英文翻译(译文)
英文名称High V oltage Direct Current Transmission-A Review, Part I 中文名称高压直流输电系统发展综述（第一部分）
学生姓名
学号
系、年级专业
指导教师
2014年3 月1 日
Electricity Transmission Co., Al Behira, EGYPT, Eng. Okba86@Gmail. com
2 Ph.D., Member, IEEE, GM, Electrical Engineering Dept., Abu-Qir Fertilizers & Chemical Industries Co., Alexandria, EGYPT,MohammedSaied((i)Gmail. com
3 Full-Prof., Member, IEEE, Electrical Engineering Dept., Faculty of Engineering, Alexandria University, Alexandria, EGYPT
摘要：高压直流（HVDC）技术和概念发展的里程碑主要是在20世纪50年代。由于采用了高功率晶闸管开关（1960-1970年），直流输电技术水平在20世纪80年代达到了一个显著的高度。经典的高压直流输电使用基于晶闸管的电流的整流转换器(LCC)技术。功率半导体开关的出现在1980 - 90年代,把开关功能特别是IGBTs IGCTs,和在这个领域持续进展,介绍了传统的(二级)电压源变换器(VSC)技术和各种各样的配置,多层次、多模块VSCs,也为电力系统应用中可行的转换技术。
直流系统由于其潜力在经历重新出现的重要程度直接也处理或促进解决大量现有和预期互相连接的交流电源系统稳态和动态问题。
HVDC技术使长距离大容量电力传输成为可能。比较评估,研究高压直流输电系统与高压交流输电系统。应用不同的高压直流系统也提供了概述。
关键词：高压直流转换器，直流输电换流技术，层次水平，高压直流输电系统组件，高压直流输电方案，高压直流输电。
I.介绍
世界上第一台发电机是直流电发电机。因此，第一个输电线路也是直流。尽管最初直流电至高无上，但交流电却因为它的用途广泛而取代了直流电。这是因为变压器、多元电路、感应电动机在1890年代和1880年代的可及性。同时电力电子技术日益渗透到电力系统主要是因为高压大功率半导体可控管的不断进步。
变压器是非常简单的机械而广泛被用来改变电压等级，输电，配电，以及电平下降。磁感应电动机是产业的耕马并且仅与交流电一起使用。这就解释了为什么交流电在商业上与国内负荷上非常有用了。对于长距离传输，直流电在经济性、技术性和环境上较之交流电更有优势。一般情况下，高压直流（HVDC）传输系统可以分为几个方面:成本、灵活性和操作要求的基础上。
最简单的直流方案是背靠背互联,它有两个转换器在同一个站点,没有输电线路。这些类型的连接是所用的两种不同的交流输电系统之间的相互关系。
复返链接连接两个换流站由单一导体线和大地或海洋用作返回的路径。最常见的直流双相链接,两个换流站与双相导体(±),和每个导体都有自己的回报。多端直流输电系统有超过两个转换器,可串联或并联连接。
II.传输系统的可靠性和可控性评估
现代电力系统的技术结构非常复杂。他们由大量相互关联的子系统和组件的交互,并影响整个系统的可靠性。可靠性的一个定义是一个组件或系统，以规定的条件下和在既定期间内执行所需的功能的
承受突然的干扰，如电短路或系统组件的非预期损失的能力。
一个可靠性模型,包括整个整个电力系统的复杂性不可能实现。分析来说过于复杂,并且结果很难解释。相反,它比单独的系统分成三个阶段(HL):发电(HLl),发电和输电(HL2),和配电(HL3)。每一阶段可以单独建模和评估。研究第二阶段也称为复合系统的可靠性评估,这可以包括充足和安全分析。高压直流系统的可靠性评估可以单独建模和评估,然后列入第二阶段评估系统整体可靠性的影响。在直流输电系统的可靠性评估,它是非常重要的知道系统的技术细节,以模型。下一个章节详细地介绍了高压直流输电系统。
