Detection and measurement of partial discharge (PD) phenomena combined with the separation and identification of PD sources is the way to achieve effective insulation integrity assessment. However, during measurement, PD signals are coupled with interferences (discrete spectral, pulsive, and white noises). Recovering PD signals from such interferences would improve PD source separation (thus identification), but still remains a challenging task. Several denoising methods have been proposed to suppress interferences. However, using a universal method to achieve interference removal is probably impossible, as the characteristics of the interferences are distinct. This study proposes a novel low-rank H-Matrix-based singular value decomposition (SVD) filter (H-SVD) that removes different types of interferences. Denoising is done by projecting the measured pulse in a lower dimensional signal space. To assess the effectiveness of the proposed method, H-SVD filter is first applied to simulated PD data and later on real-time PD data with the introduction of three different types of synthetic noises. The results of the evaluation metrics confirm that H-SVD has significant performance improvements compared to existing state-of-the-art PD denoising methods.

The mutual inductance between coils is one of the important parameters of a system of coils. However, because the theoretical analysis of mutual inductances is difficult, a precise analysis of racetrack-shaped coils has not yet been conducted. Although a method that involves the use of the finite element method for obtaining an approximation is available, this study aims to obtain the analytical solution which enables the computation of values with very high accuracy. The authors propose a novel magnetic field analysis method for deriving the analytical solution for the magnetic field and for the self-inductance of the arbitrary-shaped coils. In this study, the analytical solution is derived for the mutual inductance using a novel analysis method and numerous experiments using coils of different shapes are carried out. The experimental results were almost identical to the theoretical results, confirming the validity of the proposed analysis method. Furthermore, the authors show that even when the specifications and arrangements of the coils are changed, computational analysis can be performed in a short time.

In electrical impedance tomography, the primary task of image reconstruction process is to solve a discrete ill-posed inverse problem. The estimated solution is commonly obtained under a regularisation framework so that the noise amplified solution, which occurs during the matrix inversion process, can be avoided. The regularisation framework aims at balancing the model-data fitness while simultaneously constraining the solution space with additional prior (commonly prescribed through the regularisation matrix and solution-norm). In this study, a relationship between two robust regularisation matrices namely Newton one-step error reconstruction and fidelity-embedded regularisation is explicitly highlighted in both spatial and singular value decomposition domains. A hybrid regularisation matrix which encompasses the two prior knowledge, non-uniform sensitivity distribution and array response correlation, is then proposed as an alternative prior. Experimental results along with several evaluated performance parameters highlight the ability of the proposed prior to achieve a well-balanced and robust performance.

In order to improve the dependability and security of transformer differential protection, the energisation conditions should be effectively distinguished from internal fault conditions. In this study, a new method with a low computational burden and high robustness against measurement noises is proposed for discriminating between the transformer inrush current and internal faults. In the proposed method, sine waves are fitted to the sample points of the normalised differential currents for different phases using recursive extended least-squares (RELS) algorithm in an online manner. The extended kernel enables the algorithm to effectively estimate the dynamics of the measurement noises. Three residual signals which are defined as the differences between the normalised differential currents and the fitted sine wave signals are considered as the decision criteria. Based on the selected criteria, the energisation condition is identified in about a half-cycle of power system frequency. A large number of simulation and experimental test cases are used to demonstrate the effectiveness of the proposed method. The results show that the proposed algorithm achieves an accuracy of ∼98% and has a relatively low operating time, while the computational burden for the embedded implementation is favorably low.

The proliferation of massive non-linear loads, large motor loadings, renewable energy generation systems etc. creates many problems to the electrical power systems, such as harmonics, oscillations, which lead to the unnecessary economic cost. It is critical to implement satisfying continuous monitoring of the power quality (PQ) over the power systems. Previous works have proposed many PQ detection methodologies with promising accuracy and performance, but as more and more renewable energy is integrated into the power systems, a new challenge of frequency deviation has been raised. The accuracy of the conventional methodologies will be degraded under the frequency deviated environment, then the authors proposed in this study an instantaneous PQ indices (PQIs) detection methodology based on adaptive data resampling technique to improve the accuracy of PQIs detection within a frequency deviated environment. Finally, they validated the effectiveness of the proposal, which obtains better accuracy and performance under a frequency deviated environment with less readjustment, through simulation and measurement. Furthermore, the proposed methodology satisfies the definitions and recommendations of the IEEE Std 1459 and IEEE Std 519.

