发电机变压器组继电保护配置与定值计算研究 栗晓燕
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发电机变压器组继电保护配置与定值计算研究
栗晓燕
发电机
变压器
组继电
保护
配置
计算
研究
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发电机变压器组继电保护配置与定值计算研究 栗晓燕,发电机变压器组继电保护配置与定值计算研究,栗晓燕,发电机,变压器,组继电,保护,配置,计算,研究
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华北电力大学科技学院毕 业 设 计（论 文）开 题 报 告 学生姓名：栗晓燕 班级： 电气07K7 所在系别：电力工程系 所在专业：电气工程及其自动化 设计（论文）题目：发电机变压器组继电保护配置与定值计算研究 指导教师： 徐玉琴 2011年02月28日毕 业 设 计（论 文）开 题 报 告一、结合毕业设计（论文）课题情况，根据所查阅的文献资料，每人撰写不低于2000字的文献综述。（另附）二、本课题要研究或解决的问题和拟采用的研究手段（途径）：一、 熟悉发电机-变压器组的运行特点及其对继电保护的要求；二、 了解国家和电力部门相关的标准，掌握大型发电机-变压器组继电保护的配置原则及整定计算方法；三、 对某电厂的发电机-变压器组进行继电保护选型配置，完成继电保护整定计算，并对继电保护进行评价；四、 选择微机成套保护装置中的一种保护类型，比如差动保护，掌握其软、硬件的构成、工作原理，完成软硬件分析；五、 画出发电机-变压器组继电保护的配置图；六、 总结工作，撰写毕业设计（论文）。三、指导教师意见：1 对“文献综述”的评语： 2对学生前期工作情况的评价（包括确定的研究方法、手段是否合理等方面）：指导教师： 年 月 日关于发电机变压器组继电保护性能的文献综述一、前言：今年来随着电力系统的不断扩大，单机容量正在不断增加，单机容量迅速发展。发电机、变压器和母线是电力系统中最重要的电气设备，它们作为电能发生、转变、分配、补偿的集中电力元件，在电力系统中担当着重要的角色，其安全可靠运行和事故的正确处理对电力系统安全经济运行起着重大作用，因而受到电力行业界的重点关注。20世纪，由于技术水平的落后，电力设备运行经常受到继电保护误动、拒动的困扰。下面就发电机变压器组保护的灵敏性、双重化等性能加以分析。二、发变组保护性能的分析2.1发电机变压器组保护的灵敏性：2.1.1发电机保护：由于技术与材料的进步，现代设计的发电机组，其故障发生的几率大为减少。但是如果一旦发生，则其可能引起的灾害将较其他电力设备的故障更为严重，甚至会带来长期性的停机损失，尤其是大容量的机组。因此，一旦发觉发电机组有异常情况时，就得尽快将其与故障隔离。发电机故障一般不会破坏系统的稳定运行。如果系统的旋转备用不足，大容量电机的跳闸会导致系统频率下降，低频减载装置自动切除部分负荷。发电机故障时断路跳闸只能切断有系统供给的故障电流，灭磁开头的跳闸并不能立即使转子电流降为零，所以保护快速动作的效果受到限制。发电机是系统中最重要最昂贵的设备。定子导线嵌于铁芯槽内。同槽之间发生短路必损坏铁芯，损失重大。提高差动保护的灵敏度，减小死区，应是追求的目标。2.1.2变压器保护：唯有接有超高压系统的变压器在其高压引线上的故障才可能对系统的暂态稳定性构成威胁。对这种故障的变压器差动速断保护一定能快速灵敏的动作。长期以来变压器差动保护以低压引线上的故障作为校验灵敏性度的依据，这是十分错误的。变压器内部故障主要是匝间短路。2.2发电机变压器组保护的双重化：根据“防止电力生产重大事故的二十五项重点要求”继电保护实施细则第2.11条款规定：“保护的双重化的配置是防止保护拒动，同时减少一次设备因为保护装置异常、进修等而停运的有效措施，但保护双重化配置也造成了保护误动的几率。因此在考虑保护双重化配置时，应选用安全性高的继电保护装置，并遵循相互独立的原则。”从该条款的内容来看，实施双重化配置的目的：一是在设备设备发生故障时，防止因保护装置拒动而给故障设备造成进一步的损害;二是在保护装置故障、异常或检修时，避免因缺少缺少保护而导致一次设备不必要的停运。从经济角度说，前者保护设备的完备性，降低设备损坏而导致的直接经济损失；后者主要保证发电设备运行的连续性，提高电厂的经济效益。2.2.1发电机保护：发电机保护如果拒动，没有任何相邻设备的保护可以起到后备作用。每套微机发电机保护都包含主保护和后备保护。一旦退出或拒动就丧失全部功能。因此大型发电机的保护宜双重化。实现保护双重化的原则应当是：为防止拒动的双重化应按“或”门输出其跳闸命令：为防误动的双重化应按“或”门输出其闭锁命令。大型水力电机有多种原理的差动保护，同时采用不同原理的差动保护不是为了起到后备作用。一种差动保护只要能反应其他差动保护都不能反应的故障就应当被采用。反之，一种差动保护虽然能较其他任何一种差动保护能反应更多的故障。但如果他反应的故障已被其他几种保护联合起来的功能所覆盖，就没有必要再采用。现在故障有后备保护切除的情况是很少的，所以后备保护不需要双重化的。当每套保护都包含了主保护和后备保护时，后备保护也随着双重化了。2.2.2变压器保护：变压器有差动保护和电流保护。长期以来认为后者是前者的后备。机械型保护时代两者是由不同的硬件实现的，而且那是两者的启动电流基本相等，因此后者是可以对前者起后备保护作用的。所以在同一CPU的程序中过电流保护是不能退作为差动保护的后备的。过电流保护的作用是在外部短路时防止长时间流过短路电流烧损变压器。 由于一套微机变压器保护包含了差动保护和过流保护，在装置调试时或装置装置拒动便没有保护所以现在主张变压器保护实现双重化。实现保护双重化的原则应当是：为防止拒动的双重化应按“或”门输出其跳闸命令：为防误动的双重化应按“或”门输出其闭锁命令。2.3发电机变压器组保护的速动性：保护的速动性对大机组的自身安全及系统的稳定至关重要，考虑大机组保护速动性问题的原则应是“在保证可靠性的前提下力争满足速动性”。保护装置质量及抗干扰与速动性的问题：大机组大量使用静态保护装置，由于装置结构复杂，电子元件的使用量大，发生故障的可能性不容忽视，且易受干扰，处理不好制约保护的动作速度。实践中由于各种干扰信号和一、二次系统的暂态特性及选择性要求的制约，不能片面的追求保护的动作速度，只能根据系统的情况因地制宜，以免保护的误动。结束语：大机组的保护的速动性、灵敏性、双重化对电网的稳定和机组的安全至关重要，随着电力系统的发展和机组单机容量的增大，在保护速动性方面出现问题也会更多，对保护速动性的要求也会更高，但新的保护方式及新型保护装置也将不断出现，只有不断的分析、探讨、总结才能及时消除隐患，确保其安全运行。总结了目前机组保护配置存在的难题，我觉得，大机组实现灵敏性、双重化、速动性需要继电保护的运行、设计和研究人员解放思想、开拓思路、多做实验，以系统的观点解决和协调保护产品的设计和应用的问题。参考文献：1.发电机变压器组双重化配置方案的再讨论 吴笃贵 杨恢宏2.大容量发电机变压器组继电保护配置的若干问题 赵秀坤 张举3.大型发电机变压器组继电保护速动性浅析 吴璟岚4.大型发电机变压器组继电保护的现状与发展 王维俭 刘俊宏5.大型发电机变压器组双重化配置的问题分析 栾玉波 房培锋等6.大型发变组保护双重化配置的几个问题 刘猛7.大型变压器的继电保护配置与整定 崔荣喜 何柏娜 赵云伟8.提高大型发电机变压器组保护双重化配置可靠性方案研究 张劲松 9. 江苏电机工程 江苏省电力公司 江苏省电机工程学会10.大型机组继电保护理论基础 王维俭 侯炳蕴 编 水利水电出版社11.