Showing papers on "Voltage sag published in 2005"

A detailed comparison of system topologies for dynamic voltage restorers

Abstract: In this paper, four different system topologies for dynamic voltage restorers (DVRs) are analyzed and tested, with particular focus on the methods used to acquire the necessary energy during a voltage sag. Comparisons are made between two topologies that can be realized with a minimum amount of energy storage, with energy taken from the grid during the voltage sag, and two topologies that take energy from stored energy devices during the voltage sag. Experimental tests using a 10-kVA DVR show that the no-energy storage concept is feasible, but an improved performance can be achieved for certain voltage sags using stored energy topologies. The results of this comparison rank the no-storage topology with a passive shunt converter on the load side first, followed by the stored energy topology with a constant dc-link voltage.

Analysis, Modeling and Control of Doubly-Fed Induction Generators for Wind Turbines

Abstract: This thesis deals with the analysis, modeling, and control of the doubly-fed induction machine used as a wind turbine generator. The energy efficiency of wind turbine systems equipped with doubly-fed induction generators are compared to other wind turbine generator systems. Moreover, the current control of the doubly-fed induction generator is analyzed and finally the sensitivity of different current controllers with respect to grid disturbances are investigated. The energy efficiency of a variable-speed wind turbine system using a doubly-fed induction generator is approximately as for a fixed-speed wind turbine equipped with an induction generator. In comparison to a direct-driven permanent-magnet synchronous generator there might be a small gain in the energy efficiency, depending on the average wind-speed at the site. For a variable-speed wind turbine with an induction generator equipped with a full-power inverter, the energy efficiency can be a few percentage units smaller than for a system with a doubly-fed induction generator. The flux dynamics of the doubly-fed induction machine consist of two poorly damped poles which influence the current controller. These will cause oscillations, with a frequency close to the line frequency, in the flux and in the rotor currents. It has been found that by utilizing a suggested method combining feed-forward compensation and "active resistance," the low-frequency disturbances as well as the oscillations are suppressed better than the other methods evaluated. The maximum value of the rotor voltage will increase with the size of a voltage dip. This means that it is necessary to design the inverter so it can handle a desired value of a voltage dip. For the investigated systems the maximum rotor voltage and current, due to a voltage dip, can be reduced if the doubly-fed induction machine is magnetized from the stator circuit instead of the rotor circuit. Further, it has been found that the choice of current control method is of greater importance if the bandwidth of the current control loop is low.

Modeling and experimental verification of grid interaction of a DFIG wind turbine

Abstract: The response of the doubly fed induction generator (DFIG) wind turbine system to grid disturbances is simulated and verified experimentally. The results are compared to the response that a fixed-speed wind turbine would have given. A voltage sag to 80% (80% remaining voltage) is handled very well, which is not the case for a fixed-speed wind turbine. A second-order model for prediction of the response of DFIG wind turbines is derived, and its simulated performance is successfully verified experimentally. The power quality impact by the DFIG wind turbine system is measured and evaluated. Steady-state impact, such as flicker emission, reactive power, and harmonic emission, is measured and analyzed. It is found that the flicker emission is very low, the reactive power is close to zero in the whole operating range, and the current THD is always lower than 5%.

A generalized voltage compensation strategy for mitigating the impacts of voltage sags/swells

Abstract: Dynamic restoration of load voltage achieved through the application of voltage-injection technique is considered. The injected voltage is generated from a voltage-source-inverter-based series compensator. It is shown that during a voltage sag, the restoration process almost inevitably requires the injection of energy from the compensator to the external system. Conversely, a voltage swell event could cause the compensator to absorb energy from the external system which would then result in a rise in the dc-link voltage of the inverter. By permitting phase adjustment in the injected voltage, a generalized compensation method is proposed. The new voltage injection technique allows the magnitude of the positive phase-sequence component of the compensated load voltage to be restored to its pre-sag/swell level. It also exercises simultaneous control on the dc-link voltage. The efficacy of the proposed technique is illustrated by numerical examples.

