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1、Power System Protections The steady-state operation of a power system is frequently disturbed by various faults on electrical equipment. To maintain the proper operation of the power system, an effective, efficient and reliable protection scheme is required. Power system components are designed to o
2、perate under normal operating conditions.However, due to any reason, say a fault, there is an abnormality, it is necessary that there should be a device which senses these abnormal conditions and if so, the element or component where such an abnormality has taken place is removed, i.e. deleted from
3、the rest of the system as soon as possible. This is necessary because the power system component can never be designed to withstand the worst possible conditions due to the fact that this will make the whole system highly uneconomical. And therefore, if such an abnormality takes place in any element
4、 or component of the power system network, it is desirable that the affected element/component is removed from the rest of the system reliably and quickly in order to restore power in the remaining system under the normal condition as soon as possible. The protection scheme includes both the protect
5、ive relays and switching circuits, i.e. circuit breakers. The protective relay which functions as a brain is a very important component. The protective relay is a sensing device, which senses the fault, determines its location and then sends command to the proper circuit breaker by closing its trip
6、coil. The circuit breaker after getting command from the protective relay disconnects only the faulted element. this is why the protective relay must be reliable, maintainable and fast in operation. In early days, there used to be electromechanical relay of induction disk-type.However, very soon the
7、 disk was replaced by inverted cup, i.e.hollow cylinder and the new relay obtained was known as an induction cup or induction cylinder relay. This relay, which is still in use, possesses several important features such as higher speed; higher torque for a given power input an more uniform torque. Ho
8、wever, with the advent of electronic tubes, electronic relays having distinct features were developed during 1940s. With the discovery of solid state components during 1950s, static relays with numerous advantages were developed. The use of digital computers for protective relaying purposes has been
9、 engaging the attention of research and practicing engineers since layer 1960s and 1980s. Now, the microprocessor/mini computer-based relaying scheme, because of its numerous advantages such as self checking feature and flexibility, has been widely used in power system all over the world. The overal
10、l system protection is divided into following sections: (i)Generator protection,(ii)Transformer protection,(iii)Bus protection,(iv)Feeder protection,(v)Transmission line protection. Basic Requirements to Protective Relays Any protection scheme, which i.e. required to safeguard the power system compo
11、nents against abnormal conditions such as faults, consists basically of two elements(i)Protective relay and (ii) Circuit breaker .The protective relay which is primarily the brain behind the whole scheme plays a very important role. Therefore proper care should be taken in selecting an appropriate p
