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Transformer testing

How reliability of your power network can be enhanced

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ANALYSIS

Industry insight: Hussain Al-Juffairi, application engineer at Megger, specialising in power transformer diagnostics and testing, explains how reliability of your power network can be enhanced.

Transformers are one of the most mission critical components in the electrical grid. The need for reliable diagnostic methods drives the world’s leading experts to evaluate new technologies that improve reliability and optimise the use of the power network.

Insulation Diagnostics
Modern technology and developments in signal acquisition and analysis techniques have provided new tools for transformer diagnostics. Of particular interest are dielectric response measurements where insulation properties of oil-paper systems can be investigated.

Dielectric Frequency Response, DFR (also known as Frequency Domain Spectroscopy, FDS), was introduced more than a decade ago and has been thoroughly evaluated in a number of research projects and field tests with good results.

DFR data in combination with mathematical modeling of the oil-paper insulation is proven as an excellent tool for moisture assessment.

Since the modeling theory contains influence of temperature, DFR and modeling can be used to calculate the temperature dependence of the insulation system.

Winding resistance
Measuring a transformer’s DC resistance from one external terminal to another can reveal a great deal of information about the transformer. In addition to the obvious faulted winding (i.e., an open winding or shorted turn), more subtle problems can be detected.

The DC current, in addition to flowing through the winding, must also flow through the off-load ratio adjusting switch (RA switch), the on-load ratio adjusting switch (load tap changer or LTC), as well as numerous welded and mechanical connections. Hence, the integrity of all these components can be verified.

Turns Ratio
Another quick and simply test is to check the turns ratio of the transformer at the different tap-settings.

Transformers are subject to mechanical impact and vibration. Problems or faults occur due to poor design, assembly, handling, poor environments, overloading or poor maintenance.

Measuring the polarity and turn ratio of a transformer winding assures that the connections are correct and there are no severe mismatches or opens.

Traditional Tan Delta Measurements
The most common insulation diagnostic test is measuring capacitance and power factor at 50/60 Hz. This is the standard test performed whenever there is a need for investigating insulation properties.

Most tests are done at 10 kV (or sometimes lower, pending the voltage rating of the component), and operating temperature but there are also tests with variable voltage (tip-up/step-up testing) as well as tests where power factor versus temperature is measured. Analysis is based on (historical) statistics and comparing factory values.

Since insulation properties are pending temperature, temperature compensation has to be used for measurements not performed at 20°C, this is normally achieved by using temperature correction curve / table values for certain classes of devices.

It is obvious that the given values are approximate guidelines only. IEEE 62-1995 states; The power factors recorded for routine overall tests on older apparatus provide information regarding the general condition of the ground and inter-winding insulation of transformers and reactors.

They also provide a valuable index of dryness, and are helpful in detecting undesirable operating conditions and failure hazards resulting from moisture, carbonization of insulation, defective bushings, contamination of oil by dissolved materials or conducting particles, improperly grounded or ungrounded cores.

Dielectric Frequency Response
In DFR tests, capacitance and dissipation/power factor is measured. The measurement principle and setup is very similar to traditional 50/60 Hz testing with the difference that a lower measurement voltage is used (max 200 V) and instead of measuring at line frequency 50/60 Hz, insulation properties are measured over a wide range of frequency, typically from 1 kHz down to 1 mHz.

The capability of DFR to measure dissipation factor as function of frequency, gives the user a powerful tool for diagnostic testing. Moisture assessment is one example. High moisture levels in transformers is a serious issue since it is limiting the maximum loading capacity (IEEE Std C57.91-1995) and the aging process is accelerated.

Accurate knowledge about the actual moisture content in the transformer is necessary in order to make decisions on corrective actions, replacement/scrapping or relocation to a different site in the network with reduced loading.

Individual Temperature Correction (ITC)
DFR measurements and analysis together with modeling of the insulation system includes also temperature dependence. A new methodology (patent pending) is to perform DFR measurements and convert the results to dissipation
factor at 50/60 Hz as a function of temperature.

This technique has major advantages in measurement simplicity. Instead of time consuming heating/cooling of the bushing and doing several measurements at various temperatures, one DFR measurement is performed and the results are converted to 50/60 Hz tan delta values as a function of temperature.

Temperature correction tables such as in IEEE/C57.12.90 give average values assuming “average” conditions and are not correct for an individual transformer or bushing. This is confirmed in field experiments and some utilities try to avoid applying temperature correction by recommending performing measurements within a narrow temperature range.

How accurate the individual temperature correction can be is a valid question. Using standard tables can easily give power factor errors in the order of ± 50-100% or more. Preliminary tests with commercial test instruments and SW indicates that the inaccuracy for ITC is about ± 5-10% at the extremes of a 5-50°C temperature range i.e. correcting from 5 or 50° to 20° reference.

Oil Testing
Condition monitoring of power transformers typically starts with a regular check of the insulating oil, in particular through testing for dielectric strength, moisture content and dissolved gasses.

The latest recommendations are to specify portable oil testers offering test voltages up to 80kV, which are nowadays obtainable in field test instruments that weigh less than 20kg.

These allow immediate on-site assessment of the dielectric strength of the oil, an early indicator of any problems in the insulation of the transformer.

Bushing Diagnostics
Aging/deterioration of high-voltage bushings is a growing problem and manufacturers as well as utilities and test system providers are suggesting and testing various methods for detecting bushing problems before they turn into catastrophic failures.

This includes on-line monitoring as well as off-line diagnostic measurements. Traditional 50/60 Hz dissipation/power factor testing may give an indication of aging/high moisture content, especially if performed at various temperatures.

Increased dissipation factor at higher temperatures is a good indicator of bushing problems. Catastrophic bushing failures (explosions) are often caused by what is called “thermal runaway”.

A high dissipation factor at higher temperatures result in an increased heating of the bushing which in turn increases the losses causing additional heating which increases the losses even further and the bushing finally explodes.

Asset Management
From a productivity point of view it can be of huge benefit to have individual test instruments to cover the various tests that you wish to perform, allowing different engineers and technicians to be performing different tests around the site at the same time.

Modern PowerDB software links the different sets of test results together to form a complete database of tested assets, allowing simple reporting of completed tests, and trending of the results for diagnostic purposes.

Summary
In many countries the average age of the installed transformer base greatly exceeds its design life, and also with newly installed transformers the benefits of regular and effective maintenance ensures maximum efficiency, optimum use of the power system, and lowest lifetime equipment costs.

About the author
Hussain Al-Juffairi is an application engineer at Megger Limited specialising in power transformer diagnostics and testing.

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