Showing papers on "Voltage sag published in 2010"

Mitigation of Voltage Disturbances Using Dynamic Voltage Restorer Based on Direct Converters

Abstract: In this paper, two new topologies are proposed for three-phase dynamic voltage restorers (DVRs). These topologies are based on direct converters. The proposed topologies do not require dc-link energy storage elements. As a result, they have less volume, weight, and cost. They can also compensate long-time voltage sags and swells. The proposed DVRs can compensate several types of disturbances, such as voltage sags, swells, unbalances, harmonics, and flickers. Moreover, due to the fact that the compensation voltage for each phase is taken from all three phases, the proposed topologies can compensate one-phase outages. In the proposed topologies, three independent three-phase to single-phase direct converters are used. Each converter operates independently and, as a result, the proposed DVRs properly compensate unbalanced voltage sags and swells. The used converters can be constructed by four or six power switches. Depending on the structure of the used converters, the compensation ranges will be different. A new control method is also proposed for using direct ac/ac converters. The experimental and simulation results verify the capabilities of the proposed topologies in compensation of voltage distortions.

Interphase AC–AC Topology for Voltage Sag Supporter

Abstract: A new topology is proposed in this paper to compensate voltage sags in power distribution systems. Voltage sag is one of the major power quality problems encountered by industries. The traditional voltage sag compensator, which is a dynamic voltage restorer based on energy storage device with a series-connected voltage-source inverter, is not adequate for compensating deep and long-duration voltage sags. As per the sensitive load concern, deep and long-duration sags are more vulnerable than shallow and short-duration sags. To compensate the voltage sag, a new interphase ac-ac topology is proposed that needs no storage device. The compensator of each phase is connected on the other two phases and the power is tapped from them. A single-phase compensator is realized with two ac chopper circuits and two transformers. By controlling the duty cycle of each ac chopper, the required voltage is realized to compensate the voltage sag. Analysis, simulation, and experimental results are presented to demonstrate the proposed concept.

Voltage Control and Active Power Management of Hybrid Fuel-Cell/Energy-Storage Power Conversion System Under Unbalanced Voltage Sag Conditions

Abstract: This paper concentrates on the control of hybrid fuel-cell (FC)/energy-storage distributed generation (DG) systems under voltage sag in distribution systems. The proposed control strategy makes hybrid DG system work properly when a voltage disturbance occurs in distribution system, and it stays connected to the main grid. To distribute the power between dc power sources and stabilize the dc-link power, a Lyapunov-based neuro-fuzzy control strategy has been developed. This controller determines the supercapacitor power that should be generated according to the amount of available energy in dc-link. Also, current control strategies for the FC converter (boost) and supercapacitor converter (buck-boost converter) are designed by proportional-integral and sliding-mode control consequently. Moreover, a complementary control strategy for voltage source converter based on positive and negative symmetrical components is presented to investigate the voltage sag ride-through and voltage control capability. The hybrid system is studied under unbalance voltage sag condition. Simulation results are given to show the overall system performance including active power control and voltage sag ride-through capability of the hybrid DG system.

New Control Scheme for a Unified Power-Quality Compensator-Q With Minimum Active Power Injection

Abstract: Voltage sags are one of the most frequently occurring power-quality problems challenging power systems today. The unified power-quality conditioner (UPQCs) is one of the major custom power solutions capable of mitigating the effect of supply voltage sags at the point of common coupling (PCC) or load. A UPQC-Q employs a control method in which the series compensator injects a voltage that leads the supply current by 90° so that the series compensator at steady state consumes no active power. However, the UPQC-Q has some disadvantages. First, there are limitations in rating when using UPQC-Q for series compensation. Second, there is a phase difference between the input and output voltage in proportion to the severity of voltage sags. As a result, it cannot offer effective compensation for voltage drops. This paper proposes a new control scheme for the UPQC-Q that offers minimum active power injection. The proposed power injection method takes into consideration the limitations of the rated voltage capacity of the series compensator, and the phase difference during voltage sag events. The validity of the proposed control scheme has been investigated by simulation and experimental results for less than 50% voltage sag conditions.

