外文资料

　Power System Protections

　Introduction

　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 operate under normal operating conditions. However, if 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 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 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 protective 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 send command to the proper circuit breaker by closing its trip 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 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 and more uniform torque.

　However, 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 engaging the attention of research and practicing engaging the attention of research and practicing engineers since late 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 systems all over the world.

　The overall 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 is required to safeguard the power system components 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 protective 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 normally 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 maloperation of the relay can be in two ways. One is the failure to operate in case a fault occurs and second is 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 relays is to sense the fault and energize the trip coil of the circuit breaker. The following types of protective relays are used for the apparatus such as synchronous machines, bus bar, transformer and the other apparatus and transmission line protection.

　(1)Overcurrent relays.

　(2)Undervoltage relays.

　(3)Underfrequency 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 relays, 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 are: (i) electromechanical type, (ii) thermal relays, (iii) transductor relays, (iv) rectifier bridge relay, (v) electronic relays, (vi) static relays, (vii) digital relaying schemes.

　Faults and Their Damages on Power Systems

　Faults on Transmission Lines

　Because transmission lines are exposed to lightning and other atmospheric hazards, faults on them occur more frequently than those in apparatus. The types of faults taking place on a transmission line are listed, in the order of severity, as following:

　(1)3-φ fault (LLL fault) or 3-φ to ground fault (LLLG fault) with or without fault impedance. This fault which is most severe but least common is only one in number.

　(2)Double line to ground (LLG) fault with or without fault impedance. This fault is less severe but more common than 3-φ fault. However, this type of faults are three in number.

　(3)Line to line (LL) fault. This fault is more common but less severe than the above faults. These faults are also three in number.

　(4)Single line to ground (LG) fault. This fault is the least severe but the most common one. These faults are also three in number.

　From the above, we conclude that are four types of faults which are ten in number. The first three faults such as LLL or LLLG, LLG and LL faults involving two or more phases are known as phase fault while the fourth fault, namely, LG fault, is called ground fault. All of the line faults will bring the system into abnormal operating conditions, and may damage electrical equipment. Therefore, the faulty lines must be isolated from the system by protection relays.

　Faults in Synchronous Machines

　Generators are subjected to varieties of possible hazards when they are in operation. The possible hazards or faults which may occur in a synchronous generator can broadly be classified into two categories: (i) internal faults within the generator, (ii) abnormal operating and/or abnormal system conditions caused by external faults. Internal faults of a generator mainly include stator faults and rotor faults.

　Stator Faults----Within the stator winding, faults can occur due to failure of insulation (i.e. dielectric) and open circuit of conductor. Failure of insulation can lead to the short circuit between: (i) two or more phases, (ii) phase and core, (iii) two or more turns of the same phase (i.e. interturn fault).

　Failure of insulation can occur due to overvoltage, overheating caused by unbalanced loading, by overloading, by ventilation troubles, and by improper cooling of lubrication oil. It may also be caused by conductor movement due to forces exerted by short circuit currents or out of step operation. The most common fault in the stator winding is ground fault; about 85% of the faults are phase to ground faults in any generator winding. Phase to ground fault if persists may lead to phase to phase fault and even to phase-phase-phase fault (three-phase short circuit), which is the most severe fault though least common. The cause of overvoltage which ultimately results into failure of insulation can be due to overspeed of the prime mover, or due to defective voltage regulator; however, these days governors and voltage regulators act very fast and prevent any damage to the winding insulation.

　Rotor Faults----In the rotor winding also failure of insulation between field winding and core or two or more turns can occur. These faults may ultimately result in unbalanced currents and heating of the rotor. If the rotor is ungrounded, first earth fault does not show any effect but a second earth fault increases the current in the affected portion of winding which may cause distortion and permanent damage. It is advisable to open the field circuit breaker even with single earth fault to avoid second earth fault to avoid second earth fault so as to prevent local heating.

　Abnormal operating conditions / miscellaneous faults----There are a number of abnormal conditions which do not occur in the stator or rotor winding, but are undesirable since they can damage the generator. Each of these conditions is discussed in the following.

