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华北电力大学科技学院毕业设计(论文)任务书所在院系 电力工程系 专业班号 电气07K2 学生姓名 王炳林 指导教师签名 审批人签字 毕业设计(论文)题目 110/35/10kV降压变电所电气一次系统设计 2010年 4 月 6 日一、毕业设计(论文)主要内容1、选择主变压器的容量和型号； 2、电气主接线设计3、计算短路电流；4、一次选择电气设备；5、防雷系统设计；6、屋内外配电装置设计7、总平面布置二、基本要求1、变电站类型：变电站类型地方降压变电所2、电压等级：110/35/10kV3、负荷情况：35kV：最大40MW，最小30MW，Tmax = 5600小时，cos= 0.85 10kV：最大18MW，最小12MW，Tmax = 5400小时，cos= 0.854、进出线情况：(1) 110kV侧：2回（架空线） (2) 35kV侧： 8回（架空线） 10kV侧：12回（电缆）。5、系统情况：(1) 系统110kV母线电压满足常调压要求；(2) 系统110kV母线短路电抗标幺值为0. 3（SB100MVA UB=平均额定电压）。6、环境条件：(1)最高温度40，最低温度-30，年平均温度20；(2)土壤电阻率 400 欧米； (3)当地雷暴日40日/年。三、设计(论文)进度序号设计项目名称完成时间备注1阅读文献，了解变电站电气一次设计的流程和相关技术问题，完成开题报告毕业设计的13周2内容1、2、43内容3564内容4785内容5、69-106撰写毕业论文 11参考文献1. 发电厂电气部分课程设计参考资料，黄纯华编，天津大学2. 电力工程设计手册（1-4）西北、东北电力设计院3. 电力系统课程设计及毕业设计参考资料，曹绳敏编，北京水利电力出版社，19954. 电力系统课程设计参考资料，梁志瑞编，华北电力学院设计(论文)预计完成时间： 2011 年6 月19 日华 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：071901010221 姓 名： 王炳林 所在系别： 电力工程系 专业班级： 电气07k2班 指导教师：王炳林原文标题：Power Quality Enhancement by TCSC Application toMitigate the Impact of Transformer Inrush Current2011年 6 月 10 日通过可控串联电容补偿器减轻变压器励磁涌流的影响来提高电能质量摘要：变压器增压时会产生过多的瞬时励磁涌流，特别是在切换之后变压器铁芯还有磁通量增加。励磁涌流作用于变压器附近的电能使系统电压下降。电能质量降低的程度取决于短路电流在母线回路的震荡程度和瞬间电流的衰减时间系数。本文研究可控串联补偿器的应用减轻变压器励磁涌流对电能质量的影响。首先，它描述可控串联补偿器对母线短路电流震荡的影响，然后描述可控串联补偿器可以减缓电能下降的原理。它也研究和举例阐述了可控串联电容补偿器对变压器励磁涌流不同的操作方式。可控串联电容补偿器限制故障电流的能力是通过在变压器励磁涌流故障条件产生的阻抗特性来研究的。关键词：励磁涌流；电能质量；可控串联电容补偿器；变压器；电压降落1介绍电能质量是一直是电力公共事业和电力用户关注的问题。电能质量术语包含很多方面，这些方面也不是新近研究出来的。但是随着它作为一种现代化产业发展中的能量越来越受人们的关注。电能质量问题的投诉大多是关于不能长时间储存电能，这也是输电及配电系统造成短路或者变压器产生励磁涌流的原因。一般都在使用这些条款。在电力系统中典型的电压骤降或者下降是0.1%-0.9%半个周期每分钟。现在电控系统的电压暂降应用在采矿业，对于造纸工业和半导体制造业有不利影响。减少10额定电压以及缩短2或3个周期都能影响关键设备，对于生产流程有不利影响。举个例子来说，当交流电被应用到一个调速驱动时，它可以转换成脉冲直流。脉冲储存在一个使其平缓的脉冲电容器中。当电容器储存能量时，它允许系统产生一些电压骤降等。但是如果电压骤降比较严重有频发的话就会使电容电压低于临界水平(通常是电压骤降开始之后个周期内),最后导致过程中断。电能质量的低下对于持续能量的中断有很大的影响。有关这方面的一些产业为此付出了高额的费用。因此，很有必要减轻电能质量的涌流。励磁涌流引起电流骤降是因为能源和变压器之间发生了阻抗。假如励磁变压器总线电压很低(或源阻抗高)，那么产生的电压骤降就会很显著。减少电压骤降就要减少这种情况的发生。 电压骤降局限于敏感工业的承受力，但是目前还存在不足的标准判断电压凹陷承受能力是否为高压工业所能负载。最普遍公认的基准是ITIC的(信息技术产业委员会)特性曲线，更正式的名称为CBEMA特性曲线。该基准定义了曲线过电压相当有限的敏感负荷，即电压敏感信息技术设备可以在120 V额定电压下进行操作。同时设备能够承受电压中断持续20ms与电压骤降到80%时的额定电压持续0.5秒。晶闸管控制电容(TCSC)是一种新兴技术,它结合了常规系列电容器与晶闸管控制的反应堆(TCR),可连续自动控制系统的TCSC电抗引入。晶闸管控制电容TCSC已经运用于功率流控制、瞬时动力补偿、缓解及共振，由于设计的不同他们也可以通过调整他们的阻抗电感值大的动态被用来限制故障电流。晶闸管控制电容错被用为电流限制器已被调查和报告。本文主要目的是为了证明TCSC对于控制励磁涌流有很大的作用。论文论述了影响励磁涌流的的因素，并说明使用PSCAD / EMTDC仿真软件13可以缓解其带来的影响，并对电压互感器涌流等情况进行了调查。2励磁涌流在过去的几十年中, 卸变压器的涌流的现象收到了很多关注。涌流大量涌入一些系统的保护系统中，特别是在高新技术产业。所以，一些复合技术提出了减轻大量电流涌入的限制3，6。一般来说,变压器电流涌动主要取决于接通电源方面以及原来的大小和方向残余助焊剂。图1显示的等效电路14的单相变压器。总机Xp是初级绕组阻力和泄漏电抗引入。同样地，拉尔夫-舒马赫和x二次绕组及泄漏电阻电抗引入分别。Xc的分流电抗引入代表磁化特性的变压器铁心，而Rc代表铁芯(迟滞和涡流)。目前，我在分流电抗的励磁涌流需设置变压器铁心稳定的必要的流量。这个分流电抗引入具有非线性流量(或电压与电流特性)与kneepoint(通常是0.05-0.15 p.u。)超过正常1 p.u操作流量，变压器铁心产生很小一部分励磁涌流来维持这个正常通量。同时，电流或流量滞后也会产生涌流在一次侧电压大约90度稳态运行。当变压器被断电时剩下的流量被限制在核心。 假设变压器图1显示的是电压器被断电，然后打开的时候瞬时值的电压是几乎为零,所要求的流量(或者瞬间电流)的磁化分支机构必须在峰值或 滞后90才可满足稳态平衡。如果通量不是立刻上升至峰值,在经过转换之后，它开始从零达到1p.u. 4周期之后持续增加,直到它变成大约2 p.u。这种现象通常是指流量的双倍作用。在流量转换，方向交换极性之后,膜通量的积累达到最大流量，最大流量甚至可以超过2 p.u。现有的非线性磁图特点表明当流量是2 p.u时，变压器进入极度饱和。变压器铁心要求励磁涌流来支持这种通量变化。 基本上,铁芯在饱或者类似无分流电抗引入核心时电感变得非常小。流量和电流慢慢走向一个稳态值这依赖于电感电阻率电路的来源、组成，以及充沛、磁化分支阻抗绕组程度。 一个简单的近似方法可用来获得估计最大涌流和预期电压骤降。由于从电压暂降的等效电路，大量涌入电流不能超过1 /(X + Xp + Xc(min)部件，在X的母线的电抗引入来源于能量充沛的变压器。