DC microgrid stability control with constant power load:a review

The DC microgrid has the advantages of high energy conversion efficiency,high energy transmission density,no reactive power flow,and grid-connected synchronization.It is an essential component of the future intelligent power distribution system.Constant power load(CPL)will degrade the stability of the DC microgrid and cause system voltage oscillation due to its negative resistance characteristics.As a result,the stability of DC microgrids with CPL has become a problem.At present,the research on the stability of DC microgrid is mainly focused on unipolar DC microgrid,while the research on bipolar DC microgrid lacks systematic discussion.The stability of DC microgrid using CPL was studied first,and then the current stability criteria of DC microgrid were summarized,and its research trend was analyzed.On this basis,aiming at the stability problem caused by CPL,the existing control methods were summarized from the perspective of source converter output impedance and load converter input impedance,and the current control methods were outlined as active and passive control methods.Lastly,the research path of bipolar DC microgrid stability with CPL was prospected.

Robust Nonlinear Current Sensorless Control of the Boost Converter with Constant Power Load

The boost converter feeding a constant power load (CPL) is a non-minimum phase system that is prone to the destabilizing effects of the negative incremental resistance of the CPL and presents a major challenge in the design of stabilizing controllers. A PWM-based current-sensorless robust sliding mode controller is developed that requires only the measurement of the output voltage. An extended state observer is developed to estimate a lumped uncertainty signal that comprises the uncertain load power and the input voltage, the converter parasitics, the component uncertainties and the estimation of the derivative of the output voltage needed in the implementation of the controller. A linear sliding surface is used to derive the controller, which is simple in its design and yet exhibits excellent features in terms of robustness to external disturbances, parameter uncertainties, and parasitics despite the absence of the inductor’s current feedback. The robustness of the controller is validated by computer simulations.

A Computational Study on the Discharge Characteristics of Atmospheric Dielectric Barrier Discharges at a Constant Power

The study on homogeneous DBDs at atmospheric pressure has attracted much attention for their advantages in applications. Tremendous work has been conducted both experimentally and numerically at a constant applied voltage or driving frequency. However the investigation of dielectric barrier discharges is still scarce for a constant power or power density. In this work, a new computational approach for DBDs is developed to explore atmospheric DBDs at a constant power based on a one-dimensional fluid model. The frequency and gap spacing effects on the atmospheric plasmas are systematically analyzed based on computational data. The computational results show that at a constant power both the current density and the amplitude of the applied voltage decrease, whereas the current pulse width increases, with increasing frequency. The simulation also indicates that as the gap spacing is raised with a fixed power and frequency, the current density and electron density increase initially, then reach their peak values, and then decrease, which means that there are maximum values for both of them. These results are significant for many industrial applications, as they can be used to optimize plasma devices of DBDs with the consideration of power consumption.

State-of-charge Balance Control and Safe Region Analysis for Distributed Energy Storage Systems with Constant Power Loads

This paper presents a fully distributed state-of-charge balance control (DSBC) strategy for a distributed energy storage system (DESS). In this framework, each energy storage unit (ESU) processes the state-of-charge (SoC) information from its neighbors locally and adjusts the virtual impedance of the droop controller in real-time to change the current sharing. It is shown that the SoC balance of all ESUs can be achieved. Due to virtual impedance, voltage deviation of the bus occurs inevitably and increases with load power. Meanwhile, widespread of the constant power load (CPL) in the power system may cause instability. To ensure reliable operation of DESS under the proposed DSBC, the concept of the safe region is put forward. Within the safe region, DESS is stable and voltage deviation is acceptable. The boundary conditions of the safe region are derived from the equivalent model of DESS, in which stability is analyzed in terms of modified Brayton-Moser's criterion. Both simulations and hardware experiments verify the accuracy of the safe region and effectiveness of the proposed DSBC strategy.

