Voltage Tolerance Curve Research Articles

Voltage sag sensitive load type identification based on power quality monitoring data

This paper focuses on identifying voltage sag-sensitive loads within unknown load type or when the load is already in operation. To achieve this, a new method of sensitive load identification based on power quality monitoring data is proposed. Firstly, the active power RMS monitoring data is used as the base data. The Hodrick-Prescott filtering and sliding mean segmentation are used to divide the period of the voltage sag event. Next, based on the division result, the differences of steady-state power quality monitoring data before and after each event are calculated as the dataset. The dynamic K-means is used to divide various load action areas. Finally, the voltage tolerance curves of each action area are fitted and compared with the preset curves, then according to the constituted rules in this paper, the type of sensitive load contained by user is recognized. The feasibility and accuracy of the proposed method are verified by analyzing the simulation examples and actual power quality monitoring data.© 2017 Elsevier Inc. All rights reserved.

Economic analysis of annual load loss due to voltage sags in industrial distribution networks with distributed PVs

In an industry, analysis of load disconnection indices is essential to determine the annual economic loss because of the high cost of load interruptions. With the interconnection of PVs in modern industrial networks, this analysis gets more complicated due to controls associated with PV inverters for reactive power support. Hence, the paper proposes a unique method for such analysis that relies on dynamic simulation to calculate load disconnections. The dynamic simulation allows the integration of the voltage tolerance curve of loads and controls associated with PVs making the scenarios more realistic. The indices are determined by employing statistical experiments which account for annual variability of daily solar and load profiles with the least possible runs of dynamic simulation. The proposed method is applied to calculate load disconnection indices for an IEEE 399-1997 industrial distribution system. A comparative analysis of the cost of disconnection under a variety of solar penetration and operation scenarios is presented. Network planners and reliability engineers can potentially utilize the proposed method to identify PV inverter controls, placement, and size effects on the annual load disconnection indices due to voltage sags.

A Low-Data-Dependence Assessment Method of Industrial Process Interruption Probability Due to Voltage Sag

Information about the interruption probability of sensitive industrial processes is important for devising mitigation strategies against voltage sags. However, the existing method for assessing interruption probability is less practical, owing to the difficulty in collecting technical information and data from industrial users. This study proposes a low-data-dependence assessment method for the interruption probability of an industrial process. First, it establishes a general assessment model for the tripping probability of sensitive equipment. Second, it proposes a parameter estimation method to determine the probability density function of the voltage tolerance curve of sensitive equipment in the uncertainty region, according to the equipment trip information recorded by the user. This improves the accuracy of the assessment. Third, this study establishes the logic function expression form of the industrial process structure to describe the relationship between the process interruption probability and the equipment tripping probability. Finally, it proposes a solution method for the process interruption probability based on a heuristic search algorithm, which overcomes the problem of technological process information acquisition, such as the logical relationship among various equipment in the industrial process. The rationality and applicability of the proposed method are verified through a case study.

Tolerance of Electromagnetic Relay to Voltage Sags and Short Interruptions

This paper studies the tolerance of electromagnetic relay (EMR) under voltage sag and short interruptions on the basis of response mechanism analysis and the extensive tests. First, it introduces the structure of EMR and proposes response mechanism of EMR under voltage sag. Then, a detailed test plan is presented, including the information of test platform, testing condition, EMRs used in test, list of test, test procedure, and the measured waveforms. Magnitude and duration of the sags are not only the characteristics to be considered to investigate EMR’s tolerance. The other factors, which may have significance influence on tolerance of EMR, are considered here, including point-on-wave (POW), phase angle jump (PAJ), harmonic, magnitude variation in pre- and post-sag segments, two-stage sag, and slow recovery sag. Extensive tests results are presented in the form of voltage-tolerance curves (VTCs). Besides magnitude and duration, POW, PAJ, and two-stage sag have a significant influence on the tolerance of EMR. Other factors only have a tiny impact on the tolerance of EMR. The results show that the magnitude tolerance of EMR is 48–74% of Unom, and duration tolerance is 5–28 ms; they are useful for the technical assessment of EMR’s tolerance to voltage sags and short interruptions, and for the economic assessment of the industry process trip due to its disengagement. Test results also benefit for choosing proper EMR and mitigation device in the complicated operating environment.

An evaluation system for calculating load loss caused by the tripping of low voltage releases

Voltage sag may cause large load losses due to the acting of low voltage releasers, resulting in serious economic losses. This paper proposes a load loss evaluation system to calculate the load loss caused by the acting of low voltage releasers. In the proposed evaluation system, the voltage tolerance curves are presented to express the operating features of low voltage releasers. The impact of low voltage releasers with different parameters on the load loss is fully considered based on the voltage tolerance curves. The results illustrate that the proposed load loss evaluation system can accurately calculate the load loss caused by the acting of low voltage releasers.

