Showing papers on "Voltage sag published in 2008"

Comparison of Three Single-Phase PLL Algorithms for UPS Applications

Abstract: In this paper, the performance assessment of three software single-phase phase-locked loop (PLL) algorithms is carried out by means of dynamic analysis and experimental results. Several line disturbances such as phase-angle jump, voltage sag, frequency step, and harmonics are generated by a DSP together with a D/A converter and applied to each PLL. The actual minus the estimated phase-angle values are displayed, providing a refined method for performance evaluation and comparison. Guidelines for parameters adjustments are also presented. In addition, practical implementation issues such as computational delay effects, ride-through, and computational load are addressed. The developed models proved to accurately represent the PLLs under real test conditions.

Optimized Control Strategy for a Medium-Voltage DVR—Theoretical Investigations and Experimental Results

Abstract: Most power quality problems in distribution systems are related to voltage sags. Therefore, different solutions have been examined to compensate these sags to avoid production losses at sensitive loads. Dynamic voltage restorers (DVRs) have been proposed to provide higher power quality. Currently, a system wide integration of DVRs is hampered because of their high cost, in particular, due to the expensive DC-link energy storage devices. The cost of these DC-link capacitors remains high because the DVR requires a minimum DC-link voltage to be able to operate and to compensate a sag. As a result, only a small fraction of the energy stored in the DC-link capacitor is used, which makes it impractical for DVRs to compensate relatively long voltage sags. Present control strategies are only able to minimize the distortions at the load or to allow a better utilization of the storage system by minimizing the needed voltage amplitude. To avoid this drawback, an optimized control strategy is presented in this paper, which is able to reduce the needed injection voltage of the DVR and concurrently to mitigate the transient distortions at the load side. In the following paper, a brief introduction of the basic DVR principle will be given. Next, three standard control strategies will be compared and an optimized control strategy is developed in this paper. Finally, experimental results using a medium-voltage 10-kV DVR setup will be shown to verify and prove the functionality of the presented control strategy in both symmetrical and asymmetrical voltage sag conditions.

Dynamic voltage sag correction

Abstract: A voltage sag correction device includes an input terminal adapted to receive a first operating signal having a line-to-neutral voltage. The first operating signal is provided to a load through an output terminal. A regulator module includes a rectifying device adapted to rectify a line-to-line input signal, a storage unit adapted to store energy corresponding to the rectified line-to-line input signal, and an inverter switching device adapted to use the stored energy to generate a correction signal during at least a portion of a voltage sag. An injection transformer in electrical communication with the regulator module is adapted to reduce a voltage of the correction signal. A bypass switch is in a closed position during a normal operating condition such that the injection transformer is bypassed. The bypass switch is in an open position during at least a portion of the voltage sag such that the injection transformer is energized.

A Novel Configuration for a Cascade Inverter-Based Dynamic Voltage Restorer With Reduced Energy Storage Requirements

Abstract: This paper introduces a new configuration for a cascade (H-bridge) converter-based dynamic voltage regulator in which the basic cascade converter is supplemented with a shunt thyristor-switched inductor. The proposed topology is shown to possess the ability of mitigating a severe and long duration voltage sag with a significantly smaller energy demand from the cascade converter. A suitable control system is designed, and the operation of the new device is analyzed using electromagnetic transients simulation as well as mathematical analysis. Simulation and experimental results are presented to demonstrate the feasibility and the practicality of the proposed novel dynamic voltage restorer topology.

