Sag Characteristics Research Articles

Fractional Flow Theory Applicable to Non-Newtonian Behavior in EOR Processes

The method of characteristics, or fractional-flow theory, is extremely useful in understanding complex Enhanced Oil Recovery (EOR) processes and in calibrating simulators. One limitation has been its restriction to Newtonian rheology except in rectilinear flow. Its inability to deal with non-Newtonian rheology in polymer and foam EOR has been a serious limitation. We extend fractional flow methods for two-phase flow to non-Newtonian fluids in one-dimensional cylindrical flow, where rheology changes with distance from injection well. The fractional flow curve is then a function of position and we analyze the characteristic equations for two applications—polymer and foam floods. For polymer flooding, we present a semi-analytical solution for the changing fractional flow curve where characteristics and shocks collide. The semi-analytical solution is shown to give good agreement with the finite-difference simulation thus helping us understand the development and resolution of shocks. We discuss two separate cases of foam injection with or without preflush. We observe that the fractional flow solutions are more accurate than finite-difference simulations on a comparable grid and hence the method can be used to calibrate simulators. For SAG (alternating-slug) foam injection, characteristics and shocks collide, making the fractional-flow solution complex. Nonetheless, one can solve exactly for changing mobility near the well, to greater accuracy than with conventional simulation. The fractional-flow method extended to non-Newtonian flow can be useful both for its insights for scale-up of laboratory experiments and to calibrate computer simulators involving non-Newtonian EOR. It can also be an input to streamline simulations.

Effect of protection device coordination on voltage sag characteristics of distribution networks

In this paper, ATP (a powerful software for simulation of power system transients) is used to assess the performance of a test distribution network in faulty conditions. This study includes the modeling of a distribution network considering the random nature of faults. Fault parameters are produced using a Monte Carlo procedure and initiated before network simulation. To investigate the voltage sag characteristics of the network in faulty conditions, four strategies are introduced for coordination of protective devices. Simulation results are employed to study the network behavior under these protection strategies for fault durations of 200 ms, 600 ms and 1 s. The results are analyzed and compared with ITIC curves where fuses and reclosers are used as protection devices. Simulation results show that protection device coordination has a significant effect on voltage sag characteristics of distribution systems.

Affect of Voltage Sags on Electro-magnetic Contactor -A New Parameter for Sag Characteristics-

Affect of Voltage Sags on Electro-magnetic Contactor -A New Parameter for Sag Characteristics-

Voltage Sag Effects on the Process Continuity of a Refinery with Induction Motors Loads

Problem statement: Process continuity of industrial plants (like a refinery) is subjected to several shutdowns due to voltage sags causing large induction motors to trip, either by undervoltage or by overcurrent relays, sometimes by the mechanical protection. These unplanned shutdowns cost the plant tens of thousands of dollars per shutdown in addition to material damage costs, restart charges and any penalties due to delay in product delivery and shipping. Approach: The behavior of induction motors during voltage sags was investigated. In addition to the well-known parameters of voltage sags (time and duration), other parameters such as three-phase unbalance, point on the wave of sag occurrence, harmonics, loading percentage and pre-sag voltage were also examined through computer simulations using the MATLAB/SIMULINK toolbox. Motors’ ride-through capability during sags was also studied and guidelines for adjusting the protection relays of the induction motors were highlighted. Results: Results showed that response of induction motor to voltage sags is dependant on sag characteristics, as well as motor and load parameters. Three-phase voltage sags and sags occurring at zero crossing were the most severe events. Transient currents occurring at the instants of voltage sag and voltage recovery are directly proportional to the voltage drop, not to the remaining voltage magnitude. Transient currents and torques induced at the instant of sag recovery are higher than those induced at the instant of sag occurrence. Lightly-loaded motors and motors operated at voltages higher than the nominal voltage are less affected by voltage sags. Conclusion: Manufacturers of induction motors impose strict protection settings to protect their machines from any possible damage. However, the starting process of the motor inherits starting currents and torques much higher than those occurring during voltage sags. Readjusting of the protection relay settings especially the undervoltage relay may be adequate to counteract voltage sags. No conditioning equipment was required.

