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This paper presents an analytical approach to the analysis of the interaction between induction motors (IMs) and voltage sags. The presented methodology enables quick assessment of the influence of IM on sag characteristics and avoids time-consuming transient simulations. The accuracy of the derived analytical formulae is verified through classical transient simulations using the PSCAD/EMTDC package. This paper further proposes a simple, yet efficient transformation of resulting nonrectangular voltage sags, based on the loss of voltage index, into equivalent rectangular sags suitable for conventional voltage sag performance benchmarking. Index Terms--Induction motors (IMs), power quality (PQ), voltage sags.

The paper describes genetic-algorithm (GA)-based optimization software for reconfiguration of a distribution network in order to minimize financial losses due to voltage sags. The developed methodology starts with a selected number of switches which generate various topologies. Load flows are then performed to evaluate the feasibility of topologies. For each feasible topology, fault analysis is performed to first calculate voltage sags at different buses in the network and then to calculate financial losses incurred by voltage sags at buses with sensitive industrial processes. The result of the optimization is the topology which yields the lowest voltage sag cost to customers. The main features of the GA include double-point crossover and adaptive mutation. The developed software tool is applied to the 295-bus generic distribution system. Index Terms--Distribution networks, genetic algorithms (GAs), optimization, reconfiguration, voltage sag costs.

This paper presents a stochastic aggregate harmonic load (AHL) model built from physical load parameters. Parameters used include nonlinear load composition, diversity factors, and nonlinear load participation in total demand. They are obtained from samples of customer load database of different sectors of customers. The aggregate load modules are developed for harmonic simulation of distribution network using commercially available software. Uncertainties in loading conditions, load composition and aggregate harmonic load parameters are taken into account separately and simultaneously to determine voltage harmonic distortion levels in distribution network. The simulations performed on a large, realistic distribution network using developed AHL modules give realistic voltage distortion levels/ranges. The results of simulations are in agreement with field measurement results. Index Terms--Distribution network, harmonics, stochastic aggregate harmonic load model.

The second part of this paper presents a practical implementation and application of the methodology for the stochastic assessment of the annual financial losses due to interruptions and voltage sags discussed in the first part of this paper. The costs of interruptions and voltage sags are determined separately and then combined in order to estimate the total financial losses in the network. The methodology is illustrated on a generic realistic distribution network with all relevant network components modeled appropriately. Finally, different network topologies are compared taking into account total financial losses in the network. It is shown that the inclusion of the financial losses due to voltage sags that may account for up to about 20% of the interruption costs may alter the network topology ranking. Index Terms--Financial losses, interruptions, power quality (PQ), reliability, voltage sags.

This paper presents a generalized methodology for stochastic/probabilistic assessment of the annual financial losses due to interruptions and voltage sags. The costs of interruptions and voltage sags are treated separately and then combined in order to estimate the total financial losses in the network per annum. The methodology proposed is applicable to both assessment of individual customer losses and assessment of total network losses. It takes into account in a probabilistic manner all the uncertainties associated with the voltage sag calculation, sensitivity of customers' equipment to voltage sags, the interconnection of the equipment within an industrial process, and customer types and the location of the process in the network. Index Terms--Financial losses, interruptions, power quality (PQ), reliability, voltage sags.

This paper discusses the uncertainty involved in the behavior of sensitive equipment used in various industrial processes and the methodology to incorporate this effect in quantifying the equipment trips due to voltage sags over a specified time period. The exact and more precise counting of equipment trips is essential for the accurate economic assessment of power quality. Existing standards and previously published test results on the sensitivity of this equipment against voltage sags are critically reviewed and possible ranges for variation in equipment sensitivity are investigated. Four different probable behaviors within respective ranges were considered for the stochastic assessment of equipment trips due to voltage sags. This study is performed on a generic power system. Index Terms--Power quality, voltage sags, voltage-tolerance curves.

