Showing papers on "Power system simulation published in 2006"

A computationally efficient mixed-integer linear formulation for the thermal unit commitment problem

Abstract: This paper presents a new mixed-integer linear formulation for the unit commitment problem of thermal units. The formulation proposed requires fewer binary variables and constraints than previously reported models, yielding a significant computational saving. Furthermore, the modeling framework provided by the new formulation allows including a precise description of time-dependent startup costs and intertemporal constraints such as ramping limits and minimum up and down times. A commercially available mixed-integer linear programming algorithm has been applied to efficiently solve the unit commitment problem for practical large-scale cases. Simulation results back these conclusions

Modeling of the wind turbine with a doubly fed induction generator for grid integration studies

Abstract: Due to its many advantages such as the improved power quality, high energy efficiency and controllability, etc. the variable speed wind turbine using a doubly fed induction generator (DFIG) is becoming a popular concept and thus the modeling of the DFIG based wind turbine becomes an interesting research topic. Fundamental frequency models have been presented but these models are often complex with significant numerical overhead as the power converter block consisting of power control, rotor side and grid side converter control and DC link are often simulated in detail. This paper develops a simple DFIG wind turbine model in which the power converter is simulated as a controlled voltage source, regulating the rotor current to meet the command of real and reactive power production. This model has the form of traditional generator model and hence is easy to integrate into the power system simulation tool such as PSS/E. As an example, the interaction between the Arklow Bank Wind Farm and the Irish National Grid was simulated using the proposed model. The model performance and accuracy was also compared with the detailed model developed by DIgSILENT. Considering the simplification adopted for the model development, the limitation and applicability of the model were also discussed in this paper.

Interfacing Renewable Energy Sources to the Utility Grid Using a Three-Level Inverter

Abstract: This paper presents a novel approach for the connection of renewable energy sources to the utility grid. Due to the increasing power capability of the available generation systems, a three-level three-phase neutral-point-clamped voltage-source inverter is selected as the heart of the interfacing system. A multivariable control law is used for the regulator because of the intrinsic multivariable structure of the system. A current source (playing the role of a generic renewable energy source) is connected to the grid using a three-level inverter in order to verify the good performance of the proposed approach. Large- and small-signal d-q state-space averaged models of the system are obtained and used to calculate the multivariable controller based on the linear quadratic regulator technique. This controller simultaneously regulates the dc-link voltage (to operate at the maximum power point of the renewable energy source), the mains power factor (the power is delivered to the grid at unity power factor), and the dc-link neutral-point voltage balance. With the model and regulator presented, a specific switching strategy to control the dc-link neutral-point voltage is not required. The proposed controller can be used for any application, since its nature makes possible the control of any system variable. The good performance of the presented interfacing solution in both steady-state and transient operation is verified through simulation and experimentation using a 1-kW neutral-point-clamped voltage-source-inverter prototype, where a PC-embedded digital signal processor board is used for the controller implementation

Usefulness of DC power flow for active power flow analysis with flow controlling devices

Abstract: DC power flow is a commonly used tool for contingency analysis. Recently, due to its simplicity and robustness, it also becomes increasingly used for the real-time dispatch and techno-economic analysis of power systems. It is a simplification of a full power flow looking only at active power. Aspects such as voltage support and reactive power management are possible to analyse. However, such simplifications cannot always be justified and sometimes lead to unrealistic results. Especially the implementation of power flow controlling devices is not trivial since standard DC power flow fundamentally neglects their effects. Until recently, this was not an issue as the application of power flow controlling devices in the European grid was limited. However, with the liberalisation of European electricity market and the introduction of large wind energy systems, the need for real power flow control has emerged and therefore, the use of these devices has been reconsidered. Several phase shifting transformers (PST) are being installed or planned in order to control flows. Therefore, it is important to fundamentally re-validate the fast, but less accurate, DC power flow method. In this paper the assumptions of DC power flow are analysed, and its validity is assessed by comparing the results of power flow simulations using both the DC and AC approaches on a modified IEEE 300 bus system with PSTs.