电气与电子工程师协会标准是评估高压直流输电系统变电站的指南。这个标准推动并定义了高压直流输电系统的生命周期内所有阶段的可靠性、可用性、可维护性的基本概念。介绍高压直流输电系统可靠性、可用性、可维护性的目的就在于帮助：i)改善电站服务的可靠性、可用性、可维护性。ii)计算并比较考量不同高压直流输电系统的可靠性、可用性、可维护性。iii)减少损耗。iv)减少多于设计。v)提升高压直流输电系统整流器的规格。在一些出版的资料中涵盖了将高压直流输电系统作为单一系统的评估研究。
从另一方面讲，高压直流输电环节的可控性提供了坚定的传输容量没有限制，由于网络拥塞或平行的路径上循环流动。可控性允许的高压直流输电为'跨越式'多'瓶颈点“，或在交流电网旁路顺序路径的限制。因此，高压直流输电线路的利用率通常高于针对超高压（EHV）交流输电每兆瓦时降低了传输成本。通过消除循环流动，可控释放为服务中间负载，并提供出口，用于本地产生的预期目的，并行传输容量。
III.交流传输与直流传输
随着可再生能源发电的快速发展，如风力和太阳能发电，并在长的距离所产生的高电能，它是通过紧急经济和环境的方式来养活这些分布式能量回馈电网。
实际上，交流电是非常熟悉的工业和家用负载，但它长距离传输有一定的局限性。另外，作为市电的负载增加，电网的容量需要扩大，尽管架空交流线已经占据太多空间传输。在一个字，一个新的传输方法是必要解决这些问题和其他问题.DC传输，它是在几个项目被使用。
例如，开关操作浪涌，是严重的瞬态过电压的高压输电线路。交流传动的高峰值正常峰值电压的2 - 3倍,在直流输电是正常电压的1.7倍。交流传动的高峰值正常峰值电压的2 - 3倍,在直流输电是正常电压的1.7倍。此外，在特高压直流输电比高压交流输电线路具有较小的电晕和无线电干扰在下面的部分中，是在高压直流输电系统与传统的交流传输系统的比较中进行。
A. 传输损耗比较
所有交流或直流的费用送电线通常包括主要部分的费用，例如;优先权(行)在塔的设施期间，是相当数量风景也许被占领，指挥，绝缘体，终端设备。除业务成本之外例如送电线损失。对于交流和直流线路给予操作上的限制，直流线路必须进行尽可能多的权力有两个导体的交流线路与相同大小的三根导线的能力。
此外，直流线路基础设施的要求比交流线路较少，这将从而减少直流线路安装成本。
抵消换流站的成本就越高。Fig.l。一个显示典型的曲线之间的交流和直流传输成本比较,考虑: 终端站的成本；
线损；
亏损资本化价值。
直流曲线不像交流曲线陡峭,因此大大降低线成本每公里。长距离交流线,中间无功补偿的成本必须考虑。盈亏平衡距离在500到800公里的范围取决于其他因素,如特定国家的成本元素,为项目融资利率、损失评估、通行成本等。
图1.显示了发电容量和距离交流和直流系统。
1)环境问题
直流输电系统基本上是环境友好,因为改进能源传输可能导致一个更高效的利用现有的发电厂。土地覆盖和直流架空输电线路的通行权相关的成本不是像交流线那样高。这减少了视觉冲击和节省土地赔偿新项目。还可以提高现有线路的电力传输能力。
(a)
(b)
在文献[49]的工程方法，工具和设计解决方案的概要介绍。在高压直流换流站的设计考虑声学要求使用的验证方法也解释了。
IV. 高压直流输电对高压交流电压稳定性
长传输线路被要求提供的电源，主要负荷中心或现有的传输网络的最近的连接点。对于长距离传输的大容量电源的几个技术和经济问题必须要考虑可制成最佳的决定。电压稳定一般来说是将被考虑的其中一个主要技术问题[50] - [51]。几种方法,用于获得稳定的直流系统中,也提出了在文献[51]-[59]。用于HVDC系统的最共同的电压稳定索引是最大值可利用的力量(地图)，重要有效的短路比率(CESCR)和电压刚性系数(VSF)。
最大功率方法,它决定了地图和电压灵敏度方法确定最佳[52]中描述粘胶短纤。这两个方法一致,即地图点达到时粘胶短纤接近无限,如果转换器操作不断的消光角和恒功率控制模式。的基本曲线稳定性方程方程也派生考虑负荷特性和系统参数。这些方法应用于[53]确定最不利荷载特征对降低功率/电压稳定的利润。这是通过分析负荷特性的影响最大功率不稳定(dP / dI)和直流系统的绘制。短路比率(SCR)或CESCR也被考虑当HVDC系统的刚性系数，但是只合适评估AC系统的冲击对稳定裕度HVDC [60]。