Active stabilization methods of electric power systems with constant power loads:a review

Modern electric power systems have increased the usage of switching power converters.These tightly regulated switching power converters behave as constant power loads(CPLs).They exhibit a negative incremental impedance in small signal analysis.This negative impedance degrades the stability margin of the interaction between CPLs and their feeders,which is known as the negative impedance instability problem.The feeder can be an LC input filter or an upstream switching converter.Active damping methods are preferred for the stabilization of the system.This is due to their higher power efficiency over passive damping methods.Based on different sources of damping effect,this paper summarizes and classifies existing active damping methods into three categories.The paper further analyzes and compares the advantages and disadvantages of each active damping method.

Optimal and constant power allocation for joint transmission in HetNet

This paper investigates the power allocation issues for joint transmission in heterogeneous network （HetNet）, which is characterized by severe cross-tier interference. The optimization problem of maximizing the HetNet throughput is formulated. The original problem turns out to be a non-convex problem, the global optima of which cannot be obtained by conventional optimization methods. This paper develops a novel method to achieve the global optima by tuming the original problem into quasi-convex problem. In addition, this paper considers a constant power allocation scheme, as a tradeoff between the system throughput and computational complexity. Based on duality gap theory, the bound of constant power allocation scheme is mathematically derived. Numerical results under different system parameters indicate that both the proposed schemes outperform conventional interference coordination schemes.

Numerical Simulation and Analysis of Power Consumption and Metzner-Otto Constant for Impeller of 6PBT

Majority of non-Newtonian fluids are pseudoplastic with shear-thinning property, which means that the viscosity will be different in different parts of the stirred tank. In such mixing process, it is difficult to predict accurately the power consumption and mean shear rate for designing novel impeller. Metzner-Otto method is a widely accepted method to solve these questions in mixing non-Newtonian fluids. As a result, Metzner-Otto constant will become a key factor to achieve an optimum way of economical mixing. In this paper, taking glycerine and xanthan gum solutions as research system, the power consumption, stirred by the impeller composed of perturbed six-bent-bladed turbine （6PBT） with differently geometrical characteristics in a cylindrical vessel, is studied by means of computational fluid dynamics （CFD）. The flow is modeled as laminar and a multiple reference frame （MRF） approach is used to solve the discretized equations of motion. In order to determine the capability of CFD to forecast the flow process, the torque test experiment is used to measure the glycerine solution power consumption. The theological properties of the xanthan gum solutions are determined by a Brookfleld rheometer. It is observed that the power consumption predicted by numerical simulation agrees well with those measured using torque experiment method in stirring glycerine solution, which validate the numerical model. Metzner-Otto constant is almost not correlated with the flow behavior index of pseudoplastic fluids. This paper establishes the complete correlations of power constant and Metzner-Otto constant with impeller geometrical characteristics through linear regression analysis, which provides the valuable instructions and references for accurately predicting the power consumption and mean shear rate of pseudoplastic fluids in laminar flow, comparatively.

Effect of lens constants optimization on the accuracy of intraocular lens power calculation formulas for highly myopic eyes

AIM: To evaluate the effect of different lens constant optimization methods on the accuracy of intraocular lens(IOL) power calculation formulas for highly myopic eyes.METHODS: This study comprised 108 eyes of 94 consecutive patients with axial length(AL) over 26 mm undergoing phacoemulsification and implantation of a Rayner(Hove, UK) 920H IOL. Formulas were evaluated using the following lens constants: manufacturer’s lens constant, User Group for Laser Interference Biometry(ULIB) constant, and optimized constant for long eyes. Results were compared with Barrett Universal II formula, original Wang-Koch AL adjustment method, and modified Wang-Koch AL adjustment method. The outcomes assessed were mean absolute error(MAE) and percentage of eyes with IOL prediction errors within ±0.25, ±0.50, and ±1.0 diopter(D). The nonparametric method, Friedman test, was used to compare MAE performance among constants.RESULTS: Optimized constants could significantly reduce the MAE of SRK/T, Hoffer Q, and Holladay 1 formulas compared with manufacturer’s lens constant, whereas the percentage of eyes with IOL prediction errors within ±0.25, ±0.50, and ±1.0 D had no statistically significant differences. Optimized lens constant for long eyes alone showed non-significant refractive advantages over the ULIB constant. Barrett Universal II formula and formulas with AL adjustment showed significantly higher accuracy in highly myopic eyes(P<0.001). CONCLUSION: Lens constant optimization for the subset of long eyes reduces the refractive error only to a limited extent for highly myopic eyes.

