Showing papers in "Iet Science Measurement & Technology in 2014"

Photovoltaic power forecasting using statistical methods: impact of weather data

Abstract: An important issue for the growth and management of grid-connected photovoltaic (PV) systems is the possibility to forecast the power output over different horizons. In this work, statistical methods based on multiregression analysis and the Elmann artificial neural network (ANN) have been developed in order to predict power production of a 960 kWP grid-connected PV plant installed in Italy. Different combinations of the time series of produced PV power and measured meteorological variables were used as inputs of the ANN. Several statistical error measures are evaluated to estimate the accuracy of the forecasting methods. A decomposition of the standard deviation error has been carried out to identify the amplitude and phase error. The skewness and kurtosis parameters allow a detailed analysis of the distribution error.

Precision irrigation based on wireless sensor network

Abstract: The main purpose of this study is to present a complete irrigation solution for the farmer based on wireless sensor network. The challenge is to create an automated irrigation system which can reduce in the same time the water's waste and is cost effective. Different parameters are important to measure in order to calculate the efficient quantity of water needed by plants. In this study, the proposed solution is the use of low-cost sensor nodes having reduced power consumption able to realise necessary requirements. The system is composed of different types of nodes. Each node consists of a TelosB mote and adequate sensors or actuators. Soil nodes are used to detect the level of moisture and temperature in soil. Weather nodes monitor the climatic changes. Other nodes are connected to actuators which are able to control the opening of the irrigation valve if necessary.

Rolling element bearing fault diagnosis based on non-local means de-noising and empirical mode decomposition

Abstract: The presence of faults in the bearings of rotating machinery is usually observed with impulses in the vibration signals. However, the vibration signals are generally non-stationary and usually contaminated by noise because of the compounded background noise present in the measuring device and the effect of interference from other machine elements. Therefore in order to enhance monitoring condition, the vibration signal needs to be properly de-noised before analysis. In this study, a novel fault diagnosis method for rolling element bearings is proposed based on a hybrid technique of non-local means (NLM) de-noising and empirical mode decomposition (EMD). An NLM which removes the noise with minimal signal distortion is first employed to eliminate or at least reduce the background noise present in the measuring device. This de-noised signal is then decomposed into a finite number of stationary intrinsic mode functions (IMF) to extract the impulsive fault features from the effect of interferences from other machine elements. Finally, envelope analyses are performed for IMFs to allow for easier detection of such characteristic fault frequencies. The results of simulated and real bearing vibration signal analyses show that the hybrid feature extraction technique of NLM de-noising, EMD and envelope analyses successfully extract impulsive features from noise signals.

Intelligent stochastic framework to solve the reconfiguration problem from the reliability view

Abstract: This study proposes a new method to employ the distribution feeder reconfiguration as a reinforcement strategy to enhance the reliability of the distribution systems. Also, several wind power sources are considered to assess their effects on the reliability indices. In order to make the final results more dependable, a stochastic framework based on the probabilistic power flow is utilised to consider the uncertainty of forecast/measurement error of the active and reactive loads, failure rate, repair rate and output generation of wind units, concurrently. The objective functions to be investigated are (i) system average interruption frequency index, (ii) average energy not supplied, (iii) active power losses and (iv) total cost. Moreover, a new modified optimisation method based on harmony search algorithm is proposed to improve the total ability of the algorithm to explore the problem search space globally. The effectiveness and efficiency of the proposed method are examined through two radial distribution test systems.

Optimal stochastic capacitor placement problem from the reliability and cost views using firefly algorithm

Abstract: This study suggests a new stochastic framework based on point estimate method to capture the uncertainty associated with the forecast error of the active and reactive loads and failure rate and repair rate values. The objective functions to be investigated are system interruption frequency index, system average interruption duration index and average energy not supplied. Meanwhile, since the total MW cost is an attractive issue to the power utilities, the active power losses is also considered as an objective function. In order to explore the problem search space globally, a novel self-adaptive modification method based on firefly algorithm is proposed. The feasibility and effectiveness of the proposed method are examined on the IEEE 34-bus test system. According to the simulation results, the proposed framework based on capacitor placement problem can improve the reliability of the system effectively.

