【作者】 金鹏；

【导师】 艾欣；

【作者基本信息】 华北电力大学 ， 电力系统及其自动化， 2014， 博士

【摘要】 随着能源危机加剧、环境日益恶化,加之传统的集中供电方式暴露出诸多弊端,分布式发电技术逐渐受到关注。该技术有助于推动可再生能源的利用,并在一定程度上为传统电力系统提供了有效补充。然而在诸多优势的背后,其出力的随机性和惯性较小的问题限制了其应用范围,大规模接入更是会对电网安全稳定带来负面影响。因此微电网应运而生,成为分布式电源(DG)“友好”接入电网的途径。与以同步发电机为主的传统电力系统不同,微电网包含多种类型的DG,它们的特性和控制方式差距较大,难以标准化。当微电网进行分层控制后,二级控制和三级控制可以依据微电网中的各种测量值进行系统级调节。只需要借助微电网内部的通信,一些难以在DG本地侧实现的功能可由更高级的控制执行。这将有效的简化本地控制结构,有利于实现微电网控制的标准化。本文在国家重点基础计划项目(“973”计划)的资助下完成,研究内容涉及以下四个方面：DG的初级控制、改善无功分布的二级控制、抑制功率脉动的二级控制以及微电网的经济运行,本文的具体内容如下：(1)从微电网的架构和多种DG的数学模型出发,提出了一种微电网初级控制策略,协调了不同调频性质DG的控制,实现了储能、柴油发电机、风力发电以及光伏发电在微电网内共同运行。根据平均值建模方法,对微电网内部可控的电力电子器件以受控电压源和受控电流源形式进行建模,加快了多DG微电网仿真的速度。仿真结果验证了所提控制策略在光伏出力波动、负荷突变、孤岛与并网模式切换等情况下的可行性。(2)针对传统下垂控制无功分配不均的问题,从下垂增益、负荷侧电压以及微电网传输线路阻抗特性等方面分析了经典电压/无功下垂控制无功负荷分配的机理。在此基础上,通过上传至微电网中央控制器(MGCC)的DG功率信息,建立了基于二级控制的微电网无功控制策略。该策略利用无功出力分配关系和空载输出电压的偏离程度构造势函数。MGCC根据势函数对各DG的空载输出电压进行集中调整,达到改善无功出力分配的目的。该控制方法不需要对经典的下垂控制进行改动,只需借助于MGCC与DG间的通信。另外,在仿真的研究中利用正则环流无功概念量化了环流的程度,并在不同负荷下考察了环流的特点和抑制效果。(3)微电网内会包含一定比例的单相DG和负荷,这会导致微电网公共连接点(PCC)电压的不对称。在不对称电压下PQ控制的DG会产生有功和无功功率的二倍工频波动。通过对有功和无功脉动的机理分析,首次推导了有功和无功功率脉动的解析关系,并进一步揭示了正负序电气量对功率脉动的根本性影响。在此基础上,提出一种抑制功率脉动的二级控制策略。该方案根据最优化理论,以功率脉动幅值为优化目标,以功率脉动率和PQ控制的功率给定值为约束建立优化模型。利用优化结果,二级控制能够在无功脉动较大时实现对有功和无功脉动的同时抑制,达到功率脉动幅值和脉动率指标的深度优化。(4)经济运行控制根据优化结果调整各DG的功率输出给定值,可视为微电网的顶层控制策略。由于微电网包含较大比例的间歇性能源,其出力波动对制定微电网发电计划带来较大困难。因此提出了基于机会约束规划的微电网经济运行模型,该模型考虑了储能充放电、备用容量配置以及功率平衡方面的约束。另外,光伏发电采用了一种考虑云量和地理因素的概率模型,该模型能够更真实地反映光伏出力的波动变化。在求解机会约束过程中,利用概率序列理论对光伏和风力发电的概率模型进行了离散化处理,将机会约束转化成了确定性的约束形式,使之可以通过TOMLAB等商业优化软件求解。仿真结果表明,与随机模拟求解相比,该方法具有计算速度快且结果稳定的优势。此外,为了考察概率性序列的合理性,还讨论了不同离散化步长对计算结果的影响,并进一步验证了所提出方法的有效性。更多还原

