New inverter topology for ground current suppression in transformerless photovoltaic system application

An interesting inverter topology is proposed in this paper.It is similar to the typical three-phase full bridge inverter from the topology point of view,but smartly designed for the ground current reduction in single-phase photovoltaic(PV)inverter applications.Theoretical analysis is conducted to clarify the operation mechanism of the proposed topology.Performance evaluation is carried out to verify the effectiveness of the proposed topology for the ground current suppression.

Residual current compensation for single-phase grounding fault in coal mine power network

The way of neutral point to earth via full compensation arc suppression coil can solve the problem of residual current compensation in coal mine power network effectively. Based on the analysis on the grounding current detection results of Xieqiao coal mine, the conclusion that harmonic component of grounding current is dominated by higher harmonics with complex harmonic sources in coal mine power network system was obtained. The influences of harmonic source type and fault point position on harmonic voltage and harmonic current were analyzed theoretically. The influences of earthed fault feeder detection result and the estimation errors of parameters to earth on residual current compensation were analyzed. A new thought of residual current prediction and the selections of model method and control method were proposed on this basis. The simulation results prove that harmonic amplitudes of zero sequence voltage and zero sequence current are determined by harmonic source type as well as fault point position in coal mine power network, and also prove that zero sequence voltage detection can avoid the unstable problem of coal mine power network system caused by undercompensation of capacitive current. Finally, the experimental device of full compensation arc suppression coil is introduced.

Effects of Transformer DC Bias and the Relationship Between UHVDC Grounding Current and Grounding Resistances

Recently years,UHVDC transmission system is paid more attention to in the field of China's power system.It takes key part in the China electrical power development stratagem.But,many problems are caused by UHVDC system,such as DC bias,corrosion of metal underground and so on.DC bias is harm to the transformers nearby UHVDC grounding polar.In this paper,the influences of DC grounding current on transformer are introduced and some suggestions of DC bias solution were provided.And,the relationship between UHVDC Grounding Current and grounding Resistance of Substation was analyzed.Firstly,two-part network circuit was used to equivalent the grounding circuit.Then,an analysis of rules was done between DC bias current and grounding resistance.Finally,the conclusion is given that DC bias current rises fast as DC grounding resistance or AC grounding resistance rises.It drops when resistance of AC transmission line or interaction resistance between DC grounding system and AC grounding system rises.Decreasing AC grounding resistance and DC grounding resistance is important to restrain DC bias current.Increasing resistance of AC transmission line such as adding resistance into transformer neutral-point grounding is a useful way to limit DC bias current.

Simulation on DC Current Distribution in AC Power Grid under HVDC Ground-Return-Mode

This paper focus on the Modeling and Calculation of DC current distribution in AC power grid induced under HVDC Ground-Return-Mode. Applying complex image method and boundary element method, a new field-circuit coupling model was set up. Based on the calculation result with complex image method, this paper derived the modification factor for induced earth potential from practical measurement, which increased the accuracy of calculation. The modification method is helpful for evaluation on the effect of means used for blocking the dc-bias current in transformer neutral and also useful for the forecast of the DC current distribution when the power grid is in different line connection mode. The DC distribution character in Guangdong power grid is shown and suggestion is proposed that the mitigation of dc-bias should start from those substations whose earth-potential is highest.

The Study of Single Station Inverting the Sea Surface Current by HF Ground Wave Radar Based on Adjoint Assimilation Technology

This paper introduces the assimilation technology in an ocean dynamics model and discusses the feasibility of inverting the sea surface current in the detection zone by assimilating the sea current radial velocity detected by single station HF ground wave radar in ocean dynamics model. Based on the adjoint assimilation and POM model, the paper successfully inverts the sea surface current through single station HF ground wave radar in the Zhoushan sea area. The single station HF radar inversion results are also compared with the bistatic HF radar composite results and the fixed point measured results by Annderaa current meter. The error analysis shows that acquisition of flow velocity and flow direction data from the single station HF radar based on adjoint assimilation and POM model is viable and the data obtained have a high correlation and consistency with the flow field observed by HF radar.

