On Single-Phase Earth Faults in a 20-kV Power Network

Single-phase earth faults in per-phase shielded cables of a 20-kV electrical network that is relatively new in Russia are studied experimentally. In contrast to the widely used 6- to 10-kV electrical network with an isolated or compensated neutral, the 20-kV network has low-resistance neutral grounding. Therefore, calculated conditions for substantiation and choice of nominal parameters of per-phase shielded cables for the 6- to 10- and 20-kV networks can significantly differ. It is shown that shielding for 6- to 10-kV cables is chosen for the case of a double-phase to earth fault (two phases connected with the earth), whereas, for 20-kV cables, this is a single-phase earth fault, which is significantly less strict with respect to thermal stability. Double-phase earth faults in 20 kV networks are practically excluded due to significantly lower levels of overloads in the network upon commutations and a comparatively short time (0.25–1.0 s) of faulted-line tripping. It is established experimentally that the single-phase earth fault in 20-kV per-phase shielded cables does not transforms to a multiphase earth fault. It is substantiated that the choice of shields for these cables based on the current of single-phase earth fault eases the requirements to thermal stability of the cables, which is of principal importance for lowering the cable production cost and power and energy losses in electrical networks.     

Toward improved sensitivity of overcurrent protection of 6- to 10-kV lines

The sensitivity of overcurrent protection of 6- to 10-kV lines is considered. Special cases of insufficient sensitivity of line overcurrent protection to single-phase faults behind the transformers connected to the line are displayed. Appropriate measuring circuitry is offered as a method of improving protection sensitivity.     

On technical requirements for Arc Suppression Reactors in the distribution circuit of 6–35 kV

Development of the technical requirements for arc suppression reactors (ASRs) used for the capacitive current compensation of the single-phase earth fault is a critical task. The corresponding GOST (State Standard) 19470–74 “Oil-Filled Arc Suppression Reactors. Technical Requirements” is no more valid; automatically controlled reactors (with adjustable spacing controlled by magnetic biasing) are the mainstream; and completely new reactor designs, with the capacitor adjustment in particular, have become available. The standard guide for the earth-fault current compensation [1], not taking into account new technologies, has become obsolete. This paper considers the current criteria for the ASR application in the 6- to 35-kV networks and proposes new ones. It is proposed to use the residual earth-fault current, but not the ASR detuning coefficient, as the major criterion for capacitive current compensation of the single-phase earth fault; in this case, new requirements for the ASR and the network conditions characterized by active phase-to-ground and harmonics content currents are specified taking into account the electrical safety regulations. The problem of capacitance asymmetry is considered absolutely and relatively, and it is shown that the use of ASRs controlled by magnetic biasing allows omitting the network balancing and limitation of the asymmetry voltage at the level of 0.75% of the phase voltage. It is proposed to take into account these recommendations during development of an earth-fault current compensation guide and general technical requirements for ASRs and automatic tuning systems.     

Corrosion protection of 6-10-35-110-220-kV cable lines in St. Petersburg

Data on the corrosion protection system of the 6-10-35-110-220-kV power cable lines in St. Petersburg are given. Information on how to select equipment for units of cathode protection of 110-220-kV, oil-filled cable lines is given.     

Zero sequence directional earth-fault protection with improved characteristics for compensated distribution networks

Selectivity of directional active power (‘wattmetric’) relays for zero sequence earth-fault protection of compensated medium voltage distribution networks is analysed. The analysis is based on estimation of zero sequence steady-state currents in compensated networks. Maloperations caused by imbalance currents of zero sequence current filters are taken into account. A new method to improve the selectivity is proposed.     

The effect of the operating regime of 6(10)-kV power cables on parameters of partial discharges in insulation

The effect of the operating regime of 6(10)-kV power cables of urban power-distribution networks on the parameters of partial discharges in insulation is investigated. Operating regimes of the cable, regimes after high-voltage testing and short circuits, and the ability to diagnose a single-phase fault in an urban 6(10)-kV power-distribution network are analyzed. Dependences of changes in partial discharge intensity in these regimes are defined.     

Time-delayed current cutoffs in cable lines

The factors affecting the operating conditions of (6–10)-kV cable lines are considered. In some instances, zones characterized by instability under the thermal action of the short-circuit current appear in power-line development are noted. Elimination of such instability by means of time-delayed current cutoffs is proposed.     

