Fault Detection inside transformer
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 powerplant 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.
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 EPRI’s Guidelines for the Life Extension of Substations , section 3. See also IEEE 62-1995™ , section 220.127.116.11.