Showing papers on "Electric power system published in 2007"

Modeling, Analysis and Testing of Autonomous Operation of an Inverter-Based Microgrid

Abstract: The analysis of the small-signal stability of conventional power systems is well established, but for inverter based microgrids there is a need to establish how circuit and control features give rise to particular oscillatory modes and which of these have poor damping. This paper develops the modeling and analysis of autonomous operation of inverter-based microgrids. Each sub-module is modeled in state-space form and all are combined together on a common reference frame. The model captures the detail of the control loops of the inverter but not the switching action. Some inverter modes are found at relatively high frequency and so a full dynamic model of the network (rather than an algebraic impedance model) is used. The complete model is linearized around an operating point and the resulting system matrix is used to derive the eigenvalues. The eigenvalues (termed "modes") indicate the frequency and damping of oscillatory components in the transient response. A sensitivity analysis is also presented which helps identifying the origin of each of the modes and identify possible feedback signals for design of controllers to improve the system stability. With experience it is possible to simplify the model (reduce the order) if particular modes are not of interest as is the case with synchronous machine models. Experimental results from a microgrid of three 10-kW inverters are used to verify the results obtained from the model

Instantaneous power theory and applications to power conditioning

Abstract: Preface. 1. Introduction. 1.1. Concepts and Evolution of Electric Power Theory. 1.2. Applications of the p-q Theory to Power Electronics Equipment. 1.3. Harmonic Voltages in Power Systems. 1.4. Identified and Unidentified Harmonic-Producing Loads. 1.5. Harmonic Current and Voltage Sources. 1.6. Basic Principles of Harmonic Compensation. 1.7. Basic Principles of Power Flow Control. References. 2. Electric Power Definitions: Background. 2.1. Power Definitions Under Sinusoidal Conditions. 2.2. Voltage and Current Phasors and the Complex Impedance. 2.3. Complex Power and Power Factor. 2.4. Concepts of Power Under Non-Sinusoidal Conditions -Conventional Approaches. 2.5. Electric Power in Three-Phase Systems. 2.6. Summary. References. 3 The Instantaneous Power Theory. 3.1. Basis of the p-q Theory. 3.2. The p-q Theory in Three-Phase, Three-Wire Systems. 3.3. The p-q Theory in Three-Phase, Four-Wire Systems. 3.4. Instantaneous abc Theory. 3.5. Comparisons between the p-q Theory and the abc Theory. 3.6. Summary. References. 4 Shunt Active Filters. 4.1. General Description of Shunt Active Filters. 4.2. Three-Phase, Three-Wire Shunt Active Filters. 4.3. Three-Phase, Four-Wire Shunt Active Filters. 4.4. Shunt Selective Harmonic Compensation. 4.5. Summary. References. 5 Hybrid and Series Active Filters. 5.1. Basic Series Active Filter. 5.2. Combined Series Active Filter and Shunt Passive Filter. 5.3. Series Active Filter Integrated with a Double-Series Diode Rectifier. 5.4. Comparisons Between Hybrid and Pure Active Filters. 5.5. Conclusions. References. 6 Combined Series and Shunt Power Conditioners. 6.1. The Unified Power Flow Controller (UPFC). 6.2. The Unified Power Quality Conditioner (UPQC). 6.3. The Universal Active Power Line Conditioner (UPLC). 6.4. Summary. References. Index.

Optimal Power Flow of Multiple Energy Carriers

Abstract: This paper presents an approach for combined optimization of coupled power flows of different energy infrastructures such as electricity, gas, and district heating systems. A steady state power flow model is presented that includes conversion and transmission of an arbitrary number of energy carriers. The couplings between the different infrastructures are explicitly taken into account based on the new concept of energy hubs. With this model, combined economic dispatch and optimal power flow problems are stated covering transmission and conversion of energy. A general optimality condition for optimal dispatch of multiple energy carriers is derived, and the approach is compared with the standard method used for electrical power systems. Finally, the developed tools are demonstrated in examples

Input-Admittance Calculation and Shaping for Controlled Voltage-Source Converters

Abstract: A controlled power electronic converter can cause local instabilities when interacting with other dynamic subsystems in a power system. Oscillations at a certain frequency cannot, however, build up if the converter differential input admittance has a positive conductance (real part) at that frequency, since power is then dissipated. In this paper, input-admittance expressions for a voltage-source converter are derived. It is seen how the admittance can be shaped in order to get a positive real part in the desired frequency regions by adjusting the controller parameters.

