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A method is proposed in this paper to determine the harmonic impedances in low-voltage networks in a stochastic way. The consequences of resonances for harmonic propagation and stability of power converters are summarized. By using Monte Carlo simulation, the method includes the uncertainties in customer impedances, specifically due to electronic loads and local generation. The uncertainty in customer impedance is included by considering probability distribution for the resistive, inductive, and capacitive parts of the impedance. The concept of transfer impedance is used for phase-to-neutral connections. A method is developed and applied to two existing low-voltage networks in Sweden. Results show that, for these two networks, the resonant frequencies decrease around 28% once photovoltaic panels are installed. This paper includes a discussion of some of the practical aspects of applying the proposed method. [ABSTRACT FROM AUTHOR]

Harmonic distortions at a point of common coupling (PCC) are caused by the harmonic sources in both supply system and customer facilities. In order to verify compliance with harmonic limits for an operating facility, it is essential to determine the contribution of a facility to the harmonic distortions measured at the PCC. Over the past many years, a significant amount of research work has been conducted in developing different harmonic analysis methods and especially harmonic contribution assessment methods. A common need of this line of research is consistent test systems that can be used to verify/validate a proposed method or to compare different approaches. To address this gap, a benchmark test system representing a typical medium-voltage/low-voltage network supplying industrial loads is proposed in this paper. This paper also includes examples on how to use the system to assess the performance of harmonic contribution determination methods. [ABSTRACT FROM AUTHOR]

This paper proposes a novel method for voltage dip classification using deep convolutional neural networks. The main contributions of this paper include: 1) to propose a new effective deep convolutional neural network architecture for automatically learning voltage dip features, rather than extracting hand-crafted features; 2) to employ the deep learning in an effective two-dimensional (2-D) transform domain, under space-phasor model (SPM), for efficient learning of dip features; 3) to characterize voltage dips by 2-D SPM-based deep learning, which leads to voltage dip features independent of the duration and sampling frequency of dip recordings; and 4) to develop robust automatically-extracted features that are insensitive to training and test datasets measured from different countries/regions. Experiments were conducted on datasets containing about 6000 measured voltage dips spread over seven classes measured from several different countries. Results have shown good performance of the proposed method: average classification rate is about 97% and false alarm rate is about 0.50%. The test results from the proposed method are compared with the results from two existing dip classification methods. The proposed method is shown to outperform these existing methods. [ABSTRACT FROM AUTHOR]

This paper proposes a novel islanding detection method (IDM) for grid-connected photovoltaic systems (GCPVSs) through a disturbance injection in the maximum power point tracking (MPPT) algorithm. When an absolute deviation of the output voltage exceeds a threshold, the applied disturbance shifts system operating point from its maximum power point (MPP) condition. This leads to a sharp active power output reduction and consequently, a significant voltage drop in islanded mode beyond the standard voltage limit. The proposed algorithm is defined in a way that the distributed generator (DG) can be restored to MPP after islanding classification. It is thereby effective in microgrid in where the power injection at maximum level to cater the critical loads and maintain the stability of the isolated area are pursued. An intentional time delay has also been considered to avoid nuisance tripping in short-circuit faults which do not require tripping. The assessment of the proposed technique has been conducted for a sample network containing two GCPVSs in a real-time platform including actual relays in hardware-in-the-loop (HiL). The provided results under extensive islanding scenarios defined in islanding standards endorse timely and accurately detection with negligible non-detection zone (NDZ) as well as no false tripping in non-islanding disturbances. The comparative analysis of the presented scheme with a few recent IDMs for GCPVS highlights its overall superiorities, including very small NDZ, fast detection, thresholds self-standing determination, no adverse effect on power quality, and simple and inexpensive integration. [ABSTRACT FROM AUTHOR]

Point-on-wave (POW) is a single-event characteristic of voltage dips, influencing the performance of sensitive equipment, such as AC contactor and wind turbines. However, definition and calculation of POW are unsolved issues. The existing definition of POW is for single-phase dips; it does not consider three-phase dips. Moreover, existing POW calculation methods are inaccurate. This paper proposes a definition of POW as a single-event characteristic for three-phase voltage dips. Then, it proposes a POW calculation method based on space vector, including POW detection and angle calculation. The proposed method is based on the ellipse characteristics during the dip-stage and non-dip stage. The proposed method does not depend on zero crossing for angle reference. To verify the performance of the proposed method, a power-system simulation model is built to generate voltage dips with different origin. Furthermore, the accuracy of the proposed method is verified by 425 sets measured voltage dips, and compared with the published methods. [ABSTRACT FROM AUTHOR]