作者[59]介绍了一个新的索引(dqt/eig_min)电压对AC/DC系统的稳定性分析的。该指数是用于将系统划分为柔软且非软模态系统。后者被定义为系统常数dQt / eig_分钟前的发展,反之亦然。
这个指数也作为基础来决定的类型无功补偿和直流控制策略。
直流电塔：
图2。典型的输电线路结构大约1000兆瓦。
虽然上述指标可以用来比较直流系统之间的电压稳定的利润,他们并不适用于高压交流和直流的比较。在文献[56]中，作者延续崩溃（POC）的方法来测定系统，包括高压直流输电线路鞍结分岔的开
一个优化问题。
然而,需要更深入的分析解释,需要考虑和直流输电系统的控制问题。不恰当的控制计划发射,灭绝,和重叠角导致换向失败或雅可比矩阵的奇异性。因此,PoC基于这种方法是不可靠的用于直流电压稳定和暖通空调系统的比较。在一个特定的总线的DVAC / DQ的因素是在AC和DC系统常用的电压稳定性指标[51]，[54] - [55]。然而，它从未被用于高压交流和直流输电系统之间的比较。
V.优点和高压直流输电的后发劣势
虽然直流的理由选择通常是经济,可能还有其他原因选择。
在许多情况下需要更多的交流线路提供相同的权力在同一距离由于系统稳定性的局限性。此外,长途AC 线路通常需要中间交换站和无功补偿。这可以增加交流输电变电站成本的地步，这是媲美的特高压直流输电[29]。
直流可能是唯一可行的方式互连两个异步网络,减少故障电流,利用长电缆电路,绕过网络拥塞,分享实用征地没有退化的可靠性,减轻环境问题。在所有这些应用中,直流交流输电系统起到很好的补充作用。下面这些强调了高压直流输电系统的优缺点。
A.优点
1) 每一路导线能承担较大的电量
2)基站建设更简单，电力塔更小。
3)双极式高压直流输电系统的线路只需要两座绝缘整流器而不是三座。
4) 更窄的通行权。
5）要求只有三分之一的导体的绝缘套为双回路交流线路。
6) 节省大约在线路施工30％。
7）接地回路都可以使用。
8）每根导线可以操作作为一个独立的电路。
9）在稳定状态下没有充电电流。
10）无集肤效应。
11）降低线路损耗。
12)线路功率因数总是统一的。
13)线不需要无功补偿。
14)同步操作不是必需的。
15)有限距离不稳定。
16)互连不同频率的交流系统。
17) 在DC线的低短路潮流。
18)不会产生交流系统的短路电流。
19)可控性允许直流“超越”多“薄弱点”。
20)没有物理限制限制距离或功率电平直流地下或海底电缆
1）转换器是昂贵的。
2）转换器需要大量的无功功率。
3）多终端或网络操作是不容易的。
4）转换器产生的谐波，因此，需要过滤器。
5)盈亏平衡距离影响通行权的成本和线路建设的典型值500公里[38]-[40]。
VI.高压直流输电系统的应用
自1954年中国内地瑞典哥特兰岛之间的第一个商业项目[30]的调试，高压直流输电已逐渐成为一项成熟的技术交流系统互连。高压直流输电技术的应用是由一些特殊的条件下高压直流输电是最可行的，也可能是唯一的解决办法有道理的。这些应用包括大容量输电长距离，子船用电缆传输，异步系统间的连接[64]。特高压直流输电的应用程序可细分为以下不同的基本类型[29]，[37]和[64] 。
A.远距离大容量输电
如上图所示，高压直流输电系统通常提供了更经济的替代交流输电，对利用在长的距离，大容量电力输送的清洁远程资源，如产生的高电能;水电开发，坑口电厂，太阳能，大型风力发电场，或大热岩地热energy.This传输与使用较少的高压直流输电线路比交流输电成立。
B.电缆传输
不像在AC电缆的情况下，不存在物理限制限制了HVDC地下或海底电缆的距离或功率电平。地下电缆可用于共享行与其他实用程序，无需在使用公共走廊的影响可靠性的担忧。
地下和海底电缆系统的节能优势，此前已证明，明知这取决于功率电平进行传输，这些节省可以抵消在40公里以上的距离更高的换流站的成本。
另一方面，为在距离的交流输电，有在缆绳容量的下车由于当前的费用它易反应的组分，因为缆绳比AC架空线有更高的电容并且降低感应性。虽然这可以由中间分流器报偿补偿对地下缆绳以增加的费用，如此做就不是实用的为水下缆绳[65]- [66]。
C.异步关系
随着高压直流输电系统，互连可以异步网络之间进行更多的经济和可靠的系统运行。异步互连允许在互惠互利的互连，同时提供了两个系统之间的缓冲区。通常，这些互连使用背到后端转换器没有传输线[67]。
