Showing papers on "Voltage sag published in 1999"

Application of Morlet wavelets to supervise power system disturbances

Abstract: A wavelet transform approach using a Morlet basis function is proposed to supervise power system disturbances in this paper. With the time-frequency localization characteristics embedded in wavelets, the time and frequency information of a waveform can be presented as a visualized scheme. Different from the fast Fourier transform, the wavelet transform approach is more efficient in monitoring various disturbances as time varies. The method has been tested on the detection of various simulated disturbances including voltage sag, voltage swell, momentary interruption and oscillatory transients and on the harmonic analysis of the arc furnace from the field test data. Testing results demonstrated the practicality and advantages of the proposed method for the applications.

Control of energy optimized dynamic voltage restorer

Abstract: The dynamic voltage restorer (DVR) is a custom power device used for voltage compensation of sensitive loads against voltage disturbances in power distribution lines. This paper illustrates a correction technique, which draws a minimum amount of energy from the DVR during the process of compensation of a voltage sag or swell. Using the proposed method it can be shown that a particular disturbance can be corrected with less amount of storage energy compared to that of existing in-phase boosting method. The paper also discusses a multiloop feedback control method applicable for the DVR to obtain good dynamic performance.

Effect of voltage sags on adjustable-speed drives: a critical evaluation and an approach to improve performance

Abstract: In this paper, a critical evaluation of the effect of voltage sags on adjustable-speed drives (ASDs) is presented. In particular, the DC-link voltage variation under voltage sag and its dependence on source impedance, DC-link inductance and output load is computed. It is shown that, for larger source impedance, the DC-link voltage variation under a voltage sag is also large and increases the susceptibility of an ASD and may result in a nuisance trip. The results from the analysis are plotted in per-unit quantities and serve as a design guide to assess ASD performance for a variety of sags. In order to improve the performance of ASDs, this paper proposes an integrated boost converter approach. This approach provides ride-through to critical ASD load during voltage sags without any additional energy storage device. Upon detection of a voltage sag, the boost converter operates with suitable duty ratio and maintains the DC-link voltage within acceptable limits. This prevents nuisance tripping and facilitates continuous operation of critical ASD load at rated torque. The proposed integrated boost converter does not introduce any additional semiconductors in the series path of the power flow and is low in cost. A commercially available 480 V 22 kVA ASD is modified with the integrated boost converter approach, and details are discussed. Analysis, simulation, and experimental performance of the ride-through approach are presented.

An instantaneous phase angle detection algorithm under unbalanced line voltage condition

Abstract: If a negative sequence is generated by a voltage sag and/or unbalance, it appears as an oscillating error in a synchronous reference frame (SRF). In power conditioning equipment, an exact value of positive sequence is needed for achieving the desired goal of the system, e.g., unity power factor and constant output voltage, whereas the exact value of negative sequence is needed for compensation. To measure the positive sequence separately from the negative sequence, one uses normally a low pass filter having a narrow bandwidth. However, such a filter causes a lot of phase delay or measurement delay, thus the response time of the system tends to be lengthened. The authors propose a method of estimating the positive and the negative sequence voltages separately without a significant delay by utilizing the weighted least-squares estimation (WLSE) method having the covariance resetting technique. They demonstrate through simulation and experiment the superior performance of the proposed scheme in measuring the positive and the negative sequence voltages at the time of abrupt transition. This method can be applied to UPSs, PWM AC/DC converters, active filters, series voltage compensators, etc.

Method and system for ride-through of an adjustable speed drive for voltage sags and short-term power interruption

Abstract: In one embodiment of the present invention, a system for ride-through of an adjustable speed drive for voltage sags is provided. The system comprises an adjustable speed drive including a three phase electric utility, a diode rectifier, a dc-link, an inverter, and a motor. Coupled to the adjustable speed drive is a ride-through circuit which includes a controller having a first input and a second input, the first input operable to receive the voltage from each phase of the three phase electric utility and determine the magnitude and phase of the voltage sag, and the second input operable to receive the dc-link voltage to ensure that the rated dc-link voltage is not exceeded. Also included is an IGBT coupled to an output of the controller and operable to switch on and off rapidly based on the amount of voltage sag. An inductor associated with the IGBT and operable to store energy when the IGBT is switched on and to transfer energy to the dc-link when the IGBT is off is also included. This maintains the dc-link voltage to provide ride-through operation of the adjustable speed drive.