This paper presents a comprehensive method for stochastic prediction of the number and characteristics of voltage sags in large distribution networks. The novelty of the proposed approach is in probabilistic modeling of the failure of the protection system. The probability of the failure of the protection system is determined using a straightforward, decision tree based method. The paper highlights the effects of the protection system failure on the number and characteristics of voltage sags. The method proposed and its capabilities are demonstrated on the realistic size generic distribution system. The results presented in the paper show that the proposed method is superior to the traditional techniques used for the assessment of voltage sags as it results in higher accuracy of the prediction of the number and characteristics of voltage sags. Index Terms--Fault tree method, power system protection, stochastic assessment, voltage sags.

This paper presents the influence of different types of transformer winding connections on the propagation of voltage sags caused by symmetrical and asymmetrical faults in the power system. Single and multiple transformers are considered. A particular propagation of voltage sags from customer buses to industry facilities through the transformers employed at the entrance of the facilities is highlighted in the study. Different winding connections, dominantly used in the U.K. distribution networks, were modeled at the service transformers. The study was performed on a generic distribution network, and a bus from the 11-kV distribution network was randomly selected. It was found that the performance of voltage sags inside the industry facility is a function of transformer connections used at the service transformer. Index Terms--Power quality, propagation of voltage sags, transformer winding connections, voltage sags.

This paper describes a new method for classification and presentation of voltage sag and short interruption performance information for individual sites and entire systems. The method expands on traditional methods by including information about the individual phase voltages during the disturbance, rather than just a single magnitude and duration. This improved method provides better information for correlation with equipment impacts associated with the voltage sags and short interruptions. The method is validated with sag and interruption data measured/recorded in actual power systems and with results of simulations on an example system. Index Terms--Classification and presentation of voltage sags and short interruptions, fault rate, power quality, system/site sag performance.

This paper analyzes the influence of modeling of fault distribution along transmission line on the assessment of number and characteristics of voltage sags. The generic distribution network was used in all simulations. Different types of transformer winding connections were modeled and different (symmetrical and asymmetrical) types of faults were simulated. A line was selected from the previously identified area of vulnerability for a given bus and different faults having different distributions along the line were simulated. It was shown that depending on the fault distribution (uniform, normal, exponential) along the line, different numbers and characteristics of voltage sags could be expected at the selected bus. Index Terms--Fault distribution, power quality, stochastic assessment, voltage sags.

This paper analyzes the influence of modeling of fault distribution along the boundary crossing lines of the area of vulnerability on the stochastic prediction of voltage sags and their characteristics. The study was performed on a generic distribution network, and all types of faults were simulated throughout the network. Two buses from the 11-kV distribution system were randomly selected and the areas of vulnerability due to each type of fault for specific sensitive equipment at the selected buses were determined. These areas were determined by assessing voltage sag performance at each phase separately. Finally, the influence of different fault distributions along the vulnerability area boundary crossing lines was considered. It was shown that the fault distribution along those lines influences strongly the number and characteristics of voltage sags at the customer connection point. Index Terms--Area of vulnerability, boundary crossing lines, fault distribution, power quality, stochastic assessment, voltage sags.

This paper discusses the sensitivity of ac coil contactors to voltage sags, short interruptions, and undervoltage transients on the basis of extensive test results. Existing standards and previously published work are reviewed, and a detailed description of the testing procedure used is presented. The following tests were performed: sensitivity to rectangular voltage sags with ideal and nonideal supply voltage, testing with two-stage voltage sags, and voltage sags which occur during starting of large motors, as well as testing against measured voltage sag. The importance of voltage sag characteristics besides magnitude and duration was also investigated. Generic sensitivity curves are proposed. Index Terms--Power quality, short interruption, undervoltage transient, voltage sag, voltage-tolerance curve.

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]

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]

This paper investigates the economic value of voltage sag mitigation devices to end users. The focus is on sensitive industrial plants. Different types of power system faults are simulated on a typical British distribution network model to obtain voltage sag profiles at various locations in the network. The cost-effectiveness of various mitigation devices is investigated for industrial-size end users with various process characteristics and sensitivities, to obtain the optimal investment profile for each end-user plant. The sensitivity of the assessment to various levels of initial financial loss resulting from process interruption and network fault rates is also investigated. The findings would serve as valuable references for industrial end users who are considering investment in mitigation devices to reduce the financial losses caused by voltage sags. [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]