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1、UHF Characteristic of PD in Oil Paper Insulation under DC Voltage Wenrong Si Junhao Li Peng Yuan and Yanming Li School of Electrical Engineering Xi an Jiaotong University Xi an 710049 China E mail siwenrong Abstract A test system with UHF and UWB detection technologies developed to measure PD under
2、DC voltage is introduced in this paper According to the typical defects in oil paper insulation of converter transformer six corresponding PD models are designed The test results of PD UWB detection under DC voltage show that the PD current pulses have a very short rise time which are capable of rad
3、iating UHF signals at frequencies up to at least 300 MHz and these electrical magnetic waves can be picked up by the UHF sensor The DBM spectrum analysis is made for all UHF signals of the models which shows that they differ with various defects and the maximum amplitudes of the energy distribution
4、against the UHF are also different These indicate the on line monitoring for the UHF PD signal under DC voltage should be realized utilizing the narrow band filter with adaptive center frequency Index Terms PD UHF detection DBM spectrum DC oil paper insulation I INTRODUCTION he partial discharge PD
5、would not lead to instantaneous breakdown but it is always the ultimate cause to insulation deterioration and eventually breakdown Therefore PD detection becomes the most popular method to evaluate dielectric insulation condition under AC voltage application But PD detection and analysis under DC vo
6、ltage are not as well developed as under AC voltage for that there are just a few reports 1 5 can be found And all of that were only concentrated on the conventional method PD pulse current detection recommended by the IEC 60270 which for the detection of PD under DC voltage no special facilities ar
7、e needed and in principle any AC PD test system can be used In this paper a test and detection system for studying the ultra high frequency UHF signals radiated by PD under DC voltage was designed Lots of experimental tests were made on the PD models according to the typical defects in oil paper ins
8、ulation of converter transformer and the DBM spectrum analysis of the UHF signals of all PD models is presented II EXPERIMENTALSYSTEM The test system constructed with both UWB pulse current method and UHF method to detect PD signal under HVDC is shown in figure 1 The role of UWB pulse current detect
9、ion method is to ensure that the signal detected by the UHF sensor This work was supported by the High Voltage Division School of Electrical Engineering Xi an Jiaotong University is radiated by the PD model For PD current pulse in oil having a very short rise time which is capable of radiating UHF s
10、ignals at frequency up to at least 1GHz and these E M waves can be picked up by UHF sensors The detection system was developed resorting an oscilloscope which provides a large bandwidth 0 500MHz and a fast sampling rate up to 2 5 Gs S with memory length up to 10MB The sensitivity ranges from 1mV div
11、 to 5V div And the UHF sensor see figure 2 an antenna has superior performance in an effective wide band of 300 1500MHz Fig 1 Schematic of PD test and detection system under DC voltage Fig 2 UHF sensor III PD MODELS AND PD SIGNAL To simulate the classical defects in oil paper insulation six correspo
12、nding PD models were constructed shown in figure 3 They are defect models named needle plate parallel plates surface discharge bound cavity wedge shaped electrode and suspension electrode all of which are installed in a sealed plexiglas cylinder separately T 2008 International Conference on Conditio
13、n Monitoring and Diagnosis Beijing China April 21 24 2008 978 1 4244 1622 6 08 25 00 2007 IEEE Fig 3 Oil paper insulation PD models a needle to plate b parallel plates c surface discharge d bound cavity e wedge shaped electrode f suspension electrode A large number of experimental detection of PD si
14、gnals was done in the high voltage test laboratory with the system shown in the figure 1 During the whole detection the test voltage was controlled as a steady DC voltage The signals of the needle plate model detected are shown in figure 4 as an example to describe the test results At first when the
15、 voltage had just reached the inception voltage only a few single PD signal could be detected see the figure 4 a and the PD repetition rate was very low After the voltage increased to a certain value the PD repetition rate become to very high Besides this a lot of single PD signal could be detected
16、even some PD sequence developed in a very short time could also be detected occasionally figure 4 b But all of the PD UHF signals have the approximate shape figure 4 c in time domain except the amplitude Here focus is made on the UHF signal in this paper And other five PD models also have the same phenomenon mentioned above And there is also little difference between the UHF PD signals in time domain As we all know the selections of center frequency and bandwidth are very important to detection
