Showing papers on "AC power published in 2002"

Voltage rise: the big issue when connecting embedded generation to long 11 kV overhead lines

Abstract: There has been much interest in embedding small generators deep within distribution systems. The steady-state voltage rise resulting from the connection of these generators can be a major obstacle to their connection at the lower voltage levels. This article summarises the results of some generic studies, explaining this voltage rise issue and how it may be overcome. Methods discussed to counteract voltage rise are primary substation voltage reduction, reactive power import, autotransformers installation, conductor upgrading, and generation constraints.

Optimal power flow by enhanced genetic algorithm

Abstract: This paper presents an enhanced genetic algorithm (EGA) for the solution of the optimal power flow (OPF) with both continuous and discrete control variables. The continuous control variables modeled are unit active power outputs and generator-bus voltage magnitudes, while the discrete ones are transformer-tap settings and switchable shunt devices. A number of functional operating constraints, such as branch flow limits, load bus voltage magnitude limits, and generator reactive capabilities, are included as penalties in the GA fitness function (FF). Advanced and problem-specific operators are introduced in order to enhance the algorithm's efficiency and accuracy. Numerical results on two test systems are presented and compared with results of other approaches.

A method of tracking the peak power points for a variable speed wind energy conversion system

Abstract: In this paper, a method of tracking the peak power in a wind energy conversion system (WECS) is proposed, which is independent of the turbine parameters and air density. The algorithm searches for the peak power by varying the speed in the desired direction. The generator is operated in the speed control mode with the speed reference being dynamically modified in accordance with the magnitude and direction of change of active power. The peak power points in the P-/spl omega/ curve correspond to dP/d/spl omega/=0. This fact is made use of in the optimum point search algorithm. The generator considered is a wound rotor induction machine whose stator is connected directly to the grid and the rotor is fed through back-to-back pulse-width-modulation (PWM) converters. Stator flux-oriented vector control is applied to control the active and reactive current loops independently. The turbine characteristics are generated by a DC motor fed from a commercial DC drive. All of the control loops are executed by a single-chip digital signal processor (DSP) controller TMS320F240. Experimental results show that the performance of the control algorithm compares well with the conventional torque control method.

Control system and method for electric vehicle

Abstract: An electric traction vehicle is described herein which may be used to provide power to off-board electric power-consuming systems or devices. The electric traction vehicle may provide 250 kilowatts or more of three phase AC power to an off-board electric power consuming system. The electric traction vehicle may also include an electrical power storage device which can be selectively discharged to allow the vehicle to be serviced.

Fuzzy logic controlled shunt active power filter for power quality improvement

Abstract: The simulation and experimental study of a fuzzy logic controlled, three-phase shunt active power filter to improve power quality by compensating harmonics and reactive power required by a nonlinear load is presented. The advantage of fuzzy control is that it is based on a linguistic description and does not require a mathematical model of the system. The fuzzy control scheme is realised on an inexpensive dedicated micro-controller (INTEL 8031) based system. The compensation process is based on sensing line currents only, an approach different from conventional methods, which require harmonics or reactive volt-ampere requirement of the load. The performance of the fuzzy logic controller is compared with a conventional PI controller. The dynamic behavior of the fuzzy controller is found to be better than the conventional PI controller. PWM pattern generation is based on carrierless hysteresis based current control to obtain the switching signals. Various simulation and experimental results are presented under steady state and transient conditions.

Diode-clamped multilevel converters: a practicable way to balance DC-link voltages

Abstract: The converter topologies identified as diode-clamped multilevel (DCM) or, equivalently, as multipoint clamped (MPC), are rarely used in industrial applications, owing to some serious drawbacks involving mainly the stacked bank of capacitors that constitutes their multilevel DC link. The balance of the capacitor voltages is not possible in all operating conditions when the MPC converter possesses a passive front end. On the other hand, in AC/DC/AC power conversion, the back-to-back connection of a multilevel rectifier with a multilevel inverter allows the balance of the DC-link capacitor voltages and, at the same time, it offers the power-factor-correction capability at the mains AC input. An effective balancing strategy suitable for MPC conversion systems with any number of DC-link capacitors is presented here. The strategy has been carefully studied to optimize the converter efficiency. The simulation results related to a high-power conversion system (up to 10 MW) characterized by four intermediate DC-link capacitors are shown.

