Single-phase-to-ground fault protection based on zero-sequence current ratio coefficient for low-resistance grounding distribution network

Definite-time zero-sequence over-current protection is presently used in systems whose neutral point is grounded by a low resistance(low-resistance grounding systems).These systems frequently malfunction owing to their high settings of the action value when a high-impedance grounding fault occurs.In this study,the relationship between the zero-sequence currents of each feeder and the neutral branch was analyzed.Then,a grounding protection method was proposed on the basis of the zero-sequence current ratio coefficient.It is defined as the ratio of the zero-sequence current of the feeder to that of the neutral branch.Nonetheless,both zero-sequence voltage and zero-sequence current are affected by the transition resistance,The influence of transition resistance can be eliminated by calculating this coefficient.Therefore,a method based on the zero-sequence current ratio coefficient was proposed considering the significant difference between the faulty feeder and healthy feeder.Furthermore,unbalanced current can be prevented by setting the starting current.PSCAD simulation results reveal that the proposed method shows high reliability and sensitivity when a high-resistance grounding fault occurs.

Zero-sequence Current Suppressing Strategy for Dual Three-phase Permanent Magnet Synchronous Machines Connected with Single Neutral Point

Dual three-phase permanent-magnet synchronous machines(DTP-PMSM)connected with a single neutral point provide a loop for zero-sequence current(ZSC).This paper proposes a novel space vector pulse width modulation(SVPWM)strategy to suppress the ZSC.Five vectors are selected as basic voltage vectors in one switching period.The fundamental and harmonic planes and the zero-sequence plane are taken into consideration to synthesis the reference voltage vector.To suppress the ZSC,a non-zero zero-sequence voltage(ZSV)is generated to compensate the third harmonic back-EMF.Rather than triangular carrier modulation,the sawtooth carrier modulation strategy is used to generate asymmetric PWM signals.The modulation range is investigated to explore the variation of modulation range caused by considering the zero-sequence plane.With the proposed method,the ZSC can be considerably reduced.The simulated and experimental results are presented to validate the effectiveness of the proposed modulation strategy.

Protection of Zero-Sequence Power Variation in Mountain Wind Farm Collector Lines Based on Multi-Mode Grounding

The arc-suppression coil(ASC)in parallel low resistance(LR)multi-mode grounding is adopted in the mountain wind farm to cope with the phenomenon that is misoperation or refusal of zero-sequence protection in LR grounding wind farm.If the fault disappears before LR is put into the system,it is judged as an instantaneous fault;while the fault does not disappear after LR is put into the system,it is judged as a permanent fault;the single-phase grounding fault(SLG)protection criterion based on zerosequence power variation is proposed to identify the instantaneous-permanent fault.Firstly,the distribution characteristic of zero-sequence voltage(ZSV)and zero-sequence current(ZSC)are analyzed after SLGfault occurs in multi-mode grounding.Then,according to the characteristics that zero-sequence power variation of non-fault collector line is small,while the zero-sequence power variation of fault collector line can reflect the active power component of fault resistance,the protection criterion based on zero-sequence power variation is constructed.The theoretical analysis and simulation results show that the protection criterion can distinguish the property of fault only by using the single terminal information,which has high reliability.

Zero-Sequence Current Suppression Strategy for Open-End Winding Permanent Magnet Synchronous Motor Based on Model Predictive Control

Compared with the traditional three-phase star connection winding,the open-end winding permanent magnet synchronous motor(OW-PMSM)system with a common direct current(DC)bus has a zero-sequence circuit,which makes the common-mode voltage and the back electromotive force(EMF)harmonic generated by the inverters produce the zero-sequence current in the zero-sequence circuit,and the zero-sequence current has great influence on the operation efficiency and stability of the motor control system.A zero-sequence current suppression strategy is presented based on model predictive current control for OW-PMSM.Through the mathematical model of OW-PMSM to establish the predictive model and the zero-sequence circuit model,the common-mode voltage under different voltage vector combinations is fully considered during vector selection and action time calculation.Then zero-sequence loop constraints are established,so as to suppress the zero-sequence current.In the end,the control strategy proposed in this paper is verified by simulation experiments.

