The partial discharges (PD) measurement approach in power equipments is a very important inspection technique for insulation deterioration assessment. The PD based approach possesses the greatest potential for further development. This study proposes a noncontact type acoustic measurement system. We first investigate an acoustic measurement method in the laboratory. To prove the accuracy of the acoustic measurements, we proceed with, in the laboratory, signal-pattern comparison between the acoustic measurement method and the pulse current method. This study creates polar-coordinate and discharge type identification patterns. We propose the use of the q-£p-t patterns, the polar-coordinate patterns and discharge type identification patterns, with mutual cross-reference, to identify the discharge type. Then this study applies the wavelet transform to suppress noises; a wavelet mother function most similar to the acoustic PD signals is chosen and then set the filtering threshold value for the wavelet transform. The signals' features will be extracted after the noises are eliminated. The experimental results indicate that the application of wavelet transform can effectively eliminate the field noises. Next, the features will be used to build the training database for the back-propagation neural network (BNN) to construct the discharge patterns' recognition and identification system. Finally, we apply the finished neural networks to field signal-pattern identification. The proposed acoustic measurement system is applied on line to epoxy-resin transformers, power distributors, and the like. The superior measurement results we obtained shall be able to correctly identify power equipment's PD fault types.

Superconductivity Centennial Conference 2011- EUCAS–ISEC–ICMC (18-23 Sep 2011, The Hague, The Netherlands)

The insulation lifetime of power cables is determined by several factors. One of the most important of these is the occurrence of partial discharge (PD) at the dielectric. The ability to detect and locate a PD source is limited by attenuation of the high frequency PD pulses as they propagate through the cable to the sensor. Therefore it is necessary to understand the high frequency response of such cables. The ultimate aim of this thesis is to develop an accurate frequency-dependent cable model for detecting and locating degraded insulation regions on power cables, caused by partial discharge activities. Numerical methods can calculate field distribution in the vicinity of a cavity of non-standard shape which generates PDs, and is difficult to calculate by analytical methods. The simulated results show the important influence of the shape of cavity on the electric stress within it. The cavity stress enhancement increases as the permittivity of the dielectric increases. The increase is greater for cavities with large diameter to thickness ratios. A cavity with its axis parallel to the applied field direction has a higher stress enhancement. In addition the stress distribution in the cavity is smaller for spherical cavities than for cylindrical types. The research results show that the semi-conducting layers response voltage increases as frequency increases. This indicates that the semi-conducting layers can have high sensitivity for detection of partial discharge signals and this may be a useful feature to incorporate in the design of cables and in the application of cable models. By using ATPDraw, FEM and EMTP-RV techniques, three different types of cable models are developed. The simulated results give a good agreement with the measured results on the single and three phase power cable. The developed cable model can use for reconstruction of PD source signal by using the receiving signal captured at the cable ends. It is important to use the true pulse shape because it is characteristic of the PD type and location. An investigation into the possibility of detecting different PD patterns and signals when conducting PD tests using different sensor bandwidths is also presented in this thesis. The occurrence of discharge activity was created by an artificial defect manufactured in the single core cable insulation. The artificial defect generated internal discharge and was used to investigate the PD signal propagation on cross-linked polyethylene (XLPE) cable. Capacitance coupled external sensors have been applied for the PD detection measurements and the results show that these external sensors have a number of advantages compared to high frequency current transformer (HF-CT) sensors for the detection of PD pulses. In addition, development of a method to detect cross-coupling of PD signals between phases of a three core cable and location of the PD source on the three phase cable. In order to visualize recorded data gained by PD measurement of three phase cable under test, the 3PARD diagram was used. Each data pulse is assigned to a single dot in the (scatter plot) diagram. The measured results show that the 3PARD diagram allowed the user to identify the fault between phases with PD location. The model used for reconstruction which includes the effect of semicon material in the losses provides accurate reproduction of the propagation characteristics of high frequency PD pulses and the thesis work had used such a model to reconstruct PD waveforms of site PDs from the measured signal for the first time. The use of the original waveform is important for PD identification and location in the practical situation.

Mineral oil has been widely used in liquid insulation of power transformers. However, it is poorly biodegradable and could cause serious contamination to the environment if a spill occurs. With increasingly strict environmental rules and regulations, there is considerable interest from the Utilities to apply esters in power transformers as substitutions to mineral oil. In order to use esters in large power transformers, their dielectric properties should be thoroughly investigated. This PhD thesis covers the experimental studies on the dielectric properties of a type of synthetic ester (Midel 7131) and a type of natural ester (FR3) in both uniform and divergent electric fields, using a mineral oil (Gemini X) as the benchmark. The dielectric properties of transformer liquids in uniform fields were investigated using breakdown voltage tests. The breakdown voltages of esters should be at least similar to that of mineral oil to allow a replacement in transformers. To obtain a fair comparison, the AC breakdown voltages of well-processed transformer liquids were tested, and their distributions were statistically analyzed. Since the breakdowns of transformer liquids in uniform fields are caused by the weakest-links, conditions representative of in-service transformer liquids were also considered by testing the effects of extraneous factors, such as particles, water and electrode area. The divergent fields were produced by sharp needle electrodes with tip curvatures of a few micrometers. The dielectric properties of transformer liquids in such fields were studied using several methods. A traditional PD detector was used to study the partial discharge characteristics of insulating liquids, such as the inception voltages and the repetition rates. A high speed camera was utilized to identify the streamer generation, propagation and breakdown phenomena. An oscilloscope was used to investigate the current signals associated with these phenomena. The breakdown voltages of transformer liquids were also determined in the divergent field at various gaps. Furthermore, the fault gases in transformer liquids under partial discharge faults were determined and analyzed. The following findings and conclusions can be made from the research in this thesis: * The AC dielectric strengths of esters in uniform fields are similar to that of mineral oil when they are in a well-processed condition. When practical liquid conditions are considered, the AC dielectric strengths of esters are higher than that of mineral oil. * The partial discharge behaviours at overstressed voltages can be used to differentiate various transformer liquids. Esters are relatively inferior to mineral oil in terms of higher discharge amplitude, higher discharge repetition rate and more negative partial discharges.* Mineral oil possesses a higher ability to suppress the propagation of negative streamers. Thus, the AC dielectric strength of mineral oil in the divergent field is relatively higher than those of esters. * Esters generate the same types of fault gases due to electrical discharge as mineral oil, but in relatively larger amounts.

