Comparative Study of Single-phase Phase-locked Loops for Grid-connected Inverters Under Non-ideal Grid Conditions

In renewable power generation systems,ensuring the synchronization of the inverter and the power grid is crucial for the stable operation of grid-connected inverters.Nowadays,the phase-locked loop(PLL)technology has become a widely used grid synchronization method because of its simple implementation and robustness under various grid conditions.Even though a lot of PLLs have been proposed,an overview and comparative analysis of multiple PLLs can be helpful for practical applications.In addition,the weak grid condition is a great challenge for the system.Therefore,this study first presents an overview of the existing PLLs together with their general structures and basic working principles.Depending on the implementation of the phase detector,the PLL can be divided into three categories:power-based PLL(pPLL),orthogonal-signalgenerator-based PLL(OSG-PLL)and adaptive-filter-based PLL(AF-PLL).Then,from the above classification,seven typical single-phase PLLs are selected for further study.Finally,some test results are given,and a comprehensive evaluation of the selected PLLs under different grid conditions is conducted.

Effect of spin on the instability of THz plasma waves in field-effect transistors under non-ideal boundary conditions

Terahertz(THz) radiation can be generated due to the instability of THz plasma waves in field-effect transistors(FETs). In this work, we discuss the instability of THz plasma waves in the channel of FETs with spin and quantum effects under non-ideal boundary conditions. We obtain a linear dispersion relation by using the hydrodynamic equation, Maxwell equation and spin equation. The influence of source capacitance, drain capacitance, spin effects, quantum effects and channel width on the instability of THz plasma waves under the non-ideal boundary conditions is investigated in great detail. The results of numerical simulation show that the THz plasma wave is unstable when the drain capacitance is smaller than the source capacitance;the oscillation frequency with asymmetric boundary conditions is smaller than that under non-ideal boundary conditions;the instability gain of THz plasma waves becomes lower under non-ideal boundary conditions. This finding provides a new idea for finding efficient THz radiation sources and opens up a new mechanism for the development of THz technology.

A truncated implicit high-order finite-difference scheme combined with boundary conditions

In this paper, first we calculate finite-difference coefficients of implicit finite- difference methods （IFDM） for the first and second-order derivatives on normal grids and first- order derivatives on staggered grids and find that small coefficients of high-order IFDMs exist. Dispersion analysis demonstrates that omitting these small coefficients can retain approximately the same order accuracy but greatly reduce computational costs. Then, we introduce a mirrorimage symmetric boundary condition to improve IFDMs accuracy and stability and adopt the hybrid absorbing boundary condition （ABC） to reduce unwanted reflections from the model boundary. Last, we give elastic wave modeling examples for homogeneous and heterogeneous models to demonstrate the advantages of the proposed scheme.

Distributed Air-Conditioning Energy Management System within the Smart Grid Context

-Air-conditioning （AC） systems are the major energy consumption units in residential and commercial buildings. In the context of smart grid, optimizing AC operations leads to substantial saving in energy consumption, reducing the consumer＇s bill and contributing to the environment by minimizing carbon emissions from generating stations. This paper presents a distributed AC energy management system for buildings by using networked master-slaves controller architecture. The proposed system was designed, simulated, and experimentally tested by using real AC units in a students＇ residence hall. Based on the students＇ class schedules, several operational scenarios were implemented and tested. The proposed system implementation leads to a 40% to 60% saving of the consumed energy by the tested units. The same energy management scheme can be applied and implemented in other commercial and residential buildings.

LOCAL AND PARALLEL FINITE ELEMENT METHOD FOR THE MIXED NAVIER-STOKES/DARCY MODEL WITH BEAVERS-JOSEPH INTERFACE CONDITIONS

In this paper, we consider the mixed Navier-Stokes/Darcy model with BeaversJoseph interface conditions. Based on two-grid discretizations, a local and parallel finite element algorithm for this mixed model is proposed and analyzed. Optimal errors are obtained and numerical experiments are presented to show the efficiency and effectiveness of the local and parallel finite element algorithm.

Accuracy of the staggered-grid finite-difference method of the acoustic wave equation for marine seismic reflection modeling

Seismic wave modeling is a cornerstone of geophysical data acquisition, processing, and interpretation, for which finite-difference methods are often applied. In this paper, we extend the velocity- pressure formulation of the acoustic wave equation to marine seismic modeling using the staggered-grid finite-difference method. The scheme is developed using a fourth-order spatial and a second-order temporal operator. Then, we define a stability coefficient (SC) and calculate its maximum value under the stability condition. Based on the dispersion relationship, we conduct a detailed dispersion analysis for submarine sediments in terms of the phase and group velocity over a range of angles, stability coefficients, and orders. We also compare the numerical solution with the exact solution for a P-wave line source in a homogeneous submarine model. Additionally, the numerical results determined by a Marmousi2 model with a rugged seafloor indicate that this method is sufficient for modeling complex submarine structures.

