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The effect of doping with holmium (Ho) and erbium (Er) rare-earth elements (REE) on the main parameters and characteristics of photocells based on p-GaSe/n-InSe heterostructures has been experimentally studied. Photo-electromotive force measurements were performed for undoped and rare-earth doped samples and obtained results comparatively analyzed. The light characteristic of the samples shows that significant improvement are observed in the rare-earth doped crystals. The possibility of targeted control, as well as increasing their reproducibility and stability by changing the content of the introduced impurity (NREE), is shown. The optimal situation is provided by alloying Er with NREE≈10−1 at.%. [ABSTRACT FROM AUTHOR]

The paper reveals the influence of Li, B and the composition of metal contacts on the processes of resistive switching in memristive structures M/NC/D/M. After field exposure in structures Cu/(Co50Fe50)x(LiNbO3)100–x/s-LiNbO3/Cu/sitall, Cu/(Co50Fe50)x(LiNbO3)100–x/d-LiNbO3/Cu/sitall and Cu/(Co40Fe40B20)x(SiO2)100–x/d LiNbO3/Cu/sitall at x < 13 was detected a residual voltage (up to 16 mV) due to the electromigration of Li ions, that leading to a "reversible" type of VAC hysteresis and instability of the time dependencies of induced resistive states. In the structures of Cu/(Co40Fe40B20)x(LiNbO3)100–x/s-LiNbO3/Cu/sitall, Cr/Cu/Cr/(Co40Fe40B20)x(LiNbO3)100–x/s-LiNbO3/Cr/Cu/Cr/sitall containing B, the residual voltage is reduced by formation of chemical compounds B with percolated Li atoms. When limiting the electromigration of Li ions, the main mechanism of resistive switching is the processes of electromigration of oxygen vacancies in the dielectric oxide layer. Suppression of residual voltage in the Cr/Cu/Cr/(Co50Fe50)x(LiNbO3)100–x/s-LiNbO3/Cr/Cu/Cr/sitall structure due to the introduction of a Cr buffer layer that does not dissolve Li leads to the absence of bipolar resistive switching in these structures. [ABSTRACT FROM AUTHOR]

To measure the spatial distribution of the magnetic field, both single Hall sensors and arrays of Hall sensors are used. In most cases, a one-dimensional ruler is sufficient. The finished device is called a measuring carriage. The rest of the article describes the first stage of its creation. This article consists in Hall sensor rejection, during which the sensors operate under extreme operating conditions (with increased electrical and thermal loads). During this so-called accelerated aging, the changes in the residual stress values, temperature coefficients, nonlinearity, and divergence of the sensitivity coefficient were monitored. This study can serve as a methodological guide in determining the selection criteria for Hall sensors for precision measuring systems. The need for aging sensors was shown to stabilize their long-term characteristics. In addition, the process of rejecting sensors according to the parameters of interest is described. [ABSTRACT FROM AUTHOR]

With the widespread application of permanent magnet synchronous motor (PMSM) drives, the demand for safety and reliability has increased. Inverter open-circuit fault is a frequent and severe failure that requires reliable detection and remedial measures. This paper proposes an open-circuit fault diagnosis technique based on voltage residual analysis that does not require additional voltage sensors. The proposed technique analyzes the characteristics of the reference voltage and the actual output voltage under both healthy and fault conditions. To achieve fast and reliable fault detection, it uses the DC component and second harmonic component in the residual voltage between estimated voltage and controller reference voltage. Once an open-circuit fault is detected, the polarity of the DC component is capable of identifying the fault location. The effectiveness and robustness of the proposed technique are verified through simulations and testing on a triple redundant PMSM under various load and speed conditions. [ABSTRACT FROM AUTHOR]

In order to study the strong electromagnetic pulse effect of critically vulnerable equipment in power systems and evaluate the survivability under high-altitude electromagnetic pulses, it is necessary to study the characteristics of the transient response of metal oxide arresters to the high-altitude electromagnetic pulse by experiment. In this paper, an experimental platform for high-altitude electromagnetic pulse conduction current injection for a typical 10 kV metal oxide arrester was set up, and the key parameters such as peak value of overshoot voltage, peak value of residual voltage, action voltage and response time were obtained by the experiment. The results show that: the action voltage of this type of metal oxide arrester is 3.53 times higher than that of its rated voltage; the peak value of overshoot voltage is 2.19 times that of the peak value of residual voltage under lightning impulse current; the peak value of residual voltage is 1.57 times that under lightning impulse; and the response time varies little with the electromagnetic pulse conduction current amplitude, averaging 46.86 nanoseconds under a high-altitude electromagnetic pulse conduction environment. [ABSTRACT FROM AUTHOR]

