Showing papers on "Voltage sag published in 2004"

Control and testing of a dynamic voltage restorer (DVR) at medium voltage level

Abstract: The dynamic voltage restorer (DVR) has become popular as a cost effective solution for the protection of sensitive loads from voltage sags. Implementations of the DVR have been proposed at both a low voltage (LV) level, as well as a medium voltage (MV) level; and give an opportunity to protect high power sensitive loads from voltage sags. This paper reports practical test results obtained on a medium voltage (10 kV) level using a DVR at a distribution test facility in Kyndby, Denmark. The DVR was designed to protect a 400-kVA load from a 0.5-p.u. maximum voltage sag. The reported DVR verifies the use of a combined feed-forward and feed-back technique of the controller and it obtains both good transient and steady-state responses. The effect of the DVR on the system is experimentally investigated under both faulted and nonfaulted system states, for a variety of linear and nonlinear loads. Variable duration voltage sags were created using a controllable LV breaker fed by a 630 kVA distribution transformer placed upstream of the sensitive load. The fault currents in excess of 12 kA were designed and created to obtain the required voltage sags. It is concluded the DVR works well in all operating conditions.

Performance evaluation of PLL algorithms for single-phase grid-connected systems

Abstract: Phase angle, frequency and amplitude of the utility voltage vector are basic information for an increasing number of grid-connected power conditioning equipments, such as PWM rectifiers, uninterruptible power systems (UPS), voltage sag compensators and the emerging distributed generation systems. For these applications, accurate tracking of the utility voltage vector is essential to ensure correct operation of the control system. This paper presents a comparative study of synchronous reference frame PLL algorithms for single-phase systems. Simulation and experimental results, including operation of the PLL structures under distorted utility conditions are presented, to allow a performance evaluation of the PLL algorithms.

Performance evaluation of PLL algorithms for single-phase grid-connected systems

Abstract: Phase angle, frequency and amplitude of the utility voltage vector are basic information for an increasing number of grid-connected power conditioning equipments, such as PWM rectifiers, uninterruptible power systems (UPS), voltage sag compensators and the emerging distributed generation systems. For these applications, accurate tracking of the utility voltage vector is essential to ensure correct operation of the control system. This paper presents a comparative study of synchronous reference frame PLL algorithms for single-phase systems. Simulation and experimental results, including operation of the PLL structures under distorted utility conditions are presented, to allow a performance evaluation of the PLL algorithms.

Analysis of series compensation and DC-link voltage controls of a transformerless self-charging dynamic voltage restorer

Abstract: This paper describes a transformerless self-charging dynamic voltage restorer (DVR) series compensation device used to mitigate voltage sags. A detailed analysis on the control of the restorer for voltage sag mitigation and dc-link voltage regulation are presented. A nonlinear element is shown to exist in the regulator, the activation of which can adversely affect its stability. Active cancellation for this element is recommended. Simulation and experimental results are presented for a 1-kVA prototype to validate the analysis as well as demonstrate the DVR's performance under both voltage restoration and self-charging operating conditions.

Sensitivity of AC coil contactors to voltage sags, short interruptions, and undervoltage transients

Abstract: 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.

Voltage sag stochastic prediction using an electromagnetic transients program

Abstract: This paper presents a summary of the work performed by the authors on voltage sag analysis using a time-domain tool. The document describes a procedure for stochastic prediction of voltage sags based on the Monte Carlo method. A medium size distribution network is used to analyze the advantages of this approach, the convergence of the Monte Carlo method, the influence of protective devices and the importance of voltage sag indices. The limitations of the present version and the future work are also discussed.

Unified approach for mitigating voltage sag and voltage flicker using the DSTATCOM

Abstract: Summary form only given. This paper introduces a novel method for the mitigation of the voltage sag and voltage flicker by using Kalman filter and its derivatives (adaptive, and extended). The Kalman filter is used as a tool to extract both the instantaneous envelope of the voltage sags, and to extract the instantaneous flicker level (IFL) of the voltage flicker. Also the paper demonstrates the advantages of using the Kalman filter instead of the existing tools for tracking and extracting voltage disturbances. Digital simulation results are presented to illustrate the mitigation of unbalanced voltage sags, and the compensation of the cyclic and noncyclic voltage flicker by employing the proposed algorithm.

Review of dynamic voltage restorer for power quality Improvement

Abstract: Power quality has always been important for customers, but with increasing applications of electronic loads and controllers sensitive to the power quality, the subject has attracted renewed interest in recent times. Power quality encompasses several aspects: harmonics, over voltage, flicker voltage sags and swells, interruptions etc., lasting only a few cycles can cause significant damage for a manufacturing process and computer hardware installations. Voltage source converter based dynamic voltage restorer (DVR) can be used effectively for mitigation of voltage sag and swells. This paper presents the operating principle, power circuit topologies, mathematical modeling, control philosophies and applications of DVR for power quality improvement along with systematic review of published literature.