A Derivative-Free Optimization Algorithm Using Sparse Grid Integration

We present a new derivative-free optimization algorithm based on the sparse grid numerical integration. The algorithm applies to a smooth nonlinear objective function where calculating its gradient is impossible and evaluating its value is also very expensive. The new algorithm has: 1) a unique starting point strategy;2) an effective global search heuristic;and 3) consistent local convergence. These are achieved through a uniform use of sparse grid numerical integration. Numerical experiment result indicates that the algorithm is accurate and efficient, and benchmarks favourably against several state-of-art derivative free algorithms.

并网逆变器非线性特性建模及稳定性研究综述

并网逆变器作为连接新能源分布式发电和电网之间的关键接口,发挥着将电能高效、高质量馈入电网的重要作用。随着新能源渗透率的持续提高,并网点处电网存在诸如高电网阻抗、丰富的低频谐波、电压幅值/频率波动甚至故障等复杂状况。鉴于并网逆变器本质上是一个多输入、强耦合的非线性系统,沿用传统的线性建模与分析方法已难以解释工程中的复杂谐波振荡现象,并网逆变器的稳定性分析及其稳定运行受到巨大挑战。为此,该文首先具体阐述了线性与非线性和系统受扰大小之间的关系,指出大干扰稳定性分析更依赖于非线性建模及分析方法的应用。在此基础上,对已见诸报道的并网逆变器稳定性分析方法进行了归纳和对比。然后,详细阐述了稳定性分析方法在并网逆变器特定非线性环节中的应用。最后,总结并讨论了现有研究的不足,以期进一步促进“双高”背景下并网装置非线性建模和稳定性分析的研究。

Load Management Scheme Using Air Conditioning Electric Power Consumption and Photovoltaic Power System Generation

In the last few decades, in the world and also in the European Union, considerable resources had been invested in the rapid development of renewable energy sources and distributed generation in general. At the same time, power consumption is continuously increasing, and consumers are becoming more complex, which ultimately requires new investments in the distribution network. Concept of smart grids is generally accepted as a possible solution. Smart grid is a concept with many elements, where monitoring and control of every element in the chain of production, transmission, distribution and final consumption enable much more efficient delivery and use of electricity. One of the elements of smart grid efficiency is the ability of real-time demand-supply balancing. This balancing is carried out by monitoring of consumption and redistribution of electricity among individual end users, according to their needs. The aim of this paper is creating algorithm for real-time load management using power measurements. Algorithm for real-time load management at the ETFOS （Faculty of Electrical Engineering in Osijek）, Croatia is created based on measurements of photovoltaic power plant production, the power consumption of air conditioning system and the faculty building total electricity consumption. Expected result of real-time re-dispatching of air conditioners consumption, depending on the level of electricity production in photovoltaic power plant is decreasing peak demand of the faculty.

面向电网调峰的盘管式冰蓄冷空调技术研究

储能是助力实现碳达峰碳中和战略目标的关键技术。当前电力供需关系的矛盾随着可再生能源高比例接入和电力需求快速增长日益尖锐,该问题在夏季用电高峰期间尤为突出。冰蓄冷空调系统在夜间利用谷电制冰储存冷量,在白天用电高峰时融冰供冷,有助于电网调峰和改善用能经济性。该文总结分析盘管式冰蓄冷空调关键技术的研究现状和发展方向。系统控制层面:对比分析冰蓄冷空调系统的各种优化控制策略;蓄冰融冰方式层面:详细阐述内融冰、外融冰和内外融冰结合3种方式特点,并对能量模型进行归纳;结构层面:分析典型强化传热方式对传热性能的影响关系。最后,归纳不同材质盘管的研究结果与应用前景。

复杂大电网非预设性运行工况下的继电保护动作适应性仿真分析

非预设性运行工况下的突发事件多,导致继电保护逻辑的适应性较差,不合理区域较多,为此,进行复杂大电网非预设性运行工况下继电保护动作适应性仿真分析。分析历史常规故障,标记故障点最常发生位置,构建零序、负序方向元件模型、距离元件模型、纵联电流差动元件模型,观察复杂大电网非预设性运行工况下的电网参数变化情况,第一时间掌握电网的耦合性,不断选择电网所需的继电保护动作逻辑,确认继电保护动作适应性。仿真实验表明,将故障点设置在保护区域内和区域外,继电保护动作能够根据故障选择不同保护模型进行故障母线切除或者保护不动作,适应性好。

