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.

Harmonic Transfer Function Based Single-input Single-output Impedance Modeling of LCC-HVDC Systems

This study presents a harmonic transfer function(HTF)based single-input single-output(SISO)impedance modeling method.The method converts an HTF from phase domain to sequence domain and then transforms it into an SISO impedance while preserving the frequency coupling information of different sequences and different harmonics.Applications of this method to a line-commutated converter based high-voltage direct current(LCC-HVDC)system are presented.The results demonstrate the accuracy of the derived SISO impedance,and a truncation-order selection is suggested.The case study shows that the proposed method facilitates simpler impedance measurements and associated stability analysis.

Equivalent Impedance Parameter Calculation of Three-phase Symmetrical Loads for Harmonic Source Location

The equivalent impedance parameters of loads have been widely used to identify and locate the harmonic sources.However,the existing calculation methods suffer from outliers caused by the zero-crossing of the denominator.These outliers can result in inaccuracy and unreliability of harmonic source location.To address this issue,this paper proposes an innovative method of equivalent impedance parameter calculation of three-phase symmetrical loads that avoid outliers.The correctness and effectiveness of the proposed method are verified by simulations on Simulink using actual monitoring data.The results show that the proposed method is not only simple and easy to implement but also highly accurate.

Joint torque-based Cartesian impedance control with friction compensations

In order to investigate the joint torque-based Cartesian impedance control strategies and the influence of compensations for friction, an experimental study on the identification of friction parameters, friction compensation and the Cartesian impedance control are developed for the harmonic drive robot, by using the sensors available in the joint itself. Different from the conventional Cartesian impedance control schemes which are mostly based on the robot end force/torque information, five joint torque-based Cartesian impedance control schemes are considered, including the force-based schemes in Cartesian/joint space, the position-based schemes in Cartesian/joint space and the stiffness control. Four of them are verified by corresponding experiments with/without friction compensations. By comparison, it is found that the force-based impedance control strategy is more suitable than the position-based one for the robot based on joint torque feedback and the friction has even a positive effect on Cartesian impedance control stability.

Model tests and numerical analyses on horizontal impedance functions of inclined single piles embedded in cohesionless soil

Horizontal impedance functions of inclined single piles are measured experimentally for model soil-pile systems with both the effects of local soil nonlinearity and resonant characteristics.Two practical pile inclinations of 5掳 and 10掳 in addition to a vertical pile embedded in cohesionless soil and subjected to lateral harmonic pile head loadings for a wide range of frequencies are considered.Results obtained with low-to-high amplitude of lateral loadings on model soil-pile systems encased in a laminar shear box show that the local nonlinearities have a profound impact on the horizontal impedance functions of piles.Horizontal impedance functions of inclined piles are found to be smaller than the vertical pile and the values decrease as the angle of pile inclination increases.Distinct values of horizontal impedance functions are obtained for the ＇positive＇ and ＇negative＇ cycles of harmonic loadings,leading to asymmetric force-displacement relationships for the inclined piles.Validation of these experimental results is carried out through three-dimensional nonlinear finite element analyses,and the results from the numerical models are in good agreement with the experimental data.Sensitivity analyses conducted on the numerical models suggest that the consideration of local nonlinearity at the vicinity of the soil-pile interface influence the response of the soil-pile systems.

Microcontroller-Based Sinusoidal Voltage Generation for Electrical Bio-Impedance Spectroscopy Applications

A sinusoidal voltage wave generator is proposed based on the use of micro-processor digital signals with programmable duty-cycles, with application to real-time Electrical Cell-substrate Impedance Spectroscopy (ECIS) assays in cell cultures. The working principle relies on the time convolution of the programmed microcontroller (μC) digital signals. The expected frequency is easily tuned on the bio-impedance spectroscopy range [100 Hz, 1 MHz] thanks to the μC clock frequency selection. This system has been simulated and tested on the 8 bits μC ArduinoTM Uno with ATmega328 version. Results obtained prove that only three digital signals are required to fit the general specification in ECIS experiments, below 1% THD accuracy, and show the appropriateness of the system for the real-time monitoring of this type of biological experiments.

