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LINE PROTECTION WITH DISTANCE RELAYSDistance relaying should be considered when overcurrent relaying is too slow or is not selective. Distance relays are generally used for phase-fault primary and back-up protection on subtransmission lines, and on transmission lines where high-speed automatic reclosing is not necessary to maintain stability and where the short time delay for end-zone faults can be tolerated. Overcurrent relays have been used generally for ground-fault primary and back-up protection, but there is a growing trend toward distance relays for ground faults also.Single-step distance relays are used for phase-fault back-up protection at the terminals of generators. Also, single-step distance relays might be used with advantage for back-up protection at power-transformer tanks, but at the present such protection is generally provided by inverse-time overcurrent relays.Distance relays are preferred to overcurrent relays because they are not nearly so much affected by changes in short-circuit-current magnitude as overcurrent relays are, and , hence , are much less affected by changes in generating capacity and in system configuration. This is because, distance relays achieve selectivity on the basis of impedance rather than current.THE CHOICE BETWEEN IMPEDANCE, REACTANCE, OR MHOBecause ground resistance can be so variable, a ground distance relay must be practically unaffected by large variations in fault resistance. Consequently, reactance relays are generally preferred for ground relaying.For phase-fault relaying, each type has certain advantages and disadvantages. For very short line sections, the reactance type is preferred for the reason that more of theline can be protected at high speed. This is because the reactance relay is practically unaffected by arc resistance which may be large compared with the line impedance, as described elsewhere in this chapter. On the other hand, reactance-type distance relays at certain locations in a system are the most likely to operate undesirably on severe synchronizing-power surges unless additional relay equipment is provided to prevent such operation.The mho type is best suited for phase-fault relaying for longer lines, and particularly where severe synchronizing-power surges may occur. It is the least likely to require additional equipment to prevent tripping on synchronizing-power surges. When mho relaying is adjusted to protect any given line section, its operating characteristic encloses the least space on the R-X diagram, which means that it will be least affected by abnormal system conditions other than line faults; in other words, it is the most selective of all distance relays. Because the mho relay is affected by arc resistance more than any other type, it is applied to longer lines. The fact that it combines both the directional and the distance-measuring functions in one unit with one contact makes it very reliable.The impedance relay is better suited for phase-fault relaying for lines of moderate length than for either very short or very long lines. Arcs affect an impedance relay more than a reactance relay but less than a mho relay. Synchronizing-power surges affect an impedance relay less than a reactance relay but more than a mho relay. If an impedance-relay characteristic is offset, so as to make it a modified• relay, it can be made to resemble either a reactance relay or a mho relay but it will always require a separate directional unit.There is no sharp dividing line between areas of application where one or another type of distance relay is best suited. Actually, there is much overlapping of these areas. Also, changes that are made in systems, such as the addition of terminals to a line, can change the type of relay best suited to a particular location. Consequently, to realizethe fullest capabilities of distance relaying, one should use the type best suited for each application. In some cases much better selectivity can be obtained between relays of the same type, but, if relays are used that are best suited to each line, different types on adjacent lines have no appreciable adverse effect on selectivity. THE ADJUSTMENT OF DISTANCE RELAYSPhase distance relays are adjusted on the basis of the positive-phase-sequence impedance between the relay location and the fault location beyond which operation of a given relay unit should stop. Ground distance relays