Electromagnetic Zero-Sequence Harmonics Blocker： Modeling and Experimental Analysis

This paper reveals new contributions to the analysis and development of devices for harmonic distortion mitigation. Considering the sequential distribution of harmonic currents, zero-sequence components could be diminished using electromagnetic devices, particularly the eZSB （electromagnetic zero-sequence blocking）. One important characteristic of this device, which has received particular attention on this research, is its robustness and low cost of construction. Theoretical and experimental results related to the behavior of the electromagnetic blocking devices are presented. The results illustrate the consistence of the theoretical aspects related with the model in the frequency domain, as well as the performance of the blocking devices, reducing zero-sequence harmonic currents, mainly by the conjunct action of the eZSF （electromagnetic zero-sequence harmonic filter）, working as a impedance coupler. In this context, aiming the evaluation of the reliability of the results obtained through mathematical modeling, experimental tests have been carried out using a low-power prototype, highlighting particular aspects related to its function as a zero-sequence harmonic blocker.

Simplified Cluster Voltage Balancing Control Based on Zero-sequence Voltage Injection for Star-connected Cascaded H-bridge STATCOM

The cluster DC voltage balancing control adopting zero-sequence voltage injection is appropriate for the starconnected cascaded H-bridge STATCOM because no zerosequence currents are generated in the three-phase three-wire system.However,as the zero-sequence voltage is expressed in trigonometric form,traditional control methods involve many complicated operations,such as the square-root,trigonometric operations,and inverse tangent operations.To simplify cluster voltage balancing control,this paper converts the zero-sequence voltage to the dq frame in a DC representation by introducing a virtually orthogonal variable,and the DC components of the zero-sequence voltage in the dq frame are regulated linearly by proportional integral regulators,rather than being calculated from uneven active powers in traditional controls.This removes all complicated operations.Finally,this paper presents simulation and experimental results for a 400 V±7.5 kvar star-connected STATCOM,in balanced and unbalanced scenarios,thereby verifying the effectiveness of the proposed control.

基于双PI控制器的模块化多电平变换器环流抑制策略

由于模块化多电平变换器(modular multilevel converter,MMC)本身的结构特点带来了桥臂环流问题,环流的存在使得MMC桥臂电流发生畸变,从而提高了器件的额定电流容量,增加了系统成本,甚至威胁功率器件和电容的正常安全运行。为了克服上述问题,首先建立了MMC的数学模型,推导出环流中主要存在直流分量和偶数次环流谐波分量,其中2次、4次谐波所占比重最大。为了实现对环流的有效抑制,提出了一种基于双PI控制器的环流抑制策略;基于该策略搭建了Matlab/Simulink仿真和实验平台,实验和仿真结果验证了环流理论分析的正确性和所提环流抑制策略的有效性。

一种基于3次谐波注入的并联三相四桥臂逆变器均流控制策略

三相四桥臂逆变器(3p4l)在三相三桥臂逆变器的基础上引入第四桥臂,使得三相能够解耦控制并具备带不对称负载能力,在此基础上采用3次谐波注入可以提高逆变器的直流电压利用率。若将多个三相四桥臂逆变器单元共直流母线并联,能够实现扩容。但是并联单元的电感电流若不采取控制,会导致环流问题,严重时会损坏逆变器。在基于平均电流均流控制策略的基础上,采用一种适用于模拟电路实现的3次谐波注入方式。由于主电路元器件参数的不对称性,并联单元各自生成的3次谐波不对称,增大了并联单元之间的零序环流。针对该问题,提出一种基于各并联单元3次谐波信号平均值法的三相四桥臂逆变器并联均流控制策略。在保留3次谐波注入的同时使得并联模块四个桥臂电感电流得到控制,消除环流,实现了并联桥臂均流。最后通过仿真和实验验证了控制策略的正确性。

耦合电抗器在并联型三相并网逆变器中的应用

为提高三相并网逆变器的功率等级,提出一种由耦合电抗器组成的新型并联型三相并网逆变器,通过对耦合电抗器去耦得出并联型逆变器的等效电路,并分别在三相静止坐标系和同步旋转坐标系下建立平均模型,阐明耦合电抗器抑制零序环流的根本机理,即耦合电抗器可以增大零序环流阻抗。根据所得平均模型设计电流解耦控制策略,抑制了零序环流,解决了零序环流引起的不均流、波形畸变问题,提高了系统的可靠性和效率。最后,对一组1.5 MW并联型三相并网逆变器进行了仿真验证,结果证明了上述分析及控制策略的正确性。

错位无环流可逆调速系统的过渡过程分析

错位无环流可逆调速系统，采用错位控制，消除了静态环流；采用电压负反馈，防止了动态环流．本文详细分析了该系统正反组可控硅切换过程中，各主要参量的变化过程及各调节器输出限幅的整定原则，并绘制了过渡过程的波形图。