基于地理国情监测数据的景观生态评价研究

本文以黄河三角洲国家级自然保护区作为研究对象,应用景观生态学理论,提出基于地理国情监测数据的人为干扰度划分方法,选取合适指标,构建景观生态评价指标体系。本文以500 m×500 m格网作为评价单元,采用2015―2020年六期地理国情监测数据,运用熵权法确定单个指标的权重,得到景观生态安全指数(LESI),并从空间和时间维度对保护区内景观生态情况进行分析评价。研究结果表明,2015―2020年期间,保护区的景观生态状况得到逐步改善,尤其是2020年,不仅生态安全指数全部提升到良好及以上水平,空间分布也呈现出更好的趋势。

电网不对称故障下MMC自适应相功率均衡控制策略

交流电网不对称故障工况下模块化多电平换流器(modular multilevel converter,MMC)存在相功率不均衡的问题,常规的基于直流环流注入的调控方法会引起桥臂电流不对称,导致各相电流应力不相等;而基于零序电压注入的方法可能导致系统过调制,危害系统安全稳定运行。针对传统相功率均衡控制策略的局限性,提出一种交流电网不对称故障下MMC自适应相功率均衡控制策略。首先,分析基于零序电压和直流环流注入的MMC相间功率调控原理,指出不同方法单独进行相间功率均衡的局限性。其次,研究零序电压注入方法的过调制边界,引入相功率分配系数,给出不同故障下相功率系数优化方法,提出基于零序电压和直流环流注入协调的MMC自适应相功率均衡控制策略。最后,通过仿真验证了机理分析与所提控制策略的正确性和有效性。

考虑条件风险价值的交直流系统两阶段分布鲁棒低碳经济优化调度

考虑到海上风电出力的随机性以及日益突出的生态环境问题,以含柔性直流输电技术(voltagesource converter high voltage direct current,VSC-HVDC)的交直流系统为研究对象,提出了考虑条件风险价值(conditional valueatrisk,CVaR)的两阶段分布鲁棒低碳经济优化模型,构建了基于Kullback-Leibler(KL)散度的概率分布模糊集,同时利用条件风险价值量化了极端场景下的尾部风险,使得模型能够同时考虑概率分布不确定性以及处于最坏概率分布中极端场景下的尾部损失;此外,将阶梯型碳交易机制并入所提分布鲁棒模型中,通过合理利用柔性资源和储能装置,增强系统运行的灵活性,在兼顾运行风险的前提下,降低碳排放量的目标。再者,为了提高计算效率,在列和约束生成算法(column-and-constraint generation method,C&CG)和Multi-cut Benders分解算法的基础上提出了双循环分解算法。最后,在基于改进的IEEE RTS 79测试系统中验证了所提模型及算法的有效性。

有限元强度折减法分析软基边坡稳定性

有限元强度折减法在均质土坡和岩石边坡中已有较多的研究和应用,但应用到强度较低的软基边坡,其模型网格划分和边界条件,与均质边坡有所不同。经对比研究得出软基边坡模型的网格节点密度达到每米2点较为适宜,模型边界应以修正后的坡高H来界定。结合具体工程案例,利用有限元强度折减法复核其稳定,并与极限平衡法进行了对比,结果表明前述网格划分和模型边界设置较为适宜,且有限元强度折减法可以较好的反映边坡应力应变分布。

不同降雨工况条件下的崩滑地质灾害危险性评价

为针对性地采取预防、避让、治理等地质灾害防治与管控提供依据,完善在地质灾害危险性评价中将降雨作为单一诱发因子参与评价体系的弊端,在大雨、暴雨、大暴雨和特大暴雨4种不同降雨工况条件下进行了研究区崩滑地质灾害危险性评价。以云南省元阳县作为研究区域,以栅格单元作为评价单元,选取地貌类型、高程、坡度、坡向、曲率、工程地质岩组、土地利用类型、断层距离和河流距离9个评价因子,采用主观的层次分析法与客观的信息量模型相结合的加权信息量模型对元阳县崩塌、滑坡进行了地质灾害易发性评价。研究结果表明:基于自然间断点法元阳县域可分为低、中、高、极高4类易发区,4类易发区面积分别占元阳县面积的21.55%,35.46%,25.53%和17.16%。利用ROC曲线得出区划结果精度AUC值为0.812,表明区划结果很好。在易发性评价基础上,以年平均最大日降雨量为诱发因素,分别对大雨、暴雨、大暴雨和特大暴雨4种工况条件下的研究区进行了崩塌、滑坡地质灾害危险性评价,得到了大雨([25,50) mm)工况、暴雨([50,100) mm)工况、大暴雨([100,250]mm)工况和特大暴雨(>250 mm)工况4种条件下基于极值假设的研究区崩滑地质灾害危险性评价结果,并对不同降雨工况条件下的地质灾害危险性评价结果进行了对比分析,确定了地质灾害危险性评价结果在不同降雨条件下的空间合理性。通过与实际调查结果的对比表明,4种不同降雨工况条件下的崩滑地质灾害危险性评价结果与实际高度吻合,说明评价结果具有较高的可靠性与合理性。