Inductive oil debris sensors have been widely used in the health monitoring of mechanical equipment. Such sensors are important for fault diagnosis, life prediction, and prevention of catastrophic failures. In this article, the concept that the residual voltage caused by the asymmetry of two incentive coils hinders the improvement of the sensor resolution is theoretically demonstrated for the first time. In order to achieve ultrasensitive detection, a novel two-stage autoasymmetrical compensation circuit (AACC) is introduced into the triple-coil debris sensor to automatically suppress the residual voltage in real time. The experimental results show the successful detection of 70 μm (diameter or D) ferrous particles and 165 μm (D) nonferrous particles in a sensing channel with a diameter of 43 mm. The AACC is verified effectively to maintain the residual voltage at an ultrasmall value even in the presence of environmental fluctuations, and the sensitivity of the sensor using AACC is more than 31 times that of the sensor without AACC. [ABSTRACT FROM AUTHOR]

Based on the triggered lightning technology, this study carried out observational tests regarding the impulses of ground potential rise (GPR) to surge protective devices (SPD), and emphatically analyzed the impulse and damage effects of the initial long continuous current processes of triggered lightning on the SPDs. The results showed that after the triggered lightning currents were injected into grounding grids, the SPDs with rated flow rates were easily damaged under the joint actions of the initial long continuous currents of the lightning and the subsequent return strokes. It was observed that when the energy flowing through the SPDs had accumulated to a certain level, only the initial long continuous current processes had caused damages to the SPDs. The impulse effects on the SPDs were determined to be closely related to the different waveforms of the initial long continuous current processes. For example, when the long continuous current processes were superimposed by the initial continuous current pulses (ICCP), with fast rising edges and large amplitudes, then the energy passing through the SPDs tended to rapidly increase. This was believed to likely be the most critical factor related to the damages to the SPDs during the long continuous current processes. [ABSTRACT FROM AUTHOR]

Multilevel inverter topologies have been widely used in applications of wide-power range and fault diagnosis of such circuits is becoming more and more important. T-type three-level inverters have advantages in efficiency compared to common neutral-point-clamped inverters. This paper proposes a new diagnosis method of an open-circuit (OC) fault for a T-type three-level inverter. The method is implemented by measuring the phase voltage of the circuit and comparing it with the reference voltage of the circuit to obtain a residual voltage. Firstly, according to the analysis of different current conduction paths of the circuit, the voltage level changes of the circuit bridge voltage and phase voltage in different device open modes are studied. Then, based on the above analysis, a device open-circuit fault diagnosis scheme is proposed based on the residual voltage. Finally, simulation and experimental results show that the proposed method can identify the faulty switch exactly and quickly. [ABSTRACT FROM AUTHOR]

This paper addresses one of the new and most important issues arising when low-power voltage transformers (LPVTs) are used in power network substations for evaluating, among others, the residual voltage measurement. Conversely to open-triangle inductive instrument transformers, the use of phase voltage transformers for measuring the residual voltage gets challenging due to the very high accuracy required for the three LPVTs. In this paper, a general expression to estimate the residual voltage measurement uncertainty, starting from the LPVTs accuracy, is presented. The effectiveness of the proposed approach is then confirmed with both Monte Carlo simulations and actual measurements on a general three-phase system. [ABSTRACT FROM AUTHOR]

Voltage sag frequency estimation is necessary for understanding the voltage sag severity in power system and offering full information for the interested parties to mitigate voltage sag. The high penetration of wind power in the power system and the uncertainty of the fault distribution raise new challenges to accurate voltage sag frequency estimation. This study presents a systematic voltage sag frequency estimation method, considering the fault distribution density, fault ride‐through (FRT) process of wind turbines during voltage sag and the interval characteristic of voltage sag frequency. First, this study proposes a fault distribution estimation model based on adaptive kernel density. Second, this study proposes a method for calculating the residual voltage and duration of voltage sag during FRT and combines the common distance protection action to analyse the effect on voltage sag by FRT process of wind turbines. Lastly, this study proposes an interval‐valued voltage sag estimation method considering the interval characteristics of fault rate in the power system. IEEE 30‐bus test system is used to verify the proposed method, the estimation results show better performance of the proposed method compared with the typical estimation methods. [ABSTRACT FROM AUTHOR]

• The fuzziness of residual voltage, the correlation of duration and the chaos of occurrence time are proposed. • Three prediction models to predict voltage sag characteristics is proposed. • Fuzzy membership function is used to describe the voltage sag fluctuation of nodes under time-varying conditions. • A evidence theory synthesis formula is proposed to enhance the predictability of duration. • The phase space reconstruction is used to describe the multi-dimensional factors affecting the occurrence of voltage sag. An effective method of assisting utilities and users to avoid the risk of voltage sags is the accurate prediction of voltage sags. This study analyzes the predictability and proposes the prediction models for three characteristics of voltage sags based on measured data. First, this study defines the time series of voltage sag and analyzes the predictability of the residual voltage, duration, and time of occurrence. Second, this study proposes a fuzzy logic prediction model of the residual voltage to address the fuzziness of the residual voltage. Third, this study proposes an evidence theory prediction model of the duration according to the correlation between the residual voltage and duration. Further, this study proposes a chaotic signal prediction model of the occurrence time by reconstructing the phase space of the occurrence time. Finally, the measured voltage sag data from 10 sites are used to analyze and validate the proposed prediction models. [ABSTRACT FROM AUTHOR]

