Identification of Acoustic Emission Signals Originating from the Core Magnetization of Power Oil Transformer

Abstract

In this paper, the properties of AE signals originating from phenomena occurring during magnetization of ferromagnetic materials which are used to construct power transformer cores are presented. The AE signals in a selected power oil transformer were recorded and analyzed. The analysis included, i.e., time, frequency, and time-frequency analyses, calculations of amplitude distributions of the signals and defined AE descriptors, determination of the descriptor map on the side walls of transformers, as well as a detailed analysis of selected part of the signals.The maps of descriptors were analyzed in the frequency bands of 20-70 kHz, 70-100 kHz, and 100-200 kHz. The analysis of the properties of the signals was performed in time and frequency domains. Based on the analysis, there were identified the AE signals originating from the phenomena occurring during the core magnetization of a power oil transformer. To identify those phenomena, the maps of the ADC descriptor calculated in the band of 20-70 kHz when selecting the measurement points in which there were no AE sources from partial discharges were used.An analysis of magnetoacoustic emission signals in the bands of 70-100 kHz and 100-200 kHz was also performed. The analysis of the signal properties in such an extended frequency range allowed determining the properties of the magnetoacoustic signals coming from core sheets of power oil transformers.Keywords: power oil transformers; magnetization of transformers cores; magnetoacoustic emission. Magnetic phenomena in ferromagnetic materialsPhenomena in ferromagnetic materials are one of the sources of acoustic signals in power oil transformers. During the magnetization of ferromagnetic materials which are used to construct transformer core laminations, there occurs the reversible movement of domain boundaries (the so-called Bloch walls), irreversible movement of domain boundaries (Barkhausen effect), and rotation of magnetization vectors in the direction of the external magnetic field. Within these processes, there occur processes of creation of domain boundaries, movement of these boundaries, and their annihilation.The Barkhausen jump results in (i) the generation of an electromagnetic wave (Barkhausen field effect -HEB), (ii) a local change of the magnetization and defects in the crystal lattice and (iii) the generation of an acoustic wave (magnetoacoustic emission effect).The phenomenon of magnetoacoustic emission (MAE) consists in generating a sound wave by a jump motion of a 90• domain wall in a material with nonzero magnetostriction (Augustyniak, 1999). A MAE signal depends not only on the microstructure, stresses or the magnetization frequency, but also on the geometry of the sample and its demagnetization. MAE has a relatively short history. It was first reported by A.E. Lord in the middle 1970 -s (Lord,1975. He called that effect "the acoustic Barkhausen effect" by analogy with "the classic Barkhausen effect". Elastic waves generated during MAE are caused by changes in magnetostrictive stresses during t...

Frequency Diagnostics of Insulating System of Power Transformers

“…Vibrations in the core and winding are the main sources of acoustic emission from a transformer [10], [11]. The vibrations are mostly caused by the following reason -the loosening of winding clamping may cause coil vibrations.…”

“…Similarly, loosening of core clamping may cause core vibration. Other sources of AE in the core include such phenomena as magnetostriction or magnetoacoustic emission by the Barkhausen effect [10], [11]. The vibrations in the core and coil produce AE with maximal energy in the audible frequency range.…”

“…100 Hz, and spectrum of the AE is rich in higher harmonics. In addition, the noise generated by magnetoacoustic emission causes random high-frequency fluctuations in the acoustic waveform [11].…”

A Classification Method for Select Defects in Power Transformers Based on the Acoustic Signals

“…Apart from typical low-frequency acoustic signals emitted by the transformer core, which are related to its vibrations (usually up to a few kHz) [6], ultrasound signals may be emitted as well. They are often related to magnetostriction phenomenon and/or loose elements around the iron sheets of the core; these kinds of defect have their dominant frequency bands at around 15–40 kHz [37]. AEs emitted by oil flow.…”

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“…100 Hz, and spectrum of the AE is rich on higher harmonics. In addition, noise generated by magnetoacoustic emission causes random high-frequency fluctuations in the acoustic waveform [14], [15].…”