A virtual lock-in amplifier,spectrum analyzer,impedance meter and semiconductor analyzer implemented on an SR7265 hardware target

Lock-in amplifiers are used to detect and measure very small alternating current(AC)signals down to the range of nVs.Accurate measurements can be made even when the small signals are buried by noise thousands of times larger.With the digital signal processing(DSP)technology involved in modern instrumentation,a lock-in amplifier is more versatile in sensing and recovering small signals.Combining the virtual instrumentation technology,we reorganize the functional blocks of a programmable lock-in amplifier and build it as a virtual spectrum analyzer,virtual impedance meter,virtual network analyzer,virtual semiconductor parameter analyzer,signal generator,etc.A 4 layer model is used to implement these virtual instruments.The same virtual instrument can also be implemented on a general purpose FPGA developing board.

VIRTUAL-RAY METHOD AND ITS APPLICATION IN THE PLANE WAVE SCATTERING BY AN IMPEDANCE WEDGE

The virtual-ray method for treating HF electromagnetic scattering problems is derived from the plane wave of free space, and using this the plane wave scattering by an impedance wedge is studied. In the solution process a novel concept of generalized circle is introduced so that the complete amplitude function is obtained. And a reasonable physical interpretation for the term W2, which was neglected previously, is given. The calculated results agree well with those of the analytical solution obtained by G.D. Maliuzhinets(1958).

Harmonic State Space Based Impedance Modeling and Virtual Impedance Based Stability Enhancement Control for LCC-HVDC Systems

Line commutated converter based high-voltage direct-current(LCC-HVDC)transmissions are prone to harmonic oscillation under weak grids.Impedance modeling is an effective method for assessing interaction stability.Firstly,this paper proposes an improved calculation method for the DC voltage and AC currents of commutation stations to address the complex linearization of the commutation process and constructs an overall harmonic state-space(HSS)model of an LCC-HVDC.Based on the HSS model,the closed-loop AC impedances on the LCC-HVDC sending and receiving ends are then derived and verified.The impedance characteristics of the LCC-HVDC are then analyzed to provide a physical explanation for the harmonic oscillation of the system.The effects of the grid strength and control parameters on system stability are also analyzed.To improve the impedance characteristics and operating stability of the LCC-HVDC system,a virtual impedance based stability enhancement control is proposed,and a parameter design method is considered to ensure satisfactory phase margins at both the sending and receiving ends.Finally,simulation results are presented to verify the validity of the impedance model and virtual impedance based stability enhancement control.

Robust force tracking control via backstepping sliding mode control and virtual damping control for hydraulic quadruped robots

In order to improve the force tracking performance of hydraulic quadruped robots in uncertain and unstructured environments,an impedance-based adaptive reference trajectory generation scheme is used.Secondly,in order to improve the robustness to environmental changes and reduce the contact force errors caused by trajectory tracking errors,the backstepping sliding mode controller is combined with the adaptive reference trajectory generator.Finally,a virtual damping control based on velocity and pressure feedback is proposed to solve the problem of contact force disappearance and stall caused by sudden environmental change.The simulation results show that the proposed scheme has higher contact force tracking accuracy when the environment is unchanged;the contact force error can always be guaranteed within an acceptable range when the environment is reasonably changed;when the environment suddenly changes,the drive unit can move slowly until the robot re-contacts the environment.

An Integrated Control Strategy Adopting Droop Control with Virtual Inductance in Microgrid

As there exists sorts of distributed generators in microgrid, an integrated control strategy containing different control methods against corresponding generators should be applied. The strategy in this paper involves PQ control and droop control methods. The former aims at letting generators like PV output maximum power. The latter stems from inverter parallel technique and applies to controlling generators which can keep the network voltage steady to make the parallel system reach the minimum circulation point. Due to the unworthiness of droop control applied in low-voltage microgrid of which the impedance ratio is rather high, the paper adopts the droop control introducing virtual generator and virtual impedance. Based on theoretical analysis, simulation in Matlab is also implemented to verify the feasibility of the strategy.

