Scheduling of generation in deficit power systems

Scheduling studies are carried out on a deficit hydrothermal power system. The problem is formulated as one of minimizing the severity of power cuts. The maintenance schedule of the thermal plants is selected by minimizing its deviation from an ideal, unrealizable schedule. The thermal plants are production-scheduled on the basis of their maintenance schedule. The influence of the stochastic nature of the load and the hydro inflows on the scheduling is evaluated. The hydro plants are scheduled through the extension of the techniques of the surplus case. Illustrative studies on an actual deficit hydro-thermal system are described     

Optimization-based scheduling of hydrothermal power systems withpumped-storage units

This paper presents an optimization-based method for scheduling hydrothermal systems based on the Lagrangian relaxation technique. After system-wide constraints are relaxed by Lagrange multipliers, the problem is converted into the scheduling of individual units. This paper concentrates on the solution methodology for pumped-storage units. There are, many constraints limiting the operation of a pumped-storage unit, such as pond level dynamics and constraints, and discontinuous generation and pumping regions. The most challenging issue in solving pumped-storage subproblems within the Lagrangian relaxation framework is the integrated consideration of these constraints. The basic idea of the method is to relax the pond level dynamics and constraints by using another set of multipliers. The subproblem is then converted into the optimization of generation or pumping; levels for each operating state at individual hours, and the optimization of operating states across hours. The optimal generation or pumping level for a particular operating state at each hour can be obtained by optimizing a single variable function without discretizing pond levels. Dynamic programming is then used to optimize operating states across hours with only a few number of states and transitions. A subgradient algorithm is used to update the pond level Lagrangian multipliers. This method provides an efficient way to solve a class of subproblems involving continuous dynamics and constraints, discontinuous operating regions, and discrete operating states     

Long-term expansion planning for hydrothermal electric power systems

This paper shows some aspects of the applicability of mixed integer programming (MIP) in expansion planning for regional electric utility systems, considering both power plants and the transmission network. The hydrothermal generation system may include significant run-of-river and reservoir power stations, and also pumped storage stations. Some essential elements of the model are explained to demonstrate the MIP modelling. The application of the model to the long-term expansion planning problem of a hydrothermal power system is also described.     

Market power assessment and mitigation in hydrothermal systems

The objective of this work is to investigate market power issues in bid-based hydrothermal scheduling. Initially, market power is simulated with a single stage Nash-Cournot equilibrium model. Market power assessment for multiple stages is then carried through a stochastic dynamic programming scheme. The decision in each stage and state is the equilibrium of a multi-agent game. Thereafter, mitigation measures, especially bilateral contracts, are investigated. Case studies with data taken from the Brazilian system are presented and discussed     

Selection of objectives in minimum loss power flow in hydrothermal electric power systems

Two formulations of the optimal minimum loss hydrothermal power flow problem in electric power systems are discussed. The formulations — one based on Kron's loss objective function and the other a network-based loss formulation — are treated in detail and the results of the two approaches are compared using two standard test systems. For the systems investigated, Kron's loss minimization schedules higher thermal generation than that required by the network loss minimization during lower demand. Kron's loss minimization involves lower active power transmission losses than those involved by the network-based loss minimization during low demand, and higher active power transmission losses than those required by the network-based loss minimization during higher demand. The total energy loss involved in the outcome of the Kron's loss minimization results is higher than that due to the network-based loss minimization schedule. There is a negligible difference in voltage magnitudes. Reactive power generation results favour the network-based loss approach. On the basis of the experiments, it is concluded that a network-based loss model is preferable in this application.     

A scenario simulation approach for market power analysis in hydrothermal systems

This paper focuses on markets based on physical bilateral contracts combined with a power exchange (PX). The main feature to be analyzed is the behavior of the hydrothermal system where one or more companies have a big reservoir with capacity for inter annual regulation. In this work, inter annual regulation means the ability of a reservoir to allocate water from consecutive years. The use of the reservoirs in a context of a power exchange market with competition is the main objective of this study. Results on the Chilean Central Interconnected System using stochastic dynamic programming are presented and discussed.     