电力系统主设备继电保护实验文伯瑜 姜龙华 编 中国电力出版社12.高压电网继电保护原理与技术 朱声石 编著 中国电力出版社13. 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Journal of Hua zhong University of Science & Technology, 1999, 27 ( 6) : 16 -18.华北电力大学科技学院本科毕业设计（论文）华 北 电 力 大 学 科 技 学 院毕 业 设 计（论 文）附 件外 文 文 献 翻 译学 号： 071914030110 姓 名： 栗晓燕 所在系别： 电力工程系 专业班级： 电气07K7班 指导教师： 徐玉琴 原文标题： Electric power systems protective relays 2011年 6月 6日电力系统继电保护查尔斯W.布赖斯继电保护3.1介绍和基础保护继电器是对故障和异常状况如短路、低频等提供保护以避免严重损害重大件设备，如线，变压器和发电机的设备。保护继电器系统检测故障并将跳闸信号传送到断路器和其他开关，而开关通过它来清除、孤立、中断故障。尽管限制设备因故障受损是必要的，但更关心的是保护其余系统免受故障。例如，受线路短路运行，如果故障不及时清除，线路会继续遭受损害。继电器的速动性，选择性，可靠性，随着断路器的可靠快速动作往往会防止这种破坏，甚至更重要的是将防止这种损害延到变电站母线和变压器。显然，继电保护系统必须精心设计，以实现各要素的适当平衡如可靠性，速动性，选择性和经济性。一种保护，即暂态过电压的潮流保护需要避雷器的，不包括在本课题内。相反，本专题侧重于保护继电器系统的过流保护和其他异常情况的保护。一个过电压继电器可用于防止过压状态持续作用电气设备，但不能保护由断路器操作或闪电引起的暂态潮流3.1.1保护的重要性显然保护短路引起的故障很重要，但为了强调它的重要性，认为一个广泛的架空传输系统可受到由于雷击引起的闪络临时故障和由物理伤害引起永久性故障，冰、风荷载同杆塔意外破坏造成的破坏力一样。显然，线路暴露在外面，在正常条件下这些故障的频率是每天会发生数次。在风暴最坏的情况下，故障可能在几个小时内发生几百次。同样系统在各种异常的条件下的保护也是非常重要的。例如，系统低频率可出现汽轮机叶片的严重损坏，发电机升压变压器可能在极低频率运行过励磁时，继电器的反应可能在短暂波动扰动下传输给断路器造成发电机、线路跳闸。所有这些，和其他的情况都必须被保护继电器工程师预见到。3.1.2短路当系统的任何部分突然短路，无论是三相短路或者是接地故障或者是相间故障，发生短路可能会比正常负载电流大许多。短路保护是最基本的形式保护，适用于所有的系统级：发电，输电，变电，配电和用电。短路电流的计算是电力系统分析的标准问题，许多计算机程序可用于执行这种计算。显然，这个话题大大超出了我们当前的利益，但它会证明指导，审查的简单计算的高效一些，留下更复杂的情况给计算机。当同步发电机是突然受到三相短路，它产生一个大的瞬时电流然后衰减，先是迅速，然后缓慢。为了表述这个复杂的现象，我们通常计算交流分量的初始值（尽管我们从经验中得知将有直流偏移量的衰减）。交流短路电流的初始值是瞬态电流，并且是使用常量后面的发电机电压源瞬变电抗计算的（也就是俗称的发电机制造商）。为了计算出在系统中输电和配电的电流，广泛的网络计算必须执行（这是计算机方案由来）。要计算不平衡的短路电流，对称分量法理论通常被使用。这个理论，首先由Fortescue在初期的20世纪开发了一部分，陈述了三相不平衡电路可以由三个互连的电路解决，这两个一个是对称的三相短路（具有不同的相位序列）和一个是单相短路（实际上是一个与所有相电流的三相电路相并在全相位相电压）。这三个电路被称为正，负，和零序网络。如果没有发展这个理论，我们说，一个单相接地短路（也称为单相接地短路），可能可以解决互连正序网络，负序网络，并在系列零序网络在点故障。同样地，相间短路，可通过正极网络的互连并同时在负序网络的故障点和两相接地通过连接故障时的故障点的所有三个平行的顺序网络解决。图3.1.1显示了正序网络由于计算三相短路电流，而且序网图可以把简单的包含两母线和两台发电机的单相接地和相间短路互联。很多情况下这些数字可能简化为一个等效电路。对于单相接地短路，总故障电流为零序电流的三倍。对于相间短路，总故障电流是正序故障电流的1.73倍。对于两相接地短路，总故障电流是正序故障电流的1.57倍，总故障电流是零序故障电流的3倍。相量图3.1.2给出了相间短路和接地短路近似的结果。典型的保护是用接地继电保护来防止单相接地短路和相间保护来防止相间短路和三相短路。3.1.3显示了三段继电器和接地继电器连接到一个电流互感器（简称CT）。图3.1.3。三个电流互感器对过流继电器提供反馈。请注意， 51N的51-1为接地保护的第1段，51-2为第二段，51-3为第3段。三相短路和相间短路，将被相间继电器检测到，接地短路将被接地保护检测到。这些数字（51和52）是ANSI标准的设备功能编号的例子。这些数字和他们所代表的设备的几个例子见于表（见完整列表的ANSI C37- 2）。在下面的章节我们将会在很多关于这方面和其他方面有更多的话要说。21距离保护装置25同期装置27欠压继电器 32方向继电器46逆相继电器47相序继电器49热继电器50瞬时过电流继电器51交流时限过流继电器52交流断路器59过电压继电器 67交流方向过流继电器68闭锁继电器72直流断路器76直流过流继电器77个脉冲传感器78相位角或失步继电器79交流输出重合闸继电器81频率继电器85载波或试验线接收继电器86闭锁继电器87差动继电器3.1.3相位和极性继电器的工程师的最有价值和最有力的两个工具是电流和电压的相量图和相对极性。这些工具是相辅相成的，也就是说，相量图不完整，甚至有些含糊，除非一个变量图（示意图）可完成所有电压和电流变量得极性标志。在这些图中，我们使用双下标符号：电流由a到b表示为Iab,电压从b下降到c表示为Vbc。在电路图zhong ，传统方向的电流用指示箭头表示，而电压的极性则由 “+“和“ - ”标记。图3.1.4给出了电流和电压（在相量形式）的简单例子。相量要么是字母或要么是数字表示，如ABC，XYZ，或用321代表相序。由于没有普遍接受的符号，切记，单相一端用A 另一端用x表示，两相就是用数字，例如分别用3和1来代表相位。相顺序很重要，因为它代表一个物理量（例如，旋转方向的三相交流电机）。有没有标准，和例子会有时有时使用ABC有时使用321。图3.1.5说明了这种对应关系。图3.1.4。简单的例子，显示当前的电流参考方向，电压极性和相量图。图3.1.5。相量图显示三相使用不同的电压系统命名为相同的电压。由于系统是同步的，最上面用A，中间用3，下面用Y。该相序是ABC或321或XYZ。3.1.4接地系统以前，许多电力系统的不接地系统用三角-三角连接的变压器。不接地系统遇到一些问题，包括两个避雷器为单相变压器用瞬态过电压（每边一绕组），并与快速，非常常见的麻烦相自动清除地面短电路。未接地系统连接到地的杂散电容而已，所以单相接地短路故障电流很小绘制。为了快速检测和清除接地故障，并减少绝缘避雷器成本，接地系统比不接地系统越来越普遍。在许多情况下，接地系统的电源变压器用星形-三角形连接，在星侧中性点接地。发电机和降压变压器负荷三相负载往往是通过一个电阻接地，以限制接地故障电流使之在一个较小的值。发电机升压变压器和降压变压器服务四线配电系统通常原边和副边都用三角形连接。记住，原边是输入副边是输出，因此升压变压器的原边是低电压的一面，降压变压器原边是电压高的一端。