Application of Atomic Sparse Decomposition to Feature Extraction of the Fault Signal in Small Current Grounding System

Applying the atomic sparse decomposition in the distribution network with harmonics and small current grounding to decompose the transient zero sequence current that appears after the single phase to ground fault occurred. Based on dictionary of Gabor atoms and matching pursuit algorithm, the method extracts the atomic components iteratively from the feature signals and translated them to damped sinusoidal components. Then we can obtain the parametrical and analytical representation of atomic components. The termination condition of decomposing iteration is determined by the threshold of the initial residual energy with the purpose of extract the features more effectively. Accordingly, the proposed method can extract the starting and ending moment of disturbances precisely as well as their magnitudes, frequencies and other features. The numerical examples demonstrate its effectiveness.

Application of ESPRIT in Broad Beam HF Ground Wave Radar Sea Surface Current Mapping

HF surface wave radar system OSMAR2000 is a broad-beam sea-state detecting radar. ESPRIT (Estimation of Signal Parameters via Rotational Invariance Technique) algorithm is proposed to apply in DOA (direction of arrival) determination of sea echoes. The algorithm of ESPRIT is briefly introduced first. Then discussions are made on the technique for application in the OSMAR2000 framework. Numerical simulation results are presented to demonstrate the feasibility of radial current mapping based on this method. The algorithm manifests significant performance and computational advantages compared with that of MUSIC. Data acquired by OSMAR2000 are processed to give radial current map and the synthesized vector currents are compared with the in-situ measurement with traditional means. The results show the validity of ESPRIT application in DOA determination for broad-beam radar. Key words HF ground wave radar - sea surface current - ESPRIT - MUSIC CLC number TN 911.72 - TN 958.95 Foundation item: Supported by the National Natural Science Foundation of China (60201003) and the National High Technology Development 863 Program of China (863-818-01-02)Biography: Liu Dan-hong (1976-), female, Master candidate, research direction: radar signal processing.

Experimental Analysis on Shielding Effect of the Grounding Electrodes Under Impulse-current

Generally,the flow of a lightning impulse current from a grounding electrode into ground is a very complicated process determined by many factors.In order to analyze the mechanism of the impulse current dissipating in the earth from grounding electrode,the experiments that had been carried out by other authors almost used a single horizontal grounding wire or vertical grounding rod for sake of simplicity.However,in practical conditions,most of the grounding systems are constructed of grounding electrodes with branches in different directions.In this study,basing on the principle of dimensional similarity,impulse simulation experiments are performed on the common ground electrodes with conductor branches.This paper focuses on analyzing the impulse current dispersal regularity of different branches when injecting at one point.Comparing with the leakage current distribution of a single ground electrode,it is found that the leakage currents along the branches increase with the distance to the current feed point,and the more conductors near the injection point,the more uneven the leakage current distribution is.This work indicates that shielding effect should be taken into account when analyzing the impulse characteristics of grounding electrodes.

海上风电单芯海缆接地环流影响因素分析

单芯海缆在生产、施工方面相对于三芯海缆更具优势,但是其接地环流异常引起的海缆故障频繁发生。单芯海缆接地环流运行机理及检测策略有待深入研究。建立了基于阻抗矩阵的单芯海缆数学模型,揭示了铠装电阻率、铠装层相对磁导率、接地电阻、短路电阻等因素对阻抗矩阵内参数和接地环流的影响规律。结合实际工程案例分析了海缆接地环流分配不均的机理,为海缆的运行状态评估及检修提供了参考依据。

高灵敏微波载荷卫星出舱电缆及舱体屏蔽的EMC设计

针对高灵敏微波载荷卫星对舱板电磁屏蔽效能的高需求与出舱电缆对舱体屏蔽效能巨大破坏性之间的矛盾,文章首先提出航天器理想舱板(无出舱电缆)的电磁屏蔽效能分析方法,给出提高对于1 GHz以上干扰电磁波屏蔽效能的设计原则;然后分析出舱电缆成为影响航天器外舱板对P波段(150 MHz~1 GHz)电磁波屏蔽效能主要因素的机理,给出干扰电流与干扰场强关系计算公式;最后给出出舱电缆及舱体屏蔽的电磁兼容(EMC)设计方法,提出用接地电阻测量法验证出舱电缆屏蔽皮接地的有效性,为总体总装EMC实施找到关键控制手段,并通过整星试验数据确认此方法能有效提升整星EMC试验效率。

Study on Earth Surface Potential and DC Current Distribution around DC Grounding Electrode

DC magnetic biasing problem,caused by the DC grounding electrode, threatened the safe operation of AC power grid. In this paper, the characteristics of the soil stratification near DC grounding electrode was researched. The AC-DC interconnected large-scale system model under the monopole operation mode was established. The earth surface potential and DC current distribution in various stations under the different surface thickness was calculated. Some useful conclusions are drawn from the analyzed results.

