A three-dimensional numerical calculation of the wind-driven thermohaline and tide-induced Lagrangian residual current in the Bohai Sea

On the basis of a three-dimensional weakly nonliear theory of Lagrangian residual current in the Baroclinic shallow seas, a diagnostic numerical calculation of wind-driven, thermohaline and tide-induced Lagrangian residual current in the Bohai Sea is made. The model involves the Richardson number in the eddy viscosity coefficient, wind, thcrmolialine and tidal effects in the focing terms. The runoff of the Huanghe River and a part of the Huanghai Warm Water coming from the Huanghai Sea through the Bohai Sea Strait is also considered. The velocity-splitting method is adopted. The wind-driven circu lation, thermohaline circulation and the tide-induced Lagrangian residual circulation are also obtained individually and analysed. The dynamics of the three main eddies in the Lagrangian mean circulation is discussed. Finally, the numerical result is partly verified with the observed data.

New Method for 3D Transient Eddy Current Field Calculation and Its Application in Magneto-Acoustic Tomography

A new method of 3D transient eddy current field calculation is proposed. The Maxwell equations with time component elimination （METCE） are derived under the assumption of magnetic quasi static approximation, especially for the sample of low conductivity. Based on METCE, we deduce a more efficient reconstruction algorithm of a 3D transient eddy current field. The computational burden is greatly reduced through the new algorithm, and the computational efficiency is improved. This new algorithm decompounds the space-time variables into two individual variables. The idea is to solve the spatial vector component firstly, and then multiply it by the corresponded time component. The iterative methods based on METCE are introduced to recover the distribution of conductivity in magneto-acoustic tomography. The reconstructed images of conductivity are consistent with the original distribution, which validate the new method.

Calculation of currents in Taiwan Strait during summerⅠ.Three-dimensional diagnostic calculation

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Numerical calculation of drift current in wind waves

-Drift current induced by wind and waves is investigated with phase-averaged Navier-Stokes equation in which the Reynolds stress is closed by k-ε model. The governing equations are solved by the finite volume method in a system of nonorthogonal coordinates which is fitted to the phase-averaged wave surface. The predicted drift current is fairly reasonable and the drag coefficient of sea-surface predicted with the newly developed interface conditions shows good agreement with previous measurements when breaking waves do not exist.

An Analysis of Observed Ocean Currents Data and Numerical Simulation in the Offshore Area of Jiaonan

Based on the diurnal consecutively observed data in the offshore area of Jiaonan in 2005, the paper tries to make a preliminary analysis of the specificity of ocean currents, tidal current property and residual current property in the area in observing dates. Then on the basis of observed data analysis and by employing the split-step method, the paper conducts a numerical simulation of the tidal current field, which can show the M2 tidal constituent tidal wave system, current ellipse distribution, maximum current velocity distribution and time-dependent current field. The calculated results agree well with the observed data, which can on the one hand reflect the basic specificities of temporal and spatial distribution of the M2 tidal constituent current field to some extent, and, on the other hand, offer more information about the hydrodynamic condition. So the paper would provide a scientific basis for the making of sea environment protection plans in the offshore area of Jiaonan under certain conditions.

基于三有源桥串并联潮流控制器的双极直流配电网主动限流策略

直流潮流控制器是解决环网式直流配电网的线路潮流不完全可控的有效技术手段。然而,现有方法未能充分发掘其在故障限流中的潜力。该文建立了三有源桥串并联潮流控制器(triple active bridge power flow controller,TAB-PFC)的故障模量分析模型,提出一种基于TAB-PFC的双极直流配电网主动限流策略。首先阐述了TAB-PFC的限流原理,提出基于TAB-PFC的主动限流控制策略。然后对TAB-PFC不同故障阶段进行建模,并计及极间互感构建含TAB-PFC的双极直流配电网故障模量等效模型。在此基础上,分析不同参数对TAB-PFC的限流能力的影响,为其参数选取提供依据。在MATLAB/Simulink搭建了含TAB-PFC的双极直流配电网模型,验证了所提主动限流策略的有效性及故障等效电路模型和参数分析的正确性。

Asymmetrical Fault Current Calculation Method and Influencing Factors Analysis of Droop-Controlled Inverters

Since the fault dynamic of droop-controlled inverter is different from synchronous generators (SGs), protection devices may become invalid, and the fault overcurrent may damage power electronic devices and threaten the safety of the microgrid. Therefore, it is imperative to conduct a comprehensive fault analysis of the inverter to guide the design of protection schemes. However, due to the complexity of droop control strategy, existing literatures have simplified asymmetric fault analysis of droop-controlled inverters to varying degrees. Therefore, accurate fault analysis of a droop-controlled inverter is needed. In this paper, by analyzing the control system, an accurate fault model is established. Based on this, a calculation method for instantaneous asymmetrical fault current is proposed. In addition, the current components and current characteristics are analyzed. It was determined that fault currents are affected by control loops, fault types, fault distance and nonlinear limiters. In particular, the influences of limiters on the fault model, fault current calculation and fault current characteristics were analyzed. Through detailed analysis, it was found that dynamics of the control loop cannot be ignored, the fault type and fault distance determine fault current level, and part of the limiters will totally change the fault current trend. Finally, calculation and experimental results verify the correctness of the proposed method.

