Simulations of transients are an obliged step for the insulation coordination of cable lines. Although a frequency-dependent power cable line model better takes into account real conditions since resistive and inductive parameters vary with frequency, a distributed parameter with lumped losses power cable models can be useful to simplify and give higher flexibility to the insulation coordination study. In such a case, a dilemma rises up when a frequency shall be selected for the determination of the cable parameters. In this study, a comparative analysis between the two models has been performed considering a typical configuration layout, i.e. an overvoltage coming from an overhead line and stressing the cable line terminated on a transformer. The overvoltages stressing both cable insulation wall and cable thermoplastic jacket are evaluated using the same system configuration with both frequency-dependent and distributed parameter with lumped losses power cable line models, in the latter case sweeping different frequency values for the determination of the cable parameters. A frequency range that minimise the differences between the overvoltage estimation using the two models is suggested.

Ambience during thermal ageing of ester liquid has a high impact on corona inception voltage (CIV) under AC and DC voltages. The CIV is high with ester liquid thermally aged in nitrogen ambience and is the least with air aged. CIV is high under negative DC voltage followed by positive DC and AC voltages. CIV reduces with increase in harmonic voltage and the reduction is predominant with harmonic voltages with higher total harmonic distortions. Corona activity radiates ultra-high-frequency (UHF) signal with its dominant frequency at 1 GHz. Phase-resolved partial discharge analysis carried out using UHF signal indicates that corona discharges occur at the peak of the voltage waveform. The magnitude of discharges is less in thermally aged liquid but the number of discharges increases. Fourier transform infrared spectroscopy analysis indicates the formation of carbonaceous particles with thermal ageing. Thermal ageing of ester liquid causes to form new chromophoric moieties causing red shift of absorption band in ultraviolet–visible spectra. Fluorescence spectroscopy traces the characteristic variations due to thermal ageing, which can be adopted for condition monitoring.

Umbrella-type structure, the material of insulators and surface positions influence the wetting process of polluted insulator surfaces tremendously. Using a custom-made phase-angle-difference (PAD) detection device, the PAD of the different positions of different polluted insulators is measured, and the PAD curves and partial pollution surface electrical conductivity γ ₂₀ varying with time are plotted. The results show that there is a certain difference in the wetting characteristics of different positions. The saturated wetting time of different positions of LXAY-70 insulators is 12.5–15 min; that for LXHY5-70 insulators is 12–25 min; that for XWP2-70 insulators is 8–14 min; and that for FXBW-10/70 insulators is 18–26.5 min. The umbrella-type structure is shown to have a direct effect on the wetting process of polluted insulators. The complicated umbrella-type structure, especially the existence of umbrella ribs, causes an uneven degree of wetting between the upper and lower surfaces, and leads to the increase of the difference in wetting state between the different positions. Furthermore, there is a certain difference in the wetting speed of insulators comprised of different materials. Hydrophilic porcelain and glass insulators are more easily wetted than composite insulators, even though the hydrophobicity class of composite insulators is 7.

In this study, a method is presented for implanting controlled switching taking into account simultaneously the exact values of trapped charge and pre-strike voltage when Circuit Breaker (CB) contacts are approaching each other. This approach is implemented in the EMTP/ATPDraw environment using Transient Analysis of Control Systems routine. By presenting a new approach, the proposed method determines the optimum closing point for CB contacts without imposing any limitation on line-side fluctuations. In order to perform statistical switching, the presented method models the uncertainties of controller circuit, the mechanical circuit of CB, and trapped charge voltage. Unlike the conventional approaches in which the statistical parameters were selected by default, in the presented statistical switching these parameters are estimated based on CB parameters. Furthermore, the presented method is used to calculate the insulation risk caused by controlled switching overvoltage. Simulation results show the accurate and efficient performance of the proposed controlled switching approach in significantly decreasing the risk and the maximum overvoltage created along the line, compared to the conventional CB. So that, insulation risk has decreased to zero and the maximum overvoltage created along the line has decreased to 46%.

Partial discharge (PD)-based diagnosis is extensively employed in condition assessment of electrical equipment. In the case of multiple PD sources, discrimination of mixed signals is significant for reliable PD interpretation. To improve the separation performance of three or more PD sources, a multi-step discrimination method is proposed. The cumulative energy functions are exploited to characterise wave shapes of PD signals, and width and sharpness features are extracted and classified to separate mixed patterns. With the objective of maximising a novel evaluation parameter of separation capability, the oblique line and length of structure element are optimised in the feature extraction stage. For the multi-step discrimination method, the feature extraction and clustering procedures are repeatedly applied to the whole dataset, sub-classes, sub-sub-classes etc., until no more clusters are generated. To evaluate the separation performance of the proposed algorithm, a mathematical model for PD pulse is proposed, which is the multiplication of Heidler enveloping function and an oscillating function whose frequency spectrum confirms Gaussian distribution. In the end, the multi-step discrimination method is tested with PD current pulses and ultra-high-frequency signals of three artificial defects in transformer and gas-insulated system, and the results prove the effectiveness of the proposed algorithm.