Complex systems analysis of series of blackouts: Cascading failure, critical points, and self-organization

Abstract: We give an overview of a complex systems approach to large blackouts of electric power transmission systems caused by cascading failure. Instead of looking at the details of particular blackouts, we study the statistics and dynamics of series of blackouts with approximate global models. Blackout data from several countries suggest that the frequency of large blackouts is governed by a power law. The power law makes the risk of large blackouts consequential and is consistent with the power system being a complex system designed and operated near a critical point. Power system overall loading or stress relative to operating limits is a key factor affecting the risk of cascading failure. Power system blackout models and abstract models of cascading failure show critical points with power law behavior as load is increased. To explain why the power system is operated near these critical points and inspired by concepts from self-organized criticality, we suggest that power system operating margins evolve slowly to near a critical point and confirm this idea using a power system model. The slow evolution of the power system is driven by a steady increase in electric loading, economic pressures to maximize the use of the grid, and the engineering responses to blackouts that upgrade the system. Mitigation of blackout risk should account for dynamical effects in complex self-organized critical systems. For example, some methods of suppressing small blackouts could ultimately increase the risk of large blackouts.

Power System Analysis and Design

Abstract: From the Publisher: Glover and Sarma offer broad coverage of power system concepts, combined with chapter opening case studies, updated exercises, and new problems for solutions with computers and programmable calculators.

Virtual power plant and system integration of distributed energy resources

Abstract: A concept is presented along with the overarching structure of the virtual power plant (VPP), the primary vehicle for delivering cost efficient integration of distributed energy resources (DER) into the existing power systems. The growing pressure, primarily driven by environmental concerns, for generating more electricity from renewables and improving energy efficiency have promoted the application of DER into electricity systems. So far, DER have been used to displace energy from conventional generating plants but not to displace their capacity as they are not visible to system operators. If this continues, this will lead to problematic over-capacity issues and under-utilisation of the assets, reduce overall system efficiency and eventually increase the electricity cost that needs to be paid by society. The concept of VPP was developed to enhance the visibility and control of DER to system operators and other market actors by providing an appropriate interface between these system components. The technical and commercial functionality facilitated through the VPP are described and concludes with case studies demonstrating the benefit of aggregation (VPP concept) and the use of the optimal power flow algorithm to characterise VPP

The ABCs of HVDC transmission technologies

Abstract: An overview of high voltage direct current systems and applications is presented in this paper. The favorable economics of long-distance bulk-power transmission with HVDC together with its controllability make it an interesting alternative or complement to AC transmission. The higher voltage levels, mature technology, and new converter designs have significantly increased the interest in HVDC transmission and expanded the range of applications

A Survey of Frequency and Voltage Control Ancillary Services—Part I: Technical Features

Abstract: This two-part paper surveys the frequency and voltage control ancillary services in power systems from various parts of the world. In this second part, essential economic features that must be taken into account when designing markets for ancillary services are first discussed. The methods adopted in eight different systems (Australia, France, Germany, Great Britain, New Zealand, PJM, Spain, and Sweden) for trading frequency and voltage control ancillary services are then compared. Lastly, new indicators for comparing volumes and costs of ancillary services across systems are proposed and calculated for the systems included in this survey. The companion paper surveys the technical features of these ancillary services

Flexible Active Power Control of Distributed Power Generation Systems During Grid Faults

Abstract: The increasing penetration of distributed power generation into the power system leads to a continuous evolution of grid interconnection requirements. In particular, active power control will play an important role both during grid faults (low-voltage ride-through capability and controlled current injection) and in normal conditions (reserve function and frequency regulation). The aim of this paper is to propose a flexible active power control based on a fast current controller and a reconfigurable reference current selector. Several strategies to select the current reference are studied and compared using experimental results that are obtained during an unsymmetrical voltage fault. The results of the analysis allow selection of the best reference current in every condition. The proposed methods facilitate multiple choices for fault ride through by simply changing the reference selection criteria.