Understanding the effects of structured self-assessment in directed,self-regulated simulation-based training of mastoidectomy:A mixed methods study

Objective:Self-directed training represents a challenge in simulation-based training as low cognitive effort can occur when learners overrate their own level of performance.This study aims to explore the mechanisms underlying the positive effects of a structured self-assessment intervention during simulation-based training of mastoidectomy.Methods:A prospective,educational cohort study of a novice training program consisting of directed,self-regulated learning with distributed practice(5x3 procedures)in a virtual reality temporal bone simulator.The intervention consisted of structured self-assessment after each procedure using a rating form supported by small videos.Semi-structured telephone interviews upon completion of training were conducted with 13 out of 15 participants.Interviews were analysed using directed content analysis and triangulated with quantitative data on secondary task reaction time for cognitive load estimation and participants’self-assessment scores.Results:Six major themes were identified in the interviews:goal-directed behaviour,use of learning supports for scaffolding of the training,cognitive engagement,motivation from self-assessment,selfassessment bias,and feedback on self-assessment(validation).Participants seemed to self-regulate their learning by forming individual sub-goals and strategies within the overall goal of the procedure.They scaffolded their learning through the available learning supports.Finally,structured self-assessment was reported to increase the participants’cognitive engagement,which was further supported by a quantitative increase in cognitive load.Conclusions:Structured self-assessment in simulation-based surgical training of mastoidectomy seems to promote cognitive engagement and motivation in the learning task and to facilitate self-regulated learning.

Systematic design method for PI controller with Virtual Resistor-based Active Damping of LCL filter

The Virtual Resistor based Active Damping(VR-AD) is widely employed in converters connected to the grid via LCL filters in order to mitigate the inherent resonance of the filters. Nevertheless, in digitally controlled systems, the PWM and the calculating delays modify the system characteristics in terms of frequency and phase, thus destabilizing the system and degrading the VR-AD performances, mainly in low switching frequencies. Moreover, the stability of the system is greatly affected under weak grid operation characterized by large grid impedance variation. This paper solves these problems by proposing a systematic, robust and optimized design procedure of voltage oriented PI control(VOC) with VRAD. The considered design procedure ensures robust control(sufficient stability margins) and high quality of grid current(reduced steady-state error and minimized THD value) despite the negative impact of digital time delay, grid impedance variation and filter parameters change. Simulation and experimental results are presented to show robustness and efficiency of the suggested design procedure.

Research on Virtual DC Generator-Based Control Strategy of DCMicrogrid with Photovoltaic and Energy Storage

With the penetration of a large number of photovoltaic power generation units and power electronic converters,the DC microgrid shows low inertia characteristics,which might affect the stable operation of the microgrid in extreme cases.In order to enhance the“flexible features”of the interface converter connected to the DC bus,a control strategy of DCmicrogrid with photovoltaic and energy storage based on the virtual DC generator(VDCG)is proposed in this paper.The interface converters of the photovoltaic power generation system and the energy storage system simulates the inertia and damping characteristics of the DC generator to improve the stability of the DC bus voltage.The impedance ratio of DC microgrid was obtained by establishing the small-signal model of photovoltaic power generation system and energy storage system,and the Nyquist curves was applied to analyze the small-signal stability of the system.Finally,the simulation results were verified with MATLAB/Simulink.The results show that the proposed control strategy can slow down the fluctuation of bus voltage under the conditions of photovoltaic power fluctuation and load mutation,thus enhancing the system stability.

基于电路等效的并网逆变器失稳分析与稳定控制

以并网逆变器为功率接口的新能源发电系统在弱电网条件下易发生振荡失稳问题。该文将并网逆变器的控制回路可视化为电路元件组成的虚拟阻抗,基于该电路模型分析了弱电网条件下电流内环与锁相环交互作用导致并网逆变器振荡失稳的机理,在此基础上,提出了基于有源阻尼的稳定控制设计方法,并对不同有源阻尼控制的电路特性以及稳定性提升能力进行了对比分析。研究结果表明,针对锁相环引入负电阻造成的振荡失稳问题,阻抗-高通滤波器型有源阻尼控制策略具有更优的稳定性提升能力。最后通过PSCAD/EMTDC仿真和远宽StarSim控制器硬件在环实验对比了不同有源阻尼控制策略的振荡抑制效果,并验证了阻抗-高通滤波器型有源阻尼控制的动态性能。结果表明,所设计的稳定控制能够在200 ms内有效抑制系统振荡,并且可实现在短路比为1的极弱电网条件下稳定运行。