The development of high-voltage direct current circuit breaker (HVDCCB) has progressed rapidly. One of the major reasons for the failure of a circuit breaker is the metallic particle contamination. An enhanced mathematical model is proposed to simulate the motion dynamics of a spherical copper particle in the break of sulphur hexafluoride (SF₆) HVDCCB. By linking COMSOL and MATLAB, the interaction between particle motion and electric field distribution and plastic random movement in the collision process is considered. The effects of voltage magnitude and particle size on the particle motion are discussed. By means of statistical methods, the obtained findings show that, as voltage magnitude decreases or particle size increases, the particle–anode collision frequency decreases, and the particle–anode collision region increases in the x -direction but decreases in the y -direction. The accuracy of the proposed model is verified by experimental observations via high-speed video camera.

In order to study the stress detection method on long-distance oil and gas pipeline, the distribution characteristics of the surface remanence signals in the stress concentration regions must be known. They were studied by using the magnetic domain model in the non-magnetic saturation state. The finite element method was used herein with the aim to analyse the static and mechanical characteristics of a ferromagnetic specimen. The variation law of remanence signal in stress concentration regions was simulated. The results show that a residue signal in the stress concentration region exists. In addition, a one-to-one correspondence in the non-magnetic saturation environment is evident. In the case of magnetic saturation, the remanence signal of the stress concentration region is covered and the signal cannot be recognised.

Corona inception voltage (CIV) due to water droplet sitting over the surface of epoxy nanocomposite material depends on supply voltage frequency, the conductivity of water droplet and the contact angle of the test specimens. The contact angle of the specimen and CIV due to water droplet has a direct correlation. It is realised that the CIV is high under negative DC and the least under AC voltages. Surface charge accumulation studies indicate that the accumulated charge and its decay time constant reduces in the damage-caused zone due to corona activity. The ultra-high frequency (UHF) signal generated due to water droplet-initiated corona activity has frequency content in the range of 0.8–1 GHz. The localisation of incipient discharges is demonstrated by using the non-iterative technique and the cross recurrence plot (CRP) technique is used to estimate the time difference of arrival (TDOA) of UHF signals generated due to water droplet-initiated discharge. Laser-induced breakdown spectroscopy (LIBS) depicts the elemental composition and reveals the difference in plasma temperature and threshold fluence between all the test specimens. In short, the performance of ion trapping particle filled epoxy nanocomposite performance is found to be best followed by titania filled epoxy nanocomposite and the base epoxy resin.

Pressboard is the main building material of a power transformer winding structure. Through-thickness compression behaviour of pressboard has a direct influence on the transformer winding clamping pressure. Being a cellulose based polymeric material with a porous structure, the through-thickness compression behaviour of pressboard is highly non-linear. Also, the mechanical properties of pressboard are moisture, temperature, and ageing status dependent. However, information on the exact behaviour of the through-thickness compressibility of pressboard under typical operating conditions of a power transformer has not been well documented in the literature. This has posed difficulties in developing reliable techniques to detect loose clamping conditions in power transformer windings. This study presents the results of laboratory measurements on the effect of moisture, temperature and ageing on the through-thickness compression behaviour of oil-impregnated pressboard. From the measured results, it was found that the compressibility of pressboard could increase with increasing temperature, ageing and moisture ingress. It was also found that moisture and ageing tend to exaggerate the thermal softening of pressboard. Subsequent finite element analysis confirmed that transformer clamping pressure has significantly altered with the pressboard condition.

In this study, two types of samples, i.e. a disc-shaped cavity embedded within solid dielectrics and the other on an electrode surface, were used to investigate partial discharge (PD) behaviours at DC voltage. It has been found that the discharge frequency of the sample with one metallic surface was much larger under a positive voltage than under a negative one, whereas it was not sensitive to voltage polarities for the sample with two insulating surfaces. In order to explain experimental results, a simulation model based on continuity equations was established to obtain PD sequences at DC voltage, in which surface charge decay and discharge time lag were taken into account. Simulation results showed that positive surface charges dissipated faster than the negative, and the discharge time lag kept almost unchanged at the same voltage amplitude. The discharge time interval consisted of field recovery time and discharge time lag, and the former was determined by surface charge decay rate. In terms of these, the effect of surface charge decay on discharge frequency was discussed. The research is helpful to clarify PD mechanism at DC voltage.

