基于三次谐波电流的并列运行发电机定子单相接地保护

针对中小型水电站并列运行的多台发电机,提出一种具有选择性的发电机定子单相接地保护方法,测量发电机中性点侧和机端侧各相的三次谐波电流,计算定子绕组对地泄漏电流三次谐波分量的相间差,检测故障发电机,实现选择性定子接地保护.理论分析和EMTP仿真结果表明,该保护方法具有良好的选择性,能有效检测中性点侧及机端侧故障,具有鲁棒性强、精度高等优点.     

励磁绕组两点接地故障对横联差动保护的干扰分析

励磁绕组两点接地故障与匝间短路故障一样,不仅使转子励磁磁势出现偶次谐波分量,而且在气隙中出现恒定磁势,后者的作用在定子绕组中出现横联差动电流,从而对差动保护带来干扰。利用这种现象,还可实现励磁绕组匝间短路故障的在线监测。     

Ground distance relay Compensation based on fault resistance calculation

The fault resistance introduces an error in the fault distance estimate, and hence may create an unreliable operation of a distance relay. A new compensation method based on fault resistance calculation is presented. The fault resistance calculation is based on monitoring the active power at the relay point. The compensated fault impedance measures accurately the impedance between the relay location and the fault point. The relay has shown satisfactory performances under various fault conditions especially for the ground faults with high fault resistance. This new compensation method avoids the under-reach problem in ground distance relays.     

谐波电场法带电检测直流绝缘子

挂网运行的直流绝缘子数量越来越多,其检测问题日益突出。分析认为,合成绝缘子内导通性故障及零值瓷绝缘子是最难发现、最危险的故障。文中研究了这类故障直流绝缘子的带电检测问题,提出利用直流绝缘子串周围谐波电场带电检测直流绝缘子的新方法,即谐波电场法,并进行了可行性试验与灵敏度分析。现场与实验室可行性试验表明,谐波电场法能够对直流绝缘子进行在线检测。理论分析表明,该方法灵敏度较高,能有效地给出低于一定阻值的零值瓷绝缘子和合成绝缘子内绝缘导通性故障信息。谐波电场法的提出为直流绝缘子带电检测提供了有力手段。     

First harmonic analysis: a circuit theory point of view

The first harmonic method is usually applied to nonlinear system analysis, being particularly adequate for the study of oscillations. As shown in this paper, oscillators analysis can be performed by using only basic concepts of circuit theory. The nonlinearities present in the oscillator electric circuits are approximated, via the first harmonic method, by resistance or source-controlled equivalent circuit elements, and the resulting linear circuits are entirely analyzed via conventional circuit theory. As a result, the analysis of electric oscillators and the first harmonic method are introduced in a comprehensible electric circuit context     

Grounding of Generators Connected to Industrial Plant Distribution Buses

Increasing ``energy consciousness' in industry is causing expanded interest in in-plant generators, which are normally connected to the main plant distribution bus. Electrical system reliability requires the proper application of neutral grounding and ground fault protection for these machines. Mechanical limits on safe generator ground fault currents, possible circulating harmonic currents in multiunit installations, and the effects of internal ground faults add complexity.     

接地异常引起的设备故障分析处理

以一起变电站接地异常引起的设备故障为例,分析了造成设备故障的原因,提出下一步急需对所有变电站接地网开展普测和开挖检查工作,对锈蚀严重、接地电阻超标的立即进行接地网改造等针对性防范措施,有效避免因接地异常引起设备损坏事件的重复发生。     

小电流接地系统单相接地机理分析

主要从配电网单相接地的等效电路入手,分析了中性点不接地系统或经消弧线圈接地系统单相接地时,各相对地电压和中性点位移电压的变化特性,其波零序电流和五次谐波零序电流的分布规律,为接地相和接地线路的判别提供了理论依据。     

Stray Voltage: Sources and Solutions

Stray voltage is caused by voltage drop and ground faults and may have its origin on the primary electrical distribution system or on the customer's secondary electrical system. The rms value of the neutral-to-earth voltage along a primary distribution line may be at a value of zero some distance from the substation depending on the condition of the conductor resistances, grounding resistances, and the amount of load. Neutral-to-earth resistance is not the cause of stray voltage; however, the value of this resistance to earth at a particular location will affect the level of stray voltage. A four-wire single-phase feeder system supplying farm buildings from a single metering point is effective in preventing on-farm secondary neutral voltage drop, provided the four-wire system is extended to all farm loads, and provided no high-magnitude ground faults are present. Isolation of the primary and secondary neutral systems at the distribution transformer is effective in preventing off-farm sources from entering the customer's system. This separation may be accomplished using a number of commercially available devices.     