Highlights • A novel method to monitor the neutral grounding resistor of unit-connected generators is proposed. • The proposed method uses the third harmonic of neutral and residual voltages to detect the resistor failure. • The proposed technique has been restrained against load variation, step-up transformer saturation and ground faults. • Maloperation of generator stator ground protection due to neutral grounding resistor failure has been resolved. Abstract Neutral grounding resistors are used to control transient overvoltages and overcurrents in power systems. Failure of these critical assets revives many disadvantages of ungrounded and solidly grounded traditional systems. Therefore, monitoring the neutral grounding resistors is necessary. A new continuous monitoring technique is proposed for such a resistor located at the neutral of unit-connected generators. It detects resistor failure by comparing the third harmonic of neutral and residual voltages. This proposed scheme demonstrates reliable operation under various conditions of the system and resistor as observed by PSCAD in conjunction with MATLAB. Further performance validation has been carried out using an industrial generator protection relay as well. In contrast to existing methods, this scheme does not require any additional measurement instruments. Thereby, it is easily retrofitted to installed unit-connected generator protection infrastructures. Moreover, this continuous monitoring technique prevents maloperation of the generator stator ground protection due to the neutral grounding resistor failure. [ABSTRACT FROM AUTHOR]

• Electromagnetic transient model for varistor-based surge protective devices. • Resistive, inductive, and capacitive behavior of varistor-based SPDs. • Residual voltage of SPDs depends significantly on their base configuration. • Excellent simulation results for a wide range of very fast-front impulse currents. • The model can be used for insulation coordination studies and dimensioning of SPDs. This work provides a simplified transient model for low-voltage DIN rail surge protective devices (SPDs), accounting for their resistive, inductive, and capacitive behavior. The time-domain modeling approach is based on impulse current and sinusoidal voltage experiments. The lumped-circuit model is implemented in the ATP-EMTP environment and yields results in satisfactory agreement with the experimentally derived residual voltage and energy absorption of commercially available DIN rail SPDs for single- and three-phase installations. The proposed model can be used for evaluating the protection level and maximum residual voltage of SPDs under standard and non-standard surge currents and can be an effective tool employed in insulation coordination studies of power systems. [ABSTRACT FROM AUTHOR]

Lightning overvoltage may affect the signal transmission and damage terminal equipment. In order to solve this problem, the signal system Surge Protection Devices (SPDs) is designed by analyzing the signal transmission characteristic and the design theory of SPDs. The impulse tests are performed on the SPDs using a combined wave generator. Residual voltages and sharing currents are measured and discussed by considering impulse voltages, DC breakdown voltages of downstream SPDs and decoupling resistances. The blind zone of the signal system SPDs is also discussed in detail. In addition, the SPDs are equivalent to a two-port network, and the influence of distributed capacitance of the SPDs on signal transmission is analyzed for the first time through experiments. Cut-off frequency is measured and discussed by considering DC breakdown voltages of downstream SPDs and decoupling resistances. From the two experiments, the results indicate that increasing the decoupling resistance can decrease residual voltage and sharing current, but will reduce the cut-off frequency. Decreasing the DC voltage of downstream SPDs will have a similar effect. So the decoupling resistance and the DC breakdown voltage of downstream SPDs resistance need to be considered carefully in designing signal system SPDs. The sufficient research of signal system SPDs in this paper has a certain reference value in lightning protection. [ABSTRACT FROM AUTHOR]

The object of research is an analytical expression for representing the switching current impulse of a surge arrester. Any current impulse (both lightning and switching) is characterized by such parameters as the virtual front time and the virtual time to halfvalue on the tail. According to the standard IEC 60099-4:2014, switching current impulse has a virtual time to half-value on the tail of roughly twice the virtual front time. This requirement is one of the most problematic places in this task. The existing approaches used to represent lightning current impulses are not suitable in this case, since these impulses have virtual time to half-value on the tail of two and a half times the virtual front time. This problem can be solved with a help of analytical piecewise continuous functions. It is shown how to describe switching current impulses of the surge arresters with a help of analytical piecewise continuous functions. In contrast to other expressions, the resulting expressions for the switching impulse have only one parameter (angular frequency). Instead of approximate calculation, the front time of the resulting impulse is calculated by an analytically exact formula. Hence, the tolerance of virtual front time is equal to zero. The time to half-value on the tail of the resulting impulses is determined with some error that can be reduced by some complication of the original expression. The proposed functions satisfy the requirements of the IEC 60099-4:2014 standard regarding switching current impulses of surge arresters. These functions allow representing current impulses having virtual time to half-value on the tail of roughly twice the virtual front time (30/60 or 45/90 microseconds). In such cases, minimal tolerance of time to half-value on the tail is +3.78 %. Additional study shows that one of proposed functions allows representing current impulses having virtual time to half-value on the tail of two and a half times the virtual front time (8/20 or 4/10 microseconds). Tolerance of time to half-value on the tail for such impulses is 0.55 %. The obtained functions are intended for study of various models of metal-oxide surge arresters on personal computers. [ABSTRACT FROM AUTHOR]