A comparison of high-power converter topologies for the implementation of FACTS controllers

Abstract: This paper compares four power converter topologies for the implementation of flexible AC transmission system (FACTS) controllers: three multilevel topologies (multipoint clamped (MPC), chain, and nested cell) and the well-established multipulse topology. In keeping with the need to implement very-high-power inverters, switching frequency is restricted to line frequency. The study addresses device count, DC filter ratings, restrictions on voltage control, active power transfer through the DC link, and balancing of DC-link voltages. Emphasis is placed on capacitor sizing because of its impact on the cost and size of the FACTS controller. A method for the dimensioning the DC capacitor filter is presented. It is found that the chain converter is attractive for the implementation of a static compensator or a static synchronous series compensator. The MPC converter is attractive for the implementation of a unified power flow controller or an interline power flow controller, but a special arrangement is required to overcome the limitations on voltage control.

Subthreshold leakage modeling and reduction techniques

Abstract: As technology scales, subthreshold leakage currents grow exponentially and become an increasingly large component of total power dissipation. CAD tools to help model and manage subthreshold leakage currents will be needed for developing ultra low power and high performance integrated circuits. This paper gives an overview of current research to control leakage currents, with an emphasis on areas where CAD improvements will be needed. The first part of the paper explores techniques to model subthreshold leakage currents at the device, circuit, and system levels. Next, circuit techniques such as source biasing, dual Vt partitioning, MTCMOS, and VTCMOS are described. These techniques reduce leakage currents during standby states and minimize power consumption. This paper also explores ways to reduce total active power by limiting leakage currents and optimally trading off between dynamic and leakage power components.

A 175 mV multiply-accumulate unit using an adaptive supply voltage and body bias (ASB) architecture

Abstract: In order to minimize total active power consumption in digital circuits, one must take into account subthreshold leakage currents that grow exponentially as technology scales. This research develops a theoretical model to predict how dynamic power and subthreshold power must be balanced to give an optimal V/sub DD//V/sub t/ operating point that minimizes total active power consumption for different workload and operating conditions. A 175-mV multiply-accumulate test chip using a triple-well technology with tunable supply and body bias values is measured to experimentally verify the tradeoffs between the various sources of power. The test chip shows that there is an optimum V/sub DD//V/sub t/ operating point, although it differs from the theoretical limit because of excessive forward bias currents. Finally, we propose a preliminary automatic supply and body biasing architecture (ASB) that automatically configures a circuit to operate with the lowest possible active power consumption.

Towards a Competitive Market for Reactive Power

Abstract: This paper presents the design of a competitive market for reactive power ancillary services. Generator reactive power capability characteristics are used to analyze the reactive power costs and subsequently to construct a bidding framework. The reactive power market is settled on uniform price auction, using a compromise programming approach based on a modified optimal power flow model. The paper examines market power issues in these markets and identifies locations where strategic market power advantages are present that need to be removed through investments in reactive power devices.

Instantaneous power compensation in three-phase systems by using p-q-r theory

Abstract: This paper proposes a novel power compensation algorithm in three-phase four-wire systems by using p-q-r theory. The p-q-r theory is compared with two previous instantaneous power theories, p-q theory and cross vector theory. The p-q-r theory provides two-degrees of freedom to control the system currents by only compensating the instantaneous imaginary power without using any energy storage element. The definition of powers maintains power conservation, and agrees well with the general understanding of power. Simulation results show the superiority of p-q-r theory both in definition and compensation.

