internal inductances and capacitances, which are related to the transformer’s internal geometry. Variations in results, either with respect to previous results, or phase to phase short circuit results, needs detailed investigation and explanation as they imply some movement or distortion.

In-Service Tests

The following tests are conducted with the transformer energized to provide an energy source for discharges or heat for the oil. These are sometimes performed as part of general inspections of substation equipment.

Acoustic Emission (AE)

AE is a specialized test where acoustic sensors are used to try to pinpoint sources of partial discharge (PD) within a transformer. The aim is to differentiate between PD in places where it may degenerate and lead to transformer failure, such as involves insulation, and PD which may be benign, such as involving an ungrounded support structure. The prognosis in each case may be substantially different.

This test produces very accurate determinations for location in some cases, but may be less clear in other cases.

Radio Frequency interference (rfi)

RFI is used as a general indicator of discharge on a substation, and is used to hone in on suspect apparatus. It can be used to detect the presence of discharge within a transformer, or in components of a transformer, as a means of triggering further tests and analysis.

Infrared Thermography

This is a general test for substation equipment that has shown success in detecting loose connections, faulty pumps and blocked cooler systems. An obvious advantage of infrared scans is that they can be done while transformers and other electrical equipment are energized, therefore reducing costs involved with outages and equipment preparation. This is a specialized test that requires an operator who has undergone training and should be certified.

The motivation for field testing should always be determined before arbitrarily performing a variety of tests. Routine maintenance testing will vary from the acceptance testing performed on a transformer, the latter being very inclusive with regards to the number of tests performed, and will often differ in the specificity typical of investigative testing. In all cases, test results should be reviewed meticulously and should be taken seriously in assessing the condition of transformers. If field testing is performed properly and results are used for trending purposes, transformer owners can rely on this data to make good judgments regarding their transformers.

Adapted from a technical paper by Jill Duplessis and Tony McGrail, Doble Engineering Company, “FIELD TESTING OF POWER TRANSFORMERS,” presented February 2006 at “The Life of a Transformer” Seminar.

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