Global Minimization of Financial Losses Due to Voltage Sags With FACTS Based Devices

Abstract: FACTS based devices are proven to be an efficient mitigation solution for voltage sag prevention. The high cost of FACTS based devices often prohibits their wider deployment within power networks. This paper presents an approach for comprehensively assessing the financial benefits to the network resulting from their use. The annual financial losses of the entire network due to voltage sags are used as a savings target. The three most widely used FACTS based devices for voltage sag mitigation, namely, static VAr compensator (SVC), static compensator (STATCOM) and dynamic voltage restorer (DVR) are then optimally placed using a Niching genetic algorithm (NGA). The aim is to reduce overall financial losses in the network due to voltage sags. The cost of the individual devices along with their installation costs and annual maintenance are taken into account in the optimization procedure. Since this methodology is largely based on an economic evaluation of the solution, several conventional economic analysis methods are utilized. Simulations are performed on a 295-bus generic distribution network (GDN).

Implementation and Performance Evaluation of a Fast Dynamic Control Scheme for Capacitor-Supported Interline DVR

Abstract: The implementation of a fast dynamic control scheme for capacitor-supported interline dynamic voltage restorer (DVR) is presented in this paper. The power stage of the DVR consists of three inverters sharing the same dc link via a capacitor bank. Each inverter has an individual inner control loop for generating the gate signals for the switches. The inner loop is formed by a boundary controller with second-order switching surface, which can make the load voltage ideally revert to the steady state in two switching actions after supply voltage sags, and also gives output of low harmonic distortion. The load-voltage phase reference is common to all three inner loops and is generated by an outer control loop for regulating the dc-link capacitor voltage. Such structure can make the unsagged phase(s) and the dc-link capacitor to restore the sagged phase(s). Based on the steady-state and small-signal characteristics of the control loops, a set of design procedures will be provided. A 1.5-kVA, 220-V, 50-Hz prototype has been built and tested. The dynamic behaviors of the prototype under different sagged and swelled conditions and depths will be investigated. The quality of the load voltage under unbalanced and distorted phase voltages, and nonlinear inductive loads will be studied.

Modeling of FACTS Devices for Voltage Sag Mitigation Studies in Large Power Systems

Abstract: Flexible ac transmission system (FACTS) devices or their subderivative custom power devices are efficient and often used and recommended power-electronics-based devices for mitigation of voltage sags in electrical power system. With FACTS devices installed, the overall system (and individual bus) sag performance could significantly change depending on the type of the device used. In order to assess this change in sag performance in realistic large power systems, the classical (essentially static) fault calculation procedure should be amended to incorporate the effects of these devices on bus voltages. This paper presents a new approach of quasistatic sag analysis using a system impedance (Z _bus) matrix that incorporates FACTS devices. Three types of FACTS devices which are most often used in practical applications are considered in this study: static compensator, static var compensator, and the dynamic voltage restorer. The case studies based on the 295-bus generic distribution system are used to illustrate the modeling method and the effectiveness of these devices in sag mitigation.

Evaluation of a Multilevel Cascaded-Type Dynamic Voltage Restorer Employing Discontinuous Space Vector Modulation

Abstract: In this paper, the application of a cascaded multilevel inverter as a dynamic voltage restorer (DVR) is investigated. Two discontinuous multilevel space vector modulation (SVM) techniques are implemented for DVR control and are shown to reduce inverter switching losses while maintaining virtually the same harmonic performance as the conventional multilevel SVM at a high number of levels. This paper also presents a mathematical relationship for computing the distortion at the point of common coupling (PCC) as a function of the distortion of the DVR. This enables the selection of the number of levels required for a certain application. An extended sag duration support compared to the two-level DVR is another advantage of the DVR with a cascaded multilevel inverter. The common-mode voltage (CMV) at the PCC has been evaluated for the three SVM techniques (the conventional multilevel SVM and the two discontinuous SVM), presenting a lower CMV for the second discontinuous SVM. A design example is presented for an 11-kV 5-MVA DVR multilevel cascaded inverter for up to 17 levels, employing the conventional multilevel SVM and the two discontinuous SVM techniques.

