Showing papers on "Electric power system published in 2005"
TL;DR: In this article, the authors examine the systems and processes needed to tap energy in vehicles and implement V2G and quantitatively compare today's light vehicle fleet with the electric power system.
2,022 citations
TL;DR: Basic features, algorithms, and a variety of case studies are presented in this paper to illustrate the capabilities of the presented tool and its suitability for educational and research purposes.
Abstract: This paper describes the Power System Analysis Toolbox (PSAT), an open source Matlab and GNU/Octave-based software package for analysis and design of small to medium size electric power systems. PSAT includes power flow, continuation power flow, optimal power flow, small-signal stability analysis, and time-domain simulation, as well as several static and dynamic models, including nonconventional loads, synchronous and asynchronous machines, regulators, and FACTS. PSAT is also provided with a complete set of user-friendly graphical interfaces and a Simulink-based editor of one-line network diagrams. Basic features, algorithms, and a variety of case studies are presented in this paper to illustrate the capabilities of the presented tool and its suitability for educational and research purposes.
890 citations
TL;DR: In this paper, the authors present a critical literature review and an up-to-date and exhaustive bibliography on the AGC of power systems, highlighting various control aspects concerning the AGG problem.
Abstract: An attempt is made in This work to present critical literature review and an up-to-date and exhaustive bibliography on the AGC of power systems. Various control aspects concerning the AGC problem have been highlighted. AGC schemes based on parameters, such as linear and nonlinear power system models, classical and optimal control, and centralized, decentralized, and multilevel control, are discussed. AGC strategies based on digital, self-tuning control, adaptive, VSS systems, and intelligent/soft computing control have been included. Finally, the investigations on AGC systems incorporating BES/SMES, wind turbines, FACTS devices, and PV systems have also been discussed.
836 citations
TL;DR: In this article, a new methodology is presented which quantifies the reserve needed on a system taking into account the uncertain nature of the wind power, and the reliability of the system is used as an objective measure to determine the effect of increasing wind power penetration.
Abstract: With wind power capacities increasing in many electricity systems across the world, operators are faced with new problems related to the uncertain nature of wind power. Foremost of these is the quantification and provision of system reserve. In this paper a new methodology is presented which quantifies the reserve needed on a system taking into account the uncertain nature of the wind power. Generator outage rates and load and wind power forecasts are taken into consideration when quantifying the amount of reserve needed. The reliability of the system is used as an objective measure to determine the effect of increasing wind power penetration. The methodology is applied to a model of the all Ireland electricity system, and results show that as wind power capacity increases, the system must increase the amount of reserve carried or face a measurable decrease in reliability.
795 citations
TL;DR: In this article, a multiobjective formulation for the siting and sizing of DG resources into existing distribution networks is proposed, which permits the planner to decide the best compromise between cost of network upgrading, cost of power losses, and cost of energy not supplied.
Abstract: In the restructured electricity industry, the engineering aspects of planning need to be reformulated even though the goal to attain remains substantially the same, requiring various objectives to be simultaneously accomplished to achieve the optimality of the power system development and operation. In many cases, these objectives contradict each other and cannot be handled by conventional single optimization techniques. In this paper, a multiobjective formulation for the siting and sizing of DG resources into existing distribution networks is proposed. The methodology adopted permits the planner to decide the best compromise between cost of network upgrading, cost of power losses, cost of energy not supplied, and cost of energy required by the served customers. The implemented technique is based on a genetic algorithm and an /spl epsiv/-constrained method that allows obtaining a set of noninferior solutions. Application examples are presented to demonstrate the effectiveness of the proposed procedure.
767 citations
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21 Jun 2005
TL;DR: In this article, the authors proposed a mixed sensitivity approach using Linear Matrix Inequalities (LMIIN) for loop-shaping in power systems. And they also proposed a control for time-delayed systems.
Abstract: Power System Oscillations.- Linear Control in Power Systems.- Test System Model.- Power System Stabilizers.- Multiple-Model Adaptive Control Approach.- Simultaneous Stabilization.- Mixed-Sensitivity Approach Using Linear Matrix Inequalities.- Normalized ?? Loop-Shaping Using Linear Matrix Inequalities.- ?? Control For Time-Delayed Systems.