A DFIG Wind-Turbine Ride-Through System Influence on the Energy Production

Abstract: The influence on the energy production of a voltage sag ride-through systems, for a doubly-fed induction generator (DFIG) wind turbine, is investigated. Based on simulations a candidate ride-through system, based on IGBT modules with high current rating and anti-parallel thyristors that can disconnect the stator of the DFIG from the grid, is developed. It has been found that by increasing the current rating, of the converters IGBTs, the losses of the converter can be reduced. However, if anti-parallel thyristors has to be connected between the stator and the grid, in order to achieve a fast disconnection, the over all losses of the total system increase instead.

A unified power quality conditioner for voltage regulation of critical load bus

Abstract: The paper discusses the operation and performance of a unified power quality conditioner (UPQC). A UPQC is a versatile device that can be used for various functions. The main aim of the paper is to operate the UPQC in such a way that it tightly regulates the bus voltage of critical loads against unbalance, harmonics, voltage sag/swell and other disturbances occurring in a distribution system. The performance of the UPQC is compared to that of a dynamic voltage restorer (DVR). Both the UPQC and the DVR are operated in the closed-loop state feedback switching control. Through simulation studies using PSCAD/EMTDC it is shown that the UPQC performs significantly better than the DVR.

Design of a prototype D-STATCOM for voltage sag mitigation

Abstract: This paper presents the design of a prototype distribution static compensator (D-STATCOM) for voltage sag mitigation in an unbalanced distribution system. The D-STATCOM is intended to replace the widely used static Var compensator (SVC). For fast response requirement, the feedforward compensation scheme is employed to enable the D-STATCOM to mitigate voltage sag and at the same time correct the power factor, thus acting as a load compensator. The compensation scheme of the D-STATCOM is derived using the symmetrical component method. In this work, the 12-pulse D-STATCOM configuration with IGBT has been designed and the graphic based models of the D-STATCOM have been developed using the PSCAD/EMTDC electromagnetic transient simulation program. Accordingly, simulations are first carried out to illustrate the use of D-STATCOM in mitigating voltage sag in a distribution system. Simulation results prove that the D-STATCOM is capable of mitigating voltage sag as well as improving power quality of a system.

Propagation of asymmetrical sags and the influence of boundary crossing lines on voltage sag prediction

Abstract: 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.

Dynamic voltage restorer against balanced and unbalanced voltage sags: modelling and simulation

Abstract: This paper presents modelling aspects of several types of dynamic voltage restorer (DVR) working against various voltage sags by simulation in PSCAD/EMTDC. It then provides analyses of working performance of the device, including capability and quality of compensation. Comparisons of working performance among these different types of DVR are also addressed. Significant simulation results show that these several types of the modelled device can work very well against balanced and/or unbalanced voltages caused by faults in a distribution system. Detailed analyses illustrate that with suitable parameter setting these devices can deal with different levels of voltage sag severity. In addition, appropriate ways to obtain a good quality output voltage by a DVR during voltage sag is also presented.

Supervisory control of dynamic voltage restorers

Abstract: The impact of system issues on the performance and power rating of dynamics voltage restorers is examined. Algorithms to restore a network voltage sag or swell to given set points have been the subject of significant research in the past. The supervisory control scheme is examined to determine these set points. The control scheme's effect on device power rating (cost) and its interaction with active loads are investigated using analytical methods, simulation and experimental tests. The influence of the choice of the supervisory control algorithm on the selection of protection scheme and the interaction between the protection scheme and the device power rating are also examined.

Frequency-response characteristics and error estimation in RMS measurement

Abstract: Root-mean-square (rms) calculation is a popular method adopted in power system parameter classification. Typical examples are its application in voltage sag classification and relay protection. Starting from the viewpoint of frequency response, this paper seeks to study the characteristics of the rms method, for both the single-frequency and mixed-frequency signals. Analysis is made on its dependence on sampling rate, sampling window size, as well as point-on-wave through strict mathematical deductions. Interesting discoveries include the locations of the "pitfall" frequencies, the minimum sampling points for correct rms output as well as the fluctuation of rms magnitude. Estimation is made on the maximum rms output errors due to the presence of harmonics as well as frequency deviation. Reasonable approximation is adopted in such estimation. Such error estimations are useful in measurement of power system voltages, or any other parameters with relatively higher fundamental but lower harmonics levels.

