Showing papers on "Power system simulation published in 2004"

Harmonic interaction between a large number of distributed power inverters and the distribution network

Abstract: Power quality problems associated with distributed power (DP) inverters, implemented in large numbers onto the same distribution network, are investigated. Currently, these power quality problems are mainly found in projects with large penetration of photovoltaics (PV) on rooftops of houses and commercial buildings. The main object of this paper is to analyze the observed phenomena of harmonic interference of large populations of these inverters and to compare the network interaction of different inverter topologies and control options. These power quality phenomenons are investigated by using extensive laboratory experiments, as well as computer modeling of different inverter topologies. A complete network simulation study on an existing residential network with large penetration of PVs, is included.

Design, analysis, and real-time testing of a controller for multibus microgrid system

Abstract: This paper concentrates on the design and analysis of a controller for multibus microgrid system. The controller proposed for use with each distributed generation (DG) system in the microgrid contains inner voltage and current loops for regulating the three-phase grid-interfacing inverter, and external power control loops for controlling real and reactive power flow and for facilitating power sharing between the paralleled DG systems when a utility fault occurs and the microgrid islands. The controller also incorporates synchronization algorithms for ensuring smooth and safe reconnection of the micro and utility grids when the fault is cleared. With the implementation of the unified controller, the multibus microgrid system is able to switch between islanding and grid-connected modes without disrupting the critical loads connected to it. The performance of this unified controller has been verified in simulation using a real-time digital simulator and experimentally using a scaled laboratory prototype.

Optimum control strategies in energy conversion of PMSG wind turbine system without mechanical sensors

Abstract: The amount of energy obtained from a wind energy conversion system (WECS) depends not only on the characteristics of the wind regime at the site, but it also depends on the control strategy used for the WECS. In order to determine the gain in energy derived from one concept as compared against another, models of several autonomous WECS have been developed using Matlab Simulink software. These allow easy performance evaluations and comparisons on different control strategies used, and determine the amount of energy injected to the grid in the case of the grid-connected systems. This paper also proposes a prototype version of the control strategy of a 20-kW permanent-magnet synchronous generator (PMSG) for maximum power tracking and compares with the results produced by previous strategies. Advantages of this mechanical sensorless control strategy for maximum power estimation are demonstrated by digital simulation of the system.

A model of PV generation suitable for stability analysis

Abstract: This paper describes a model of photovoltaic (PV) generation suitable for studying its interactions with the power system. Experimental results suggest that the maximum power point tracking part of the control system of the PV generator dominates the dynamic behavior of the system. These experimental results are used to develop and validate the proposed model. It is shown that the model accurately reflects the behavior of the generator following both small and fast changes in irradiance and AC grid voltage. The proposed model is designed to be integrated in a dynamic simulation program.

A comparison of the AC and DC power flow models for LMP calculations

Abstract: The paper examines the tradeoffs between using a full ac model versus the less exact, but much faster, dc power flow model for LMP-based market calculations. The paper first provides a general discussion of the approximations associated with using a dc model, with an emphasis on the impact these approximations will have on security constrained OPF (SCOPF) results and LMP values. Then, since the impact of the approximations can be quite system specific, the paper provides case studies using both a small 37 bus system and a somewhat larger 12,965 bus model of the Midwest U.S. transmission grid. Results are provided comparing both the accuracy and the computational requirements of the two models. The general conclusion is that while there is some loss of accuracy using the dc approximation, the results actually match fairly closely with the full ac solution.