Investigation on the performance of UPQC-Q for voltage sag mitigation and power quality improvement at a critical load point

Abstract: The unified power quality conditioner (UPQC) is one of the major custom power solutions, which is capable of mitigating the effect of supply voltage sag at the load end or at the point of common coupling (PCC) in a distributed network. It also prevents the propagation of the load current harmonics to the utility and improves the input power factor of the load. The control of series compensator (SERC) of the UPQC is such that it injects voltage in quadrature advance to the supply current. Thus, the SERC consumes no active power at steady state. The other advantage of the proposed control scheme is that the SERC can share the lagging VAR demand of the load with the shunt compensator (SHUC) and can ease its loading. The UPQC employing this type of quadrature voltage injection in series is termed as UPQC-Q. The VA requirement issues of SERC and SHUCs of a UPQC-Q are discussed. A PC-based new hybrid control has been proposed and the performance of the UPQC-Q is verified in a laboratory prototype. The phasor diagram, control block diagram, simulations and experimental results are presented to confirm the validity of the theory.

A new operation method for grid-connected PV system considering voltage regulation in distribution system

Abstract: This paper presents a grid-connected photovoltaic (PV) system with the new operation method considering voltage regulation in distribution system. In proposed system, PV system adjusts real power and reactive power injection at connection point, in order to maintain the overall system voltage within acceptable range and improve power quality. It consists of modeling of PV array, inverter and inverter controller using PSCAD/EMTDC, and development of operation method of PV generation for voltage regulation in distribution system. PV array modeling is accomplished using 2-diode model which is generally used, and inverter with control system is implemented using proposed operation method based on instantaneous power theory and hysteresis current control. Proposed operation method consists of 3 stages control and using that operation method, overall system effectiveness is improved. To validate the effectiveness of the proposed system, it is compared with system with conventional operation under variable case such that connection, disconnection, load changes and voltage sag using PSCAD/EMTDC.

Power maximization and voltage sag/swell ride-through capability of PMSG based variable speed wind energy conversion system

Abstract: The increasing penetration level of wind farms in power system network requires better understanding of short-term voltage stability and ride through capability before their interconnection to grid The future wind turbines must stay connected irrespective of any type of disturbance in grid To achieve reliable operation and control, the wind turbines must be actively controlled in order to meet the stringent grid interconnection requirements This paper presents the modeling and performance analysis of wind energy conversion system (WECS) under different kind of grid disturbances A model of directly driven, variable speed wind turbine based on multi-pole permanent magnet synchronous generator (PMSG) is developed and simulated in MATLAB/SPS environment The simulation analysis demonstrates the maximum power extraction and enhanced ride through capability of WECS under three phase voltage sag/swell conditions

Improving distribution system performance with integrated STATCOM and supercapacitor energy storage system

Abstract: The STATCOM (synchronous static compensator) based on voltage source converter (VSC) is used for voltage regulation in transmission and distribution systems. However, strict requirements of STATCOM losses and total system loss penalty preclude the use of high frequency PWM (pulse-width modulation) for VSC based STATCOM applications. This constraint of implementing VSC without PWM results in VSC DC voltage dip, over-currents and trips of the STATCOM during and after system disturbances and faults, when its VAR support functionality is most required. In this paper, the integration and control of energy storage systems (ESSs), such as supercapacitor (ultracapacitor - UCAP) into a D-STATCOM (Distribution system STATCOM) is developed to mitigate such problems, enhance power quality and improve distribution system reliability. This paper develops the control concepts to charge/discharge the UCAP by the D-STATCOM, and validate the performance of an integrated D-STATCOM/UCAP system for improving distribution system performance under all types of system related disturbances and system faults - such as single-line to ground fault (SLG), line-line fault and 3-phase system faults. Simulation results of a 125 kVA D-STATCOM validate that integrated the D-STATCOM with 600 V, 1.0 Farad UCAP is suited for distribution system voltage regulation and voltage sag mitigation. In case of a system fault, UCAP based energy storage will aid to keep the D-STATCOM DC voltage constant and avoid over-current and trips of the VSC based D-STATCOM.

The Influence of Induction Motors on Voltage Sag Propagation—Part I: Accounting for the Change in Sag Characteristics

Abstract: This paper presents an analytical approach to the analysis of the interaction between induction motors (IMs) and voltage sags. The presented methodology enables quick assessment of the influence of IM on sag characteristics and avoids time-consuming transient simulations. The accuracy of the derived analytical formulae is verified through classical transient simulations using the PSCAD/EMTDC package. This paper further proposes a simple, yet efficient transformation of resulting nonrectangular voltage sags, based on the loss of voltage index, into equivalent rectangular sags suitable for conventional voltage sag performance benchmarking.