Wireless sensor network-based solution for environmental monitoring: water quality assessment case study

Abstract: The challenges of climate change, population growth, demographic change, urbanization and resource depletion mean that the world's great cities need to adapt to survive and thrive over the coming decades. Slashing greenhouse gas emissions to prevent catastrophic climate change, while maintaining or increasing quality of life, can be a costly and difficult process. Two factors that directly affect the life quality in the XXI century cities are the water and air quality that can be monitored using the combination of low cost sensing modules, machine to machine (M2M) and internet of things (IoT) technologies. In this context, this study presents a wireless sensor network architecture that combines low cost sensing nodes and a low cost multi-parameters sensing probe for reliable monitoring of water quality parameters of surface waters (lakes, estuaries and rivers) in urban areas. A particular attention is dedicated to the design of the conductivity, temperature and turbidity signal conditioning circuits, highlighting important issues related to linearisation, measuring dynamic range and low-cost implementation by using commercial off-the-shelf components and devices. Several issues related to the wireless sensor network implementation are included in this study, as well as several simulation and experimental results.

Dynamic voltage restorer control using bi-objective optimisation to improve power quality's indices

Abstract: Dynamic voltage restorer (DVR) is one of the custom power devices for compensating power quality indices which is used. The main function of DVR that is discussed in many studies is to compensate voltage sag at times when faults occur. For the first time, a new control structure is presented for considering the voltage sag as the main objective and voltage total harmonic distortion (THD) as the second objective of DVR controller. In this strategy, a new and powerful optimisation algorithm (known as chaotic accelerated particle swarm optimisation (CAPSO)) which is an improved version of particle swarm optimisation algorithm is used for determining the coefficients of the proportional-integral controller of DVR. These coefficients are determined in a way that voltage sag is considered as the main objective of optimisation algorithm and voltage THD is considered as its second objective. By fuzzifying the objectives, an appropriate objective function is proposed for the optimisation process. Results obtained from simulation and a comparison made between these results and those of other controllers show that the proposed strategy outperforms other strategies.

Effect of a micro dielectric barrier discharge plasma actuator on quiescent flow

Abstract: A dielectric barrier discharge plasma actuator (PA) was designed and manufactured with microscale dimensions using photolithographic process on fibre glass substrate. AC operation under sinusoidal voltage was investigated experimentally by means of electrical characterisation, smoke flow visualisations and particle image velocimetry. The performances of the micro PA were evaluated and compared with the ones of a macro PA found in this literature. The velocity induced by the micro PA was comparable with the macro PA one, but with lower applied voltage, electrical power dissipation and actuator size. This is particularly interesting for potential applications in turbomachinery.

Development of multi-band ultra-high-frequency sensor for partial discharge monitoring based on the meandering technique

Abstract: To design high-sensitivity sensors is one of the critical issues to be solved for ultra-high-frequency (UHF) partial discharge (PD) detection in substations. Commonly-used UHF sensors usually use ultra-wideband antennas for the frequency bands ranging from 300 MHz to 1.5 GHz. To avoid interference in the frequency bands, such as signals generated from mobile phones, a new multi-band UHF sensor is proposed based on the loop antenna theory and meandering technique, which reduces the sensor size, provides high sensitivity and exhibits omnidirectional performance. The sensor works in the bandwidth ranges of 480–520, 800–850 and 1100–1200 MHz, and has sensitivity of more than 10 mm. The PD detection platform was set up, three typical insulation defects, such as corona discharge, surface discharge and free metal particle discharge, were designed, and then the tests were performed to compare the performance of the multi-band sensor and broadband sensor. The results show that the multi-band sensor's bandwidth covers the main spectra of PD signals, thereby can be used for detecting most kinds of PD signals. The sensor's sensitivity is higher than that of the broadband sensor with its size occupying only 5% of the latter, meeting the requirements for detection of PD sources in substations.