Gas insulated substation generates very fast transient overvoltages (VFTOs) in power systems because of disconnector switching operations. The present design of metal–oxide arrester may not be effective against these VFTs and so the arrester fails to operate under VFTO. At this point, it becomes essential to improve the electrical behaviour of arrester. Some of the possible measures for improving the electrical behaviour of arrester blocks are the procurement of higher‐voltage gradient, reduction of arrester capacitive effect and reduction in the time lag between residual voltage and current surge. With this background, the microstructural and electrical parameters of arrester block are modified by doping scandium trioxide with basic composition and the samples are prepared using different sintering processes. Subsequently, the voltage gradient, capacitance and the time lag are found for the prepared samples. The performance of new arrester blocks are investigated against VFTO and the results are compared with conventional arrester blocks. From that, it is perceived that the new arrester samples have improved parameters and might be used for protection of power system against VFTO. [ABSTRACT FROM AUTHOR]

Operating experience shows that residual DC voltage remains in GIS bus bar for a long time after opening of disconnectors. This DC voltage will not only cause accumulation of charges and contaminants on the insulator surface, but will also easily lead to motion of free metallic particles in SF6 gas gap, weakening the insulation ability of GIS. At the time of disconnectors reclosing, impulse voltage could be generated in the bus bar and superimpose on the pre-existing DC voltage. As a result, GIS will be under combined voltage of DC and impulse. This paper experimentally studies the insulation characteristics of GIS under combined voltage of DC and lightning impulse. It is found that, for non-contaminated insulators, when DC and LI voltage are of the same polarity, flashover voltage under combined voltage is nearly the same as the flashover voltage under LI alone. However, when they are of the opposite polarity, flashover voltage decreases with increasing DC voltage. In cases there are metallic particle or powder around insulator, pre-stressed DC voltage will cause adhesion of contaminants on the insulator surface and lead to decrease of flashover voltage. Specially, for the particle-contaminated insulator, it is found that the surface charge accumulation caused by DC voltage will have big influence on the flashover voltage, and the most critical condition is when DC and LI voltage are of the opposite polarity. In addition, the breakdown characteristics of SF6 gas gap with free metallic particle are investigated. It is found that under pre-stressed DC voltage, particle will do standing motion or bouncing motion. And breakdown voltage becomes lowest when particle is doing standing motion. The results of the experiments show that residual DC voltage in GIS could deteriorate its insulation ability and the combined voltage of DC and impulse is found to be a critical condition for GIS insulation. Moreover, the combined voltage is shown to be more sensitive to detect some insulation defects in GIS than applying impulse voltage alone, especially for free metallic particles, which are commonly found in GIS but up to now no effective detection method is proposed. Therefore combined voltage of DC and impulse could be considered to serve as a supplement of field test for GIS. [ABSTRACT FROM PUBLISHER]

The model of surge arrester, which consists of two nonlinear resistors, connected with each other via two inductive and a resistive element is studied. To determine the model parameters are only required by the data given in the catalogs of surge arresters manufacturers. Until now, in the simulation of surge arresters insufficiently investigated the effect of currents typical for lightning surges. It is shown that the model of surge arrester can be investigated by comparing the results of computer simulation with the data given in the catalogs of surge arresters. The study will use the following data, specific both switching and lightning surges: the residual voltage at switching impulse current, the residual voltage at lightning impulse current, as well as the residual voltage at steep current pulse. Residual voltages corresponding to the 250-2000 A standard switching current pulses, 1.5-20.0 kA standard lightning current pulses and 10.0 kA steep current pulses were defined using Micro-Cap model of surge arrester. Residual voltages obtained in the simulation were compared with the corresponding values given in catalogs of surge arresters. It is shown that the smallest error takes place under the influence of lightning current pulses on the model of surge arrester. The model of surge arrester in Micro-Cap program allows high accuracy determination of the residual voltage on the surge arrester during the flow of discharge pulses, which are typical for lightning surges. The model also allows to calculate the transients during the flow of the lightning currents, which are composed with several successive pulses, which can pose a significant danger to the surge arresters. The results of the research can specify the parameters of substation protection against different overvoltages and, thus, reduce economic losses due to possible faults. [ABSTRACT FROM AUTHOR]

• The single-phase reclosing technique should not be used in half-wavelength transmission systems. • The residual voltage of half wavelength transmission system can be used to determine the existence of faults. • The frequency difference of residual voltage can be used to determine the existence of faults. • We propose a complete and feasible three-phase adaptive reclosure scheme for half wavelength transmission system. UHV AC transmission lines use adaptive reclosure technology not only to avoid reclosing in the permanent faults, but also effectively prevent the emergence of reclosing overvoltage, adaptive reclosure is of great significance, and its technical principle is based on the study and judgment of the existence of the fault. The reactive power of half-wavelength transmission lines along the line was self-balanced, no shunt reactor equipment, there is no "beat frequency" phenomenon, and very little information is available after the three-phase trip. In this paper, we analyze the fault traveling wave's frequency characteristics and identify the fault's characteristics based on the residual voltage frequency difference of the line after the three-phase trip. After the disappearance of the line fault, the stable frequency difference obtained by CVT reflects the entire length of the line, which is not affected by the system boundary conditions and can complete the reliable identification of the disappearance of the line fault. The two-terminal criterion is given for the short-line dead zone problem, and then a three-phase adaptive reclosure scheme for half-wavelength transmission lines based on the frequency difference to identify fault disappearance is proposed. Extensive RTDS digital-analog hybrid tests show that the method is correct and robust. [ABSTRACT FROM AUTHOR]