Impacts of Wind Power on Thermal Generation Unit Commitment and Dispatch

Abstract: This paper proposes a new simulation method that can fully assess the impacts of large-scale wind power on system operations from cost, reliability, and environmental perspectives. The method uses a time series of observed and predicted 15-min average wind speeds at foreseen onshore- and offshore-wind farm locations. A Unit Commitment and Economic Dispatch (UC-ED) tool is adapted to allow for frequent revisions of conventional generation unit schedules, using information on current wind energy output and forecasts for the next 36 h. This is deemed the most faithful way of simulating actual operations and short-term planning activities for a system with large wind power penetration. The problem formulation includes ramp-rate constraints for generation schedules and for reserve activation, and minimum up-time and down-time of conventional units. Results are shown for a realistic future scenario of the Dutch power system. It is shown that problems such as insufficient regulating and reserve power-which are typically associated with the variability and limited predictability of wind power-can only be assessed in conjunction with the specifics of the conventional generation system that wind power is integrated into. For the thermal system with a large share of combined heat and power (CHP) investigated here, wind power forecasting does not provide significant benefits for optimal unit commitment and dispatch. Minimum load problems do occur, which result in wasted wind in amounts increasing with the wind power installed

Point Estimate Schemes to Solve the Probabilistic Power Flow

Abstract: This paper analyzes the behavior of Hong's point estimate methods to account for uncertainties on the probabilistic power flow problem. This uncertainty may arise from different sources as load demand or generation unit outages. Point estimate methods constitute a remarkable tool to handle stochastic power system problems because good results can be achieved by using the same routines as those corresponding to deterministic problems, while keeping low the computational burden. In previous works related to power systems, only the two-point estimate method has been considered. In this paper, four different Hong's point estimate schemes are presented and tested on the probabilistic power flow problem. Binomial and normal distributions are used to model input random variables. Results for two different case studies, based on the IEEE 14-bus and IEEE 118-bus test systems, respectively, are presented and compared against those obtained from the Monte Carlo simulation. Particularly, this paper shows that the use of the scheme provides the best performance when a high number of random variables, both continuous and discrete, are considered.

An Advanced Statistical Method for Wind Power Forecasting

Abstract: This paper presents an advanced statistical method for wind power forecasting based on artificial intelligence techniques. The method requires as input past power measurements and meteorological forecasts of wind speed and direction interpolated at the site of the wind farm. A self-organized map is trained to classify the forecasted local wind speed provided by the meteorological services. A unique feature of the method is that following a preliminary wind power prediction, it provides an estimation of the quality of the meteorological forecasts that is subsequently used to improve predictions. The proposed method is suitable for operational planning of power systems with increased wind power penetration, i.e., forecasting horizon of 48 h ahead and for wind farm operators trading in electricity markets. Application of the forecasting method on the power production of an actual wind farm shows the validity of the method

Low-Voltage DC Distribution System for Commercial Power Systems With Sensitive Electronic Loads

Abstract: In this paper, the use of DC power system to supply sensitive electronic loads is treated. First, general design issues regarding DC power systems are discussed, and then the measurement results from a scaled laboratory setup are presented. The results show that it is possible to supply sensitive electronic loads through an AC/DC interface, and to keep them online during grid transients. The use of a DC power system to supply sensitive electronic loads will have lower losses compared with a conventional AC uninterruptible power-supply solution due to fewer power conversion steps.