Coordinated Control for Harmonic Mitigation of Parallel Voltage-source Inverters

The increasing use of power electronic devices can deteriorate the power quality by introducing voltage and current harmonics.In islanded microgrids,the presence of nonlinear loads can distort the point of common coupling(PCC)voltage,while the dead-time effect can also bring additional circulating current harmonics among parallel inverters.To simultaneously attenuate the PCC voltage harmonics and suppress the dead-time induced circulating current harmonics,this paper proposes a coordinated control strategy for harmonic mitigation of parallel inverters.The proposed control strategy allows inverter impedances to be properly reshaped at selective harmonic frequencies.As a consequence,the PCC voltage harmonics are filtered by the inverter operating in the harmonic compensation mode(HCM),whereas the dead-time induced circulating current harmonics are suppressed by the inverter operating in the harmonic rejection mode(HRM).Experimental results from an islanded microgrid prototype with two parallel inverters are provided to validate the effectiveness of the proposed control strategy.

Nonlinear Magnetoimpedance Effect in FeCoNi Ferromagnetic Tubes

The very high (up to 820% of the magnetoimpedance ratio) and sensitive nonlinear giant magnetoimpedance effect has been studied in the FeCo1Ni magnetic tubes electroplated onto Cu(3%)Be nonmagnetic wirefor frequencies from 1-10MHz. Special annealing was carried out in order to induce the magnetic anisotropy. The high harmonic generation was observed and the harmonics show larger variations with the external magnetic field than the fundamental frequency. The super high sensitivity of the harmonics is promising as regards the increase of the sensitivity of magnetoimpedance sensors.

Influence of the Electrical Vehicle Charging on the Grid Harmonic

This paper described the impact of the electrical vehicle（EV） charging on the grid harmonic. In view of the randomness of the EV charging process, the harmonic admittance matrix method and superposition method were used to build the single and multiple EVs charging simulation model. By using Matlab as a simulation tool, we analyzed harmonic currents of single and multiple EVs chargers. The results show that the harmonic ratio is beyond the scope of the national harmonic standard. Finally a parallel hybrid active filter（PHAPF） was introduced for governance of harmonic.The experimental results show that net side harmonic currents are significantly reduced by using the PHAPF and meet the national standard GB/Z17625.6-2003 regulations limit.

Compensation of Unbalanced Impedance of Asymmetric Wind Power PMSG Compensated by External Circuits in Series

The unbalanced impedance of the asymmetric 3-phase wind power permanent magnet synchronous generator(PMSG)compensated by external circuits in series with the 3-phase windings is investigated in this paper.The asymmetric impedance includes the unbalanced resistances,unbalanced self-inductances,and unbalanced mutual inductances.From the perspective of the second harmonic inductances in dq-frame and from the perspective of the second harmonic power,it is theoretically demonstrated that the original asymmetric 3-phase system with asymmetric impedance can be modified to a balanced system by external circuits consisting of resistances and inductances.Therefore,the second harmonic power and DC bus voltage due to the asymmetries can be suppressed naturally without any software modifications.The feasibility of this compensation method is validated by elaborate experiments at different speeds and under different load condition,although the effectiveness might be slightly affected by the non-linearity of the compensation inductance in practice.

基于盲源分离的工业谐波源负荷分类识别方法

针对传统谐波源辨识方法无法实现工业用户谐波源具体类型的非侵入式识别的问题,该文提出一种基于盲源分离的工业谐波源负荷分类识别方法。该方法仅依据工业用户进线处电压、电流数据即可实现谐波源负荷具体类型的非侵入式识别。首先,从负荷等值阻抗模型入手,建立工业用户多负荷等值阻抗并联电路模型;其次,采用集合经验模态分解与奇异值分解相结合的方法确定构成用户进线处监测点综合等值阻抗信号的源阻抗信号数目;然后,采用快速独立分量分析实现将谐波源负荷等值阻抗信号从综合负荷等值阻抗信号中分离;最后,将分离出谐波源负荷等值阻抗信号频率特征与典型谐波源负荷进行匹配,进而实现分类识别。仿真与实测实验结果均表明,所提方法能够准确识别工业用户所含多种谐波源负荷的具体类型,具有较好的可行性和实用性。