冲击式水电机组不同工况下孤网运行的稳定性分析

我国西南建有大量的冲击式水电机组的水电站,这些水电站与电网联系薄弱,孤网运行稳定性问题突出。对此建立了冲击式水轮机的数学模型、完善了冲击式水电机组孤网分析模型,研究了孤网条件下冲击式水电机组的控制参数稳定域。首先,根据电站实际情况建立冲击式水轮机的非线性模型。其次,根据李雅普诺夫第一方法开展稳定性分析,在不同运行工况点对冲击式水轮机进行线性化,获得相关传递系数,代入孤网分析模型建立状态方程,根据临界稳定条件求出稳定域。最后,通过非线性模型仿真对分析结果进行验证。结果表明,喷针开度变化时对运行稳定域的影响较大,不能忽视;调速器参数整定时,需对所有运行工况进行分析,按最小稳定域工况进行参数优化。

基于高精度热泵模型的电热协同独立微网设备优化配置

为实现含丰富工业余热、数据中心余热、生活污水余热等低品位热源的区域协同供能,以及提高区域清洁能源利用效率,针对区域独立微网供能系统的设备优化配置策略进行研究。基于多类型低品位热源的不同温度品位特征,提出用于提升热源温度品位的关键电热泵设备多参数拟合模型,并生成热泵拟合模型的误差不确定性场景;考虑微网在实际运行不确定性场景下产生的供热舒适度补偿、切负荷与切光伏风险,提出基于多条件风险价值(CVaR)理论的独立微网设备容量优化配置双层模型,上层模型基于全寿命周期成本费用年值与年CVaR值优化设备配置方案,下层模型基于场景分析法优化运行成本;采用遗传算法对双层模型进行求解。算例结果证明:考虑热泵拟合模型误差有效降低了系统运行风险,且低品位热源的回收利用具有较大经济性潜力。

含微分平坦控制的柔直输电系统稳定性分析

交流系统不对称运行时换流站控制参数不合理的取值会影响柔性直流输电系统稳定运行。为此,首先提出不对称工况下含微分平坦控制的换流站小信号模型,同时结合模块化多电平换流器(modular multilevel converter,MMC)内部特性、环流抑制器以及直流线路等各子部分的动态模型,建立两端柔性直流输电系统全局小信号模型;然后在此基础上,依据微分平坦控制器内、外环控制参数根轨迹的变化情况,分析其对系统稳定性的影响,确定其合理取值范围;最后在PSCAD/EMTDC中搭建含微分平坦控制的两端柔性直流输电测试系统,验证所提小信号模型以及结论的正确性,为实际工程中控制器参数整定提供理论依据。

基于可塑性面积单元问题的建成环境对交通状态的影响

针对分析建成环境对交通状态影响时网格单元划分问题,以济南市为研究区域,选取人口密度、购物商场密度、公交站点密度作为济南市建成环境的评价指标,选取车辆轨迹点平均速度作为济南市交通状态的评价指标,考虑可塑性面积单元问题(modifiable areal unit problem,MAUP),分别采用1 km×1 km、5 km×5 km、10 km×10 km的3种正方形网格及1 km×5 km、1 km×10 km、5 km×10 km的3种长方形网格划分研究区域,通过皮尔逊相关系数、普通最小二乘法(ordinary least squares,OLS)、空间自相关分析3种方法分析不同划区方案下建成环境与交通状态间的相关性,通过计算并比较建成环境与交通状态间的皮尔逊相关系数、OLS拟合优度、莫兰指数确定最佳划区方案。结果表明:以5 km×5 km的正方形网格单元划分研究区域时,交通状态和建成环境间的相关性最大,可较好地反映建成环境对交通状态的影响。