Assessment of transmission cost recovery applying marginal pricing in hydrothermal power systems

This paper describes an integrated framework to evaluate short-run marginal costs (SRMC) in hydrothermal systems, taking into account the chronological aspects of reservoir operation, transmission constraints, equipment failures, hydrological variation and load uncertainty. The resulting SRMC values are used to calculate circuit revenues, which are then compared with investment requirements. It is shown that the representation of these probabilistic factors substantially increases revenues, in contrast with the widely reported under-recovery found in studies which only represent normal operating conditions. Case studies with the Brazilian North-Northeastern system are presented and discussed.     

水火电力系统短期发电计划优化方法综述

水火电力系统的短期发电计划问题在电力系统的安全可靠和经济运行中发挥着越来越重要的作用，由于其本身的复杂性，很难从理论上找到全局最优解。深入探讨各种优化算法，并加以分类，详细综述各种优化方法在水火电力系统短期发电计划问题中所取得的研究成果和存在的不足之处。     

Scheduling hydro power systems with restricted operating zones anddischarge ramping constraints

An optimization-based algorithm is presented for scheduling hydro power systems with restricted operating zones and discharge ramping constraints. Hydro watershed scheduling problems are difficult to solve because many constraints, continuous and discrete, including hydraulic coupling of cascaded reservoirs have to be considered. Restricted or forbidden operating zones as well as minimum generation limits of hydro units result in discontinuous preferred operating regions, and hinder direct applications of efficient continuous optimization methods such as network flow algorithms. Discharge ramping constraints due to navigational, environmental and recreational requirements in a hydro system add another dimension of difficulty since they couple generation or water discharge across time horizon. Integrated consideration of the above constraints is very challenging. The key idea of this paper is to use additional sets of multipliers to relax discontinuous operating region and discharge ramping constraints on individual hydro units so that a two-level optimization structure is formed. The low level consists of a continuous discharge scheduling subproblem determining the generation levels of all units in the entire watershed, and a number of pure integer scheduling subproblems determining the hydro operating states, one for each unit. The discharge subproblem is solved by a network flow algorithm, and the integer scheduling problems are solved by dynamic programming with a small number of states and well-structured transitions     

Optimal variable structure controller for the load-frequency control of interconnected hydrothermal power systems

A new approach is presented for the load-frequency control of interconnected power systems using the theory of variable-structure systems and linear optimal control theory. A systematic procedure for the selection of the switching hyperplane, which is of vital importance in the design of variable-structure controllers, is developed by minimizing a performance index in the sliding mode operation. The proposed control scheme is illustrated by digital simulation of an interconnected power system consisting of a hydro power plant and a steam power plant.     

Analytical modeling of chronological reservoir operation inprobabilistic production costing [of hydrothermal power systems]

This paper describes an extension of the Baleriaux methodology that represents chronological aspects of hydrothermal power system reservoir operation. The production costing problem is decomposed into a sequence of chronological hydro reliability problems. Those reliability problems are analytically solved by a sequence of convolutions of reservoir storage levels, demand, inflows and generation capacity. The methodology is illustrated in a case study with a utility-derived system, and the results are compared with existing solution schemes     

Opposition-based differential evolution for hydrothermal power system

This paper presents opposition-based differential evolution to determine the optimal hourly schedule of power generation in a hydrothermal system. Differential evolution (DE) is a population-based stochastic parallel search evolutionary algorithm. Opposition-based differential evolution has been used here to improve the effectiveness and quality of the solution. The proposed opposition-based differential evolution (ODE) employs opposition-based learning (OBL) for population initialization and also for generation jumping. The effectiveness of the proposed method has been verified on two test problems, two fixed head hydrothermal test systems and three hydrothermal multi-reservoir cascaded hydroelectric test systems having prohibited operating zones and thermal units with valve point loading. The results of the proposed approach are compared with those obtained by other evolutionary methods. It is found that the proposed opposition-based differential evolution based approach is able to provide better solution     

考虑分时电价的虚拟发电厂调度策略

虚拟发电厂大规模有效集成地域分散、类型各异的分布式电源以及用户负荷,能够更加充分地发挥分布式电源效益,提高用户供电可靠性。虚拟发电厂内部调度策略应适应配电网运行环境。在考虑分时电价的基础上,分析虚拟发电厂调度策略,研究在不同峰谷电价时段,结合分布式电源时序发电波动性、可控电源发电计划特性,充分利用峰谷电价差来减少虚拟发电厂发电运行成本。     