Validity and Reproducibility of the Garmin Vector Power Meter When Compared to the SRM Device

Mobile power meters allow for cyclists to monitor power output （PO） during training and competition. The Garrnin Vector power meter （VPM） measures PO at the pedal compared to the crank and has been tested in only a few limited studies. The purpose of this study was to determine the validity and reproducibility of the VPM by comparing it to the SRM. The VPM validity was tested by （1） a submaximal incremental test, （2） submaximal constant power test, （3） sprint test, and （4） a field test. The reliability of the VPM was tested by repeating the laboratory tests 10 times over a 6 week span. Significant differences （P = 0.046） were found between the mean POSRM （178 ± 1.8 W） and POVPM （163.5 ± 14.7 W） for the submaximal constant-power test. No significant differences were found between the POMAX SRM and the POMAx VPM. The reproducibility of the VPM was lower than the SRM （CV = 8.52 ±4.0 vs 3.48 ± 1.9, 10.66% vs 5.50%, and 67.7% vs 55.3% for the submaximal incremental test, submaximal constant-power test, and field test respectively）. The POVPM appears to underestimate the POSRM and is less valid and reliable across various cycling efforts.

A Constant-bandwidth MPPT Strategy for Variable-speed WECS

对变速风力发电机组在额定风速以下实施最大功率跟踪控制有利于提高机组的发电效率，进而提高机组的年发电量。目前广泛运用的最大功率跟踪控制方法为功率反馈最大功率跟踪策略。该文从最大功率跟踪的跟踪带宽角度出发，研究发现功率反馈最大功率跟踪控制策略存在跟踪带宽低，且随着风速的减小而进一步恶化的缺点。之后，针对该问题，提出一种恒定带宽的最大功率跟踪控制方法，该方法通过在功率反馈最大功率跟踪控制策略中引入误差功率前馈信号，实现了最大功率跟踪带宽恒定的目的。在实验室内建立了一套1．2kW的变速风力发电实验系统，实验结果验证了所提方法的正确性和可行性。

抽油机井恒功率变频控制系统研究

抽油机作为陆上油田最重要的机械采油装备之一,近年来已发展出一系列的变频节能技术,但其研究主要集中在硬件方面,缺乏相关基础理论,导致现场应用普适性差,应用效果参差不齐。为此,研究有杆泵变频测控机理,优选了主要部件,对测试数据提出了要求,形成了有杆泵抽油变频控制系统。构建BP神经网络系统对正常井和漏失、柱塞碰泵、气体影响等异常井进行识别,分类提出变频计算模型:考虑负扭矩和抽油机冲次界限等建立了正常井恒功率变频计算模型,实例对比分析了变频后抽油机速度、加速度变化规律,产液量一定时,负功功率和有功功率分别下降了65%和44%。建立了上行程漏失的异常井变频计算模型,针对性地提高上行程冲次,形成“上快下慢”运行模式。实例计算表明:该系统缩短上行程时间2.6 s,较好地解决有杆泵上行程漏失和变频节能的问题。研究成果可为抽油机变频技术提供理论支持,对油田节能和提产具有指导意义。

微波含水率检测技术在复杂油水混合形态下的应用研究

在油水混合物中会出现不同的乳化形态,不同的乳化形态对应的电导率和介电常数也具有显著差异,从而导致通过检测电导率和介电常数值间接测量含水率的测量结果不准确。针对复杂油水混合状态下含水率难以测准的问题,提出一种基于微波技术的复杂油水两相流含水率检测技术,建立了基于微波技术的“油包水”和“水包油”状态分析方法。首先通过检测微波信号穿过不同介质后信号幅值的衰减和相位的偏移,推测油水混合物的含水率值。再利用不同输入功率的微波信号在不同介电常数下功率衰减比不同的原理,来区分油水混合物的乳化状态,优化含水率的测量。室内验证和矿场试验结果表明,该系统可有效区分乳化液的混合状态,通过含水率标定值对实测数据进行修正,最终测量的含水率误差可控制在1个百分点以内,实现了复杂油水混合形态下含水率的高精度测量。