【Abstract】 With the influence of energy crisis, deteriorating environment and defect of centralized power supply, distributed generation technologies have been paid for attention gradually. By promoting utilization of renewable energy, distributed generation technologies become effective complements to the traditional power system. However, large-scale grid connected distributed generations can cause negative effects on the security and stability because of randomness and inertialess, so their application range is still limited. Under this circumstance, microgrid is proposed and becomes an approach for distributed generator (DG) to connect to utility grid friendly.Unlike traditional power system dominated by synchronous machines, there are many differences on characteristics and control modes among numerous types of DGs in microgrid, so the standardization of control is difficult to achieve. Under the architecture of hierarchical control, secondary control and tertiary control conduct global adjustment by the measurement in microgrid, and some control targets which cannot be met locally are achieved by communication. In addition, structure of local control can be simplified and control structure of microgrid could be standardized by hierarchical control. This dissertation which is supported by Special Fund of the National Basic Research Program of China (973Program) covers four aspects:local control of DGs, reactive power sharing enhancement based on secondary control, power oscillations suppression based on secondary control and economic operation of microgrid. The main content of this dissertation is as follows:(1)According to architecture of microgrid and models of DGs, the primary control strategy which coordinates DGs with different frequency regulation modes is proposed, and a microgrid containing energy storage, diesel generator, wind power generation and photovoltaic power generation can operate under the primary control. By average modeling technique, controllable power electronic devices could be transformed into combination models of controlled voltage source and controlled current source, simulations that contains several DG models are accelerated. Furthermore, the proposed control strategy is verified under circumstances such as power fluctuation of photovoltaic, load variation and switch between grid-connected mode and island mode.(2)In order to deal with uneven sharing of reactive power in classic droop control, mechanism of reactive power sharing is analyzed from the aspect of droop gain, load voltage and line impedance characteristics. By power information of DGs that is uploaded to the microgrid central controller(MGCC), a reactive control strategy based on secondary control is constructed. Potential functions which have the objective of reactive power sharing and deviation of output voltage at no-load are established in the proposed method. MGCC centrally regulates output voltage at no-load of DGs via potential functions, and reactive power sharing among distributed generation units is effectively improved. The classical structure of droop control need not be changed under secondary control, and communication between DGs and MGCC is needed. Furthermore, the magnitude of circulation can be quantified by the concept of circulating normalised reactive power, circulation characteristics and circulation suppression are tested under different load conditions.(3)Due to the presence of single-phase DGs and load in microgrid, unsymmetrical voltage in a three-phase system causes100-Hz power oscillations of DGs. By analyzing the mechanism of the active and reactive power oscillations, Analytic formula of power oscillations is firstly derived, and the impact of positive and negative sequence electrical quantities on power oscillations is revealed. Accordingly, a secondary control is proposed to control power oscillations. An optimization model is built following optimization theory. The model aims at minimizing the amplitude of oscillations, subject to constraints which include power oscillations rate and power reference of PQ control. By optimized results, the secondary control can simultaneously reduce active and reactive oscillations while reactive oscillations are large. Therefore, the indexes of oscillations amplitude and oscillations rate reach the depth optimization.(4) According to optimized results, the economic operation control adjusts power given values of each DG and can be considered as superior control strategy of microgrid. Due to large proportion of intermittent energy contained in microgrid, the power generation plan is difficult to draw up because of randomness fluctuation. An economic operation model of microgrid based on chance constrained programming is put forward, and charge-discharge constraint of energy storage, reserve capacity constraint and power balance constraint are considered in the model. In addition, a probabilistic model based on cloud amount and geographical factors is adopted for photovoltaic generation, randomness fluctuations is truly reflected. On the basis of sequence operation theory, probability sequences are calculated by probability distribution of wind power generation and photovoltaic power generation, the chance constraints of the proposed model can be transformed into equivalence class of deterministic constraints. Accordingly, the proposed model is compatible with solvers such as TOMLAB, the results of the scheme are stable and can be quickly calculated compared with stochastic simulations. Futhermore, in order to investigate the rationality of probability sequence, the impact of different discretized steps on results is discussed and the proposed method is verified.更多还原