EPOCHS: a platform for agent-based electric power and communication simulation built from commercial off-the-shelf components

Abstract: This paper reports on the development and subsequent use of the electric power and communication synchronizing simulator (EPOCHS), a distributed simulation environment. Existing electric power simulation tools accurately model power systems of the past, which were controlled as large regional power pools without significant communication elements. However, as power systems increasingly turn to protection and control systems that make use of computer networks, these simulators are less and less capable of predicting the likely behavior of the resulting power grids. Similarly, the tools used to evaluate new communication protocols and systems have been developed without attention to the roles they might play in power scenarios. EPOCHS integrates multiple research and commercial off-the-shelf systems to bridge the gap.

Real-Time Identification of Optimal Operating Points in Photovoltaic Power Systems

Abstract: Photovoltaic power systems are usually integrated with some specific control algorithms to deliver the maximum possible power. Several maximum power point tracking (MPPT) methods that force the operating point to oscillate have been presented in the past few decades. In the MPPT system, the ideal operation is to determine the maximum power point (MPP) of the photovoltaic (PV) array directly rather than to track it by using the active operation of trial and error, which causes undesirable oscillation around the MPP. Since the output features of a PV cell vary with environment changes in irradiance and temperature from time to time, real-time operation is required to trace the variations of local MPPs in PV power systems. The method of real-time estimation proposed in this paper uses polynomials to demonstrate the power-voltage relationship of PV panels and implements the recursive least-squares method and Newton-Raphson method to identify the voltage of the optimal operating point. The effectiveness of the proposed methods is successfully demonstrated by computer simulations and experimental evaluations of two major types of PV panels, namely: 1) crystalline silicon and 2) copper-indium-diselenide thin film

Power quality and stability improvement of a wind farm using STATCOM supported with hybrid battery energy storage

Abstract: A large penetration of wind generation info the power system will mean that poor power quality and poor stability margins cannot be tolerated from wind farms This requires that methods to improve power qualify and stability for such systems be found The static compensator (STATCOM) with hybrid battery energy storage (BES) has great potential to fulfil this role, though considerable advances in the control of this system are still to be made From an economic point of view, rating the STATCOM for steady-state power-quality improvement duty is appropriate Rating the STATCOM to absorb large amounts of additional power in excess of its transient overload capability during network faults is inappropriate A hybrid of BES and braking resistor is therefore proposed A new hybrid STATCOM–BES control technique is developed and discussed in the context of improving the stability and power quality to fixed speed, induction generator, wind turbines The variation of the network voltage, active and reactive power with the fluctuation of the wind generation is studied A wind generation system with a STATCOM battery energy storage unit and the new control was simulated and the results demonstrate that both power quality and the stability margin can be improved significantly for wind farms

Modeling Strategy for Back-to-Back Three-Level Converters Applied to High-Power Wind Turbines

Abstract: Three-level converters are becoming a realistic alternative to the conventional converters in high-power wind-energy applications. In this paper, a complete analytical strategy to model a back-to-back three-level converter is described. This tool permits us to adapt the control strategy to the specific application. Moreover, the model of different loads can be incorporated to the overall model. Both control strategy and load models are included in the complete system model. The proposed model pays special attention to the unbalance in the capacitors' voltage of three-level converters, including the dynamics of the capacitors' voltage. In order to validate the model and the control strategy proposed in this paper, a 3-MW three-level back-to-back power converter used as a power conditioning system of a variable speed wind turbine has been simulated. Finally, the described strategy has been implemented in a 50-kVA scalable prototype as well, providing a satisfactory performance

Development of a Unified Design, Test, and Research Platform for Wind Energy Systems Based on Hardware-in-the-Loop Real-Time Simulation

Abstract: Traditionally, offline modeling and simulation has been the tool of choice for improving wind energy system control strategies and their utility system integration. This paper exploits how a newly established real-time hardware-in-the-loop (HIL) test facility, which is designed for testing all-electric ship propulsion systems, can be utilized for wind energy research. The test site uses two 2.5-MW/220-rpm dynamometers and a 5-MW variable voltage and frequency converter to emulate a realistic dynamic environment, both mechanically and electrically. The facility is controlled by a digital real-time electric power system simulator that is capable of simulating electrical networks and control systems of substantial complexity, typically with a 50-mus time step. Substantial input/output allows the feedback of measured quantities into the simulation. A 15-kW mock-up motor-generator set is used to demonstrate some critical aspects of the concept including the implementation of a proposed neural-network-based sensorless maximum wind energy capture control. From the dynamic test results presented, it is concluded that the proposed system shows great potential for the development of a unified wind energy design, test, and research platform