A Dynamic LVRT Solution for Doubly Fed Induction Generators

Abstract: Doubly fed induction generators have become the most common type of wind turbine generators. However, this type of generator is susceptible to grid-side low voltage and short circuits due to existence of a power electronics converter on the rotor side. When a short circuit or voltage sag happens on the grid side, the rotor current of the generator tends to rise, which could cause damage to the rotor converter. Design and implementation of a series converter on the stator side is presented in this paper to limit the current rise in the rotor. This system includes an active AC/DC inverter, three series transformers, and a DC-bus capacitor. To lower the rating of the components and make the system viable for practical solutions, an exponential decaying sinusoidal voltage, instead of a pure sinusoidal voltage, is applied by the converter during short circuit.

Fast sag/swell detection method for fuzzy logic controlled dynamic voltage restorer

Abstract: In this study, the design and analysis of a fuzzy logic (FL) controlled dynamic voltage restorer (DVR) are presented and extended to perform fast fault detection. A new control method for DVR is proposed by combining FL with a carrier modulated PWM inverter. The proposed control scheme is simple to design and has excellent voltage compensation capabilities. The proposed method for voltage sag/swell detection has the ability of detecting different kinds of power disturbances faster than conventional detection methods. Effectiveness of the proposed detection method is shown by comparison with the conventional methods in the literature. Simulation results under unbalanced supply voltage are presented to evaluate the performance of the designed DVR.

Global Voltage Sag Mitigation With FACTS-Based Devices

Abstract: This paper presents an approach to optimally select and allocate flexible ac transmission (FACTS) devices in a distribution network in order to minimize the number of voltage sags at network buses. The method proposed is based on the optimization of a preselected objective function using simple and niching genetic algorithms (GA). The objective of the optimization is to achieve the improvement in overall system sag performance of the network. Using proposed GA-based optimization, the location, the type and the rating of six (in total) FACTS devices are optimized simultaneously. Three types of FACTS devices are implemented in this study, namely, static var compensator, static compensator, and dynamic voltage restorer. The performance of the proposed algorithm is tested and illustrated on 295-bus generic distribution system.

Voltage-Sag Tolerance of DFIG Wind Turbine With a Series Grid Side Passive-Impedance Network

Abstract: Due to the increase of the number of wind turbines connected directly to the electric utility grid, new regulator codes have been issued that require low-voltage ride-through capability for wind turbines so that they can remain online and support the electric grid during voltage sags. Conventional ride-through techniques for the doubly fed induction generator (DFIG) architecture result in compromised control of the turbine shaft and grid current during fault events. In this paper, a series passive-impedance network at the stator side of a DFIG wind turbine is presented. It is easy to control, capable of off-line operation for high efficiency, and low cost for manufacturing and maintenance. The balanced and unbalanced fault responses of a DFIG wind turbine with a series grid side passive-impedance network are examined using computer simulations and hardware experiments.

Voltage Quality Improvement Using DVR

Abstract: The problem of voltage sags and Swells and its severe impact on sensitive loads is well known. To solve this problem, custom power devices are used. One of those devices is the Dynamic Voltage Restorer (DVR), which is one of the most efficient and effective modern custom power devices used in power distribution networks. This paper described DVR principles and voltage correction methods for balanced and/or unbalanced voltage sags and swells in a distribution system. Simulation results were presented to illustrate and understand the performances of DVR under voltage sags/swells conditions. The results obtained by simulation using MATLAB confirmed the effectiveness of this device in compensating voltage sags and swells with very fast response (relative to voltage sag/swell time).

