Coordinated control between a grid forming inverter and grid following inverters suppling power in a standalone microgrid

Battery energy storage systems,fuel cells,and photovoltaic generators are being adopted in distribution networks to achieve CO;emission reduction.These power sources require inverters to connect to AC networks.However,sub-networks,such as a microgrids containing inverter power sources,may be disconnected from the utility network and operated in a standalone mode during extended blackouts.This study focuses on a standalone microgrid supplied by inverter power sources without a synchronous generator and proposes a new microgrid inverter control.In this inverter control,a single grid forming inverter is operated as a master power source to determine frequency and voltage,and other inverters,called grid following inverters,are operated as subordinate power sources with active and reactive power control.The necessary functions in the energy management system are examined,and the coordinated operation of all inverters in the microgrid is demonstrated via simulation.

Grid Forming Converters in Renewable Energy Sources Dominated Power Grid:Control Strategy,Stability,Application,and Challenges

The renewable energy sources(RESs)dominated power grid is an envisaged infrastructure of the future power system,where the commonly used grid following(GFL)control for grid-tied converters suffers from lacking grid support capability,low stability,etc.Recently,emerging grid forming(GFM)control methods have been proposed to improve the dynamic performance and stability of grid-tied converters.This paper reviews existing GFM control methods for the grid-tied converters and compares them in terms of control structure,grid support capability,fault current limiting,and stability.Considering the impact of fault current limiting strategies,a comprehensive transient stability analysis is provided.In addition,this paper explores the typical applications of GFM converters,such as AC microgrid and offshore wind farm high-voltage direct current(OWF-HVDC)integration systems.Finally,the challenges to the GFM converters in future applications are discussed.

An automatic grid generation approach over free-form surface for architectural design

An essential step for the realization of free-form surface structures is to create an efficient structural gird that satisfies not only the architectural aesthetics,but also the structural performance.Employing the main stress trajectories as the representation of force flows on a free-form surface,an automatic grid generation approach is proposed for the architectural design.The algorithm automatically plots the main stress trajectories on a 3D free-form surface,and adopts a modified advancing front meshing technique to generate the structural grid.Based on the proposed algorithm,an automatic grid generator named "St-Surmesh" is developed for the practical architectural design of free-form surface structure.The surface geometry of one of the Sun Valleys in Expo Axis for the Expo Shanghai 2010 is selected as a numerical example for validating the proposed approach.Comparative studies are performed to demonstrate how different structural grids affect the design of a free-form surface structure.

PV Based Microgrid with Grid-Support Grid-Forming Inverter Control-(Simulation and Analysis)

Microgrid (MG) is a small entity of electrical network which comprises of various Distributed Generation (DG) sources, storage devices, and group of loads in various classes. MG provides reliable and secure energy supply to the critical loads of communities while operating either in on-grid or off-grid mode. In this study, a coordinated power management control strategy for a typical low voltage (LV) MG network with integration of solar Photovoltaic (PV) and storage facility has been developed and analysed in Matlab-Simu-link software environment at various modes (on-grid, off-grid, and on-grid to off-grid transition) of MG operation. Solar PV and battery power inverters are considered as grid-support grid-forming (GsGfm) Voltage Source Inverter (VSI) with the implementation of modified droop and virtual output impedance control strategies. Proposed control strategy supports coordinated control operation between PV units and battery storage, equal power sharing among the DG sources, and smooth MG mode transition with regulation of voltage and frequency level in MG network. In addition, voltage and current THD level were analysed and verified as per the standard of AS4777.

A two-grid algorithm based on Newton iteration for the stream function form of the Navier-Stokes equations

In this paper, we propose a two-grid algorithm for solving the stream function formulation of the stationary Navies-Stokes equations. The algorithm is constructed by reducing the original system to one small, nonlinear system on the coarse mesh space and two similar linear systems （with same stiffness matrix but different right-hand side） on the fine mesh space. The convergence analysis and error estimation of the algorithm are given for the case of conforming elements. Furthermore, the Mgorithm produces a numerical solution with the optimal asymptotic H^2-error. Finally, we give a numerical illustration to demonstrate the effectiveness of the two-grid algorithm for solving the Navier-Stokes equations.