最小的磁化电抗引入、Xc(min)，代表变压器加压之后的最大可能的通量。虽然Xc(min)可以从测试和变压器铁心得到预测，但也并不是唾手可得的数据。当变压器铁芯高度饱和类似空心时，普遍认为是典型的两倍。短路阻抗即Xc(min)可以被假设为典型2(Xp + x)或2XT。XT为最初和其后漏损阻抗，它是可从显见数据中获得。取代典型值Xc(min)，涌入电流不能超过1 /(X + Xp + 2XT)p.u虽然在这个过程中会出现漏损抗阻，并且这些抗阻是不可得到的。但是这是他们认为最实用的研究，即以Xp或x等于XT / 2(15)。因此，最大的电压暂降即可估计为X /(X + 2.5XT)p.u 3，把励磁涌流的影响降到最低，控制断路器的计算时间和策略合作关系，在电力变压器发生涌流时，SCR发射角度被压抑启动。然而，变压器的瞬时涌流和方向还是难以测量。此外，弹簧力特性的断路器对确定正确的机械关闭时间也有很大的影响。除了被动的解决方案，inverter-based系列提出了利用补偿电流型控制来减少非正常涌流启动过程的模式。这个系列补偿电流在二次绕组变压器注入补偿，提供电流补偿。提供与变压器流失电流相反的电流。通过这种方法，励磁涌流被很好的控制。在电压稳定模式、系列补偿的区别注入和预期的用途电压，负载电压通过系列变压器负载电压维持在正常价值。3利用结构和运作模式TCSCA. TCSC模式TCSC的基本结构显示在图1中。它主要由四部分组成：系列补偿电容C，旁路电感L,可单向控制晶晶闸管、氧化锌电压器MOV等。TCSC的基础程度由补偿能力大小的电容器控制，主要功能是通过电感L减少绕过了短路电流和能量吸收MOV等表现出来。可单向控制晶闸管可控硅用于改造TCSC阻抗的需求,满足各类电力系统的状态，如提高稳定性，提高传动性能、控制磁共振等通过控制脉冲，TCSC可以触发晶闸管的触发角转变。随后，当前的价值会随着TCSC电感神经元控制的转变而不断变化,并将全部等效阻抗。.一般来说，当触发角145-180时，TCSC、等效阻抗出现电容。当触发角为90- 140时，TCSC等效阻抗电感的出现为限制短路电流特性，从而系统失效。在TCSC模块来绕过线路断路器安装，如果发生严重过失或设备故障。在电容器操作时，电感电流被限制，Ld两级电路的限制和频率的电容电流。B.稳定状态下操作方式在正常条件下,有四种操作模式：阻断模式；促进模式；电容模式；归纳刺激模式(图2)。当晶闸管阀引发不了晶闸管运转，并保持在不运转状态，TCSC运作就处在阻断模式。在这种情况下，TCSC就如同固定系列电容器。在促进模式下，晶闸管阀被不断触发和阀门停留的时间很短，这时TCSC表现得像个并联系列电感、电容器，并在晶闸管阀的分支。在这种模式下，产生电压稳定状态，TCSC归纳和阀门电流是稍大于线电流产生的电流电容器组的。对实际TCSC XC比例在XL /0.1 - 0.3之间的电容电压范围内，给定线的电流是非常低的。因此，利用旁路模式作为一种手段来减少错误的电容压力。为晶闸管提供一个触发器在其具有正向电压电容器穿过零电压之前，所以一个电容放电电流脉冲会传阅通过并联电感分支。 放电电流脉冲增加了线电流通过，增加线电流电压造成电容器电流增加。电容器电压达到峰值电压，通过双向晶闸管分支会增加比例的费用。这个基本电压费用比例也增加了。从系统的角度看，从这种模式的线插入电容器到近三倍固定电容器是正常的TCSC操作模式。在电感电流模式，促进循环的TCSC晶闸管部门大于线电流。在这种大晶闸管电流模式下，电容器的电压波形的正弦波形状会非常扭曲。这个峰值电压接近转弯时，极小的波形和高阀压力感应刺激模式能够稳态操作。此模式增加电感线，所以它和应用TCSC减少快速增长载荷率的优势是相反的。同时，这种模式是非常有用的在电路故障电流下降期间。这种模式作为一个current-limiting系统，有助于降低电压暂降在断层是正常的。4系统研究图3演示单行的简化图样本138千伏系统。变压器提供电能质量敏感负载。变压器的阻抗数据是138/21 MVA，阻抗，14.9%千伏和绕组，一面是用138千伏接地。从使用邻近的变压器到转变为大的客户，由于电压凹陷的涌流在加压，具有电压敏感负载的评估以决定是否需要以及如何使用变压器是很有必要的，因为时间是有限的。A. TCSC电容退出模式可以通过补偿效应的一系列传输线来缓解涌流电力品质。短路级别的系统常见的连接当假定为1850年供电量132千伏特时，它给出一个正序阻抗大约是9.4。该系统称具有操作132千伏特的电压，变压器漏电抗引入约高达有9位。这个电抗引入线的假设j47为最高赔偿的70%。当AB线的补偿，采用近似的服务，在第二部分讨论的技术,最大可能的是补偿线路电压暂降忽视估计94 /(9)的2.54 +部件或29%。 当线AB是在不同的服务时，TCSC退出服务。当uncompensated线在服务,例如,TCSC也退出服务，将会产生源阻抗47 = 7.83 9.4，最大电压骤降7.83近似为7.8级+ 2.5/(9)。B. TCSC电容刺激模式当TCSC在游标模式和线型时，最大程度的补偿产生的来源阻抗将会达到20%。C. TCSC阻挡模式当TCSC在阻断模式中时，此过程中晶闸管比起最高值来补偿程度较轻,因此产生的源阻比会以前的情况抗会更高。通常，电容是电容的刺激模式三分之一。从以上情况可以看出，能源和力量的阻抗网络在励磁涌流中扮演了主要的角色。TCSC减少阻抗来源，导致较少的临时电压下降从而使变压器加压。5模拟结果如图3所示，系统模拟了元素的系统建模PSCAD / EMTDC全部的细节。如图所示的变压器系统通电后0.2秒后开始模拟。假定它和不同线路连接到一个母线A上。这意味着变压器励磁涌流通过不同的线条，不一定从线TCSC补偿。因此，TCSC仍在游标不会从目前的电容器中触发MOV等过压。图4显示了变压器被加压后的一个在母线,电压是被1655 PSCAD / EMTDC引起的。在这种情况下，没有线路补偿，线AB也未得到TCSC的补偿。从这个数字可以得出，电压下降是相当敏感的载荷的影响,不一定与母线有关。值得注意的是,如图4，电压是相中性的额定电压的部件。其标称峰值(2)(132千伏特* 3108.8kV)。图5显示励磁涌流超过7倍正常流量。变压器的电流骤降时间常数假设为1.0秒。图6母线电压以全线补偿。作为可以推断，从数字4和6看出TCSC的电压暂降明显。从图七TCSC模式中可以很明显的看到，TCSC在影响变压器电压骤降并加压。作为推荐的实践,经营者可以设置最大限度地满足之前变压器赔偿的线。6 总结 在这篇文章中显示TCSC可以减轻变压器励磁涌流的电压骤降的影响。由于能源和通电的变压器之间的阻抗网络，电流骤降引暂时性的电压下降。如果短路MVA使用变压器母线是低电压(或变压器的源阻抗高)产生的电压骤降会显著，电压下降造成励磁涌流。结果显示TCSC可以减少源阻抗,从而减少相关电压下降。一种近似方法即把最大限度的价值的电压骤降进行评估，敏感负载在变压器加压时产生严重的问题及时与客户联系。TCSC可以有助于减轻了电压骤降相关的励磁涌流,使我们没有必要去通过复杂和昂贵的方法来补救。通过分析和模拟这种结果是正确的。