Line-Interactive Single-Phase Dynamic Voltage Restorer With Novel Sag Detection Algorithm

Abstract: This paper describes the development of a line-interactive single-phase dynamic voltage restorer (DVR) with a novel sag detection algorithm. The developed detection algorithm has a hybrid structure composed of an instantaneous detection part and the root-mean-square (rms) variation detection part. The source voltage passes through the instantaneous sag detection part. If instantaneous sag is detected, the rms variation detector-1 is selected to calculate the rms variation. The rms variation detector-2 is selected when the instantaneous sag occurs under the operation of the rms variation detector-1. The feasibility of proposed algorithm was verified through computer simulations and experimental works with a 3-kVA-rating prototype of line-interactive DVR. The line-interactive DVR with the proposed algorithm can compensate the input voltage sag or interruption within 2.0-ms delay. The developed DVR can be effectively utilized for the sensitive loads, such as a computer, communication device, and automation device.

An Inrush Mitigation Technique of Load Transformers for the Series Voltage Sag Compensator

Abstract: Survey results suggest that 92% of interruption at industrial facilities is voltage sag related. The voltage sag compensator, based on a transformer-coupled series-connected voltage-source inverter, is among the most cost-effective solution against voltage sags. When voltage sags happen, the transformers, which are often installed in front of critical loads for electrical isolation, are exposed to the disfigured voltages and a dc offset will occur in its flux linkage. When the compensator restores the load voltage, the flux linkage will be driven to the level of magnetic saturation and severe inrush current occurs. The compensator is likely to be interrupted because of its own overcurrent protection, and eventually, the compensation fails, and the critical loads are interrupted by the voltage sag. This paper proposes an inrush current mitigation technique together with a state-feedback controller for the voltage sag compensator. The operation principles of the proposed method are specifically presented, and experiments are provided to validate the proposed approach.

Transformer based voltage sag generator to perform LVRT and HVRT tests in the laboratory

Abstract: A transformer based voltage sag generator to test renewable energy systems is presented. Design considerations concerning overvoltages and overcurrents during switching action are adressed. Measurement results of a 30 kW laboratory prototype testing a line side PWM converter and a DFIG are presented. Both measurement results for investigations on the transformer based sag generator at the line side converter and the DFIG show the superb functionality of the sag generator. Voltage dips of variable magnitude, duration time and fault type can be generated to perform certification of LVRT capability. The device is also able to perform HVRT tests.

Novel Sag Detection Method for Line-Interactive Dynamic Voltage Restorer

Abstract: This letter proposes a novel sag detection method for the line-interactive dynamic voltage restorer (DVR). The DVR with proposed detection method can compensate the voltage sag or interruption within 2-ms delay, which is faster than the existing delay time of 4 ms. The feasibility of proposed method was verified through computer simulations. The DVR with proposed detection method can effectively compensate the voltage sag or interruption for sensitive loads.

A Dynamic Voltage Restorer for Voltage Sag Mitigation in a Refinery with Induction Motors Loads