Deforming analysis of sheet metal based on stereo vision and coordinate grid

A new approach based on stereo vision technology is introduced to analyzesheet metal deformation. By measuring the deformed circle grids that are printed on the sheetsurface before forming, the strain distribution of the workpiece is obtained. The measurement andanalysis results can be used to verify numerical simulation results and guide production. To getgood accuracy, some new techniques are employed: camera calibration based on genetic algorithm,feature abstraction based on self-adaptive technology, image matching based on structure feature andcamera modeling pre-constrains, and parameter calculation based on curve and surface optimization.The experimental values show that the approach proposed is rational and practical, which can providebetter measurement accuracy with less time than the conventional method.

提高新能源场站稳定性的构网型与跟网型变流器容量配比估算

大规模新能源接入使得电网短路比降低,系统电压支撑强度下降,导致静态电压稳定和次/超同步振荡等稳定问题凸显。构网型变流器具有较强的电压支撑能力,在新能源场站配置一定比例的构网型变流器可以提升系统稳定性,但由于构网型变流器容量与稳定裕度的解析关系不明,导致构网型变流器容量合理占比的理论值估算十分困难。该文针对新能源基地接入低短路比弱电网场景,从小干扰稳定视角探讨构网型与跟网型变流器容量配比的估算方法及其典型值。首先,对构网型变流器进行电压源等值,并通过广义短路比指标分析了构网型变流器配置比例对系统强度以及稳定裕度的影响规律;其次,针对改造构网型风电机组和额外加装构网型变流器两种技术方案,以行业相关标准为边界条件并结合典型升压变压器参数,估算了实际工程中容量配比的典型值;最后,利用多风电场系统案例验证了结论的有效性。

大容量高压级联储能系统分型融合控制方法

储能的规模化应用为可再生能源高占比的新型电力系统提供重要支撑。高压级联储能系统将电池布置于级联H桥模块直流侧,是提高储能设备单机容量的有效手段。储能变流器控制方式包括跟网型控制和构网型控制两类,其中跟网型控制功率响应快速,但在弱网中易振荡失稳;构网型控制具有主动支撑能力,但存在强网适应性问题。该文提出了分型融合控制方法,实现了跟网特性与构网特性的柔性融合和灵活调节,提高了高压级联储能系统的电网强度适应性。通过仿真分析验证了所提控制方法的实用性与有效性。

基于遗传算法的自由曲面单层空间结构网格参数化自动生成及方向优化

自由曲面空间网格结构的力学性能和网格方向之间的联系复杂,难以快速直接地找到最优方向,若对结构网格方向进行多角度建模分析,则存在工作量大且准确性不易保证等问题。为了快速确定自由曲面空间网格结构的最优网格方向,提出一种基于遗传算法的自由曲面空间结构网格方向优化方法,以结构网格方向为优化变量,结合参数化建模软件Grasshopper和有限元分析软件SAP2000实现参数化分析,以结构应变能和结构最大位移两项力学指标作为优化目标,通过最小化目标函数实现结构网格方向优化。通过参数化编程,可实现快速自动优化,并结合工程案例,应用自动优化方法进行网格优化。结果表明:基于遗传算法的自由曲面空间结构网格方向自动优化法具有实用性和高效性,具有较高的工程应用价值。

构网型变流器:物理本质与特征

全球能源产业变革促进了电力系统电力电子化转型,以新能源为主体的电力电子化电力系统中同步机占比低,系统呈现低惯量、弱阻尼特征,引发了一系列运行问题。构网型变流器本质为电压源,能够主动构建电压频率,为系统提供惯量支撑,有利于改善系统稳定性,是提高新能源主动支撑能力的有效手段,近年来成为研究热点。该文从电力电子化电力系统的运行问题出发,梳理了下垂控制、虚拟同步机控制等典型构网型控制策略的实现原理及内在联系,建立构网型控制策略的统一数学模型;通过类比稳态条件下同步机与构网型变流器的电压频率构建原理,揭示了构网型变流器的物理本质,从电压频率构建、惯量阻尼支撑、同步机制等角度解释了构网型变流器的内涵与特性,可为构网型变流器的稳定与控制提供理论参考。