在四线系统相的配电变压器采用中性点接地（单相）或在原边用星形连接（三相）。三相配电变压器的二次侧可连接成星形或三角形。在星形星形配电变压器用于四线（三相导线和零线）系统，星三角接线的配电变压器经常与不接地系统连接。如果中性点接地，绕组接地，当接地故障的发生原边，分布绕组将提供接地电流。这通常会导致中性点接地的星形-三角形连接方式的变压器保险丝大量烧毁。绕组中性点不接地可以解决这一问题，但可能会造成过电压问题，如产生谐振。星-星连接的绕组避免了谐振的问题，因为两侧绕组都有固定的接地中性点。在许多情况下，星形连接的自耦变压器和星形星形电力变压器广泛应用。Protective Relays ELECTRIC POWER SYSTEMS PART 3 PROTECTIVE RELAYS Charles W. Brice August 2002 Protective Relays -3.1.1- 3.1 Introduction and Basics Protective relays are the devices that provide protection against faults, such as short circuits, and abnormal system conditions, such as low frequency, to avoid serious damage to vital pieces of equipment such as lines, transformers, and generators. The protective relay system detects the fault and sends trip signals to circuit breakers and other switchgear, while the switchgear clears the fault by interrupting it and isolating the faulty equipment. Although it is desirable to limit damage to the equipment subjected to the fault, the overriding concern is to protect the rest of the system from the fault. For example, a line subjected to a short circuit will often suffer damage if the short circuit is not promptly cleared. Relays that are fast, selective, and reliable along with fast, reliable circuit breakers will often prevent such damage, and even more importantly will prevent the damage from spreading to the substation bus and the transformer. Obviously, the protective relay system must be carefully designed to achieve the proper balance among factors such as reliability, speed, selectivity, and economics. One aspect of protection, namely transient overvoltage protection by application of surge arresters, is not covered in this course. Rather, the present topic focuses on protective relay systems for overcurrent protection and protection from other abnormal conditions. An overvoltage relay might be used to protect some apparatus from sustained overvoltage condition, but not to protect the apparatus from a transient surge due to switching or lightning. 3.1.1 Importance of protection Protection of the system from damaging short circuit currents is obviously important, but to underscore its importance consider that an extensive overhead transmission system may be subject to temporary faults due to lightning-induced flashover and permanent faults due to physical damage from ice and wind loading as well as accidental destruction of poles and towers. The frequency of these faults is obviously a function of the lines overall exposure to the damage, but faults may occur several times a day during normal conditions. In the worst cases of storm damage, hundreds of faults may occur in a few hours. Also of great importance is protection from various abnormal system conditions. For example, severe damage to steam turbine blades can occur at low system frequencies, generator step-up transformers may be overexcited at very low frequency operation as the unit starts up, certain relays may respond to transient generator swings during disturbances resulting in transmission line tripping. All these, and other, conditions must be foreseen by the protective relay engineer. 