A new formulation for FACTS allocation for security enhancement against voltage collapse

Abstract: This paper proposes a new formulation for reactive power (VAr) planning problem including the allocation of flexible ac transmission systems (FACTS) devices. A new feature of the formulation lies in the treatment of security issues. Different from existing formulations, we directly take into account the expected cost for voltage collapse and corrective controls, where the control effects by the devices to be installed are evaluated together with the other controls such as load shedding in contingencies to compute an optimal VAr planning. The inclusion of load shedding into the formulation guarantees the feasibility of the problem. The optimal allocation by the proposed method implies that the investment is optimized, taking into account its effects on security in terms of the cost for power system operation under possible events occurring probabilistically. The problem is formulated as a mixed integer nonlinear programming problem of a large dimension. The Benders decomposition technique is tested where the original problem is decomposed into multiple subproblems. The numerical examinations are carried out using AEP-14 bus system to demonstrate the effectiveness of the proposed method.

Optimal Power Flow Using Refined Genetic Algorithm

Abstract: This article presents a simple, effective, and reliable refined genetic algorithm (RGA) for solving the optimal power flow (OPF) problem. This genetic algorithm with the exponential variation of crossover probability, mutation probability, anddynamic hierarchy of the coding system has the ability to code a large number of control variables in a practical system, within a reasonable length of chromosome, and the algorithm is less sensitive to starting points. It is, therefore, able to regulate the active power outputs of generators, bus voltages, shunt capacitors/reactors, and transformer tap-settings to minimize the fuel cost. The feasibility of the algorithm is demonstrated using the IEEE 6-bus and 30-bus systems.

An investigation on the validity of power-direction method for harmonic source determination

Abstract: The power-direction method has been used widely to identify the locations of harmonic sources in a power system. A number of utility-customer disputes over who is responsible for harmonic distortions have been settled with the help of the method. A closer examination of the method, however, reveals that it is unable to fulfil the task of harmonic source detection. Case studies can easily show that the method yields incorrect results. In this paper, problems associated with the method are investigated using case studies and mathematical analysis. The results show that the power direction method is theoretically incorrect and should not be used to determine harmonic source locations. The main cause of the problem is that the direction of active power flow is a function of the phase-angle difference between the two sources. The direction of reactive power flow, on the other hand, has a better correlation with the source magnitudes.

Voltage sag compensation with energy optimized dynamic voltage restorer

Abstract: The compensation capability of a dynamic voltage restorer (DVR) depends primarily on the maximum voltage injection ability and the amount of stored energy available within the restorer. A new phase advance compensation (PAC) strategy for the DVR is proposed in order to enhance the voltage restoration property of the device. The scheme requires only an optimum amount of energy injection from the DVR to correct a given voltage sag. Supply voltage amplitude and phase detection scheme as well as a phase advance determination scheme are also included. The resulting DVR design is shown to be superior in terms of lower storage energy need compared to the conventional in-phase boosting method. The analytical results are validated by laboratory tests carried out on a prototype of the restorer. The efficacy of the proposed method is illustrated.

An extended nonlinear primal-dual interior-point algorithm for reactive-power optimization of large-scale power systems with discrete control variables

Abstract: This paper presents a new algorithm for reactive-power optimization of large-scale power systems involving both discrete and continuous variables. This algorithm realizes successive discretization of the discrete control variables in the optimization process by incorporating a penalty function into the nonlinear primal-dual interior-point algorithm. The principle of handling these discrete variables by the penalty function, the timing of introducing the penalty function during iterations, and the setting of penalty factors are discussed in detail. To solve the high-dimension linear correction equation speedily and efficiently in each iteration, a novel data structure rearrangement is proposed. Compared with the existing data structures, it can effectively reduce the number of nonzero fill-in elements and does not give rise to difficulty in triangular factorization. The numerical results of test systems that range in size from 14 to 538 buses have shown that the proposed method can give nearly optimum solutions, has good convergence, and is suitable for large-scale system applications.

