Time domain analysis of switching transient fields in high voltage substations

Switching operations of circuit breakers and disconnect switches generate transient currents propagating along the substation busbars. At the moment of switching, the busbars temporarily acts as antennae radiating transient electromagnetic fields within the substations. The radiated fields may interfere and disrupt normal operations of electronic equipment used within the substation for measurement, control and communication purposes. Hence there is the need to fully characterise the substation electromagnetic environment as early as the design stage of substation planning and operation to ensure safe operations of the electronic equipment. This paper deals with the computation of transient electromagnetic fields due to switching within a high voltage air-insulated substation (AIS) using the finite difference time domain (FDTD) metho

A model for calculating EM field in layered medium with application to biological implants

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.Modern wireless telecommunication devices (GSM Mobile system) (cellular telephones and wireless modems on laptop computers) have the potential to interfere with implantable medical devices/prostheses and cause possible malfunction. An implant of resonant dimensions within a homogeneous dielectric lossy sphere can enhance local values of SAR (the specific absorption rate). Also antenna radiation pattern and other characteristics are significantly altered by the presence of the composite dielectric entities such as the human body. Besides, the current safety limits do not take into account the possible effect of hot spots arising from metallic implants resonant at mobile phone frequencies. Although considerable attention has been given to study and measurement of scattering from a dielectric sphere, no rigorous treatment using electromagnetic theory has been given to the implanted dielectric spherical head/cylindrical body. This thesis aims to deal with the scattering of a plane electromagnetic wave from a perfectly conducting or dielectric spherical/cylindrical implant of electrically small radius (of resonant length), embedded eccentrically into a dielectric spherical head model. The method of dyadic Green's function (DGF) for spherical vector wave functions is used. Analytical expressions for the scattered fields of both cylindrical and spherical implants as well as layered spherical head and cylindrical torso models are obtained separately in different chapters. The whole structure is assumed to be uniform along the propagation direction. To further check the accuracy of the proposed solution, the numerical results from the analytical expressions computed for the problem of implanted head/body are compared with the numerical results from the Finite-Difference Time-Domain (FDTD) method using the EMU-FDTD Electromagnetic simulator. Good agreement is observed between the numerical results based on the proposed method and the FDTD numerical technique. This research presents a new approach, away from simulation work, to the study of exact computation of EM fields in biological systems. Its salient characteristics are its simplicity, the saving in memory and CPU computational time and speed.Cochlear UK Limited and EPSR

Absolute calibration of radiometric partial discharge sensors for insulation condition monitoring in electrical substations

Measurement of partial discharge (PD) is an important tool in the monitoring of insulation integrity in high voltage (HV) equipment. Partial discharge is measured traditionally using galvanic contact techniques based on IEC 60270 standard or near field coupling [1]. Freespace radiometric (FSR) detection of PD is a relatively new technique. This work advances calibration method for FSR measurements and proposer a methodology for FSR measurement of absolute PD intensity. Until now, it has been believed that absolute measurement of partial discharge intensity using radiometric method is not possible. In this thesis it is demonstrated that such measurement is possible and the first ever such absolute measurements are presented. Partial discharge sources have been specially constructed. These included a floating electrode PD emulator, an acrylic cylinder internal PD emulator and an epoxy dielectric internal PD emulator. Radiated signals are captured using a wideband biconical antenna [1]. Free-space radiometric and galvanic contact measurement techniques are compared. Discharge pulse shape and PD characteristics under high voltage DC and AC conditions are obtained. A comparison shows greater similarity between the two measurements than was expected. It is inferred that the dominant mechanism in shaping the spectrum is the band-limiting effect of the radiating structure rather than band limiting by the receiving antenna. The cumulative energies of PD pulses in both time and frequency domains are also considered [2]. The frequency spectrum is obtained by FFT analysis of time-domain pulses. The relative spectral densities in the frequency bands 50 MHz – 290 MHz, 290 MHz – 470 MHz and 470 MHz – 800 MHz are determined. The calibration of the PD sources for used in the development of Wireless Sensor Network (WSN) is presented. A method of estimating absolute PD activity level from a radiometric measurement by relating effective radiated power (ERP) to PD intensity using a PD calibration device is proposed and demonstrated. The PD sources have been simulated using CST Microwave Studio. The simulations are used to establish a relationship between radiated PD signals and PD intensity as defined by apparent charge transfer. To this end, the radiated fields predicted in the simulations are compared with measurements. There is sufficient agreement between simulations and measurements to suggest the simulations could be used to investigate the relationship between PD intensity and the field strength of radiated signals [3]