　(1)Loss of synchronism. This condition can occur either due to loss of field excitation or governor becomes defective. During out of step condition, as the swing angle between the generated voltage of the machine and that of other units in the system changes, the current in any such unit varies in magnitude. The current surges that result are cyclical in nature, their frequency being a function of reactive rate of slip of the poles in the machine. The resulted high peak currents and off-frequency operation can cause winding stresses, and pulsating torques which can excite mechanical resonances that can be potentially damaging to the generator and to the shifts. Thus generator should be tripped without any delay within the first slip cycle to avoid any major damage.

　(2)Overspeed. The cause of overspeed is sudden loss of a very large load; sometimes this happens due to tripping of circuit breaker near the generator end. In the case of steam turbine, the steam can be shut off immediately but in case of hydro turbine, the water flow cannot be stopped quickly, due to the mechanical and hydraulic inertia. The governor controls the over speeding so as to avoid any high voltage, high frequency and mecheanical damage to the generators. The setting of an overspeed rating may be 115% for steam turbines and 140% for hydro-turbiners.

　(3)Motoring. In a multi-generator system, when prime mover fails to provide required speed, the generator may act as a motor, drawing power from the system, instead of supplying power. Generally motoring is prevented by sensitive reverse power relay which operates on about 0.5% reverse power.

　(4)Underspeed. Due to failure of steam or water supply to the prime mover, the speed of the generator will reduce and if the reverse power relay fails, then underspeed and/or underfrequency relay comes into picture and trips the circuit breaker.

　(5)Loss of excitation. Excitation failure may be caused by a faulty field circuit breaker or failure of the exciter. It can be detected by an undercurrent dc relay. Due to failure of excitation, the synchronous generator may act as an induction generator thereby absorbing reactive power (i.e. sink of reactive power). Turbine generator tends to overheat the rotor and the slot wedges under these conditions because of heavy currents in these parts and sometimes arcing occurs at metal wedges in the slots.

　(6)Overvoltage. This may be caused due to overspeed or overexcitation when speed governor or voltage regulator fails to act as desired.

　(7)Stator overheating. Overheating may occur due to bearing failure, overloading, inadequate lubrication, or improper cooling of lubricating oil, etc. Overheating affects the dielectric strength of insulation.

　(8)External faults. Whenever abnormal conditions occur beyond the generator protection zone, the generator is also affected since the very source of power to the external fault is the generator itself. These conditions can be detected by the magnitude of negative sequence current, second harmonic current in field current and line overcurrent relay.

　中文译文

　电力系统继电保护

　简介

　电力系统的稳态运行是电力设备频繁的被多种故障扰动。为了维持电力系统正常的运行，需要一个高效、可靠的保障计划。设计电力系统元件是为了使其在正常的运行条件下运行。然而，由于某原因，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的其他部分相隔离。这是必要的，因为电力系统的元件被设计不是为了承受最糟糕的可能的条件而使整个的系统运行在高度不经济的状态。因此，如果在电力系统网络的任何元件或者设备出现这样的非正常运行状态，为了尽可能的在处于正常的运行状态下剩余的电力系统内存贮电能，从电力系统中迅速、可靠的移除故障元件或者设备是必须的。

　这个计划包括保护继电器和转换电路，例如断路器。作为首脑功能的保护继电器是一个非常重要的元件。保护继电器是一个感知装置，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。断路器从保护继电器得到命令之后仅仅断开故障元件。这就是保护继电器必须在运行状态下是可靠的、稳定的、快速的原因。

　在早期，曾经有感性磁盘式的电磁继电器。然而，不久，这个磁盘式被倒杯式取代，例如，空心圆柱和新的继电器被作为感应杯式或者感应圆柱式继电器而众所周知。这种继电器拥有许多重要的功能，例如更高的速度，对于一个给定的能量的输入有更大的转距而且转矩也更均匀，现在这种继电器仍就在使用。