电力系统黑启动过程中线路继电保护的配置与整定

针对电力系统黑启动过程的特殊要求,提出了其继电保护配置的基本原则;研究了适用于黑启动初期单电源运行方式下以零序电流保护和距离保护为主的线路保护配置方案;在分析不同运行方式下零序故障电流特点的基础上,研究并提出了零序电流保护的整定方法。     

微机式自适应馈线保护装置

介绍了ＷＸＢ－３２型微机式自适应馈线保护装置的特点、功能,硬软件及使用方法。能在只利用当地故障信息的条件下,在线识别系统的运行方式和故障状态,并在此基础上自动计算整定速断和过电流保护的定值,用户可以根据不同的要求选用不同功能的速断和过电流保护,达到了一机多用的效果;     

柔性直流输电系统桥臂过流保护定值配合方法

针对交流故障下柔性直流桥臂过流保护可能误动导致故障穿越失败,直流故障下桥臂过流保护拒动或慢动可能导致开关器件过流等问题,讨论了柔性直流换流阀桥臂过流保护功能在阀控和直流保护装置中的配置和定值配合关系。借鉴交流保护定值配合图的概念,提出了桥臂过流保护功能在阀控装置与直流保护装置中的优化配合方法。最后,在PSCAD/EMTDC中搭建了仿真平台,结果表明所述桥臂过流保护配合关系和定值设置方法可在交流故障下成功实现故障穿越,并可在直流极间短路等严重故障下可靠闭锁换流器并触发旁路晶闸管,降低电力电子器件的过流风险。     

A novel scheme for current-only directional overcurrent relay

Overcurrent relays are widely used as main protection in sub-transmission and distribution systems. In mesh and multi-source networks, application of directional relay is unavoidable. Traditional directional overcurrent relays use the reference voltage phasor as the polarizing quantity to estimate the direction of the fault. Traditional direction distinguishing scheme is unreliable in the case of close-in faults. In this paper, a novel algorithm for directional overcurrent relay is proposed. The new algorithm uses only current signals for determining the fault direction. It uses superimposed component of the current signal and does not require phasor estimation. This new algorithm uses pre-fault current signal as the polarizing quantity. The proposed method is tested on simple power system in different situations. The results show it leads to fast and reliable directional protection.     

基于改进灰狼算法的DG接入配电网反时限过电流保护定值优化

反时限过电流保护在含分布式电源的配电网中应用广泛,但其定值整定大多依赖工程经验,限制了实际的工程应用。为此,提出一种基于改进灰狼算法(GWO)的DG接入配电网反时限过电流保护定值优化方法,在保护动作特性方程和对保护灵敏性、速度性、选择性要求的基础上建立优化模型。为克服灰狼算法容易早熟、收敛精度低等缺陷,在保留可调参数少这一优势的前提下,引入了佳点集初始化、收敛因子指数衰减、变异和启发式参数确定这4种策略对灰狼算法做了改进。仿真算例验证了所提模型在两相和三相故障下的可行性与优越性,并探讨了时间级差对整定的影响、将来算法优化和工程应用的方向,为大规模DG接入配电网的保护定值优化问题提供了研究思路。     

一起进口500 kV变压器夹件接地电流过大的处理

介绍了一起进口500kV变压器夹件接地电流过大的现场处理情况。早期进口的变压器铁心、夹件接地点在油箱顶部或上部,应对铁心、夹件的接地线引至油箱下部接地,便于运行中监测接地电流,及时发现铁心、夹件接地故障。当出现夹件或铁心多点接地故障后,在不具备条件彻底消除故障的情况下,通过对故障性质的综合分析判断,采用串入限流电阻的方法来限制接地电流是可行的。     

一种同步型分布式电源输出短路电流抑制技术

随着分布式电源（DG）在配电网渗透率的不断提高,其对继电保护尤其是过流保护的影响日益显著。针对短路电流输出能力强的同步DG,提出了一种基于全控型器件的励磁控制技术以抑制DG对配电网过流保护的影响。首先,分析了同步发电机机端短路时输出电流同励磁电流间的数学关联。然后,通过分析电流型逆变器调制策略下励磁能量泄放回路时间常数的影响因素,建立了面向配电网过流保护的同步DG励磁控制方案。最后,通过仿真分析验证了该励磁控制方案的有效性及其对转子匝间绝缘的影响。该方法只需要通过对全控型器件变流器的控制,即可抑制同步DG对过流保护的影响。     