混合遗传算法在接地电阻测量中的应用

针对双钳法测量接地电阻时,接地极数量和接地极腐蚀对测量结果造成较大误差的问题,提出了一种基于混合遗传算法(HGA)的接地电阻测量方法。在接地电阻等效模型基础上构建接地方程组,利用遗传算法和梯度下降法优化目标函数,根据测量数据R_m求解接地电阻等效模型,实现对接地电阻的测量。仿真结果表明:所提方法得到的接地电阻最大误差为1.76%,在接地极腐蚀,接地回路不可靠的情况下能够提高接地极电阻的测量精度,基本解决双钳法测量接地电阻受接地极数量影响的问题。     

大型超临界汽轮发电机匝间短路故障分析及主保护研究

大型超临界汽轮发电机组将是我国的主力机组,电抗的增大和定子绕组电阻的减少将使短路电流水平下降,保护灵敏度降低.同时由于结构设计的原因,综合性能较好的一些保护方案如横差保护和不完全纵差保护等将不能应用于大型超临界汽轮发电机.文中重点讨论了匝间短路后各电气量的变化对继电保护的影响.研究表明,发电机发生匝间短路故障后,随着短路匝数的增加,转子二次谐波电流以及单元件横差电流均有较大的变化,以此构成的相应保护方案将能够可靠动作.在此基础上,提出故障分量负序功率方向保护可以作为一重主保护方案,并就该方案在实际应用中所涉及的主要问题提出了看法和建议.     

Transmission line fault location for double phase-to-earth fault onnon-direct-ground neutral system

An accurate algorithm for locating double phase-to-earth faults on transmission lines of nondirect ground neutral systems is presented. The algorithm employs the faulted phase network and zero-sequence network as the fault location model. It effectively eliminates the effect of load flow and fault resistance on the accuracy of fault location. The technique embodies an accurate location by measuring only one local end data. The algorithm is used in a procedure that provides the automatic determination of faulted line and phase, rather than requires engineer to specify them. Simulation results have shown the effectiveness of the algorithm under the condition of earth faults     

Contact-failure phenomena and their development in gas-insulated switchgear

To explain contact failure phenomena and their development in the gas-insulated switchgear (GIS). tests were conducted by simulating abnormal current-carrying conditions using an actual 300-kV, 2000-A GIS. This paper describes the process observed. ranging from contact failures to ground faults, deteriorative phenomena and some detection methods for initial contact failure conditions. A summary of this study is as follows: (1) The contact resistances are greatly dependent on the materials and construction of contacts. (2) The relation between the initial contact resistance and the carried current resulting in arcing is derived with approximately constant Joule-heat generation. (3) Ground faults result from sagging of a shield melted by arcing or a combination of two causes: (i) scattering of molten metal caused by arcing; and (ii) sagging of a conductor caused by bolt melt down. (4) To detect initial contact failures, it is recommended to observe the contact resistance, the temperature rise on the tank surface, or the electrical partial discharges.     

Influence of difference system parameters on operating conditions of third harmonic ground-fault protection system of unit-connected generator

In the paper, the influence of unbalance of the measuring element of the unit-connected generator stator ground-fault protection system excited by the ratio of third harmonic voltage to ground in the generator neutral and at the generator terminals on the value of the voltage supplied to the measuring element during ground faults is analyzed. The analysis is made for currently used ground-fault protection systems in which the measuring element is balanced by an autotransformer voltage divider or an impedance bridge.     