Utility Wind Integration and Operating Impact State of the Art

Abstract: In only six years, from 2000 to 2006, wind energy has become a significant resource on many electric utility systems, with nearly 74 000 MW of nameplate capacity installed worldwide at the end of 2006. Wind energy is now "utility scale" and can affect utility system planning and operations for both generation and transmission. The utility industry in general, and transmission system operators in particular, are beginning to take note. At the end of 2005, the Power Engineering Society (PES) published a special issue of its Power & Energy Magazine that focused on integrating wind into the power system. This paper provides a summary and update on many of the salient points from that special issue about the current state of knowledge regarding utility wind integration issues.

Costs and Emissions Associated with Plug-In Hybrid Electric Vehicle Charging in the Xcel Energy Colorado Service Territory

Abstract: The combination of high oil costs, concerns about oil security and availability, and air quality issues related to vehicle emissions are driving interest in plug-in hybrid electric vehicles (PHEVs). PHEVs are similar to conventional hybrid electric vehicles, but feature a larger battery and plug-in charger that allows electricity from the grid to replace a portion of the petroleum-fueled drive energy. PHEVs may derive a substantial fraction of their miles from grid-derived electricity, but without the range restrictions of pure battery electric vehicles. As of early 2007, production of PHEVs is essentially limited to demonstration vehicles and prototypes. However, the technology has received considerable attention from the media, national security interests, environmental organizations, and the electric power industry. The use of PHEVs would represent a significant potential shift in the use of electricity and the operation of electric power systems. Electrification of the transportation sector could increase generation capacity and transmission and distribution (T&D) requirements, especially if vehicles are charged during periods of high demand. This study is designed to evaluate several of these PHEV-charging impacts on utility system operations within the Xcel Energy Colorado service territory.

Predictive & Adaptive MPPT Perturb and Observe Method

Abstract: The perturb and observe (P&O) best operation conditions are investigated in order to identify the edge efficiency performances of this most popular maximum power point tracking (MPPT) technique for photovoltaic (PV) applications. It is shown that P&O may guarantee top-level efficiency, provided that a proper predictive (by means of a parabolic interpolation of the last three operating points) and adaptive (based on the measure of the actual power) hill climbing strategy is adopted. The approach proposed is aimed at realizing, in addition to absolute best tracking performances, high robustness and promptness both in sunny and cloudy weather conditions. The power gain with respect to standard P&O technique is proved by means of simulation results and experimental measurements performed on a low power system. Besides the performance improvements, it is shown that the proposed approach allows possible reduction of hardware costs of analog-to-digital (A/D) converters used in the MPPT control circuitry.

Multi-Input Inverter for Grid-Connected Hybrid PV/Wind Power System

Abstract: The objective of this paper is to propose a novel multi-input inverter for the grid-connected hybrid photovoltaic (PV)/wind power system in order to simplify the power system and reduce the cost. The proposed multi-input inverter consists of a buck/buck-boost fused multi-input dc-dc converter and a full-bridge dc-ac inverter. The output power characteristics of the PV array and the wind turbine are introduced. The perturbation and observation method is used to accomplish the maximum power point tracking algorithm for input sources. The operational principle of the proposed multi-input inverter is explained. The control circuit is realized by using a digital signal processor and auxiliary analog circuits. For practical applications, functions of soft-start and circuit protection are implemented. Experimental results have shown the performance of the proposed multi-input inverter with desired features

An Adaptive Control System for a Dc Microgrid for Data Centers

Abstract: In this paper, an adaptive control system for a dc microgrid for data centers is proposed. Data centers call for electric power with high availability and a possibility to reduce the electric losses and hence the need for cooling. By using local energy sources, high reliability can be achieved, and by using dc the number of conversion steps, and therefore also the losses, can be reduced. The dc microgrid can also be used to supply closely located sensitive ac loads during outages on the utility grid. The proposed dc microgrid can be operated in eight different operation modes described here, resulting in 23 transitions. The adaptive control system coordinates the control of converters, sources and switches used in the dc microgrid. The adaptive control system is tested in the simulation software packages PSCAD/EMTDC, and the results of the most interesting operation modes and transitions are presented. The results show that it is possible to use the proposed dc microgrid to supply sensitive electronic loads, and also during utility grid outages, supply closely located sensitive ac loads. To reduce transients in the voltage experienced by the sensitive ac loads, the dc microgrid requires fast utility outage detection and fast switches.