Power Quality Enhancement by TCSCApplication to Mitigate the Impact ofTransformer Inrush CurrentMojtaba Khederzadeh, Senior Member, IEEEAbstract- Excessive transient inrush current occurs during transformers energization, especially when the transformer core has remnant flux that adds to the flux build-up after switching. System voltage sag due to inrush current affects the power quality of the network in proximity of the transformer. The extent to which power quality is degraded depends on short circuit MVA at the source bus, and the magnitude and decay time constant of the transient current. This paper investigates the application of Thyristor Controlled Series Capacitor (TCSC) at the line ends to mitigate the impact of transformer inrush current on power quality. First, it describes the influence of TCSC on the short circuit MVA at the source bus, and then the mechanism by which the TCSC is able to mitigate voltage sags. It also investigates and illustrates different TCSCs modes of operation on transformer inrush current. The ability of TCSC to limit fault currents by developing high inductive impedance under fault conditions with respect to transformer inrush current is also investigated. Index Terms- Inrush current, Power Quality, Thyristor Controlled Series Capacitor (TCSC), Transformer, Voltage Sag.I. INTRODUCTIONPower quality is an issue that is interesting for both electric utilities and end-users. The term power quality comprises a multitude of aspects, which is not new in essence, but has a growing concern due to the dissemination of sensitive loads (e.g. industrial plants) that use power electronics as a means of modernizing their manufacturing OWER quality is an issue that is interesting for both electric utilities and end-users. The term power quality comprises a multitude of aspects, which is not new in essence, but has a growing concern due to the dissemination of sensitive loads (e.g. industrial plants) that use power electronics as a means of modernizing their manufacturing processes 1.Voltage sags on present day electronically-controlled machinery used in mining, pulp and paper industries andsemiconductor manufacturing have detrimental effects. Voltage sags of as little as 10% of the nominal voltage and of duration as short as 2 or 3 cycles can affect critical equipment and adversely impact the production processes. As an example, ac voltage is rectified and converted to pulsed dc when it is applied to an adjustable speed drive. This pulsing dc is stored in a capacitor, which in turn supplies smooth dc. Since the capacitor stores energy, it allows the system to ride through some voltage sags. But if voltage sag is of sufficient depth and duration, the capacitor voltage will drop below a critical level (typically several cycles after the sag begins), at which point the drive may misoperate or simply shut down, resulting in process disruptions. Outages due to poor power quality can have as detrimental impact as sustained power interruptions. Some of these industries even pay a premium price for high quality power. It is, therefore, necessary to mitigate the impact of inrush current on power quality 3.The inrush current causes a temporary voltage drop due to impedance of the network between the sources and the energized transformer. If the short circuit MVA available at the transformer bus is low (or the source impedance is high), the resulting voltage drop can be significant. The