Monitoring partial discharge (PD) activities inside electrical substation using few ultra high frequency (UHF) sensors mounted around the periphery of the substation could be a useful and economical tool. In this work, an experimental study has been conducted in the laboratory and in substations to investigate the localisation ability of the UHF PD detection system. For localisation, one needs to calculate time difference of arrivals (TDOAs) between the UHF waveforms obtained from the multiple sensors mounted around the substation. After that these TDOAs are used for localisation of the PD sources. In this paper a novel joint entropy (JE)-based TDOAs calculation method is proposed and also compared with the existing techniques. For localisation of PD sources, a modified direction of arrival (DOA)-based method for the near field is proposed, as found to be suitable when the sensors are mounted around the substation. Obtained results in TDOA estimation using the proposed JE method is in line with other technique; however, it was found to provide consistent results and is useful in an extreme situation. Modified DOA-based localisation algorithm found to provide improved performance due to the elimination of approximation as required in previously reported far-field-based localisation system for the air-insulated substations.

This study presents experimental testing of samples from two uni-directional carbon fibre composite (CFC) manufactured panels to determine the factors that influence the electrical properties of CFCs and to also determine the temperature coefficient of resistance. The CFC panels were manufactured by two different techniques to compare the impact of the manufacturing process on the electrical properties. Various electrical conductivity measurement methods were evaluated to determine the most consistent and accurate technique. The influence of sample geometry on the measured electrical conductivity was also investigated. Thermal imaging was used to image resistive losses and illustrate the current paths through the fibres within the CFC test samples. Finally, the effects of increasing temperature on the CFC samples are presented, illustrating that CFCs have a negative temperature coefficient.

This study focuses on the effect of accelerated thermal and electrical ageing on the dielectric properties of two vegetable oils (olive and soya oils) and a mineral oil which are thermally aged in sealed bottles at 110°C for durations up to 720 h and electrically submitted up to 2500 low-energy discharges under DC voltage. The investigated properties are the real part of the complex relative permittivity, dissipation factor and conductivity in a large frequency range (20 Hz–10 MHz) using frequency domain spectroscopy. This study also discusses the discharge currents generated in these oils and their corresponding fast Fourier transforms followed by power spectral density analysis. The electrical discharges are applied to the oil samples in a plane–plane configuration electrodes and 2.5 mm gap under DC voltage. The frequency-domain investigated descriptors are: maximum peak, mean value, rms value, peak factor, variance and standard deviation of magnitude spectrum, in addition to the resonance and bandwidth frequency of discharge currents. Furthermore, the spectrograms of these discharge currents are presented, illustrating how the frequency composition and the intensity of each frequency change with time.

In this study, the authors propose a hybrid computational model for the reliable simulation of electromagnetic-wave phenomena emerging in graphene structures, which incorporates two variations of an unconditionally-stable finite-difference time-domain algorithm. The new approach features: (a) a dispersive model that relies on the auxiliary differential equation technique and takes into consideration the graphene's surface conductivity, and (b) a modified discretisation algorithm implementing error-optimised spatial approximations, which is appropriate for graphene-free mesh nodes. The resulting update equations are devoid of time-step restrictions, thus formulating a suitable and useful computational framework for the investigation of contemporary graphene-based problems, which commonly call for densely-sampled grids and/or prolonged simulations. After theoretically assessing the fundamental features of the proposed algorithm, the propagation properties of surface waves on graphene are extracted numerically and compared with analytical estimations, so that computational validation is provided. In addition, a realistic surface-wave coupler configuration is simulated and its main characteristics are extracted accurately, exemplifying the method's robustness on more complex setups.