功率耦合和电流限幅影响下构网型变流器的暂态同步稳定分析

在有限过流能力的构网型变流器中,有功和无功环之间的功率耦合以及电压电流环之间的电流限幅,会极大地影响变流器与电网的同步过程。因此,该文致力于研究大扰动故障下功率耦合和电流限幅对变流器暂态同步稳定性的影响。首先,考虑上述环节建立构网型变流器并网系统的降阶模型;然后,构建包含构网型控制主要状态变量(功角、频率和输出电压)的能量函数,并在此基础上通过拉塞尔理论估计变流器的吸引域及其对应的暂态稳定裕度。分析表明,功率耦合和电流限幅分别通过影响等效势能和阻尼分布削弱变流器的暂态同步稳定性。由此,该文提出一种基于虚拟阻抗和功率解耦的构网型控制优化策略,以抑制功率耦合和电流限幅引入的负面影响。最后,仿真和实验结果验证上述理论分析的正确性。

构网型柔性直流系统阻尼环节控制策略优化研究

采用构网控制的柔性直流输电系统可模拟同步电机运行特性,为电网提供频率和电压支撑,而传统的功率同步型构网控制阻尼环节引入了调频功能,使柔直功率输出随电网频率变化而波动。该文基于同步发电机调频和阻尼环节的机理辨析,提出了功率同步型构网控制阻尼环节改进方法,可同时满足并网运行稳态功率精准控制和频率突变的阻尼需求,并基于小信号稳定分析给出了控制参数设计原则,基于电磁暂态仿真平台以及渝鄂柔性直流工程现场试验验证了改进策略的有效性。

基于改进虚拟同步发电机控制的功率交互振荡抑制策略

在指令功率发生扰动时,虚拟同步发电机控制环路易产生功率交互振荡,威胁系统的安全稳定运行。为此,提出一种基于改进虚拟同步发电机控制的功率交互振荡抑制策略。基于相对增益矩阵原理研究不同系统参数变化时虚拟同步发电机输出功率间交互作用的变化规律;采用转速阻尼功率高通反馈改进虚拟同步发电机有功功率-频率控制环路,并通过绘制Bode图对比传统虚拟同步发电机控制策略与改进虚拟同步发电机控制策略的暂态和稳态性能;利用相对增益矩阵原理分析改进虚拟同步发电机控制策略的有功功率环路交互作用情况。仿真及实验结果表明,所提策略可有效实现虚拟同步发电机交互振荡的抑制。

构网型并网逆变器状态空间建模及稳定性分析

随着“双碳”目标的提出与逐步实施,作为新能源发电单元接入电力网络的重要接口元件的并网逆变器稳定性问题凸显。目前,电网大部分采用跟网型并网逆变器,然而弱电网中跟网型并网逆变器与锁相环、电网间存在耦合,严重影响系统稳定性。构网型并网逆变器在弱电网中较跟网型并网逆变器表现出更强的适应性,但现阶段构网型并网逆变器状态空间模型的相关论述较少,基于状态空间模型的构网型并网逆变器稳定性分析也不够深入。针对这一空缺,首先详细呈现构网型并网逆变器状态空间建模的相关细节,包括功率控制环、电压控制环、时滞环节和LCL滤波环节;然后,复现构网型并网逆变器失稳现象,并基于所提模型开展参与因子分析,验证模型有效性;其次,基于模型对构网型并网逆变器主要环节参数变化引起的失稳问题进行分析;最后,在Matlab/Simulink中搭建改进的IEEE 4节点三相平衡电网系统进行仿真,进一步验证模型有效性和实用性。

单电压环构网型并网逆变器暂态稳定性分析

随着电力电子化电力系统的快速发展,构网型并网逆变器作为新型电源得到了广泛关注,其中单电压环构网型并网逆变器因具有更强的小信号稳定性而在新能源并网中具有独特优势。同时,构网型并网逆变器在大扰动下呈现与同步电机不同的暂态响应,容易产生暂态失稳。针对这一问题,以单电压环构网型并网逆变器大扰动模型为基础,采用相平面图的方法分析了大扰动下功率控制环、电压控制环对逆变器的暂态作用,给出了关键控制器参数对暂态稳定性的影响,刻画了功角的暂态响应特性。同时,揭示了有功控制环、无功控制环和单电压环在暂态期间会改变功角超调量、逆变器输出电压幅值和变化率,进而影响并网逆变器的暂态稳定性。基于上述分析,给出了减小有功和无功下垂系数,增大电压积分系数和低通滤波器截止角频率的控制参数优化方法,可增强单电压环构网型并网逆变器暂态稳定性。最后,通过Matlab/Simulink仿真验证了分析的准确性与可行性。

