摘要
由于能源分布和经济发展的不平衡,高电压、大容量、远距离输电和大电网的互联已经成为世界各国现代电力系统发展的主要趋势之一。随着电网的快速发展,如何在保证电网安全稳定运行的前提下,提高其运行的有效性引起了国内外学者的广泛关注。
FACTS技术的诞生和发展为实现这一目标提供了新的控制技术和有效手段。在FACTS技术发展和市场推广的过程中,研究人员逐渐认识到：对于一个大型电网,FACTS技术的应用效果很大程度上取决于其在电网中的配置是否合理；针对电力系统的实际需求,如何合理地选择FACTS装置在电网中的接入点、容量及类型是一个亟待解决的问题。它直接关系到电网的安全稳定运行和FACTS装置的投资效益。
目前,电网中FACTS装置的配置主要采用仿真试探或工程经验所得,这显然不能满足现代电网发展和FACTS装置推广应用的要求,有必要对FACTS装置在电网中接入点、容量及类型的选择方法进行系统化的理论研究。论文以优化配置FACTS装置提高电网运行有效性为主线,以典型FACTS装置为研究对象,对其在电力系统中的优化配置力法进行了深入探讨,并应用于实际电网FACTS装置接入点、容量及类型的选择,从而初步建立了FACTS装置在电网中优化配置理论体系,主要的研究内容及成果如下
从基本原理出发,介绍了典型FACTS装置在提高电网静态电压稳定性、输电能力、暂态稳定性等方面的功能特性；在此基础上,通过比较输出特性曲线、串/并联无功补偿与线路传输有功功率增量之间的关系、对系统暂态稳定的影响程度,分析了它们各自的适用范围。结合国内外调研情况,建立了FACTS装置投资费用函数,以指导其类型的选择。给出了适用于提高电网运行有效性的评估指标,为FACTS装置接入点、容量的选择提供了分析依据。
以寻找电网中电压稳定薄弱母线、提高输电能力关键线路、发生故障对电网危害程度较大的地点为目的,分别采用连续潮流法、模态分析法、扩展等面积法则求取负荷节点电压稳定裕度、线路参与因子、系统暂态稳定裕度,并以此为依据对电网中节点、线路安装FACTS装置的优先次序进行排序,指导FACTS装置接入点的选择。
在无功就地平衡和分层平衡原则的基础上,把传输无功功率造成的有功网损最小作为约束条件,利用经济压差的概念,建立了传输无功功率造成的有功损耗、节点电压、无功补偿容量之间严格的数学关系式,以合理地确定无功补偿装置容量;通过灵敏度指标反映FACTS装置补偿容量变化对电网暂态稳定裕度的影响程度,进而将大扰动下FACTS装置补偿容量的确定处理成保证系统稳定的前提下,使补偿代价最小的量化问题。
在FACTS装置接入点、容量及类型选择方法研究的基础上,针对提高电网静态电压稳定性、输电能力、暂态稳定性等多个目标,设计了FACTS装置在电网中的优化配置模型。结合不同FACTS装置自身特性和投资费用函数,给出了求解单目标最优解的计算方法。提出了利用sPareto解求取多目标函数Pareto前沿最精简表示形式的方法,直接反映FACTS装置各配置方案之间的折衷,实现了方案的优化选择。
为了验证本文所述方法的正确性和模型的有效性,在实际电网中,研究了采用FACTS装置提高其运行有效性的配置方案,并在仿真中考虑了负荷模型对分析结论的影响,为工程决策提供了理论分析依据。对于极端故障情况,论文综合考虑了无功补偿、直流功率提升和发电端切机等提高系统暂态稳定性的措施,给出了包含多种措施的综合应对方案,为电网在该故障下的稳定运行提出了具体的指导意见。
Due to unbalanced energy distribution and economic development, high voltage, large capacity and long distance power transmission and interconnection of bulk power systems have become one of the major development trends of modern power systems all over the world. With the rapid development of power grids, how to improve their operational effectiveness without compromising the reliability has caused widespread concerns of domestic and foreign scholars.
The birth and development of FACTS provide new control technology and effective tools to achieve the above goal. During the development of FACTS technology and its marketing, researchers gradually recognize that the effects of applying FACTS in a bulk power grid heavily depend on the placement of FACTS devices. Therefore, how to select the locations, capacities and types of FACTS devices according to the practical demands of a power grid is an immediate problem needed to be solved, which has direct effects on reliability of grid operation and profitability of investments in FACTS devices.
Currently, placement of FACTS devices in power grids is mainly relied on simulation trials or project experience, which obviously can not meet the demand of popularization of FACTS devices in a modern power system; therefore, it is necessary to make a systematical study on selection of locations, capacities and types of FACTS devices. Organized around the optimal placement of FACTS devices to improve operational effectiveness of a power grid with typical FACTS devices as the research subjects, the dissertation discusses the optimal placement methods thoroughly and applies them to a real power grid. As a result the theoretical architecture on the optimal placement methods of FACTS devices is built. The main research contents and accomplishments of the dissertation are summarized as follows.
Based on basic principles, functional properties of typical FACTS devices such as improving static voltage stability, transfer capability and transient stability of power systems are introduced. Then the application range of a device is analyzed by comparing output characteristic curves, the relationship between series (or shunt) compensation and active power increment of transmission line, and the impact on power system transient stability. The cost function of FACTS devices is established based on domestic and international case studies to instruct the choice of device types. The evaluation index of improving operational effectiveness of power grid is given, which provides guidelines for selection of locations and capacities of FACTS devices.
For purposes of targeting weak buses of voltage stability, key lines of improving transfer capability and locations with severe impacts on the grid during fault contingencies, voltage stability margin of load buses, line participation factors and system transient stability margin are obtained by continuation power flow, modal analysis method and extended equal area criterion respectively. Based on the above calculations, buses and lines in the power grid are ordered by their priority of installing FACTS devices to guide the selection of locations.
Based on local and hierarchical balance of reactive power and taking minimum active power losses caused by transferring reactive power as the constraints, the strict mathematical relationship among active power losses caused by transferring reactive power, bus voltage and reactive compensation capacity is established using economic voltage difference to determine capacities of FACTS devices reasonably. Through the sensitivity index which reflects the impact of compensation capacity variation on transient stability margin, determination of compensation capacity under large disturbance is treated as the quantity problem of minimizing compensation cost with the constraints of system stability.
Aimed at improving static voltage stability, transfer capability and transient stability of power grids, a mathematical model of optimal FACTS devices placement is designed based on the study of selection of locations, capacities and types of FACTS devices. Calculation method is given to obtain single objective optimal solutions considering functional characteristics of several typical FACTS devices and the cost function. A method is proposed to obtain a minimal representation of Pareto frontier of multi-objective function using smart Pareto filter, which reflects the tradeoff among different schemes to yield an optimal choice.
Placement scheme of FACTS devices to improve operational effectiveness of real power grids is studied to validate the proposed method and model with impacts of the load model on the study considered in simulation. The study provides theoretical basis for engineering decision-making. For extreme faults, a synthetic strategy including various countermeasures to enhance transient stability such as reactive power compensation, HVDC power emergency support and generator tripping is given, and guidelines for reliable grid operation in contingency are presented.
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