Ultrasonic and Sonic Fault Detection
This test should be performed when hydrogen is increasing markedly in the DGA. High hydrogen generation indicates partial discharge occurring inside the transformer. Other gases such as methane, ethane, and ethylene may also be increasing. Acetylene may also be present, if arcing is occurring, and may also be increasing. Ultrasonic contact (in contact with the tank) fault detection can detect partial discharge (corona) and full discharge (arcing) inside the transformer. This test can also detect loose parts inside the transformer. Partial discharges emit energy in the order of 20 kHz to 200 kHz. These frequencies are above levels that can be audibly detected. The test equipment receives the signals and converts them electronically into audible signals. Headphones are provided to eliminate spurious noise from the power plant and other sources. The equipment logs data for future reference. A baseline test should be conducted and compared with future test data. This test method has some limitations. If a partial discharge is located deep within the windings, external detectors may not be sensitive enough to detect and locate the problem. However, partial discharges most often occur near the top of the transformer in areas of high-voltage stress which can readily be located by this method. These defects can sometimes be easily remedied, extending transformer service life.
Process
Magnetic piezoelectric crystal transducers, sized and tuned to the appropriate frequencies, are placed on the outside of the tank, and signals are recorded. If discharges are detected, the location is triangulated so that, during an internal inspection, the inspector will know the general area to search for a problem. Likewise, sonic (audible ranges) fault detection can find mechanical problems, such as noisy bearings in pumps or fans, nitrogen leaks, loose shields, or other loose parts inside the transformer tank, etc. See also IEEE 62-1995™.
Vibration Analysis
Vibration analysis by itself cannot predict many faults associated with transformers, but it is another useful tool to help determine transformer condition. Vibration can result from loose transformer core segments, loose windings, shield problems, loose parts, or bad bearings on oil cooling pumps or fans. Exercise extreme care in evaluating the source of vibration. Many times, a loose panel cover, door, or bolts/screws lying in control panels, or loose on the outside have been misdiagnosed as problems inside the tank. There are several instruments available from various manufacturers, and the technology is advancing quickly. Every transformer is different, therefore, to detect this, baseline vibration tests should be run and data recorded for comparison with future tests.
Process
For a normal transformer in good condition, vibration data is normally two times line frequency (120 Hz) and also appears as multiples of two times line frequency; that is, four times 60 (240 Hz), six times 60 (360 Hz), etc. The 120 Hz is always the largest and has an amplitude of less than 0.5 inch per second (ips) and greater than 0.1 ips. The next peak of interest is the four times line frequency or 240 Hz. The amplitude of this peak should not exceed 0.5 ips. None of the remaining harmonic peaks should exceed 0.15 ips in amplitude.
Turns Ratio Test
This test only needs to be performed if a problem is suspected from the DGA, Doble testing, or relay operation. The turns ratio test detects shorted turns, which indicate insulation failure by determining if the correct turns ratio exists. Shorted turns may result from short circuits or dielectric (insulation) failures.
Process
Measurements are taken by applying a known low voltage across one winding and measuring the induced-voltage on the corresponding winding. The low voltage is normally applied across a high-voltage winding so that the induced-voltage is lower, reducing hazards while performing the test. The voltage ratio obtained by the test is compared to the nameplate voltage ratio. The ratio obtained from the field test should agree with the factory within 0.5%. New transformers of good quality normally compare to the nameplate within 0.1%.
For three-phase delta/wye or wye/delta connected transformers, a three-phase equivalency test should be performed. The test is performed and calculated across corresponding single windings. Look at the nameplate phasor diagram to find out what winding on the primary corresponds to a particular winding on the secondary. Calculate the ratio of each three-phase winding based on the line to neutral voltage of the wye winding. Divide the line-to-line winding voltage by 1.732 to obtain the correct line-to-neutral voltage. Check the tap changer position to make sure it is set at the position where the nameplate voltage is based. Otherwise, the turns ratio test information cannot be compared with the nameplate. Nameplate information for Reclamation transformers is normally based on the tap three position of the tap changer. See the manufacturer’s instruction manual for the specific turns ratio tester for details. See IEEE 62-1995™.
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