VSC输出电流反馈型虚拟同步控制技术

基于输出功率反馈的虚拟同步机(VSG)技术在网侧故障时,难以保持电压源暂态特性并实现精准暂态冲击电流限制,同时极易出现暂态同步失稳问题。为解决上述问题,提出一种基于电压源型变流器(VSC)输出电流反馈型VSG技术。外环控制直接采用VSC输出电流作为反馈控制量,不仅能实现构网和电压/频率主动支撑功能,还能保证在故障状态下外环控制具备稳定平衡点,降低暂态同步失稳风险,同时还能实现稳态下的电流限幅控制。电压内环采用基于相量分析的虚拟阻抗自适应选取方法,依赖就地量测信息进行虚拟阻抗快速自适应调整,以抑制故障瞬间暂态冲击电流。在PSCAD/EMTDC中搭建详细电磁暂态仿真模型,并构建相应的物理验证平台,仿真和实验结果表明,所提方法可以较好地抑制故障瞬间暂态冲击电流。

构网控制目标下系统故障特征及保护性能

分析了构网目标下的典型同步控制及故障穿越策略,提出了构网型变流器在故障分析时的等效电路。在此基础上,在相量平面分析了含构网型变流器系统稳态运行及各类故障工况下的电气量特征,指出构网控制目标下的故障特征与传统同步机系统的异同。重点针对当前电网中主流配置的电流差动保护与距离保护开展构网控制目标下的动作性能评估,并通过构建PSCAD仿真模型验证理论研究结果。分析表明,相比于传统跟网型控制,构网变流器控制技术通过模拟同步发电机的基本特性,为系统中的继电保护创造了一个相对良好的运行环境,有利于提高保护整体动作性能。

基于虚拟阻抗自适应的孤岛型微电网并联逆变器下垂控制策略

当前微电网并联逆变器下垂控制机制一般设定为独立式,控制的范围较难扩展,导致控制补偿差增加。为此,提出对基于虚拟阻抗自适应的孤岛型微电网并联逆变器下垂控制策略的设计与实践。先进行下垂控制特性提取,采用交互的方式,扩大控制的范围,设计交互控制机制。在此基础上,构建虚拟阻抗自适应微电网并联逆变器下垂控制模型,采用补偿核验的方式确保下垂控制效果。针对选定的5个测试逆变器,经过2个周期的测定得出的控制补偿差被较好地控制在1.05以下,说明此次在虚拟阻抗自适应原理的辅助下,所设计的微电网并联逆变器下垂控制方法更为高效,具有实际的应用价值。

一种基于虚拟功率的虚拟同步机功率解耦策略

针对虚拟同步发电机(virtual synchronous generator,VSG)控制的分布式能源(distributed generation,DG)和电网之间的阻抗比过高,引起的功率耦合问题。对功率耦合机理进行了分析,通过相对静态增益系数建立了功率与阻抗比及功角的耦合度关系,提出了一种基于虚拟负阻抗的虚拟功率控制技术,利用虚拟阻抗计算虚拟电压,并将得到的虚拟电压反馈到功率计算部分,形成虚拟功率实现功率解耦,并基于耦合度约束条件给出了虚拟电阻和电感具体的求解方法。最后利用MATLAB/Simulink仿真软件搭建仿真模型,通过与传统虚拟阻抗技术进行比较,验证了所提出的虚拟功率控制策略对减弱有功环与无功环之间的耦合有明显作用。

基于改进虚拟同步发电机的构网型并联储能逆变器控制研究

针对构网型变换器预同步过程时电力系统频率电压稳定性低导致变换器并网失败的问题,提出一种基于改进线性自抗扰控制(LADRC)的构网型变换器预同步控制策略。首先,采用无锁相环(PLL)控制策略,通过相位偏差的反馈控制对网侧电压的相位和幅值进行快速同步追踪,可避免因PLL精度较低和响应速度慢而引起的电力系统稳定性降低的问题。在此基础上,在有功频率支路模块的角频率输出端引入LADRC,这可以有效解决预同步控制过程中变换器输出电压频率存在过冲的问题,从而确保构网型变换器预同步控制顺利进行最终实现成功并网。最后,在MATLAB/Simulink仿真平台中搭建基于改进LADRC的构网型变换器预同步控制模型并进行仿真验证,结果表明所提控制策略可以有效抑制系统频率震荡,并能够加速系统预同步进程,确保构网型变换器安全运行最终实现成功并网,仿真结果验证了所提方法的有效性。