土壤电阻率的负温特性及冻土对接地极冲击响应的影响

西藏地区由于独特的气候与地理环境,在其冻土地区存在雷电与低温共存现象。低温下冻土作为含冰复杂体系,其在雷击下的冲击散流性能仍不明晰,因此该文就冻土电阻率的负温特性以及接地极的冲击特性开展研究。首先研究了不同含水量、含盐量的土壤电阻率随温度的变化规律;然后在冲击电流作用下,以永冻土试品的表层土壤融化厚度和季节性冻土试品的表层土壤冻结厚度为变量,探究了其对相应冻土中垂直接地极暂态电位的影响规律;最后结合冻结条件下土壤冲击放电的形貌特征与地中电场强度和电流密度的分布特性,探讨了负温下接地极暂态电位变化的本质原因。试验研究表明:考虑盐分对土壤的影响,当其温度在一次冻结温度Tf与二次冻结温度Ts之间时,电阻率随温度下降缓慢上升,只有当温度降至二次冻结温度时,才存在突增现象;在永冻土中,当冻土温度高于Ts时,接地极仍具有较好的散流性能。由此可知降低Ts可以减小水分冻结对土壤电阻率的影响,为避免极寒地区接地极失效提供了新思路。

某型地面发控点火电流异常分析和电路改进

针对某型地面发控系统在使用过程中出现的点火电流异常问题,采用数学方法研究点火电流与元器件参数的变化规律,发现地面发控点火电流在某些环境下不稳定的根本原因。最后提出一种闭环控制方案,增加电流反馈环节,解决点火电流异常问题。实际应用验证表明,改进后的点火电路能适应更大的点火负载,有效提高了点火的可靠性。

基于单端分支系数的四端柔直环网线路保护方案

为提升直流线路保护在柔性直流环网中的速动性与可靠性,提出一种基于单端分支系数的保护方案。分析了不同故障位置下各换流阀提供的故障电流分量,并根据单端换流阀极线电流突变量构造分支系数,结果表明线路保护区内故障下的单端分支系数小于区外故障,利用该特征构造区内、外故障识别元件。在线路高阻接地致使主保护失效时,根据桥臂最大允许电流决定线路纵联后备保护的开放时限,以优化现有主、后备保护方案之间的时序配合方案,保障各种直流线路故障工况下,换流阀不间断运行。保护方案不依赖线路边界元件,算法简单,大量仿真结果表明,该方案有良好的耐受过渡电阻能力,能在换流阀闭锁前实现故障区域可靠识别。

极低频电流场透地通信路径损耗建模与分析

针对水平方向地电极电流场无线强穿透信息传输需求,研究了基于极低频水平双电极电流场信道传输特性。考虑到电极半径、电极入土深度、电极间距、发射信号频率和信号传输距离等参数对透地(Through-The-Earth,TTE)通信传输性能的影响,建立了地电极电流场TTE通信路径损耗模型,分析了各参数对路径损耗的影响,确定了信号传输最佳工作参数。根据选定参数搭建了极低频电流场TTE通信系统,在200 m和400 m通信距离下测试了3~10 Hz信号的路径损耗,通过试验数据与仿真结果对比分析验证了模型的准确性。

双制式列车接地系统的车-地联合牵引供电计算

为研究双制式列车在不同供电制式区段下的车体环流和车体—轴端电位分布,针对某型双制式列车构建列车接地系统的链式电路模型,并提出车-地一体化的交、直流区段联合牵引供电计算方法;同时,建立列车接地保护电阻优化模型,并分析2种列车接地系统配置方案.以国内某条双制式线路为算例进行仿真验证,研究结果表明:方案1相较方案2,列车的车体—轴端电位最大值降低36.58%~41.04%,车体环流最大值降低18.49%~22.97%;并且在方案1中,头尾车保护电阻设置为20 mΩ,可使列车的车体—轴端电位最大值为1.15 V,车体电流最大值为50.30 A,达到抑制车体—轴端电位的最优效果.该车-地一体化的交、直流区段联合牵引供电计算方法可适用于单一供电制式或者多供电制式列车接地系统的分析.