3.1.2 Short circuits When any part of the system is suddenly shorted, whether all three phases, phase to ground, or phase to phase, the current that flows just after the occurrence of the short circuit may be quite large compared to normal load current. Short-circuit protection is the most basic form of protection, and is applied at all levels of the system: generation, transmission, substation, distribution, and utilization. Calculation of short-circuit currents is a standard problem in power system analysis, and many computer programs are available to perform the calculation. Obviously, this topic is one that goes substantially beyond our current interest, but it will prove instructive to review some of the C. W. Brice August 2002 -3.1.2- high points of simple calculations, leaving the more complicated cases to the computer. When a synchronous generator is subjected to a sudden three-phase short circuit, it responds with a large transient current that decays, first rapidly, then more slowly. To simply represent this complicated phenomenon, we usually calculate the initial value of the AC component (although we know from experience that there will be a DC offset that decays as well). The initial value of the AC short-circuit current is the subtransient current, and is calculated by using a constant voltage source behind the generator subtransient reactance (which is commonly tabulated by generator manufacturers). To calculate the current out in the transmission and distribution systems, an extensive network calculation must be performed (this is where the computer program comes in). To calculate the current drawn by an unbalanced short circuit, the theory of symmetrical components is usually used. This theory, first developed by Fortescue in the early part of the 20TH century, states that an unbalanced three-phase circuit may be solved by interconnecting three circuits, two that are balanced three-phase circuits (with differing phase sequences) and one that is a single-phase circuit (actually a three-phase circuit with all phase currents in phase and all phase voltages in phase). These three circuits are called the positive, the negative, and the zero sequence networks. Without developing the theory, we state that a single phase to ground short circuit (also called a single line to ground short circuit) may be solved by interconnecting the positive sequence network, the negative sequence network, and the zero sequence network in series at the point of the fault. Likewise, a phase to phase short circuit may be solved by interconnecting the positive and negative sequence networks in parallel at the point of the fault, and a double phase to ground fault by connecting all three sequence networks in parallel at the point of the fault. Figure 3.1.1 shows the positive sequence network used for calculating the three-phase short-circuit current, and the sequence network interconnections for single phase to ground and phase to phase short circuits on a simple system consisting of two busses and two generators. Many cases may be simplified to an equivalent circuit that has that figure. For a single phase to ground short circuit, the total fault current on the faulted phase is three times the zero sequence current. For a phase to phase short circuit, the faulted phases deliver 1.73 times the positive-sequence fault current. For a double phase to