A wavelet-based procedure for automatically determining the beginning and end of transmission system voltage sags

Abstract: Voltage sag disturbances are characterized by several parameters, including but not limited to sag duration and depth. An accurate determination of fault inception and clearing times is needed to properly determine these and other characterizing parameters, such as point-on-wave, and to verify proper breaker operation. This paper presents a discrete wavelet-based approach for determining the beginning and end times of sags. Several types of wavelets are compared using recorded transmission sag data. A recommendation is made for the best overall wavelet choice. The procedure is fully described, and the results are compared with other methods for determining sag duration, such as the specified RMS voltage and waveform envelope methods.

Automatic voltage regulator using an AC voltage-voltage converter

Abstract: Voltage sags and extended undervoltages are one of the main concerns of industries today. These voltage sags could cause high negative impact on productivity, which is certainly an undesirable aspect in industrial and commercial applications. Current tap-changing transformers used in distribution systems have proven to be inadequate in solving these problems related to line regulation. A solution to these problems is to install an AC voltage-voltage power converter that has been developed primarily for voltage sag correction. This system incorporates high-speed IGBT switching technology and was designed to provide the speed and efficiency required by industrial customers. Furthermore, the system will provide the flexibility of installation with or without the incorporation of current tap-changing transformers. Simulation results show the operations involved in voltage sag correction of the AC voltage-voltage power converter. A single-phase system can be readily developed into a three-phase converter system based on the exact principle of operation.

Voltage sag: an overview of IEC and IEEE standards and application criteria

Abstract: The establishment of of standards and guidelines to cope with power quality issues has become evident. Among these, voltage sags are the most important cause of interruption of industrial processes. Sag is heavily dependent on the supply systems configuration, process equipment design, systems switching and protection practices, among others. In this environment, the development and the proper use of standards to regulate custumers, equipment manufacturers and utilities relations regarding sags issues is a most important question. This paper presents a discussion on the IEEE and IEC power quality related standards, and their applicability to voltage sag evaluation in a industrial or commercial system, utility system, and to equipment.

Effect of voltage sags on adjustable speed drives-a critical evaluation and an approach to improve its performance

Abstract: In this paper, a critical evaluation of the effect of voltage sags on adjustable speed drives (ASDs) is presented. In particular the DC-link voltage variation under voltage sag and its dependence on source impedance, DC-link inductance and output load is computed. It is shown that for larger source impedance, the DC-link voltage variation under a voltage sag is also large and increases the susceptibility of an ASD and may result in a nuisance trip. The results from the analysis are plotted in per unit quantities and serve as a design guide to assess ASD performance for a variety of sags. In order to improve the performance of ASDs, this paper proposes an integrated boost converter approach. This approach provides ridethrough to critical ASD load during voltage sags without any additional energy storage device. Upon detection of a voltage sag the boost converter operates with suitable duty ratio and maintains the DC-link voltage within acceptable limits. This prevents nuisance tripping and facilitates continuous operation of critical ASD load at rated torque. The proposed integrated boost converter does not introduce any additional semiconductors in the series path of the power flow and is low in cost. A commercially available 480 V, 22 kVA ASD is modified with the integrated boost converter approach and details are discussed. Analysis, simulation and experimental performance of the ridethrough approach are presented.

Algorithm for the study of voltage sags on induction machines

Abstract: This paper proposes an algorithm for the approximate calculation of the effects caused by a voltage sag in the induction machine supply system. The algorithm computes the current and torque peaks, and the mechanical speed loss for an extensive range of voltage sags. It has a high calculation speed because it makes two simplifications. The first supposes that speed varies insignificantly during the first cycles after the voltage drop and recovery points to solve analytically the electrical transients in these cycles. The second neglects the electrical transient during the sag duration for a quick evaluation of the speed loss. Machine sensitivity to voltage sags is graphically shown. The curves can be applied to protective relay coordination.

Using voltage sag and interruption indices in distribution planning

Abstract: A traditional approach to distribution planning calls for the most economical system upgrades, timed to meet projected capacity needs. In a more competitive environment, there is also value in improving the power quality. Power quality impacts, especially in the areas of RMS voltage variations and sustained interruptions, can alter the economic evaluation of investment options for capacity expansion. While it may appear attractive to feed new load from a particular substation, an increased sag rate may result, favoring a different solution. This paper describes a planning process that includes power quality impacts. The process is dependent on service quality indices that measure the performance of the system. Methods for estimating the indices from measurements and simulations are discussed. An example is presented that illustrates how consideration for the cost of RMS variations and sustained interruptions can alter the planning decision.