Sensitivity, Approximation, and Uncertainty in Power System Dynamic Simulation

Abstract: Parameters of power system models, in particular load models, are seldom known exactly, yet dynamic security assessment relies upon simulation of those uncertain models. This paper proposes a computationally feasible approach to assessing the influence of uncertainty in simulations of power system dynamic behavior. It is shown that trajectory sensitivities can be used to generate accurate first-order approximations of trajectories that arise from perturbed parameter sets. The computational cost of obtaining the sensitivities and perturbed trajectories is minimal. The mathematical structure of the trajectory approximations allows the effects of uncertainty to be quantified and visualized using worst-case analysis and probabilistic approaches

A Stochastic Unit-commitment Model for the Evaluation of the Impacts of Integration of Large Amounts of Intermittent Wind Power

Abstract: A large share of integrated wind power causes technical and financial impacts on the operation of the existing electricity system due to the fluctuating behaviour and unpredictability of wind power. The presented stochastic bottom-up electricity market model optimises the unit commitment considering five kinds of markets and taking explicitly into account the stochastic behaviour of the wind power generation and of the prediction error. It can be used for the evaluation of varying electricity prices and system costs due to wind power integration and for the investigation of integration measures.

Distributed Generation Micro-Grid Operation: Control and Protection

Abstract: Performing intentional islanding or micro-grid operation of Distributed Generators (DGs) can improve the power system service quality and increase the power system reliability. Despite the benefits micro-grid operation can bring to the power system, many challenges and technical issues constraint its operation. This paper addresses two main challenges associated with the operation of micro-grids: voltage/frequency control and protection. The main aim of this paper is three-fold. First, a control strategy for inverter based DGs is proposed to control both voltage and frequency during islanded operation. Secondly, a protection scheme is proposed to protect both the lines and DGs during islanded operation. Lastly, both the control scheme and the protection scheme are coordinated to avoid nuisance tripping of the DGs and non-critical loads. The study is performed using a digital computer simulation approach PSCAD/EMTDC.

Modeling and analysis of a flywheel energy storage system for Voltage sag correction

Abstract: The U.S. Navy is looking for methods to maximize the survivability of combat ships during battle conditions. A shipboard power distribution system is a stiff isolated power system that is vulnerable to voltage sags, which arise due to faults or pulsed loads, which can cause interruptions of critical loads. A series voltage injection type flywheel energy storage system (FESS) is used to mitigate voltage sags and maximize the survivability of the ship. The basic circuit consists of an energy storage system, power electronic interface, and a series injection transformer. In this case, the energy storage system consists of a flywheel coupled to an induction machine. The stored energy is used for sag correction for the critical load. Indirect field-oriented control (IDFOC) with space-vector pulsewidth modulation (SVPWM) is used to control the induction machine. Sinusoidal PWM is used for controlling the power system side converter. This paper presents the modeling, simulation, and analysis of a FESS with a power converter interface using PSCAD/EMTDC.

Measurement-based dynamic load models: derivation, comparison, and validation

Abstract: Accurate load modeling is essential for power system dynamic simulation. In this paper, four dynamic load models are identified based on multiple online measurement data from the Taiwan Power System. The performances in modeling real and reactive power behaviors by dynamic and selected static load models are evaluated. Parameter variation with respect to different loading conditions is analyzed. A simple and efficient method is presented to estimate a representative parameter set for different loading conditions. The cross-validation technique is applied to validate the four dynamic load models in order to obtain a better estimate of their performance. Numerical studies indicate that linear dynamic load models studied in this paper give better results than two nonlinear dynamic load models in modeling reactive power behaviors during disturbance while they are comparable in modeling real power behaviors

Contribution of DFIG-based wind farms to power system short-term frequency regulation

Abstract: A control strategy that provides a doubly-fed induction generator (DFIG)-based wind farm with the capability to provide short-term frequency regulation is investigated. The controller manipulates dynamically the position of the DFIG rotor flux vector to slow down the generator allowing for a temporary surge in the power output, which helps to reduce the frequency drop following the transient period after the loss of network generation. A generic network that combines synchronous and wind-farm generation has been modelled and used for control system design and dynamic assessment. Studies are conducted for the DFIG operating at different speed and power output conditions. Simulation results are described which illustrate the contributions to frequency support of DFIG-based wind farms controlled with the proposed strategy. Machine data and control parameters are provided.