Sizing equipment grounding conductors based on thermal damagecurves

In solidly grounded power systems, where separate equipment grounding conductors are used inside conduit or other raceways, sizing the equipment grounding conductor (EGC) per the minimum specified in NEC Article 250-122, Table 25D-122, can result in an inadequate ground-fault circuit design. Depending upon the magnitude of the ground-fault current and the specific rating or settings of the overcurrent protective device, an EGC can be damaged or completely destroyed before an upstream overcurrent device can clear the fault. Maximum thermal damage to an EGC often results from low-level ground faults, where currents fall below the short delay pickup setting for the protective circuit breaker. It should be recognized that Table 250-122 of the NEC serves only as a guide in determining the minimum size of an EGC. This is emphasized by the note added beneath the cable, which states "Equipment Grounding Conductors may need to be sized larger than specified in this cable in order to comply with Section 250-2(d)." Section 250-2(d), states than an effective grounding path shall "have the capacity to conduct safely any fault current likely to be imposed on it." Unfortunately, Table 250-122 is often used as the only basis for selecting an EGC. Here we present an example that illustrates how selecting an EGC based only on Table 250-122 can result in a ground-fault circuit design that may violate Section 250-2(d)     

A novel complex current ratio-based technique for transmission line protection

With respect to sensitivity, selectivity and speed of operation, the current differential scheme is a better way to protect transmission lines than overcurrent and distance-based schemes. However, the protection scheme can be severely influenced by the Line Charging Capacitive Current (LCCC) with increased voltage level and Current Transformer (CT) saturation under external close-in faults. This paper presents a new UHV/EHV current-based protection scheme using the ratio of phasor summation of the two-end currents to the local end current, instead of summation of the two-end currents, to discriminate the internal faults. The accuracy and effectiveness of the proposed protection technique are tested on the 110 kV Western System Coordinating Council (WSCC) 9-bus system using PSCAD/MATLAB. The simulation results confirm the reliable operation of the proposed scheme during internal/external faults and its independence from fault location, fault resistance, type of fault, and variations in source impedance. Finally, the effectiveness of the proposed scheme is also verified with faults during power swing and in series compensated lines.     

高压电动机负序过电流保护整定计算

分析、计算了各种运行情况下电动机的负序电流,分析了各种运行方式下出现负序电流对电动机的危害,粗略估算了各种运行方式下出现负序电流的允许时间,并分析了各种运行方式下正序电流与负序电流、负序功率方向的关系,指出无负序功率方向闭锁负序过电流时常用整定值存在的问题,在区外不对称短路时易引起误动。对不同负序过电流保护动作判据、不...     

Minimization of high harmonics in the current of a biasing controlled arc suppressing reactor

Compensation of single-phase earth fault currents in 6- to 35-kV networks is one of the purposes of using arc suppressing reactors (ASRs). Nonlinear distortion current of the reactor contributes to the residual fault current, for which reason its minimization is a critical task during ASR design. Operation under the technical saturation mode of electrotechnical steel is a characteristic feature of biasing controlled ASRs. Moreover, the biasing current of the device contains the high harmonics components; the share of these components depends on the structure of the magnetic system and on the properties of the electrotechnical steel. This paper proposes an approach to optimization of the magnetic system of a biasing controlled ASR based on adjustment of the biasing characteristics of the magnetic core according to the results of experimental measurements of the harmonic composition of reactor currents within a wide biasing range. Analysis of the influence of the parameters of the combined shell–core magnetic system on the reactor distortion current is carried out. Results of experiments for obtaining a biasing characteristic aiming at calculation of the harmonic composition of the reactor current with minimum 10% error are given. An optimization calculation of the parameters of the RUOM reactor magnetic system sections was carried out that allowed reducing the distortion current to 4% within the whole control range (10–120%) of the reactor nominal power and to 1% within the range of 60–100% of the reactor nominal power.     

110 kV主变压器间隙保护的整定及运行研究

结合国内频繁发生110 kV线路接地故障,而线路末端110 kV主变间隙保护动作跳主变三侧开关的事件,重点分析了110 kV主变间隙保护的整定和110 kV线路保护的配合问题,对保护配置与整定提出了建议。按常规设计的变压器中性点间隙距离选择、间隙电流保护整定配合和对有关保护整定配合进行了改进,提出了切实可行的措施。     

基于EPON通信的配电网保护与控制的实践

针对配电网络结构日益复杂,导致阶段式过流保护时限配合困难且无法实现全线速动的问题,结合嘉善配电网“三双”改造的试点,提出了一种基于EPON通信网络的10kV线路保护与控制方法,能够在变电站10kV馈线出口开关保护动作之前实现故障的快速定位、隔离,可有效解决配电网上下级保护定值不易配合的问题,同时通过远方备自投实现非故障区域的供电恢复,提高了供电可靠性。