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  Proceedings of 2005 nlernational Symposium on Electrical Insulating Materials June 5-9,2005, Kitakyushu, Japan A6-3 The UHF Method for Measurement of Partial Discharge and Interference Suppression Qingguo Chen, Yonghong Wang, Xinlao Wei School of Electrical and Electronic Engineering, Harbin University of Science and Technology, Harbin, 150040, China Email: qgchen@ii, Abstract: In recent years, the ultra high frequency (UHF) method is employed to reduce and avoid the noises effects on partial discharge (PD) detection for the power transformer on-site. In order to know the UHF signal radiation fiom the partial discharge in oil impregnated insulation and its propagation characteristics in a transformer tank, the experiments have been carried out in high voltage laboratory. The current impulse waveforms of PD and its UHF signals are measured through the partial discharge detecting circuit and a coupler, respectively. The results show that the electromagnetic wave can be excited from the partial discharge in oil-impregnated insulation with the higher frequency than that in air. The propagation and attenuation characteristics of UHF signals in a metal tank are different from that in opening air. The experimental results also show that the UHF signal can be radiated from the inside of a metal tank through a wire, which makes it possible to measure partial discharge for the power transformer with the coupler fixed at the outside of the tank. INTRODUCTION In power system, the failure of major elements can cause disruptions, and result in very expensive losses. To improve the reliability of power system, in recent years, some kinds of condition monitoring devices have been developed 11-21 to detect the partial discharge in transformer on-site, based on the electrical, acroustic or chemical method. For its higher sensitivity to PD activities, the electrical method [3-41 is widely used. The key to on-line PD monitoring of power transformer using electrical methods is t suppress interferences. Despites great efforts have been made, it is still the critical task to suppress the interferences for building an on-line PD monitoring system of the transformer. According to the experience of PD detection in gas insulaion substation GIS)[5-6], The ultra high fkequency UHF) method is proposed to detect the partial discharge in transformer[7-9]. Considering the different partial discharge activities, as well as the different structure configuations between the GIS and the transformer, it is very important to know: 1) Can the UHF signal be excited fiom the partial discharge orgins in transformer? 2 Does the complex inner structure of transformer effect the propagation and attenuation of UHF signals? 3) Can the UHF coulper receive the UHF sisals from the outside of the transformer tank? To answer the obove questions, the experimental researching works are carried out in this paper. UHF COUPLERS To coulpe the UHF signals excited from the PD srcins, the dual-arm broadband Archimedean antenna[lO] is employed, shown in Fig. I with the diameter equal to hlrc and the deepth of back cavity selected to hJ4. The impedance of dual-arm Archimedean antenna is about 164R. Ln order to matching this impedance to the coaxial cable whose impedance is 50 2, the printed circuit balun is used, shown in Fig.2 , or transforming a balanced to an unbalanced transmission line over a wide frequency range. Figure : ?he structure of dual-arm Archimedean antenna Figure 2 The printed circuit balun In this paper, the broadband Archimedean antennas are designed with band width from lOOMHz to 3 GHz, and with the center frequency 800 MHz. The UHF signals are 29 Authorized licensed use limited to: INDIAN INSTITUTE OF TECHNOLOGY MADRAS. Downloaded on February 16, 2009 at 22:57 from IEEE Xplore. Restrictions apply.  