are adjusted in the same way, although some types may respond to the zero-phase-sequence impedance. This impedance, or the corresponding distance, is called the "reach" of the relay or unit. For purposes of rough approximation, it is customary to assume an average positive-phase-sequence-reactance value of about 0.8 ohm per mile for open transmission-line construction, and to neglect resistance. Accurate data are available in textbooks devoted to power-system analysis.To convert primary impedance to a secondary value for use in adjusting a phase or ground distance relay, the following formula is used:where the CT ratio is the ratio of the high-voltage phase current to the relay phase current, and the VT ratio is the ratio of the high-voltage phase-to-phase voltage to the relay phase-to-phase voltage–all under balanced three-phase conditions. Thus, for a 50-mile, 138-kv line with 600/5 wye-connected CT’s, the secondary positive-phase-sequence reactance is aboutIt is the practice to adjust the first, or high-speed, zone of distance relays to reach to80% to 90% of the length of a two-ended line or to 80% to 90% of the distance to the nearest terminal of a multiterminal line. There is no time-delay adjustment for this unit.The principal purpose of the second-zone unit of a distance relay is to provide protection for the rest of the line beyond the reach of the first-zone unit. It should be adjusted so that it will be able to operate even for arcing faults at the end of the line. To do this, the unit must reach beyond the end of the line. Even if arcing faults did not have to be considered, one would have to take into account an underreaching tendency because of the effect of intermediate current sources, and of errors in: (1) the data on which adjustments are based, (2) the current and voltage transformers, and (3) the relays. It is customary to try to have the second-zone unit reach to at least 20% of an adjoining line section; the farther this can be extended into the adjoining line section, the more leeway is allowed in the reach of the third-zone unit of the next line-section back that must be selective with this second-zone unit.The maximum value of the second-zone reach also has a limit. Under conditions of maximum overreach, the second-zone reach should be short enough to be selective with the second-zone units of distance relays on the shortest adjoining line sections, as illustrated in Fig. 1. Transient overreach need not be considered with relays having a high ratio of reset to pickup because the transient that causes overreach will have expired before the second-zone tripping time. However, if the ratio of reset to pickup is low, the second-zone unit must be set either (1) with a reach short enough so that its overreach will not extend beyond the reach of the first-zone unit of the adjoining linesection under the same conditions, or (2) with a time delay long enough to be selective with the second-zone time of the adjoining section, as shown in Fig. 2. In this connection, any underreaching tendencies of the relays on the adjoining line sections must be taken into account. When an adjoining line is so short that it is impossible to get the required selectivity on the basis of react, it becomes necessary to increase the time delay, as illustrated in Fig. 2. Otherwise, the time delay of the second-zone unit should be long enough to provide selectivity with the slowest of (1) bus-differential relays of the bus at the other end of the line(2)transformer-differential relays of transformers on the bus at the other end of the line,or (3) line relays of adjoining line sections. The interrupting time of the circuit breakers of these various elements will also affect the second-zone time. This second-zone time is normally about 0.2 second to 0.5 second.The third-zone unit provides back-up protection for faults in adjoining line sections.So far as possible, its reach should extend beyond the end of the longest adjoining line section under the conditions that cause the maximum amount of underreach, namely, arcs and intermediate current sources. Figure 3 shows a normal back-up characteristic. The third-zone time delay is usually about 0.4 second to 1.0 second. To reach beyond the end of a long adjoining line and still be selective with the relays of a short line, it may be necessary to get this selectivity with additional time delay, as in Fig. 4.THE EFFECT OF ARCS ON DISTANCE-RELAY