ground short circuit, the faulted phases deliver 1.57 times the positive-sequence fault current, and the total fault current is 3 times the zero-sequence fault current. Phasor diagrams shown in Figure 3.1.2 give approximate results for phase and ground short circuits. Protective Relays -3.1.3- GEN 1BUS 1TRANSF 1BUS 2TRANSF 2GEN 2POSITIVE SEQUENCE NETWORKNEGATIVE SEQUENCE NETWORKZERO SEQUENCE NETWORKIoPOSITIVE SEQUENCE NETWORK FOR THREE-PHASE SHORT CIRCUITFAULT LOCATIONPOSITIVE SEQUENCE NETWORK NEGATIVE SEQUENCE NETWORKPHASE TO PHASE SHORT CIRCUITSINGLE PHASE TO GROUNDSHORT CIRCUIT. FAULTEDPHASE CURRENT IS 3 Io121112222211 Figure 3.1.1. Some fault calculations on a simple system. C. W. Brice August 2002 -3.1.4- ABCABCABCABCABCNORMAL THREE-PHASESHORT CIRCUIT SINGLE PHASE TOGROUND SHORT CIRCUITPHASE TO PHASESHORT CIRCUIT DOUBLE PHASE TOGROUND SHORT CIRCUITVabVanIaVabIaIbIcVabIaVanIcIbIbIc Figure 3.1.2. Typical phasor diagrams for phase and ground short circuits. Typical protective relay practice is to use ground relays to protect against single phase to ground short circuits and phase relays to protect against phase to phase short circuits and three phase short circuits. Figure 3.1.3 shows three phase relays and one ground relay connected to current transformers (abbreviated CTs). Protective Relays -3.1.5- 321AC BUS51-151-351N52DC SOURCE51-151-1 SI51-1 SI51-251-2 SI51-2 SI51-351-3 SI51-3 SI51N51N SI51N SI51 TIME OVERCURRENT RELAY52 POWER CIRCUIT BREAKER51-252TC52aTC TRIP COILSI SEAL-INa BREAKER AUX. CONTACT: closed only when the main contacts are closedRELAY OPERATING COILRELAY CONTACTS Figure 3.1.3. Three current transformers feeding overcurrent relays. Note that the ground relay is device 51N and the phase relays are 51-1 for phase 1, 51-2 for phase 2, and 51-3 for phase 3. Three-phase and phase to phase short circuits will be detected by the phase relays, and phase to ground short circuits will be detected by the ground relay. C. W. Brice August 2002 -3.1.6- The device numbers (51 and 52) are examples of ANSI standard device function numbers. A few examples of these numbers and the devices that they represent are given in the table below (see ANSI C37-2 for a complete list). We will have much more to say about this and other connections in the following sections. 21 Distance Relay 25 Synchronizing Device 27 Undervoltage Relay 32 Directional Power Relay 46 Reverse Phase Relay 47 Phase Sequence Relay 49 Thermal Relay 50 Instantaneous Overcurrent Relay 51 AC Time Overcurrent Relay 52 AC Circuit Breaker 59 Overvoltage Relay 67 AC Directional Overcurrent Relay 68 Blocking Relay 72 DC Circuit Breaker 76 DC Overcurrent Relay 77 Pulse Transmitter 78 Phase Angle or Out of Step Relay 79 AC Reclosing Relay 81 Frequency Relay 85 Carrier or Pilot-Wire Receiver Relay 86 Lockout Relay 87 Differential Relay 3.1.3 Phasing and polarity Two of the relay engineers most valuable and powerful tools are phasor diagrams and relative polarity of currents and voltage. These tools go hand in hand; that is, the phasor diagram is incomplete and somewhat ambiguous unless an elementary (schematic) diagram is available complete with polarity marks for all voltage and current variables. On these diagrams, we use the double subscript