voltage drop reduces with decay of the inrush 4-8. The voltage sags associated with inrush currents must berestricted to the withstand capability of sensitive industrial loads. But there is lack of standards which quantify voltagesag withstand capability for high voltage industrial loads. The most widely recognized benchmark is based on the ITIC (Information Technology Industries Council) curve, formallycalled CBEMA (Computer Business Equipment Manufacturers Association) curve. The curve defines overvoltage and under-voltage susceptibility for a very limited segment of voltage sensitive loads, namely, information technology equipment operating at 120 V nominal voltages.Based on this curve, the equipment is designed to withstand a voltage interruption lasting 20 ms and a voltage sag to 80% of nominal voltage lasting 0.5 seconds.The thyristor controlled series capacitor (TCSC) is an emerging technology that combines conventional series capacitor with thyristor controlled reactor (TCR), which allows continuous control of the TCSC reactance. TCSCs are already used for power flow control, transient dynamic compensation, sub-synchronous resonance mitigation, and they may also be used for limiting fault current by adjusting their impedance dynamically to a large inductive value that depends upon the TCSC design. The use of TCSCs as fault current limiters has been investigated and reported 9-12. The main objective of this paper is to demonstrate in detail the capability of TCSC to mitigate the voltage sags associated with transformer inrush current. The paper discusses factors that influence the inrush current. Using the PSCAD/EMTDC simulation software 13, the alleviation of the impact of transformer inrush current on voltage sags is investigated.II. INRUSH CURRENTOn the past several decades, inrush current phenomena generated from unloaded transformer have received much attention. Inrush currents are highly undesirable for some protective system, especially in high-tech industries. So, a few echniques of mitigating inrush currents have been proposed to limit the inrush currents 3-6. Generally, the transformer inrush currents are mainly determined by the power-on angle and the magnitude and direction of the original residual flux.Fig. 1 shows the equivalent circuit 14 of a single-phasetransformer. Rp and Xp are primary winding resistance and leakage reactance, respectively. Likewise, Rs and Xs are secondary winding resistance and leakage reactance, respectively. The shunt reactance Xc represents the magnetizing characteristic of the transformer iron core, whereas Rc represents iron core (hysteresis and eddy current) losses. The current, Im, in the shunt reactance is the magnetizing current required to set-up the necessary flux in the transformer core. This shunt reactance has a nonlinear flux (or voltage) versus current characteristic with a kneepoint which is just (typically 0.050.15 p.u.) above the normal operating flux of 1 p.u. The transformer core (or shunt reactance) draws very little magnetizing current to maintain this normal flux. Also, the magnetizing current or flux lags the applied voltage on the primary side by approximately 90during steady-state operation. When the transformer