Novel designs of wideband Rogowski coils for high pulsed current measurement

Abstract: This study presents design, calibration and testing of six novel flat-spiral self-integrating Rogowski coils wound in two layers These coils are two varnished-wire wound coils with air and ferrite cores (RC1 and RC2, respectively), two flat-twin wire wound coils with air and ferrite cores (RC3 and RC4, respectively) and two coaxial-cable wound coils with and without the insulating jacket and with ferrite core and RC5 and RC6, respectively All coil parameters are calculated and good agreement is found between these values and the corresponding measured ones Coil sensitivity decreases as coil-to-wire separation increases and so does mutual inductance between the coil and the current-carrying wire Linearity of all coil sensitivities are checked for different separations and waveforms, and good agreement is found Designed coils with ferrite core give better response than those with air core because the low cutoff frequency decreases On the contrary, the droop effect is slightly observed for coils wound on wooden core, where RC4 and RC6 give the best response Finally, the frequency response analysis for all coils shows that the percentage relative errors of the measured and the calculated coil sensitivities are within ±01%

Direct adaptive general type-2 fuzzy control for a class of uncertain non-linear systems

Abstract: In this study, a stable direct adaptive general type-2 fuzzy logic controller (DAG2FLC) is introduced for a class of non-linear systems. The proposed controller uses advantages of general type-2 fuzzy logic systems (GT2FLSs) in handling dynamic uncertainties to approximate unknown non-linear actions. Implementing general type-2 fuzzy systems is computationally costly; however, by using a recently introduced α-plane representation, a GT2FLS can be seen as composition of several interval type-2 fuzzy logic systems with a corresponding level of α for each. Linguistic rules are directly incorporated into the DAG2FLC controller and a H ∞ compensator is added to attenuate external disturbance and fuzzy approximation error. Also general type-2 fuzzy adaptation laws are derived using Lyapunov approach, and the stability of the closed-loop system has been proven by mathematical analysis. In order to evaluate the performance of the proposed controller, the results are compared with those obtained by direct adaptive type-1 fuzzy logic controller and a direct adaptive interval type-2 fuzzy logic controller, which are the latest researches in the problem in hand. The proposed controller is applied to a chaotic Gyro system as a case study. Simulation reveals the effectiveness of the proposed controller in presence of dynamic uncertainties and external disturbances.

Moisture and electrical discharges effect on naphthenic mineral oil properties

Abstract: The physicochemical properties of three transformer mineral oils of different levels of degradation, resulting from exposure to moisture and electrical discharges, are investigated. The first oil is new and untreated whereas the second is taken from a transformer after one year of normal operation; and the third oil is extracted from a 15-year-old operating transformer. The considered parameters are breakdown voltage, dielectric constant e r, dissipation factor (tan δ) and resistivity. Isothermal charging currents have also been measured under dc for long application times. It is shown that the physicochemical characteristics present major differences depending on the considered oil sample. An easy method enabling to determine the water solubility in oil is proposed; this method is based on the measurement of oil resistivity. To validate a part of the results obtained in laboratory, measurements on an on-load operating transformer have also been achieved. These show that the properties of oils with dissolved water in laboratory are similar to those of oils with really dissolved water inside operating transformers.

Real-time measurement of meteorological parameters for estimating low-altitude atmospheric turbulence strength (C n 2)

Abstract: The major factor that limits the performance of Free Space Optical Communication is atmospheric turbulence which fluctuates over time in accordance with the variations in local meteorological parameters. Estimating the atmospheric turbulence strength C n 2 with the measurement data becomes significant to find the data rate the system is capable of operating under different outdoor local environmental conditions. Hence, a low cost customised system for continuously measuring the local meteorological data is developed and presented in this study. A field test scintillometer setup is established for a link range of 0.5 km at an altitude of 15.25 m. Specialised sensors are interfaced to the digital architectures to acquire the real-time data corresponding to atmospheric changes. The accuracy and performance of the measurement system are tested against standard instruments and the maximum correlation coefficients of 99.92, 99.63, 99.73 and 99.88% are achieved for wind speed, temperature, relative humidity and pressure, respectively. An experimental model to estimate C n 2 using measured meteorological data is developed and the atmospheric turbulence strength is estimated. The validations of the estimated results with the scintillometer measurement are also analysed. The weather profile and corresponding C n 2 variations at our test field for different seasons in 1 year period are presented and the results are analysed.