基于动态补偿下垂法的并联逆变器控制研究

提出了一种基于动态补偿下垂法的新型逆变器并联策略,解决了传统并联逆变器下垂控制策略中频率跌落,无功功率不均分,有功、无功计算结果不准确等问题。该策略包括在功率环中采用CSOGI-QSG模块滤除直流干扰;在传统有功控制环中引入自适应频率反馈策略补偿频率跌落;在无功控制环中引入虚拟电感改善无功功率的分配,并采用自适应电压补偿策略改善母线电压跌落。通过搭建MATLAB仿真和实验平台验证,证明了该控制策略可以显著改善并联逆变器中的功率均分效果。

基于虚拟阻抗VPI谐振控制器的MMC环流抑制策略

针对MMC相间环流包含的二倍频以及偶次高倍频问题,该文提出了基于虚拟阻抗VPI谐振控制器的新型环流抑制策略,该策略通过虚拟阻抗与VPI谐振控制器结合,从而有效抑制环流中二倍频及偶次高倍频。首先建立了模块化多电平换流器(modular multilevel converter,MMC)的数学模型,分析了内部环流;其次将环流二倍频分量利用陷波器精准提取之后采用VPI谐振控制器进行抑制;最后设计了虚拟阻抗,抑制环流中的高频谐波分量。利用Matlab/Simulink搭建三相MMC仿真模型并进行验证。

电压控制型虚拟同步发电机的高频谐振特性分析与抑制策略

虚拟同步发电机(VSG)模拟同步发电机的运行机制,引入了有功调频和无功调压控制环节,与传统电流控制型变流器不同,是一种电压控制型策略。文中立足电压控制型虚拟同步发电机控制结构和网源协调特性,构建小信号分析模型,分析在不同控制参数与电网参数场景下的振荡模态,进而掌握电压控制型虚拟同步发电机并网系统的高频谐振特性,并针对性提出高频谐振抑制策略。首先,分别建立电压控制型虚拟同步发电机、传统变流器各控制和电气环节的小信号分析模型,通过特征根分析方法,定量分析所有特征根的振荡模态和阻尼比,发现电压控制型虚拟同步发电机存在与传统变流器类似的高频振荡模态;同时分析虚拟同步发电机控制参数变化、电网强弱场景对其高频谐振阻尼作用机理。其次,基于电压控制型虚拟同步发电机控制策略,以有源阻尼方法为解决思路,提出基于电流内环前端通道引入虚拟阻抗控制策略的谐振抑制方法,抑制其高频谐振。最后,利用Matlab/Simulink仿真和RT-LAB控制器在环实验结果验证所提出的控制策略,结果表明虚拟阻抗控制环节起到有效正阻尼作用,相比于无源阻尼方法,在无需额外增加测点和外部控制量的情况下,电压控制型VSG网侧阻尼特性改善,暂态响应能力增强,高频失稳现象得到有效抑制。

多换流器直流系统的虚拟惯性评估与控制参数协调设计

针对基于类虚拟同步发电机(AVSG)的多换流器直流系统遇到的非预期虚拟惯性问题,提出了一种多换流器直流系统虚拟惯性通用分析方法,有效修正了系统级等效AVSG外特性。首先,将多个AVSG控制回路等效为一个等值AVSG控制回路,建立了直流系统的等值单台AVSG模型。其次,提出了一种系统级虚拟惯性评估方法,从等值单台AVSG模型的输出阻抗频域响应特性角度对系统级虚拟惯性进行评估。然后,讨论了电压比例-积分(PI)控制参数对虚拟惯性特性的影响,并开展了电压PI控制参数和虚拟惯性系数间的协调设计。最后,在PLECS仿真软件及RT-BOX硬件在环实验平台上搭建了多换流器直流系统的开关模型,通过多种实验工况验证了等值单台AVSG模型和控制参数协调设计的有效性和可行性。

基于电流源和电压源的储能系统双模式控制策略研究

本文针对储能系统在电网辅助服务(调频)、差价套利、新能源发电功率管理等不同应用场景下的适应性问题,提出了一种基于电流源和电压源的双模式切换策略。该策略采用基于PI的电压和相位预同步补偿及虚拟阻抗软限流策略解决并网时可能造成的系统过流冲击。此外,通过对电压控制积分器进行复位和数据切换,实现了模式间的无缝切换;通过在Simulink仿真软件中搭建储能微网模型,对其在并网运行、离网运行、离并网转换、多机并联等典型工况进行了仿真验证。验证表明,该控制策略能够满足并离网要求,而且在多机并联时具备黑启动能力,启动完成后将自动转入并网运行。