基于末端机组电压前推的逆变型新能源发电场站故障等值计算

场站级的故障等值建模与分析方法是新能源集中并网地区的保护原理设计与定值计算研究的基础。传统的场站等值建模多采用单机倍乘和分群聚类算法,选取特征参数等值最终将场站模型简化,或者基于机组控制策略采用迭代算法进行求解,存在等值参数误差和计算不收敛的问题。因此,该文首先分析逆变型电源故障后电流的控制策略;然后通过两点迭代的方法求取线路电压与机组输出电流的对应曲线,进而通过遍历末端机组电压,运用前推的方式得到场站并网点电压与电流相量的对应关系,求得逆变型新能源发电场站的等值曲线;最后通过仿真计算及实际短路试验数据验证显示,对于大规模逆变型新能源场站故障稳态的等值计算,所提方法无需复杂迭代计算,计算精度高,与试验录波数据平均等值计算误差为3.28%。

双极短路故障下基于暂态能量抑制的MMC-HVDC电网限流设备的参数优化

模块化多电平换流器高压直流输电电网发生直流短路故障时,具有故障电流上升速率快、峰值高等特征,会严重危害电网安全。为了抑制故障电流发展,缓和高压直流断路器开断电流大小与故障切除时间之间的矛盾,需要在直流侧配置限流设备。首先,研究双极短路故障下不同类型限流设备的工作机理,并从暂态能量的角度探讨能量释放源的分布情况、暂态能量的演化特征及限流设备参数对其限流能力的影响;其次,提出基于NSGA-Ⅱ优化算法的故障限流器与直流电抗器参数优化配置计算方法;最后,在PSCAD/EMTDC仿真系统中搭建四端双极柔性直流电网模型,针对不同电气需求的应用场景,设计相应的限流设备参数优化方案。仿真测试结果表明,该优化方法能够正确地配置限流设备的参数,提高限流设备对暂态能量的抑制效率,验证了所提方法的有效性与可靠性。

基于风光相位特性的新能源电网短路电流计算方法

针对传统短路电流计算方法在新能源电网中的局限性,提出了基于风光相位特性的新能源电网短路电流计算方法。构建了风光互补模型,以确定风力发电和光伏发电之间的相位差。通过对直流电网等效模型中的动态元件进行离散化处理,使用梯形积分法进行数值计算。采用改进节点法建立网络方程,并通过迭代计算求解故障支路在不同时刻下的短路电流值。实验结果表明,设计方法误差最高仅为1.4%,说明设计方法在新能源电网短路电流计算方面具有更高的准确性和实用性。

Wave Transformation and Breaking in Diagonal Opposing Current

When waves propagate into diagonal opposing current from non-current area, not only the wave parameters but also the direction of wave propagation will change, that is, wave refraction will occur. The authors have calculated the changes of wave parameters, including wave refraction, by Linear Wave Theory, and have also done systematic study on wave transformation and breaking in opposing current by means of experimental analysis and theoretical calculation. In order to know the effect of wave refraction, computation is done in this paper about wave transformation and breaking on gentle slopes in diagonal opposing current.

Design of a Hardware-Implemented Phase Calculating System for Feedback Control in the LHCD Experiments on EAST

A fully hardware-implemented phase calculating system for the feedback control in the lower-hybrid current drive （LHCD） experiments is presented in this paper. By taking advantages of field programmable gate array （FPGA） chips with embedded digital signal processing （DSP） cores and the Matlab-aided design method, the phase calculating algorithm with a square root operation and parallel process are efficiently implemented in a single FPGA chip to complete the calculation of phase differences fast and accurately in the lower-hybrid wave （LHW） system on EAST.