Sensitivity of personal computers to voltage sags and short interruptions

Abstract: This paper discusses the sensitivity of personal computers (PCs) to voltage sags and short interruptions on the basis of the extensive test results. Existing standards and previously published works are reviewed, and a description of a used testing procedure is presented. The following tests were performed: sensitivity to rectangular voltage sags with ideal and nonideal supply characteristics, and sensitivity to voltage sags caused by the starting of large motors. The results obtained emphasize the importance of clear definition of the malfunction criteria for this equipment.

A resistance sign-based method for voltage sag source detection

Abstract: A method to determine the origin of voltage sag disturbance is proposed in this paper. Its principle is to estimate the equivalent impedance of the nondisturbance side by utilizing the voltage and current changes caused by the disturbance. The sign of the real part of the estimated impedance can reveal if the disturbance is from upstream or downstream. The proposed method can be easily implemented into power quality meters for troubleshooting applications or into digital revenue meters to document the responsibility for disturbances. Extensive field and laboratory test results have proven the effectiveness and robustness of the proposed method.

Applying distance relay for voltage sag source detection

Abstract: This paper proposes the application of distance relay information for a voltage sag source detection problem. It is found that the magnitude and angle of impedance before and after the sag event clearly indicate on which side of the power-quality (PQ) monitor, the sag source lies. The required information on impedance can be obtained by the PQ monitor using the estimates of distance relay at that point or applying the algorithm of such relay.

PSCAD/EMTDC simulation of unified series-shunt compensator for power quality improvement

Abstract: This paper deals with the simulation of a unified series-shunt compensator (USSC) aimed at examining its capability in improving power quality in a power distribution systems. The USSC simulation model comprises of two 12-pulse inverters which are connected in series and in shunt to the system. A generalized sinusoidal pulse width modulation switching technique is developed in the proposed controller design for fast control action of the USSC. Simulations were carried out using the PSCAD/EMTDC electromagnetic transient program to validate the performance of the USSC model. Simulation results verify the capabilities of the USSC in performing voltage sag compensation, flicker reduction, voltage unbalance mitigation, UPS mode, power-flow control and harmonics elimination. A comparison of the USSC with other custom power devices shows that the USSC gives a better performance in power-quality mitigation.

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

Abstract: 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, this 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.

Symmetrical and unsymmetrical voltage sag effects on three-phase transformers

Abstract: This paper studies the effects of symmetrical and unsymmetrical voltage sags on three-phase three-legged transformers. The transformer model includes saturation and the parameters have been obtained from experimental measurements. The study shows that sags can produce transformer saturation when voltage recovers. This saturation produces an inrush current that is similar to the inrush current produced during the transformer energizing. The study takes into account that the voltage recovery instant can take only discrete values, since the fault-clearing is produced in the natural current zeroes. The paper also studies the influence of the sag type, duration, depth and fault current angle on the inrush current peak value. The current peak has a periodical dependence on sag duration, and a linear dependence on depth. Unsymmetrical sags can result in current peaks as high as those of symmetrical sags.

Coordination of voltage sag and overcurrent protection in DG systems

Abstract: A study of a distribution system is presented where the effect of distributed generation (DG) presence is analyzed. The system is typical of a rural, industrial, and low-power (weak) distribution scheme. The concept of specific energy for voltage sag immunity of sensitive equipment (SE) under the new scenario created by the DG presence is applied, especially studying the double magnitude/duration sag that is caused by the change in source impedance due to the response of dissimilar protection devices. The new protection coordination concepts of the not-anymore-radial distribution systems are described. A new methodology for the study of SE behavior under voltage sag when connected to a weak distribution system with DG is proposed.

Optimized Control Strategy for a Medium-Voltage DVR

Abstract: Voltage sags are a major problem in present distribution systems. Therefore, different solutions are examined to compensate these sags and to avoid production losses at sensitive loads. Especially, dynamic voltage restorers (DVR) are one solution to realize this goal. Presently, a system wide integration of DVRs is hampered because of costs. Voltage sag compensation requires energy storage, the amount of which depends greatly on the required voltage level (VDVR). Present control strategies are either able to minimize the needed voltage, to allow a better utilization of the storage system or to minimize the distortions at the load. To avoid this drawback an optimized control strategy is presented in this paper, which is able to reduce the voltage amplitude and causes low distortions at the load side. In the following paper, a brief introduction of the basic DVR principle is given. After this, different control strategies are briefly described, before an optimized strategy is presented. Finally, simulation results are shown to verify and prove the functionality of the presented control strategy

Voltage sag state estimation for power distribution systems

Abstract: The increased awareness on power quality has resulted in the need to quantify the voltage 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, estimation 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).