Switching power supply circuit

Abstract: PROBLEM TO BE SOLVED: To propose a switching power supply circuit at a relatively low cost, which does not open circuit a filter capacitor SOLUTION: In the switching power supply circuit connected to an AC power supply 1 and having a fuse 2 on a current path and controlling an on-duration of a switching element 6 for switching a current in a coil 5a and stabilizing the output DC voltage, an on-state of the switching element 6 is continued and the fuse 2 blows out when a voltage across the filter capacitor 4 connected to DC voltage output terminals of a rectifier circuit for rectifying an AC voltage from the AC power supply 1 and outputting the DC voltage exceeds a preset upper limit

Power flow control approach to power systems with embedded FACTS devices

Abstract: This paper focuses on developing an approach to steady-state power flow control of flexible AC transmission systems (FACTS) device-equipped power systems. Based on a power-injection model of FACTS devices and an optimal power flow model, a novel versatile power flow control approach is formulated, which is capable of implementing power flow control incorporating any FACTS device flexibly. Different from existing FACTS device control approaches, the active and (or) reactive power injections are taken as independent control variables. Therefore, using this method, Jacobian matrix need not be changed, although various FACTS devices possess different physical models and different control parameters. Furthermore, it enables the integration of FACTS devices into the existing power system analysis and control programs efficiently. Physical limits of the FACTS devices are also considered in the model. Numerical results on a reduced practical system and a 1500-bus practical system with various FACTS devices are presented to illustrate the vigorousness of the proposed approach.

