Acoustic testing of power transformers

The acoustic method is based on the detection of sound impulses generated by electrical discharges, with sensors installed on the wall of the tank. Modern ultrasound sensors can register discharge processes with energies up to 10- 7 J. This method is promptness and allows locating the defect, accompanied by discharges. In addition, a range of very efficient methods of monitoring of power equipment is based on this method. In the world this method is being developed, and currently a new international standard (IEC 62478) for measuring partial discharges with acoustic method is being prepared.

In electrical equipment there can be simple or complex conditions of ultrasound. In high-voltage bushings and instrument transformers there are usually simple conditions of ultrasound, when the sound of the discharge is distributed in almost homogeneous medium at distances of hundreds of wavelengths and therefore is attenuated slightly. In power transformers, a source of electrical discharge may be in the depths of the equipment. In this case, ultrasound passes a number of obstacles and is considerably attenuated. If a small quantity of oil-filled objects has almost the same values of acoustic signal at any point on the surface, in examination of the power transformer this difference is more significant, and it is necessary to look for surface area with a maximum signal while moving the sensor.

Sensor Positions in the acoustic antenna. R = 120 mm. Waveforms.

More than 30 years ago in the USSR there were released about a hundred АПЧР type sensors that were used in TCI Zaporozhye transformer plant, Moscow power plant, as well as in operation of transformers. In Russia, in the last 15 years, about 2000 domestic РЧРш type sensors were permanently installed in Mosenergo substations to control gas-insulated equipment, cable couplings and power transformers. These sensors are used for periodic inspection of the equipment. The world’s most famous and similar sensors are type R6I, R15I, VS150-RI sensors sold by "Physical Acoustics Corporation", "Vallen systeme" and "Ultraprobe".

РЧРш type sensors

For registration and analysis of signals, generic digital oscilloscopes are commonly used in Russian power engineering. For example, these are models by Tektronix (U.S.) or "AKTAKOM" (Russia), which are possible to record waveform and spectrum. Imported and domestic instruments for the analysis of acoustic emission signals are not applied in the Russian energy sector due to several reasons, including the high costs of equipment. "Technoservice-Electro" uses the ultrasonic moderator "Dolphin", which main advantage is the transmission of sound image of the defect. Similar audio units to record the signals are part of the apparatus of acoustic emission. Ultrasound moderator allows to record, slow down and listen to the ultrasound signal. Operating methods of this device are based on a person's ability to recognize the sound images of defects. Sensor’s operating frequency range 60-130 kHz when slowing down 100 times is translated to one octave of audible range of 600 Hz - 1.3 kHz. Recording time is two periods of the line frequency – 40 ms, playback time – 4 s. In addition, it is possible to use common computer programs for processing and editing digital audio, such as “Sound forge”, “Gold wave”

"Dolphin" ultrasonic moderator

To understand methods of work with received and slowed down ultrasound, let’s recall the known truth. A man has two ears, located at a distance of 15 cm, which corresponds to half the wavelength of sound in air at a frequency of maximum sensitivity f = 1 kHz. Wavelength is λh = C / f = 33 cm, where C is the speed of sound (in air C = 330 m/s).

The speed of sound in transformer oil is D = 1390 m/s, the operating frequency of the sensor is in the area of 100 kHz, so the required wavelength is  λ ~ 1,5 cm, which is about 10-20 times smaller than λh. People can recognize a sound in the familiar audio range, focused on λch = 33 cm Therefore, listening to a power transformer signals with "Dolphin" or "Dolphin stereo", we “change” the scale of surrounding space, and we need to imagine a transformer 10 times bigger than it is.

Usually the discharge sound level in the transformer insulation comes from a distance of about 1 m, the broadcast length can be measured in almost a hundred of wavelengths. In familiar sound pattern at a frequency of 1 kHz in the air the distance of 100 wavelengths is 30 meters. As the result of reflection the sound creates a reverberation image. In addition, while passing the tank walls, the longitudinal sound wave from a liquid turns into transverse shear and surface waves, which are converted with ultrasonic transducer into an electrical signal. Considering all transfer functions of environment and sensor, and restore the true sound of the discharge is still impossible. However, even the distorted acoustic signals can be reliably interpreted.

Slowed down and audible discharge sounds are clicks, usually with a period of 1 s. Waveform of the sound contains 5-15 periods of damped oscillations. This sound is short (~ 5 ms) and is not a music or speech. For digital analysis of this sound the wavelet transform should be applied. Classification of audio waveforms and wavelet transformations is performed with the mathematical methods of pattern recognition. And in general, more important for recognition are not the frequencies and tone but the signs called signs of maximum prospect. At present, both the human and the program set out the following classification: a single partial discharge in oil and sulfur hexafluoride, a single spark discharge in oil, numerous discharges in oil, vibro mechanical process.

Waveform of the discharge sound in a drop of water in the oil, and its wavelet transform (plane and solid)

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