Voltage sags mitigation has become a significant concern for utilities due to the pressure from both customers and regulators recently. This paper proposes a novel multi-objective optimization formulation for STATCOM allocation problem to mitigate voltage sags. The allocation of STATCOM is formulated as a multi-objective optimization problem (MOP) with two conflicting objectives including the voltage sag performance and annual investment costs of STATCOM. To analytically evaluate the voltage sag performance in the optimization process, a mathematical voltage sag profile calculation model of power system with multiple STATCOMs is proposed and solved by nonlinear programming method. System Average RMS Variation Frequency index-X index is chosen to quantify the voltage sag performance for optimization. Sensitivity-based candidate bus selection is also performed as an essential step to reduce problem size. An improved multi-objective evolutionary algorithm (MOEA) called SPEA2+SDE (Strength Pareto Evolutionary Algorithm with shifting density estimation) is utilized to solve the proposed MOP model, so a set of Pareto optimal STATCOM allocations for voltage sag mitigation can be derived demonstrating the trade-off between two objectives and better supporting the decision-making. IEEE 30-bus system and IEEE 118-bus system are tested to demonstrate the effectiveness and feasibility of proposed method. [ABSTRACT FROM AUTHOR]

Voltage imbalances are one of the most severe challenges in electrical networks, which negatively affect their loads and other connected equipment. This paper proposes a voltage support control strategy to mitigate the voltage imbalance in inverter-based low voltage distribution networks. The control scheme is derived taking in mind the following control objectives: a) to increase the positive sequence voltage as much as possible, b) to decrease the negative sequence voltage as much as possible, c) to inject the power generated by the primary source, and d) to minimize the output current of the inverter. The innovative contribution of the proposed solution is based on the design of a control algorithm that meets the aforementioned objectives without resorting to communications with other grid components. The theoretical results are experimentally validated by selected tests on a laboratory setup with X/R ratio close to one. [ABSTRACT FROM AUTHOR]

The parameters of a harmonic source change with the fluctuating background voltage and current in electric power system, which makes it difficult to establish accurate harmonic models. To solve this problem, a dynamic parameter model (DPM) for harmonic source networks is proposed in this paper. By equating the parameters of a harmonic source as a polynomial of voltage and current, the proposed model can adjust parameters accordingly. The model is not only suitable for a single harmonic source but also for complex harmonic source networks. The process of modeling need no internal information of the harmonic source. The performance of the model is, for different configurations of the harmonic source, compared to the performance of other harmonic models. The results show that the proposed model has higher accuracy and is more suitable for practical power system. [ABSTRACT FROM AUTHOR]

In this paper, typical strengths, fault levels, and source impedances are thoroughly analyzed and calculated for the study of quality of supply in 230/400 V 50 Hz distribution systems. Considering all the disparity in distribution network design, this study is based on a comprehensive database containing typical arrangements and equipment in U.K./European systems, as well as on fully documented generic network models supplying four residential load subsectors in the U.K., i.e., from metropolitan to rural areas. Thus, this paper proposes an alternative method for determining reference values of network supply impedances and short-circuit fault levels at different points and locations of the medium-to-low voltage distribution system. The aim of this study is to provide a wider range of benchmark values than those stipulated in the IEC 60725 Standard, which only defines a single-reference threshold of public supply impedances for all types of distribution systems and residential customers. In order to assist network operators in the planning and design of their distribution systems, these values are further disaggregated and classified in this paper according to network/demand type. [ABSTRACT FROM PUBLISHER]

This paper proposes a probabilistic method aimed at estimating the voltage sag frequency of nonmonitored sites given the number of sags recorded by a limited set of voltage sag monitors. The proposed method is based on simulating a representative database of feasible pseudomeasurements by using probabilistic values of fault coefficient, fault impedance, fault type, and fault location. This information is processed through Bayesian filtering based on the Bayes' probabilistic theorem. This enables obtaining the most probable voltage sag index at nonmonitored busbars given the index measured at monitored busbars. This statistical estimation of sag indices at locations where there are no monitors constitutes the main contribution of this paper. This study is developed in three power systems: the IEEE 24-busbar reliability test system, the IEEE 118-busbar test system, and the transmission network of Ecuador with 357 busbars. [ABSTRACT FROM PUBLISHER]

This paper compares different methods for voltage-dip characterization. Those methods are based on earlier proposed algorithms for extracting three-phase characteristics (dip type, characteristic voltage, and so-called “PN factor”). The difference between the 12 methods being studied in this paper is in the way in which the time variation of those characteristics is treated to result in single-event characteristics. The methods are applied to 259 measured voltage dips, and the performance of the different methods is compared. It is found that small differences in the method can result in big differences in results. From the comparison, two methods are selected and recommended for inclusion in international standards. [ABSTRACT FROM PUBLISHER]

In this paper, we present a new approach for solving the problem of power quality monitors (PQMs) optimal allocation, for the monitoring of short-duration voltage variations caused by a fault condition in a power grid system. The problem is treated in a multiobjective perspective with two optimal criteria: minimization of the number of PQMs and maximization of the number of identified faults. Non-identification of an event can occur as a result of symmetry conditions in the network, i.e., in cases where two or more faults generate the same signals in some buses, which leads to ambiguity in the monitoring results. Symmetry increases the complexity of both the problem formulation and solution. The problem is described as a multiobjective discrete optimization problem and is solved by the algorithm for bicriteria discrete optimization within reasonable computational time. That approach was tested in power grids of different characteristics and sizes. The results demonstrate the proposed methodology applicability for solving the problem in real-size networks. [ABSTRACT FROM AUTHOR]