Abstract: Problem statement: ANRPC is a refinery based in Alexandria, Egypt, The plant was subjected to several shutdowns due to tripping of large induction motors, either by under voltage or by over current relays, sometimes by the mechanical protection. The main cause for such unplanned shutdowns was voltage sags. The Dynamic Voltage Restorer (DVR) has recently been introduced to protect the industrial facilities from voltage sags and other voltage disturbances. Existing configurations and control techniques for the DVR aim at protecting industries of high-tech, loads with adjustable speed drives and other power-electronic based loads. Industries with induction motors loads require a complete different approach for the design and control of a suitable DVR. Owing to the inherit inertia of the induction motors and their capability to withstand short-duration, shallow sags, in addition to its tolerance to phase jumps, a DVR with low cost, fast response and simple controller could be configured to fulfill the voltage restoration requirements. Approach: In this study, a simple DVR was proposed, which utilized the classical Fourier Transform (FT) for sag detection and quantification, a controller based on feed-foreword technique which utilized the error signal (difference between the reference voltage and actual measured voltage) to trigger the switches of an inverter using a Pulse Width Modulation (PWM) scheme. The proposed DVR utilized energy from other available feeder or from energy storage unit through a rectifier. Modeling and simulation of the proposed DVR was implemented in the Matlab/Simulink workspace. Results: Simulation results showed that the proposed DVR was efficient in mitigating balanced, unbalanced, multistage and consecutive sags, as well as swells (over-voltages). The main shortcoming of the DVR, being a series device, was its inability to mitigate complete interruptions. Conclusion: The DVR should be configured with respect to the load requirements. With respect to induction motors loads with inherit inertia and insensitivity to phase jumps, the proposed DVR would be of lower cost, simpler controller and faster response. Key words: Power quality, voltage sag, Dynamic Voltage Restorer (DVR), custom power INTRODUCTION Problem statement: Alexandria National Refining and Petrochemicals Co. (ANRPC) is a refinery based in Alexandria, Egypt, with the purpose of producing lead-free, high-octane gasoline. With an average load of 10 MW, the plant suffers from several shutdowns due to voltage sags. Although lasting for durations in the range of a quarter second to slightly more than one second, these voltage sags cause large induction motors connected directly to the supply bus to trip, either by undervoltage or by overcurrent relays, sometimes by the mechanical protection. As the motor torque is directly proportional to the square of the supply voltage, a decrease (sag) to 70% of the rated voltage will cause the motor torque to decrease to 49%, which may not be sufficient for driving the load. To avoid the risk of damage of the motors’ shafts during voltage sags, strict protection settings are applied, leading to numerous (sometimes unnecessary) shutdowns. A previous study on the effects of voltage sags on IMs showed that IMs are insensitive to very short duration sags (and interruptions). IMs were also unaffected by phase angle jumps associated with most of the voltage sags (ElShennawy et al ., 2009).

Assessment of system voltage sag performance based on the concept of area of severity

Abstract: This study presents a new approach for voltage sag assessment based on the concept of ‘area of severity’ (AOS). The network regions, where the fault occurrences will simultaneously lead to voltage sags at different sensitive load points, can be determined by performing an AOS analysis. The impact rankings of the network lines and buses are also addressed. The impact rankings are determined according to the contribution degree of each line and bus faults to the total expected number of voltage sags in the power system. The concepts of AOS and the impact ranking are useful in establishing efficient planning for the mitigation of voltage sags, and for evaluating the relationship between sensitive load points and system voltage sag performance.

High voltage power line communication system using an energy harvesting power supply

Abstract: A communication and control system for a high voltage power line using an energy harvesting power supply to avoid batteries in the communication components maintained at line voltage. The energy harvesting power supply utilizes scavenged backscatter power received from a transceiver maintained at ground potential or a power supply with a super-saturating magnetic flux core, such as a mu-metal core, to harvest electromagnetic field energy from the power line. The communication equipment maintained at line voltage communicates information to the transceiver maintained at ground potential by modulating backscatter energy reflected from the beam transmitted by the ground level transceiver to minimize the power requirement of the communication equipment maintained at line voltage. Response equipment includes capacitors, voltage regulator, voltage sag supporter, circuit interrupter, remote communication equipment, reporting and analysis system.

Analysis of a Distributed Grid-Connected Fuel Cell During Fault Conditions

Abstract: The effect of a short circuit fault and a voltage sag fault on a distributed grid-connected solid oxide fuel cell (SOFC) is investigated in this paper. The fuel cell is modeled in Simulink and the performance is verified against experimental load testing data. Grid faults are simulated, and conclusions are drawn on the fuel cell response and the effects of the fault condition on internal fuel cell parameters.