Detection of voltage sag sources based on instantaneous voltage and current vectors and orthogonal Clarke's transformation

Abstract: Different methods for voltage sag source detection are discussed. They are based either on disturbance energy, voltage-current characteristic or active (real) current component. It is shown that, in the cases of asymmetrical voltage sags, both current-based methods known from literature do not work well. These two methods are therefore generalised using a vector-space approach. Furthermore, three new methods are introduced using orthogonal Clarke's transformation, which can be used to detect the sources of those voltage sags provoked by earth faults. All the discussed methods for voltage sag source detection have been tested by applying extensive simulations, laboratory tests and field testing. The results obtained show the very high effectiveness of the proposed methods, which are superior to the methods known from literature, especially for detecting sources of asymmetrical voltage sags.

On Tracking the Source Location of Voltage Sags and Utility Shunt Capacitor Switching Transients

Abstract: This paper presents a new procedure to track the disturbance source location of voltage sags and shunt capacitor switching transients in a power system based on branch current measurements. In the proposed method, the power-quality metering locations are first determined by the sensitivity of the system fault-current level to the fault voltage. Then, a current deviation index for representing the change of each measured branch current rms magnitudes before and after the disturbance is calculated to track the disturbance source location within an area confined by the directions of the top-priority branches. The IEEE 30-bus benchmark system and an actual transmission system with recorded power-quality disturbance events are used to illustrate the usefulness of the proposed method. Simulation results obtained by the proposed method are also compared with those obtained by a previously developed approach and by actual recorded data. By observing the results, it shows that the proposed method is efficient and accurate to track the source area of voltage sags or shunt capacitor switching transients.

Factor affecting characteristic of voltage sag due to fault in the power system

Abstract: Faults in the power system are the most common reason for the occurrence of important power quality problem, voltage sag in the system. Due to increasing use of sensitive and sophisticated control in almost all modern devices at the industrial and residential consumer level, the Voltage sag which causes severe problems to these devices needs to be analyzed. Factors which affect the characteristics of voltage sag as a type of fault in the system, location of fault in the system, X/R ratio of transmission lines, type of transmission as single or double circuit transmission, Point on wave of sag initiation are studied in this paper. PSCAD/EMTDC is used as a simulation tool.

An Approach to Eliminating DC Magnetic Flux From the Series Transformer of a Dynamic Voltage Restorer

Abstract: This paper proposes a control method for eliminating dc magnetic flux from the series transformer of a dynamic voltage restorer. The method is characterized by intentionally injecting no compensating voltage during a one-sixth line cycle (=3.3 ms at 50 Hz). The control method is independent of an initial phase angle of a voltage sag, thus requiring no flux detection. The period, during which no compensating voltage is injected, is one-third as short as that of an existing method. This paper confirms the effectiveness and viability of the proposed method. A 200-V 5-kW laboratory system verifies experimentally that the proposed method brings no dc magnetic flux to the series transformer. Moreover, the proposed method brings a smaller dc voltage drop to the dc load of a three-phase diode rectifier than the existing method does in the same conditions.

Rating and design issues of DVR injection transformer

Abstract: The load side voltage disturbances can be effectively mitigated by connecting a series compensating device called dynamic voltage restorer (DVR). Generally, DVR is realized by a voltage source inverter with dc storage device and a series injection transformer. Rating and design issues of an injection transformer are of much concern for the proper operation of the DVR. Voltage underrating of the transformer may lead to its saturation whereas overrating increases the cost and size of the DVR. The voltage applied to the primary winding of the transformer is a combination of several components of different frequencies. In view of above, rating and design issues are presented in this paper. A novel method is proposed to find the voltage rating of the transformer as a function of the fundamental frequency. Furthermore, to reduce the effects of current harmonics in the transformer, A-factor is used. A PSCAD model is used to validate the efficacy of the proposed design method to find the voltage rating, and overall performance of the DVR. The experimental studies confirm the proposed method.