notation: the current flow from a to b is represented as Iab and the voltage drop from b to c is represented as Vbc. On circuit diagrams, the direction of conventional current flow is indicated by an arrow, while voltage polarity is indicated by + and - marks. Figure 3.1.4 gives a simple example of currents and voltages (in phasor form). Phases will either be labeled with letters or numbers, such as ABC, xyz, or 321 with the order representing the phase sequence. Since there is no universally accepted notation, bear in mind that one utility may use A to represent the same phase that another calls x. Likewise, two utilities that use numbers, may represent the same phase with 3 and 1 respectively. The phase sequence is important, since it represents a physical quantity (the direction of rotation of the shaft of a three-phase AC motor, for example). There is no standard, and the examples will sometimes use ABC and sometimes 321. Figure 3.1.5 illustrates this correspondence. Protective Relays -3.1.7- VRXlIXcpqrsVqrVrsIVpqPHASOR DIAGRAMVVpqVqrVrsPHASOR SUM OF VOLTAGE DROPS Figure 3.1.4. Simple example circuit showing current reference direction, voltage polarity, and phasor diagram. C. W. Brice August 2002 -3.1.8- ABC321NYZXN Figure 3.1.5. Phasor diagrams showing phase to neutral voltages of three systems using different nomenclature for the same phases. Since the systems are synchronized, the phase A in the top figure is identical to the phase 3 in the middle figure and phase Y in the bottom figure. The phase sequence is ABC or 321 or XYZ. 3.1.4 Grounding systems Formerly, many power systems were ungrounded systems using delta-delta transformers. Ungrounded systems suffer from several problems, including transient overvoltages that Protective Relays -3.1.9- necessitate the use of two surge arresters for a single-phase transformer (one on each side of the winding), and trouble with rapid, automatic clearing of the very common phase to ground short circuit. An ungrounded system is connected to ground only by stray capacitances, so a single phase to ground short circuit draws very little fault current. To allow rapid detection and automatic clearing of ground faults, and to reduce insulation and arrester costs, grounded systems have become more common than ungrounded ones. In many cases, the power transformers in a grounded system are connected delta wye, with the wye neutral grounded to the substation ground mat. Generators and step-down transformers serving only three-phase loads are often grounded through a resistance to limit the ground fault current to low values. Generator step-up transformers and step-down transformers serving four-wire distribution systems are usually connected delta on the primary and wye on the secondary. Remember primary means input and secondary means output, so a step-up transformer primary is the low-voltage side but a step-down transformer primary is the high-voltage side. Distribution transformers on four-wire systems are connected phase to neutral (single phase) or wye (three phase) on the primary side. The secondary side of three-phase distribution transformers may be connected in either wye or delta. The wye-wye distribution transformer is used for four-wire (three phase conductors