New Control Technique Applied in Dynamic Voltage Restorer for Voltage Sag Mitigation

Abstract: The Dynamic Voltage Restorer (DVR) was a power electronics device that was able to compensate voltage sags on critical loads dynamically. The DVR consists of VSC, injection transformers, passive filters and energy storage (lead acid battery). By injecting an appropriate voltage, the DVR restores a voltage waveform and ensures constant load voltage. There were so many types of the control techniques being used in DVR for mitigating voltage sags. The efficiency of the DVR depends on the efficiency of the control technique involved in switching the inverter. Problem statement: Simulation and experimental investigation toward new algorithms development based on SVPWM. Understanding the nature of DVR and performance comparisons between the various controller technologies available. The proposed controller using space vector modulation techniques obtain higher amplitude modulation indexes if compared with conventional SPWM techniques. Moreover, space vector modulation techniques can be easily implemented using digital processors. Space vector PWM can produce about 15% higher output voltage than standard Sinusoidal PWM. Approach: The purpose of this research was to study the implementation of SVPWM in DVR. The proposed control algorithm was investigated through computer simulation by using PSCAD/EMTDC software. Results: From simulation and experimental results showed the effectiveness and efficiency of the proposed controller based on SVPWM in mitigating voltage sags in low voltage distribution systems. It was concluded that its controller also works well both in balance and unbalance conditions of voltages. Conclusion/Recommendations: The simulation and experimental results of a DVR using PSCAD/EMTDC software based on SVPWM technique showed clearly the performance of the DVR in mitigating voltage sags. The DVR operates without any difficulties to inject the appropriate voltage component to correct rapidly any anomaly in the supply voltage to keep the load voltage balanced and constant at the nominal value.

A novel Dynamic Voltage Restorer based on matrix converters

Abstract: In this paper a Dynamic Voltage Restorer (DVR) based on a matrix converter without energy storage devices is proposed to cope with voltage fluctuations. The ac/ac power converter takes energy from the grid during voltage sag/swell. By connecting the matrix converter's input terminals on the load-side and injecting the compensation voltages on the supply-side, it is possible to hold a constant input voltage, resulting in an efficient solution for compensating deep voltage sags and swells. Thus, the proposed topology has the ability to compensate balanced and unbalanced voltage fluctuations and to eliminate the energy storage elements. The space-vector modulation (SVM) technique is utilized to fulfill the input and output requirements. Numerical simulation results are presented to validate the approach.

Opportunities for power quality improvement through DG-grid interfacing converters

Abstract: Power electronics technology is becoming an increasingly important aspect of today's power distribution system. It is the key interface to connect distributed energy resource (DER) to the utility and the local loads. With the increased penetration of power electronics based distributed generation (DG) systems, the power quality requirements are becoming more stringent. On the other hand, if controlled and regulated properly, the DG-grid interfacing converters are able to improve the system efficiency and power quality, in addition to the primary function of real power injection. This paper discusses the opportunities for power quality improvement through the DG-grid interfacing converters. While the harmonic voltage compensation is the focus of this paper, a number of other ancillary functions, such as unbalance voltage compensation, voltage sag mitigation and reactive power compensation, can be realized in a similar manner. Two alternative DG control methods, namely current controlled DG and voltage controlled DG, are considered and the associated power quality compensation strategies are developed. Simulation results and experimental results from a three-phase 5kVA laboratory DG prototype are provided.

A new fast peak detector for single or three-phase unsymmetrical voltage sags

Abstract: In this paper, a new fast peak detector for single or three-phase unsymmetrical voltage sags is proposed. The proposed detector is modified from a single-phase digital phase-locked loop based on a d-q transformation using an all-pass filter (APF). The all pass filter generates a virtual phase with 90° phase delay but the virtual phase cannot reflect a sudden change of the grid voltage in the moment of voltage sag, which causes a resulted peak value to be significantly distorted and settle down slowly. Specially, a settling time of the peak value is too long when voltage sag occurs around zero crossing such as phase 0° and 180°. This paper describes operating principle of the APF problem and proposes a modified all-pass filter (MAPF) to mitigate the inherent APF problem. In addition, a new fast peak detector employing the MAPF is proposed and the detector can calculate peak value within 0.5 msec even when voltage sag occurs around zero crossing. The proposed fast peak detector is compared with the conventional detector using APF, and shows faster detection time in the whole range of phase. Furthermore, the proposed fast peak detector can be effectively applied to unsymmetrical three-phase voltage sags. Simulation and experimental results verify the advantages of the proposed detector and MAPF.