Simplified Voltage-Sag Filler for Line-Interactive Uninterruptible Power Supplies

Abstract: There are three types of static uninterruptible power supplies (UPSs): passive standby, line interactive, and double conversion. The last one protects the load against all types of line disturbances, but it is the most expensive and the one with the lowest efficiency. On the other hand, passive-standby and line-interactive UPSs have higher efficiency and lower cost, but they show an important drawback: a switching time from normal to stored-energy mode. As a consequence, there is a notch in the UPS output voltage during this switching time. In a previous paper, the authors proposed a method for filling these voltage notches with a sinusoidal waveform generated by a switch-mode converter. In this one, a simplified notch filler is proposed. It consists of two capacitors, one charged with positive voltage and the other with negative voltage. If the fault occurs in the positive period, the positive-charged capacitor is connected to the load. This connection is then modulated in order to obtain a sinusoidal waveform at the load. In the negative period, the other capacitor is used in the same way.

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

Abstract: Regulatory standards for grid interconnection require wind generators ride-through disturbances such as faults and support the grid during such events. Conventional accommodations for providing voltage sag ride-through for doubly fed induction generator (DFIG) wind turbines result in compromised control of the turbine shaft and grid current during unbalanced faults. This paper presents analysis and control design of a DFIG wind turbine with series grid side converter for ride through during unbalanced voltage sag events. A dynamic model and control structure is developed for unbalanced operating conditions. Experimental results from 2kW laboratory hardware are used to verify the proposed concepts. Hardware results illustrate excellent ride through response of the DFIG system under various sag conditions.

A single-phase voltage sag generator for testing electrical equipments

Abstract: This paper describes a transformer-based voltage sag generator (VSG) suitable to evaluate the susceptibility of electrical equipment to voltage sag. The built VSG utilized one auto-transformer and two solid state relays (SSRs) to provide nominal voltage and sag voltage to the load. The switch statuses of two SSRs are controlled by nominal voltage and sag voltage duration signal produced by electronic circuits. The VSG operating result shows that it enables effective control of sag magnitude, duration, beginning and ending points on output voltage wave. If needed, it can work as a voltage swell generator and a voltage interruption generator. By supplying high voltage (HV) transformer from the primary side, the VSG also can provide HV sag, swell, and interruption. The presented VSG is easier to set up in the lab, and the construction cost is much lower than buying VSG products at the current market.

The Influence of Generator Scheduling and Time-Varying Fault Rates on Voltage Sag Prediction

Abstract: This paper discusses the influence of generator scheduling and time-varying fault rates on the stochastic prediction of voltage sags. Typically, in the stochastic prediction of voltage sags, the annual expected sag frequencies (ESFs) at sensitive load points are calculated by assuming that the operating conditions and topology of the power system remain unchanged and fault rates of system components are constant throughout a year. In this paper, in order to obtain reasonable accuracy in predicting the annual ESFs at sensitive load points, the variation of fault rates due to adverse weather and the effect of generator scheduling are considered. The study was performed on the IEEE 30-bus test system. Two buses were randomly selected and the ESFs at the selected buses were calculated for different cases (i.e., with and without incorporation of time-varying fault rates and the operation schedule of generators in the system).