Measurement and Simulation of Induced Voltage and Current on 110 KV Crossing Transmission Lines under UHV AC Transmission Lines

The induced electricity of 110 kV transmission lines which cross the UHV AC transmission lines may threaten personal safety of the maintenance staff. In this paper, field measurement of the induced voltage and induced current on a 110 kV crossing line inside Jinhua in Zhejiang province is performed. The electrostatic induced voltage on the measured line is 12.24 kV. The power frequency electromagnetic field simulation model is established, and the calculation results are consistent with the measured. Influence factors analysis shows that the electrostatic induced voltage on the 110 kV line is 12.78 kV, the electromagnetic induced voltage is 12.3 V, the induced current through ground wire is less than 1A when the UHV lines operate at full load. The induced voltage and current decrease while the crossing distance increases. Parallel lines induction is much higher than crossing lines. The electromagnetic induced voltage after ground knife-switch shut down would exceed the human safety voltage 36 V while the crossing angle is less than 30?, so the temporary ground wire must be hanged to ensure safety of the maintenance staff.

同轴多层筒式空心线圈自感和互感解析计算方法

在电磁耦合的无线电能传输系统中,准确获取线圈电感参数是设计和优化无线电能传输系统的重要理论依据。现有的无线电能传输系统研究大多针对平面板结构线圈,同轴多层筒式空心线圈相比于平面板线圈具有更高的传输功率和传输效率。因此,该文针对同轴多层筒式空心线圈,提出了一种考虑高频涡流效应的自感和互感解析计算方法。首先,将单匝线圈截面离散等效为数量众多、形状相同的网格带电圆环单元,采用离散等效求和法推导出单匝线圈的自感和线圈间互感的解析式;然后,基于筒式空心线圈的整体结构以及考虑高频涡流效应,推导出基于第一类、第二类完全椭圆积分和开尔文函数的一、二次线圈自感和总互感计算的理论解析式;最后,以两种不同排布方式的筒式空心线圈为例,对比解析计算、有限元计算和实验测量结果,验证了所提解析法的可行性,同时也为筒式空心线圈的设计提供参考依据。

脉冲晶闸管热网络模型及运行状态评估方法

针对脉冲晶闸管承受强流脉冲导致的瞬态电磁热力联合冲击产生的热疲劳累积突发性失效问题,提出了在脉冲晶闸管内布置温度传感器的状态监测方法,建立了脉冲晶闸管的高阶瞬态热阻抗网络模型,分析了强流脉冲作用下脉冲晶闸管内部不同层的温度分布规律,并通过理论计算验证了分析方法的有效性。计算结果表明,热网络模型计算相对偏差小于5%。可为脉冲晶闸管型强流开关的状态评估提供理论指导。

混合级联特高压直流功率损失计算方法研究

混合级联特高压直流(UHVDC)在降低直流换相失败风险的同时提升了直流输电的灵活性,但由于LCC与VSC混合拓扑导致不同阀组闭锁后健全阀组的功率传输能力存在不确定性,影响了直流功率损失准确计算。此处提出了一种适用于混合级联UHVDC的功率损失计算方法。直流控制系统以极为单位计算阀组闭锁后的直流功率并发送安控系统计算直流功率损失,设计了合理接口与数据处理方式保证阀组单次及相继故障直流功率损失计算的准确性。最后通过试验,验证了所提方法的正确性与有效性。

基于七阶NR法与Broyden方法的柔性直流系统潮流新算法

目的 为了解决当前柔性直流输电系统潮流算法求解效率低的问题,方法 首先提出基于七阶NR法的潮流算法,该算法相较于传统的潮流算法具有更高的收敛阶数,可以有效减少迭代次数,提高算法效率;其次,针对七阶NR法单步迭代计算量大的问题,提出基于七阶NR法与Broyden方法的改进算法,利用七阶NR法突出的收敛特性,减少迭代次数,同时保留Broyden方法单步迭代计算成本较低的优势,避免浪费计算能力,使计算效率得到较大提升;最后提出一种经验法则,以确定改进算法中高阶NR法的迭代次数。结果 使用经典和改进算法对修改后的IEEE标准算例进行仿真测试,结果表明:与经典NR法相比,交流和直流的改进算法潮流计算结果最大误差分别为1×10^(-8)和2×10^(-8);平均计算时间方面,在IEEE-14、IEEE-30、IEEE-57、IEEE-118共4种算例系统下,改进算法的平均计算时间比经典NR法的减少了约61%;当算例系统由双端变为三端和双馈入后,改进算法的迭代次数保持不变,计算时间分别增加0.352 ms和0.636 ms;在重负荷情况下,改进算法的计算效率最高,且预设迭代次数较简单,当负荷值达到2.992时,其计算时间仅为经典NR法的60%。结论 本文的改进算法具有与经典NR法相同的精确性,但在计算速度方面更有优势,且更有利于在重负荷等极端情况下使用。