Partial discharge behaviours within a void-dielectric system under square waveform applied voltage stress

Abstract: There are many factors, which influence the behaviours of partial discharge (PD) within a void located in a solid dielectric material. One of the important factors is the waveform of the applied voltage on the electrode of a system, such as its amplitude and frequency. In this study, a two-dimensional axial-symmetric model of a cylindrical void in polyethylene layers has been developed using finite element analysis software. The model was used to simulate PD activity in the void under square waveform applied voltage under different amplitudes of the voltage. The model has considered the effect of charge propagation along the void surface during a PD occurrence. The obtained simulation results were compared with the measurement results reported from previous literature to validate the PD model that has been developed in this work. It was found that the simulation and measurement results are within reasonable agreement with only slight disagreement. From the comparison, critical parameters from the model that affect PD behaviours under square waveform applied voltage were found to be the electron generation rate and the inception, extinction and critical charge propagation fields. These findings may increase an understanding of PD behaviours within a void in a dielectric material under square waveform applied voltage, which is important for insulation diagnosis.

Alternative formulation of the Kalman filter for correlated process and observation noise

Abstract: The Kalman filter for linear system with cross-correlated process and observation noises at the same epoch is revisited. As the process noise is correlated with the observation noise, the probability of the former conditioned on the latter should have a higher concentration than its original probability, implying that the conditional one becomes more accurate. Inspired by this, an alternative formulation of the filter is derived that the conditional process noise rather than the original one is used in the prediction step. The conditional mean and covariance are derived in two different ways, that is, through state augmentation and conditionally Gaussian distribution. It is also proved that the proposed formulation is theoretically equivalent to the two available frameworks. However, this formulation has the merit of explaining why the estimate is improved taking the cross-correlation into account, that is, the a posteriori covariance of the process noise is reduced compared to the original one implying that the process model becomes more accurate. A simple but illustrative example is simulated and the efficacy of the proposed framework is validated.

A multifunctional real-time power quality monitoring system using Stockwell transform

Abstract: The proliferation of power electronic devices in the power sector has intensified the power quality (PQ) issues. Identification of these PQ issues is one of the most important steps for their effective elimination. Hence, development of a real-time (hardware) PQ monitoring system is of great research interest. This study proposes a multifunctional real-time (hardware) PQ monitoring system using Stockwell transform ( S -transform). The proposed system is designed and implemented in LabVIEW environment, which can display the real-time fast Fourier transform plots, root-mean-square values, harmonic components, values of total harmonic distortion and three-dimensional plots of S -transform of the three-phase voltages. Main contribution of this study is the real-time detection and classification of different PQ disturbances which even a layman can understand through visual displays to ascertain the quality of supply, unlike other PQ analysers available commercially that require some technical expertise.

24-GHz Patch antenna array on cellulose-based materials for green wireless internet applications

Abstract: A 24-GHz patch array antenna with integrated feeding network has been fabricated exploiting a multi-layer cellulose-based (i.e. paper) substrate. The adopted microstrip circuitry exploits a copper adhesive laminate that is shaped by a photo-lithographic process and transferred to the hosting substrate using a sacrificial layer. The multi-layer structure is obtained by stacking and gluing two layers of photo-paper with an interposed copper ground plane. The measurements show an input reflection coefficient of about −29 dB at the centre frequency, an operating bandwidth with S 11 ⩽ −20 dB of 540 MHz and a gain of 7.4 dBi. The estimated radiation efficiency is 35%. The proposed design shows the feasibility of low-cost antenna systems for green wireless internet technology and applications up to the boundary between microwaves and millimetre-waves.