Investigation of Voltage Sag Impact on Wind Turbine Tower Vibrations

Abstract: To study the effect of voltage sag on the mechanical vibration of a wind turbine structure, a detailed model that considers the combined electrical, mechanical and aerodynamic aspects of the wind turbine must be used. A drawback of many previously published works in the area of wind turbine simulation is that they study either a very simple mechanical model with a detailed electrical model or vice versa. Therefore, the interactions between electrical and mechanical components are not accurately taken into account. In this paper, three simulation programs—TurbSim, FAST and Simulink—are used to model the wind, mechanical and electrical parts of a wind turbine and its controllers in detail. Simulation results obtained from the model are used to observe the interaction of all three factors affecting the operation of a wind turbine system. The focus of the paper is on the effect of the voltage sag on tower vibration, considering different power system characteristics (i.e. short circuit level and X/R ratio), mechanical parameter (i.e. mechanical tower damping) and wind turbine operating conditions. Copyright © 2008 John Wiley & Sons, Ltd. Received 13 December 2006; Revised 4 October 2007; Accepted 6 November 2007

Impact and improvement of Distributed Generation on voltage quality in Micro-grid

Abstract: This paper illuminates some voltage quality problems caused by distributed generation (DG) in microgrid, including voltage fluctuation, voltage sag and voltage swell, voltage uplifted and harmonic resonance due to large capacity capacitor switching and so on. It studies the characteristic of these kinds of voltage quality problem and their impact on microgrid. According to these characteristics, this paper presents a method which depends on the type and structure of DG in microgrid to improve the voltage quality. The studies show that the penetration of DG in microgrid has beneficial improvement of system response to voltage disturbances. Rotating type DG can mitigate voltage fluctuation caused by rapidly changing loads due to its short circuit strength. Inverter type DG also has beneficial impact on voltage fluctuation if it has voltage regulation function. Simulation and analysis on a system model show that the proposed approach is effective and promising.

Unified power quality conditioner with super-capacitor for energy storage

Abstract: This paper proposes a new configuration of UPQC (Unified Power Quality Conditioner) that consists of the DC/DC converter and the super-capacitors for compensating the voltage interruption. The proposed UPQC can compensate the reactive power, harmonic current, voltage sag and swell, voltage unbalance, and the voltage interruption. The performance of proposed system was analyzed through simulations with PSCAD/EMTDC software, and the realization of proposed system was verified through experimental works with a prototype. The proposed system can improve the power quality at the common connection point of the non-linear load and the sensitive load. Copyright © 2007 John Wiley & Sons, Ltd.

DVR with Sliding Mode Control to alleviate Voltage Sags on a Distribution System for Three Phase Short Circuit Fault

Abstract: Control of most of the industrial loads is mainly based on semiconductor devices, which causes such loads to be more sensitive against power system disturbances. Thus, the power quality problems have gained more interest recently. Voltage sags are mainly caused due to starting of large induction motors, switching of large inductive loads, short circuit faults in transmission and distribution systems and energizing of large transformers. Therefore, different solutions are examined to compensate these sags and to avoid production losses at sensitive loads. Dynamic voltage restorer (DVR) is one of the solutions to realize this goal. For a successful compensation, the DVR must be able to detect the voltage sag and control the inverter to restore the voltage. The Sliding mode control is used for the DVR. Using Sliding mode control to the DVR, additional sag detection method is eliminated. This improves the dynamic response of the DVR and also DVR is able to compensate for any variation in source voltage. In this paper, a DVR with sliding mode control strategy is used to alleviate the voltage sags caused due to faults occurring on the parallel feeders in a three-phase distribution system. The DVR along with the other parts of the distribution system are simulated using MATLAB/SIMULINK.

Voltage sag generating equipment

Abstract: The purpose of this invention is a device for generating voltage dips of a depth of up to 100% with adjustable variation switches and at as many levels of depth as wished, for connection in a wind farm or electricity generating system between the electrical energy evacuation line produced by the generators of the electrical installation in general or wind generation in particular and the system of electrical energy generation or wind turbine under test, where the voltage dip generator device controls and regulates the short circuit power detected by the electrical energy generating machinery or the wind turbine to be tested for voltage values exceeding 500 V and power exceeding 100 kW.