Power quality detection with classification enhancible wavelet-probabilistic network in a power system

Abstract: A model of disturbance detection for harmonics and voltages using wavelet-probabilistic network (WPN) is proposed, which is a two-layer architecture, containing the wavelet layer and probabilistic network. It uses the wavelet transformation (WT) to extract the features from various disturbances and probabilistic neural network (PNN) to analyse the translation patterns from time-domain distorted wave and perform classification tasks. The proposed WPN detects the disturbances of harmonics and voltages, and has been tested for the power quality problems caused by harmonics, voltage sag, voltage swell and voltage interruption. It has also been compared with wavelet networks as well as combined the WT and conventional neural networks. The test results show that this simplified network architecture enhances the classification performance and shortens the processing time for detecting disturbing events.

System voltage sag performance estimation

Abstract: This paper describes an analytical method for evaluating the voltage sag performance of a power-supply distribution network. The severity of voltage sag, as a result of system fault, is quantified by its magnitude and duration, and illustrated using a voltage sag density table. Since different loads have a different tolerance level to voltage sags, different evaluation algorithms are proposed in this paper to account for each type of sensitivity. It is illustrated that once the failure rates of the power components making up a system are known, the number and severity of the ensuing voltage sag events can be readily estimated. These estimates would be useful for predicting potential impacts on the operation of sensitive loads and for planning mitigating solutions to improve or maintain the power quality of the supply system. The possibility of incorporating other influencing factors from the behavior of the load and the fault events is also discussed.

Stabilization of Grid Connected Wind Generator by STATCOM

Abstract: Recently voltage-source or current-source inverter based various FACTS devices have been used for flexible power flow control, secure loading and damping of power system oscillation. Some of those are used also to improve transient and dynamic stability of wind power generation system (WPGS). In this paper, we propose the static reactive compensator (STATCOM) based on voltage source converter (VSC) PWM technique to stabilize grid connected squirrel cage wind generator system. A simple control strategy of STATCOM is adopted where only measurement of rms voltage at the wind generator terminal is needed, i.e. there is no need of reactive power measurement. Fuzzy logic controller is used as the control methodology of STATCOM, rather than conventional PI controller. The voltage sag and swell improvement of WPGS is compared with both fuzzy and PI controller. Moreover, the steady state capacitor value used with induction generator is reduced by certain percentage when STATCOM is used with WPGS. Comprehensive results are presented to assess the performance of the STATCOM connected with WPGS, where the simulations have been done by PSCAD/EMTDC.

A DFIG wind turbine ride‐through system. Influence on the energy production

Abstract: The influence of a voltage sag ride-through system on the energy production of a doubly-fed induction generator (DFIG) wind turbine is investigated. Using simulations, a candidate ride-through system based on insulated gate bipolar transistor (IGBT) modules with high current rating and with the option of having antiparallel thyristors, which can quickly disconnect the stator of the DFIG from the grid, is developed. It has been found that, by increasing the current rating of the converter IGBTs, the losses of the converter can be reduced. However, if antiparallel thyristors have to be connected between the stator and the grid in order to achieve a fast disconnection of the stator circuit, the overall losses of the total system increase instead. Copyright © 2005 John Wiley & Sons, Ltd.

Effects of balanced and unbalanced voltage sags on VSI-fed adjustable-speed drives

Abstract: This paper analyzes the effects of voltage sags on the three-phase rectifier of a voltage-source-inverter (VSI)-fed adjustable-speed drive. These effects are dc voltage drop and ac current peaks. As rectifier behavior is univocally characterized by two parameters (which depend on the ac reactance and the dc capacitor), their influence on the dc voltage drop and the ac current peaks is studied. Diagrams in function of these two parameters are presented in order to easily predetermine the worst effects of voltage sags originated by single-phase faults. The results of the study suggest that if the trip levels are correctly adjusted, the drive can ride through the voltage sags produced by single-phase faults (assuming the drive has typical values of L and C).