Effect of morphology on electrical treeing in low density polyethylene nanocomposites

Abstract: Influence of polymer morphology on the inception and the growth of electrical trees in unfilled low density polyethylene (LDPE) as well as LDPE filled with 1, 3 and 5% by weight nanoalumina samples stressed with 50 Hz ac voltage has been studied It is seen that there is a significant improvement in tree inception voltage with filler loading in LDPE filled with nanoparticles Tree inception voltage increased with the filler loading up to 3% by weight nanoalumina loading and showed a reduction at 5% by weight loading Change in tree growth patterns from branch to bush as well as a slower tree growth with increase in filler loading in LDPE alumina nanocomposites were observed The degree of crystallinity and change in crystalline morphology induced by the presence of alumina nanoparticles in LDPE was studied using differential scanning calorimetry (DSC) and scanning electron microscopy (SEM) DSC results show a similar melting behaviour for both unfilled LDPE and LDPE nanocomposites However, there is a reduction in the degree of crystallinity for LDPE filled with 5% by weight nanoalumina An increase in lamellae packing with increase in filler loadings and a highly disordered spherulitic structure for LDPE filled with 5% by weight nanoalumina was observed from the SEM images The slow propagation of tree growth as well as reduction in tree inception voltage with increase in filler loadings were attributed to the morphological changes observed in the LDPE nanocomposites

New self-adaptive bat-inspired algorithm for unit commitment problem

Abstract: Bat-inspired algorithm (BA) is a new evolutionary meta-heuristics algorithm inspired by a known technique of bats for finding prey This study presents a self-adaptive BA to solve the unit commitment (UC) problem The applied self-adaptive technique increases the population diversity and improves the exploration power of BA which results in better solutions and higher speed of convergence in solving the UC problem This study, also, applies simple methods to handle the minimum on-/off-time constraint and spinning reserve requirement in generation of all solutions directly and without using any penalty function The performance of the proposed method is verified by applying 10 up to 100-unit systems as well as a Taiwan power (Taipower) 38-unit system in a 24 h scheduling horizon

Calibration of triaxial MEMS vector field measurement system

Abstract: Triaxial MEMS vector field measurement system with triaxial MEMS sensors, such as accelerometer and magnetometer, is typically used to acquire navigation information in strap-down inertial navigation system (SINS). The navigation accuracy of the body is directly affected by the measurement accuracy of the measurement system. This study presents a two-step calibration algorithm for the triaxial MEMS vector field measurement system based on a detailed analysis of the measurement system error model. In the calibration procedure of this method, the first step is to calibrate the triaxial vector sensor error, including bias, scale factors and non-orthogonality using the ellipsoid fitting method; the second step is to calibrate the misalignment between the orthogonal sensor frame and the system body frame using the four-position method. Furthermore, the mathematical analysis of four-position method calibration error is done to study on the factors influencing the misalignment calibration accuracy, numerical simulation and experiments, which are performed for validating the analysis. A series of calibration experiments on a triaxial MEMS accelerometer in the measurement system show significant enhancement of the accuracy of three components measurement data in the system body frame.

Three-dimensional electrical capacitance tomography reconstruction by the Landweber iterative algorithm with fuzzy thresholding

Abstract: The image reconstruction for electrical capacitance tomography is a non-linear, underdetermined and ill-posed inverse problem. It is difficult to obtain a reconstructed image with high quality, especially in the case of three-dimensional (3D) reconstruction. An iterative image reconstruction algorithm with fuzzy thresholding is proposed in this study. The threshold value in each iteration is generated by minimising the measure of fuzziness of current reconstructed image. The algorithm proposed is tested by the noise-free and the noise-contaminated capacitance data. Extensive computer simulations demonstrate that the fuzzy thresholding can reduce low grey-level artefacts effectively. As a result, the spatial error, volume error, permittivity error and scattered artefacts of the reconstructed image are reduced obviously; not only that, the number of iterations needed to obtain a good reconstruction result is decreased greatly. The result of 3D reconstruction of a H-shape object verifies the effectiveness of the fuzzy thresholding further.