Power System Analysis: Short-Circuit Load Flow and Harmonics

Abstract: Short-Circuit Currents and Symmetrical Components Nature of Short-Circuit Currents Symmetrical Components Eigenvalues and Eigenvectors Symmetrical Component Transformation Clarke Component Transformation Characteristics of Symmetrical Components Sequence Impedance of Network Components Computer Models of Sequence Networks Structure and Nature of Electrical Power Systems Power System Studies Unsymmetrical Fault Calculations Line-to-Ground Fault Line-to-Line Fault Double Line-to-Ground Fault Three-Phase Fault Phase Shift in Three-Phase Transformers Unsymmetrical Fault Calculations System Grounding Open Conductor Faults Matrix Methods for Network Solutions Network Models Bus Admittance Matrix Bus Impedance Matrix Loop Admittance and Impedance Matrices Graph Theory Bus Admittance and Impedance Matrices by Graph Approach Algorithms for Construction of Bus Impedance Matrix Short-Circuit Calculations with Bus Impedance Matrix Solution of Large Network Equations Current Interruption in AC Networks Rheostatic Breaker AC Arc Interruption Current-Zero Breaker Transient Recovery Voltage The Terminal Fault The Short-Line Fault Interruption of Low Inductive Currents Interruption of Capacitive Currents TRV in Capacitive and Inductive Circuits Prestrikes in Breakers Overvoltages on Energizing High-Voltage Lines Out-of-Phase Closing Resistance Switching Failure Modes of Circuit Breakers Operating Mechanisms-SF6 Breakers Vacuum Interruption Stresses in Circuit Breakers Application and Ratings of Circuit Breakers and Fuses according to ANSI Standards Total and Symmetrical Current Rating Basis Asymmetrical Ratings Voltage Range Factor K Circuit Breaker Timing Diagram Maximum Peak Current Permissible Tripping Delay Service Capability Duty Requirements and Reclosing Capability Capacitance Current Switching Line-Closing Switching Surge Factor Out-of-Phase Switching Current Rating Transient Recovery Voltage Generator Circuit Breakers Specifications of High-Voltage Circuit Breakers Low-Voltage Circuit Breakers Fuses Short Circuit of Synchronous and Induction Machines Reactances of a Synchronous Machine Saturation of Reactances Time Constants of Synchronous Machines Synchronous Machine Behavior on Terminal Short Circuit Circuit Equations of Unit Machines Park's Transformation Park's Voltage Equation Circuit Model of Synchronous Machines Calculation Procedure and Examples Short Circuit of Synchronous Motors and Condensers Induction Motors Short-Circuit Calculations according to ANSI Standards Types of Calculations Accounting for Short-Circuit Current Decay Rotating Machines Model Types and Severity of System Short Circuits Calculation Methods Network Reduction Breaker Duty Calculations Generator Source Short-Circuit Current Asymmetry Calculation Procedure Examples of Calculations Thirty-Cycle Short-Circuit Currents Short-Circuit Calculations According to IEC Standards Conceptual and Analytical Differences Prefault Voltage Far-from-Generator Faults Near-to-Generator Faults Influence of Motors Comparison with ANSI Calculation Procedures Examples of Calculations and Comparison with ANSI Methods Calculations of Short-Circuit Currents in DC Systems DC Short-Circuit Current Sources Calculation Procedures Short Circuit of a Lead Acid Battery Short-Circuit Current of DC Motors and Generators Short-Circuit Current of a Rectifier Short Circuit of a Charged Capacitor Total Short-Circuit Current DC Circuit Breakers Load Flow over Power Transmission Lines Power in AC Circuits Power Flow in a Nodal Branch ABCD Constants Transmission Line Models Tuned Power Line Ferranti Effect Symmetrical Line at No Load Illustrative Examples Circle Diagrams Modal Analysis Corona on Transmission Lines System Variables in Load Flow Load Flow Methods: Part I Modeling a Two-Winding Transformer Load Flow-Bus Types Gauss and Gauss-Seidel Y-Matrix Methods Convergence in Jacobi-Type Methods Gauss-Seidel Z-Matrix Method Conversion of Y to Z Matrix Triangular Factorization Method of Load Flow Load Flow Methods: Part II Function with One Variable Simultaneous Equations Rectangular Form of Newton-Raphson Method of Load Flow Polar Form of Jacobian Matrix Simplifications of Newton-Raphson Method Decoupled Newton-Raphson Method Fast Decoupled Load Flow Model of a Phase-Shifting Transformer DC Load Flow Models Second Order Load Flow Load Models Induction Motor Models Impact Loads and Motor Starting Practical Load Flow Studies Reactive Power Flow and Control Voltage Instability Reactive Power Compensation Reactive Power Control Devices Some Examples of Reactive Power Flow Flexible AC Transmission Systems Three-Phase and Distribution System Load Flow Phase Coordinate Method Three-Phase Models Distribution System Load Flow Optimal Capacitor Locations Optimization Techniques Functions of One Variable Concave and Convex Functions Taylor's Theorem Lagrangian Method: Constrained Optimization Multiple Equality Constraints Optimal Load Sharing between Generators Inequality Constraints Kuhn-Tucker Theorem Search Methods Gradient Methods Linear Programming-Simplex Method Quadratic Programming Dynamic Programming Integer Programming Optimal Power Flow Optimal Power Flow Decoupling Real and Reactive OPF Solution Methods of OPF Generation Scheduling Considering Transmission Losses Steepest Gradient Method OPF Using the Newton Method Sequential Quadratic Programming Successive Linear Programming Interior Point Methods and Variants Security and Environmental Constrained OPF Harmonics Generation Harmonics and Sequence Components Increase in Nonlinear Loads Harmonic Factor Three-Phase Windings in Electrical Machines Tooth Ripples in Electrical Machines Synchronous Generators Transformers Saturation of Current Transformers Shunt Capacitors Sub-Harmonic Frequencies Static Power Converters Switch-Mode Power (SMP) Supplies Arc Furnaces Cycloconverters Thyristor-Controlled Reactor Thyristor-Switched Capacitors Pulse-Width Modulation Adjustable Speed Drives Pulse Burst Modulation Chopper Circuits and Electric Traction Slip Frequency Recovery Schemes Lighting Ballasts Voltage Source Converters Inter-Harmonics Effects of Harmonics Rotating Machines Transformers Cables Capacitors Harmonic Resonance Voltage Notching Electromagnetic Interference Overloading of Neutral Protective Relays and Meters Circuit Breakers and Fuses Telephone Influence Factor Harmonic Analysis Harmonic Analysis Methods Harmonic Modeling of System Components Load Models System Impedance Three-Phase Models Modeling of Networks Power Factor and Reactive Power Shunt Capacitor Bank Arrangements Unbalance Detection Study Cases Harmonic Mitigation and Filters Mitigation of Harmonics Band-Pass Filters Practical Filter Design Relations in an ST Filter Filters for a Furnace Installation Filters for an Industrial Distribution System Secondary Resonance Filter Reactors Double-Tuned Filter Damped Filters Design of a Second-Order High-Pass Filter Zero-Sequence Traps Limitations of Passive Filters Active Filters Corrections in Time Domain Corrections in the Frequency Domain Instantaneous Reactive Power Harmonic Mitigation AT Source Multilevel Converters Arc Flash Hazard Analysis Relating Short-Circuit Currents with Arc Flash and Personal Safety Arc Flash Hazard Analysis Hazard=Risk Categories System Grounding: Impact on Incident Energy Duration of an Arc Flash Event and Arc Flash Boundary Protective Relaying and Coordination Short-Circuit Currents Arc Flash Calculations in Medium-Voltage Systems Arc Flash Calculations in Low-Voltage Systems Accounting for Decaying Short-Circuit Currents Wind Power AEP 765 kV Transmission Grid Initiative in the United States Wind Energy Conversion The Cube Law Operation Wind Generators Power Electronics Reactive Power Control Harmonics Computer Modeling Appendix A: Matrix Methods Appendix B: Calculation of Line and Cable Constants Appendix C: Transformers and Reactors Appendix D: Sparsity and Optimal Ordering Appendix E: Fourier Analysis Appendix F: Limitation of Harmonics Appendix G: Estimating Line Harmonics