716 citations
TL;DR: The operational characteristics of the topologies for hybrid electric vehicles, fuel cell vehicles, and more electric vehicles (MEV) are discussed and some of the major fundamental issues that currently face these advanced vehicular technologies are highlighted.
Abstract: This paper discusses the operational characteristics of the topologies for hybrid electric vehicles (HEV), fuel cell vehicles (FCV), and more electric vehicles (MEV). A brief description of series hybrid, parallel hybrid, and fuel cell-based propulsion systems are presented. The paper also presents fuel cell propulsion applications, more specific to light-duty passenger cars as well as heavy-duty buses. Finally, some of the major fundamental issues that currently face these advanced vehicular technologies including the challenges for market penetration are highlighted.
712 citations
TL;DR: In this paper, the impact of wind turbine inertial response characteristics on frequency control of small isolated power systems is discussed. But, due to differing electromechanical characteristics, this inherent link is not present in wind turbine generators.
Abstract: Increasing levels of wind generation has resulted in an urgent need for the assessment of their impact on frequency control of power systems. Whereas increased system inertia is intrinsically linked to the addition of synchronous generation to power systems, due to differing electromechanical characteristics, this inherent link is not present in wind turbine generators. Regardless of wind turbine technology, the displacement of conventional generation with wind will result in increased rates of change of system frequency. The magnitude of the frequency excursion following a loss of generation may also increase. Amendment of reserve policies or modification of wind turbine inertial response characteristics may be necessary to facilitate increased levels of wind generation. This is particularly true in small isolated power systems.
708 citations
12 Jun 2005
TL;DR: In this paper, the optimal location to place a DG in radial as well as networked systems to minimize the power loss of the system is investigated. But, the proposed analytical methods are not suitable for the optimal placement of DGs in power systems.
Abstract: Summary form only given. Power system deregulation and the shortage of transmission capacities have led to increased interest in distributed generation (DG) sources. Proper location of DGs in power systems is important for obtaining their maximum potential benefits. This paper presents analytical methods to determine the optimal location to place a DG in radial as well as networked systems to minimize the power loss of the system. Simulation results are given to verify the proposed analytical approaches.
602 citations
28 Jan 2005
593 citations
TL;DR: A solution to the reactive power dispatch problem with a novel particle swarm optimization approach based on multiagent systems (MAPSO) is presented and it is shown that the proposed approach converges to better solutions much faster than the earlier reported approaches.
Abstract: Reactive power dispatch in power systems is a complex combinatorial optimization problem involving nonlinear functions having multiple local minima and nonlinear and discontinuous constraints. In this paper, a solution to the reactive power dispatch problem with a novel particle swarm optimization approach based on multiagent systems (MAPSO) is presented. This method integrates the multiagent system (MAS) and the particle swarm optimization (PSO) algorithm. An agent in MAPSO represents a particle to PSO and a candidate solution to the optimization problem. All agents live in a lattice-like environment, with each agent fixed on a lattice point. In order to obtain optimal solution quickly, each agent competes and cooperates with its neighbors, and it can also learn by using its knowledge. Making use of these agent-agent interactions and evolution mechanism of PSO, MAPSO realizes the purpose of optimizing the value of objective function. MAPSO applied to optimal reactive power dispatch is evaluated on an IEEE 30-bus power system and a practical 118-bus power system. Simulation results show that the proposed approach converges to better solutions much faster than the earlier reported approaches. The optimization strategy is general and can be used to solve other power system optimization problems as well.
TL;DR: The optimization is aimed at reducing the fuel consumption rate of the system while constraining it to fulfil the local energy demand and provide a certain minimum reserve power and a penalty is applied for any heat produced in excess of demand.
Abstract: A cost optimization scheme for a microgrid is presented. Prior to the optimization of the microgrid itself, several schemes for sharing power between two generators are compared. The minimization of fuel use in a microgrid with a variety of power sources is then discussed. The optimization of a small power system has important differences from the case of a large system and its traditional economic dispatch problem. Among the most important differences is the presence of a local heat demand which adds another dimension to the optimization problem. The microgrid considered in this paper consists of two reciprocating gas engines, a combined heat and power plant, a photovoltaic array and a wind generator. The optimization is aimed at reducing the fuel consumption rate of the system while constraining it to fulfil the local energy demand (both electrical and thermal) and provide a certain minimum reserve power. A penalty is applied for any heat produced in excess of demand. The solution of the optimization problem strongly supports the idea of having a communication infrastructure operating between the power sources.