fed to UHF pre-amplifier through coaxial cable. The gain of amplifier is 40dB, and its band width is selected from 300MHz to 3GHz. The frequency response of amplifier is shown in Fig. 3. IC TI -Voltage regulator, T2 -HV test transformer, C -Specimen, RI - Protective resistor, CO Coupling capacitor. Z -coaxial cable, UC UHF coupler, CRO - Tek 684C digital oscilloscope Figure 5: Schcmatic diagram oftest setup for PD detection The experiments are performed under an alternating voltage source of 50 Hz he PD current pulses and UHF signals are observed and recorded simultaneously by digital oscilloscope with the bandwidth lGWz and the maximum sampling rate SGSls Figure 3: Frequency response of amplifier EXPERIMENTALRESULTS EXPERIMENTAL ARRANGEMENT To simulate the partial discharge occurred in power transformer, the PD models are constructed (shown in Fig.4), which are used to simulate: w Corona discharge in oil gap (Fig.4a) w Discharge occurring in void of press-board insulation (Fig. 4b) a Surface discharge along the press-board (Fig.4 c Floating electrode discharge (FigAd) In order to make comparison for PD signals between in transformer oil and in air, the tests are firstly performed in air, then submerges the electrodes in transformer oil and repeated the same procedures of previous test. The test data was analyzed with a Fast Fourier Transformer (FFT). The power spectrums of the signals radiated from PD srcins in air and in transformer oil are shown in Fig6 and Fig.7 respectively. Figure 4: Models for PD n bansformer oil and in air in above models, the needle electrodes are made of stainless steel, and the plane electrodes and floating electrodes are all made of brass During the experiments, the electrodes are put into a test cell filled with the transformer oil. The experimental works were carried out in well shielding room. The schematic diagram of the experimental setup is shown in Fig. 5 bm mc Figure 6: Signal spectrum for model a) in air rrerplmq 6 3r Figure 7: Signal spectrum for model a) in oil 30 Authorized licensed use limited to: INDIAN INSTITUTE OF TECHNOLOGY MADRAS. Downloaded on February 16, 2009 at 22:57 from IEEE Xplore. Restrictions apply.  The above results reveal that the frequency of PD signals excited from PD srcins in transformer oil is higher than that in air, and the highest frequency can reaches about 1.2GHz. The same results are also obtained for the other PD models, shown in Fig.8, Fig.9 and Fig 10, respectively. heqwwy GHr j Figurc 8. Signal spectrum for model b) in ail Figure 9: Signal spectrum for mode1 (c) in oil Figurc IO: Signal spectrum for model d) in oil From the above Figures, it can be seen that the spectrum of the PD signals in transformer oil mainly concentrates on the range from O.4GHz to 1 OGHz. ATTENUATION OF UHF SIGNAL To study the propagation and attenuation characteristics of UHF signals in a transformer tank, the experimental work are carried out in well shielding room with height 4m width 4.4m and length 6m. The test setup consists of UHF signals emitting system, and UHF signals receiving system. In the emitting system, a steep-front pulse generator with a front rise time less than Ins, and a 1/4 ;i monopole antenna are employed. The receiving system consists of a dual-am Archimedean antenna and a TEK 684C digital osdliscope. During the tests, the emitting system are placed in the center of room with height lm from the ground. By moving the Archimedean antenna, the UHF signals at different positions are obtained, whose amplitude distribution are plotted versus the distance from the transmitter to receiver, shown in Fig. 11  0.0851~ 52D25303.54D4.- ? D m Figure 11' UHF ignal propagation in a metal tank Repeating the above tests in opening air the relationship between amplitudes of the received UHF signals and distance from the transmitter to receiver re also measured, which is plotted in Fig.12. i 1 q \ 3 Figure 12: UHF signal propagation in opening air From the above results, it can be seen that the propagation and attenuation of UHF signals in a metal t nk is quite different from that in opening air. In the metal tank he attenuation of UHF signals is slow with the undulations caused by continuous reflection from the metal walls. - 31 - Authorized licensed use limited to: INDIAN INSTITUTE OF TECHNOLOGY MADRAS. Downloaded on February 16, 2009 at 22:57 from IEEE Xplore. Restrictions apply.  