A Voltage Sag Severity Evaluation Method for the System Side Which Considers the Influence of the Voltage Tolerance Curve and Sag Type

Considering the influence of user equipment voltage tolerance characteristics and sag types on the evaluation results, this paper proposes a voltage sag severity evaluation method for the system side which considers the influence of the voltage tolerance curve and sag type. As such, a quantitative evaluation of the severity of voltage sag events can be achieved. Firstly, the user’s voltage tolerance curve is used to construct the comparison reference value of the energy index, in order to realize the rapid analysis of the severity of the sag event in the normal area and the abnormal area. Secondly, aiming at the problem of insufficient descriptions of the severity difference of sag events in uncertain areas, an improved energy index evaluation model combined with user tolerance characteristics is established through an interval division and interval weight calculation, so as to divide and evaluate the severity of sag events in uncertain areas. Considering the influence of the sag type on the voltage tolerance curve and user equipment, the energy index correction factor is then constructed, and the measurement function is used for an interval evaluation to obtain the ranking result of the voltage sag severity, which is more in line with the actual situation. Finally, the rationality and effectiveness of the proposed method are verified by analyzing 24 voltage sag events at a monitoring node.

Practical tables for tolerance assessment of adjustable speed drives under voltage sag

The tolerance of adjustable speed drive (ASD) under voltage sag is important for users to select equipment and take mitigation methods. However, it is not easy for users to obtain the tolerance information of ASD. To address this issue, this paper proposes practical tables for tolerance assessment of ASD under voltage sag, providing a simpler and faster way for an approximate understanding of the tolerance of ASD. First, this paper proposes a calculation method for three trip characteristics to describe the tolerance of ASD. Second, this paper presents a voltage tolerance curve (VTC) assessment model based on the three trip characteristics, and the coordinate of the “knee point” of VTC can be assessed straightforwardly. Furthermore, the practical tables for tolerance assessment of ASD are formed based on the studies mentioned above. Users can obtain the tolerance of ASDs by looking up the table when they know several key parameters. Simulations tests verify the effectiveness of the calculation method and practical tables.

부하의 전압 내성 특성을 고려한 순간전압강하 평가 연구

This paper presents a novel method to assess voltage sags by taking into account the voltage tolerance characteristic of loads. In general, the tolerance characteristic is expressed in the form of a two-dimensional curve of voltage magnitude and duration. Also, the shape of characteristic curves depend on the type of loads. In order to estimate the expected sag frequency affecting sensitive loads, it is necessary to perform a voltage sag assessment considering the voltage tolerance curves of loads. However, because most existing methods of voltage sag assessment only focus on voltage sag magnitudes, these methods have limitations in reflecting various types of tolerance curves. In this paper, an effective method for reflecting various thresholds of sag magnitude and duration is proposed. The proposed method performs voltage sag assessment based on the area of vulnerability for given voltage thresholds and sag durations are estimated using a probability model on the historical operation data of protection system. In addition, we propose an effective assessment scheme based on the concept of measuration by division for a slope-shape tolerance curve.

Evaluation Scheme of Voltage Sag Immunity in Sensitive Industrial Process

In order to enhance the effectiveness of voltage sag governance and the practicability of the process immunity assessment method, an immunity assessment method for the assembly line process is proposed, fully considering the relationship between logistics line and power line, and thoroughly portraying the relationship between equipment and its links in depth. Combined with the existing voltage tolerance curve test results, the power line immunity time is reasonably evaluated considering the response delay of each device. In addition, the fault probability is quantified considering the possibility of equipment failure and its link repairing. On this basis, the minimal cut-set method is adapted to standardize the constructed random network of logistics lines. The immunity of the whole manufacturing process is evaluated through series-parallel logical analysis. In the end, the paper mill is taken as a sample to illustrate the proposed method. The deviation between the average annual economic loss obtained from model evaluation and this typical producer’s survey loss is regarded as the basis for the measurement of the effectiveness of the method. The results show that the immunity of the process can be accurately evaluated for the deviation of the evaluation obtained by the proposed method is relatively small.

Optimal coordination of directional overcurrent relays in distribution systems with DGs and FCLs considering voltage sag energy index

Parallel analysis of power quality and protection coordination has become an important part of modern power system studies. Nowadays, the coordination of directional overcurrent relays (DOCRs) is significantly affected by the emerging of distributed generations (DGs). Using voltage sag energy index, a new approach is proposed for optimization of DOCRs coordination in distribution systems in the presence of DGs, Fault current limiters (FCLs), and voltage-sensitive buses. This approach aims to minimize an objective function which comprises three key elements: operating time of relays, voltage sag energy index, and voltage sag duration. The impedance of FCL and the time setting multiplier, current setting and characteristic of DOCRs are the optimization variables. Voltage tolerance curve and voltage sag energy index are also used to show the improvement of voltage sag characteristics and prioritization of answers. Simulation results show that, the operating time of DOCRs and voltage sag characteristics are significantly improved.