TL;DR: In this paper, a hidden failure embedded DC model of power transmission systems has been developed to study the power law distributions observed in North American blackout data, and the impacts of several model parameters on the global dynamics and evaluate possible mitigation measures.
TL;DR: An extensive study on controlling the vehicular electric power system to reduce the fuel use and emissions, by generating and storing electrical energy only at the most suitable moments is presented.
Abstract: In the near future, a significant increase in electric power consumption in vehicles is expected. To limit the associated increase in fuel consumption and exhaust emissions, smart strategies for the generation, storage/retrieval, distribution, and consumption of electric power will be used. Inspired by the research on energy management for hybrid electric vehicles (HEVs), this paper presents an extensive study on controlling the vehicular electric power system to reduce the fuel use and emissions, by generating and storing electrical energy only at the most suitable moments. For this purpose, both off-line optimization methods using knowledge of the driving pattern and on-line implementable ones are developed and tested in a simulation environment. Results show a reduction in fuel use of 2%, even without a prediction of the driving cycle being used. Simultaneously, even larger reductions of the emissions are obtained. The strategies can also be applied to a mild HEV with an integrated starter alternator (ISA), without modifications, or to other types of HEVs with slight changes in the formulation.
TL;DR: In this article, a particle swarm optimization (PSO) technique was used for loss reduction in the IEEE 118-bus system by using a developed optimal power flow based on loss minimization function by expanding the original PSO.
Abstract: This paper presents a particle swarm optimization (PSO) as a tool for loss reduction study. This issue can be formulated as a nonlinear optimization problem. The proposed application consists of using a developed optimal power flow based on loss minimization function by expanding the original PSO. The study is carried out in two steps. First, by using the tangent vector technique, the critical area of the power system is identified under the point of view of voltage instability. Second, once this area is identified, the PSO technique calculates the amount of shunt reactive power compensation that takes place in each bus. The proposed approach has been examined and tested with promising numerical results using the IEEE 118-bus system.
TL;DR: An efficient SCUC approach with ac constraints that obtains the minimum system operating cost while maintaining the security of power systems is introduced.
Abstract: In a restructured power market, the independent system operator (ISO) executes the security-constrained unit commitment (SCUC) program to plan a secure and economical hourly generation schedule for the day-ahead market. This paper introduces an efficient SCUC approach with ac constraints that obtains the minimum system operating cost while maintaining the security of power systems. The proposed approach applies the Benders decomposition for separating the unit commitment (UC) in the master problem from the network security check in subproblems. The master problem applies the augmented Lagrangian relaxation (LR) method and dynamic programming (DP) to solve UC. The subproblem checks ac network security constraints for the UC solution to determine whether a converged and secure ac power flow can be obtained. If any network violations arise, corresponding Benders cuts will be formed and added to the master problem for solving the next iteration of UC. The iterative process will continue until ac violations are eliminated and a converged optimal solution is found. In this paper, a six-bus system and the IEEE 118-bus system with 54 units are analyzed to exhibit the effectiveness of the proposed approach.
TL;DR: In this paper, an actively controlled battery/ultracapacitor hybrid has been proposed to achieve higher specific power while reducing battery current and its internal loss, which can be scaled to larger or smaller power capacities for a variety of applications.
Abstract: An actively controlled battery/ultracapacitor hybrid has broad applications in pulse-operated power systems. A converter is used to actively control the power flow from a battery, to couple the battery to an ultracapacitor for power enhancement, and to deliver the power to a load efficiently. The experimental and simulation results show that the hybrid can achieve much greater specific power while reducing battery current and its internal loss. A specific example of the hybrid built from two size 18650 lithium-ion cells and two 100-F ultracapacitors achieved a peak power of 132 W which is a three-times improvement in peak power compared to the passive hybrid power source (hybrid without a converter), and a seven times improvement as compared to the lithium-ion cells alone. The design presented here can be scaled to larger or smaller power capacities for a variety of applications.
TL;DR: In this article, the design and implementation of a novel control scheme for a doubly fed induction generator (DFIG), of the type employed with wind turbines, to provide support to power system operation is addressed.
