Showing papers on "Voltage sag published in 2007"

A Robust Control Scheme for Medium-Voltage-Level DVR Implementation

Abstract: In this paper, a robust control scheme with an outer Hinfin voltage control loop and an inner current control loop is designed and implemented on a medium-voltage (MV)-level dynamic voltage restorer (DVR) system. Through a simple selection of weighting functions, the synthesized Hinfin controller would exhibit significant gains in the vicinity of positive- and negative-sequence fundamental frequencies, and therefore, it would be able to regulate both positive- and negative-sequence components effectively, with explicit robustness in the face of system parameter variations. A detailed discussion of Hinfin controller weighting function selection, inner current loop tuning, and system disturbance rejection capability is presented. Finally, the designed control scheme is extensively tested on a laboratory 10-kV MV-level DVR system with varying voltage sag (balanced and unbalanced) and loading (linear/nonlinear load and induction motor load) conditions. It is shown that the proposed control scheme is effective in both balanced and unbalanced sag compensation and load disturbance rejection, as its robustness is explicitly specified.

A Robust Control Scheme for Medium-Voltage-Level DVR Implementation

Abstract: In this paper, a robust control scheme with an outer Hinfin voltage control loop and an inner current control loop is designed and implemented on a medium-voltage (MV)-level dynamic voltage restorer (DVR) system. Through a simple selection of weighting functions, the synthesized Hinfin controller would exhibit significant gains in the vicinity of positive- and negative-sequence fundamental frequencies, and therefore, it would be able to regulate both positive- and negative-sequence components effectively, with explicit robustness in the face of system parameter variations. A detailed discussion of Hinfin controller weighting function selection, inner current loop tuning, and system disturbance rejection capability is presented. Finally, the designed control scheme is extensively tested on a laboratory 10-kV MV-level DVR system with varying voltage sag (balanced and unbalanced) and loading (linear/nonlinear load and induction motor load) conditions. It is shown that the proposed control scheme is effective in both balanced and unbalanced sag compensation and load disturbance rejection, as its robustness is explicitly specified.

Dynamic Voltage Restorer Utilizing a Matrix Converter and Flywheel Energy Storage

Abstract: A new series power conditioning system using a matrix converter with flywheel energy storage is proposed to cope with voltage sag problem. Previous studies have highlighted the importance of providing adequate energy storage in order to compensate for deep voltage sags of long durations in weak systems. With the choice of flywheel as a preferred energy storage device, the proposed solution utilizes a single ac/ac power converter for the grid interface as opposed to a more conventional ac/dc/ac converter, leading to higher power density and increased system reliability. The paper develops the dynamic model for the complete system including the matrix converter in dual synchronous reference frames coupled to the flywheel- machine and the grid, respectively. The dynamic model is used to design a vector control system that seamless integrates functions of compensating load voltage and managing energy storage during voltage sag and idling modes. The numerical simulation results and experimental results from a laboratory-scale hardware prototype are presented to verify system performance.

A New Method of Voltage Sag and Swell Detection

Abstract: The fundamental voltage, current, and phase angle are required for a wide variety of power system applications. An algorithm that is capable of calculating or estimating these quantities in real time, in the presence of distorted waveforms, finds application in diverse areas of power systems. Techniques to detect voltage sag include the root mean square (rms), Fourier transform, and peak voltage detection methods. The problem with these methods is that they use a windowing technique and can therefore be too slow when applied to detect voltage sags for mitigation since they use historical data. Recent work in the field of signal processing has led to an algorithm that can extract a single non-stationary sinusoidal signal out of a given multi-component input signal. The algorithm is capable of estimating the amplitude, phase and frequency. In this paper, the algorithm is compared to existing methods of sag detection

Shifting Reference Values to Account for Voltage Sag

Abstract: A system and method, including software implemented techniques, can be used to adjust for sag in stored data values. Charge is applied to multiple memory cells, and each memory cell is charged to a target voltage corresponding to a data value. The memory cells include a reference cell that is charged to a predetermined voltage. A voltage level in the reference cell is detected, and voltage levels from a group of memory cells are also detected. An adjustment is performed based upon the difference between the detected voltage level in the reference cell and the predetermined voltage.