Electric field distribution depends on the geometry of internal parts of a transformer, which may experience as uniform, quasi-uniform and non-uniform fields. In the vicinity of these fields, particles present in transformer oil undergo various field stresses, especially in case of conducting particles. Any variation in local field stress causes relative change in partial discharge (PD) characteristics. Mineral oil in transformer may contain particles having different sizes, shapes and numbers. The irregular shape of particles can cause serious effects than smooth spherical particle. This study investigates the effect of irregular-shaped copper (Cu) particles in transformer oil PD characteristics under varying electric field. Furthermore, to study the effect of particle size, large-, medium- and fine-sized irregular-shaped Cu particles were used. Phase-resolved PD (PRPD) pattern, PD characteristics such as magnitude, number, discharge power, rise and duration time were analysed. The observed PRPD patterns were comparatively similar regardless of applied electric fields and particle size. The experiential PD characteristics are observed to depend on electrode configuration, particle size and applied voltages. The measured discharge power indicated that fine-sized particles required minimum applied voltage to dissipate sufficient discharge power in oil compared with medium- and large-sized particles.

The nuisance tripping of distance relay during power swing has been identified as one of the major causes for power system blackouts. Avoiding the risk of maloperation of distance relay and hence improving resilience during power swing, demands the development of a scheme capable of generating blocking signal during power swing and detecting a fault occurring with power swing from the measurement of current signal. In this regard, a protection scheme based on the hybrid framework of an ensemble of the tree, least square, and Adaline algorithm to perform the task of fault detection/classification, zone identification, location estimation, and power swing detection is proposed. The proposed scheme is based on the online estimation of DC offset information along with the fundamental component of the current signal. The performance of the proposed scheme has been analysed for diverse power swing and fault scenarios for two-machine system and Western System Coordinating Council 9-bus system. The simulation results reflect the effectiveness of the developed scheme in terms of sensitivity and reliability. Furthermore, to authenticate the effectiveness of the proposed scheme for practical scenarios, implementation, and validation of the proposed scheme has been carried out in a real-time environment using the OPAL-RT digital simulator.

The impulse voltage wave invading a transformer is not the double exponential impulse wave (DEIW) specified by the IEC but a non-standard waveform with oscillation attenuation characteristics, and this fact leads to doubts about the scientificity and optimality of insulation coordination. A key solution for this issue is the establishment of an equivalent relationship between the measured non-standard impulse voltage wave and the DEIW. In this study, a test platform of multiparameter impulse voltage was set up. Then, the authors obtained the 50% breakdown voltage (U _50%) of the oil-impregnated paper (OIP) for transformer under the bipolar oscillatory attenuated impulse wave (BOAIW) and the DEIW with different waveform parameters. The effects of waveform parameters on the breakdown characteristics of the OIP were finally revealed. On the basis of experimental data and several mathematical methods, the authors established the U _50% prediction mathematical models for the OIP under the BOAIW and DEIW. Based on the prediction models and the microfluctuation characteristics of measured overvoltage's waveform parameters, the authors proposed a method for evaluating the waveform equivalence. The characteristics of voltage amplitude and change rate were regarded as the two key factors for the waveform equivalence of this method.

Time-domain reflectometry (TDR) is an important technique for verifying the impedance and quality of signal paths in components, interconnects, and transmission lines. However, the success of channel condition assessment through on-site interpretation of TDR echoes hinges on the skill and experience of the operator. This study presents a TDR simulator to assist loop diagnostics on copper twisted pairs widely used in current digital subscriber line (DSL) access networks. The simulator is developed based on established frequency-dependent cable models to synthesise realistic TDR echoes arising from various cable discontinuities. Modelling of arbitrary loop configurations and faults is accomplished by cascading multiple sub-lines with different characteristics through ABCD parameters modelling. The usability of the simulator is enhanced using a graphical user interface and its accuracy has been verified experimentally using TDR echoes obtained from commercial TDR testers. Since lengthy twisted pairs are deployed in real DSL access networks, the TDR simulator can be used to perform on-site simulation and verification during DSL network maintenance and troubleshooting, as well as to gain mastery of TDR handling while circumventing the need for bulky and costly cabling test beds. The simulator can also be useful to supplement the teaching of electromagnetics and high-frequency measurement.