Dynamic wind turbine models in power system simulation tool DIgSILENT

Abstract: This report presents a collection of models and control strategies developed and implemented in the power system simulation tool PowerFactory DIgSILENT for different wind turbine concepts. It is the second edition of Risoe-R-1400(EN) and it gathers and describes a whole wind turbine model database built-op and developed during several national research projects, carried out at Risoe DTU National Laboratory for Sustainable Energy and Aalborg University, in the period 2001-2007. The overall objective of these projects was to create a wind turbine model database able to support the analysis of the interaction between the mechanical structure of the wind turbine and the electrical grid during different operational modes. The report provides thus a description of the wind turbines modelling, both at a component level and at a system level. The report contains both the description of DIgSILENT built-in models for the electrical components of a grid connected wind turbine (e.g. induction generators, power converters, transformers) and the models developed by the user, in the dynamic simulation language DSL of DIgSILENT, for the non-electrical components of the wind turbine (wind model, aerodynamic model, mechanical model). The initialisation issues on the wind turbine models into the power system simulation are also presented. The main attention in the report is drawn to the modelling at the system level of the following wind turbine concepts: (1) Fixed speed active stall wind turbine concept (2) Variable speed doubly-fed induction generator wind turbine concept (3) Variable speed multi-pole permanent magnet synchronous generator wind turbine concept These wind turbine concept models can be used and even extended for the study of different aspects, e.g. the assessment of power quality, control strategies, connection of the wind turbine at different types of grid and storage systems. Different control strategies have been developed and implemented for these wind turbine concepts, their performance in normal or fault operation being assessed and discussed by means of simulations. The described control strategies have different goals e.g. fast response over disturbances, optimum power efficiency over a wider range of wind speeds, voltage ride-through capability including grid support. A dynamic model of a DC connection for active stall wind farms to the grid including the control is also implemented and presented. (au)

Evaluation of uncertainty in dynamic simulations of power system models: The probabilistic collocation method

Abstract: This paper explores the use of a new technique, the probabilistic collocation method (PCM), to enable the evaluation of uncertainty in power system simulations. The PCM allows the uncertainty in transient behavior of power systems to be studied using only a handful of simulations. The relevant theory is outlined here and simple examples are used to illustrate the application of PCM in a power systems setting. In addition, an index for identification of key uncertain parameters, as well as an example with a more realistic power system, are presented.

Risk-constrained self-scheduling of a thermal power producer

Abstract: This paper addresses the self-scheduling problem of a price-taker power producer. It focuses on risk modeling, emphasizing the tradeoff existing between maximum profit and minimum risk. The paper analyzes a self-scheduling model that considers simultaneously profit and risk. This model is formulated as a mixed-integer quadratic programming problem, which is solved using commercially available software. Relevant results from a realistic case study are discussed.

A novel real-time simulation technique of photovoltaic generation systems using RTDS

Abstract: For the performance test of photovoltaic (PV) generation systems, actual system apparatuses: a solar panel, converter system, and load facilities should be installed. It is also hardly possible to compare a maximum power point tracking (MPPT) control scheme with others under the same weather and load conditions in an actual PV generation system. One of the possible alternatives is to realize a transient simulation scheme for PV generation systems under real weather conditions of insolation and surface temperature of solar cell. The authors propose a novel real-time simulation method for PV generation systems under real weather conditions using a real-time digital simulator (RTDS). V--I curves of a real PV panel are tested using electric load device, and a hypothetical network of the tested PV panel is created on the RTDS by arranging electrical components from the customized component model libraries. The real weather conditions, insolation, and temperature of the PV panel, are interfaced through the analog input ports of the RTDS for real-time simulation. The outcomes of the simulation demonstrate the effectiveness of the proposed simulation technique, and also show that cost-effective verification of availability and stability of PV generation systems is possible using the built-in simulator.

A novel active power quality compensator topology for electrified railway

Abstract: To improve the power quality of traction power system, a novel active power quality compensator (APQC) and a new compensating currents detection method are proposed. The APQC consists of a three-phase voltage source converter and a Scott transformer. The Scott transformer, taken as an isolation transformer, not only connects the three-phase converter to the traction power system, but also converts the traction power system to a nearly balanced three-phase power system. Therefore, a general three-phase converter could be used in APQC. Regarding the traction substation as a compensating object, the power quality of a traction substation can be improved integrally. Simulation and prototype experimental results show that the proposed APQC is able to compensate reactive power, harmonic, and negative-sequence currents in two feeders of a traction substation.

PSIM-based modeling of automotive power systems: conventional, electric, and hybrid electric vehicles

Abstract: Automotive manufacturers have been taking advantage of simulation tools for modeling and analyzing various types of vehicles, such as conventional, electric, and hybrid electric vehicles. These simulation tools are of great assistance to engineers and researchers to reduce product-development cycle time, improve the quality of the design, and simplify the analysis without costly and time-consuming experiments. In this paper, a modeling tool that has been developed to study automotive systems using the power electronics simulator (PSIM) software is presented. PSIM was originally made for simulating power electronic converters and motor drives. This user-friendly simulation package is able to simulate electric/electronic circuits; however, it has no capability for simulating the entire system of an automobile. This paper discusses the PSIM validity as an automotive simulation tool by creating module boxes for not only the electrical systems, but also the mechanical, energy-storage, and thermal systems of the vehicles. These modules include internal combustion engines, fuel converters, transmissions, torque couplers, and batteries. Once these modules are made and stored in the library, the user can make the car model either a conventional, an electric, or a hybrid vehicle at will, just by dragging and dropping onto a schematic blank page.