Experimental Investigation of a Single-Phase UPQC With Minimum VA Loading

Abstract: The paper deals with an optimized unified power-quality conditioner (UPQC), which aims at the integration of series active and shunt active power filters with minimum volt-ampere (VA) loading of the UPQC. The series active filter is a dynamic voltage restorer (DVR), which regulates the voltage at the load end with minimum VA loading of the overall UPQC by injecting the voltage at an optimum angle. The proposed scheme is validated by an experimental prototype in the laboratory. Selected experimental results are reported along with analytical findings, which show the effectiveness of the proposed UPQC

Interline Unified Power Quality Conditioner

Abstract: This paper proposes a new connection for a unified power quality conditioner (UPQC) to improve the power quality of two feeders in a distribution system. A UPQC consists of a series voltage-source converter (VSC) and a shunt VSC both joined together by a common dc bus. It is demonstrated how this device is connected between two independent feeders to regulate the bus voltage of one of the feeders while regulating the voltage across a sensitive load in the other feeder. Since the UPQC is connected between two different feeders (lines), this connection of the UPQC will be called an interline UPQC (IUPQC). The structure, control and capability of the IUPQC are discussed in this paper. The efficacy of the proposed configuration has been verified through simulation studies using PSCAD/EMTDC

A Dual-Functional Medium Voltage Level DVR to Limit Downstream Fault Currents

Abstract: The dynamic voltage restorer (DVR) is a modern custom power device used in power distribution networks to protect consumers from sudden sags (and swells) in grid voltage. Implemented at medium voltage level, the DVR can be used to protect a group of medium voltage or low voltage consumers. However, the DVR will therefore be tasked to mitigate even more faults involving downstream loads. Large fault currents would flow through the DVR during a downstream fault before the opening of a circuit breaker. This will cause the voltage at point of common coupling (PCC) to drop, which would affect the loads on the other parallel feeders connected to PCC. Furthermore, if not controlled properly, the DVR might also contribute to this PCC voltage sag in the process of compensating the missing voltage, thus further worsening the fault situation. To limit the flow of large line currents, and therefore restore the PCC voltage as well as protect the DVR system components, a downstream fault limiting function is proposed and integrated in the DVR operation. A flux-charge-model feedback algorithm is implemented so that the DVR would act as a large virtual inductance in series with the distribution feeder in fault situations. Controlling the DVR as a virtual inductor would also ensure zero real power absorption during the DVR compensation and thus minimize the stress in the dc link. Finally, the proposed fault current limiting algorithm has been tested in Matlab/Simulink simulation and experimentally on a medium voltage level laboratory DVR system.

Investigation and Improvement of Transient Response of DVR at Medium Voltage Level

Abstract: An area of interest for dynamic voltage restorer (DVR) research is the damping of transient oscillations initiated at the start and at the recovery instant from a voltage sag. Nonlinear loads, with harmonic currents close to the DVR filter resonance frequency, can also excite the resonance oscillations. To compensate voltage sags and dampen high-frequency oscillations simultaneously, an investigation of the transient response of DVR is first carried out. Possible control schemes and their effects on the oscillation attenuation are also studied. Such studied control schemes include the commonly used single voltage loop control, voltage feedback plus reference feedforward control, and double-loop control with an outer voltage loop and an inner current loop. Subsequently, an effective and simple resonance damping method is proposed by employing a closed-loop control with an embedded two-step Posicast controller. The proposed control methods have been extensively tested on a 10-kV DVR system. It is shown that the proposed damping methods improve both the transient and steady-state performance of the DVR.

A Reconfigurable Uninterruptible Power Supply System for Multiple Power Quality Applications

Abstract: A novel topology of a modular per-phase uninterruptible power supply (UPS) system based on reduced-switch-count configuration is proposed in this paper. The proposed power conversion methodology offers active front-end filtering capability which ensures unity input power factor (reactive power compensation) and low input total harmonic distortion (harmonic power compensation). In addition, this UPS system provides dynamic voltage sag compensation capability, which consequently eliminates any series transformer or dc-dc boost converter that is typically used in conjunction with traditional stand-alone UPS systems. Furthermore, it has the desirable characteristics of making seamless transition from normal to backup mode during power failures and vice versa, as well as providing controlled voltage charging at the dc bus link. The proposed UPS is also impervious to load variations, which enables it to operate under nonlinear load condition. Lastly, the circuit structure is conceived from a commonly used three-leg six-switch building block, thereby making it simple and cost-effective, and offering improved power conversion efficiency as compared to a conventional line-interactive or on-line UPS schemes. A complete set of simulation and sample experimental results based on a 1-kW test prototype of the proposed UPS system are presented in this paper to demonstrate its viability and efficacy.