Voltage Sags: Validating Short-Term Monitoring by Using Long-Term Stochastic Simulation

This paper presents a procedure to validate voltage sag results based on a short-term monitoring program and stochastic assessment of voltage sag characteristics. The main practical use of this methodology is to analyze the accuracy of sag characteristics obtained from short monitoring periods. With a Monte Carlo Simulation approach, probabilistic models of several factors are taken into account: lines and busbars fault rate, prefault voltage, fault-type distribution, fault-location uncertainty, and fault resistance distribution. Confidence intervals based on the percentile method and hypothesis tests are the statistical tools selected to perform the validation of voltage sags magnitude and frequency. A case study based on the evaluation of a six-month monitoring period shows the applicability of the proposed methodology.

Improved Fault Location on Distribution Feeders Based on Matching During-Fault Voltage Sags

A fault location algorithm for distribution feeders based on matching during-fault voltage sags is presented. The basic principle of the algorithm is based on the fact that when a fault occurs on the feeder, voltage sags propagate presenting different characteristics for each feeder node. Knowing the voltage sag characteristics, it is possible to locate the faulty node or the faulty area of the feeder. This voltage-based approach ensures the efficiency and robustness of the algorithm, which provides suitable and accurate results. An overhead, three-phase, three-wire, 13.8 kV, 134-node, real-life feeder model is used to evaluate the algorithm. Test results show that the algorithm is quite suitable due to not only the faulted point being located, but also a faulted area being identified providing important information to direct the maintenance crew seeking to repair any fault inflicted damage.

ADVANCED CHARACTERIZATION OF VOLTAGE SAGS AND ASSESSMENT OF EQUIPMENT SENSITIVITY

The paper proposes a new approach for characterization and classification of voltage sags and for the assessment of sag influence on sensitive equipment. The existing classifications of voltage sags mainly use the method of symmetrical components and describe voltage sags in terms of magnitude and duration. It has been recognized though, that the other sag characteristic may influence sensitive equipment operation. In this paper, the approach based on Odq-components method (Park's transformation) is used for sag description and equipment sensitivity assessment. This approach enables to represent voltage sags as a function of time, magnitude, instant of fault initiation and phase-angle jump at the same time or as a function of more than two characteristics simultaneously. Furthermore, this new representation of voltage sags facilitates the investigation of equipment sensitivity to phase-angle jump, moment of fault initiation and to any combination of these and other sag characteristics.

Algoritmo baseado em afundamentos de tensão para localização de faltas em alimentadores de distribuição de energia elétrica

Neste trabalho é proposto um algoritmo baseado nas características de afundamentos de tensão para localização de faltas em alimentadores aéreos de distribuição de energia elétrica. Quando uma falta ocorre no alimentador, afundamentos de tensão propagam-se apresentando diferentes características para cada nó do alimentador. A utilização das características dos afundamentos de tensão garante a eficiência e a robustez do algoritmo, o qual fornece resultados adequados para a localização da falta. Um alimentador aéreo, real, trifásico a três fios, 13,8 kV, com 134 nós é utilizado para avaliar o algoritmo proposto. Os resultados dos testes mostram que o algoritmo fornece não somente o provável local da falta, mas também uma área contendo este local. Essas informações são extremamente úteis para as equipes de manutenção localizarem a falta e restabelecerem o fornecimento de energia elétrica.

Validation of Voltage Sag Simulation Tools: ATP and Short-Circuit Calculation Versus Field Measurements

Two methods to calculate voltage sags are validated against actual measurements. One of the methods is a short-circuit calculation program resulting directly in sag magnitude during the fault. The other is an electromagnetic transient program resulting in voltage waveform as a function of time. Individual sag characteristics and system performance obtained by deterministic simulation and measurement are compared. The influence of the random variables (prefault voltage, fault location and fault impedance) is investigated.

The Influence of Induction Motors on Voltage Sag Propagation—Part I: Accounting for the Change in Sag Characteristics

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> 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. </para>

The Influence of Induction Motors on Voltage Sag Propagation—Part II: Accounting for the Change in Sag Performance at LV Buses

This paper describes the influence of induction motors (IMs) on voltage sag characteristics at low-voltage (LV) buses. This influence is particularly pronounced in case of low magnitude and long duration voltage sags. Even though the conventional magnitude duration representation of voltage sags in the presence of IMs is not completely accurate, it is very convenient as it can be directly linked to a standard way of recording/reporting voltage sags. The equivalent rectangular sags developed in this study take into account the effect of IMs and establish voltage sag performance more accurately at LV buses in the distribution network. The methodology developed is not limited to simulation-based voltage sag studies. It can be equally well applied as a ldquopostprocessorrdquo to finely tune the results of monitoring distribution systems as its inputs are ldquoclassicalrdquo voltage sag magnitude duration tables. The automated procedure for counting and characterization of voltage sags at LV distribution buses is illustrated on a 295-bus generic distribution network.