is deenergized, a remnant flux is trapped in the core.Fig. 1 Equivalent circuit of a single-phase transformer.Assume that the transformer shown in Fig. 1 is initially deenergized with zero remnant flux. It is then switched on when the instantaneous value of the applied voltage is close to zero, which requires that the instantaneous value of flux (or current) in the magnetizing branch must be at peak value or lag by 90 to satisfy the steady-state equilibrium. Since the flux cannot instantaneously rise to peak value, it starts from zero and reaches 1p.u. after 1/4 cycle and continues to increase until it becomes approximately 2 p.u. (peak) 1/2 cycle after switching. This phenomenon is commonly referred to as the flux doubling-effect. If there is any remnant flux present prior to switching, and its polarity is in the direction f flux build-up after switching, the maximum flux can even exceed 2 p.u.The nonlinear flux and current characteristic as in Fig. 2 shows that the transformer enters into extreme saturation when flux is 2 p.u. The transformer core requires excessive magnetizing current to support this flux build-up during switching. Basically, the iron core in saturation behaves like an air core inductor with shunt reactance becoming very small. Flux and current slowly decay to a steady-state value with the rate of decay dependant on the inductance to resistance ratio of the circuit consisting of source, the energized winding, and magnetizing branch impedances.A simple approximate method can be used to obtain an indication of the maximum inrush current and the expected voltage sag due to it from the equivalent circuit. The inrush current cannot exceed 1/(X+Xp+Xc(min) p.u., where X is the source reactance of the bus from which the transformer is energized. The minimum magnetizing reactance, Xc(min), represents the maximum possible flux build-up after the transformer energization. Although Xc(min) can be estimated from the open circuit test and the hysteresis characteristic of the transformer iron core, it is not as readily available as the nameplate data. Since the transformer iron core behaves like an air core at such a high degree of saturation, Ref. 7 suggest that the air core inductance is typically two times the short circuit impedance i.e., Xc(min) can be typically assumed as 2(Xp+Xs) or 2XT. XT is the sum of the primary and secondary leakage reactances, and it is available from the nameplate data. Substituting the typical value of Xc(min), the inrush current cannot exceed 1/(X+Xp+2XT) p.u. Although the leakage reactances of individual windings are not available, in most practical studies they are assumed as equal, i.e., Xp or Xs equal to XT/2 15. Thus, the maximum voltage sag can be estimated as X/(X+2.5XT) p.u. 3.Fig. 2 Nonlinear characteristics of Xc.To minimize the problems associated with transformer inrush current, controlled switch-on time of the circuit breaker or SCR firing angles have been proposed to suppress the startup inrush current in power transformer 3. However, it is difficult to measure instantaneous magnitude of residue flux and direction at the instant of transformer exercitation. In addition, the spring force characteristics of circuit breaker