Bio-inspired Algorithms for the Design of Multiple Optimal Power System Stabilizers: SPPSO and BFA

Abstract: Damping intra-area and interarea oscillations are critical to optimal power flow and stability in a power system. Power system stabilizers (PSSs) are effective damping devices, as they provide auxiliary control signals to the excitation systems of generators. The proper selection of PSS parameters to accommodate variations in the power system dynamics is important and is a challenging task particularly when several PSSs are involved. Two classical bio-inspired algorithms, which are small-population-based particle swarm optimization (SPPSO) and bacterial foraging algorithm (BFA), are presented in this paper for the simultaneous design of multiple optimal PSSs in two power systems. A classical PSO with a small population of particles is called SPPSO in this paper. The SPPSO uses the regeneration concept, introduced in this paper, to attain the same performance as a PSO algorithm with a large population. Both algorithms use time domain information to obtain the objective function for the determination of the optimal parameters of the PSSs. The effectiveness of the two algorithms is evaluated and compared for damping the system oscillations during small and large disturbances, and their robustness is illustrated using the transient energy analysis. In addition, the computational complexities of the two algorithms are also presented.

A Stochastic Programming Approach to Power Portfolio Optimization

Abstract: We consider a power portfolio optimization model that is intended as a decision aid for scheduling and hedging (DASH) in the wholesale power market. Our multiscale model integrates the unit commitment model with financial decision making by including the forwards and spot market activity within the scheduling decision model. The methodology is based on a multiscale stochastic programming model that selects portfolio positions that perform well on a variety of scenarios generated through statistical modeling and optimization. When compared with several commonly used fixed-mix policies, our experiments demonstrate that the DASH model provides significant advantages.

Dynamic Stability of Power Systems with Power Electronic Interfaced DG

Abstract: In this paper, the transient stability of power systems with a high penetration level of power electronic interfaced (converter connected) distributed generation is explored by means of computer simulations. Small 2 and 3 bus test systems are used. The converter is modeled as a three-phase full-bridge IGBT voltage source converter (VSC). The control setting is such that during the actual disturbance the converter connected DG stays connected to the system but with a current limiter. The simulations are performed by using MATLAB SimPowerSystems

Modeling and simulation of microturbine based distributed generation system

Abstract: Distributed generation (DG) is predicted to play an important role in the electric power system in the near future. It is widely accepted that micro turbine-generation are currently attracting lot of attention to meet users' need in the distributed generation market. In order to investigate the ability of microturbine units in distribution systems, their efficient modeling is required. This paper presents a dynamic model of a micro-turbine generator system. The model is developed in the MATLAB/Simulink and implemented in SimPowerSystems library. The model is built from the dynamics of each part with their interconnections. This simplified model is a useful tool for studying the various operational aspects of micro turbines. The performance of developed model is studied by connecting it to an isolated load.

Frequency-Adaptive Network Modeling for Integrative Simulation of Natural and Envelope Waveforms in Power Systems and Circuits