Computational methods for predicting voltage sags consider their occurrence as a Poisson process, although without confirmation from monitoring data, until now. In this paper, the stochastic nature of voltage sags is analyzed and discussed using monitoring data from 60 sites of the Portuguese Transmission Network, covering the years 2011–2015 (a total of 17 157 recorded voltage sags). A mathematical model to describe the voltage sag occurrence as a stochastic process is presented. The assumption of constant failure rates of the network elements implies that the occurrence of voltage sags is a Poisson process. However, that assumption is not valid if voltage sag clusters are included, as these imply considering time-dependent failure rates. Then, the time between voltage sags is described by an exponential distribution, if clusters are not included, and may be described by the gamma distribution, if including clusters. The boundaries of the adequacy of exponential and gamma distributions are assessed, based on monitoring data. The time of occurrence of monitored voltage sags are analyzed and results confirm that the Poisson process describes the occurrence of voltage sags when voltage sag clusters are disregarded. The gamma distribution fitting is also confirmed when clusters are included in the analysis. [ABSTRACT FROM AUTHOR]

Voltage sags and swells are power quality events commonly observed in power systems; however, none of the existing methods allows determining their point-on-wave inception and recovery instants (and consequently, their duration) accurately in all cases. The primary goal of this paper is to determine these instants with little or no delay, by calculating the absolute rms voltage difference between two adjacent sliding windows. The proposed method is based on the assumption that this difference is maximum when the sample under analysis corresponds to either the inception or the recovery instant. This method is valid for both sag and swell events, with or without transients. Evaluation of the proposed method performance for different types of events shows that it is robust and highly accurate in determining the inception and recovery instants. The estimation error for the majority of the events analyzed is either zero or one sample (each sample corresponds to a phase angle difference of 2.81 $^\circ$), while the worst performance is three samples. [ABSTRACT FROM AUTHOR]

This paper discusses the sensitivity of programmable logic controllers (PLCs) to voltage sags based on mechanistic analysis and numerous tests. First, the sag sensitivity caused by different values of rated working voltage is shown through an analysis of the power supply and discrete input/output modules of PLCs. Second, a detailed sag-tolerance experimental scheme is proposed, and PLC sensitivity curves that differ from early curves are established through several experimental tests. Experiments are conducted with nine typical PLCs. The sensitivity of the power supply and discrete input modules are researched first, and then, the effect of the key characteristics of voltage sag, namely, magnitude, duration, points-on-wave of sag initiation, odd harmonics, multiple sags, and continuous sags, is tested. The uncertain area of the PLC sag tolerance is identified after intensively processing the extensive experimental data and curve fitting. A more accurate sag influence assessment for PLCs can be made with the given curves compared to the early curves. Furthermore, experiments have revealed that certain PLCs may work in an uncertain status during voltage sags. The reason and the method of elimination with the proper choice of dc bus capacitance for PLCs are also analyzed. [ABSTRACT FROM AUTHOR]

The power direction method is a widely used method for harmonic source identification. This method can work with no harmonic impedance information but is not theoretically rigorous. To overcome this theoretical shortcoming of the traditional power direction method, six existence criteria and two dominance criteria are proposed in this paper. These are all sufficient conditions that are mathematically derived based on the widely used utility–customer equivalent circuit. The existence criteria can be used to determine whether some harmonic source exists on the customer or utility side, and the dominance criteria can be used to identify whether the customer side is the dominant harmonic source. These criteria can be regarded as an improved power direction method, and the key improvement is the establishment of a rigorous theoretical basis. These criteria were tested using a MATLAB program, and no counterexample was found. Their practical application for harmonic source identification in a transformer substation is also introduced and discussed. [ABSTRACT FROM AUTHOR]

This paper proposes a voltage sag estimation approach based on a deep convolutional neural network. The proposed approach estimates the sag magnitude at unmonitored buses regardless of the system operating conditions and fault location and characteristics. The concept of system area mapping is also introduced via the use of bus matrix, which maps different patches in input matrix to various areas in the power system network. In this way, relevant features are extracted at various local areas in the power system and used in the analysis for higher level feature extraction, before feeding into a fully-connected multiple layer neural network for sag classification. The approach has been tested on the IEEE 68-bus test network and it has been demonstrated that the various sag categories can be identified accurately regardless of the operating condition under which the sags occur. [ABSTRACT FROM AUTHOR]