Control of Reduced Rating Dynamic Voltage Restorer with Battery Energy Storage System

Abstract: In this paper, different voltage injection schemes for dynamic voltage restorers (DVRs) are analysed with particular focus on the methods used to minimize the rating of the voltage source converter (VSC) used in DVR The control and operation of a DVR is demonstrated with reduced rating VSC The reference load voltage is estimated using the unit vectors and the synchronous reference frame (SRF) theory is used for the control of DVR The compensations of sag, swell and harmonics in supply voltage using the reduced rating DVR are demonstrated using MATLAB with its Simulink power system blockset (PSB) toolboxes

A novel configuration for a Cascade Inverter Based Dynamic Voltage restorer with reduced energy storage requirements

Abstract: The paper introduces a new configuration for a cascade (H-bridge) converter based dynamic voltage regulator, in which the basic cascade converter is supplemented with a shunt thyristor switched inductor The proposed topology is shown to posses the ability of mitigating severe and long duration voltage sag with a significantly smaller energy demand from the cascade converter A suitable control system is designed , and the operation of the new device is analyzed using electromagnetic transients simulation as well as mathematical analysis Simulation and experimental results are presented to demonstrate the feasibility and the practicality of the proposed novel DVR topology

Engine Cranking System and Method

Abstract: A vehicle has an engine, a starter motor, an energy storage system (ESS) for powering an auxiliary system, and a supercapacitor module for powering the starter motor during engine cranking and starting. The supercapacitor module disconnects from the starter motor to recharge. A DC-DC booster converter increases the level of voltage supplied from the ESS so as to charge the supercapacitor module to a relatively higher voltage level, such as approximately 125 to 140 percent of the voltage level supplied by the ESS. A method for preventing voltage sag in an auxiliary system of the vehicle includes disconnecting the supercapacitor module from the starter motor when the engine is running, and then charging the supercapacitor module using the ESS until a cranking support voltage equals a stored target voltage. A detected commanded cranking and starting of the engine causes the connection of the supercapacitor module to the starter motor.

Solid-State Transformer Based on SiC JFETs for Future Energy Distribution Systems

Abstract: In order to reduce the emission of greenhouse gas and replace the limited energy sources like coal, oil or uranium, the number of renewable energy sources is constantly growing. This development results in a rising number of distributed power plants, which are principally subject to substantial energy fluctuations. In order to easily connect the new energy sources to the grid and improve the power quality by harmonic filtering, voltage sag correction and highly dynamic control of the power flow new power electronic systems so called Intelligent Universal / Solid-State Transformers (SST) are required. These interconnecting devices would enable full control of magnitude and direction of real and reactive power flow and could replace not controllable, voluminous and heavy line frequency transformers. Based on such devices a smart grid comparable to the internet, where a plug and playconnection of sources and loads, distributed energy uploads and downloads and energy routing for transferring energy from the producer to the consumer, is possible. In the top of Fig.1 a conventional interconnecting system based on a back-to-back (BTB) converter and slow IGBT devices is shown. This system consists of ac-dc/dc-ac converters and two line-frequency transformers, which provide galvanic isolation as well as voltage level conversion and which have a large volume and weight. In order to decrease the volume/weight of the system and reduce the raw material consumption new topologies [1]–[3], which replace the line frequency transformers by high frequency and high voltage transformers, have been proposed. These proposals are also based on IGBT devices, which significantly limit the feasible switching frequencies and the voltage level of the converter systems. In order to overcome these limits, new converter systems (cf. A, B & C in Fig.1) based on high voltage SiC JFETs cascades, which enable a much higher power density and a significantly higher system dynamic are presented in SectionII of this paper. Furthermore, these systems require a lower number of converter stages, what results in lower system costs and a higher reliability. In SectionIII a 5kV/50kHz bi-directional, isolated dc-dc converter, which is a key element of the solid-state transformers, is discussed in detail. There the switching behaviour of the SiC switches, the design of the transformer and the achievable performance of the dc-dc converter are presented. Finally, the system parameters of a 1MW solid-state transformer based on the SiC dc-dc converter are calculated and the performance of the different topologies with respect to power density, efficiency and realisation effort is compared in SectionIV.