New robust voltage sag disturbance detector using an adaptive prediction error filter

Abstract: This paper presents a robust power disturbance detection algorithm. The power line disturbance is extracted from the steady-state 60 Hz and harmonics background using an adaptive prediction error filter. In order to detect the disturbance, a stop-and-go CA CFAR detector algorithm is utilized. The algorithm is demonstrated via simulations, and actual power transmission line data are utilized to demonstrate its performance and to compare the performance to wavelet-based detectors.

A Study on the Improvement of Power Quality Using Wavelet Transforms

Abstract: In recent years, electric power quality has become an important issue in power systems. This is due to the fact that the usage of power electronic equipment and electronic-based loads, which are sensitive to power quality disturbances, has increased, and thus normal operations in industrial and commercial environments are becoming more vulnerable to the electric power quality disturbances. In order to improve the quality of power, this paper presents a new noise suppression method to remove the noise associated with various types of electric power disturbances using the discrete wavelet transform. Power system disturbances such as those caused by voltage sag, voltage swell, outage, waveshape fault-impulse, harmonic distortion, capacitor energisation transients and are furnace are simulated using the well known electromagnetic transients program(EMTP). The wavelet transform, which has received considerable interest in many other areas such as acoustics, communications, etc., is proposed as a fast and effective means of analysing voltage and current waveforms generated during power system disturbances; this method hinges upon realising a given noise-ridden signal into a number of other signals which represent detailed components of the original signal. The decomposition is performed using multi-resolution signal decomposition techniques. The simulation results clearly demonstrate the superiority and effectiveness of the wavelet transform in both voltage and current signal noise reduction.

Voltage sag analysis for making economic decisions on mitigation solutions

Abstract: There exist today many power enhancement devices that can improve power system voltage quality. The technologies provide varying degrees of protection for different costs, which can suit individual customer's needs. The author describes how the job of selecting the appropriate power enhancement device may seem complicated, but is a straightforward process. He details how, once the requirements of the application are understood, engineering and economic analysis identifies a cost-effective solution.

A noise suppression method for improvement of power quality using wavelet transforms

Abstract: Electric power quality has become an important issue in power systems. This is due to the fact that the usage of power electronic equipment and electronic-based loads, which are sensitive to power quality disturbances, has increased, and thus normal operations in industrial and commercial environments are becoming more vulnerable to the electric power quality disturbances. In order to improve the quality of power, this paper presents a new noise suppression method to remove the noise associated with various types of electric power disturbances using the discrete wavelet transform. Power system disturbances such as those due to voltage sag, voltage swell, outage, waveshape fault-impulse, harmonic distortion, capacitor energisation transients and arc furnace are simulated using the well known Electromagnetic Transients Program (EMTP). The wavelet transform, which has received considerable interest in many other areas such as acoustics, communications, etc., is proposed as a fast and effective means of analysing voltage and current waveforms generated during power system disturbances; this method hinges upon realising a given noise-ridden signal into a number of other signals which represent detailed components of the original signal. The decomposition is performed using multi-resolution signal decomposition techniques. The simulation results clearly demonstrate the superiority and effectiveness of the wavelet transform in both voltage and current signal noise reduction.

An enhanced method for assessing the effect of voltage sag in power distribution system

Abstract: This paper presents a method to evaluate the effect of voltage sags in power distribution systems using the historical component reliability data of electric utilities. The proposed method is based on the SCBEMA curve that expresses the representative susceptibility curve by voltage sag for each customer type. These curves are made using the CBEMA curves obtained from experiments on the customers' sensitive equipment. In order to transform SCBEMA curves into the probability damage by voltage sag, the fuzzy membership function is used in this paper. Case studies were performed to analyze the effect of voltage sag by varying parameters. The Monte Carlo simulation method and the historical reliability data in KEPCO are used for case studies.

Optimal Daubechies' wavelet bases for detection of voltage sags in electric power distribution and transmission systems

Abstract: Wavelet analysis of voltage sag completely depends on the choice of the wavelet basis. For better detection performance via wavelet analysis, the choice of the optimal wavelet basis must be provided within the constraints of the uncertainty principle which restricts arbitrary assignment of time-frequency resolution. In this paper, we describe local properties of the wavelet basis and voltage sag signal in terms of time duration and frequency bandwidth parameters. After comparison of the local properties of the wavelet basis and voltage sag signal, we suggest a set of performance indexes to measure the time-frequency resolution relation between the wavelet basis and the voltage sag signal. This procedure of determining the optimal wavelet basis can be extended to other possible applications of wavelets.