Outdoor Insulation and Gas Insulated Switchgears

This book focuses on theoretical and practical developments in the performance of high-voltage transmission line against atmospheric pollution and icing. Modifications using suitable fillers are also pinpointed to improve silicone rubber insulation materials. Very fast transient overvoltage (VFTO) mitigation techniques, along with some suggestions for reliable partial discharge measurements under DC voltage stresses inside gas-insulated switchgears, are addressed. The application of an inductor-based filter for the protective performance of surge arresters against indirect lightning strikes is also discussed

Non-conventional sensors for measuring partial discharge under DC electrical stress

Partial discharge (PD) is a micro discharge that occurs in defected regions within the insulating media. As these discharges are the main culprits that cause dielectric material aging, PD measurements have been used for assessing insulating materials, including solids, liquids, and gases for power applications. There are various methods and sensors available for measuring PD sensitive to specific characteristics and operable over a wide range of frequencies. Most PD measurement techniques provide patterns that enable PD interpretation more comfortable for users. For example, in AC applications, the phase-resolved partial discharge (PRPD) technique provides identifiable patterns for distinguishing various types of PDs. However, the establishment of meaningful patterns to multiple types of PD in DC systems requires more sensitive and accurate measurements of individual PD pulses with noise rejection functionality due to the lack of phase-resolved information. Investigating of the transient phenomena such as individual PD pulses requires well-designed circuits with sufficiently large bandwidths. Waveshapes can be easily disturbed by background noise and deformed by the frequency response of measuring circuits and data acquisition systems (DAQ). Noises are unwanted disturbances that could be suppressed by suitable filters or mathematical methods. Measurement circuits and DAQ systems consist of transmission lines, sensors, cables, connectors, DAQ hardware, and oscilloscopes. Therefore, matching the impedance of all components guarantees a reflectionree path for traveling signals and addresses most of the challenges relevant to transient measurements. In this dissertation, we proposed and designed an appropriate testbed equipped with high bandwidth transmission line and electromagnetic field sensors suitable for investigating PD under DC electrical stresses. We comprehensively used finite element analysis simulations through the COMSOL Multiphysics software to design the dimensions and evaluate the frequency response of the testbed, transmission line, and electromagnetic sensors. Furthermore, based on the new testbed, DC PD measurements were performed using conventional and non-conventional sensors. Finally, various types of DC PD were statistically classified based on the proposed testbed

Erfassung und Evaluierung von Teilentladungen in Leistungstransformatoren mit speziellen Sensoren und Diagnoseverfahren

Transformers are key elements of the power grid. Due to their importance and high initial cost, asset managers utilize monitoring and diagnostic tools to optimize their operation and extend their service life. The main objective of this thesis is to develop new methods in the field of monitoring and diagnosis of transformers in order to reduce maintenance costs and decrease the frequency of forced outages. For this purpose, two concepts are proposed. Small generator step-up transformers are essential in wind and photovoltaic parks. The first presented concept entails an online fault gas monitoring system for these transformers, specially hermetically-sealed transformers. The developed compact, maintenance-free and cost-effective monitoring system continuously tracks the level of the key leading indicators of transformer faults in the gas cushion. The second presented concept revolves around partial discharge (PD) assessment by the UHF measurement technique, which is based on capturing the electromagnetic (EM) waves emitted in case of PD in the insulation of a transformer. In this context, the complex EM system established when probes are introduced into the tank of a transformer and with PD as the excitation source is analyzed. Drawing on this foundation, a practical approach to the detection and classification of PD with the focus on the selection of the optimal frequency range for performing UHF measurements depending on the device under test is presented. The UHF measurement technique also offers the possibility of PD localization. Here, the determined arrival time (AT) of the captured signals is critical. A PD localization algorithm, based on a multi-data-set approach with a novel AT determination method, is proposed. The methods and algorithms proposed for the detection, classification and localization of PD are validated by means of practical experiments