Abstract: Algorithms for the simulation of transients in power electric systems and circuits can be classified into two major categories. For the simulation of diverse transients in ac and dc networks, the algorithms process instantaneous signals in the time domain to track natural waveforms as observed in reality. For the simulation of lower frequency transients that modulate ac carriers in ac networks, algorithms that process phasor signals to track envelope waveforms are popular. The methodology proposed in this paper uses analytic signals to bridge the merits of instantaneous and phasor signals and enable the efficient simulation of both natural and envelope waveforms as well as the smooth transition between both. The key enabling method referred to as frequency-adaptive simulation of transients (FAST) is distinguished by the introduction of the shift frequency as a simulation parameter in addition to the time-step size. This distinguishes the methodology from the known methods of power system and circuit simulation, which only use the setting of the time-step size to adapt the simulation process. By setting the shift frequency to a nonzero value, the Fourier spectra of the analytic signals are shifted and adapted according to the waveform type of interest. This adds value because different types of transients with and without an ac carrier can be simulated efficiently and accurately within one and the same simulation run. To provide compatibility with existing tools, the numerical integration is formulated to model the network branches such that nodal analysis can readily be used to construct the overall network model. Calculations of the accuracy as well as test studies that cover network energization and deenergization, angle modulation, and amplitude modulation substantiate the claims made and demonstrate the application of the methodology

Application guidelines for power swing detection on transmission systems

Abstract: Power swing detection on transmission systems is becoming more critical. Traditionally, setting relays for power swing blocking (PSB) or power swing tripping applications has been very complex and time consuming. In many cases, the settings are not correct, which is discovered when the relay operates incorrectly. This paper provides the reader with practical setting and application guidelines for traditional impedance-based PSB schemes. It shows how to set a PSB scheme without stability studies. Highlighted are some problem areas when setting and applying power swing detection elements. Application of these setting guidelines will be demonstrated using a power system modeled on a real-time digital simulator.

Design and evaluation of a directional algorithm for transmission-line protection based on positive- sequence fault components

Abstract: A directional relay algorithm for EHV transmission lines using positive-sequence fault components is presented. By comparing the phase relationship between the voltage and current measured at the relay point, the algorithm can determine correctly whether a fault is in the forward or backward direction. Specially designed techniques and logic are adopted to solve the difficult problems that exist in a real system. The signal-processing procedure for extracting the required fault components is provided in detail. Extensive simulation studies were conducted on a 500 kV system model using EMTDC. Theoretical analysis and simulation results show that the proposed algorithm provides adequate sensitivity, reliability and a fast operating response under a variety of system and fault conditions. In addition, it provides significant advantages over conventional directional relays, and these are discussed in the paper.

Model validation with hybrid dynamic simulation

Abstract: Model validation has been one of the central topics in power engineering studies for years. As model validation aims at obtaining reasonable models to represent actual behavior of power system components, it has been essential to validate models against actual measurements or known benchmark behavior. System-wide model simulation results can be compared with actual recordings. However, it is difficult to construct a simulation case for a large power system such as the WECC system and to narrow down to problematic models in a large system. Hybrid dynamic simulation with its capability of injecting external signals into dynamic simulation enables rigorous comparison of measurements and simulation in a small subsystem of interest. This paper presents such a model validation methodology with hybrid dynamic simulation. Two application examples on generator and load model validation are presented to show the validity of this model validation methodology. This methodology is further extended for automatic model validation and dichotomous subsystem model validation. A few methods to define model quality indices have been proposed to quantify model error for model validation criteria development.

Solutions to practical unit commitment problems with operational, power flow and environmental constraints

Abstract: In this paper, an algorithm to solve environmental constrained unit commitment problem (UCP) with operational and power flow constraints has been developed to plan an economic and secure generation schedule. Both economic load dispatch (ELD) and economic emission dispatch (EED) have been applied to obtain optimal fuel cost and optimal emission of generating units for the entire time horizon. The unit commitment solution for the environmental constrained problem has been formulated as a multiobjective problem by considering both ELD and EED simultaneously. The common economic emission dispatch (CEED) bi-objective problem is converted to single objective function by adding a price penalty factor. A modified price penalty factor is proposed to solve this problem. The UCP solutions without operational and power flow constraints are not practical due to secure operation of the power system network. This proposed algorithm introduces an efficient unit commitment (UC) approach considering environmental constraints along with power flow constraints that obtains the minimum operating cost satisfying both unit and network constraints. In the proposed model repeated optimal power flow (OPF) for the satisfactory unit combinations under the given study period has been carried out to obtain UC solutions with both operational and power flow constraints. This proposed algorithm has been tested for environmental constrained UCP on IEEE 30 bus and Indian utility practical systems with and without power flow constraints scheduled for 24 hours. The solutions obtained are quite encouraging and useful in the economic emission environment. The algorithm and simulation are carried through Matlab environment.