Tracking algorithm with radar and infrared sensors using a novel adaptive grid interacting multiple model

Abstract: This study presents a novel adaptive grid interacting multiple model based on modified iterated extended Kalman filter (AGIMM-MIEKF) for tracking a manoeuvreing target using radar/infrared (IR) heterogeneous sensors. This tracking algorithm is developed by aligning observation data of radar/IR sensors in time, and fusing the synthesised data before applying to AGIMM-MIEKF algorithm. Under the architecture of the proposed algorithm, the AGIMM deals with the model switching, whereas the MIEKF accounts for non-linearity in the dynamic system models. A new measurement update equation and an iterated termination criterion are derived and applied to radar/IR tracking system. The simulation results show that the presented AGIMM-MIEKF has higher tracking precision than the traditional algorithms.

Simplified skin-effect formulation for power transmission lines

Abstract: This study presents a simplified formulation to calculate the skin effect on transmission lines wires. The methodology is developed from the solution of the time-domain Maxwell's wave equations whose frequency-domain solutions are obtained using Fourier transforms. Results obtained from the proposed formulation are compared with those calculated using the conventional method based on the approach by Bessel functions. The proposed skin-effect formulation is also validated based on electromagnetic transients simulations in the time domain considering a conventional 69-kV transmission system.

Pilot study: electrical impedance based tissue classification using support vector machine classifier

Abstract: Tissue classification using computer aided diagnosis can help automated decision making to aid clinical diagnosis. Classification of breast tissue based on spectral features of impedance loci has frequently been done to classify malignant tissue with further requirement of more complex classification methodologies needed to improve the characterisation. In current study, tissue classification is done using in vivo electrical impedance data of 18 human subjects, from four quadrants of breast, palm, nail, arm, bicep and classified using algorithms involving machine learning methodologies, specifically support vector machines (SVMs) that are supervised learning models. They consist of learning algorithms based on the principal of structural risk minimisation. Two methodologies of SVM have been used in this study: with data binning and data pruning and without data binning and data pruning. Data binning and data pruning have improved the sensitivity of the SVM from 76.76 to 89.23%, but the specificity has decreased from 76.23 to 74.15%. This is a pilot study towards testing the reliability of the developed electrical impedance measuring system and developing a data mining-based decision making system into an electrical impedance spectroscopy system, to help users (physicians) with tissue classification leading to reliable objective decision making.

Non-quadratic exponential stabilisation of non-linear hyperbolic partial differential equation systems

Abstract: In this study, a new systematic approach is proposed to design the fuzzy controller for a class of Takagi–Sugeno fuzzy-partial differential equation (TS fuzzy-PDE) systems which describe the non-linear distributed parameter system formulated by first-order semi-linear hyperbolic PDEs. In this study, non-quadratic Lyapunov function is utilised and some slack matrices are introduced to derive stability conditions in terms of linear matrix inequalities (LMIs). The proposed approach has three main features. First, stability conditions are not derived in the form of spatial differential LMI. Second, conservativeness of LMI conditions is reduced. Third, there is no restriction on the form of semi-linear hyperbolic PDE systems and therefore more semi-linear systems classes can be stabilised. Also, the proposed approach is more suitable for practical implementation compared with the recently published papers.

Flexible dual-diversity wearable wireless node integrated on a dual-polarised textile patch antenna

Abstract: A new textile wearable wireless node, for operation in the 2.45 GHz industrial, scientific and medical (ISM) band, is proposed. It consists of a dual-polarised textile patch antenna with integrated microcontroller, sensor, memory and transceiver with receive diversity. Integrated into a garment, the flexible unit may serve for fall detection, as well as for patient or rescue-worker monitoring. Fragile and lossy interconnections are eliminated. They are replaced by very short radiofrequency signal paths in the antenna feed plane, reducing electromagnetic compatibility and signal integrity problems. The compact and flexible module combines sensing and wireless channel monitoring functionality with reliable and energy-efficient off-body wireless communication capability, by fully exploiting dual polarisation diversity. By integrating a battery, a fully autonomous and flexible system is obtained. This novel textile wireless node was validated, both in flat and bent state, in the anechoic chamber, assessing the characteristics of the integrated system in free-space conditions. Moreover, its performance was verified in various real-world conditions, integrated into a firefighter garment, and used as an autonomous body-centric measurement device.