Assessing voltage fluctuations in the power system is necessary to prevent negative impacts to customers and the connected equipment. A flickermeter can be used to measure a flicker severity level that is based on the total voltage fluctuation present at a measurement point. The flickermeter can only assess the total flicker level at the measurement point; it cannot identify any individual network user's contribution. For the purposes of limit compliance assessment, it is essential to have the ability to determine each user's contribution. A measurement algorithm for accomplishing this task is presented in this paper. The algorithm is based on using measured voltages and user currents to determine the user complex power, and from the complex power and an online estimate of the network impedance, estimate the voltage fluctuation that the user would produce, if acting alone, at the measurement point. These estimated voltage fluctuations are used as inputs to the flickermeter so that the resultant flicker severity value reflects only the contribution of the individual user. Fourteen weeks of field test results are correlated with user demand data to demonstrate the effectiveness of the approach, and the traditional concepts of weekly percentiles, flicker summation, and background flicker are employed to validate the results. [ABSTRACT FROM AUTHOR]

Harmonic loads and unbalanced loading in a distribution transformer lead to higher losses, an increase in temperature that leads to premature failure, and a shorter lifetime. To prevent these difficulties, the rated power of a transformer supplying nonlinear loads or unbalanced load is reduced; commonly known as derating. In this paper, an alternative to traditional loss based derating is proposed. It is shown that derating might actually not be required when contrasted with the traditional approach. A perspective is presented on how to make the kVA rating “adaptive” and utilize the same kVA rating with or without harmonics. Actual field data are presented to support this viewpoint. [ABSTRACT FROM AUTHOR]

Voltage fluctuations caused by rapid reactive power consumption of loads and photovoltaic or wind generator output variations may produce flickers and cause undesired effects on electric power components and human eyes. This paper presents a hybrid approach for voltage fluctuations assessment by using a synchrosqueezing transform-based algorithm. The proposed method first gives characterization of voltage fluctuations through accurate extraction of the measured voltage envelope by the Hilbert transform. The synchrosqueezing transform and an unsupervised clustering method called mean shift are then applied to determine the number of frequency components and corresponding frequencies. It is followed by the implementation of the bandpass filters to detect the magnitude of each frequency component. The proposed hybrid method is tested by both simulations and field measurements. Results compared with other commonly seen methods show that the proposed method provides a more accurate assessment. [ABSTRACT FROM AUTHOR]

Inductive coordination between power lines and pipelines has focused on the 60/50 Hz component in the past. Through field measurements and circuit-model based analysis, this research shows that harmonic currents in power lines can cause more significant voltage induction on pipelines. It is due to the relatively higher frequency and zero-sequence nature of some of the harmonic currents. In view of the findings, this paper presents two methods for assessing the severity of harmonic voltage induction. One is a circuit-model-based method and the other is a field measurement-based method. [ABSTRACT FROM AUTHOR]

This paper proposes solutions to the problem of voltage sags caused by distributed generators anti-islanding protection. This is a recently identified power-quality (PQ) concern resulting from the increasing penetration of distributed generators into distribution networks. This problem has been previously investigated by the authors and it was revealed that it can seriously affect PQ indices as well as distributed generators reconnection procedures. However, no solutions have been formerly proposed or investigated. In this context, this paper investigates several potential solutions to be adopted by the utility and/or by the DG owners and presents a thorough evaluation of their main advantages and disadvantages. Besides this investigation, simple step-by-step procedures for assessing the effectiveness of each solution are described. The presented studies and procedures will guide the utility engineers in decision-making process associated with the choice of reasonable mitigation techniques to this problem. [ABSTRACT FROM PUBLISHER]

Inrush currents in transformers can have very disruptive effects, such as voltage sags, false tripping of the protective devices, and mechanical stresses in the transformer windings. This paper shows that there are operating situations that may cause a transformer to draw abnormally high inrush currents. Examples include the normal operation of offline uninterruptible power-supply (UPS) systems, interruptions, voltage sags, and notching. These conditions may produce inrush-like currents of more than twice the value of the “normal” maximum inrush caused by energizing at voltage zero-crossing. For this condition, the term “phase-hop” is used in this paper. Laboratory experiments were performed on four different transformers (1 kVA) with varied characteristics and show the impact of phase-hop in the magnitude of inrush currents. The experiments are also used to validate the Electromagnetic Transients Program model used for the analysis of multiple cases. In addition, the behavior of the magnetic flux in a transformer under phase-hop is investigated and compared with different operating conditions using finite elements. The results of this paper have implications in transformer design and in the operation and design of UPS systems to prevent the damaging effects of phase-hop. [ABSTRACT FROM AUTHOR]