In addition to high-order harmonic and noise components, the fault current may contain decaying DC components which highly affect the estimated fundamental frequency phasor. This study presents a new hybrid method which aims to calculate and remove the unwanted effect of decaying DC components on fundamental phasor estimation. The main novelty of the proposed method is that the proposed method is independent of the dynamic behaviour of the current transformer (CT). In other words, unlike previously published papers, this method analytically calculates the CT time constant. To such aim, utilising frequency modulation, fault current signal is shifted. Utilising simple but comprehensive formulations based on integration and Prony-complex theory, the magnitude and time constant of decaying DC components are calculated in one cycle. After that, utilising a modified discrete Fourier transform the fundamental phasor component is calculated. Several test signals and also practical fault signals are applied to the proposed method. The simulation results illustrate the proposed method simply but comprehensively remove the unwanted effects of the decaying DC components. Also, it has quick speed as well as a high-precision response in the estimation of fundamental phasor under different simulation scenarios.

This study gives new insight into unipolar coronas with special regard to the governing parameters, of practical value, for the outer edge of the ionisation region. The shape and local radius of that interface are fully determined according to a capillary-based scheme, whose original application can be explained by a long-range collective containment effect presumably exerted by internal streamers. Properly implemented indirect experiments furnish supportive features, especially used to reconstruct interfacial anomalies. The adopted cross-sectorial approach allows traditional pre-breakdown Townsend/Meek criteria to be circumvented for the benefit of computational simplification, other than precision when strongly non-uniform fields are, as usual, involved. This represents a genuine disruptive paradigm change whose additional advantages are reflected in clearer interpretations of both Kaptsov hypothesis and maximum thrust density impressed, next to the interface, to air neutrals. Even the corona current–voltage characteristic has been called into play to sustain the capillary model with convincing arguments. These claims existence of a unifying theoretical substrate for a number of pre-breakdown interfacial phenomena, whatever the state of matter.

This study proposes an estimation procedure for thickness measurement of conductive material by investigating the skin effect phenomenon in eddy current non-destructive testing. Both three-dimensional numerical modelling using the finite volume method and experimental analysis are considered. Before starting this investigation, the material to be measured must be recognised in terms of its electrical conductivity, and hence this study presents also an electrical conductivity characterisation which is identified by using two methods; the first one is the four-probe-based technique while the second one is an eddy current-based technique.

This study comprehensively investigates power quality (PQ) identification problem and proposes the optimum combination of base wavelet and machine learning algorithm (MLA) which would yield the highest classification accuracy. Although this problem has been studied by various researchers in the recent past, the selection of appropriate base wavelet and MLA, which would give better classification accuracy, have received comparatively less attention. This study bridges this gap by investigating the classification performance of 110 wavelets and 7 well-known MLAs across various noise levels using over 3500 PQ events generated as per IEEE Standard 1159. The results of this investigation demonstrate that the choice of base wavelet does significantly affect the classification performance. Further, it was observed that a single base wavelet does not provide optimum performance across all MLAs at various noise levels. In contrast, each MLA gives the maximum accuracy with a distinct base wavelet. The robustness of MLA against noise is studied which establishes that the simple MLAs, such as decision tree and Naive-Bayes, are more robust against noise compared to the other intricate MLAs. Finally, several recommendations are drawn for the selection of base wavelet and MLA which yields the best possible accuracy.

In this study, results of polarisation and depolarisation currents measured on an insulating liquid are presented. The goal is to contribute to a better understanding of mechanisms occurring at the metal–liquid contact. Electrodes of different metallic nature are associated with the same mineral oil, the metal/liquid/metal structure is submitted to DC voltages and the transient current is recorded with a high sensitive electrometer. The current versus time feature shows that the conduction process is in good compliance with the metal work function rather than with its electric conductivity. The current versus field characteristics indicate a predominance of the ohmic behaviour for the low fields. The space charge limited conduction regime appears as soon as the electrical constraint exceeds ∼1 MV/m, which corresponds to 10% of the breakdown voltage. Schottky injection is suspected to occur from the metal into the liquid since the applied fields are uniform and non-extremely intensified.

The aim of this work is to estimate the flashover voltage of cap-and-pin insulator using a novel improvement of the pollution resistance evaluation. In cap-and-pin suspension insulators, local arcing starts around the pin where the dry bands first appear due to the evaporation of the conductive layer. In appropriate conditions, a unique dominant arc starting from the pin propagates toward the cap resulting in a complete flashover. Since the highest current density is concentrated around the arc root during propagation, the pollution layer residual resistance variation, with respect to the arc position, is non-linear. Thus an original three-dimensional model, which depicts a real insulator using COMSOL Multiphysics^TM to evaluate the residual resistance, is developed. This model takes into consideration the actual geometry of the insulator and the current restriction at the arc root for a specific arc radius. A novel mathematical model for calculating the residual resistance of this polluted insulator was elaborated for all possible arc radii. The mathematical model and the 3D simulation results are in satisfactory agreement. To evaluate the flashover voltage, a new approach based on Saddle Point Finding Method is proposed. The new approach and model have been validated using experimental results of previous studies.