also have a strong influence to determine the correct instant of mechanical closing timeIn addition to the passive solutions, inverter-based series compensator has been proposed using a current-mode control for reducing the undesired inrush current during startup mode 6. The series compensator injects a compensating current on the secondary winding of the series transformer. The compensating current supplied by the series compensator has opposite polarity to the inrush current produced by the power transformer. As a result, the inrush current is well suppressed with such a new control approach. In voltage stabilization mode, the series compensator injects the difference between the utility voltage and the desired load voltage through a series transformer to maintain the load voltage at the normal value.III. BASIC STRUCTURE AND OPERATION MODES OF TCSCA. TCSC ModelThe basic structure of TCSC is shown in Fig.1. It is mainly constituted by four parts: series compensating capacitor C, bypass inductance L, bidirection thyristor SCR and zinc oxide voltage limiter MOV. The degree of TCSC basic compensation is controlled by the capacity size of capacitor C. The main function of bypass inductance L is to reduce the short circuit current and the energy absorbed by MOV. Bidirection thyristor SCR is used to transform the equivalent impedance of TCSC which fulfill the needs in all kinds of power system condition, such as improving the stability, increasing the transmission capability, restraining hyposynchronization resonance and so on 9-10.By controlling the trigger pulse, TCSC can transform the trigger angle of thyristor. Subsequently, the current value of inductance subcircuit which controlled by TCSC can be transformed, and then the total equivalent impedance will be changed continuously. Generally, when the trigger angle is 145 o 180 o , the equivalent impedance of TCSC is appeared as capacitance. When the trigger angle is 90 o 140 o , the equivalent impedance of TCSC is appeared as inductance as which characteristic can restrict short circuit current during system failure 9-10. A circuit breaker is also installed across the TCSC module to bypass it if a severe fault or equipment malfunction occurs. A current limiting inductor, Ld, is incorporated in the circuit to restrict both the magnitude and the frequency of the capacitor current during the capacitor bypass operation. B. TCSC Modes of Operation in Steady State In normal operating conditions, there are four modes of operation; blocking mode; bypass mode; capacitive boost mode; and inductive boost mode (Fig. 2). When the thyristor valve is not triggered and the thyristors are kept in non-conducting state, the TCSC is operating in blocking mode. In this mode, the TCSC performs like a fixed series capacitor. In bypass mode the thyristor valve is triggered continuously and the valve stays conducting all the time; so the TCSC behaves like a parallel connection of the series capacitor with the inductor, Ls, in the thyristor valve branch. In this mode, the resulting voltage in the steady state across the TCSC is inductive and the valve current is somewhat bigger than the line current due to the current generation in the capacitor bank. For