• Complete VFTO(s) generation and measurement system based on (MPC) and (TLT) technologies. • Five commercial ZnO varistors of the same reference were used in the characterization. • The influence of the applied overvoltage front time on the capacitive effect of the varistor is confirmed. To study the behavior of low voltage ZnO varistors against Very Fast Transient Overvoltage (VFTO), the authors have developed a complete VFTO(s) generation and measurement system based on magnetic pulse compression (MPC) and Transmission Line Transformer (TLT). This system has an output voltage of 24 kV, rise time of 32 ns, and a bandwidth of 10 MHz. The measurement system comprises a damped resistive voltage divider with an impedance of 1000 MΩ and a maximum bandwidth of 10 MHz. The experiment was performed with five commercial zinc oxide varistors with a nominal voltage of 150 V, each one was subjected to VFTO-type voltage pulses and, by Analysis of Variance (ANOVA), a single varistor was selected to perform the case study. This work presents an experimental methodology to observe the response of low voltage ZnO varistors against VFTO. This response is the delay of the residual voltage to the applied discharge current and indicates the poor performance of these devices against VFTOs. Our methodological proposal consists of a series of laboratory tests that will allow manufacturers and consumers to take corrective measures in case of malfunction, damage of the varistor material, or permanent degradation of the device. [ABSTRACT FROM AUTHOR]

Surge Protective Devices (SPDs) are widely applied to protect the low-voltage equipment against the surges from direct lightning strikes or the lightning-induced effect. To ensure the protection effect, the users should not only consider the performance and quality of the SPDs themselves, but also the coordination between SPDs and between SPD and the protected equipment. In practice, there may be SPDs with different characteristics from many manufacturers on the same line. One SPD is installed by the contractor in charge of electrical installation and then various panel boards are used in the installation made by various panel builders. All of them may use different brands of SPDs. So it is difficult to evaluate the coordination effect between those SPDs and between SPD and the protected equipment by theoretical analysis. A solution would be to perform the coordination test. This paper provided an investigation on the proposed coordination test method for different configurations with various loads, various cable lengths, various cable enclosures and various surge waveforms through experiments in the laboratory. The sharing current and residual voltage of two cascaded SPDs were measured and discussed. The proposed coordination test method was verified to be informative for achieving successful coordination between SPDs and between SPD and the protected equipment. [ABSTRACT FROM AUTHOR]

Impulse voltage testing is generally carried out for assessing the insulation strength of power equipment using standard impulse voltage waveforms in high-voltage laboratory. However, in actual field, the power equipment experiences standard and non-standard lightning impulse voltages. The main objective of this work is to analyse the voltage stress on surge arresters under standard and non-standard lightning impulse waveforms. Surge arrester models are developed in matlab (The MathWorks, Inc., Natick, Massachusetts, United States), and residual voltage test results are compared with manufacturer data for validation. Impulse voltage test is performed on the Institute of Electrical and Electronics Engineers surge arrester model using standard and non-standard lightning impulse voltage waveforms. The obtained test results showed that non-standard impulses have the potential to develop higher voltage stress and hence pose a high risk to surge arresters. However, results of this study provide the basis for further laboratory-based investigation considering a wide variety of impulse voltage waveforms representing the real lightning overvoltages. The test results will contribute to identify the need for modifying existing test standards or introducing new standards for impulse testing of surge arrester. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The characteristics of the residual voltage of a surge protective device (SPD) connected to an overhead distribution line due to nearby natural cloud-to-ground lightning and to rocket-triggered lightning are analyzed in this paper. The residual voltage of the SPD is found to be closely related to the ground potential rise at the grounding terminal of the SPD. For natural lightning occurring 1.8 km to 10.5 km away from the SPD, the ground potential rise (GPR) is too small to influence the residual voltage of SPD. For rocket-triggered lightning with a 40-m distance between the lightning channel and the SPD, the residual voltage of SPD is primarily determined by the voltage induced in the distribution line for return stroke and large M-components process, whereas the influence of the GPR becomes strong in the minor M-components stage. For a 5-m distance between the lightning channel and the SPD, the residual voltage of SPD originates primarily from the GPR during the entire flash, exhibiting the characteristics of reversed polarity. The duration of SPD's residual voltage may be up to the order of milliseconds due to the GPR in the case of the large M-components and a long continuing current process, which may cause damage to the SPD. [ABSTRACT FROM AUTHOR]

