At present, the main method for detecting the discharge fault of the transformer is to measure the partial discharge level, and there are pulse current partial discharge amount measurement method (hereinafter referred to as pulse current method) and ultrasonic partial discharge measurement method (hereinafter referred to as ultrasonic method). The pulse current method needs to be detected by the applied voltage when the equipment is powered off. Although the discharge amount can be quantitatively measured to determine the electrical position of the discharge point, the spatial position of the discharge point cannot be determined, and the detection must be a power outage. Happening. Therefore, it is more and more important to implement timely and on-line ultrasonic partial discharge detection on transformers and cooperate with other insulation test items (such as oil chromatographic gas analysis, far infrared temperature measurement, etc.) to analyze the insulation status of transformers and determine the nature of insulation defects in time. .
1 ultrasonic partial discharge test principle
There are two types of partial discharge of insulating medium: intra-bubble discharge; medium breakdown at high field strength. Some cast and extruded insulating media are prone to air gaps or bubbles. The dielectric constant of air is smaller than that of solid media, and the field strength is inversely proportional to the dielectric constant. Therefore, air gaps or bubbles in the medium are the source of partial discharge; when the local electric field is higher, the dielectric breakdown will be caused at the weak point of the insulation. The above two partial discharges often occur simultaneously or induce each other in most cases.
When a partial discharge occurs at the test voltage (or operating voltage) of the transformer, it is accompanied by physical phenomena such as electrical pulses, ultrasonic waves, light, heat, and chemical changes. As long as there is a partial discharge inside the transformer, high frequency electrical disturbances must occur and will propagate to all electrical circuits connected to it. A partial discharge of the transformer can be quantitatively detected by receiving a discharge signal from a test device connected to the terminal of the device. At the same time, as long as there is partial discharge, in the process of discharge, with the occurrence of discharge, with the burst-like acoustic emission, ultrasonic waves are generated, and the medium is quickly propagated to the surrounding medium, and the ultrasonic wave is mounted on the outer wall of the transformer tank. When the signal is converted into an electrical signal, the partial discharge level in the transformer can be measured. This is the ultrasonic partial discharge measurement method of the transformer.
When partial discharge occurs inside the transformer, the ultrasonic wave propagates through different media (oil paper, separator, winding, oil, etc.) with the release of sound energy. The ultrasonic signal propagates to the outside of the transformer tank through a medium such as insulating paperboard or insulating oil at a certain speed, and propagates in the form of a spherical wave. The ultrasonic wave passes through the insulating medium and reaches the sensor on the wall of the transformer. There are two ways: one directly Propagation, that is, the longitudinal wave of the ultrasonic wave passes through the insulating medium, the transformer oil, etc. to the inner wall of the oil tank, and passes through the steel plate to reach the sensor; the other is transmitted to the inner wall of the oil tank by the longitudinal wave, and the rear edge of the steel plate propagates to the sensor according to the transverse wave, which is a composite wave. The ultrasonic propagation path is shown in Figure 1. The power source S generates ultrasonic waves, SA is a longitudinal wave, and SBA and SCA are complex waves.
Ultrasonic waves have a strong penetrating power, but they will cause some distortion of the waveform when penetrating various media. This distortion is mainly caused by amplitude attenuation. Table 1 shows the propagation of ultrasonic waves in different media. Speed and its relative decay rate relative to transformer oil.
Although the ultrasonic wave travels much faster in the steel plate than in the transformer oil, the attenuation of the ultrasonic wave in the steel plate is large, so the amplitude of the direct wave reaching the sensor is much larger than that of the composite wave.
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