Distributed Power Harvesting Systems Using DC Power Sources

Abstract: A system and method for combining power from DC power sources. Each power source is coupled to a converter. Each converter converts input power to output power by monitoring and maintaining the input power at a maximum power point. Substantially all input power is converted to the output power, and the controlling is performed by allowing output voltage of the converter to vary. The converters are coupled in series. An inverter is connected in parallel with the series connection of the converters and inverts a DC input to the inverter from the converters into an AC output. The inverter maintains the voltage at the inverter input at a desirable voltage by varying the amount of the series current drawn from the converters. The series current and the output power of the converters, determine the output voltage at each converter.

On Computational Issues of Market-Based Optimal Power Flow

Abstract: The deregulated electricity market calls for robust optimal power flow (OPF) tools that can provide a) deterministic convergence; b) accurate computation of nodal prices; c) support of both smooth and nonsmooth costing of a variety of resources and services, such as real energy, reactive energy, voltages support, etc.; d) full active and reactive power flow modeling of large-scale systems; and e) satisfactory worst-case performance that meets the real-time dispatching requirement. Most prior research on OPF has focused on performance issues in the context of regulated systems, without giving much emphasis to requirements a)-c). This paper discusses the computational challenges brought up by the deregulation and attempts to address them through the introduction of new OPF formulations and algorithms. Trust-region- based augmented Lagrangian method (TRALM), step-controlled primal-dual interior point method (SCIPM), and constrained cost variable (CCV) OPF formulation are proposed. The new formulations and algorithms, along with several existing ones, are tested and compared using large-scale power system models.

Microgrid Protection

Abstract: In general, a microgrid can operate in both the grid-connected mode and the islanded mode where the microgrid is interfaced to the main power system by a fast semiconductor switch called static switch, (SS). It is essential to protect a microgrid in both the grid-connected and the islanded modes of operation against all types of faults. The major issue arises in island operation with inverter-based sources. Inverter fault currents are limited by the ratings of the silicon devices to around 2 p.u. rated current. Fault currents in islanded inverter based microgrids may not have adequate magnitudes to use traditional overcurrent protection techniques. The philosophy for protection is to have the same protection strategies for both islanded and grid-connected operation. The static switch is designed to open for all faults.

Regulation of Photovoltaic Voltage

Abstract: In photovoltaic power systems, both photovoltaic modules and switching-mode converters present nonlinear and time-variant characteristics, which result in a difficult control problem. This paper presents an in-depth analysis and modeling to discover the inherent features of a photovoltaic power system. The method of successive linearization simplifies the nonlinear problem back to the linear case. This paper also presents the use of Youla parameterization to design a stable control system for regulating the photovoltaic voltage. The experimental and simulation results demonstrate the effectiveness of the presented analysis, design, and implementation.

A Fuzzy-Optimization Approach for Generation Scheduling With Wind and Solar Energy Systems

Abstract: This paper presents a fuzzy-optimization approach for solving the generation scheduling problem with consideration of wind and solar energy systems. Wind and solar energy are being considered in the power system to schedule unit power output to minimize the total thermal unit fuel cost. When performing the generation scheduling problem in conventional methods, the hourly load, available water, wind speed, solar radiation must be forecasted to prevent errors. However, actually there are always errors in these forecasted values. A characteristic feature of the proposed fuzzy-optimization approach is that the forecast hourly load, available water, wind speed and solar radiation errors can be taken into account using fuzzy sets. Fuzzy set notations in the hourly load, available water, wind speed, solar radiation, spinning reserve and total fuel cost are developed to obtain the optimal generation schedule under an uncertain environment. To demonstrate the effectiveness of the proposed method, the generation scheduling problem is performed in a simplified generation system. The results show that a proper generating schedule for each unit can be reached using the proposed method.