A Sensitive Industrial Process Model for Financial Losses Assessment Due to Voltage Sag and Short Interruptions

The financial loss assessment of industrial users due to voltage sag and short interruptions is important for distribution network performance evaluation and power quality enhancement scheme design. Existing assessment models are highly dependent on complete industrial process information, which is difficult to obtain. To solve this problem, a new sensitive industrial process model needs limited industrial processes information is proposed. In the proposed model, the sensitive equipment is modeled by voltage tolerance curves (VTC), and the truth table is used to characterize the logical relationship. As the industrial process information is limited, the boundary value of VTC and partial logical function values of the truth table are uncertain. To determine the model's uncertain parameters objectively, an optimization model with the goal of minimizing the deviation of the assessment loss is constructed, and the details of the algorithm are also given out. An example based on some investigation data is given to verify the effectiveness of the model and show how to use it.

Practical Analytical Model of Adjustable Speed Drive for Tolerance Ability Evaluation of Transient Low Voltage Disturbance

The failure of adjustable speed drive (ASD), which is sensitive to transient low voltage disturbance such as voltage sag and short duration interruption, will bring considerable loss to industrial users. Evaluating the disturbance tolerance ability of adjustable speed drives is the key to improve its tolerance. The voltage tolerance curve (VTC) is a tool to quantify the tolerance of voltage sag sensitive equipment including adjustable speed drives. However, the traditional test strategy for obtaining this curve is time-consuming and may introduce errors caused by the performance deterioration of the tested equipment. This paper proposes an analytical model of adjustable speed drive based on its response mechanism analysis to voltage sags. The model constructs a group of practical formulas for calculating the knee point of the voltage tolerance curve. The knee point can be used to quickly estimate the contour of the curve. This model can help engineers to conduct a first-cut assessment of the sag tolerance for drives to select equipment or design planning. Experimental tests are carried out to verify the effectiveness of the proposed method. Moreover, an application of the results is presented to improve the traditional test strategy.

Bayesian Network-Based Process Downtime Cost Determination of an Industrial Plant

Industrial plants utilize sensitive equipment to produce their products and meet their financial targets. Equipment downtime caused by power quality issues such as voltage sag affects production and entails cost hence poses a threat to their ability to deliver their financial objectives. This research aims to determine the response of industrial equipment to sag events and quantify the downtime cost caused by interruption in the production process. The study used the voltage tolerance curve to determine the individual equipment response to sag events and the Bayesian Network to establish the network structure of the production process. The probability of process interruption and the associated downtime losses was computed using a mathematical software. The research shows a strong relationship between the equipment’s response to voltage sag events and the production downtime cost and highlights the importance of the immunity of equipment to voltage sags.

Assessment of Equipment Trip Probability Due to Voltage Sags Based on Fuzzy Possibility Distribution Function

Assessment of equipment trip is needed for proper estimation of interruption/disruption cost and voltage sag mitigation. The equipment trip depends on the severity of voltage sag and the tolerance of the equipment toward the sag. However, the occurrence of voltage tolerance of an equipment in between the two known bound levels is uncertain in nature. The existing evaluation methods for equipment trip analysis fail to properly assess this uncertain property of voltage tolerance curves. This paper presents a novel approach to assess the equipment trip by handling the uncertainties by using fuzzy probability and possibility distribution. A new method is proposed to transform a rigorously performed statistical data into a fuzzy possibility distribution function, which eliminates the ambiguity that comes with the non-standardized selection of membership function/possibility function. With the proposed method, the statistical data are used to extract the fuzzy probability distribution of voltage sag intensity, which is given by both the magnitude and time duration of voltage sags, while the concept of fuzzy probability is used to calculate the fuzzy trip probability or equipment failure probability. The proposed method is finally applied to estimate the number of trips for six different sensitive equipments connected to two practical Indian distribution systems.

Methodology for Determination of the Number of Equipment Malfunctions Due to Voltage Sags

This article deals with the assessment of the reliability of sensitive equipment due to voltage sags. The most frequent problems of power quality are voltage sags. Equipment that cannot withstand short-term voltage sag is defined as sensitive device. Sensitivity of such equipment can be described by the voltage–tolerance curves. A device (generator) to generate voltage sags (also interruptions) with duration at least 1 ms has been designed and developed for this purpose. Equipment sensitive to voltage sags was tested using this generator. Overall, five types of sensitive equipment were tested: personal computers, fluorescent lamps, drives with speed control, programmable logic controllers, and contactors. The measured sensitivity curves of these devices have been used to determine the number of trips (failures) due to voltage sags. Two probabilistic methods (general probability method and cumulative probability method) to determine probability of equipment failure occurrence are used. These methods were applied to real node in the distribution system with its actual performance of voltage sags/swells. The calculations also contain different levels of sensitivity of the sensitive equipment.

