| The partial discharge within the power transformer is the main reason which results in its insulation aging and fault. The UHF method is widely used for PD detection due to its advantages such as good anti-interference capability, and high sensitivity. However, up to now in most PD detection methods, the UHF sensors were installed within the transformers, which causes this method not suitable for the power transformers under operation, because it is difficult to design and impose sensors inside. By researching UHF signal outside of the power transformer, this problem can be solved.The propagation characteristics of UHF signal outside of the power transformer and detection sensors design are core parts for external detection method. In this paper, the propagation characteristics of UHF signal is studied firstly. After it is confirmed that the UHF signal could be transmitted outside of the power transformer, a new kind of portable and easy-to-impose UHF signal detection sensor is designed based on the distribution, frequency characteristic of external signal and its recommended location. Finally some tests are designed to verify the feasibility and effectiveness of UHF signal detection by external sensor method. The main research results are as follows:(1) After theoretical and simulation research, the propagation characteristics of UHF signal in the paper-insulated bushing is studied. The results show that after the electromagnetic waves enter the bushing, most of the energy is restricted in the capacitor core of the bushing and the rest energy is restricted at the capacitor core port. This part could be transmitted into the free space through the capacitor core port. Moreover, most of the electromagnetic waves propagating to the outside of the bushing tend to spread parallel with the bushing porcelain. The signal attenuation rapidly when spread far from the bushing porcelain.(2) A simulation model of single-arm Archimedean spiral antenna was designed to research the effects of parameters of antenna on external posited sensor sensitivity. Then the capability of detection of external sensor was verified by testing with several typical insulation defect caused PD signal. The results show that the single-arm Archimedean spiral antenna can be directly fed by a 50 ? coaxial cable. It can also operate at the ultra-high frequency band and be utilized as a circular-polarization antenna as the double-arm Archimedean spiral antenna. The sensor is suitable for loading on other portable devices for its 5 mm section height and the net weight which is less than 150 g. The voltage standing wave ratio, which is less than 3, corresponds to the frequency band 650MHz~2.7GHz. The wideband characteristic and the detection sensitivity of the sensor is further verified by the experimental study.(3) A test platform was set up in order to verify the feasibility of the power transformer UHF signal detection by external posited sensors. The results show that there exists the cut-off effect for relatively low frequency signals when they spread through the bushings. For different types of PD, the magnitudes of output signal by external sensors vary a lot while for internal posed sensors their magnitudes are the same. So it is not able to reflect the PD conditions within power transformer simply based on output signal magnitude. The magnitude of sensor output signal around the flange of bushing is higher than that of other places, therefore the sensors are recommend to be posited near the bushing flange. |
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