Showing papers on "Voltage sag published in 2001"

Analysis and performance evaluation of a distribution STATCOM for compensating voltage fluctuations

Abstract: Controller design of a STATCOM-based voltage compensator requires a valid analytical model of the system. If phasor algebra is used for modeling, it is difficult to accurately describe the STATCOM behavior during compensation of subcycle transients in the PCC voltage. In this paper, a small-signal model of the system, with a distribution line, is derived. Predictions based on frequency-domain analysis are made, which are validated by experimental results. This model, therefore, can be used for controller design where subcycle voltage transients are to be compensated. It is shown that the voltage controller, so designed, can accomplish voltage sag mitigation. A discussion on the design of DC bus voltage controller-and experimental results showing its performance-is also included.

Compensation of distribution system voltage sag by DVR and D-STATCOM

Abstract: This paper describes the techniques of correcting the supply voltage sag in a distribution system by two power electronics based devices called DVR and D-STATCOM. A DVR injects a voltage in series with the system voltage and a D-STATCOM injects a current into the system to correct the voltage sag. The steady state performance of both DVR and D-STATCOM is determined and compared for various values of voltage sag, system fault level and load level. The minimum apparent power injection required to correct a given voltage sag by these devices is also determined and compared. The maximum voltage sag that can be corrected without injecting any active power into the system is also determined. Simulation results indicated that a DVR can correct a voltage sag with much less injected apparent power compared to that of a D-STATCOM.

Software phase-locked loop applied to dynamic voltage restorer (DVR)

Abstract: In this paper, the analytical and practical design issues of a software phase-locked loop (SPLL) for DVR are presented. A SPLL model that uses a lag/lead loop controller, is derived in order to analyse the system performance and filtering characteristic by the use of bode diagrams and root-locus methods. In DVR applications, parameters of the design of the SPLL controller are not only dependent on the steady state and dynamic state, but also on practical conditions such as utility unbalance, voltage sag/swell magnitude, voltage harmonics, phase jumps and frequency variations. Therefore, the practical aspect of the SPLL implementation has also been discussed. Experimental results demonstrate its phase tracking capability.

Reducing voltage sags through fault current limitation

Abstract: Short circuit current limitation in distribution utilities can be an effective way to improve power quality, since the expected voltage sag amplitude during faults can be dramatically reduced A simple series LC circuit tuned at the net frequency, with the capacitor shunted by a metal oxide varistor (MOV), proves to be a well suited limiter to reach the goal In the paper, the properties of such a circuit are analyzed and its operation is investigated through computer simulation The relevant pro and con aspects are outlined and discussed

Three-phase PWM boost-buck rectifiers with power-regenerating capability

Abstract: Three-phase pulsewidth-modulated (PWM) boost-buck rectifiers with power-regenerating capability are investigated. The converters under consideration are capable of: (1) both voltage step-up and step-down; (2) bidirectional power processing; and (3) almost unity-power-factor operation with nearly sinusoidal AC current. Expected advantages are: (1) applicability to lower voltage applications, e.g., direct retrofit to replace diode or thyristor rectifiers; (2) switching loss reduction in the inverter load; (3) low-order harmonic control in the inverter load output voltage; (4) blanking time effect mitigation in the inverter load; and (5) a modest level of voltage sag/swell compensation. In this paper, firstly, a step-by-step power stage derivation process is described. Then, taking the Cuk-Cuk realization as an example, its operating principle and modulation scheme are described. A steady-state model and dynamic model for controller design are also described. Representative results of circuit simulations and hardware experiments are presented. Through these procedures, the feasibility of the presented three-phase PWM boost-buck rectifier with power-regenerating capability is demonstrated.

Solutions to the power quality problem

Abstract: The problems of power quality are diverse both in nature and also in terms of the power systems that must be considered. From high-voltage transmission systems to domestic supply problems due to natural phenomena and interference from polluting loads call for a very wide range of solutions. These solutions are now becoming available with the development of new and larger solid-state switching devices. This paper discusses some of the devices available to combat power quality problems including: static VAr compensators; STATCOM (static compensators); system interconnection using DC link; UPFC for power flow control; active filters; dynamic voltage restorers for voltage sag mitigation; energy storage; and the premium power park.

A fast detection algorithm suitable for mitigation of numerous power quality disturbances

Abstract: In this paper, a fast detection method for voltage disturbances is explored. The proposed method provides reliable and fast detection for either single-phase or polyphase voltage disturbances such as voltage sags, voltage swells, flicker, frequency change in the utility voltage, and harmonic distortion. The algorithm is based on the theory that allows a set of three-phase voltages be represented as dc voltages in a d-q synchronous rotating frame. In this case, the utility input voltages are sensed and then converted to dc quantities in the d-q reference frame. Thus, any disturbance at the utility input voltages will be promptly reflected as disturbances in the d-q values. Further processing of the signals minimizes the point-on-wave effect and yields a trip signal indicating if the disturbance is a voltage sag or a voltage swell. Analysis, simulation, and experimental results are presented for a three-phase system. The proposed algorithm is implemented on a digital signal processor (DSP)-based system to provide ride-through for critical loads.