Nowadays, modern industrial processes are controlled by equipment highly sensitive to voltage sags and interruptions. Although the proposed methods in the literature allow utilities to estimate annual equipment trips due to voltage sags and permanent interruptions, they do not assess explicitly momentary interruptions. This paper proposes a probabilistic methodology that includes the attempts of reclosers to clear nonpermanent faults during the estimation of annual voltage sags and momentary interruptions at each busbar. Furthermore, voltage-tolerance ranges for each type of equipment are taken into account to quantify annual equipment trips due to voltage sags and momentary interruptions for each customer in the distribution network. The results allow the utility to identify a geographic area where sets of customers face high equipment trips during the year. The proposed methodology can be used by utilities to prioritize corrective measures to improve the quality of service to those customers most affected by annual equipment trips. [ABSTRACT FROM PUBLISHER]

Significant financial losses due to voltage sags, short duration interruptions (SDIs), and long duration interruptions (LDIs) have been reported over the past years. The methods proposed in the literature allow us to estimate the annual indices related to process trips, though, as they are confined to voltage sags and LDIs, they do not take into account SDIs. This paper proposes to estimate annual financial losses related to process trip (FLPT) for each customer in the distribution system. The methodology correlates the attempts of reclosers to clear nonpermanent faults along with the concept of process immunity time in order to separately estimate the annual FLPT due to voltage sags, SDIs, and LDIs. This division allows the utility to quantify the severity of each of them and identify geographic areas where mitigation solutions should be deployed to avoid customer complaints and penalties by regulatory agencies. [ABSTRACT FROM PUBLISHER]

Quantifying the effect of voltage fluctuations in power systems using a flickermeter is documented in IEC 61000-4-15. Modern digital implementations of a flickermeter take continuous time voltage samples as inputs to a software processing algorithm which ultimately calculates the short-term flicker severity Pst . While this process is perfectly suited for real-time measurement applications, it is less useful for off-line applications typically encountered during planning studies or any other application where continuous time sample data are not available. Because rms values are typically produced as a result of planning calculations and cycle-be-cycle rms values are much easier to store over long time periods for later use in off-line evaluations, it is appropriate to consider the performance of the flickermeter algorithm with rms voltage inputs so that additional applications can be considered. The analytical evaluation of flickermeter performance with rms voltage inputs is presented in this paper. The conclusion is that the flickermeter algorithm performs accurately with these inputs provided that the fluctuation frequency is low, but becomes inaccurate for higher frequency fluctuations. This analytical conclusion is supported by both simulation and field test results at three sites. [ABSTRACT FROM AUTHOR]

A stochastic method is presented in this paper to estimate the future voltage unbalance in a low-voltage distribution network with high-penetration of single-phase photovoltaic inverters (PVIs). The location and phase allocation of the PVIs are considered as input parameters for the stochastic simulation. The method has been applied to three different low-voltage networks: two in Sweden and one in Germany. In the Swedish networks, for 6-kW single-phase PVIs, it is likely that the contribution from single-phase photovoltaic inverters to the voltage unbalance exceeds 1%. The 2% value is unlikely to be exceeded. In the German network, for 4.6-kW single-phase PVIs, the voltage unbalance is between 1.35% and 2.62%. The risk of high-voltage unbalance can be reduced by a combination of controlled distribution over the phases and a reduction of the maximum size for a single-phase PVI. [ABSTRACT FROM PUBLISHER]

Since photovoltaic (PV) generators have no mechanical inertia, fast solar irradiance changes associated with cloud transients cause instantaneous variations in PV power output, resulting in rapid and possibly significant voltage changes in systems with high penetration of PV generators. In addition to characterizing this emerging power-quality issue, this paper proposes and investigates techniques to mitigate these fast voltage variations caused by cloud transients in distribution system with PV solar farms. The investigated approaches are based on the PV inverter capacity for fast reactive power compensation without the need to oversize the inverter or curtail the active power by using only the available reserve. Real measurement data characterize the issue and time-series power-flow simulations evaluate the mitigation techniques. [ABSTRACT FROM PUBLISHER]

This paper investigates the factors that influence the outcome of financial-loss analysis in voltage sag studies. Focusing on industrial processes, this paper considers the effects of voltage sag profile estimation, plant-load profile modelling, and process cycle modelling on the financial risk associated with process failure. Based on voltage sag monitoring records, sag profile is probabilistically modeled for the estimation of future sag characteristic. Variations in financial risk due to varying process activities in different stages of a process (process cycle) and load profile are also investigated. The effects of the individual factors are analyzed through Monte Carlo simulation. The stochastic net present value method is introduced as a financial analysis tool. [ABSTRACT FROM AUTHOR]

Voltage dips and interruptions may cause major economic damage, not only can they create considerable loss of production (manufacturing) or data (ICT), there is also loss of market, the loss of client trust, comfort, etc. that incite energy consumers to implement certain forms of protection for their systems. However, the selection of the most cost-effective mitigation method is difficult because of the wide range in available protection devices and the mostly unknown and variable interruption cost. In this paper, the theoretical selection method is compared with the currently used selection methods in industry, showing a discrepancy between theory and practice. Using an alternative selection method, an optimal mitigation method can be found. This paper describes this method and its use by means of a practical example. The case examined is the protection of a 250 kVA installation, sensitive to voltage dips (ICT load). The proposed method makes a cost-benefit analysis of several proposed mitigation solutions resulting in an overview giving the optimal solution for a certain interruption cost interval. It enables to clearly interpret the investment costs and to compare completely different mitigation methods, only using data available to an industrial customer. [ABSTRACT FROM AUTHOR]