Generalized theory of instantaneous active and reactive powers in single-phase circuits based on Hilbert transform

Abstract: This paper presents a new theory of instantaneous power in single-phase circuits, which is named "ps-qs theory" The ps-qs theory is formed based on the Hilbert transform and allows us to define the active and reactive powers instantaneously in single-phase circuits that operate in transient state In addition, a novel control strategy of active and reactive powers in single-phase circuits, which is based on the ps-qs theory, is described Some significant characteristics of the proposed strategy are shown by experimental results

System for providing assured power to a critical load

Abstract: A first AC power source comprising a fuel cell power plant (18), and a second power source, typically grid (10), are normally connected via a high speed isolation switching means (19) to provide sufficient AC power to a critical load (14).

A "critical impedance"-based method for identifying harmonic sources

Abstract: This paper proposes a new method to determine whether the utility or the customer side has more contribution to the harmonic currents measured at the point of common coupling. The method is inspired by the observation that the direction of harmonic reactive power, instead of active power, is a more reliable indicator on the location of dominant harmonic sources. The method needs approximate impedance information to operate. Mathematical analysis, simulation studies, and field measurements have shown that this is a useful, reliable, and practical solution for the harmonic source detection problem.

Use of load control to regulate voltage on distribution networks with embedded generation

Abstract: Since 1994 Econnect has, in conjunction with Northern Electric PLC, investigated the use of consumer load control as a new and innovative method to actively regulate distribution system voltage when affected by the operation of embedded generators [1],[2]. There are a number of issues that can limit the installed capacity of embedded generators; these are often voltage related, and the most common is steady state voltage rise. A number of techniques can be applied to limit steady state voltage rise, some of which are static in time (e.g., network reinforcement) and some dynamic (e.g., power factor control). This paper discusses the issue of excess steady state voltage rise and the methods of limitation that can be applied with specific reference to wind generation. The new and innovative approach using consumer load control is discussed and compared with the existing methods using a simulation case study.

Fuzzy-based reactive power and voltage control in a distribution system

Abstract: This paper presents fuzzy-based reactive power and voltage control in a distribution system. The main purpose is to find the combination of main transformer load tap changer (LTC) positions and capacitors on/off switching operations in a day, such that the voltage deviations at the secondary bus of main transformer become as small as possible, while the reactive power flows through the main transformer and the real power losses at feeders become as little as possible. To minimize system repair cost, the total number of switching operations of LTC and capacitors in a day must be kept as few as possible. From the descriptions above, the linguistic expressions such as "as small as possible," "as little as possible," and "as few as possible" are not clear. So in this paper, the reactive power and voltage control problem is first formulated with fuzzy sets then an annealing searching technique is used to find a proper combination of LTC positions and capacitors on/off switching operations in a day. To demonstrate the effectiveness of the proposed method, reactive power and voltage control in a distribution system within the service area of Yunlin District Office of Taiwan Power Company (TPC) are analyzed. It is found that a proper dispatching schedule for LTC positions and capacitors switching operations can be reached by the proposed method.