Power System Disturbance Recognition Using Wavelet and S-Transform Techniques

Abstract: Power quality issues have been a source of major concern in recent years due to extensive use of power electronic devices and non-linear loads in electrical power system and consequently sensitive detection and accurate classification of power disturbances have become very much necessary. To monitor electrical power quality disturbances, short time discrete Fourier transform (STFT) is most often used. But for non- stationary signals, the STFT does not track the signal dynamics properly due to the limitations of a fixed window width chosen a priori. This paper presents a new approach for power quality analysis using a modified wavelet transform, known as S–transform and the analysis of several power quality problems using both S–transform as well as discrete wavelet transform validates the superiority of S–transform.

Equipment failures caused by power quality disturbances

Abstract: This paper provides insight into a failure mechanism that impacts a broad range of industrial equipment. Voltage surges have often been blamed for unexplained equipment failure in the field. Extensive voltage monitoring data suggests that voltage sags occur much more frequently than voltage surges, and that current surges that accompany voltage sag recovery may be the actual culprit causing equipment damage. A serious limitation in equipment specification is highlighted, pointing to what is possibly the root-cause for a large percentage of unexplained equipment field failures. This paper also outlines the need for a standard governing the behavior of equipment under voltage sags, and suggests solutions to protect existing equipment in the field.

Voltage sag analysis and solution for an industrial plant with embedded induction motors

Abstract: In this paper a power quality (PQ) problem in an industrial plant is analyzed and its possible solutions explored, specifically regarding voltage sags. It is analyzed a plant's electrical system sensitivity regarding voltage sags, how does the magnitude depression and its duration affect the performance of the electrical loads (mainly induction motors). Several proposals are discussed and explored for voltage sag mitigation and their feasibility for the plant's PQ problem. Finally, settings of the voltage sag mitigation equipment (timer or latching relay) are made analyzing voltage recovery times after voltage sag has occurred.

Voltage sag analysis and solution for an industrial plant with embedded induction motors

Abstract: In This work a power quality (PQ) problem in an industrial plant is analyzed and its possible solutions explored, specifically regarding voltage sags. It is analyzed a plant's electrical system sensitivity regarding voltage sags, how does the magnitude depression and its duration affect the performance of the electrical loads (mainly induction motors). Several proposals are discussed and explored for voltage sag mitigation and their feasibility for the plant's PQ problem. Finally, settings of the voltage sag mitigation equipment (timer or "latching" relay) are made analyzing voltage recovery times after voltage sag has occurred.

Harmonic analysis and improvement of a new solid-state fault current limiter

Abstract: This paper presents a harmonic study on a newly developed solid-state fault current limiter. Using this device, the supply voltage sag is reduced when a short-circuit fault occurs on a cable feeder in the downstream network, hence improving the power quality. The device will eventually isolate the faulted part from the healthy network. Harmonics caused by the fault current limiter are analyzed and a method is proposed to prevent undesirable harmonic interactions. Analytical and experimental results are compared with existing regulations. It is verified that, with precautions, the operation of the solid-state fault current limiter will not cause problems to either the supply network or the loads.

Compensation control of matrix converter fed induction motor drive under abnormal input voltage conditions

Abstract: Matrix converter (MC) is a direct energy conversion device with high input power factor and regeneration capability. As a device without dc link elements, the ac line side disturbances can degrade its output characteristic and reliability, even deteriorate the drive performance of motor load. In this paper, the behaviors of the MC drive under abnormal input voltage conditions, in terms of unbalance, nonsinusoidal and short-time voltage sag, have been investigated. A feedback compensation method based on close loop control of output current is proposed to improve the output performance of MC. In addition, a feedforward compensation method by measuring instantaneous values of input voltage is developed to compensate the output voltages of MC and ensure the drive performance of induction motor under abnormal input voltage conditions. The effectiveness and feasibility of the two proposed compensation methods have been proven through numerical simulations and experimental tests.

Estimating economic impact of voltage sags

Abstract: This paper describes a systematic approach to evaluate the technical as well as the eventual economic impacts from voltage sag disturbances from a customer's perspective. It is illustrated how the influence of voltage sag problem can be evaluated from the knowledge of the system voltage sag performance and the equipment sensitivity. It also takes into consideration the role-played by each equipment towards the performance of the customer's load system. This in turn allows the monetary costs of interruption to the system operation as induced by voltage sags to be readily determined. This method should be useful for conducting cost/benefit analysis when designing or improving the configuration of the load system. It is particularly useful for evaluating between alternative power quality conditioning solutions.