This paper proposes a new connection for a unified power quality conditioner (UPQC) to improve the power quality of two feeders in a distribution system. A UPQC consists of a series voltage-source converter (VSC) and a shunt VSC both joined together by a common dc bus. It is demonstrated how this device is connected between two independent feeders to regulate the bus voltage of one of the feeders while regulating the voltage across a sensitive load in the other feeder. Since the UPQC is connected between two different feeders (lines), this connection of the UPQC will be called an interline UPQC (IUPQC). The structure, control and capability of the IUPQC are discussed in this paper. The efficacy of the proposed configuration has been verified through simulation studies using PSCAD/EMTDC. [ABSTRACT FROM AUTHOR]

This paper presents an improved design method for a novel transformer inrush current reduction scheme. The scheme energizes each phase of a transformer in sequence and uses a neutral resistor to limit the inrush current. Although experimental and simulation results have demonstrated the effectiveness of the scheme, the problem of how to select the neutral resistor for op- timal performance has not been fully solved. In this paper, an analytical method that is based on nonlinear circuit transient analysis is developed to solve this problem. The method models transformer nonlinearity using two linear circuits and derives a set of analytical equations for the waveform of the inrush current. In addition to establishing a set of formulas for optimal resistor determination, the results also reveal useful information regarding the inrush behavior of a transformer and the characteristics of the sequential energization scheme. This paper also proposed a method, the use of surge arrester, to solve the main limitation of the sequential phase energization scheme—the rise of neutral voltage. Performance of the improved scheme is presented. [ABSTRACT FROM AUTHOR]

This paper is the first part of a work aimed at predicting the voltage sag performance of distribution networks using digital simulation and an EMTP-like tool. The paper discusses modeling guidelines for representing power distribution components in voltage sag simulations and details the approaches selected in this work. Different models for representing some components can be used to obtain voltage sag characteristics with similar accuracy. Test results are included to illustrate the performance of the developed models and support the selection of different modeling approaches. [ABSTRACT FROM AUTHOR]

Power quality indices have been developed to assess the performance of a single site or an entire system. They can guide utilities to improve their systems and can be used by manufacturers to decide about equipment immunity. This paper is the third part of a joint paper aimed at predicting the voltage sag performance of distribution networks by estimating voltage sag indices. Two types of indices are analyzed in the present paper, they give, respectively, the average number of events and the average interruption duration of each kilovolt ampere served at the low-voltage level over the assessment period. Their values are obtained from a stochastic prediction using a time-domain simulation tool. The methodology used to obtain these indices and the analysis of the influence of the protective devices are the main contributions of this paper. The limitations of the present work are also discussed. [ABSTRACT FROM AUTHOR]

In contrast to most other power-quality phenomena, it requires several years of measurements to obtain a good insight of the voltage sag problems at a given grid connection point. In some countries, sag measurements at high- or medium-voltage level of several years are available. This paper analyzes how to calculate the sag characteristics at lower voltage levels if the sags measured at higher voltage levels are only characterized as the largest value of the voltage drop in each phase. Whether these values are based on phase-to-phase or phase-to-neutral voltages turns out to be an important parameter and is therefore analyzed in detail. Based on this analysis, this paper gives guidelines on how new sag meters should preferably be connected (measuring phase-to-phase or phase-to-neutral voltages). [ABSTRACT FROM PUBLISHER]

This paper presents a theory to explain the characteristics of a sequential phase energization based inrush current reduction scheme. The scheme connects a resistor at the transformer neutral point and energizes each phase of the transformer in sequence. It was found that the voltage across the breaker to be closed has a significant impact on the inrush current magnitude. By analyzing this voltage using steady-state circuit theory, the simulation and experimental results presented in a companion paper are explained. The results lead to the establishment of a guide to select the optimal value of the neutral resistor. The applicability of the proposed scheme to different transformer configurations has also been investigated in this paper. It is shown that the idea of sequential phase energization leads to a new class of techniques for limiting switching transients. [ABSTRACT FROM PUBLISHER]

An interline dynamic voltage restorer (IDVR) is a new device for sag mitigation which is made of several dynamic voltage restorers (DVRs) with a common dc link, where each DVR is connected in series with a distribution feeder. During the sag period, active power can be transferred from a feeder to another one and voltage sags with long durations can be mitigated. IDVR compensation capacity, however, depends greatly on the load power factor, and a higher load power factor causes lower performance of IDVR. To overcome this limitation, a new idea is presented in this paper which enables reducing the load power factor under sag conditions and, therefore, the compensation capacity is increased. The proposed IDVR employs two cascaded H-bridge multilevel converters to inject ac voltage with lower total harmonic distortion and eliminates the necessity to low-frequency isolation transformers in one side. The validity of the proposed configuration is verified by simulations in the PSCAD/EMTDC environment. Then, experimental results on a scaled-down IDVR are presented to confirm the theoretical and simulation results. [ABSTRACT FROM AUTHOR]