Harmonics, power factor correction and transient overvoltage analysis in a stainless steel cold rolling mill plant system caused by voltage sags

Abstract: This paper presents guidelines for analyzing harmonics, power factor correction and transient overvoltage in a stainless steel cold rolling plant using sendzimir cold rolling mill process. The rolling mill loads operate at a wide range variation of power factors and reactive power that results in penalty to charges and bus voltages variation. In addition, the nonlinear characteristics of the rolling mill drives cause significant harmonic currents flowing through the plant and utility power system. These harmonic currents result in system voltage distortion and power loss in the system. The measurement of voltage and current auto reclosing supply voltage during power line disturbances has been performed. Voltage sag and short interruption occurrence often exist in such a case study. The extended analysis of results may lead to identifying problems and possible correction measures.

Investigation of electric drive system responses to lightning related voltage sags

Abstract: Lightning strokes on utility transmission lines may cause flashovers between line to line or line to ground, which would consequently result in voltage sags at the lower AC voltage buses. A voltage sag at the AC bus of electric drives would cause a voltage drop at the drive DC bus voltage which may have an adverse impact on electric drive systems. Both AC and DC drives are sensitive to the DC bus voltage drop. Electric drive tripping, which would cause paper machine shutdown, is a frequent occurrence during lightning seasons. This paper presents the result of an investigation of the drive responses to the utility lightning strokes for a paper mill which has both AC and DC drives. Specifically, the following topics are discussed: (1) voltage sag phenomena related to the fault caused by lightning strokes on the utility transmission systems; (2) electric drive system responses to the voltage sags; a computer model on a three-phase system was developed to investigate the problems; and (3) possible system and drive modifications for riding through voltage sags. The impacts of the modifications have also been evaluated to assure proper paper machine operations.

Voltage sag limiting system and method of operation

Abstract: A system (10) for limiting the current (56) drawn by a load (22) from a power source (18) includes a control circuit (12) and a current limiting circuit (14). Control circuit (12) receives a threshold voltage (21), VTHRESHOLD, and a source voltage (19), VSOURCE, associated with power source (18), and generates a control voltage (24) in response to VSOURCE and VTHRESHOLD. Current limiting circuit (14) limits the current (56) drawn by load (22) from power source (18) in response to control voltage (24) such that VSOURCE does not decrease below VTHRESHOLD.

Modelling consumer voltage sags caused by distribution primary faults

Abstract: In recent years, managers and engineers of industrial and commercial facilities have expressed their deep concerns about the disruption of their electronic and computerized processes due to the frequent occurrence of lower voltage conditions on their power delivery systems, particularly voltage sags. The cause of these voltage sags is often attributed to line faults occurring in the utility transmission and distribution systems. This paper presents the phase and line-to-line voltage sag levels seen at an individual customer's facilities due to transmission and distribution faults occurring at various locations within the utility's network configurations. The models used to describe the transmission and distribution system operating characteristics are based on the detailed component electrical models contained in MatLab Power Blockset.

Behaviour of variable speed drives under the influence of voltage sags

Abstract: Vlll In ac variable speed drives (VSDs) having an uncontrolled rectifier front-end, the effect of voltage sags are mainly observed in the dc bus characteristics. These V S D s are susceptible to nuisance tripping due to dc under-voltage or ac over-current faults which result in production loss, material wastage and require manual interventions in industrial processes. This thesis is aimed at improving the voltage sag ride-through performance of ac V S D s by improving the control algorithm. The proposed strategy recommends maintaining the dc link voltage constant at the nominal value utilising two control modes (a) by recovering the kinetic energy available in the rotating mass at high motor speeds and (b) by recovering the magnetising energy available in the motor winding inductances at low speeds. By combining these two modes, the V S D can be configured to ride-through voltage sags at all speeds. Additional control loops are suggested for this dc link voltage control. The proposed control strategy was applied on a synchronous reluctance motor (SRM) V S D and on an induction motor (IM) V S D . In the case of an S R M V S D , both modes of this control strategy are found to work satisfactorily and sag ride-through can be achieved at all motor speeds. In an IM VSD, the first mode of the control strategy, viz. closed loop dc voltage control by recovering the kinetic energy available in the system inertia is found to work satisfactorily and sag ride-through can be achieved at high motor speeds. However, due to the inherent I M characteristics, it was found that the magnetising energy present in an I M is not recoverable. The reasons for this behaviour are analysed and an alternative ridethrough scheme is suggested by accommodating the limitations encountered in the case of an I M so that the V S D is able to ride-through voltage sags at low speeds also. This control strategy can provide a voltage sag ride-through performance at all motor speeds down to standstill. It is also shown that the transition between various control modes during a sag situation can be achieved relatively smoothly.