一种模块化多电平的改进最近电平调制策略

模块化多电平换流器(modular multilevel converter,MMC)因其模块化设计易于扩展、开关频率低等特点,在中高压大功率系统中得到越来越多的应用;而最近电平调制(nearest level modulation,NLM)作为一种实时阶梯波调制方式,常用于上百个电平数的场合。首先对MMC的基本结构和最近电平调制的运行机理进行分析,推导出该调制策略所存在的固有缺陷;在此基础上,提出了改进的最近电平调制策略。通过引入调制波与阶梯波间偏差,有效避免了在电平数较低情况下电容电压波动、环流等问题。最后在PSCAD/EMTDC中搭建了MMC仿真模型,对改进前后的最近电平调制策略进行了比较分析。仿真结果表明该改进调制策略可以提高系统的稳态运行特性,对子模块电容电压波动和环流产生抑制作用。

Estimating zero-sequence impedance of three-terminal transmission line and Thevenin impedance using relay measurement data

Current and voltage waveforms recorded by intelligent electronic devices(IEDs)are more useful than just performing post-fault analysis.The objective of this paper is to present techniques to estimate the zero-sequence line impedance of all sections of a three-terminal line and the Thevenin equivalent impedance of the transmission network upstream from the monitoring location using protective relay data collected during short-circuit ground fault events.Protective relaying data from all three terminals may not be always available.Furthermore,the data from each terminal may be unsynchronized and collected at different sampling rates with dissimilar fault time instants.Hence,this paper presents approaches which use unsynchronized measurement data from all the terminals as well as data from only two terminals to estimate the zero-sequence line impedance of all the sections of a three-terminal line.An algorithm to calculate positive-,negative-and zero-sequence Thevenin impedance of the upstream transmission network has also been presented in this paper.The efficacy of the proposed algorithms are demonstrated using a test case.The magnitude error percentage in determining the zero-sequence impedance was less than 1%in the test case presented.

不同铺设方式及屏蔽层接地方式对10 kV单芯电缆安全运行的影响

基于炼铁总降TRT电缆运行实例,分析了10 k V单芯电力电缆因金属屏蔽层中环流引起的故障。给出了几种常见铺设方式下单芯电缆金属屏蔽层感应电压幅值计算公式,及金属屏蔽层接地方式的选择。提出了保障电缆安全运行的建议。

含冗余模块和时域调制信号的多端MMC直流互联系统稳态闭环模型

该文提出可用于模块化多电平换流器（modular multilevel converter,MMC）主电路关键参数选型、功率运行极限及稳态性能计算分析的多端MMC直流互联系统稳态闭环模型。首先,分析和推导含冗余模块的单个桥臂等效电容表达式;其次,以时域调制信号（包括直流调制信号、基频调制信号的幅值和初相位及二倍频调制信号的幅值和初相位）作为外部输入量,建立含桥臂二倍频环流和任意直流网络拓扑的桥臂电流闭环模型;再次,基于功率平衡原则,等效建立以时域调制信号作为未知量的稳态控制系统模型;最后,将桥臂电流闭环模型与控制系统模型相结合,构建多端MMC系统稳态闭环模型。以一个四端MMC直流互联系统为例,通过电磁暂态模型和该文所提模型进行对比,验证所提稳态闭环模型的精确性,并在此基础上进一步研究冗余模块数对MMC主电路关键参数和稳态性能的影响。所提模型通过简单迭代计算,即可快速精确地评估MMC主电路参数选型、功率运行极限及稳态性能。

Research on Fault-feeder Selection Dvice

The compensation current of the arc-suppressing coil makes the phase and amplitude of zero-sequence measurement current of the earthed fault feeder to vary. It is very hard to detect the fault feeder by using existing detectors based on single method. In this paper, integrative feeder selection strategy—zero sequence current increment method and the direction of transient current— is put forward. Based on the integrative feeder selection strategy, the design of fault-feeder selection device for one-phase-to ground fault on resonance grounding system is presented. For the purpose of testing and validating the operating principle of the device, the experiment of single-phase-to-ground fault has been carried out on the simulation of 1.2 kV power network. The results from many repeat experiments show that stability of the fault selection device is satisfactory.