　然而，随着电子管的到来，有不同功能的电子继电器在二十世纪四十年代已经形成。随着而二十世纪五十年代固态元件的发现，具有大量优势的无触点继电器形成了。对于保护继电器的目的而言，自从二十世纪六十年代末到二十世纪八十年代，数字电脑的运用已经使人们开始关注研发和实践工程师了。现在，微处理器的计算机为基础的中继方案由于它的大量的优势，例如自检测功能和灵活性，在全世界的电力系统中已经被广泛的使用。

　这全部的保护系统被分为以下几个部分：(i)发电机保护，（ii）变压器保护，(iii)母线保护，(iv) 馈线保护，(v)传输线保护。

　保护继电器的基本要求

　任何被要求的非正常运行条件下（例如故障）用以保障电力系统的元件保护计划基本上有两部分组成：(i)保护继电器和(ii)断路器。在整个计划背后的重要的首脑的保护继电器扮演着非常重要的角色。因此在选择一个合适的并且在正常运行状态下可靠的，高效的，快速的保护继电器应该适当的关注。保护继电器必须满足以下的几个要求：

　（1）由于对于一个完美的设计和健全的系统来说的故障通常是罕见的，因此继电器仅仅偶尔才被运行。这意味着中继保护在正常运行下被闲置并且无论什么时候故障出现继电器会被启动。换句话，它必须是可靠的。

　（2）由于部分的可靠性取决于维护，因此继电器必须能被容易的维护。

　（3）继电器的误动作有两种方式。一个是故障出现继电保护不能动作，另一个是没有故障时继电保护动作了。事实上，如果故障出现继电器必须运行，如果故障没有出现继电器不需要运行。

　（4）继电保护必须能足够敏感的区别正常状态和故障状态。

　保护继电器

　保护继电器的功能是感知故障和驱动断路器的脱扣线圈。保护继电器以下的型号被用以这些设备，例如同步发电机，母线，变压器和其他的器件及传输线的保护。

　(1)过电流继电器

　(2)低电压继电器

　(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（i）负序继电器和，（ii）零序继电器。

　(7)差动继电器和比率差动继电器

　(8)距离继电器例如（i）平面阻抗继电器,（ii）角度阻抗继电器，例如欧姆或者电阻继电器，（iii）角度导纳继电器，例如姆欧继电器和，（iv）偏置和制约继电器。

　(9)控制继电器例如（i）有线控制继电器，（ii）载波控制继电器，（iii）微波控制继电器。

　由于结构的不同有不同型号的继电器。他们是：（i）机械电子型号，（ii）温度继电器，（iii）饱和电抗型继电器，（iv）整流桥型继电器，（v）电子继电器，（vi）静态继电器，（vii）数字继电保护装置。

　故障和电力系统的损害

　传输线上的故障

　由于传输线暴露在闪电和其他有危害的大气中，它们的故障出现的比其他的设备更频繁。传输线上发生主要的故障类型被列出如下：

　(1)三相故障或者是三相接地短路故障。这个最严重的但是很少出现的故障仅有一种情况。

　(2)两相短路接地故障，这个故障对于三相故障来说不太严重但是比较常见。然而，这个故障类型有三种情况。

　(3)相间短路故障。这个故障比前两个故障更普遍但是不太严重，这个故障有三种情况。

　(4)单相接地故障。这个故障是最不严重但是最常见的故障。这个故障有三种情况。

　综上所述，我们的结论是这四种故障类型总共有十种情况。起初的三个故障像三相故障或者三相接地短路故障，两相短路接地故障和相间短路故障这样涉及两个或者更多相的被称为相间故障而第四个故障，即，单相接地故障被称为接地故障。所有的线路故障将使电力系统进入非正常运行状态，并且可能损坏电力设备。因此，故障线路必须被保护继电器从电力系统中隔离。

　同步发电机的故障

　当发电机运行时，他们受各种可能的危害的影响。这些在同步发电机上可能出现的危害或者故障可以大致分为两种类型： (i)发电机内部的内部故障， (ii) 不正常运作或外部故障而导致的异常系统情况。发电机内部故障主要包括故障定子和转子故障。