Distributed mitigation of voltage sag by optimal placement of series compensation devices based on stochastic assessment

Abstract: Central mitigation does not necessarily provide the most economical solution to voltage-sag problems. Distributed mitigation with strategically placed compensation devices should provide a better alternative. Placement studies of this nature usually involve extensive evaluations of candidate compensation schemes, which can become unmanageable for large systems with stochastic voltage-sag data. A probability-based technique known as the weighted sampling method is proposed to simplify the problem. The technique is applied to a distribution system to optimize the cost of placing series compensation. The cost has two components: device cost and cost reduction to consumers due to implementation of the device(s). Series compensation devices being optimized are the two distinctly different dynamic voltage restorer and thyristor voltage regulator. These devices are placed and optimized to best complement each other. To achieve reliable and fast convergence, a genetic algorithm with innovative coding is proposed. A 34-node distribution system is studied with a wide range of voltage-sag data and consumer tolerance characteristics.

Effects of symmetrical voltage sags on three-phase three-legged transformers

Abstract: This paper describes the effects of symmetrical voltage sags on three-phase three-legged transformers. The transformer model includes saturation and is based on experimental parameters. It has been implemented in PSpice. Voltage recovery after a voltage sag can produce transformer saturation. This saturation produces an inrush current similar to that of the transformer energizing. The variables that influence the peak value of the inrush current, namely depth, duration, and initial point-on-wave are studied. A result that must be emphasized is the periodical influence of the sag duration.

Voltage sag detection based on rectified voltage processing

Abstract: An algorithm for voltage sag detection based on rectified voltage processing is presented and its performances are evaluated by means of simulated and real voltage waveforms (recorded at some low-voltage busbars and measured on a scaled prototype of a voltage support system). The algorithm is intended to detect voltage sag occurrence and to trigger the disconnection of the faulty supply network and subsequently the connection of a voltage support system. The algorithm is very convenient in terms of memory storage and computational requirements and may be implemented on low-cost platforms. The results show that the time for detection is very short and always less than a quarter of the supply voltage period.

Voltage Sag Response of PWM Rectifiers for Variable-Speed Wind Turbines

Abstract: The voltage sag response of PWM rectifiers is analyzed for a candidate dc-link voltage control system. It is shown that the voltage sag response is mainly determined by the dynamics of the dc-link voltage and the control system. PWM rectifiers are found to be robust against voltage sags, but the remaining grid voltage limits the maximum active power that can be delivered to the utility grid.

Modular power supply system including a power status signal generator to perform fast sag detection to input peak voltage

Abstract: A modular power supply system incorporating a power status signal generator circuit to detect the voltage sag of the input peak voltage within a short period is proposed. The power status signal generator circuit mainly includes a peak detector that detects a peak value of an input voltage, wherein the peak detector comprises a first comparator that compares the input voltage with a predetermined reference voltage and sends an output signal according to a comparison result thereof, a RC network connected to the first comparator and is configured to charge and discharge the capacitor according to the output signal of the first comparator, and a second comparator connected to the resistor-capacitor network that compares a terminal voltage of the capacitor of the resistor-capacitor network with the predetermined reference voltage and generates a power status signal according to a comparison result thereof.

A voltage sag supporter utilizing a PWM-switched autotransformer

Abstract: This paper describes a novel distribution-level voltage control scheme that can compensate voltage sag and swell conditions in three-phase power systems. Faults occurring in power distribution systems or facilities in plants generally cause the voltage sag or swell. Sensitivity to voltage sags and swells varies within different applications. For sensitive loads, even a voltage sag of short duration can cause serious problems. Normally, a voltage interruption triggers a protection device, which causes the shutdown of the entire load. In order to mitigate power interruptions, this paper proposes a scheme for voltage sag support based on a pulse width modulated (PWM) autotransformer. The proposed scheme is able to quickly recognize the voltage sag or swell condition, and it can correct the voltage by either boosting the input voltage during voltage sag events or reducing the voltage during swell events. The key advantage in this voltage boost topology is that it uses only one controlled switch per phase to boost the input voltage, thereby resulting in minimum cost per kVA. The paper presents the new voltage control topology, as well as the voltage detection method, the snubber design, and the commutation logic for the thyristor bypass switch. Experiments have been carried out to verify the validity of the proposed scheme.

A fuel cell system with Z-source inverters and ultracapacitors

Abstract: In this paper a fuel cell system with Z-source inverter and ultracapacitors for voltage sag compensation is proposed The structure of Z-source inverter is simple It has unique features that can boost without a DC/DC converter using only a modified switching pattern Fuel cells have unique output characteristics The fuel cell system can experience start-up transient and voltage sag Simulation results show that the proposed fuel cell system with Z-source inverter and ultracapacitors is capable of operating with stable response to the system transient and voltage sags