Cost Estimation of Voltage Dips in Small Industries Based on Equipment Sensitivity Analysis

Voltage dip is one of the detrimental power quality problems that can lead to huge financial losses in industries. Its economic impact is not only associated with the quality of the supply but also with sensitivity of electronic controls and equipment of the industry which are susceptible to voltage dips. Mitigating solutions are available but the choice depends on the careful assessment of the economic impact of voltage dips and economic gains of solutions. This paper presents an approach for estimating the economic cost of voltage dips based on sensitivity analysis. The voltage-tolerance curves of the sensitive equipment are obtained from experimental tests under different conditions. From the behavior and interaction of process equipment, different failure modes and economic sensitivity density are determined for different types of voltage dips. Voltage events monitored in the MV-network for several years are assessed to determine the frequency and severity of voltage dips at the customer terminal. The economic values of equipment and processes are assessed to get insight into alternative solutions with more rewarding measures. Then, cost-benefit analysis is performed to compare the economic gains of solutions protecting equipment or processes showing more rewarding economic values.

Identification of Weak Areas of Power Network Based on Exposure to Voltage Sags—Part I: Development of Sag Severity Index for Single-Event Characterization

This paper presents a new single-event characteristic, sag severity index (SSI), derived with respect to equipment sensitivity to voltage sags. It is more inclusive of associated uncertainties/variation in equipment response to voltage sags than existing single-event characteristics. By changing parameter settings, the index appropriately accounts for sag duration and adequately addresses the variation in equipment sensitivity. The value of the index changes continuously at the joining regions of different sag severity levels and reflects realistically the sensitivity trend of equipment embedded in voltage tolerance curves. The properties of the SSI are analyzed and discussed through comparison with characteristics of five previously reported single-event indices including four numerical indices and one fuzzy voltage sag index.

Test and analysis on sensitivity of low‐voltage releases to voltage sags

This study discusses the sensitivity (i.e. ride-through performance) of low-voltage releases to voltage sags on the basis of extensive tests, and presents two sensitivity models to assess the tripping characteristics of low-voltage releases, which have not been studied before. First, working principle of low-voltage release and existing standards are reviewed. Second, a detailed test scheme is proposed and ten kinds of low-voltage releases have been tested. Test results show that magnitude, duration and point-on-wave angle of voltage sag jointly determine the sensitivity of low-voltage releases. After extremely processing test results and applying the curve fitting theory, the functions representing the relationship between magnitude and duration at different point-on-wave angles are formed, which symbolise a detailed sensitivity model of low-voltage release. Moreover, the other simpler and more practical sensitivity model is proposed by introducing rectangular envelope lines of voltage tolerance curves at different point-on-wave angle.

A New Method of Evaluating the Sensitivity of Sensitive Load to Voltage Sag

This paper proposes a new method to evaluate the voltage sag sensitivity (VSS) of the sensitive load by the combination of several probability density function models, which can fully describe the randomness of load sensitivity. Firstly, five common probability density functions were respectively used to build the mathematical model of voltage tolerance curve (VTC) of sensitive load in uncertainty region. Secondly, the VSS of sensitive load was respectively evaluated by five models, and the weighted sum of five models’ results was calculated by unbiased variance method. In the end, the proposed method was verified to be rational according to an application example.

Evaluation of Voltage Sag Mitigation by Resonant Grounding Comparing to Existing Direct Grounding in Provincial Electricity Authority

This paper proposes a practical mitigation solution to the voltage sag problem by resonant grounding for a 22-kV distribution system of Provincial Electricity Authority (PEA) at the Patumthani 1 substation in Pathumthani province in Thailand. A computer simulation by ATP-EMTP was carried out to investigate the voltage sag impact at the low voltage side of the delta/wye distribution transformer connected at a fault location in the existing solidly grounded system and proposed resonant grounded system. The results indicate that the proposed resonant grounding can help all customers connected at the same bus survive from voltage sag problems and also keep the customers on the faulty feeder connected to the system being continuously supplied for single-to-ground faults. To evaluate the benefits of the resonant grounding, the different voltage-tolerance curves are proposed by Information Technology Institute Council (ITIC) curve and laboratory testing of Personel Computer (PC) curve, Programable Logig Coltroller (PLC) and Adjustable Speed Drive (ASD). The saving from the cost of customer interruption and voltage sag is calculated comparing between the existing solid grounding and the proposed resonant grounding with different voltage-tolerance curves