Abstract: This paper addresses the design and implementation of a novel control scheme for a doubly fed induction generator (DFIG), of the type employed with wind turbines, to provide support to power system operation. It is shown that this controller provides a DFIG-based wind farm with operational and control compatibility with conventional power stations, the ability to contribute to voltage support and recovery following network faults, the ability to provide a power system stabilizer capability that improves overall system damping, and the capability of contributing short-term frequency support following loss of network generation. A simple but realistic test network that combines synchronous and wind farm generation has been modeled and used to assess dynamic performance. Simulation results are presented and discussed that demonstrate the capabilities and contributions of the new DFIG controller to network support.
09 May 2005
TL;DR: Online demonstration of a new response-based (feedback) Wide-Area stability and voltage Control System (WACS) is described, developed as a flexible platform to prevent blackouts and facilitate electrical commerce.
Abstract: As background, we describe frequently used feedforward wide-area discontinuous power system stability controls. Then we describe online demonstration of a new response-based (feedback) Wide-Area stability and voltage Control System (WACS). The control system uses powerful discontinuous actions for power system stabilization. The control system comprises phasor measurements at many substations, fiber-optic communications, real-time deterministic computers, and transfer trip output signals to circuit breakers at many other substations and power plants. Finally, we describe future development of WACS. WACS is developed as a flexible platform to prevent blackouts and facilitate electrical commerce.
12 Jun 2005
TL;DR: In this article, the authors analyze the assumptions of DC power flow, and make an attempt at quantifying these using indexes, and answer the question of how low the X/R ratio of line parameters can be, and what is the maximal deviation from the perfect flat voltage which still allows DC Power Flow to be acceptably accurate.
Abstract: In recent days almost every study concerning the analyses of power systems for market related purposes uses DC power flow. DC power flow is a simplification of a full power flow looking only at active power flows. Aspects as voltage support and reactive power management are not considered. However, such simplifications cannot always be justified and might sometimes be unrealistic. In this paper authors analyze the assumptions of DC power flow, and make an attempt at quantifying these using indexes. Among other, the paper answers the question of how low the X/R ratio of line parameters can be, and what is the maximal deviation from the perfect flat voltage which still allows DC power flow to be acceptably accurate.
09 May 2005
TL;DR: The essence of the natural gas infrastructure for supplying the ever-increasing number of gas-powered units is discussed and security-constrained unit commitment is used to analyze the short-time impact of natural gas prices on power generation scheduling.
Abstract: The restructuring of electricity has introduced new risks associated with the security of natural gas infrastructure on a significantly large scale, which entails changes in physical capabilities of pipelines, operational procedures, sensors and communications, contracting (supply and transportation), and tariffs. This paper will discuss the essence of the natural gas infrastructure for supplying the ever-increasing number of gas-powered units and use security-constrained unit commitment to analyze the short-time impact of natural gas prices on power generation scheduling. The paper analyzes the impact of natural gas infrastructure contingencies on the operation of electric power systems. Furthermore, the paper examines the impact of renewable sources of energy such as pumped-storage units and photovoltaic/battery systems on power system security by reducing the dependence of electricity infrastructure on the natural gas infrastructure. A modified IEEE 118-bus with 12 combined-cycle units is presented for analyzing the gas/electric interdependency.
TL;DR: In this article, a hybrid power system using many wind turbine generators in isolated small islands is proposed. But the system can supply high quality power using an aqua electrolyzer, fuel cell, renewable energy and diesel generator.
Abstract: A hybrid power system uses many wind turbine generators in isolated small islands. The output power of wind turbine generators is mostly fluctuating and has an effect on system frequency. In order to solve this problem, we propose a new power system using renewable energy in small, isolated islands. The system can supply high-quality power using an aqua electrolyzer, fuel cell, renewable energy, and diesel generator. The generated hydrogen by an aqua electrolyzer is used as fuel for a fuel cell. The simulation results are given to demonstrate the availability of the proposed system in this paper.
TL;DR: In this paper, the authors proposed a real-time GPS-synchronized wide-area frequency monitoring network (FNET), which consists of frequency disturbance recorders and an information management system.