This paper presents a new method for the classification and characterization of voltage dips and swells in electricity networks. The proposed method exploits unique signatures and parameters of three-phase voltage signals extracted from the polarization ellipse in 3-D coordinates. Five ellipse parameters, which include azimuthal angle, elevation, tilt, semi-minor axis, and semi-major axis, are used to classify and characterize voltage dips and swells. Seven types of voltage dips, which include a total of 19 groups of dips incorporating different kinds of balanced (three-phase dips) and unbalanced (single-phase or double-phase) dips, are identified and successfully classified using the 3-D polarization ellipse parameters. Two types of voltage swells, which include a total of 6 groups, are also classified using the proposed method. The proposed method is validated using real measurement data, recorded waveforms provided by the IEEE 1159.2 working group, and the data of unbalanced dips associated with phase-angle jumps, voltage drops, and rotations due to loading effects. [ABSTRACT FROM AUTHOR]

This paper presents a probabilistic harmonic simulation method to study the power-quality impact of electric vehicles (EVs). The proposed method takes into account the random operating characteristics of the vehicles, such as random charging time, charging duration, and vehicle locations. A multiphase model of the primary and secondary systems is used to represent the study system. Extensive case studies have been conducted using the method and the measured EV data. The results reveal that the current versions of plug-in hybrid EVs have no significant harmonic impact on power systems. However, the Level 1 charger can increase the neutral-to-earth voltage at homes which could lead to increased stray voltage incidents. [ABSTRACT FROM AUTHOR]

Monitoring of voltages and currents at system buses gives the network operators information about the performance of their network, both for the system as a whole and for individual locations and customers. There is also demand from the customers and the regulatory agencies to provide information on the actual power-quality (PQ) level. Developments in enabling technology have made it possible to monitor at a large scale and to record virtually any PQ parameter of interest. While many network operators are installing monitoring equipment and while more and more monitors are available, there is a lack of knowledge and agreement on a number of aspects of the monitoring process and on processing the recorded data. As a response to this lack of uniformity in approach, data acquisition, and processing, in February 2011, CIGRE and CIRED established the Joint Working Group C4.112: “Guidelines for Power quality monitoring—measurement locations, processing and presentation of data.” In order to identify the current international industry practice on PQ monitoring, the group carried out a survey in 43 countries across the world. This paper summarizes the key findings from 114 responses to the questionnaire and identifies prevalent industrial practice in PQ monitoring around the world. [ABSTRACT FROM AUTHOR]

A fault-location method for radial distribution systems is proposed in this paper. The proposed method uses voltage and current phasors from feeder root and voltage sags measured at sparse nodes along the feeder, and pinpoints faults to the nearest node. Decision-tree (DT)-based fault segment identification is introduced before the process of node selection to reduce the computational complexity and improve fault-location accuracy. The method has been implemented on a practical distribution system and tested under a large number of fault scenarios. Test results are compared with those from the traditional voltage-sag-based fault-location algorithm using the same inputs, and the conclusion is that the proposed method can achieve more reliable results while maintaining computational simplicity. A quantitative method to suggest the optimal placement of measurement units based on the DT variable importance is proposed at the end. [ABSTRACT FROM PUBLISHER]

The method presented in this paper addresses the problem of voltage sag state estimation. The problem consists in estimating the voltage sags frequency at nonmonitored buses from the number of sags measured at monitored sites. Usually, due to limitations on the number of available voltage sag monitors, this is an underdetermined problem. In this approach, the mathematical formulation presented is based on the fault positions concept and is solved by means of the singular value decomposition technique. The proposed estimation method has been validated by using the IEEE 118 test system and the results obtained have been very satisfactory. [ABSTRACT FROM PUBLISHER]

This paper investigates the voltage sag caused by distributed generation anti-islanding protection. This is a new power-quality concern associated with distributed generation expansion. Anti-islanding protection aims to disconnect distributed generators immediately after the opening of a recloser. As a consequence, after the reclosing operation, voltage sag will occur in some parts of the distribution system, since the loads are energized immediately but the generators are no longer present. In this context, this work presents the findings on our investigations about this problem. The results show that the resulting voltage sag level can violate power-quality limits. Thus, a set of indices has been introduced to characterize the severity of the voltage sag, and a load flow-based method has been proposed to predict the phenomenon. Sensitivity studies are also conducted in order to identify some key factors that influence this type of voltage depression. [ABSTRACT FROM PUBLISHER]