基于低频暂态零序电流畸变率的灵活接地系统高阻接地保护

灵活接地系统中,基于稳态故障特征构成的单相接地保护普遍存在耐过渡电阻能力低的问题,并联小电阻投入后该问题进一步突出。文中研究了并联小电阻投入后故障线路与非故障线路低频暂态零序电流中暂态直流分量比值关系,发现暂态直流分量比值仅与对地电容有关。分析了故障线路与非故障线路低频暂态零序电流畸变率之间比值关系,进一步提出了一种基于低频暂态零序电流畸变率的灵活接地系统单相高阻接地故障选线方法。通过比较特定时间窗口内各馈线低频暂态零序电流畸变率的大小实现故障选线,并可准确区分母线故障与出线故障。该方法能够避免零序电压互感器断线与零序电流互感器极性反接的影响,且受故障距离影响小。同时,该方法采用暂态故障特征构成选线判据,有利于克服并联小电阻投入后稳态故障分量微弱的问题,对互感器精度要求不高。最后,通过实时数字仿真和实验平台分别验证了所述方法的准确性和有效性。

Single-phase earth fault current distribution between optical fiber composite overhead ground wire and ordinary ground wire in transmission system

It is important for the safety of transmission system to accurately calculate single-phase earth fault current distribution.Features of double sided elimination method were illustrated.Quantitative calculation of single-phase earth fault current distribution and case verification were accomplished by using the loop method.Influences of some factors,such as single-phase earth fault location and ground resistance of poles,on short-circuit current distribution were discussed.Results show that:1) results of the loop method conform to those of double sided elimination method;2) the fault location hardly influences macro-distribution of short-circuit current.However,current near fault location is evidently influenced;and 3) the short-circuit current distribution is not so sensitive to the ground resistance of poles.

计及电缆绝缘不均匀老化的接地电流相位角特性研究

交联聚乙烯(XLPE)电力电缆广泛应用于配电网和输电网,但它们在持续的电应力下遭受长期的老化退化,电缆老化容易造成绝缘击穿事故。为了提升电缆绝缘老化状态的在线评估技术,基于PSCAD建立了计及电缆绝缘不均匀老化的三芯管型电缆接地电流相位角计算模型,通过快速傅里叶变换FFT变化得到单相不均匀老化、双相不均匀老化、三相均匀老化3类情况的接电线电流相位角特性。结果表明:相比于钢铠接地线电流,屏蔽线电流相角对于老化特性的反应更为灵敏;对于单相老化,介电常数由2增加为5,屏蔽线电流相位角从-51.01°递增为-50.6°;对于双相老化,随老化程度增加,相位角增幅逐渐增大;对于三相均匀老化,相比于正常电缆的屏蔽线电流相角增值为0.24°,同时相比于介电常数同为2.8的单相老化与双相老化的屏蔽线电流相角增值为最大(单相老化为0.15°、双相老化为0.17°);通过屏蔽线电流相位幅值,不仅可以判断电缆绝缘是否老化,还可区分老化相别与老化程度。本研究结果可为通过地线电流相位角评估电缆绝缘老化等级提供重要理论依据。

基于车体接地电流的故障列车精准识别方法

针对专用回流轨直流牵引供电系统发生正极与车体短路故障时,故障列车不能被精准识别的问题,首先给出了双边供电牵引系统等值模型,其次分析了故障列车和正常列车的车体接地电流特征差异,最后提出了一种基于车体接地电流的故障列车精准识别方法。所提方法采用列车的车体接地电流幅值和方向作为故障识别判据,当其幅值小于整定值时判断列车正常,当大于等于整定值时继而采用其方向进行判断,电流为正方向的列车被判为故障车,电流为反方向的列车被判为正常车。该方法原理简单,实现方便,实用性强,采用双重判据能有效避免误判,可靠性高。通过Matlab仿真和现场试验录波验证了方法的可行性和有效性。