AC creeping discharges propagating over solid–gas interfaces

Abstract: This study is aimed at the characterisation of surface discharges propagating over different solid materials namely glass, bakelite and epoxy resin immersed in three gases and their mixtures (namely SF6, N2 and CO2, SF6/N2 and SF6/CO2) under AC voltage (50 Hz) in a point-plane electrode system. It is shown that the stopping length of discharges (Lf ) increases linearly with the applied voltage and decreases when the gas pressure is increased. Lf is longer in CO2 and N2 than in SF6 [Lf (N2) > Lf (CO2) > Lf (SF6)]; and the increase of SF6 content in the investigated mixtures leads to a significant decrease of Lf . Also, for a given gas, Lf is slightly longer with glass and bakelite than that with epoxy resin indicating that the higher the dielectric constant, the longer the discharge is. Thus, the density of the discharge branches and Lf reduce when the thickness of insulators increases and/or their dielectric constant decreases. Such observations evidence the implication of capacitive charge effect and electric field in the development of creeping discharges.

Numerical and experimental study of the effects of load and distance variation on wireless power transfer systems using magnetically coupled resonators

Abstract: This work investigates a series resonant circuit designed to wirelessly transfer power to charge an electrical vehicle battery. A typical approach assumes the load connected to the power transfer system to be constant and then the wireless link efficiency is studied. In practical engineering applications, however, the load and the distance between coils will vary and the efficiency may strongly depend on these variations. The efficiency will also be affected by the presence of massive conducting or shielding structures in the proximity of the wireless system. Here, the authors study these effects with the help of an equivalent circuit extracted from a full wave simulation and correlated with measured results. The authors demonstrate that by changing the load resistance the efficiency of the system can be improved, even for a large separation between the two magnetically coupled resonators; however, the maximum efficiency point may not correspond to the maximum power that can be handled by the system. They then analyse the primary and secondary voltages and currents in support of the above findings.

Effect of the grounding resistance to the behaviour of high-voltage transmission lines' surge arresters

Abstract: External overvoltages constitute a typical cause of faults, damages and interruptions in the electrical networks’ high-voltage transmission lines. In order to protect the lines and their equipment against lightning, overhead ground wires in combination with surge arresters are installed, improving the lightning performance of the system and reducing the annual failure rate. The achievement of low values of earth resistance is an essential requirement, since a good grounding system reduces considerably the corresponding insulation breakdowns. However, the behaviour of surge arresters in relation to the grounding resistance is different, as it depends on the position of the lightning hit. The current work is devoted to the sensitivity analysis of the failure probability of surge arresters with relation to the grounding resistance, investigating concurrently the effect of the arresters’ installation interval and the energy absorption capability.

Unbalanced three-phase distribution system frequency estimation using least mean squares method and positive voltage sequence

Abstract: The subject of this study is a frequency estimation algorithm suitable for grid-connected power converters placed at a weak coupling point of a three-phase electrical distribution system. An upgraded version of the widely used complex least mean squares (CLMS) algorithm for frequency estimation is introduced to cope with different voltage amplitude unbalance and harmonic distortion levels, both frequently present in power system at distribution level. First, it is suggested that the CLMS algorithm uses only a positive phase-sequence component of voltage vector, the component that is inherently symmetrical and by cancelling the phase unbalance preserves the circular vector trajectory in a two-phase αβ-plane. This study shows that it is even possible to use the positive voltage phase-sequence vector extracted using a constant delay block, thus avoiding potential instability issues in the case of signal frequency feedback loop. Second, possible high signal harmonics and signal measurement noise are both removed using low-pass filters prior to CLMS algorithm deployment. Computer simulations and experiments are performed under a variety of conditions to validate the effectiveness of the proposed technique. Experimental results are achieved using the dataset sampled from the actual three-phase grid voltage at distributed level and with data processing done in the LabVIEW software environment.