Fuzzy approach for optimal placement and sizing of capacitor banks in the presence of harmonics

Abstract: This paper presents a fuzzy-based approach for optimal placement and sizing of fixed capacitor banks in radial distribution networks in the presence of voltage and current harmonics. The objective function includes the cost of power losses, energy losses, and capacitor banks. Constraints include voltage limits, number/size and locations of installed capacitors, and the power quality limits of IEEE-519 standard. Candidate buses for capacitor placement are selected using the sensitivities of constraints and the objective function with respect to reactive power injection at each bus. Using fuzzy set theory, a suitable combination of objective function and constraints is generated as a criterion to select the most suitable bus for capacitor placement. The /spl alpha/-cut process is applied at each iteration to guarantee simultaneous improvements of objective function and satisfying given constraints. Simulation results for the 18 bus IEEE distorted network show the advantages of the proposed method as compared to the maximum sensitivities selection algorithm.

Power system modeling for inverse problems

Abstract: Large disturbances in power systems often initiate complex interactions between continuous dynamics and discrete events. The paper develops a hybrid automaton that describes such behavior. Hybrid systems can be modeled in a systematic way by a set of differential-algebraic equations, modified to incorporate impulse (state reset) action and constraint switching. This differential-algebraic impulsive-switched (DAIS) model is a realization of the hybrid automaton. The paper presents a practical object-oriented approach to implementing the DAIS model. Each component of a system is modeled autonomously. Connections between components are established by simple algebraic equations. The systematic nature of the DAIS model enables efficient computation of trajectory sensitivities, which in turn facilitate algorithms for solving inverse problems. The paper outlines a number of inverse problems, including parameter uncertainty, parameter estimation, grazing bifurcations, boundary value problems, and dynamic embedded optimization.

Investigation of anti-islanding protection of power converter based distributed generators using frequency domain analysis

Abstract: The anti-islanding algorithm proposed by the Sandia national laboratories is analyzed in this study because this scheme, also known as the Sandia scheme, is considered to be effective in detecting islanding of distributed generation systems. Previously, other than heuristic approaches, there has not been any quantitative analysis for tuning the control gains of the algorithm based on the power rating and bandwidth of the distributed generation (DG) power converter. The paper interprets the components of the algorithm that affect the voltage magnitude and frequency into block diagrams that can be linearized and studied using continuous time approximations. This paper uses a frequency domain approach to analyze the range for the gains required by anti-islanding algorithm to effectively determine the disconnection of the mains grid within an acceptable time duration. The analysis provides guidelines for using Sandia's national laboratory schemes under different application conditions. The results are validated using detailed time domain DG and power system simulations.

Large power system stability enhancement using wide-area signals based hierarchical controller

Abstract: A two-level hierarchical structure is proposed to improve the stability of multimachine power systems. The solution consists of a local controller for each generator at the first level helped by a multivariable central one at the secondary level. The secondary-level controller uses remote signals from all generators to synthesize decoupling control signals that improve the local controllers' performances. The first-level controllers, on the other hand, use only local signals to dampen local oscillations. A systematic procedure for the design of the wide-area signals based central controller is given. It is based on a reformulation of the multimachine power system model into a suitable and closed form. Our control strategy leads to a robust wide-area controller having two major components: a model-based decoupling nonlinear control and an adaptation module. The nonlinear control parameters are updated so that the controller sensitivity to power systems operating point and structure variation is considerably reduced. The hierarchical structure is used on a large power system and simulation results reveal that the system stability is considerably improved: a power system, unstable under conventional controllers (AVR-PSS/GOVERNOR), is rendered stable when combining the central and local controllers' actions.