Behavior Investigations of Superconducting Fault Current Limiters in Power Systems

Abstract: When electric power systems are interconnected in operation, the fault current levels increase. This problem can be solved by deploying a bus-tie Superconducting Fault Current Limiter (SFCL) in distribution substations which allows two buses to be tied without significantly raising fault current levels. The Alternative Transient Program (ATP) version of Electromagnetic Transient Program (EMTP), ATPDraw and The Output Processor (TOP) are applied to investigate the operational behavior of the SFCL in a parallel connection of radial supply systems. The case study showed that an SFCL on bus tie location can not only suppress the fault current levels effectively but also improve the power quality and the reliability of the supply network. The simulation results will be presented and discussed in detail

Markov State Model for Optimization of Maintenance and Renewal of Hydro Power Components

Abstract: In this paper a reliability model is presented which can be used for scheduling and optimization of maintenance and renewal. The deterioration process of technical equipment is modeled by a Markov chain. A framework is proposed how the parameters in the Markov process can be estimated based on a description of the technical condition of components and systems in hydro power plants according to the Norwegian Electricity Industry Association. A time dependent solution of the Markov model is presented. Imperfect periodic inspection can be modeled by the proposed approach. The length of the inspection interval depends on the system condition revealed by the previous inspection. The model can be used to compute performance measures and operational costs over a finite time horizon. Finally, simulation results for a dataset for a Norwegian hydro power plant are presented.

PV inverter simulation using MATLAB/Simulink graphical environment and PLECS blockset

Abstract: In this paper a photovoltaic (PV) energy conversion system is simulated jointly with its control. The simulation of the system is developed for testing control algorithm before a real-time implementation. The control part is developed using MATLAB/Simulink in order to ensure a direct generation of the real-time code for the dSPACE control board. The simulation of the power system is first realized using MATLAB/Simulink. In a second step, the simulation of the power system is realized using the PLECS toolbox. Both simulation models are tested and selective simulation results are provided for a comparative study.

Power System Adequacy and Security Calculations Using Monte Carlo Simulation incorporating Intelligent System Methodology

Abstract: Monte Carlo Simulation has been extensively used in reliability evaluation of electric power systems. One of the issues with this approach has been the computational time for convergence of indices being estimated, especially when the systems are highly reliable. Perhaps the most commonly used approach to deal with this problem has been some version of variance reduction techniques. Recently some publications have proposed use of intelligent systems techniques such as self-organizing maps and linear vector quantization to tackle this problem. This paper will provide a perspective on this hybrid approach using Monte Carlo Simulation and intelligent system methods. The philosophy of this hybridization as well some results will be discussed.

Behavioral modeling of dc-dc converters for large-signal simulation of distributed power systems

Abstract: System designers normally use commercial dc-dc converters to build the power system. Simulation plays an important role in the configuration, analysis and design of the system, but the lack of available models of commercial dc-dc converters is a major drawback. The aim of this work is to develop a generic model of dc-dc converters based on the information provided by manufacturers in their datasheet. The model is intended to be able to predict power consumption, efficiency, system stability and large signal behavior including inrush current, protections, start-up and remote on-off control.

Wind Farm Modeling in Reliability Assessment of Power System

Abstract: A wind farm is a collection of wind turbines built in an area to provide electricity. Wind power is a renewable energy resource and an alternative to nonrenewable fossil fuels. In this paper impact of wind farm in power system reliability is investigated and a new procedure for reliability assessment of power systems with wind farm in HL? is proposed. In proposed procedure, a wind farm is given as a generation with probabilistic and stochastic generation. Historical wind velocities in past years are used for extraction of probabilistic distribution function for velocity and therefore for output electric power. Using this function, the output power of wind farm is divided into multi state. Each state shows a certain output electric power with certain probability for occurrence. The effect of wind farm on composite system reliability is evaluated with consideration proposed model for wind power and mathematical expectation concepts. As case study a portion of Iran power system grid in south east of Iran is selected. As historical data, wind velocity for twenty past years is gathered and used for modeling. The results in this real network are reported in this paper.