Impact of input voltage sag and unbalance on dc link inductor and capacitor stress in adjustable speed drives

Abstract: This paper investigates the impact of input voltage unbalance and sags on stresses in the dc-bus choke inductor and dc-bus electrolytic capacitors of adjustable-speed drives (ASDs). These stresses are primarily attributable to the rectifier's transition into single-phase operation, giving rise to low-order harmonic voltages (120 Hz, 240 Hz, etc.) that are applied to the dc-link filter components. These harmonics elevate the ac-flux densities in the dc choke core material significantly above values experienced during normal balanced-excitation conditions, causing additional core losses and potential magnetic saturation of the core. It is shown that the effects of voltage unbalances and sags on the dc-link capacitor lifetime will be the same when either line inductors or a dc-link choke inductor are used if the dc choke-inductance value is twice the value of the line inductance. Simulations and experimental tests are used to verify the accuracy of predictions provided by closed-form analysis and simulation for a 5-hp or 3730-W ASD system.

Dynamic Voltage Restorer For Voltage Sag Compensation

Abstract: The Dynamic Voltage Restorer (DVR) is a power electronic device that is used to inject 3-phase voltage in series and in synchronism with the distribution feeder voltages in order to compensate for voltage sag. In this paper the operation of a DVR is presented. The power circuit of a DVR together with the control techniques used for compensation is explained. The operation of the Software Phase Locked Loop (SPLL) and its stability are analyzed. The DVR control system is described and verified using simulation. The control algorithm implemented using DSP was tested using generated input voltage signals and the results are also presented. The micro-cycle effect of battery current for balanced and unbalanced sag is also investigated.

A novel structure for multilevel converters

Abstract: In this paper a new structure for asymmetric multilevel converters is presented. The new topology is consisting of N series-connected of two interconnected multilevel converters per phase. The presented circuit is an optimized multilevel converter with minimum DC voltage sources and switches. The investigated topology can generate a large number of levels (odd and even) without increasing the number of switches and DC voltage sources. For this structure a new algorithm for calculating the levels of DC voltage sources is proposed. Also a complete analysis is given for calculating the harmonic distortion and THD. The proposed converter has higher efficiency because the number of switches is reduced and the devices which can be switched at low frequency. Then for showing the ability of the proposed converter, we use this converter as the core of a DVR, which is based on this converter. This DVR has been modeled and the simulation results show that the DVR can solve power quality problems such as: voltage sag, voltage swell, unbalanced and distorted voltages

Improvement of transient stability margin in power systems with integrated wind generation using a STATCOM: an experimental verification

Abstract: The impact of the wind generation on the power systems is no longer negligible if high penetration levels are going to be reached. Significant barriers to interconnection are being perceived already with the severe requirements of the new emerged grid codes. Depending on the generator technologies, different solutions are found to support behavior in case of voltage sags. Voltage source static VAr compensator such as the STATCOM can be used to regulate voltage as shunt compensator with directly connected asynchronous wind generators. This paper has analyzed the extent to which the transient stability margin can be increased by the use of a STATCOM, first by simulations and then with an experimental verification on a setup of 7.5 kW. Measurement results confirm that the STATCOM provides a clear stability margin increase and with adequate rating it becomes possible to ride through severe faults

A new method for power quality detection based on hht

Abstract: Hilbert-Huang Transform is introduced for detection and analysis of power quality disturbances (voltage sag and swell, voltage interruption, transient interruption and oscillation etc) and harmonics (integer and non-integer harmonics) The method is composed of Empirical Mode Decomposition (EMD) and Hilbert Transform The signal is decomposed into Intrinsic Mode Function (IMF) by the EMD method Then amplitude and frequency of corresponding time can be detected accurately and quantitatively It can detect time, amplitude and frequency of the power quality disturbances and also detect amplitude and frequency of harmonics by this method Simulation results show that this method has good performance to analyze non-stationary and stationary signals

An effective SSC control scheme for voltage sag compensation

Abstract: Voltage sags are very frequent events associated with faults in the network, with energization of transformers or starting of large motors. Although their duration is very short, they can represent a big problem for industrial plants, especially for those characterized by the presence of sensitive loads, like power electronics devices. In this paper, the problem of the design of a device able to compensate voltage sags, namely a Static Series Compensator (SSC), is thoroughly analyzed, both from the point of view of the choice of the components and their rating and from the stand-point of the control system. The model has been implemented with the aid of the electromagnetic code PSCAD-EMTDC and has been tested in a radial distribution network for the protection of a nonlinear load consisting of a diode rectifier.