Transient overvoltage represents a high magnitude voltage surge of a short duration which has to be considered when performing proper insulation coordination. Overhead line energisation is found to be a fundamental cause of transients. Both normal and faulty conditions are possible during line energisation. The latter implies the existence of an asymmetrical phase-to-ground fault and facing with two asymmetrical processes which take place concurrently. This type of energisation results in severe overvoltage, as well as in a voltage rise at the line receiving end. However, in practice, measuring devices are usually placed at the line sending end, which points to difficulties in determination of phase-voltages at the opposite end of the line. This study offers a solution for the aforementioned issue by proposing the frequency-dependent voltage transfer functions (TFs). By using the functions any voltage value at the sending line end will obtain the corresponding value at the receiving end. The functions are derived through complex mathematical modelling and by signal processing based on the empirical mode decomposition method. The TFs validity is confirmed through performed simulation in MATLAB Simulink and real-site measurements.

In this study, the bearing, as well as the electric faults of an induction motor, are diagnosed using the fuzzy-integral data-fusion method in feature level with high reliability. Time domain of various features is computed using the induction motor three-phase current and voltage measurements. Appropriate features are extracted by means of the proposed method and then classified by the fuzzy C-means algorithm. The fuzzy membership functions show the relation between a feature set and a fault to establish the mappings between the features and the given faults. Finally, different features are fused using the fuzzy-integral method to produce diagnostic results. The technique is validated experimentally on an induction motor coupled with a centrifugal pump. The capability of the proposed technique is also evaluated in the presence of disturbances and simultaneous occurrence of different faults. The results indicate an increase in the reliability in fault detection and isolation.

To study effects of pressure, humidity and aging time on the corona aging process, aging characteristics of silicone rubber composite insulators were researched by orthogonal tests. The test platform of adjustable pressure and humidity was constructed. TSC, SEM and FTIR tests were selected for aged samples. By the range analysis, it showed that the effect of pressure on trapped charges was related to humidity and aging time. There were more trapped charges when the humidity was higher, and the influence of humidity was more obvious along with the longer aging time at a low pressure. As the pressure increased, however, the effect of humidity on trapped charges was gradually reduced. But the influence of aging time was more significant, while aged degree of silicone rubber was deepened with increasing aging time. Furthermore, the aging state of materials was the collaboration effect of charged particles and nitric acid. Moreover, it could be seen from TSC, SEM and FTIR tests, sorting results of aging states were inconsistent. Results showed that the TSC test was comprehensively embodied the corona aging characteristics caused by physical and chemical defects. Therefore, results of TSC tests could be selected as an effective criterion for aging evaluations.

A clear understanding of the partial discharge (PD) phenomena occurring in cavities at various radial locations in the insulation is essential for proper monitoring of cables. This work presents a comparison of finite element method (FEM) model and capacitive model for simulation of PD in the insulation and an attempt to identify reliable parameters for monitoring the condition of cable insulation is done. A three-dimensional FEM-based model of the cable is developed in ANSYS software by including the effects of surface charge accumulation and propagation along with the phenomena of charge decay at the void surface. The induced charge concept which is correlated with changes in capacitance is employed to model a capacitive model for simulation of PD by considering the free electron availability for simulation of PDs in cables in EMTP software. The experimental validation of developed models is done by PD measurement on 11 kV cross-linked polyethylene cables. Analysis of the simulation results recommends the relevance of the capacitive model to be employed for condition assessment of cable insulation in view of its simplicity and appropriateness in the estimation of model parameters from actual cable measurements.

Analogue circuits are one of the most commonly used components in industrial equipment, but circuit failure may lead to significant causalities and even enormous financial losses. To address this issue, in this work the authors propose a new feature extraction scheme based on cross-wavelet transform (XWT) and variational Bayesian matrix factorisation (VBMF). Primarily, fault signals acquired from defect circuits are collected and processed by using XWT to obtain the joint time-frequency representation. VBMF is utilised to fetch the time-frequency information of the fault signal. A nine-dimensional feature vector is then constructed. Finally, a support vector machine optimised by a flower pollination algorithm is introduced to locate faults. Results show that the proposed approach can effectively locate the different kinds of defection while achieving a higher accuracy.