Factor affecting characteristic of voltage sag due to fault in the power system

Faults in the power system are the most common reason for the occurrence of important power quality problem, voltage sag in the system. Due to increasing use of sensitive and sophisticated control in almost all modern devices at the industrial and residential consumer level, the Voltage sag which causes severe problems to these devices needs to be analyzed. Factors which affect the characteristics of voltage sag as a type of fault in the system, location of fault in the system, X/R ratio of transmission lines, type of transmission as single or double circuit transmission, Point on wave of sag initiation are studied in this paper. PSCAD/EMTDC is used as a simulation tool.

Voltage Sag Studies in Distribution Networks—Part I: System Modeling

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.

Voltage Sag Studies in Distribution Networks—Part II: Voltage Sag Assessment

This paper is the second part of a joint paper aimed at predicting the voltage sag performance of distribution networks by estimating voltage sag indices. This part explores the characteristics of voltage sags caused by faults in a distribution network considering the effect of the protective devices installed in the network. The work is based on a Monte Carlo procedure implemented in a time-domain simulation tool. Several scenarios with a different coordination between protective devices have been analyzed. As expected, the main conclusions are that the protection system can have a significant influence on voltage sag characteristics and proper coordination between protective devices can reduce voltage sag impact on end-user equipment.

Implementation of a voltage sag characterization based on a DSP platform

Voltage sags are nowadays the first power quality problem in the distribution power system. This work evaluates the real implementation of a voltage sag characterization based on a DSP platform. Several classification methods for voltage sags have been presented in the literature. Here the Bollen’s classification is tested. Automation of sag assessment is desirable because manual analysis may be quite difficult. Thus, the fuzzy logic technique is chosen for this aim. Th is logic system generalizes the classical two-valued logic for reasoning under uncertainty. The fuzzy algorithm was first tested by simulation and then it was translated into C code in order to be implemented in a DSP based measurement platform, obtaining a satisfactory result.

Stochastic Prediction of Voltage Sags by Considering the Probability of the Failure of the Protection System

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.

Advanced voltage sag characterisation. Part I: Phase shift

The authors discuss an important, but often neglected voltage sag characteristic: the phase shift. Firstly, a critical review of existing methodologies and approaches is presented. Then, theoretical and experimental results on the ranges and relationships of phase shift values are reviewed. A numerical example and several recorded ‘non-conventional’ voltage sag waveforms are used to highlight some of the problems encountered in calculating a phase shift. Equipment and procedures to test single-phase and three-phase equipment sensitivity to voltage sags are also discussed from a phase shift point of view.

Voltage Sag State Estimation for Power Distribution Systems

The increased awareness on power quality has resulted in the need to quantify the sag performance of a distribution feeder, similar to what has been done on characterizing the reliability performance of a feeder. Since it is impossible to measure the sag level at every node of a distribution feeder, of sag characteristics at unmetered nodes becomes necessary. This paper proposes the concept of voltage sag state estimation and associated algorithms to achieve this goal. The proposed method has the following characteristics: 1) It makes use of the radial connection characteristic of a distribution feeder, 2) it is based on a limited number of metering points, and 3) it employs a least-square method to predict the sag profile along a distribution line. The results of the proposed sag state estimator can be used to calculate the feeder power quality performance indices such as the System Average RMS Frequency Index (SARFIx).

How to Connect a Voltage Sag-Measuring Device: Phase to Phase or Phase to Neutral?

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).

Qualitative and quantitative analysis of voltage sags in networks with significant penetration of embedded generation

This paper summarizes the findings of the research carried out in the area of the voltage sag (dip) propagation in distribution networks. The emphasis is on the influence of embedded generation on voltage sag characteristics and propagation. Two different, realistic, network topologies are used in the analysis. Different types of faults and different levels of embedded generation penetration are considered. The influence of embedded generation on symmetrical and asymmetrical voltage sags is assessed. Variations in voltage sag magnitude, phase-angle jump and sag propagation through the network as a function of the level and type of embedded generation are illustrated. New indices for the assessment of voltage sag propagation are proposed. The results of the research have shown that the effects, either beneficial or detrimental, of the embedded generation on voltage sag propagation and characteristics are closely coupled with the level of their penetration into the network and the location of the generators. Copyright © 2005 John Wiley & Sons, Ltd.