Reacceleration is a method of automatically restarting motors after an unexpected deacceleration caused by system voltage events such as dips, outages, or bus transfers. Reacceleration schemes are designed to minimize process disruptions by rapid detection of supply loss, recovery/ monitoring of acceptable transient torque limits, and then automatic reclosure of the motor contactors. Depending on the connected load and minimum available fault current, the reacceleration may be “instantaneous” or a staged event designed to assure that the bus voltage is maintained at an acceptable level during the process restart. Poor coordination of the reacceleration process may cause further system outages if the nominal bus voltage drops below tolerable levels. Special consideration is required to prioritize loads, to maintain production, to prevent lifting of safety valves, and to avert equipment damage. Various processes differ in their ability to withstand temporary outages, from milliseconds (ms) to several seconds. Today's protection and control intelligent electronic devices (IEDs) provide settings, dedicated measurement, and timing circuits to allow these variances to be preprogrammed and activated based on outage duration and magnitude. Short-time outages within millisecond range combined with electrically held contactors provide the least complex situations for enabling a reacceleration system. More complex situations arise when motors are still decelerating and when the supply is restored, especially if the contactors were maintained closed during the outage. Certain more advanced IEDs also provide multiple reacceleration schemes that are automatically deployed based on longer outages—typically up to 30 min. [ABSTRACT FROM AUTHOR]

Transferring motor loads from one bus to another bus, due to a source disconnection from a main–tie–main arrangement, is not a trivial action. The transfer is dependent on detection of breakers' status, switch positions, and accurate measurements of line and bus voltages. The mechanism of transferring loads from one bus to the other imposes a number of requirements for the relay controlling the closing of the tie-breaker. The main requirement in this process is to accurately measure both the bus voltage from the “healthy” source and the decaying voltage from the bus disconnected from its supplying source. This is done to assure accurate and smooth transition of the motors to the healthy source. To perform accurate ac measurements, many protective devices are built to track the power system frequency from one voltage source. However, when the source for the frequency tracking is lost, the accuracy of the measurements associated with the voltages and currents from the healthy source is jeopardized. In such conditions, the relay may declare an absence of healthy bus voltage and could prematurely issue a close command to the tie-breaker, while the magnitudes of both bus voltages are still high and out of phase. Closing of the tie-breaker under such conditions can stress the motors and damage them due to a high torque. This paper reveals a method for accurate voltage measurement and shows a number of tests proving that inaccurate voltage measurement, due to decaying frequency in existing relay designs, could cause incorrect tie-breaker closure during transfer operations. Both scenarios of decaying voltage used for frequency tracking and decaying voltage from the other bus are explored. This paper discusses alternative methods for accurate bus voltage measurements during frequency decay of the reference voltage and makes recommendations for optimal relay performance. [ABSTRACT FROM AUTHOR]

It is well known that metal–oxide arresters (MOAs) do not have the capability of protecting the equipment in extra-high voltage networks, when they face the threats from very fast transients (VFTs). This “inaction” on the part of MOA is mainly brought by stray capacitance, which not only delays the conduction of MOA under VFTs but also brings a nonuniform voltage distribution across its structure and thereby impacts its life, especially, that of its top unit. This phenomenon prompts an urgent take on the factors that threaten the functioning of MOA under VFTs. After performing a thorough review of the studies made earlier, this study focuses on the further improvements required. This paper mainly deals with the accurate calculation of stray capacitance, its adverse impacts on MOA during its conduction phase, voltage distribution across the arrester with the aid of a finite-element method (FEM) and the Electromagnetic Transient Program (EMTP), and, finally, it explores the modifications required to get adequate protection for the equipment exposed to VFTs. [ABSTRACT FROM AUTHOR]

Voltages induced on a 1300-m overhead power line by 51 return strokes in a thunderstorm process in Guangzhou Field Experiment Site during the summer of 2008 have been measured. As to a group of surge protective devices (SPDs) were installed at the end of the overhead power line, the peak values of induced voltages were all below 1 kV, and most of induced voltage waveforms show typical residual voltages. The interval time of adjacent return strokes ranged from 14.0 to 185.6 ms. The peak values of induced voltages at the SPD site were obviously under attenuation compared to that at the household site. The lightning location data indicate that the induced voltage might be of either positive or of negative polarity depending on the location of the lightning strike points. The peak value of induced voltage tends to saturate when the ratio of peak value of return stroke current to distance from the stroke location to the measuring point exceeds 4 kA/km. [ABSTRACT FROM PUBLISHER]

One of the common characteristics of the electrothermal breakdown in an underwater discharge acoustic source (UDAS) is the existence of a pre-breakdown-heating phase. In our experiment, two phenomena were observed: (1) the breakdown time that takes on high randomicity and obeys a "double-peak" stochastic distribution; (2) the higher salt concentration that reduces the residual voltage and causes 100% non-breakdown. The mechanism of electrothermal breakdown is analysed. To specify the end of the pre-breakdown-heating phase, a "border boiling" assumption is proposed, in which the breakdown time is assumed to be the time needed to heat the border water around the initial arc to 773 K. Based on this 'border boiling' assumption, the numerical simulation is performed to evaluate the effects of two heating mechanisms: the Joule heating from the ionic current, and the radiation heating from the initial arc. The simulation results verify the theoretical explanations to these two experiment phenomena: (1) the stochastic distribution of the radius of the initial arc results in the randomicity of the breakdown time; (2) the difference in efficiency between the radiation heating and the Joule heating determines that, in the case of higher salt concentration, more energy will be consumed in the pre-breakdown-heating phase. [ABSTRACT FROM AUTHOR]