Dynamic frequency control with increasing wind generation

Abstract: Frequency control is essential for the secure and stable operation of a power system. With wind penetration increasing rapidly in many power systems, ensuring continuous power system security is vital. The frequency response to a disturbance on the all Ireland system is simulated for a range of installed wind capacities under different system conditions. The purpose of this study is to assess the effects of increased wind generation on system frequency, and the security of the system following such disturbances.

Real time digital simulation for control and protection system testing

Abstract: Today's power system environment is becoming more and more complex. Electrical power networks are being pushed closer to their limits, while at the same time higher reliability and efficiency are demanded. As a result, electrical equipment manufacturers are developing advanced equipment and installations to meet these requirements. Part of the challenge in designing such apparatus is the need to accurately simulate (prove) and test these devices before they are installed in an actual power system. The real time digital simulator (RTDS) allows developers to accurately and efficiently simulate electrical power systems and their ideas to improve them. The RTDS simulator operates in real time, therefore not only allowing the simulation of the power system, but also making it possible to test physical protection and control equipment. This gives developers the means to prove their ideas, prototypes and final products in a realistic environment. This paper will summarize the fundamental design of the RTDS simulator, with particular emphasis on recent developments and enhancements. It will also describe practical applications of the simulator in the various stages of power system design, testing and implementation. A brief description of ongoing development for the real time simulation of voltage source converter (VSC) converters is included.

Influence of the variable speed wind generators in transient stability margin of the conventional generators integrated in electrical grids

Abstract: Summary form only given. This work demonstrates that the integration of variable speed wind systems with doubly fed induction generators and a four quadrant AC-to-AC converter connected to the rotor windings (DFIG), increases the transient stability margin of the electrical grids, when compared with the case where the fixed speed wind systems with cage generators are used. The results clearly show that the DFIG improves the short-term postfault stability of the conventional synchronous generators in the network. It is due to the influence of the two dedicated rotor current regulators of the DFIG on the dynamic behavior of the other generators in the system. The reduction in dynamic reactive compensation demands was also observed when using the DFIG, which helps to avoid other problems as voltage collapse in the power system. Besides, adequate models to represent the behavior of the DFIG in transient stability studies are presented. From the simulation results some important conclusions can be extracted to guide the integration of the wind farms on weak or strong grids.

Design of a real-time digital Simulator for a D-STATCOM system

Abstract: Real-time digital simulation of power electronic systems requires significant computational resources due to increasingly complex system configurations, control algorithms, and higher switching frequency. Consequently, it is prudent to exploit various computer resources for optimizing the design of simulators/controllers for such systems. This paper presents the design and implementation details of a real-time digital simulator for a Voltage-Source-Converter-based Distribution STATic COMpensator (D-STATCOM) power system. The design process adopts a modular approach utilizing distributed digital signal processor/field-programmable gate array resources of a digital processing platform. The design has been validated by using an experimental setup of a 5-kVA D-STATCOM system.

A new PMU-based fault detection/location technique for transmission lines with consideration of arcing fault discrimination-part II: performance evaluation

Abstract: The theory and algorithms of the proposed technique have been presented in Part I of this two-paper set. In Part II of this two-paper set, the proposed technique is evaluated by considerable simulation cases simulated by the Matlab/Power system Blockset simulator. For the proposed fault detector, the trip time achieved can be up to 3.25 ms and the average value of trip times is about 8 ms for both permanent and arcing faults on transmission lines. For the proposed fault locator, the accuracy can be up to 99.99% and the error does not exceed 0.45%. Moreover, the proposed arcing fault discriminator can discriminate between arcing and permanent faults within four cycles after fault inception. It has proven to be an effective tool to block reclosing on the permanent faults in the computer simulations. The simulation results also demonstrate that the presented extended discrete Fourier transform algorithm eliminates effectively the error caused by exponentially decaying dc offset on fundamental and harmonic phasor computations. Finally, a test case using the real-life measured data proves the feasibility of the proposed technique.

An efficient multirate Simulation technique for power-electronic-based systems

Abstract: A novel multirate method of simulating power-electronic-based systems containing a wide range of time scales is presented. In this method, any suitable integration algorithm, with fixed or variable time-step, can be applied to the fast and/or slow subsystems. The subsystems exchange coupling variables at a communication interval that can be fixed or varied dynamically depending upon the state of the system variables. The proposed multirate method is applied to two example power systems that include power-electronic subsystems. Increases in simulation speed of 183-281% over established single-rate integration algorithms are demonstrated.