Systematic tool to plan and evaluate demand side strategies during sustained energy crises in hydrothermal power systems

HARE, a systematic tool to evaluate demand side measures to face sustained energy supply risk in hydrothermal power systems is presented in this paper. The main focus of the paper is to help centralized planners to systematically discuss, select, and plan the measures that better respond to the variety of critical situations that can arise due to expected energy shortage, integrate them into the usual medium-term scheduling tool and consequently keep the associated overall costs as low as possible. A medium-term definition of the system state is proposed as a decision-making aid, as well as a set of general energy saving measures that can be applied with their corresponding attributes (time delays, costs of implementation, and energy saving impact). The tool is demonstrated and applied to a simplified version of Chilean’s medium-term hydrothermal scheduling model and to a specific risk scenario experienced during 2011. The results show that it is possible to define various sets of demand side measures that avoid the impacts on the system and subsequently to select among them those with least expected implementation costs. This tool seems mainly useful for hydro-electric systems, which are more vulnerable to sustained energy supply risk. Every power system will have to go through a detailed review and planning process to implement this type of tool.     

Practical aspects in solving the medium-term operation planning problem of hydrothermal power systems by using the progressive hedging method

The medium-term operation planning (MTOP) of hydrothermal systems aims to define generation targets that minimize the expected operation cost over the planning period, considering a set of constraints with respect to the power system and the generating units. Mathematically, this problem can be characterized as a linear, stochastic and large scale, which requires the application of suitable optimization tools. The purpose of this paper is to apply the progressive hedging (PH) method to solve the MTOP problem. For that, two critical aspects of the PH are addressed: the use of warm start and the choice of a suitable penalty parameter. In addition, this paper compares the performance of the PH to other popular methods, the full high-scale linear programming problem, also known as deterministic equivalent, and the nested decomposition, when solving similar problems. As a result we show that the PH is a competitive method to solve the MTOP problem.     

Non-uniform composite representation hydroelectric systems forlong-term hydrothermal scheduling

This paper presents a nonuniform composite representation of hydroelectric power systems for use in long-term hydrothermal generation scheduling. This representation was developed from reservoir operational rules based on optimal reservoir trajectories obtained with a deterministic hydrothermal scheduling algorithm. A test system consisting of 7 large hydroelectric power plants of the Southeast Brazilian power system with 12572 MW of installed power capacity was selected for a case study. Operational cost comparisons with the classical uniform composite representation reveal significant savings     

Evaluation of the operating reserve in hydrothermal generation systems

The demand for an accurate evaluation of power reserves has become very important especially since the installation of large generating units such as nuclear power plants. The features of the power reserve and its partition into non-spinning and spinning components are discussed. Attention is paid to the regulating margin of the generating units and to the problem of evaluating the magnitude and duration of the operating reserve in a hydrothermal generating system under uncertain conditions.Practical examples are given to illustrate evaluation of the fast and slow operating reserves for the hydro and thermal subsystems of the Egyptian Unified Power System. The relation between the uncertainty in the amount of the required operating reserve on the one hand, and the amount of statistical data and size of added generating units on the other are discussed.     

Effect of deregulation on hydrothermal systems with transmission constraints

In this paper, a procedure is presented to compute power transactions among hydrothermal utilities in deregulated power pools. The paper examines the problem of price definition in utilities with hydro and thermal resources. The approach is tested on an example and the results are discussed.     

独立发电企业水火协调优化运行研究综述

介绍了在电力市场环境中,当独立发电企业同时拥有水电和火电的情况下,国内外关于研究合理安排水火电发电计划的理论和方法,从而使发电企业运行总成本最少。同时,详细阐述了各种方法取得的进展和以后的研究方向。     

独立发电企业水火协调优化运行研究综述

介绍了在电力市场环境中,当独立发电企业同时拥有水电和火电的情况下,国内外关于研究合理安排水火电发电计划的理论和方法,从而使发电企业运行总成本最少.同时,详细阐述了各种方法取得的进展和以后的研究方向.