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Sat 9 Aug 2008 04:00 AM

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Trouble shooting transformers

Michael Krüger examines new diagnostic tools for power transformers.

Michael Krüger examines new diagnostic tools for power transformers.

Due to ever-increasing pressure to reduce costs, the power industry is forced to keep old power facilities in operation as long as possible. In most European countries, about one third of the transformers are older than 30 years.

With the advancing age of transformers, a regular check of the operating conditions becomes more and more important.

The Dissolved Gas Analysis (DGA) is a proven and meaningful method such that if increased proportions of hydrocarbon gases are found in the oil, the fault must be located as soon as possible. This way, important preventative maintenance can be performed in time to avoid an unexpected total failure.

The most frequent sources of faults are the tap changers, bushings, the paper-oil insulation and the accessory equipment.

Fault location

In order to find out the reason for high gas values, further tests have to be performed for the transformer. Common test methods are:

• Winding resistance measurement

• On-Load Tap Changer (OLTC) test

• Turns ratio measurement

• Excitation current measurement

• Measurement of leakage reactance

• Frequency Response Analysis (FRA)

• Capacitance and Dissipation factor measurement

The test system used for most of the described tests, has a power amplifier, which generates currents and voltages in a frequency range of 15 to 400 Hz. Therefore tests do not have to be made at line frequency only, but can be made in this frequency range. Using frequencies other than 50/60 Hz and the harmonics, precise results can be obtained even in substations with high electromagnetic interference.

Winding resistance

Winding resistances are measured in the field to check for loose connections, broken strands and high contact resistance in tap changers. Additionally, the dynamic resistance measurement enables an analysis of the transient switching operation of the diverter switch. For a better understanding of the resistance measurements, it is necessary to understand the method of operation of the tap changer.

In most cases, the tap changer consists of two units. The first unit is the tap selector, which is located inside the transformer tank and switches to the next higher or lower tap without carrying current. The second unit is the diverter switch, which switches without any interruption from one tap to the next while carrying load current.

The commutation resistances limit the short circuit current between the taps which could otherwise become very high due to the interruption-free switching of the contacts. The switching process between two taps takes approximately 40 - 80 ms.

A winding resistance measurement was performed a 220 kV / 110 kV - 100 MVA transformer manufactured in 1955. The transformer under test was found to have conspicuously high quantities of gas in the oil, from which the conclusion was drawn of inner overheating.

Except for the middle tap all taps showed a significant increase compared to the original measured values. The differences were more than 10% or, in absolute values, up to 70 mΩ.

The deviations between switching upwards and switching downwards are likewise clearly significant. This shows that the high contact resistances are actually caused by the switching contacts of the tap selector. No silver-plated contacts were originally used and the copper contact surface was now coated by oil carbon.

After a full maintenance of the tap selector, no significant difference to the values measured at the factory in 1954 could be observed. The difference before contact maintenance was up to 30 mΩ (5%) and after it was below 1mΩ (0.18%).

Dynamic switch

To date, only the static behaviour of the contact resistances has been taken into account in maintenance testing. With a dynamic resistance measurement, the dynamic behaviour of the diverter switch can be analysed.

For the dynamic resistance measurement, the test current should be as low as possible otherwise short interruptions or bouncing of the diverter switch contacts cannot be detected. In this case, the initiated arc has the effect of shortening the open contacts internally.

Comparison to "fingerprint" results, which were taken when the item was in a known (good) condition and to the other phases, allows for an efficient analysis.

A glitch detector measures the peak of the ripple and the slope of the measuring current, as these are important criteria for correct switching. If the switching process is interrupted, even for less than 500 us, the ripple and the slope of the current change dramatically.

Analysis of stray losses

The frequency response measurement of stray losses is a tool to determine short circuits of parallel strands. The resistive part of the short circuit impedance is measured over a frequency range from 15Hz up to 400Hz. The resistance curves of the three phases are compared.

The 15Hz values are very similar to the DC values of the primary winding resistance plus the resistance of the secondary winding multiplied by the square of the ratio. If the curve of one phase is more than 2-3% different from the other phases a short circuit fault between parallel strands can be the reason for this behaviour.

Diagnosis of a defective transformer

A 220kV / 110kV / 10kV 100MVA transformer was damaged by a marten. The through-fault current was 54kA on the 10kV side. Although the transformer was switched off within 100ms, Phase A of the tertiary winding showed a short circuit connection to the core. The transformer was insured, so the insurance had to pay the repair costs. Due to the transformer's age, it was not repaired.

A diagnosis of the internal fault was the basis for the sum, which had to be paid by the insurance. First of all the ratio was measured. The large difference of approximately 20% indicated a failure with more than 20% of the turns.

The excitation current of the defective phase was 340mA whereas the excitation current of the intact phases was 10mA.

Leakage reactance measurement

As a second test the leakage reactance was measured. Changes in leakage reactance and in capacitance serve as an excellent indicator of winding movement and structural problems (displaced wedging, buckling etc.).

The conclusion was that the faulty winding was interrupted and parts of the winding were contacting the core. So a part of the secondary short circuit current was flowing through the iron core. With higher frequencies the current was displaced to the core surface due to the skin effect.

Three months after the measurements the transformer was opened, revealing the totally deformed 10kV winding and interruptions of the 10kV winding. So the conclusions were valid.

Capacitance and dissipation factor

In the past, the dissipation or power factor was measured at line frequency. It is now possible to make these insulation measurements in a wide frequency range. Beside the possibility to apply frequency sweeps, measurements can be made at frequencies different from the line frequency and their harmonics. With this principle, measurements are possible also in the presence of high electromagnetic interference in high voltage substations.

The high voltage bushings are critical components of the power transformer and particularly, capacitive high voltage bushings need care and regular tests to avoid sudden failures.

Most of these bushings have a measurement tap-point at their base and both the capacitance between this tap and the inner conductor (normally called C1) and the capacitance between the tap and ground (normally called C2) can be measured. An increase of C1 indicates partial breakdowns of the internal layers.

To determine bushing losses, dissipation factor tests are also performed. Most of bushing failures may be attributed to moisture ingress. Analysis of bushing insulation is much more detailed when frequency scans are performed, but it is necessary to compare the curves to fingerprint measurements. This way it will be possible to detect changes in insulation at a very early stage.

With advancing age transformers require regular checks of the operating conditions become more and more important. The analysis of the gas in oil is a well-proven method of analysis but must be complemented by efforts to locate any faults indicated by excess hydrocarbon gases in the oil. This way important maintenance can be performed in time to avoid a sudden total failure.

The fault location can be successfully performed using simple electrical methods, such as resistance, winding ratio, short circuit impedance and C-tan δ measurements. Modern power amplifiers, enabled for measurements in wide frequency ranges can be used for new diagnosis methods.

Particularly comparing DF curves to fingerprints it will be possible to detect degradation in insulation at a very early stage with a more detailed analysis. Additionally, excellent suppression of electromagnetic interference is guaranteed.

Michael Krüger is product line manager for primary testing with OMICRON electronics.

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