基于功率恒定的防饱和在线取电方法

基于厄流线圈取电装置提出了一种防止铁磁磁心饱和的方法,该方法使用旁路泄流电路将流入电流互感器过多的电流引入到旁路电路中消耗掉,将输出功率作为反馈值来调节旁路电路中开关管的关断,通过控制输出的功率大小,输电线路在宽范围电流内实现恒功率输出和防止电流互感器磁心到达饱和。根据电流互感器各方面的要求,建立电流互感器3D模型,将3D模型导入到Maxwell中对电流互感器本身进行有限元仿真分析,得到电流互感器内部各项参数,再将经过有限元分析的电流互感器模型导入到ANSYS中与外部电路组成取电电路进行联合仿真,通过仿真结果验证电流互感器的取电能力。再加入本次对电流互感器取电优化部分,通过电流互感器二次侧谐振的方式增加一次侧输电线路的等效阻抗,考虑到电流互感器磁心饱和的问题,采用旁路泄流电路将过多的电流通过旁路绕过电流互感器一次侧回到输电线路当中的方法解决。

自动化视角下长纤磨浆机PLC控制系统设计研究

与加工木浆、草浆等短纤的磨浆机相比,以亚麻纤维、棉絮等为主要加工材料的长纤磨浆机天然具有较为苛刻的生产条件。为提升长纤磨浆机的设备适应性,基于自动化视角设计了一种针对长纤磨浆机的PLC控制系统。该系统在传统PLC控制系统的基础上增设了一个脉冲控制器,可以帮助磨浆机从启动状态快速进入磨浆状态,以及从短纤磨浆状态快速切换为长纤磨浆状态,与传统磨浆机相比在工作效率、设备适应性等方面具有更为显著的优势。

Constant envelope FrFT OFDM: spectral and energy efficiency analysis

Constant envelope with a fractional Fourier transformorthogonal frequency division multiplexing(CE-FrFT-OFDM)is a special case of a constant envelope OFDM(CE-OFDM),both being energy efficient wireless communication techniques with a 0 dB peak to average power ratio(PAPR).However,with the proper selection of fractional order,the first technique has a high bit error rate(BER)performance in the frequency-time selective channels.This paper performs further analysis of CE-FrFT-OFDM by examining its spectral efficiency(SE)and energy efficiency(EE)and compare to the famous OFDM and FrFT-OFDM techniques.Analytical and comprehensive simulations conducted show that,the CE-FrFT-OFDM has five times the EE of OFDM and FrFT-OFDM systems with a slightly less SE.Increasing CE-FrFT-OFDM’s transmission power by increasing its amplitude to 1.7 increases its SE to match that of the OFDM and FrFT-OFDM systems while slightly reducing its EE by 20%to be four times that of OFDM and FrFTOFDM systems.OFDM and FrFT-OFDM’s amplitude fluctuations cause rapid changing output back-off(OBO)power requirements and further reduce power amplifier(PA)efficiency while CE-FrFTOFDM stable operational linear range makes it a better candidate and outperforms the other techniques when their OBO exceeds 1.7.Higher EE and low BER in time-frequency selective channel are attracting features for CE-FrFT-OFDM deployment in mobile devices.