OPERATIONThe critical arc location is just short of the point on a line at which a distance relay's operation changes from high-speed to intermediate time or from intermediate time to back-up time. We are concerned with the possibility that an arc within the high-speed zone will make the relay operate in intermediate time, that an arc within the intermediate zone will make the relay operate in back-up time, or that an arc within the back-up zone will prevent relay operation completely. In other words, the effect of an arc may be to cause a distance relay to underreach.For an arc just short of the end of the first- or high-speed zone, it is the initial characteristic of the arc that concerns us. A distance relay's first-zone unit is so fast that, if the impedance is such that the unit can operate immediately when the arc is struck, it will do so before the arc can stretch appreciably and thereby increase itsresistance. Therefore, we can calculate the arc characteristic for a length equal to the distance between conductors for phase-to-phase faults, or across an insulator string for phase-to-ground faults. On the other hand, for arcs in the intermediate-time or back-up zones, the effect of wind stretching the arc should be considered, and then the operating time for which the relay is adjusted has an important bearing on the outcome.Tending to offset the longer time an arc has to stretch in the wind when it is in the intermediate or back-up zones is the fact that, the farther an arcing fault is from a relay, the less will its effect be on the relay's operation. In other words, the more line impedance there is between the relay and the fault, the less change there will be in the total impedance when the arc resistance is added. On the other hand, the farther away an arc is, the higher its apparent resistance will be because the current contribution from the relay end of the line will be smaller, as considered later.A small reduction in the high-speed-zone reach because of an arc is objectionable, but it can be tolerated if necessary. One can always use a reactance-type or modified-impendance type distance relay to minimize such reduction. The intermediate-zone reach must not be reduced by an arc to the point at which relays of the next line back will not be selective; of course, they too will be affected by the arc, but not so much. Reactance-type or modified-impendance-type distance relays are useful here also for assuring the minimum reduction in second-zone reach. Figure 5 shows how an impedance or mho characteristic can be offset to minimize its susceptibility to an arc. One can also help the situation by making the second-zone reach as long as possible so that a certain amount of reach reduction by an arc is permissible. Conventional relays do not use the reactance unit for the back-up zone; instead, they use either an impedance unit, a modified-impendance unit, or a mho unit. If failure of the back-up unit to operate because of an arc extended by the wind is a problem, the modified-impendance unit can be used or the mho–or "starting"–unitcharacteristic can also be shifted to make its operation less affected by arc resistance. The low-reset characteristic of some types of distance relay is advantageous in preventing reset as the wind stretches out an arc.Although an arc itself is practically all resistance, it may have a capacitive-reactance or an inductive-reactance component when viewed from the end of a line where the relays are. The impedance of an arc (ZA) has the appearance:where I1 = the complex expression for the current flowing into the arc from the end of the line where the relays under consideration are.I2= the complex expression for the current flowing into the arc from the other end of the line.R A = the arc resistance with current (I1 + I2) flowing into it.Of more practical significance is the fact that, as shown by the equation, the arc resistance will appear to be higher than it actually is, and it may be very much higher. After the other end of the line trips, the arc resistance will be higher because the arccurrent will be lower. However, its appearance to the relays will no longer be magnified, because I2 will be zero. Whether its resistance will appear to the relays to be higher or lower than before will depend on the relative and actual magnitudes of the currents before and after the distant breaker opens.输电线路的距离保护在过电流保护灵敏度低或选择性差时，应当考虑采用距离保护。