This paper presents a new algorithm, namely, a three-phase-angle algorithm, for accurate characterization of three-phase unbalanced voltage sags, associated with line-to-line and single-line-to-ground faults, from recorded voltage waveforms. This new algorithm uses the relation between the angles of three-phase voltages and the angle of characteristic voltage. The performance of the proposed algorithm is evaluated by considering numerical examples and recorded voltage waveforms available with the IEEE database. A thorough comparison of the results, obtained using the proposed algorithm and other existing algorithms, is made to demonstrate its efficiency. The results suggest that the proposed algorithm is advantageous in accurate characterization of voltage sags in two ways. First, it is proficient under large/small phase-angle jump as well as for solidly and resistance-grounded systems. Second, it is capable of extracting the information about faults, associated with voltage sags. [ABSTRACT FROM PUBLISHER]

This paper proposes voltage-shift acceleration control for islanding detection of distributed-generation (DG) inverters. The proposed method is conceptually based on a traditional dq-based voltage positive feedback control. A main improvement of the proposed control is that the anti-islanding performance and its impact on inverter output control can be consistent regardless of inverter operation points. An analytical approach based on a small-signal model of an inverter controller is used to prove the proposed control. Also, an analytical design method is proposed to obtain proper control gain that satisfies two conflicting issues: successful anti-islanding and limited impact on inverter control. Simulation and experimental tests were conducted to validate the proposed detection algorithm and its design method. In addition, simulation results on a two DGs system are shown to investigate the performance in multiple DGs operation. [ABSTRACT FROM AUTHOR]

In this paper, voltage sag compensation of point of common coupling (PCC) using a new structure of fault current limiter (FCL) is proposed. The proposed structure prevents voltage sag and phase-angle jump of the substation PCC after fault occurrence. This structure has a simple control method. Using the semiconductor switch (insulated-gate bipolar transistor or gate turnoff thyristor at dc current rout leads to fast operation of the proposed FCL and, consequently, dc reactor value is reduced. On the other hand, the proposed structure reduces the total harmonic distortion on load voltage and it has low ac losses in normal operation. As a result, other feeders, which are connected to the substation PCC, will have good power quality. Analytical analysis and simulation results using PSCAD/EMTDC software and experimental results are presented to validate the effectiveness of this structure. [ABSTRACT FROM AUTHOR]

This paper discusses some of the electromagnetic-compatibility issues concerning the use of power lines for communication. Based on a series of measurements performed in a laboratory environment and in the field, five different types of interactions between communication and end-user equipment are distinguished. Measurement examples are shown for several of these types. It is concluded that the creation of a low-impedance path by end-user equipment is the main cause of interference. [ABSTRACT FROM AUTHOR]

The impact of motor starting on power quality can be assessed using detailed computer simulation studies. However, not every motor installation case needs such an extensive assessment. Utility planners are interested in quick evaluation of the potential impact of a motor installation proposal. Based on the findings, they can then determine if detailed case studies and what types of case studies are necessary. This paper presents three charts for motor starting planning according to three power quality concerns. These concerns are the amount of voltage drop caused by motor starting, the compliance to the ITIC curve, and the compliance to the IEC flicker meter limits. These charts can help utility planers to conduct quick and first-cut assessment of a motor starting situation. They also reveal the key factors affecting the motor starting related power quality concerns. The principles behind these charts are explained. Examples are given to show how to use them for quick assessment of motor starting impact. [ABSTRACT FROM AUTHOR]

This paper presents a high-efficiency voltage regulator, which combines robustness, low costs and easy maintenance without power electronics components. These characteristics make it suitable for rural networks, where investments and operational cost in power quality improvement are limited. The regulator consists of a multi-winding reduced-power transformer, and provides serial voltage compensation. Different voltage compensation steps are obtained by modifying the connection and the polarity between the primary and secondary windings. The transformer design has been optimized to obtain a high-efficiency and low-cost regulator. An automatic controller monitors the output voltage and sets the optimal compensation step. At present more than 400 units of the voltage regulator are in operation. Field test results are presented to show the operation of the voltage regulator. [ABSTRACT FROM AUTHOR]

FACTS based devices are proven to be an efficient mitigation solution for voltage sag prevention. The high cost of FACTS based devices often prohibits their wider deployment within power networks. This paper presents an approach for comprehensively assessing the financial benefits to the network resulting from their use. The annual financial losses of the entire network due to voltage sags are used as a savings target. The three most widely used FACTS based devices for voltage sag mitigation, namely, Static VA Compensator (SVC), Static Compensator (STATCOM) and Dynamic Voltage Restorer (DVR) are then optimally placed using a Niching Genetic Algorithm (NGA). The aim is to reduce overall financial losses in the network due to voltage sags. The cost of the individual devices along with their installation costs and annual maintenance are taken into account in the optimization procedure. Since this methodology is largely based on an economic evaluation of the solution, several conventional economic analysis methods are utilized. Simulations are performed on a 295-bus generic distribution network (GDN). [ABSTRACT FROM AUTHOR]