Abstract: Frequency dynamics is one of the most important measures of an electrical power system status. To better understand power system dynamics, an accurately measured wide-area frequency is needed. The concept of building an Internet-based real-time GPS-synchronized wide-area frequency monitoring network (FNET) was proposed in 2000 by Qiu et al., and this concept has been realized. The FNET system consists of frequency disturbance recorders and an information management system. The FNET has made the synchronized observations of the entire U.S. power network possible with very little cost for the first time. This paper summarizes the implementation of the FNET system and shows some preliminary observations and analyses of the data that have been collected from the FNET.
TL;DR: The paper shows that the application of this novel sensorless current flyback inverter to an MPPT-operated PV system exhibits satisfactory MPPT performance similar to the one exhibited by the system with a dc current sensor as well.
Abstract: This paper presents a flyback inverter controlled by sensorless current maximum power point tracking (MPPT) for a small photovoltaic (PV) power system. Although the proposed system has small output power such as 300 W, a few sets of small PV power systems can be easily connected in parallel to yield higher output power. When a PV power system is constructed with a number of small power systems, the total system cost will increase and will be a matter of concern. To overcome this difficulty, this paper proposes a PV system that uses no expensive dc current sensor but utilizes the method of estimating the PV current from the PV voltage. The paper shows that the application of this novel sensorless current flyback inverter to an MPPT-operated PV system exhibits satisfactory MPPT performance similar to the one exhibited by the system with a dc current sensor as well. This paper also deals with the design method and the operation of the unique flyback inverter with center-tapped secondary winding.
TL;DR: A standardized protocol for the evaluation of short-term windpower prediction systems is proposed and a number of reference prediction models are described, and their use for performance comparison is analysed.
Abstract: Short-term wind power prediction is a primary requirement for efficient large-scale integration of wind generation in power systems and electricity markets. The choice of an appropriate prediction model among the numerous available models is not trivial, and has to be based on an objective evaluation of model performance.This paper proposes a standardized protocol for the evaluation of short-term wind-power prediction systems. A number of reference prediction models are also described, and their use for performance comparison is analysed. The use of the protocol is demonstrated, using results from both on-shore and offshore wind farms. The work was developed in the frame of the Anemos project (EU R&D project) where the protocol has been used to evaluate more than 10 prediction systems.
09 May 2005
TL;DR: The thesis of this paper is that the changing environment requires a new look at the operation of the power grid and a complete redesign of the control, communication and computation infrastructure.
Abstract: The power grid is not only a network interconnecting generators and loads through a transmission and distribution system, but is overlaid with a communication and control system that enables economic and secure operation. This multilayered infrastructure has evolved over many decades utilizing new technologies as they have appeared. This evolution has been slow and incremental, as the operation of the power system consisting of vertically integrated utilities has, until recently, changed very little. The monitoring of the grid is still done by a hierarchical design with polling for data at scanning rates in seconds that reflects the conceptual design of the 1960s. This design was adequate for vertically integrated utilities with limited feedback and wide-area controls; however, the thesis of this paper is that the changing environment, in both policy and technology, requires a new look at the operation of the power grid and a complete redesign of the control, communication and computation infrastructure. We provide several example novel control and communication regimes for such a new infrastructure.
25 May 2005
TL;DR: In this paper, the authors present a power management and distribution system for hybrid electric vehicles based on a brushless DC motor driven by electric motors and brushless brushless electric motors.