　定子故障----在定子绕组内部，由于导体的未能绝缘 （即介质）和开路而发生故障。绝缘故障可以导致短路： (i) 两个或多个相，(ii) 相和铁心，(iii) 同一相中两个或者更多的转机（即匝间故障）。

　偏载，重载，通风故障，和不正常的润滑油的冷却产生过电压、 过热而引起绝缘故障。它也可能被短路电流和失步运作而施加的力而导致的导体运动所引起的。定子绕组最常见的故障是接地故障；在任何发电机绕组中的相地故障的 85%左右。如果相地故障持续可能导致相相故障甚至是相相相故障 （三相短路），虽然最不常见但是最严重的故障。最终导致绝缘故障的过电压引起的原因可能是由于原动机的超速，或者由于缺陷的稳压器；不过，目前调节器和稳压器动作非常快并且防止对任何绝缘线圈的损坏。

　转子故障----在转子绕组中，在励磁绕组和铁芯或者转机之间可能出现绝缘故障。这些故障可能会最终导致电流的不平衡和转子过热。如果转子不接地，第一个接地故障不会显示任何影响，但第二次的接地故障使受可能会导致失真和永久损坏的受影响部分的绕组的电流增加。明智的做法是断开单接地故障断路器领域甚至避免二次接地故障，以避免第二次接地故障，防止局部加热。

　异常工况 / 杂项故障---有一些不正常的条件，不会发生在定子或者转子绕组，但是是不可取的，因为它们会损坏发电机。这些条件的讨论每一个在下面。

　（1）丧失同步。这种情况可能会发生由于失磁或者调速器出现故障。在失步条件下，它作为在发电机电压和在系统改变中的其它单位之间的摆动的夹角，在任何单位的电流以一定大小变化。这一结果的浪涌电流在本质上是周期性的，其频率在机器中作为一个与两极滑差的成反比的函数。所产生的峰值很高的电流和异步运行可在绕组中引起过度得应力，并且产生可能激发机械共振的脉动转矩，而这种共振可能破坏发电机和它的轴系。因此，发电机应该在第一个事故周期被断开并且没有任何的延时，以避免重大损失。

　（2）超速。超速的原因是一个非常大的负荷突然丧失，有时出现这种情况由于靠近发电机端电路断路器跳闸。在蒸汽涡轮机的情况下，蒸汽可立即关闭，但在水轮机的情况下，水流量不能尽快停止，由于惯性的机械和液压。调节器控制超速，以避免任何高电压，高频率和对发电机的机械的损坏。一个超速等级设置可能是115％的汽轮机和140％水轮机。

　（3）发电。在一个多发电系统，当原动机未能提供所需的速度，发电机可以作为一个马达，从系统中提取能量，而不是供应能量。通常由敏感的运行在大约0.5％反相功率的逆功率继电器防止发电。

　（4）速度不足。由于提供给原动机的蒸汽或水供应的不足，发电机的速度会降低，如果逆功率继电器出现故障，那么速度不足和/或低频继电器将处于险要的地位并且断路器跳闸。

　（5）失磁。励磁断路器的故障或者励磁机的故障可能导致励磁故障。这个故障可能被一个欠电流的直流继电器检测。由于失磁，同步发电机可作为一个感应发电机从而吸收无功功率（例如阻抗接收器）。涡轮发电机趋向于使转子和这种条件下的槽楔过热，因为在这部分大的电流感应和有时金属槽楔电弧的产生。

　（6）过电压。这可能是由于当调速器或稳压器无法担任需要时引起的超速或过励磁。

　（7）定子过热。过热的出现可能由于轴承故障，超载，润滑不足，或润滑油冷却不当等。过热影响绝缘强度。

　（8）外部故障。每当超出了发电机保护区范围的异常情况发生时，发电机受影响恰恰是因为外部故障的电源是发电机本身。这些情况可以通过负序电流，二次谐波电流的磁场电流和线路过流继电器的大小来检测。

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