and a neutral conductor) systems, and the wye-delta distribution transformer is usually connected with the bank neutral not connected to ground. If the neutral is grounded, the bank tries to ground the feeder, and when ground faults occur on the primary side, the distribution banks will supply ground current. This usually causes a large number of blown fuses on wye-delta distribution transformers with grounded neutrals. Floating the bank neutral alleviates the problem, but may cause overvoltage problems such as ferroresonance. Wye-wye banks avoid the ferroresonance problem since they invariably have grounded neutrals on both sides. In many cases, wye-connected autotransformers and wye-wye power transformers may be desired. These are often provided with a delta tertiary winding so that the transformer will serve as a grounding bank. Figure 3.1.6 shows a typical system, and Figure 3.1.7 shows the zero sequence networks for delta-wye, wye-wye, and wye-wye-delta transformer banks. Note that wye-wye banks merely pass through the source system ground (if any) to the secondary system, while the other cases provide a zero-sequence connection to ground. As Figure 3.1.8 shows, this zero-sequence connection to ground is quite significant for ground fault calculations. The grounded system of Figure 3.16 has a generator grounded through an impedance, a delta-wye step-up transformer with wye solidly grounded, delta-wye step-down transformers with wye solidly grounded, and a grounded wye autotransformer with delta tertiary. Note that delta-wye and wye-wye-delta banks with grounded wyes are grounding transformers, capable of grounding an ungrounded system. The grounded wye autotransformer with delta tertiary has the same zero-sequence network as the grounded wye-wye-delta. C. W. Brice August 2002 -3.1.10- TRANSF 1TRANSF 2230 kV13.8 kV230 kV23 kVAUTOTRANSFTERTIARY230 kV115 kV13.2 kV1234512.5 kV67 Figure 3.1.6. Typical grounded system. Protective Relays -3.1.11- DELTA - GROUNDED WYEGROUNDED WYE - GROUNDED WYEGROUNDED WYE - GROUNDED WYE - DELTA TERTIARY Figure 3.1.7. Zero-sequence networks for delta-wye, wye-wye, and wye-wye-delta transformers. C. W. Brice August 2002 -3.1.12- 1234512345123453RnjXgo jXtojX12ojXg2jX12jXtjXtjX12jXgEgIfoFAULTED PHASE TO GROUND CURRENT = 3 Ifo Figure 3.1.8. Sequence networks connected in series at the point of a single phase to ground short circuit. The system is that of Figure 3.1.6 with the fault on bus 5. Note that the zero-sequence networks of the transformers have a great effect on the available ground fault current. 1.5 Instrument transformers Instrument transformers are the input transducers for protective relays, but also metering (both for revenue metering and for data acquisition). The two main types of instrument transformers are current transformers (CTs) and potential transformers (PTs). The latter are also called voltage transformers (VTs), with no difference intended. Some current transformers use the main current-carrying conductor as the primary (usually one turn) through a toroidal iron core that is wrapped by a large number of secondary turns. Other current transformers have wound primaries and secondaries in lumped coils on iron core legs. In either case, the CT acts