基于谐波状态空间理论的12脉动换流站阻抗建模

为分析谐波对电网换相型换流器高压直流输电系统(line commutated converter based high voltage direct current,LCC-HVDC)稳定性的影响,亟需建立一个考虑谐波耦合效应的LCC-HVDC精确模型。基于谐波状态空间(harmonic state space,HSS)理论,考虑频率耦合效应和控制系统建立了12脉动LCC的阻抗模型,所建交直流谐波阻抗模型能在更宽的频带与扫频结果吻合。最后通过PSCAD电磁暂态仿真结果与HSS阻抗模型计算结果对比,验证了所提出的LCC-HSS阻抗模型的正确性。LCC-HSS阻抗建模方法提高了对LCC换流站进行数学建模的精确性,且可以适应多种模式下LCC换流站阻抗建模,为LCC系统稳定性分析及参数优化提供了较为精确的模型。

带自适应谐波补偿功能的谐波阻抗控制策略

针对工业园区高比例电力电子负荷接入带来的电能质量问题,提出一种基于APF的带自适应谐波补偿限流功能的谐波阻抗控制策略。谐波阻抗控制策略可以对系统的指令电流进行无差控制并对各谐波分量进行独立控制,自适应谐波补偿限流可以有针对性地进行谐波补偿,两者相结合后可以对选取的各谐波分量进行单独控制,使限流后的谐波补偿效果明显提高。该控制策略在系统容量不超限时在Matlab/Simulink中搭建仿真模型,结果表明,该控制策略响应速度快,治理后的谐波总畸变率为2.91%,减少了27.08%,三相不平衡度为8.52%,减少了16.15%,补偿效果明显。

并网逆变器整体序阻抗建模方法及其稳定性分析

在高比例新能源电力系统中,并网逆变器与电网的动态交互作用使得系统谐波谐振问题凸显且日趋复杂。以三相平衡系统为研究对象,研究abc坐标系下考虑正负序频率耦合以及逆变器与电网阻抗交互影响的并网系统整体序阻抗建模方法,以避免行列式计算,并率先提出并网系统整体阻抗测量方法;进而在整体阻抗模型基础上,提出一种频域阻抗稳定性判据和振荡频率预测方法,可建立频域阻抗判据与并网系统物理环节的联系,以大幅降低频域阻抗判据误判的风险;最后,通过MATLAB/Simulink仿真扫频验证了系统整体阻抗模型的正确性,并通过RTLAB硬件在环实验证明其可用于新能源并网系统的稳定性判断,并具有谐振频率预测精度高的优点。

基于多电平滞环电流控制的牵引供电系统可控宽频带阻抗测量技术

精确获取牵引供电系统的宽频带阻抗特性,是分析车-网耦合系统的宽频带振荡问题的关键途径,对保障电气化铁路的安全稳定运行具有重要意义。目前的阻抗测量技术大多难以兼顾大功率与频带可控的测量要求,因此,该文提出一种基于多电平滞环电流控制的牵引供电系统可控宽频带阻抗测量技术,测量频带为10 Hz~5 kHz,可通过一次谐波扰动注入快速获取目标频带的系统阻抗,相比于传统的滞环电流控制方法,具有更低的开关频率,并且能有效避免死区效应,改善输出电流的波形,更适用于宽频信号的跟踪控制。最后,通过仿真和硬件在环实验验证了该测量技术的有效性。

计及交直流谐波传递的MMC-BTB系统阻抗特性分析及环流配合策略

基于模块化多电平换流器的背靠背系统(back-to-back system based on modular multilevel converter,MMC-BTB)中换流器间控制交互密切,导致振荡事故频发。现有研究中忽略谐波传递的单端模型无法准确刻画MMC-BTB阻抗特性,也较少从振荡抑制层面考虑两换流器间的协同配合。为此,该文考虑MMC-BTB中换流器间谐波传递与衰减机理,依据多种环流控制方式的阻抗等价性提出一种基于环流协调控制的振荡抑制策略。首先,基于等效电路定性分析谐波的流通路径,并定量推导MMC-BTB的双端模型;然后,沿谐波路径对两侧环流控制的交互机理开展分析,证明其对改善MMC-BTB阻抗特性的关键作用,并提出环流控制的协调配合与参数优化策略;最终,通过Matlab/Simulink仿真验证理论分析与所提策略的正确性。