基于恒功率约束的换挡毂数控铣削加工控制系统设计与实现

【目的】汽车双离合变速箱(Double Clutch Transmission,DCT)的核心零部件换挡毂加工制造精度要求高,针对数控铣削工艺的非线性问题,通过优化切削参数,以提高生产效率及延长刀具寿命。【方法】以主轴负载恒功率为约束条件,设计一种基于模糊控制算法的铣削加工控制系统。采用CP1E N30作为主控制器、CX-programmer开发下位机程序、Kingview开发上位机系统,完成基于主轴功率信号反馈的智能控制器的开发。【结果】在XHK716加工中心机床上实现铣削工艺的恒功率控制,通过换挡毂粗铣B平面工艺过程的应用,可知实时功率保持在目标功率附近。【结论】该控制系统提高了铣削加工效率,实现了保护刀具的目标,对延长刀具寿命有较好的参考价值。

基于谐波通讯的无线电能与反向信号同步传输技术

针对现有无线电能与反向信号同步传输(simultaneous wireless power and reverse signal transmission,SWPRST)系统存在较大无功功率、负载电压易受信号传输发生波动或需要额外增加高频信号源等问题,提出一种基于谐波通讯的SWPRST技术,通过利用逆变器输出方波电压中的基波分量传输电能,三次谐波分量传输信号。不需要外加高频信号发射电路,实现了可靠的电能与反向信号同步传输。首先,给出基于谐波通讯的SWPRST系统结构,对其工作模式和基本原理进行分析;接着,建立系统等效数学模型,分析系统参数取值对信号与电能传输之间的互扰影响;然后,对信号的调制解调电路进行设计,分析信号检测通道参数对信号传输速率的影响;最后,搭建实验平台对理论分析进行验证,实验结果表明,该方法在有效实现了无线电能与反向信号同步传输的同时,信号无误码率传输速率可达5 kbps,同时系统具有无功小,输出负载电压几乎无波动(电压波动率0.33%)等优点。该方法采用谐波作为信号载体,为多频利用式实现电能与反向信号同步传输系统提供一种新的思路,具有较好的理论意义与实际工程应用价值。

基于偏航机动的常推力定点着陆制导方法

实现定点着陆终端位置速度六分量控制通常需要发动机推力可调。提出了一种通过偏航机动实现固定推力下的定点着陆制导方法。该方法通过偏转航向角,调节发动机输出在下降轨迹平面内的分量,从而实现航程控制;为了消除推力方向偏转产生的横向位置、速度偏差,设计了偏转角符号调整策略,使得着陆器在水平面内呈“之”字形运动,以一定推进剂为代价,最终保证了下降着陆终端位置、速度的全部可控。仿真表明,所提方法简单有效,易于工程实施。

光伏发电系统的改进型快速GMPPT算法

为提高局部阴影条件下光伏发电的能量利用率,提出一种改进型快速全局最大功率点跟踪(global maximum power point tracking,GMPPT)算法.首先,研究局部阴影条件下光伏阵列的输出特性,并根据光伏阵列输出曲线中膝点与开路电压的关系,将其划分为恒流区和恒压区;其次,分析传统的最大功率梯形(maximum power trapezium,MPT)算法和以MPT算法为基础的改进型快速GMPPT算法的工作原理,改进型快速GMPPT算法利用电压的动态上、下限来限定搜索区间,并跳过调整时间较长的恒流区,以提高跟踪速度;最后,通过仿真与实验验证算法的有效性.实验结果表明:改进型快速GMPPT算法的最短跟踪时间为4.0 s,扫描电压与能量损失分别为17.34 V和98.19 J;与传统全局扫描算法相比,跟踪时间缩短68.25%,扫描电压降低74.86%,能量损失减少58.19%;与MPT算法相比,跟踪时间缩短68.00%,扫描电压降低75.63%,能量损失减少62.31%.

基于LCL-S型两接收端无线电能传输系统传输特性研究

随着WPT应用的发展,多接收WPT系统因一个发射端可以为多个接收端同时供电,近年来受到广泛关注。对LCL-S型两接收端WPT系统的功率分配进行研究,并对LCL-S谐振补偿电路的两接收端WPT系统的传输特性进行分析,提出一种双PI功率控制,实现了发射电源恒功率控制。同时,通过移相控制调节各接收端的输出功率来实现按预设比例进行功率分配。仿真实验验证了所提出的方法实现恒功率输出和按预设比例分配接收端电能的可行性和有效性。