Positive- and Zero-Sequence Impedance Estimation of YNyn0+d-Connected Main Power Transformers in Wind Power Applications

With the increased penetration of non-conventionalrenewable energy resources, such as wind power plants (WPPs),into the power system, a lot of interest has been placed on studyingtheir impact for system protection, planning and operationsto ensure continued reliability of the electric grid. The impactof WPPs on the grid is typically identified during generationinterconnection studies. One of the challenges faced by protectionand planning engineers, at an early stage in this process, isthat many of the studies involving these power plants must beperformed without specific data available because the equipmentis not built yet. Among all the needed data, a particular interest inthis study is placed on the positive- and zero-sequence impedancesof the main power transformer (MPT) that interconnects a windpower plant to the transmission system due to its contribution toshort-circuit current during ground faults. The most commonlyfound in practice MPT winding configurations is a high-voltagewinding in wye with neutral bushing;a low-voltage windingin wye with neutral bushing without phase shift between thesewindings, and a stabilizing tertiary winding connected in deltathat is not designed for external loading designated as YNyn0+d.Thus, this paper proposes a methodology to estimate the sequenceimpedances of YNyn0+d transformers from a statistical point ofview applying clustering and regression analyses on existing MPTtest report data.

High-impedance Fault Section Location for Distribution Networks Based on t-distributed Stochastic Neighbor Embedding and Variable Mode Decomposition

When high-impedance faults(HIFs)occur in resonant grounded distribution networks,the current that flows is extremely weak,and the noise interference caused by the distribution network operation and the sampling error of the measurement devices further masks the fault characteristics.Consequently,locating a fault section with high sensitivity is difficult.Unlike existing technologies,this study presents a novel fault feature identification framework that addresses this issue.The framework includes three key steps:(1)utilizing the variable mode decomposition(VMD)method to denoise the fault transient zero-sequence current(TZSC);(2)employing a manifold learning algorithm based on t-distributed stochastic neighbor embedding(t-SNE)to further reduce the redundant information of the TZSC after denoising and to visualize fault information in high-dimensional 2D space;and(3)classifying the signal of each measurement point based on the fuzzy clustering method and combining the network topology structure to determine the fault section location.Numerical simulations and field testing confirm that the proposed method accurately detects the fault location,even under the influence of strong noise interference.

SOME OSCILLATION THEOREMS OF HIGHER ORDER NON－HOMOGENEOUS LINEAR DIFFERENTIAL EQUATIONS WITH TRANSCENDENTAL MEROMORPIC COEFFICIENTS

In this paper, we investigate the complex oscillation of the higher order differential equation where B0, ...,Bk-1,,F 0 are transcendental meromorpic functions having only finitely many poles. We obtain some precise estimates of the exponent of convergence of the zero sequence of meromorphic solutions for the above equation.

Single-phase-to-ground Fault Detection with Distributed Parameters Analysis in Non-direct Grounded Systems

The fast and accurate detection of the single-phaseto-ground fault is of great significance for the reliability and safety of the power supply.In this paper,novel algorithms for distribution network protection were proposed with distributed parameters analysis in non-direct grounded systems.At first,novel generating mechanisms of zero-sequence voltage and residual current were proposed.Then the compositions of residue parameters,including residual current and residual admittances,were decomposed in detail.After that,an improved algorithm for a fault resistance calculation of a single phase-to-earth fault was also proposed,and the algorithm is much more convenient as it only needs to measure the variation of the zero-sequence voltage and does not need the prerequisites of the faulty feeder selection.Furthermore,the fault feeder can also be selected by an improved calculation algorithm of zero-sequence admittance of the faulty feeder,which cannot be affected by the asymmetry of the network.Theoretical analysis and the MATALB/Simulink simulation results demonstrate the effectiveness of the proposed algorithms.

Arc flash fault detection in wind farm collection feeders based on current waveform analysis

High impedance faults(HIFs) are easy to occur in collective feeders in wind farms and may cause the cascading of wind generators tripping. This kind of faults is difficult to be detected by traditional relay or fuse due to the limited fault current values and the situation is worse in wind farms. The mostly adopted HIF detection algorithms are based on the 3rd harmonic characteristic of the fault zero-sequence currents, whereas these 3rd harmonics are very easy to be polluted by wind power back-to-back converters. In response to this problem, the typical harmonic characteristic of HIF arc flash based on Mayr’s arc model is first analyzed, and the typical fault waveforms of HIF in wind farm are presented. Then the performance of the harmonic based HIF detection algorithm is discussed,and a novel detection algorithm is proposed from the viewpoint of time domain, focusing on the convex and concave characteristic of zero-sequence current at zerocrossing points. A HIFs detection(HIFD) prototype implementing the proposed algorithm has been developed.The sensitivity and security of the algorithm are proved byfield data and RTDS experiments.