A Voltage Sag Supporter Utilizing a PWM-Switched Autotransformer

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

Coordination of Large Pulsed Loads on Future Electric Ships

Abstract: Part of the technical versatility of future all-electric ships is the potential ability to share large amounts of power among a variety of high-power loads. To help evaluate this potential and to provide information to help guide technology selection, a physics-based model of a power train for an electric ship has been developed and implemented on three modeling platforms. Using this model, three different investigations have been carried out to explore aspects of the behavior of a rotating machine power source for a shipboard rail launcher. These were: 1) influence of rapid charging of the rotating machine system on the ship power system; 2) use of the stored energy in the rotating machines to improve ship power quality; and 3) use of the stored energy in the rotating machines to power a pulsed free-electron laser. Each study highlighted different integration opportunities and challenges. The first showed that, because the charging of the rail launchers was through 5-MW motors, there could be a voltage sag for a few cycles, but this could easily be managed so that the sag could be reduced to an inconsequential level. The second study showed that, with appropriate power electronics, the stored energy in the rail launcher power supply can be used to correct power quality problems introduced by other ship systems. Finally, the stored energy in the launcher power supply can be used to fire a free electron laser for ship defense. This feature opens the possibility of routine operation of the entire ship at highest efficiency, i.e., with the smallest number of gas turbines operating near full power, while providing stored energy needed for ship defense

Minimization of Voltage Sag Costs by Optimal Reconfiguration of Distribution Network Using Genetic Algorithms

Abstract: The paper describes genetic-algorithm (GA)-based optimization software for reconfiguration of a distribution network in order to minimize financial losses due to voltage sags. The developed methodology starts with a selected number of switches which generate various topologies. Load flows are then performed to evaluate the feasibility of topologies. For each feasible topology, fault analysis is performed to first calculate voltage sags at different buses in the network and then to calculate financial losses incurred by voltage sags at buses with sensitive industrial processes. The result of the optimization is the topology which yields the lowest voltage sag cost to customers. The main features of the GA include double-point crossover and adaptive mutation. The developed software tool is applied to the 295-bus generic distribution system.

Simulation of d-statcom and dvr in power systems

Abstract: A Power quality problem is an occurrence manifested as a nonstandard voltage, current or frequency that results in a failure or a mis-operation of end user equipments. Utility distribution networks, sensitive industrial loads and critical commercial operations suffer from various types of outages and service interruptions which can cost significant financial losses. With the restructuring of power systems and with shifting trend towards distributed and dispersed generation, the issue of power quality is going to take newer dimensions. In developing countries like India, where the variation of power frequency and many such other determinants of power quality are themselves a serious question, it is very vital to take positive steps in this direction .The present work is to identify the prominent concerns in this area and hence the measures that can enhance the quality of the power are recommended. This work describes the techniques of correcting the supply voltage sag, swell and interruption in a distributed system. At present, a wide range of very flexible controllers, which capitalize on newly available power electronics components, are emerging for custom power applications. Among these, the distribution static compensator and the dynamic voltage restorer are most effective devices, both of them based on the VSC principle. 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, swell and interruption. Comprehensive results are presented to assess the performance of each device as a potential custom power solution.

A Method for Voltage Sag State Estimation in Power Systems

Abstract: This paper presents a new technique for voltage sag state estimation. The proposed method is based on estimating the number of voltage sags occurring at nonmonitored buses from the number of sags recorded at a limited number of monitored buses. This problem, contrary to the traditional state estimation where redundant measurements are available, is formulated as an underdetermined system of equations. In this paper, linear integer programming techniques are employed to solve this estimation problem. The performance of the proposed method is tested by means of several case studies applied in the IEEE 24-bus reliability test system (RTS) and in the IEEE 118-bus test system.

Power conditioning architecture for a wind turbine

Abstract: A wind turbine 36 has a back-to-back AC/DC/AC power electronic converter chain in which the grid side converter 48 is connected in series with DFIG stator windings 64. The machine side converter 56 is fed from the rotor windings 54 of the DFIG 44. Series connection of the grid side converter 48 enables voltage sag ride-through capability via control of the stator flux. In the event of a grid voltage sag, the series converter allows for a controlled response in the stator flux and electromagnetic shaft torque, protects the machine side converter and enables continued power delivery to the grid.

System Studies of the Superconducting Fault Current Limiter in Electrical Distribution Grids

Abstract: A superconducting fault current limiter (SFCL) in series with a downstream circuit breaker could provide a viable solution to controlling fault current levels in electrical distribution networks. In order to integrate the SFCL into power grids, we need a way to conveniently predict the performance of the SFCL in a given scenario. In this paper, short circuit analysis based on the Electromagnetic Transient Program was used to investigate the operational behavior of the SFCL installed in an electrical distribution grid. System studies show that the SFCL can not only limit the fault current to an acceptable value, but also mitigate the voltage sag. The transient recovery voltage (TRV) could be remarkably damped and improved by the presence of the SFCL after the circuit breaker is opened to clear the fault.