Control loop and method for variable speed drive ride-through capability improvement

Abstract: A variable speed drive (VSD) having a rectifier, an inverter, an inverter modulator, an inverter controller, and a control loop, for controlling a motor load, wherein the control loop reduces an amount of power transferred to the inverter during a voltage sag. The control loop may include a reference generator, a filter, a regulator, and a ride-through corrective algorithm. A method for controlling a VSD to improve voltage sag ride-through by monitoring a voltage applied to the VSD, generating a control signal representative of losses in a motor load when a voltage sag is detected in the voltage applied to the VSD and applying less power to the load of the inverter.

The distribution STATCOM for reducing the effect of voltage sag and swell

Abstract: The concepts of flexible alternating current transmission systems (FACTs) and custom power have been widely studied. FACTs use power electronic devices and methods to control the high-voltage side of the network for improving power flow. Custom power is for low-voltage distribution, and improving poor power quality and reliability of supplies affecting factories, offices and homes. Power quality and reliability are becoming important issues for critical and sensitive loads after the introduction of the term of custom power by Hingorani in the early 1980s. Custom power devices is classified into three categories by their structures: dynamic voltage restorers (DVRs), distribution STATCOMs (DSTATCOMs) and unified power quality compensators (UPQCs). Among these devices, the main purpose of a DVR that injects voltage in series with a distribution feeder is reducing the effect of short-term voltage sags, dips, swells and momentary interruptions. The DVR is capable of generating and absorbing voltage independently controllable real and reactive power. It consists of a three-phase voltage source inverter, injection transformer, DC link and rectifier for charging the DC link or battery. As is known, rectifiers generate harmonic problems in distribution lines. Rectifiers or devices for charging the DC link are useless in this proposed system due to their structure. The proposed system has the function of generating and absorbing voltage using a self-charging control technique. This system has three states: (1) normal operation; (2) charging operation; and (3) recharging operation. The paper discusses control issues and the proposed control algorithm. The proposed control technique is applied to a DSTATCOM for protecting voltage sags, swells and momentary interruptions. The dynamic performance is analysed and verified through simulation.

The control and analysis of zero sequence components in DVR system

Abstract: The magnitudes and phases of the compensating voltages in a dynamic voltage restorer (DVR) system depend on the voltage sag in the phases affected by the fault and on the influence of the zero sequence components. If the delta connection of the transformer is used, the zero sequence components do not appear on the load side. But nowadays, wye connected transformers with grounded neutral are usually used. Therefore the zero-sequence components occur during faults. The zero-sequence components result in the high insulation costs and the asymmetry of the phases and magnitudes of the terminal voltages. This paper proposes a method that can mitigate the zero-sequence under the unbalance faults causing voltage sags and phase angle jumps. Moreover, the energy optimizing method is considered in this paper.

Comparison of system topologies for dynamic voltage restorers

Abstract: In this paper, different system topologies for dynamic voltage restorers are analyzed with the emphasis put on methods to acquire the necessary energy during a voltage sag Four different system topologies are analyzed and tested Two, which can be realized with insignificant energy storage and the energy is taken from the grid and two topologies, which are based on stored energy are compared Experimental tests on a 10 kVA rated dynamic voltage restorer shows that the no-energy storage concept is feasible, but an improved performance can for certain voltage sags, be achieved with stored energy In the comparison, the no-storage topology with a passive shunt converter at the load side is ranked highest followed by an energy storage topology with constant DC-link voltage

Impact of voltage sags in practical power system networks

Abstract: Voltage sags are momentary dips in voltage that may cause misoperations to the customer's sensitive loads of the power utility and are recognised as the most important power quality problems. The scope of the work presented in this paper includes: (i) critical awareness on the importance of characterization and impact of voltage sags, (ii) identification of a suitable algorithm for voltage sag analysis, and (iii) demonstration of the chosen methodology on a practical power system case study. The ability to estimate voltage sag characteristics offers an opportunity to evaluate alternate configurations. The overall methodology proposed utilizes the standard software packages of load flow analysis, short circuit studies and interface modules developed specifically for this problem. Methodology for sag analysis includes the characteristics of sag, like magnitude, duration, phase angle jump, sag frequency, some of the important factors affecting sag magnitude, and area of vulnerability for radial distribution systems.