抑制电网背景谐波的变流器导纳重塑方法

电网电压中的背景谐波作用在并网变流器上产生谐波电流,而并网变流器的频率耦合特性还会映射出新的谐波电流成分。为抑制电网背景谐波对并网电流的影响,文中提出改善并网电流质量的导纳重塑方法。首先,为了消除变流器的频率耦合特性,减少并网电流谐波成分,构造出内、外环导纳重塑因子的一般形式,将并网变流器重塑为dq坐标系下的对称系统;然后,从减少谐波含量出发,对导纳重塑因子进行合适选择,通过降低变流器导纳值来改善并网电流质量,实现消除变流器频率耦合特性的同时抑制电网背景谐波的目的;此外,变流器序导纳模值减小,使得其与电网阻抗构成的回路比曲线更难在幅相平面包围(-1,0)点,有益于并网系统的稳定。仿真和实验均验证了所提导纳重塑方法的可行性和有效性。

基于光伏逆变器的电网谐波阻抗测量新技术

针对新型电力系统背景下电网谐波阻抗的测量需求,提出了一种基于光伏逆变器的新型电网谐波阻抗测量方法。所提方法不会对电网产生扰动,无需安装额外的测量设备,并且还能在阻抗测量过程中治理电网谐波。该方法在基波下控制光伏逆变器实现正常发电功能,在谐波下控制其等效为虚拟电阻。通过改变逆变器的等效虚拟谐波电阻值,测量并网点谐波电压的幅值,并结合最小二乘法来估算系统的谐波阻抗幅值。利用硬件在环实时仿真试验平台进行阻抗测量试验,测量结果与设定值相符,系统5次谐波阻抗的测量精度达99%,7次谐波阻抗的测量精度达97%,验证了所提方法不仅可测量系统多次谐波阻抗幅值,且能保证较高的测量精度。

基于系统谐波阻抗控制的新型有源电力滤波器

针对传统有源电力滤波器滤波效果受谐波电流检测精度影响,导致滤波不彻底的问题,提出了一种基于系统谐波阻抗控制原理的新型有源电力滤波器。滤波器的第1部分为串联在电网中的单相变压器二次侧接逆变器,第2部分为单相变压器和负载之间并联的无源滤波器。通过控制使串联变压器的谐波等效阻抗很大,无源滤波器的谐波等效阻抗相对很小,由分流原理可知此时各次谐波电流将被迫流入无源滤波器支路,从而实现了各次谐波的滤除。为实现系统谐波阻抗的控制,设计了无源滤波器参数和逆变器控制参数。最后,在Matlab/Simulink中搭建了仿真模型,结果表明可将系统侧电流总谐波畸变率由29.37%降低到0.69%,满足国家谐波标准。该研究在谐波抑制方面具有一定的应用前景。

一种提高虚拟同步机电流质量的电压-电流级联闭环控制方案

当虚拟同步机(VSG)接入畸变电网时,其输出电流质量恶化。现有基于公共耦合点(PCC)电压采样的各种谐波治理方案受限于线路阻抗的大小而抑制效果不佳。为此,该文提出一种电压-电流级联闭环控制策略实现了VSG基波电压和线路谐波电流的协调闭环控制。在基波频率下,VSG跟随功率控制回路的基波电压指令实现功率传输;在谐波频率下,通过单个一阶复矢量滤波器对谐波电流总和进行整体提取,并采用闭环输出谐波电压补偿的参考量,从而等效增强了谐波电压补偿量的参考信号,大幅提升了VSG输出电流的谐波抑制效果。相比于现有方案,该方法克服了由于线路阻抗过低而抑制效果不佳的问题,能在不增加额外电感器件的条件下提高VSG在畸变电网下的输出电流质量。最后,该文搭建15 kW的样机进行实验,并与现有的三类方法进行对比,较为全面地验证了该方法的有效性且增强了谐波的抑制效果。