Yes / A microstrip line-fed wideband printed antenna is presented for radio detection of partial discharge (PD). The novel simple structure antenna has compact size of 24 × 20 × 0.16 cm3 (0.28λs × 0.23 λs × 0.002 λs) and suitable for radiometric PD wireless sensor nodes, where λs is the wavelength of the lowest frequency of the band (i.e., 0.35 GHz). The stepped and beveled radiation patch is used in combination with a slotted ground plane to achieve a wide fractional bandwidth of 119% (0.35 to 1.38 12 GHz) for a return loss better than 10 dB. Good radiation pattern characteristics are obtained across the frequency band of interest. The match between simulated and experimental results suggests that the design is sound and robust.

Partial discharge testing is a nondestructive method of identifying possible premature insulation breakdown. Magnet wires are used in various electrical equipment like injection coils, solenoids, small transformers and motors. Winding wires used in high voltage systems are subjected to several stresses during operation, which considerably lowers their lifetime. A prolonged combined effect of these stresses causes aging of the wires, inception of partial discharge activity, and degradation of the organic material in the enamel coating and varnish used by the manufacturer. The tests were conducted for AWG 30 and AWG 31 twisted magnet wire samples. The samples are aged under accelerated conditions of high frequency, temperature, and pulsed voltages. As well as conventional sinusoidal voltages. This thesis is focused on the results of voltage, frequency, and temperature stresses on the magnet wires. The partial discharge inception voltage (PDIV) and breakdown voltages are measured for different twisted wire samples. A software tool is utilized to study these partial discharge patterns. Evaluation of the parameters charge intensity, pulse count, and pulse phase position are presented in 2- and 3-dimensional plots.

During the fabrication of solid electrical insulation, small cavities known as micro voids may form in the material. As electrical stress increases in this micro void, the breakdown probability also increases. This type of electrical breakdown is commonly known as partial discharge. Magnitudes of partial discharge currents are typically small but enough to cause degradation of the electrical insulation. To study degradation for fast-rise time voltage square pulse train, partial discharge measurement is needed. In current studies, partial discharge pulse widths have been measured in the range of nanoseconds. The best approach for measurement at ultra wide band frequencies is a bridge type measurement system, to reduce external noise and improve sensitivity to PD currents. The bridge configuration can be used with samples instead of one sample and one coupling capacitor. Identically created samples will have a close match for impedance and frequency response. This type of bridge also helps to reduce other sources of measured current such as the high displacement currents due to fast rise time square pulse voltage on the samples. Further improvement includes simultaneous measurements using a “linked” bridge configuration, where bridges share a common sample. A directly connected measurement current shunt should be used for high sensitivity with a uniform ultra wide band frequency response. Post-measurement digital signal processing (DSP) algorithms will perform the task of pulse discrimination and time delay from the pulse front. This research presents a method to improve the measurement of partial discharge when applied voltage is non-sinusoidal, with high frequency components. The improvements are apparent when square pulse voltage rise times are less than 50 ns. Ultra wide band measurements of physical samples will be performed for short time duration with a digital storage oscilloscope. A DSP algorithm is used to filter residual noise from the partial discharge current. The presented measurement technique for samples for this study is an original approach. Sample results demonstrate the effectiveness of the technique.

For the purpose of developing a new highly-sensitive and reliable fiber optical acoustic sensor capable of real-time on-line detection of acoustic emissions in power transformers, this dissertation presents the comprehensive research work on the theory, modeling, design, instrumentation, noise analysis, and performance evaluation of a diaphragm-based optical fiber acoustic (DOFIA) sensor system. The optical interference theory and the diaphragm dynamic vibration analysis form the two foundation stones of the diaphragm-based optical fiber interferomtric acoustic (DOFIA) sensor. Combining these two principles, the pressure sensitivity and frequency response of the acoustic sensor system is analyzed quantitatively, which provides guidance for the practical design for the DOFIA sensor probe and system. To meet all the technical requirements for partial discharge detection, semiconductor process technologies are applied, for the first time to our knowledge, in fabricating the micro-caved diaphragm (MCD) used for the DOFIA sensor probe. The novel controlled thermal bonding method was proposed, designed, and developed to fabricate high performance DOFIA sensor probes with excellent mechanical strength and temperature stability. In addition, the signal processing unit is designed and implemented with high gain, wide band response, and ultra low noise. A systematic noise analysis is also presented to provide a better understanding of the performance limitations of the DOFIA sensor system. Based on the system noise analysis results, optimization measures are proposed to improve the system performance. Extensive experiments, including the field testing in a power transformer, have also been conducted to systematically evaluate the performance of the instrumentation systems and the sensor probes. These results clearly demonstrated the feasibility of the developed DOFIA sensor for the detection of partial discharges inside electrical power transformers, with unique advantages of non-electrically conducting, high sensitivity, high frequency response, and immunity to the electro-magnetic interference (EMI). / Ph. D.

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