Advanced voltage sag characterisation ii: point on wave

Abstract: Two important but often neglected characteristics of voltage sags and short interruptions are: point on wave of sag/interruption initiation and point on wave of sag/interruption ending A critical review of existing approaches is presented and an appropriate definition of point-on-wave values proposed An identification of point-on-wave values in monitoring applications is discussed and problems related to identification/assessment of point-on-wave values highlighted Examples of equipment sensitivity to these phenomena are analysed The results related to identified ranges and relationships of point-on-wave values are reviewed, using examples of sags and interruptions recorded in power supply systems Adequate procedure for testing single-phase and three-phase equipment to sags and interruptions are discussed with respect to point-on-wave values

A Method to Improve PLL Performance Under Abnormal Grid Conditions

Abstract: Phase tracking system or phase locked loop (PLL) is an important part of the control unit of a grid connected power converter. Popular methods such as zero crossing detection (ZCD) and synchronous reference frame (SRF) PLL do not produce accurate phase information when grid is non-ideal. In this work a method to obtain accurate phase information when the grid voltages are unbalanced by adding a moving average filter to a SRF PLL has been proposed. The performances of the PLL have been experimentally verified for abnormal grid conditions such as unbalance, harmonics, voltage sag and swell, faults etc. Generally, during a fault phase voltages become unbalanced so this method can give better fault ride through capability to a power converter.

A Voltage Sag Index Considering Compatibility Between Equipment and Supply

Abstract: Reliability indices have been used for many years to quantify the effects of sustained interruptions on the electric power system. Criteria for representing and reporting the voltage performance are still under development. A proper voltage sag index would be useful for indicating the system performance experienced at different locations. This paper presents an application of fuzzy-logic techniques to quantify voltage sags disturbance levels. Retained voltage magnitude and sag duration are the inputs of the proposed system. The output is an index that indicates the relative severity of a voltage sag event. The proposed framework takes network vulnerability, equipment sensitivity, and uncertainties in measuring voltage sags into account, thereby providing meaningful information for both the utility and customers. Fuzzy sets membership functions and inference rules involved in the inference process are described and field data are used to illustrate the analytical results. Using the proposed method, site performance indicators can be obtained from the single event indices of all events recorded. The proposed method brings new analytical insights into the assessment of service performance

Modeling and Simulation of Dynamic Voltage Restorer (DVR) Based on Hysteresis Voltage Control

Abstract: Dynamic Voltage Restorers are utilized to protect sensitive loads from voltage sags and swells which take place in the distribution systems. In this paper, a Dynamic Voltage Restorer (DVR) based on hysteresis voltage control is proposed. The DVR is modeled using Simulink'SimPowerSystem Toolbox. Discrete Fourier Transform (DFT) is used to detect the magnitude and phase jump of the voltage sag and swell. The influence of the band of the hysteresis voltage controller on the quality of the load voltage and DVR voltage is studied. The DVR is tested for three- phase voltage sag and swell with phase jump to validate the proposed control scheme.

Evaluation of Voltage Sag Ride-Through of a Doubly Fed Induction Generator Wind Turbine with Series Grid Side Converter

Abstract: With steadily increasing wind turbine penetration, regulatory standards for grid interconnection have evolved to require that wind generation systems ride- through disturbances such as faults and support the grid during such events. Conventional ride-through accommodations result in high currents and torque or are unable to ride-through down to zero volts. This paper explores two DFIG configurations in which a series grid side converter (SGSC) is employed exclusively for voltage sag ride-through. A control strategy applicable to both configurations is developed whereby the SGSC engages to stabilize the stator flux at the beginning and resolution of the sag event. Simulation results show excellent zero voltage ride-through response.

Control strategy for dynamic voltage restorers to achieve minimum power injection without introducing sudden phase shift

Abstract: A power injection control strategy is proposed for dynamic voltage restorers (DVR) under voltage sag or swell conditions. Under such a strategy, the error between the voltage of the energy storage capacitor in the DVR and a given reference voltage is extracted to control the injected power such that during an upstream voltage disturbance, zero or minimum power injection can be achieved automatically, while sudden phase jump of the load-side voltage can also be avoided. The proposed control strategy is analysed in detail and numerical examples are provided to demonstrate the effectiveness of the scheme.

Study on the Effect of Fault Current Limiter in Power System With Dispersed Generators

Abstract: Dispersed generators are being introduced to power systems to secure the electric power supply. However, a short-circuit capacity of the power system increases with the introduction of dispersed generators. As a result, there is concern that a fault current will increase further, and instantaneous voltage sag might be caused. It is proposed to apply a superconducting fault current limiter in the distribution system with dispersed generators to solve these issues. In this paper, the effect of superconducting fault current limiter in this power system is studied by the analysis with EMTP and the confirmation test is conducted with a simulator.