计及控制切换的逆变电源并网系统故障计算方法

并网逆变电源的故障特征受控制策略影响极大,非线性特性显著,传统故障分析方法无法适用。为精确描述故障期间系统电压和电流的变化过程,该文考虑了电力电子换流器的通流能力、故障后的控制策略切换、控制系统的非线性、状态变量的耦合性等因素,提出了一种适用于描述系统时域响应的全状态变量接续求解法。通过控制系统和一次电路的数值积分和微分方程组解析的接续求解得到系统状态量的时域变化全过程。该文以构网型逆变电源并网系统为例,建立PSCAD仿真模型和数学模型,通过仿真结果与该文所提方法的计算结果进行分析和对比,验证了方法的有效性和正确性。

电力电子组网系统中构网型换流器间的协同限流研究

新能源快速发展下带来的100%电力电子设备组网独立运行的场景逐渐增多,由于缺乏传统同步发电机的惯性和阻尼,当基于不同控制类型的构网型换流器为系统提供电压和频率支撑时,其对外部干扰较敏感,负荷大扰动情况下,系统存在明显的过流问题。针对该问题,首先对单端构网型换流器设计了一种基于电流饱和算法的过电流抑制方法,以限制端口换流器的暂态过电流。其次,在多端口换流器协同限流方面,提出了一种基于不同控制类型的构网型换流器间协同限流控制方法,解决了不同构网型换流器组网时由于扰动造成的输出过电流问题,并实现限流控制与正常工作模式间的平滑切换。同时,建立系统的小信号模型,利用根轨迹分析主要控制参数对系统稳定性的影响。最后,搭建了控制器级硬件在环测试平台,对理论分析和所提方法进行了硬件在环测试,验证了所提方法的有效性和优越性。

构网型储能与调相机的暂态过电压抑制能力对比研究

高压直流输电系统出现闭锁、换相失败等故障时,会引起送端换流站附近新能源电站的瞬时电压波动并易导致其脱网。首先,分析了构网型储能开环控制策略的运行原理、响应特性和关键参数影响。然后,分析了调相机的无功响应特性,并将构网型储能无功控制模型与调相机的励磁调节控制模型进行对比。最后,搭建了包含特高压直流、光伏、调相机、构网型储能的半实物仿真系统,进行了直流换相失败故障下的交流暂态过电压抑制效果对比试验。结果表明,容量相同的构网型储能和分布式调相机在无功瞬时响应速度、抑制交流暂态过电压能力方面可以实现相近的效果。

构网型直驱风机的小信号建模及动态频率支撑策略

针对传统构网型直驱风机在大扰动下的频率和功率振荡较大的问题,提出一种动态频率支撑策略。首先对构网型直驱风机进行了详细的小信号建模。其次,通过主导特征根轨迹分析有功频率控制以及交流电压控制环节中参数对频率变化特性的影响,进而提出一种灵活改变控制参数的动态频率支撑策略。最后,在Matlab/Simulink仿真平台上搭建仿真模型,验证了小信号建模的正确性和所提策略的有效性,表明所提策略能有效减小动态过程中的频率和功率振荡。

基于虚拟电容的构网型MMC快速平滑切换策略

为减小构网型模块化多电平换流器黑启动完成后的并网瞬间冲击,提出一种综合考虑并网点电压的幅值、频率及相位的最优并网控制策略。通过实时检测并网两端的电压幅值、频率及相位,达到阈值后开始并网。同时,针对该策略同步时间过长的问题,提出一种基于虚拟电容的构网型模块化多电平换流器快速并网切换控制策略,在控制环节引入虚拟电容快速调节构网型模块化多电平换流器的电压相位,节省储能型模块化多电平换流器的备用容量。最后,通过搭建硬件在环测试平台验证最优并网控制和快速并网切换控制策略的有效性和正确性。