In this study, an attempt was made to evaluate very small water content (below 0.05% in volume) in the ‘20–30’ mineral oil using both electrochemical impedance spectroscopy (EIS) and dielectric constant calculation based on the measured impedance values. Measurements were performed with the use of laboratory EIS system and were designed to verify experimentally that it is possible to develop an IS-based measuring system that would be able to work in situ, be relatively cheap and fast alternative compared with systems widely used nowadays. Known amounts of water were added to the oil samples, which were not treated by any other factors, e.g. solvents, contrary to other researchers’ findings. Impedance of the samples was measured in the frequency range of 0.01–100 Hz. Equivalent circuit was proposed and used to reveal how electric properties of the oil change due to water addition. Both calibration curves based on the value of an equivalent circuit element and dielectric constant showed linear correlation of their values with the water amount. Thus, experimental results confirm that with some discussed limitations, EIS and dielectric constant calculation can be used to estimate even such small quantities of water in mineral oil.

The measurement of the echo time delays and phases of the surface acoustic wave (SAW) tag by the corresponding reader should be more accurate with the increase of the coding capacity, and the design of the high-speed data acquisition module (DAM) is the key factor for the improvement of the reader performance. Aiming at the large capacity SAW tag, which combines pulse time delay and phase coding, the radio-frequency (RF) band-pass sampling and digital quadrature demodulation scheme is utilised for the reader design. The design of the high-speed DAM includes the analysis, calculation and determination of the sampling frequency, the sampling clock design with cascaded phase-locked loop structure and the driver circuit design etc. Finally, the SAW RFID system is established, and the time delays and phases of the tag reflectors are tested. The measurement results show the effectiveness of the high-speed DAM of the reader.

The dissipation factor (i.e. tanδ) and insulation capacitance (IC) measurements are conventional monitoring methods for assessing the aging level of insulation systems. These quantities provide an invaluable indication of the dielectric losses within the insulating materials. However, how these values are affected by the aging processes due to thermal stresses have until today never been investigated fully. Thus, this study exhibits the influence of thermal aging on tanδ and IC of windings for electrical machines (EMs). The work is performed for class 200, round enamelled magnet wire specimens. The study aims at improving the design process of EMs for short duty cycle applications; hence, its outcome might be included at the design stage for enhancing reliability and lifetime. Random wound coils are chosen in the performed study, because they are the most common winding arrangement for low-voltage EMs, which are employed in a wide range of applications (e.g. from home appliances to aerospace motors). Based on the collected data, considerations regarding the impact of relative humidity on both the dissipation factor and IC are presented. Finally, the correlation between the partial discharge inception voltage and the diagnostic measurements is experimentally verified.

The value of paper conductivity provides quantitative evaluation of transformer insulation health. However, proper identification of paper conductivity requires complete profile of polarisation–depolarisation current (PDC). PDC measurement being a time-consuming offline process generally takes several hours to complete. Furthermore, magnitude of PDC becomes very low at larger value of time, which makes it sensitive to changes in environmental conditions and field noise. Hence, accuracy of paper conductivity identification can be ensured by conducting multiple measurements. This in-turn prolongs shutdown time of equipment and become less advantageous to utilities. Here, a method is proposed which is capable of estimating paper conductivity using PDC data recorded for only 800 s. The proposed technique is tested on data collected from several real-life in-service transformers. In order to illustrate the accuracy of the proposed technique, paper conductivities (calculated from short duration PDC) were compared with those computed using PDC measured for 10,000 s.

This study focuses on the analysis of the non-linear behaviour of magnetisation inductance during a transformer frequency response test in open-circuit configuration. For this work, frequency response measurements were made on a three-phase transformer and a transformer model was used to simulate the response curve and to estimate the magnetisation inductance curve as a function of frequency. It was observed that the magnetisation inductance increases with increase of the test voltage. Also observed was an initial decrease at a very low frequency of the magnetisation inductance, caused by the displacement of the work point by the first elbow of the magnetisation curve. Additionally, the magnetisation inductance acquires a linear behaviour when the input small signal is sufficiently low. Finally, it was observed that the remanent magnetisation level in the core reduces the value of the magnetisation inductance.