Third harmonic ground-fault protection system of unit-connected generator with two parallel branches per phase

The operating conditions of a ground-fault protection reacting to a difference in 3rd harmonic voltages at the star point and at the output terminals of a unit-connected generator with a dipole stator winding formed by two parallel branches in phase are analysed. The voltage feeding the measuring element of the ground-fault protection in the generator is determined for different ways in which its star point is grounded. The paper, analyse the operating conditions of the above protection systems in the unit-connected generator with two parallel branches in stator windings phase and one pair of poles, on the basis of simulation studies. The simulations were run for real generator of 120 MW equipped with generator breakers with additional capacitances to ground. Realistic values of the breakdown channel in the main isolation of the generator stator windings were taken into account. This resistance was determined in experimental tests conducted in real conditions during arcing ground faults.     

纵向零序电压保护新方案及其应用

由于结构设计的原因,综合性能较好的一些保护方案如横差保护和不完全纵差保护等将不能应用于大型超临界汽轮发电机。文中重点讨论了纵向零序电压保护方案的改进及其在应用中的问题。研究表明,发电机发生匝间短路故障时3次谐波电势减小,而机组外部故障时发电机3次谐波电势增大。因此,为了提高纵向零序电压保护方案的性能,可以根据故障时3次谐波电压增量的特性作为选取制动量的依据,从而提高保护性能。亦即：当3次谐波电压表现为负增量时,制动量取为0或接近0;而当3次谐波电压表现为正增量时,以该增量值作为制动量。此外,对匝间保护纵向零序电压元件增加一个小延时是可行的,它可以提高保护躲暂态3次谐波影响的能力。仿真计算表明,比例制动式纵向零序电压保护能可靠区分内外部故障,符合大型超临界汽轮发电机的保护要求。     

A new method for discrimination of ground faults causes in distribution lines

Since most problems in distribution lines are ground faults, the technology to distinguish the causes in the distribution lines will be very useful in preventing faults beforehand or, at least finding any problems at an early stage. The waveforms of ground faults which were collected from data concerning problems in the distribution lines and ground fault experiment are analyzed. The technology which could predict classified groups drawn from the scale of I₀ (the waveform of ground fault current) and V₀ (the waveform of ground fault voltage), and both of them in the ground faults is studied. The results demonstrated that I₀ could be classified into three groups, that is, leaking ground faults are sine curve shaped, gap discharge ground faults are triangular, and intermittent discharge ground faults are needle shaped. Also, simulated equivalent impedance of the causes of ground faults by EMTP is calculated. Furthermore, the influence caused by the condition of the distribution lines is investigated. With these results, a technique is proposed which could predict the cause of problems in ground faults.     

A new digital ground-fault protection system for generator–transformer unit

Ground faults are one of most often reasons of damages in stator windings of large generators. Under certain conditions, as a result of ground-fault protection systems maloperation, ground faults convert into high-current faults, causing severe failures in power system. Numerous publications in renowned journals and magazines testify about ground-fault matter importance and problems reported by exploitators confirm opinions, that some issues concerning ground-fault protection of large generators have not been solved yet or have been solved insufficiently. In this paper a new conception of a digital ground-fault protection system for stator winding of large generator was proposed. The process of intermittent arc ground fault in stator winding has been briefly discussed and actual ground-fault voltage waveforms were presented. A new relaying algorithm, based on third harmonic voltage measurement was also drawn and the methods of its implementation and testing were described.     

与平衡桥有关的几种典型接地故障的处理与分析

分析了变电站直流电源系统几种比较典型的、与平衡桥电阻有关的接地故障的处理过程与方法,包括全接地故障、断开负荷发生接地故障、2套直流系统交替出现接地故障、蓄电池接地故障等;提出了用于变电站直流电源系统接地故障检测的平衡桥电阻的配置原则及注意事项。     

Development of a fault locator for a branch line

Formerly, only pulse radar and the Murray loop bridge could be utilized in a cable fault location method that is applicable to branch lines. Those methods need a terminal connection at the far end. Therefore, it is difficult in fault location involving branch lines. Pulse radar is applicable only for high‐resistance ground faults and the Murray loop bridge is applicable only for low‐resistance ground faults. We have developed current‐detection pulse radar and tested it. In high‐resistance ground fault location, the measured result includes discharge delay error. Therefore, we have proposed an error correction method and confirmed it. In addition, we have designed a new cable fault location method of pulse radar and tested it. © 2000 Scripta Technica, Electr Eng Jpn, 134(2): 19–28, 2001