IEEE C57.127-2000 pdf download IEEE Trial-Use Guide for the Detection of Acoustic Emissions from Partial Discharges in Oil-Immersed Power Transformers
3.1 Sensing transducer
The sensor is a piezo-electric displacement transducer operating in its compression mode and has a resonantfrequency (for longitudinal waves) in the 120-160 kHz range. Because the sensor is a piezo-electric device,it will also respond to varying electromagnetic fields, such as those found in substations. To minimize thiseffect, the transducer can be either a “differential” type utilizing two crystals (mounted out of phase) or ashielded single crystal transducer with an integral preamplifier circuit. The latter is the preferred and mostcommon configuration because its comparatively high-amplitude, low-impedance output is less susceptibleto degradation due to noise pickup in the connecting cables.
The acoustic impedance of a sensing crystal differs from that of the steel transformer wall; therefore, foreficient transfer of the signal from the steel to the crystal, some users interpose a “matching piece.Although several materials may be used for this purpose, a hard-epoxy resin is convenient because it alsoprovides some thermal and electrical isolation. However, care should be taken to select a resin that exhibitslow acoustic attenuation (usually a function of the fillers used) so that it does not adversely affect theamplitude of the transmitted signal. Furthermore, as the acoustic impedance of epoxy resin does notnumerically fall between that of steel and crystal, the thickness of the matching piece shall be equivalent tohalf the wavelength of the signal propagating in it-in this case, 150 kHz longitudinal waves.
The acoustic couplant gel or grease, defined in 2.1, is applied to the face of the transducer or matching piecejust prior to the test.
3.2 Preamplifier
The preamplifier circuit can be either an integral part of the sensor package or a separate unit. The preferredconfiguration is the integral amplifier discussed in 3.1. In either case, the preamplifier circuit should accepthigh-impedance (approximately 10 000 2), low-amplitude (less than 100 uV) signals, provide a gain ofabout 40 dB, and be capable of working into a 50 2 load. To preserve the integrity of the signal, the inherentnoise produced by the preamplifier itself shall not exceed 3 uV referred to its input.
3.3 Filter
The filter is a band-pass type with lower and upper cutoff frequencies of F, and Fp. These are frequencies atwhich the response to a constant sinusoidal input voltage has fallen by 3 dB from the maximum value. Inthis case, Fr, should be about 100 kHz, and Fy should be about 300 kHz. The roll-off characteristics of thefilter shall be a minimum of 48 dB/octave (240 dB/decade) for the high-pass section. This means that.relative to the signal of interest (150 kHz), a 50 kHz signal would be attenuated by 48 dB. The low-passfilter should roll off at not less than 24 dB/octave (120 dB/decade) so that a 600 kHz signal would beattenuated by 24 dB.
The purpose of the filter is to negate as many of the effects as possible of signals that are not associated withPDs. These include vibrations caused by the magnetostrictive action of the core (Barkhausen noise), pumps.and fans. Most of these fall below 30 kHz: however, the Barkhausen noise emanating from the core has beenfound to be in the 50 kHz range. Hence, a 100 kHz high-pass section with a rapid, roll-off responsecharacteristic is needed. The reasonably generous band-pass (200 kHz) allows for variations betweendifferent transducers, in so far as their resonant frequencies are concermed.
IEEE C57.127-2000 pdf download
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