practical TCSCs with XL/XC ratio between 0.1 to 0.3 range, the capacitor voltage at a given line current is much lower in bypass than in blocking mode. Therefore, the bypass mode is utilized as a means to reduce the capacitor stress during faults.In capacitive boost mode a trigger pulse is supplied to the thyristor having forward voltage just before the capacitor voltage crosses the zero line, so a capacitor discharge current pulse will circulate through the parallel inductive branch. The discharge current pulse adds to the line current through the capacitor and causes a capacitor voltage that adds to the voltage caused by the line current. The capacitor peak voltage thus will be increased in proportion to the charge that passes through the thyristor branch. The fundamental voltage also increases almost proportionally to the charge. From the system point of view, this mode inserts capacitors to the line up to nearly three times the fixed capacitor. This is the normal operating mode of TCSC.In inductive boost mode the circulating current in the TCSC thyristor branch is bigger than the line current. In this mode, large thyristor currents result and further the capacitor voltage waveform is very much distorted from its sinusoidal shape. The peak voltage appears close to the turn on. The poor waveform and the high valve stress make the inductive boost mode less attractive for steady state operation. This mode increases the inductance of the line, so it is in contrast to the advantages associated with the application of TCSC for increasing the line loadability by decreasing the line impedance. Meanwhile, this mode is useful during short circuits to decrease the fault current. This mode is normally used as a current-limiting system, helping to reduce the voltage sag during the faults.Fig. 2 TCSC Control Modes.IV. SYSTEM UNDER STUDYFig. 3 shows a simplified single-line diagram of a sample 138 kV system. Power quality sensitive loads are supplied from the transformer. The data of the transformer are 315 MVA, 138/21 kV, 14.9% impedance and star-delta windings with 138 kV star side being solidly grounded.Proximity of the transformer to a very large customerhaving voltage sensitive loads necessitate the assessment of the voltage sag due to inrush current during energization of the transformer in order to determine if, and how, it needed to be limited. The effect of a series compensated transmission line to mitigate the inrush current impact on power quality is investigated. The short circuit level of the system at the point of common coupling is assumed 1850 MVA at 132 kV, which gives a positive sequence Thevenin impedance of approximately 9.4. The system has a nominal operating voltage of 132 kV. Transformer leakage reactance is about 9 at the high side. The reactance of the line is assumed to be j47 with the maximum compensation of 70%. When the compensated line AB is out of service, using the approximate technique discussed in Section II, the maximum possible voltage sag neglecting he compensated line is estimated as 9.4/(9.4+2.59) p.u. or 29%. When line AB is in service different cases are considered:A.TCSC out of serviceWhen the uncompensated line is at service, i.e., TCSC out of servi