Voltage sag propagation in systems with embedded generation and induction motors

Abstract: The paper present results of the analysis of the voltage sag propagation in a distribution system. The assessment of the voltage sag propagation has been made on the generic distribution network. Different network topologies of a generic network with and without embedded generators and induction motors are used in the study. Symmetrical and asymmetrical faults were simulated at various voltage levels and locations. New indices for assessment of voltage sag propagation are proposed. The influence of embedded generators and induction motors on voltage sag characteristics and propagation is also analysed.

Voltage Sag Correction by Dynamic Voltage Restorer with Minimum Power Injection

Abstract: A dynamic voltage restorer is a power quality device used to correct the voltage disturbances by injecting voltage as well as power into the system. A method of determining the exact amount of voltage in- jection required to systematically correct a specific voltage drop with minimum active power injection is described in this letter. Analytical ex- pressions for both the magnitude and angle of the injected voltage are also derived. It has been found that a voltage drop of less than () 1 − pf pu can easily be corrected without injecting any active power. For higher voltage drop, injection of active power is essential but its value can be minimized by drawing power from the supply at unity power factor. Keywords: DVR, power quality, voltage sag. Introduction: Voltage magnitude, waveform, and frequency are the major factors that dictate the quality of a power supply. Use of ex- tensive nonlinear power electronic loads is one of the major reasons of deteriorating the quality of power supply. Faults at either the transmis- sion or distribution level may also cause transient voltage sag or swell in the entire system or a large part of it. Also, under heavy load condi- tions, a significant voltage drop may occur in the system. Such voltage variations are not desirable for sensitive loads.

Unified power quality conditioner with a novel control algorithm based on wavelet transform

Abstract: Power quality problems have received great attention nowadays because of their bad economical impacts on both utilities and customers. The current harmonics are the most common power quality problem, and the voltage sag is the most severe one. This paper introduces a new control algorithm based on wavelet transform for unified power quality conditioner (UPQC) to suppress current harmonics and voltage sags. The proposed control algorithm is verified by digital simulation results using PSCAD/EMTDC.

Experimental investigation of performance of a single phase UPQC for voltage sensitive and non-linear loads

Abstract: A unified power quality conditioner (UPQC) is proposed in this paper. It protects the consumer at the load end from supply voltage sag, and provides unity power factor condition at the utility for different values of load power factor. A PC-based closed loop control scheme is proposed and experimental investigation is carried out in the laboratory. Selected experimental results are reported along with control circuit and phasor diagram to validate the proposition.

Wavelet approach to power quality monitoring

Abstract: Power quality issues have become more concerned with the extensive use of power electronic devices and nonlinear loads in the electric power system, the sensitive detection and accurate classification of power disturbances attracts more attention from both utilities and industrial customers. A wavelet transform based new approach to the detection of power disturbance and measurement of power quality is proposed in this paper. This approach adopts a series of complex wavelet functions as multi-channel band pass FIR filters to detect the occurrence of disturbance, and precisely estimate the instantaneous average magnitudes and frequencies in their respective frequency bands as time varies, which are essential to power quality assessment. The performance evaluation of the proposed approach has been conducted under a variety of disturbances including momentary interruption, sag, swell, flicker, harmonics, frequency deviation, transients etc., on the basis of simulations in MATLAB environment. The evaluation results demonstrate the effectiveness and advantages of the proposed approach.

Estimating the annual frequency and cost of voltage sags for customers of five Finnish distribution companies

Abstract: Frequency and cost of voltage sags for customers of five Finnish distribution companies were evaluated. For voltage sag frequency calculations the method of fault positions was applied. Only sags caused by short circuit faults in medium voltage distribution networks were considered. Sag frequency below considered 50% magnitude showed quite low numbers. However the cost related to this sag magnitude revealed that voltage sags are a relevant power quality issue having economic significance.

A simple methodology for estimating the effect of voltage sags produced by induction motor starting cycles on sensitive equipment

Abstract: Voltage sag produced by induction motor starting current is one of the main causes of sensitive equipment dropout. The use of a motor starter reduces the voltage sag depth but increases its duration. The subsequent connection to full voltage produces a new sag separated from the first one by a few seconds. A simple method is proposed, which allows to transform the information of starting current/time characteristics to voltage sag depth/time characteristics that are directly comparable with the sensitive equipment susceptibility curves (CBEMA and other curves). The methodology also allows to consider motor repetitive starts as well as different starting cycles.