to step the current down, say from 1000 A on the primary to 5 A on the secondary. This would give a CT ratio of 1000:5 A or 200:1 A, and would require 200 turns on the secondary (assuming one primary turn). Figure 3.1.9 shows a view of a toroidal current transformer and its circuit diagram showing polarity markings. Protective Relays -3.1.13- Current transformer secondaries are typically rated at 5 A, although some are rated at 1 A. This is the continuous rating, and it may be exceeded by a factor of 10 or 20 for a short time, especially during a short circuit in supplying relays that provide overcurrent protection. TO RELAYSIsIpSYMBOL FOR CURRENT TRANSFORMERSHOWING POLARITY MARKSPRIMARY CONDUCTORCURRENT TRANSFORMERWINDOW OR BAR TYPE CURRENT TRANSFORMER Figure 3.1.9. Current transformer and its circuit diagram. The polarity marks mean that primary current into the mark induces secondary side current out of the mark. Now Figure 3.1.10 shows the equivalent circuit of a transformer, which also applies to the case of a current transformer. Notice that the magnetizing branch, which is often neglected, must present a high impedance for the device to produce a secondary (output) current approximately proportional to the primary (input) current. The shunt branch should be represented as a nonlinear reactance, since the iron core may saturate at high input levels. If the core saturates, the apparent impedance of the shunt branch will drop, since very little additional flux is produced by an increment in the current. This will cause the secondary current to fail to track the primary current. DC offset currents, which exist in the short-circuit currents of synchronous generators, will aggravate the problem, since the saturation of the iron core is dependent on the total instantaneous magnetizing current (whether AC or DC). As we will see CT saturation presents great problems to protective relaying, especially differential relays. C. W. Brice August 2002 -3.1.14- TO RELAYSCURRENT TRANSFORMER EQUIVALENT CIRCUITIpIsImX1X2XmImCOREFLUXCORE MAGNETIZATION CURVE Figure 3.1.10. Equivalent circuit of a current transformer and its magnetization curve. Note that large DC offsets in the primary current will drive the core far into saturation, causing the secondary current to deviate from the ideal. Current transformers for relaying purposes are given accuracy class ratings, such as C400 or T400 (sometimes written as 10C400 or 10T400). The letter C indicates that its performance may be calculated from its magnetization curve. These units are window-type CTs (primary conductor fed straight through the core window) with the secondary distributed all around the core, producing negligible secondary leakage flux and primary leakage flux paths entirely in air. The letter T indicates that its performance must be determined by test, since the primary may be wound or the secondary may be lumped in two coils. In that case, the secondary leakage flux may pass partly through the iron, complicating any calculations beyond ordinary practicalities. The first number (if given) indicates a 10% accuracy class (maximum 10% CT ratio error), which is invariably used for relaying work, so the num
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