Abstract: AUTOMOTIVE POWER SYSTEMS Conventional Cars Roberto Giral-Castillon, Luis Martinez-Salamero, and Javier Maixe-Altes Hybrid Electric Vehicles John M. Miller Hybrid Drivetrains M. Ehsani and Yimin Gao Electric Vehicles Ramesh C. Bansal Optimal Power Management and Distribution in Automotive Systems Zheng John Shen, X. Chen, A. Masrur, V.K. Garg, and A. Soltis AUTOMOTIVE SEMICONDUCTOR DEVICES, COMPONENTS, AND SENSORS Automotive Power Semiconductor Devices Zheng John Shen Ultracapacitors John M. Miller Flywheels John M. Miller ESD Protection for Automotive Electronics Albert Z.H. Wang Sensors Mario Manana Canteli AUTOMOTIVE POWER ELECTRONIC CONVERTERS DC-DC Converters James P. Johnson AC-DC Rectifiers Byoung-Kuk Lee and Chung-Yean Won Unbalanced Operation of Three-Phase Boost Type Rectifiers Ana V. Stankovic DC/AC Inverters Mohan Aware AC/AC Converters Mehrdad Kazerani Power Electronics and Control for Hybrid and Fuel Cell Vehicles Kaushik Rajashekara AUTOMOTIVE MOTOR DRIVES Brushed-DC Electric Machinery for Automotive Applications Babak Fahimi Induction Motor Drives Khaled Nigim DSP-Based Implementation of Vector Control of Induction Motor Drives Hossein Salehfar Switched Reluctance Motor Drives Babak Fahimi and Chris Edrington Noise and Vibration in SRMs William Cai and Pragasen Pillay Modeling and Parameter Identification of Electric Machines Ali Keyhani, Wenzhe Lu, and Bogdan Proca Brushless DC Drives James P. Johnson Testing of Electric Motors and Controllers for Electric and Hybrid Electric Vehicles Sung Chul Oh OTHER AUTOMOTIVE APPLICATIONS Integrated Starter Alternator William Cai Fault Tolerant Adjustable Speed Motor Drives for Automotive Applications Babak Fahimi Automotive Steering Systems Tomy Sebastian, Mohammad S. Islam, and Sayeed Mir Current Intensive Motor Drives: A New Challenge for Modern Vehicular Technology Babak Fahimi Power Electronics Applications in Vehicle and Passenger Safety D.M.G. Preethichandra and Saman Kumara Halgamuge Drive and Control System for Hybrid Electric Vehicles Weng Keong Kevin Lim, Saman Kumara Halgamuge, and Harry Charles Watson Battery Technology for Automotive Applications Dell A. Crouch INDEX
TL;DR: In this article, a model of the doubly fed induction generator (DFIG) suitable for transient stability studies has been proposed, where the main assumption is that the current control loops, which are much faster than the electromechanic transients under study, do not have a significant influence on the transient stability of the power system and may be considered instantaneous.
Abstract: This paper proposes a model of the doubly fed induction generator (DFIG) suitable for transient stability studies. The main assumption adopted in the model is that the current control loops, which are much faster than the electromechanic transients under study, do not have a significant influence on the transient stability of the power system and may be considered instantaneous. The proposed DFIG model is a set of algebraic equations which are solved using an iterative procedure. A method is also proposed to calculate the DFIG initial conditions. A detailed variable-speed windmill model has been developed using the proposed DFIG model. This windmill model has been integrated in a transient stability simulation program in order to demonstrate its feasibility. Several simulations have been performed using a base case which includes a small grid, a wind farm represented by a single windmill, and different operation points. The evolution of several electric variables during the simulations is shown and discussed.
TL;DR: In this paper, a model of a field-oriented controlled doubly-fed induction generator based on a fifth-order induction generator model is described and implemented in a reference frame that allows the factors affecting the inertial response of a doubly fed induction generator to be easily examined.
Abstract: The inertial response of a generator is influenced by the sensitivity of the generator's electromagnetic torque to changes in the power system frequency. This paper deals with the inertial response of wind turbines employing induction-machine-based generators. A model of a field-oriented controlled doubly fed induction generator based on a fifth-order induction-generator model is described. The proposed model is implemented in a reference frame that allows the factors affecting the inertial response of a doubly fed induction generator to be easily examined. A comparison between the inertial response of a squirrel-cage and doubly fed induction-machine-based wind-turbine generator is performed using the developed models. It is found that the inertial response of a doubly fed induction generator employing field-oriented control is strongly influenced by the rotor current-controller bandwidth.
09 May 2005
TL;DR: There seems to be a great potential for advanced wide-area protection and control systems, based on powerful, flexible and reliable system protection terminals, high speed, communication, and GPS synchronization in conjunction with careful and skilled engineering by power system analysts and protection engineers in cooperation.
Abstract: System-wide disturbances in power systems are a challenging problem for the utility industry because of the large scale and the complexity of the power system. When a major power system disturbance occurs, protection and control actions are required to stop the power system degradation, restore the system to a normal state, and minimize the impact of the disturbance. In some cases, the present control actions are not designed for a fast-developing disturbance and may be too slow. The report explores special protection schemes and new technologies for advanced, wide-area protection. There seems to be a great potential for advanced wide-area protection and control systems, based on powerful, flexible and reliable system protection terminals, high speed, communication, and GPS synchronization in conjunction with careful and skilled engineering by power system analysts and protection engineers in cooperation.