ACKNOWLEDGE RADIATION OF UHF SIGNALS For old substations, sometimes it is impossible Lo mount the UHF couplers into a transformer body. In this case, the UHF signals excited from the inner PD source must be detected from the outside of transformer tank. The external UHF method [ 11-12] has successfully been employed in PD detection of CIS with the coupler mounted on CIS window. For the power transformer, however, no such unscreened window is available. According to the structure of a power transformer, the UHF signals can only be picked up near the grounding conductors, such as the ferrite core grounding wire, the grounding wire of the phase winding neutral point, HV bushing tap grounding wire, etc. To test the radiated ability of UHF signals from a grounding wire, the experimental works are also carried out in laboratory. During the tests, the UHF signal transmitter is positioned at the inside of the screened room, a thin copper wire is connected from the inside to the outside. The inside end of wire parted the transmitter 2m. The receiver without the preamplifier is located at the outside of screened room to receive the UHF signals. Table 1 gives the measured results. Table : Received signal amplitudes Distance Amplitudes 12.5 17.8 18.7 29 6 mv) The test results reveal that the UHF signals can be radiated through a conducting wire, and its amplitudes are smaller than those received from the inside of shielding room, caused by the attenuation. So n this case, an amplifier must be applied to improve the sensitivity of PD detection. CONCLUSIONS According to the above experimental works, some conclusions can be drawn as: H UHF signals can be exited from the PD sources occurred in oil-paper insulation apparatus, with the frequency higher than that excited from PD in air. The attenuation of UHF signal in a metal tank is slower than that in opening air. .And signal amplitude distribution fluctuates with the distance from the transmitter to the receiver. The UHF signal can be radiated from the inside of a metal t nk through a wire, which makes it possible to measure partial discharge for the power transformer with a coupler fixed at the outside of the transformer tank. The authors gratefully acknowledge the financial support of the Natural Science Foundation of Heilongjiang province (023220802). REFERENCES [ ] Barry H. Ward, Stan Lindgren, “A Survey of Develop-ments in Insulation Monitoring of Power Transformers”. IEEE International Symposium on EI, Anaheim, 141-144,2000 [2] F. Q Li, L liang, D.H. Zhu, K. X. Tan et al, “A New Stationary On-line PD Monitoring System for Power Transformers”. IEEE Conference on EI, Anaheim, 686- 689,2000 [3] M. Pompili, C Mazzetti, et al, “Simultaneous Measure-ments of PD in Oil Using Conventional Narrow Band and UItra Wide Band Detection Systems”. ICDL, Roma, pp. 185-188, 1996 [4] Reinhold Braunlich, Sartin hassig et al, “Assessment of Insultiaon Condition of Large Power Transfonners by On-Site Electrical Diagnostic Methods”. IEEE Intematioal Symposium on EI. Anaheim, 372-375, 2000 [5] Imagawa hiroshi, Emoto Kunio, Koyama Hiroshi, et al. “Simplified Simulation Method for Partial Discharge Phenomena in GIS”. Proceedings of the IEEE Power Engineering Society on Transmission and Distribution, London, 534-540, 1999 [6] De Kock Nicholas, Coric, Branko, pietsch, Ralf, “UHF PD Detection in Gas-insulated Switchgear - Suitability and Sensitivity of the UHF Method in Comparison with the IEC 270 Method”. IEEE Electrical Insulation Magazine, vol. 12, No. 6 20-26, 1996 [7] M D Judd, B M Pryor, S C Kelly, et al, “Transformer Monitoring Using the UHF Technique”. ISH, London, 5.362.P5, 1999. [XJ Alain F. S Levy, Orsino B Oliveira Filho et ai, “Ultra Wideband Measurements of Partial Discharges in Air and Insulating Oil”. ICDL Roma, pp. 218-221, 1996. [9] W. R. Rutgers, Y. H. Fu, “UHF PD-Detection in a Power Transformer”. ISH, Quebec, 1997 [10]Y. T. Lo S W. Lee. “Antenna Handbook: Theory, Applications, and Design”. New York, 1988 [ll] M. D. Judd, “Dielectric Windows Improve Sensitivity of Partial Discharge Detection at UHF”. ISEI, Anaheim, pp. 304307 2000. [12] Sander Meijer, Robert G A. Zoetmulder, et al, “Development and Application of a Portable Multipurpose On-site / On-line PD Measuring System”. Conference Publication No. 473, pp. 133-138,2000 32 Authorized licensed use limited to: INDIAN INSTITUTE OF TECHNOLOGY MADRAS. Downloaded on February 16, 2009 at 22:57 from IEEE Xplore. Restrictions apply.
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