高電圧絶縁診断のための高いノイズ耐性を有する革新的な広帯域UHFセンサの開発

Partial discharge (PD) is a pre-breakdown phenomenon that can cause the insulation deterioration of HV equipment. Thus, detection of PD signals enables us to evaluate the insulation condition and avoid catastrophic breakdown. This also brings the reliable operation of the electric power system consisting of the HV equipment. The UHF method, one of the PD detection methods on the PD-emitted EM wave in the UHF bands, has been widely used in the electric power system because this method has many advantages, such as a non-direct connection with the HV circuit, high sensitivity, quick response and so on. However, the rapid development of ICT and digital technologies could impact the PD detection by the UHF method in view of the noise interference by the communication waves, consequently reducing the UHF sensor’s sensitivity and detection ability. Thus, eliminating the communication noise is strongly required in parallel with the wideband PD detection in the UHF method. Under this background, this dissertation dealt with the development of the wideband UHF sensor with high noise resistance ability for PD detection under noise conditions. The Multiple Narrow Band (MNB) antenna, a newly proposed and developed wideband UHF sensor with high noise resistance, was designed and optimized based on the theoretical and experimental discussion. The MNB antenna satisfies the main requirements of the UHF sensor and has excellent properties; wideband detection (more than 1 GHz) and high noise resistance for the communication and ISM bands (S11 > -6 dB). The design rule was summarized as a flowchart to develop the MNB antenna with different detection and elimination frequencies depending on the purpose (Chapter 3). The discharge detection property of the MNB antenna was clarified and compared with the HA based on the PD experiments. As a result, it was proved that the MNB antenna could detect different PD signals from the different insulations and discharge types, and the signal intensity, i.e., the peak to peak amplitude of the antenna output, of the MNB antenna showed a good correlation with that of the HA (Chapter 4). The noise resistance ability of the MNB antenna was extracted using the PD in SF6 gas under the noise source experiment. The MNB antenna can detect the PD signal under noise conditions with a higher signal-noise ratio (SNR) than HA under elimination frequency noises. The PD in SF6 gas under drone noise (2.4 GHz, Wi-Fi) experiment was performed to make practical conditions. The 100 times measurement result showed that the PD detection rate under drone noise was 1% and 94% for HA and MNB antenna, respectively. The PD signal and drone noise repetition rates were investigated to understand the advantage of MNB antenna compared with HA. As a result, it was observed that the PD signal repetition was much lower than drone noise repetition for HA, while the drone noise repetition was lower than PD signal repetition for the MNB antenna because of the noise resistance ability (Chapter 5). The communication noise elimination ability of the MNB antenna was further improved by adding the low-pass filtering function without size extension. The Filtered Multiple Narrow Band (FMNB) antenna was designed with higher noise resistance than the MNB antenna. This enhancement increased the Wi-Fi noise resistance ability from 15 dBm to more than 30 dBm. Due to this noise reduction, the FMNB antenna detected the PD signal under drone noise with a noticeable higher SNR than the MNB antenna. In addition, the microstrip filter can be designed for different frequency bands to resist certain noises. Moreover, the MNB antenna can be designed for different PD signals, and different noise condition depends on the purpose by adjusting the detection and elimination frequencies. It means the MNB antenna can be used in all types of HV equipment as a UHF sensor. Also, the noise resistance ability makes the MNB antenna is suitable to cooperate with other ICT and digital technology as a wireless and smart sensor. (Chapter 6).九州工業大学博士学位論文 学位記番号: 工博甲第551号 学位授与年月日: 令和4年3月25日1 Introduction|2 Experimental Setup and Procedure|3 Design of a Novel UHF Sensor for PD Detection|4 Discharge Detection Properties of a Multiple Narrow Band Antenna|5 Noise Resistance Properties of a Multiple Narrow Band antenna|6 Enhanced Filtered Multiple Narrow Band Antenna|7 Conclusions九州工業大学令和3年

Effect of water on electrical properties of Refined, Bleached, and Deodorized Palm Oil (RBDPO) as electrical insulating material

This paper describes the properties of refined, bleached, deodorized palm oil (RBDPO) as having the potential to be used as insulating liquid. There are several important properties such as electrical breakdown, dielectric dissipation factor, specific gravity, flash point, viscosity and pour point of RBDPO that was measured and compared to commercial mineral oil which is largely in current use as insulating liquid in power transformers. Experimental results of the electrical properties revealed that the average breakdown voltage of the RBDPO sample, without the addition of water at room temperature, is 13.368 kV. The result also revealed that due to effect of water, the breakdown voltage is lower than that of commercial mineral oil (Hyrax). However, the flash point and the pour point of RBDPO is very high compared to mineral oil thus giving it advantageous possibility to be used safely as insulating liquid. The results showed that RBDPO is greatly influenced by water, causing the breakdown voltage to decrease and the dissipation factor to increase; this is attributable to the high amounts of dissolved water

Propagation of high frequency partial discharge signal in power cables

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