New functionalities of the unified power quality conditioner

Abstract: Power quality problems have received much attention because of their bad economical impacts on both utilities and customers. Current harmonics are the most common power quality problem, while the voltage sag is the most severe. This paper introduces new functionalities for the unified power quality conditioner (UPQC), which never introduced before by one mitigating device (UPQC). This device can suppress current harmonics and compensate the current unbalance, which might be produced in the distribution system. It also mitigates different voltage sags, which might result from different faults in the system. The performance of the proposed UPQC is verified by digital simulation results PSCAD/EMTDC.

Method and system for providing voltage support to a load connected to a utility power network

Abstract: A voltage recovery system and its method of operation provides voltage protection to a load connected to a distribution network of a utility power system or network by boosting the voltage on a distribution line, during a momentary voltage sag caused by a fault or other contingency. The method includes voltage protection from a voltage recovery system to a load connected to a distribution network of a utility power network. A voltage protection characteristic required by the load is selected. On the basis of electrical characteristics of the voltage recovery system and the distribution network, a determination is made as to whether the voltage recovery system is capable of providing the required first voltage protection characteristic. Determining whether the voltage recovery system is capable of providing the required voltage protection characteristic includes determining a fault current capability characteristic of the distribution network and the maximum voltage improvement characteristic that the voltage recovery system can provide. If the voltage protection characteristic is greater than the maximum voltage improvement characteristic, the fault current capability characteristic and the maximum voltage improvement characteristic are used to determine an impedance in the form of an inductor to be added to the distribution network.

Sag generator with switch-mode impedance

Abstract: A voltage sag generator for alternating current power systems intentionally creates power quality disturbances. The sag generator has a switch-mode impedance between its input and its output. Varying the duty-cycle of the switch-mode impedance causes voltage sags.

Voltage sag analysis taken into account in distribution network design

Abstract: Voltage sag distributions were calculated for six different 20 kV electricity distribution systems. Results are highly dependent on the distribution system. The percentage of deep severe sags is much higher in urban systems while the annual number of sags is bigger in rural areas. AM/FM-GIS has proved to be an efficient tool in distribution system planning. Often costs are included in these studies. As a new development step, sag analysis was introduced to be integrated to these tools. A pilot software has already been developed in this project.

Application of converter-based series device for voltage sag mitigation to induction motor load

Abstract: This paper deals with the application of a static series-connected device for voltage sag mitigation to an induction motor. A control system capable to handle unbalanced voltage sags is presented and explained. The aim of this study is to verify that satisfactory performance can be obtained also in presence of dynamic loads. Simulated results, obtained by using actual voltage sag recordings, are presented.

A survey on high tech industry power quality requirements

Abstract: Power quality (PQ) is an important issue in the modern manufacturing and process control environment. Energy efficient and electronically controlled automatic machines are sensitive to the deviations in the supply voltage. The quality of power supply has caused a direct economic impact on many industrial customers. At facilities where a high degree of power quality and reliability is critical, any voltage surge, sag and momentary outage on the supply lasting more than a few cycles could cause several of the tools and production lines to malfunction. This paper summarizes the results of two surveys on high tech industry power quality requirements. Six categories of high tech industry are included in this survey, they are: semiconductor, computer and peripherals, telecommunications, optoelectronics, precision machinery, and biotechnology. The perception of high tech companies on the power quality problems and their strategies of dealing with the problems are discussed.

Voltage sags mitigation on distribution utilities

Abstract: Voltage sags are important to industrial reliability. Modern process controls are often sensitive to voltage sags, that may cause significant production outages. World-wide experience proves that a main origin of voltage sags affecting customers are short circuits on utility lines, with special reference to the overhead radial distribution ones. Since the voltage sag during the fault is proportional to the short-circuit current, an effective fault-current limitation by means of a device connected at the beginning of the most exposed radial feeders will limit the expected voltage sag amplitude and improve the system power quality. In the paper a proposal for a simple but effective fault-current limiter is given and discussed.

Automated analysis of voltage sags, their causes and impacts

Abstract: This paper focuses on voltage sags. It introduces an automated approach to the analysis of voltage sags, their causes and impacts. The sag analysis is performed using software tools developed for this purpose. First, the software performs detection, classification and characterization of voltage sags. Next, if the voltage sag is caused by a fault, the software finds the fault location. Finally the software allows replaying of the sag waveforms for the purpose of evaluating of the impacts on equipment operation.

Characteristics of voltage sags in radial networks with dynamic loads and embedded generators

Abstract: This paper is concerned with the analysis of voltage sags characteristics and propagation in radial networks following the faults in the system. Both symmetrical and asymmetrical faults are simulated at the different voltage levels and different locations in the network. Realistic pre-fault voltages and voltage phase-angles obtained from load flow study are used in all calculations. The influence of the size of the embedded generation, dynamic loads, load structure and composition on voltage sags characteristics is also investigated.