Stochastic evaluation of voltage sag and unbalance in transmission systems

Abstract: Voltage sags, also known as dips, are important to industrial reliability. This paper presents a Monte Carlo based approach to evaluate the maximum voltage sag magnitudes as well as the voltage unbalance in transmission systems. In this context, investigations have been conducted on a system model taking into consideration the uncertainty in several factors associated with the practical operation of a power system. MATLAB is used in the simulation studies.

Voltage sag state estimation for power distribution systems

Abstract: Summary form only given. The increased awareness on power quality has resulted in the need to quantify the voltage 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, estimation of sag characteristics at un-metered 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, 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).

Power Quality Disturbance Detection Based on Mathematical Morphology and Fractal Technique

Abstract: Based on mathematical morphology and grille fractal, a novel approach on power quality disturbance detection and location is presented in this paper. At first a parallel composite morphological filter with multiple-structure elements is designed to filter the random noise and impulse noise in power quality disturbance signals. Then to the filtered curves, an easy implementation criterion for singularity detection based on the change regularity of grille fractal is proposed to locate the start and end time that disturbance occurs. The voltage sag, swell, harmonic and their combined disturbances are used to verify the validity of the proposed filter-location approach. Numerical results show that the proposed approach is valid and effective

A modified sag characterization using voltage tolerance curve for power quality diagnosis

Abstract: This paper newly defines a voltage sag duration considering the voltage tolerance characteristics of individual electrical device for power quality (PQ) diagnosis. The conventional sag characterizing method has the possibility to overestimate voltage sag in case of nonrectangular sag. Furthermore, it cannot take the voltage tolerance characteristics of individual device into account. The proposed method utilizes the monitored parameters (V/sub sag/,V/sub avg/,d) at PQ monitor and the minimum voltage (V/sub min/) of voltage tolerance curve. The voltage sag profile is approximated using k/sup th/ order radical root function and finally the sag duration is modified from this function. The proposed method is applied to the nonrectangular sag due to induction motor reacceleration. It effectively evaluates the actual effects of voltage sag on the customer equipments whether the sag is rectangular or not.

Voltage sags compensation using a superconducting flywheel energy storage system

Abstract: This paper presents a voltage sag compensator, which uses a flywheel energy storage system with superconducting magnetic axial thrust bearing (SMB) and a permanent magnet radial bearing (PMB). The SMB was built with Nd-Fe-B magnet and YBCO superconducting blocks, refrigerated with liquid Nitrogen. The magnets are assembled with magnetic flux shapers in order to increase the levitation force and the stiffness. The radial PMB is used to positioning the vertically arranged switched reluctance machine (SRM) used as motor/generator. Simulations of the power electronics and SRM show that the system can work up to 30,000 rpm supplying the required energy during disturbances.

Power conversion and voltage sag correction with regenerative loads

Abstract: In dynamic voltage sag correctors and other power conversion equipment having a DC bus that is subject to over-voltage conditions due to power being fed back by a regenerative load, a bus discharge switching device and a discharge resistor are connected in series across the DC bus lines The discharge switching devices are switched on to discharge the energy storage device connected to the bus, or to separately discharge the two capacitors of a split capacitor DC bus, to eliminate the over-voltage conditions The discharge switching devices may be switched on and off periodically with a selected duty cycle to discharge energy from the energy storage capacitor or other energy storage device through the discharge resistor at a rate which does not exceed the power rating of the discharge resistor The discharge switching device may be switched with a fixed duty cycle or with a variable duty cycle that is based on the dynamic power dissipation characteristics of the discharge resistor