A lightning arrester installed in an electrical power system is designed to protect some electrical equipment against damage due to lightning impulse strike. However, if there is a multiple impulse current striking a lightning arrester, it may damage the lightning arrester itself and then, as a result, it cannot provide lightning protection to the electrical equipment anymore. In this research, three peak values of lightning impulse current,which were 600, 1300, and 2500A, each of which had 5 repetitive impulse current, was applied to a 5000A (1p.u.) ZnO block of a 24kV nominal voltage lightning arrester having seven ZnO block units. The results of research showed that by applying impulse current of the order of 600(0.12), 1300(0.26), dan 2500(0.5) A(p.u.), making the ZnO block damage as much as 26.58, 50, and 100% concecutively. In addition, the damage of the ZnO block appeared as permanen low value of residual voltage as low as more than 5% of the new ZnO block residual voltage. If the ZnO block is still being used in a lightning arrester in electrical power system, it may result in the opening of a circuit breaker and possibly making electrical power outages from a number of electrical power customers. [ABSTRACT FROM AUTHOR]

Abstract: Powell’s optimization method has been used for the evaluation of the surge arrester models parameters. The proper modelling of metal-oxide surge arresters and the right selection of equivalent circuit parameters are very significant issues, since quality and reliability of lightning performance studies can be improved with the more efficient representation of the arresters’ dynamic behavior. The proposed approach selects optimum arrester model equivalent circuit parameter values, minimizing the error between the simulated peak residual voltage value and this given by the manufacturer. Application of the method in performed on a 120 kV metal oxide arrester. The use of the obtained optimum parameter values reduces significantly the relative error between the simulated and manufacturer’s peak residual voltage value, presenting the effectiveness of the method. [Copyright &y& Elsevier]

• The electrostatic field and voltage distribution analyses are carried out on 245 kV surge arrester by considering various shield designs using numerical techniques. • Design of grading shield and development of GISA is verified by performing dielectric duties as per IEC 62,271–203. • The lightning and switching impulse discharge current tests were conducted on the developed 245 kV GISA assembly and residual voltages were recorded. • V-I characteristics of GISA is established by measuring residual voltages for the entire current range. The voltage distribution across gas insulated surge arrester (GISA) is highly non-uniform as compared to conventional porcelain-clad surge arresters. The aim of the present paper is to optimize design of grading shield in such a way that the non-uniformity voltage factor across GISA must be less than ±10%. To achieve this order of non-uniformity factors, special grading shields like window based cylindrical and conical shields are proposed. In the present paper, 245 kV GISA model has been considered to analyze voltage distribution across the metal-oxide (MO) blocks. The electrostatic field and voltage distribution analyses are carried out on surge arrester by considering various shield configurations using numerical techniques. Further, design of shield and development is verified by performing dielectric duties on GISA assembly without MO blocks as per IEC 62,271–203. The lightning and switching impulse discharge current tests were carried out on the developed 245 kV GISA residual voltages were recorded. These voltage levels for above discharge currents are found to be in the order of 2.8 – 3.2 p. u. and are at acceptable levels as per IEC 60,099–4. Finally, protection level or V-I characteristics of GISA is established by measuring residual voltages for the entire current range. [ABSTRACT FROM AUTHOR]

A scheme of single-phase-to-ground fault feeder identification in distribution networks with the application of a wavelet transform technique is presented in this paper. The scheme uses zero-sequence current traveling waves to identify the faulted feeder, and the busbar residual voltage to determine an event caused by fault or switch operation. The current traveling waves measured by zero-sequence current transducers are decomposed using wavelet multiresolution analysis. The local modulus maxima of the wavelet transform are extracted to determine the time of the initial traveling wave. The wavelet transforms on all feeders at the time are compared in magnitude and polarity with each other to identify the faulted feeder. The feeder identification is independent of the network neutral-point grounding mode. The proposed scheme was implemented and verified using Electromagnetic Transients Program (EMTP)-generated signals. The scheme proved to be robust against transients generated during normal events, such as feeder energizing and de-energizing as well as capacitor bank switching. [ABSTRACT FROM AUTHOR]

Surge protective devices (SPDs) are installed at the boundaries of lightning protection zones to damp the surges to the protection level of the SPD. If the circuit between the SPD and the apparatus to be protected is too long, the propagation of the surges leads to oscillation phenomenon. In this case, the overvoltage can be increased at the apparatus terminals. The increase is taken into account by an enhancement factor which gives the ratio of the maximum overvoltage at the apparatus terminals to the SPD protective level. According to IEC 62305-4, the maximum enhancement factor is considered to be equal to two in the worst case of the open-circuit condition. The objective of this paper is to check this relation for equipment connected to low-voltage (LV) power systems. The LV power system is considered as a TN-S system with different routings in three- and six-story buildings. The terminals of apparatus are substituted by a variety of different loads. All Maxwell's equations are solved by the method of moments and the voltage is calculated at the apparatus terminals. The SPD itself is simulated by a voltage source at the ground floor of the building. The results reveal that the voltage at the apparatus terminals may overshoot the SPD protection level by a factor of 3. [ABSTRACT FROM AUTHOR]