12、rotective relay which is reliable, efficient and fast in operation. The protective relay must satisfy the following requirements:1 since faults on a well designed and healthy system are normally rare, the relays are called upon to operate only occasionally. This means that the relaying scheme is nor
13、mally idle and must operate whenever fault occurs. In other words, it must be reliable.2 Since the reliability partly depends upon the maintenance, the relay must be easily maintainable.3 The palpation of the relay can be in two ways. One is the failure to operate in case a fault occurs an second is
14、 the relay operation when there is no fault. As a matter of fact, relay must operate if there is a fault and must not operate if there is no fault. 4 Relaying scheme must be sensitive enough to distinguish between normal and the faulty system.Protective Relays The function of the protective relay is
15、 to sense the fault and energize the trip coil of the circuit breaker. The following types of the protective relays are used for the apparatus such as synchronous machines, bus bar, transformer and the other apparatus and transmission line protection. (1) Over current relays, (2) Under voltage relay
16、s,(3) Under frequency relays,(4) Directional relays,(5) Thermal relays,(6) Phase sequence relays such as(i)negative sequence relays and, (ii)zero sequence relays,(7) Differential relays and percentage differential relays, (8) Distance relays such as (I)plane impedance relays,(ii)angle impedance rela
17、y, i.e. Ohm or reactance relays,(iii)angle admittance relays,i.e. Mho relays and ,(iv)offset and restricted relays,(9)Pilot relays such as (i) wire pilot relays,(ii)carrier channel pilot relays,(iii)microwave pilot relays. There are different types of the relaying scheme based on construction. They
18、are:(i)electromechanical type,(ii)thermal relays,(iii) transduction relays,(iv)rectifier bridge relay,(v)electronic relays,(vi)digital relaying schemes. 电力系统继电保护电力系统的稳态运行经常会因各种电力设备配故障原因而被扰乱。 为了保证电力系统可靠运行， 有效的、充足的、可靠的保护机制是必不可少的部分。电力系统元件通常是按照在正常工作条件下 运行。然而，由于任何原因，比如说故障，出现非正常运行状态，有必要采用一个装置来感知这种 非正常状态，然
19、后这种出现非正常状态的元件可以被切除，也就是说，尽可能快地将该元件与系统 其它部分隔离。这是必须的，因为现实情况下，为避免造成很大的经济损失，电力系统元件并未按 照故障运行情况下的耐受程度设计。因此，如果电网当中的任何元件发生异常状况，受影响的元件 将会同其余部分可靠快速的切除，以便于电力系统在正常工作条件下尽可能快的恢复。保护体系包括包括保护继电器以及跳合闸回路，例如，断路器。继电包括就如同大脑的作用一 样，是十分重要的。保护继电器是一个感知元件，它感知故障、确定故障的位置，并且通过闭合相 应断路器的跳闸线圈来发出跳闸命令。断路器在接收到保护继电器的命令之后切除故障元件。这就 是为什么保护继
20、电器必须可靠、快速的运行。 以前，常用的电磁继电器是一种电感性质的，不久就被一种反转型的替代。例如将中空的圆筒 和新型保护继电器做成一种为人们所知的电感型的或者磁感应继电器。这种继电器至今还被延用， 并且拥有很多种重要特性，例如高速、大转矩作为能量输入以及更多的均衡的转矩。 然而，1940 年，电子管的出现，使电力继电器的特性得到很快的发展。1950 年，伴随着固态元 件的发现，静态继电器的许多优点有又被得以很好的发展。 1960 年至 1980 年，计算机被用于继电 器保护中，它能很好的参与并在实践中得到很好的利用。当今，微机或者袖珍型计算机已经成为了 继电保护的主题，由于其自身有很多的优点
21、例如自检功能和强的适应能力，所以，它现在已经广泛 的被用于全球的电力系统。 全球的的电力系统保护分为以下几个部分 ： （1）发电机保护； （2）变压器保护;(3）母线保护； （4）馈线保护； （5）输电线路保护。 保护继电器的基本构成任何一种保护， 均被用来保护电力系统异常情况下发生的故障， 它包括两个基本元件分别是 （1） 保护继电器； （2）断路器。保护继电器在保护当中起着十分重要的作用。因此在选择合适的继电器 时应当投入相当大的精力，以满足继电器在运行中的可靠性、快速性、灵敏性。保护继电器必须满 足下列条件。 任何设计完善能够正常运行的系统都或多或少的存在少许的问题，因此这就要求保护继电
22、器 动作的灵敏性要高。 这就是说继电保护系统平常是空载运行的但无论何时一旦有故障发生必须运行。 胡句话说，它应当有相当的可靠性。 可靠性很大的程度上取决于它是否易于维护，因此，继电器应当便于维护。 继电器的误操作有两种可能，一种是故障发生时运行失败，另一种是系统正常运行时，继电 器动作。现实运行中要求，继电器在故障时必须可靠动作，在无故障是必须可靠不动作。5 保护系统必须能够具有充足的灵敏性去判断区分故障系统与正常运行系统。 保护继电器继电器的作用是感应故障并与断路器配合工作。同步奠基，母线，变压器，其他电气设备和输电线路保护中通常使用下列几种继电器。 （1） 过流继电器， （2） 过压继电器， （3） 低频继电器， （4） 方向继电器， （5） 温度继电器， （6） 相序继电器例如负序继电器以及零序继电器， （7） 差动继电器和比率差动继电器， （8） 距离继电器，包括平面阻抗继电器，角度阻抗继电器如欧姆或阻抗继电器，角度导纳继 电器如母欧继电器，偏置继电器等。 （9） 控制继电器，例如有线控制继电器，载波控制继电器，微波控制继电器。 继电保护按照结构可分为很多不同的种类。分别是：电磁继电器，热继电器，整流桥型继电器， 电子继电器，静态继电器，数字式保护继电器。