Large-scale wind power integration and voltage stability limits in regional networks

Abstract: When planning and developing large-scale wind power plants in areas distant from the main power transmission system, voltage control assessments and reactive power compensation are increasingly important. Voltage stability of the regional network may be a main limitation with respect to maximum rating and operation of the wind power plant Technical constraints in relation to wind power integration in weak grids may in general be associated with limited thermal capacity in parts of the grid and/or the adverse effect wind power can have on voltage quality and stability. In certain situations, however, local constraints regarding development of new transmission lines or upgrading of existing lines can make it interesting to utilise the existing lines to a level which in worst case may imply operation beyond the normal technical constraints of the system. In this work, challenges and opportunities arising from situations as described above are analysed, and viable measures to enable secure and acceptable operation of large wind farms in remote areas close to the thermal capacity and stability limits of the power system, are pointed out. The paper presents results from computer analyses of a simplified, yet realistic, electrical power system with wind power integration, illustrating possible solutions to achieve this.

Understanding and solving short-term voltage stability problems

Abstract: Based on actual incidents, short-term voltage instability is an increasing, but often overlooked, industry concern. A common scenario is a large disturbance such as a multi-phase fault near a load center that decelerate motor loads. Following fault clearing with transmission outages, motors raw very high current while simultaneously attempting to reaccelerate, and may stall if the power system is weak. Massive loss of load and possibly area instability and voltage collapse may follow. The authors describe actual incidents. Fast-acting generator excitation controls, fast-acting reactive power support devices (SVC, STATCOM, SMES), or fast load shedding can prevent voltage collapse. Proper analysis requires dynamic modeling of aggregated motor loads, with equivalents for distribution feeders. Power electronic based voltage support devices must be realistically modeled to determine required size, location, number, and type. Based on simulations, they conclude that voltage-sourced converter devices (STATCOM, SMES) are attractive countermeasures against load loss and voltage collapse. Factory built distribution-connected distributed devices may be cost-effective compared to larger transmission-connected devices.

A general three-phase PFC controller for rectifiers with a parallel-connected dual boost topology

Abstract: A general constant-frequency power-factor-correction (PFC) controller is proposed for three-phase rectifiers with parallel-connected dual-boost topologies. This paper shows that unity power factor and low current distortion in all three phases can be realized by one-cycle control using one integrator with reset along with a few near and logic components. This new extension of one-cycle control provides the core PFC function to the dual-boost topologies. It does not require multipliers, as used in most other control approaches to scale the current reference according to the output power level. In each 60/spl deg/ of AC line cycle, only two switches are switched at high frequency; therefore the switching losses are significantly reduced. All switches are switched at low current, which results in reduced current ratings. This control method is simple and general. It is applicable to three-phase rectifiers that can be decoupled into parallel-connected dual-boost topologies by slight modification of the logic circuit. This control method is verified by experimental results. The proposed controller is suitable to be integrated into a three-phase PFC control chip.

Shunt active power filter synthesizing resistive loads

Abstract: The paper discusses the use of a shunt active power filter to compensate for the line current distortion and to improve the power factor. The advantages of the resistive load synthesis over the sinusoidal current synthesis when the filter is used in a system where the voltage is not perfectly sinusoidal are presented. The control circuit is based on analogic multipliers, and the currents follow the same waveforms of the respective line voltages. Experimental results of connecting a three-phase active power filter to a nonsinusoidal grid are presented.

Security Constrained Optimal Power Flow for an Economical Operation of FACTS Devices in Liberalized Energy Markets

Abstract: Among the ongoing liberalization of the global energy markets, several technical developments in the field of power electronic based transmission devices lead to a significant restructuring of the electric power supply. These FACTS-devices (Flexible A_C T_ransmission S_stems) enable a continuous and wear-resistant control of active and reactive power flows as well as voltage control. The incorporation of the control capabilities provided by FACTS-devices into an optimal power flow (OPF) opens up an unused potential for reducing power losses as well as cost for power transmission of the power system operator. This paper describes the optimization method basing on sequential quadratic programming (SQP) including models of FACTS-devices suitable for a gradient based optimization. Furthermore, the economically efficient operation of FACTS-devices is shown using different scenarios.