Hybrid systems modeling for power systems

Abstract: In this work a framework for modeling power systems using hybrid input/output automata (HIOA) is proposed. The system is assumed to consist of several distinct components. Some of them drive the continuous dynamics while others exhibit event-driven discrete dynamics. Such behavior is characterized by interactions between continuous dynamics and discrete events. Therefore the power systems are an important example of hybrid systems. This hybrid modeling process is applied to a simple power system.

ObjectStab-an educational tool for power system stability studies

Abstract: Traditionally, the simulation of transient and voltage stability in power systems has been constrained to domain-specific tools such as Simpow, PSS/E, ETMSP, and EuroStag. While being efficient and thereby able to simulate large systems, their component models are often encapsulated and difficult or impossible to examine and modify. Also, these simulators often require substantial training and are therefore not ideal for normal classroom use. For academic and educational use, it is more important that the component modeling be transparent and flexible, and that students quickly get started with their simulations. This paper describes a freely available power system library called ObjectStab intended for power system stability simulations written in Modelica, a general-purpose object-oriented modeling language. All component models are transparent and can easily be modified or extended. Power system topology and parameter data are entered in one-line diagram form using a graphical editor. The component library has been validated using comparative simulations with EuroStag.

Two-phase neural network based modelling framework of constrained economic load dispatch

Abstract: A two-phase optimisation neural network based modelling framework and a solution technique is proposed for solving the economic load dispatch problem in large-scale systems. The method is based on the solution of a set of differential equations obtained from transformation of an augmented Lagrangian energy function. The main objective is to minimise the total cost of generation while meeting the load demand and satisfying a number of constraints like power balance, unit generation limits, maximum ramp-rate limits, network losses and prohibited zone avoidance. It compares the proposed technique with the lambda iteration and genetic algorithm methods while investigating its applicability to large-scale power systems. The technique has shown the potential for achieving improved and feasible results with proper selection of control parameters.

Comparative study of maximum power strategy in wind turbines

Abstract: Power characteristics of wind turbines are nonlinear It is particularly true for Savonius vertical axis systems whose the provided power is very sensitive to the load Thus, controlling its operating is essential to optimise the energetic behaviour Several control strategies (maximum power point tracking) can be used for energy conversion If the wind turbine characteristic (Cp(/spl lambda/)) is supposed to be a priori known, it can be used for an optimal control of the torque, speed or of the output current On the contrary, if this characteristic is unknown, an operational seeking algorithm such as fuzzy logic has to be implemented Several structures are also possible following the AC-DC converter which can be a PWM converter or a simple diode bridge A comparative study of these control strategies and architectures is proposed in this paper The analysis is based on simulations and on some experiments

Digital control of a voltage source inverter in photovoltaic applications

Abstract: Digital control techniques are used to control a voltage source inverter in a photovoltaic system. A digital signal processor is implemented in the voltage source inverter which provides rapid maximum power point tracking through the control of the inverter output current. In addition to this, the digital signal processor provides other necessary system functions including calculation of the inverter switching instants used for current regulation and maximum power point tracking and synchronization with the grid. The system is presented as a fully operational unit and the inclusion of simulation results, analysis of the control implementation technique and experimental verification exhibit the system's functionality.

Analysis and validation of a real-time AC drive simulator

Abstract: This paper presents the analysis and validation of a real-time AC drive simulator. Real-time simulation has been used for over a decade in power systems engineering to test controllers thoroughly, efficiently, and safely. The development and testing of large DC drives could benefit from this type of simulation, but real-time power system simulators have modeling restrictions preventing stable and accurate simulation of isolated drives switching at high frequencies. These restrictions can be overcome by combining in a single model both the power converter and motor models. After discussing the salient issues related to the real-time simulation of DC drives, a prototype real-time simulator for drives is described. Its ability to accurately emulate the behavior of large drives is demonstrated through two case studies. A first case study demonstrates the feasibility of simulating a drive of typical complexity in real-time. The second case study demonstrates how the controller for a PWM VSI fed induction motor, switching at 4 kHz, is designed, implemented in a microcontroller and tested in real-time using a simulated inverter, motor and load. Comparisons against results obtained with another simulation tool, which uses extremely accurate variable-step integration algorithms, demonstrates the validity of our approach.