电力系统分析要点与习题第二版简介《电力系统分析要点与习题第二版》是一本介绍电力系统分析相关知识的教材。
本书从电力系统的基础知识、电力负荷和电力市场开始，逐步深入到电力系统的稳态与稳定分析、电力系统的暂态分析、功率系统的控制与保护等方面。
在每个章节中，本书给出了大量的例子和习题，以帮助读者全面掌握电力系统分析的核心知识。
电力系统的基础知识电力系统的基础知识包括电力系统的组成、电力系统的运行方式以及电力系统的负荷分布。
在这一章节中，本书详细介绍了电力系统的不同组成部分，包括发电机、变压器、开关和输电线路等。
同时，本书还介绍了电力系统的运行方式，包括传统的主动力平衡（AC）系统和现代的直流输电（HVDC）系统。
另外，本章节还详细介绍了电力系统的负荷分布，包括短时和长时的负荷曲线。
电力负荷和电力市场在本章节中，本书介绍了电力负荷和电力市场的概念，以及不同电力市场之间的区别。
本书还介绍了电力市场中不同标准的电力，包括质量、计量和价格等方面的标准。
电力系统的稳态与稳定分析电力系统的稳态与稳定分析是电力系统分析的核心内容之一。
在这一章节中，本书详细介绍了电力系统的稳态和稳定性的定义、计算方法和评价方法。
同时，本书还介绍了电力系统的稳定分析中常见的各种不稳定状态，包括短路、缺相和失稳等状态。
电力系统的暂态分析电力系统的暂态分析是电力系统分析的另一个核心内容。
在这一章节中，本书详细介绍了电力系统的暂态分析的原理、方法和计算技术。
本书还通过大量的例子说明了电力系统暂态分析的实践应用。
功率系统的控制与保护功率系统的控制与保护是电力系统分析的重要内容之一。
在这一章节中，本书介绍了功率系统控制和保护的原则、方法和技术。
本书还详细介绍了电力系统故障诊断和故障恢复的技术，以及各种电力系统保护装置的原理和应用。
习题解答本书的章节中，均配有大量的例子和习题，以帮助读者掌握电力系统分析的核心知识。
在这一章节中，本书提供了对所有习题的详细解答，以帮助读者加深对所学知识的理解。
电力系统分析习题集（第二章） 【例2-1】 利用牛顿法计算图2-8所示系统的潮流分布。 【解】按照图2-7所示牛顿法潮流程序原理框图进行计算。迭代计算以前的准备工作包括形成导纳矩阵和送电压初值。 由例1-1可知，该系统的导纳矩阵为： 4235
15=0
V5=1.05 V4=1.05
P4=5
1:1.051.05:1
j0.03j0.015
2+j1
1.6+j0.8
0.08+j0.300.1+j0.353.7+j1.3
j0.25j0.25j0.25j0.25
图2-8 简单模型系统0.04+j0.25
图2-8 简单模型系统 Y=33333.3300000.074603.3100000.066667.6600000.049206.6300000.074603.3100000.073786.3558459.111203.382987.064150.275471.049206.6300000.011203.382987.098082.6645390.190015.362402.064150.275471.090015.362402.029166.637874.1jjjjjjjjjjjjjjj 各节点的电压初值如表2-1所示: 表2-1电压初值 节点 1 2 3 4 5
)0(e 1.00000 1.00000 1.00000 1.05000 1.05000
)0(f 0.00000 0.00000 0.00000 0.00000 0.00000
根据式(2-52)，(2-53)可建立修正方程式常数项(误差项)的算式： )]()()[(313313212212111111111fBeGfBeGfBeGePPs )]()()[(3133132122121111111eBfGeBfGeBfGf )]()()[(313313212212111111111fBeGfBeGfBeGfQQs )]()()[(3133132122121111111eBfGeBfGeBfGe …… …… )]()[(444444242242444fBeGfBeGePPs )]()[(4444442422424eBfGeBfGf )(24242424feVVs 根据式(2-55)，(2-56)可以得到雅可比矩阵各元素的算式： 1111113133132122121111111
电力系统电力系统介绍随着电力工业的增长，与用于生成和处理当今大规模电能消费的电力生产、传输、分配系统相关的经济、工程问题也随之增多。
这些系统构成了一个完整的电力系统。
应该着重提到的是生成电能的工业，它与众不同之处在于其产品应按顾客要求即需即用。
生成电的能源以煤、石油，或水库和湖泊中水的形式储存起来，以备将来所有需。
但这并不会降低用户对发电机容量的需求。
显然，对电力系统而言服务的连续性至关重要。
没有哪种服务能完全避免可能出现的失误，而系统的成本明显依赖于其稳定性。
因此，必须在稳定性与成本之间找到平衡点，而最终的选择应是负载大小、特点、可能出现中断的原因、用户要求等的综合体现。
然而，网络可靠性的增加是通过应用一定数量的生成单元和在发电站港湾各分区间以及在国内、国际电网传输线路中使用自动断路器得以实现的。
事实上大型系统包括众多的发电站和由高容量传输线路连接的负载。
这样，在不中断总体服务的前提下可以停止单个发电单元或一套输电线路的运作。
当今生成和传输电力最普遍的系统是三相系统。
相对于其他交流系统而言，它具有简便、节能的优点。
尤其是在特定导体间电压、传输功率、传输距离和线耗的情况下，三相系统所需铜或铝仅为单相系统的75%。
三相系统另一个重要优点是三相电机比单相电机效率更高。
大规模电力生产的能源有：1.从常规燃料（煤、石油或天然气）、城市废料燃烧或核燃料应用中得到的蒸汽；2.水；3.石油中的柴油动力。
其他可能的能源有太阳能、风能、潮汐能等，但没有一种超越了试点发电站阶段。
在大型蒸汽发电站中，蒸汽中的热能通过涡轮轮转换为功。
涡轮必须包括安装在轴承上并封闭于汽缸中的轴或转子。
转子由汽缸四周喷嘴喷射出的蒸汽流带动而平衡地转动。
蒸汽流撞击轴上的叶片。
中央电站采用冷凝涡轮，即蒸汽在离开涡轮后会通过一冷凝器。
冷凝器通过其导管中大量冷水的循环来达到冷凝的效果，从而提高蒸汽的膨胀率、后继效率及涡轮的输出功率。
而涡轮则直接与大型发电机相连。
Good morning, Professor Zhou and Professor Ma. Hello every one. Thank you for your listening. My name is houzhaohao, and my interest lies in the technology of renewable energy, particularly in wind power system.Electric Power System , in my personal opinion, is a huge and complex subject and can be classified by different characteristics. But in brief , it components that transform other types of energy into electrical energy and transmit this energy to a consumer. The production and transmission of electricity is relatively efficient and inexpensive, although unlike other forms of energy, electricity is not easily stored and thus must generally be used as it being produced.The power system can be roughly separated into three parts. The first part is generation, and the vast majority of generation is produced by synchronous generators. The second part is transmission and distribution, which consists of power transformers, transmission lines, capacitors, reactors and protection devices. The third part is load. Loads consist of a large number of, and a diverse assortment, of devices, from home appliances, lights, motors, computers to heavy industrial or manufacturing processes and equipments.Many products and processes are sensitive not only to the continuity of power supply, but also to the constancy of electrical frequency and voltage, which is also called powerquality['kwɔləti]. So,for both commercial and technical purpose['pə:pəs], all of the three electrically connected or inter-tied parts must be operated in an electric balance.Power system automation refers to using“instrumentation and control”(I&C) system or devices to perform automatic decision making, and control of the power system.That's all, thank you very much. Thank you, every one!。