An attempt is made to understand the effect of thermal ageing on natural ester oil impregnated pressboard material. Surface discharge inception voltage of natural ester oil impregnated pressboard material is reduced drastically under AC, harmonic AC voltages and with DC voltages with high ripple content. Phase-resolved partial discharge studies show an increase in the number of discharges for natural ester oil impregnated pressboard (OIP) aged at higher temperature, under AC and harmonic AC voltages with different total harmonic distortions. Surface potential measurement study complies with results obtained by surface discharge inception voltage. It indicates the presence of high-density charge traps evolved as a function of ageing time and temperature. Laser-induced breakdown spectroscopy analysis is carried out to understand the characteristic variation with thermally aged OIP material. A decrease in plasma temperature is observed for sample aged at higher temperature. Differential scanning calorimetric studies indicate reduction in peak temperature with thermally aged specimens. Ageing observed to affect mechanical strength of the cellulosic material, as indicated by tensile test. Dielectric response spectroscopy study indicates that with an increase in thermal ageing, the permittivity, tan(δ), DC conductivity and relaxation time have increased. A reduction in activation energy of the aged material is observed, which indicates the status of the OIP material.

Functionally graded materials (FGMs) are expected to be useful in the electrical industry, especially for achieving an efficient electric field stress control. However, as these are relatively new materials, there are a number of unanswered research questions around them. A novel method to obtain an optimised spatial distribution of permittivity or electrical conductivity of FGMs used as insulators for electric stress control is presented in this study. The problem is envisaged as the realisation of two objectives viz., (i) to minimise maximum stress and (ii) to achieve a more uniform electric stress distribution. Permittivity or electrical conductivity of FGMs is considered as variables of optimisation and based on this a unique stochastically-controlled multi-objective particle swarm optimisation technique is developed. The spatial optimisation of the properties has been carried out in 2-dimensions, which, unlike previous studies is more realistic. After validation with results available in the literature, it has been found that the electric field distribution in a cable termination is greatly improved by using stress control tube (SCT) having optimised permittivity gradation, which further improves by using SCT of optimised conductivity gradation.

Corona discharges on high-voltage transmission lines cause the problems of energy waste and electromagnetic interferences. Previous studies on negative DC corona discharges are mainly on the effects of various environmental factors on the corona inception voltage, Trichel pulse frequency etc. However, few researches are focused on Trichel pulses-pulseless glow mode transition, and their transition mechanisms are still lacking. In this paper, a vacuum vessel is built up in which the humidity and pressure can be accurately controlled to simulate corona discharges under different climatic conditions. The pressure range is 50–101 kPa and the relative humidity (RH) is 10–90%. A needle-plane electrode with different radius of curvature is used to produce negative corona discharges. It is found that the transition voltage increases with the RH and air pressure. A higher humidity results in the formation of hydrated ions with lower ion mobility and the increase in the gas temperature around the needle electrode. The experimental results demonstrate that the effect of reduced ion mobility is greater than the gas temperature rise. Furthermore, an interim process, in which the Trichel pulses and the pulseless glow exist simultaneously, appears during the discharge modes transition for the needle electrode with a larger radius of curvature.

The ampacity of a material is defined as its maximum electrical current capacity. In an overhead transmission line, it is related to the maximum sag that does not compromise its safe operation. As the sag depends on the conductor's temperature, with this parameter it is possible to estimate its ampacity, increasing the efficiency of the electrical system via real-time energy dispatch management. With the Brillouin backscattering phenomena, the distributed temperature measurement in the fibre inside the optical ground wire (OPGW) or optical phase conductor is possible. However, it is common for overhead power transmission lines to be composed by more than one optical fibre. A calibration algorithm was developed to enable the application of a distributed temperature sensing equipment to monitor the temperature of an OPGW with more than two different fibres. The 230 kV Ibiporã-Londrina Transmission Line was used as a case study, being approximately 20 km long and composed of three different fibres spliced in series along with its extension. Through comparison with calibrated sensors, the technique and algorithm were validated. It was possible to characterize the system and perform the temperature monitoring with a spatial resolution of 1 m and amplitude error of less than 6°C.