Abstract: The effect of the sample thickness d on the residual voltage ratio Kr indicates the existence of the dimensional effect in the commercial ZnO varistors. The empirical relations derived are based on experimental findings for Kr and breakdown electric field strength E1 mA cm−2 and the microstructural parameter average grain size μ. It is observed that Kr decreases with increasing E1 mA cm−2, while it increases with increasing μ. An integrated hybrid parameter σ2μ, obtained as the product of average grain size μ and grain size variance σ2, is found to be a representative relation between electrical properties and microstructure of the ZnO varistors. A microstructural model is used for computer simulation that also verified the dimensional effect in these varistors. The simulated results are consistent with the experimental results. [Copyright &y& Elsevier]

Generally a model of Metal Oxide Surge Arrester (MOSA) for numerical analysis uses a nonlinear resistance. But actual Voltage-Current (V-I) characteristics of MOSA have a hysteresis loop in the time domain like the i-Φ characteristic of a transformer and frequency dependency. The authors have investigated the relation between the actual V-I hysteresis characteristics obtained by some current waveforms and the static V-I characteristics. From the voltage difference between the above two characteristics, an equation was derived and a new model of MOSA was developed. This model consists of a nonlinear resistance representing the fundamental V-I characteristic, a linear inductance, and a voltage source that depends on the absorbed energy. The calculated results by the proposed model are compared with measurement results by using the waveform of standard impulse current, steep front current, and oscillated current. The accuracy of the model has been confirmed to be satisfactory. The model is expected to be useful to investigate insulation coordination of power systems. © 1997 Scripta Technica, Inc. Electr Eng Jpn, 121(1): 35–42, 1997 [ABSTRACT FROM AUTHOR]

This paper describes the fundamental characteristics of the residual voltage and the relationship between irradiation dose and residual voltage in electric power cable used in nuclear power generating stations. The residual voltage of unirradiated cable increases with time, tending to saturate. The saturated value of residual voltage is nearly proportional to bias voltage. This result supports the residual voltage of cable being caused by depolarization. From theoretical and experimental investigations we found that the residual voltage is not influenced by cable size or length. This is an important merit of residual voltage as the nondestructive diagnosis. When dose is more than 500 kGy, the residual voltages exhibit a peak. As the irradiation dose is increased, the peak moves to shorter time. Therefore, we can determine the degree of radiation degradation from the location of the peak. © 1997 Scripta Technica, Inc. Electr Eng Jpn 119(2): 18–25, 1997 [ABSTRACT FROM AUTHOR]

The demand for electric double-layer capacitors, which have high capacity and are maintenance-free, for use in a variety of devices has increased. Nevertheless, it is important to know the degradation behavior of these capacitors at high temperatures because they are expected to be used in severe environments. Therefore, degradation tests at 25 °C and 80 °C were carried out in the current study to analyze the degradation behavior. Steam-activated carbon, Ketjen black, and PTFE were used as the electrodes, conductive material, and binder, respectively, and KOH was used as the electrolyte. The impedance and capacitance were calculated from the voltage and current in the device using the alternating current (AC) impedance method. The results showed that the impedance increased and the capacitance decreased over 14 days at 80 °C, which is the inverse of what we observed at 25 °C. Rapid degradation was also confirmed from the 80 °C degradation test. The residual voltage after measuring the current and voltage was a prominent factor influencing this rapid degradation. [ABSTRACT FROM AUTHOR]

• Pulse characteristics is investigated in a fixed pumping rate and higher frequency. • A novel Q-switching method is proposed to output a stable pulse train. • Different Q states are controlled by a developed high-voltage controller. High-frequency, high-energy sequential-pulse lasers play an important role in high-speed photography. However, pump rates cannot increase infinitely with the increase in frequency. Studying the stability of a pulse train at a fixed pump rate and high frequency is of great importance. This paper presents a novel Q-switching method for improving output stability at high frequencies and a fixed pump rate. The mechanism of the method was analyzed and demonstrated. Experiments were carried out with a newly developed high-voltage control system. Experimental results were in good agreement with the results of theoretical analysis, which showed that the output pulse trains could be stabilized by using the proposed method. This work provides an important reference for high-frequency, high-energy pulse train control. [ABSTRACT FROM AUTHOR]

Presently, due to several factors, metal–oxide arresters find very limited applications against very-fast transient overvoltages (VFTOs). To enhance the utilization of these arresters and to meet the needs of the dynamic performance of arrester under VFTOs, an improved model for the prorated block is proposed and its performance is analyzed by applying current surges of different front times (5 ns to 8 \mus), tail times, and peak values. There is a time lag between the residual voltage and discharge current due to block capacitance. The delay thus observed in the initial peak of residual voltage is experimentally verified. [ABSTRACT FROM AUTHOR]