Dynamic Phasor Modeling of the Doubly-fed Induction Generator under Unbalanced Conditions

Abstract: This paper proposes the use of the dynamic phasor approach for studying the behavior of doubly-fed induction generators during faults and unbalanced conditions. The dynamic phasor approach provides more accurate models than the quasi- stationary ones and, at the same time, is computationally more efficient than detailed EMTP models. Two contingencies are taken as examples to study the wind turbine behavior: balanced and unbalanced voltage sags. Results are compared with standard electromechanical and electromagnetical models.

Identification of Fuse and Recloser Operations in a Radial Distribution System

Abstract: Recloser and fuses are the main overcurrent protective devices on distribution systems. Poor coordination adversely impacts the overall power quality especially from the momentary voltage interruption and voltage sag perspectives. Therefore, the purpose of this paper is to develop a novel approach to detect and identify which protective devices operate to clear a short-circuit condition using power-quality waveform measurement data. The proposed approach is intended to evaluate performance coordination of overcurrent protective devices and help locate faults on the feeder. The effectiveness of the techniques is demonstrated using simulation and real-world data. Results indicate a promising potential for real-world deployment and applications.

A New Control Scheme for Unified Power Quality Compensator-Q with Minimum Power Injection

Abstract: Voltage sags are one of the most frequently occurring power quality problems challenging power systems today. The Unified Power Quality Conditioner (UPQC) is one of the major custom power solutions that are capable of mitigating the effect of supply voltage sags at the load or Point of Common Coupling (PCC). 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 the disadvantage that its series compensator needs to be overrated. Thus it cannot offer effective compensation. This paper proposes a new control scheme for the UPQC-Q that offers minimum power injection. The proposed minimum power injection method takes into consideration the limits on the rated voltage capacity of the series compensator and its control scheme. The validity of the proposed control scheme is investigated through simulation and experimental results.

Design of Power Electronic Transformer based on Cascaded H-bridge Multilevel Converter

Abstract: One key component of the future automation in electrical network is the replacement of conventional distribution transformers by an all-solid-state (power- electronic) alternative. In this paper, the optimum design of a power electronic transformer (PET) based on state of the art cascaded H-bridge multilevel converter is investigated. In the design process, a new and simple control method for balancing the cascaded H-bridge DC buses has been introduced. The proposed PET is extremely modular and can be extended for different voltage and power levels. It performs typical functions and has advantages such as power factor correction, elimination of voltage sag and swell, and reduction of voltage flicker in load side. Also in comparison to conventional transformers, it has lower weight, lower volume and eliminates necessity for toxic dielectric coolants.

A Transformer Inrush Mitigation Method for Series Voltage Sag Compensators

Abstract: Voltage sags are major power quality problems encountered by industries. Especially for high-tech companies inside industry parks, any voltage sag event in the distribution system could affect many manufacturers and inflict significant losses. The voltage sag compensator, based on a transformer-coupled series-connected voltage source inverter, is among the most cost- effective solution to protect sensitive loads. One important issue of this system is the transformer inrush at the start of sag compensation. The inrush current may trigger the over-current protection of the compensator inverter, and the compensation voltage is greatly reduced due the magnetic saturation of the coupling transformer. Oversizing the transformer is a common approach to avoid the inrush current. However, this would dramatically increase the size and weight of the sag compensation system. This paper proposes a new technique for mitigating the inrush of the coupling transformer and preserving the output voltage for effective sag compensation. Detailed explanations of the proposed inrush mitigation technique are presented, and the effectiveness of the proposed scheme is verified by laboratory test results. The inverter-assisted commutation sequence of the bypass thyristors for fast start of the compensator is also presented and verified by test results.

Voltage Sag Disturbance Detection Based on Short Time Fourier Transform

Abstract: Short time Fourier transform (STFT) was employed to measure the amplitude of voltage sag, detect interference instant and locate interference source. The amplitude of voltage sag was determined by the amplitude characteristics of fundamental frequency voltage and the high frequency signal produced when voltage sag starts and disappears was employed to identify the voltage sag inception and disappearance instant. A method based on the amplitude of fundamental frequency component and the number of signal singularities is proposed to locate interference source. The method can distinguish the voltage sag induced by fault from that induced by induction motor starting effectively. Simulation tests show that the method has high sensitivity and has better immunity to harmonics and noise than the methods based on wavelet transform.