异步直流环节有效地采取行动对付从传递到另一个网络级联在一个网络中断传播。
更高的功率传输可以实现的，在弱电系统的应用提高了电压稳定，采用电容整流转换器。动态电压支撑和改善电压稳定性，而不一样需要交流系统增援电压源换流器（VSC）的转换器允许更高的功率传输提供。VSC转换器不受换向失败,允许从附近的交流的缺点快速复苏。
可不受任何限制的经济力量时间表，其中反向功率方向，因为没有最小功率或电流限制[68]。
D.离岸的传送
自励式，动态电压控制，以及黑启动能力，使允许紧凑VSC隔离和孤立的岛屿上负载,或海上钻井和生产平台长途海底电缆。此功能可以消除需要运行不经济的或昂贵的本地生成或提供一个出口等近海从
域。许多更好的风网站具有更高的容量因子均位于境外。基于VSC的HVDC输电不仅可以有效地利用长距离陆地或海底电缆，而且还提供无功支持，风力发电和复杂的互联点[29]。
E.对大市区的功率传输
大城市的电源取决于地方一代和力量进口能力。当地一代比较陈旧,而且效率不及新单位位于远程。空气质量法规可能限制这些老单位的可用性。新的传输到大城市很难网站由于通行权的限制和土地使用的限制。协定基于VSC的地下传输电路在现有的两用优先权可以被安置带来力量，以及提供电压支持允许更加经济的电源，不用妥协的可靠性。接收终端像给予力量，提供电压规则和动力的一台虚拟的发电机一样行动有反应的力量储备。
电站结构紧凑，主要安置在室内进行选址在市区比较容易。此外， VSC提供的动态电压支持可能经常增加毗邻交流输电[29的]能力。
这些应用可以被总结如下：
I) 大块能量输电通过长途架空线。
2)通过海底电缆输电的大部分能量。
3)快速和精确地控制能量流在背靠背直流链接,创建一个积极的机电振荡阻尼,,通过调节发射功率并提高网络的稳定性。
4)连接两个交流系统和不同频率使用异步背靠背直流链接,没有约束对系统频率或相位角度。
5)多端直流链接用于遍历地区提供必要的战略和政治关系的潜在合作伙伴,当权力从远程传送一代的位置,在不同的国家,或在一个国家的不同地区。
6) 当很远从消费者时，位于连接可更新的能源，例如水力发电，矿嘴、太阳，风力场或者热石地热能。7）脉冲宽度调制（PWM）可用于基于VSC的HVDC技术相对于基于晶闸管常规高压直流。这种技术非常适用于风电连接到电网。
8）连接两个交流系统在不增加短路功率，无功功率没有得到过直流链路传输。这种技术是在发电机的连接，在图3所示的直流输电系统的各种应用中是有用的。
VII. 不同的高压直流输电系统方案 [69]-[73]
A.背靠背转换器
“背靠背”表明,整流器和逆变器位于同一车站。背靠背转换器主要用于电力传输之间相邻的交流电网不能同步。他们也可以使用在一个网状网格为了实现一个定义的功率流。
B .单极的远距离传输
为很长很长的距离,特别是海电缆传输,返回路径与地面/海电极将最可行的解决方案。在许多情况下,现有的基础设施或环境约束防止电极的使用。在这种情况下,使用金属返回路径,尽管增加了成本和损失。
C.双极远距离传输
两个独立的两极的双相结合,共同低压返回路径,如果可用,只能携带一个小不平衡电流在正常操作。如果使用这个配置所需的传输容量超过一个单极。也使用它如果需要更高的能源可用性或降低甩负荷能力使我们有必要对两极分裂的能力。在维护或中断一个杆,它仍然是有可能的传输功率的一部分。超过50%的传输能力可以利用,剩余的实际过载容量限制,而只需要三分之一绝缘的导体集相比,双电路交流。
双相情感障碍的解决方案在解决方案的其他优点有两个单极子是降低成本,由于一个共同的或没有返回路径,降低损失。在[74],[76]双极直流输电系统配置建模。可靠性模型在这些三篇论文相似,但论文的目标是不同的。
1)双相情感与地面返回路径:
这是一个常用的配置为双极传输系统。该解决方案提供相对于操作过程中突发事件和维护能力降低，在一个单极故障具有高度灵活性，声音极的电流将被接管的接地回路和故障极将被隔离。以下所引起的转换器的极点中断时，电流可以从接地返回路径被换向到由故障极的高压直流输电导体提供一金属返回路径。
2）双极专用金属返回路径
单极操作：
如果有限制甚至临时使用的电极，或者如果传输距离比较短，有专门的LVDC金属回路导体可以被认为是作为一种替代有电极的接地返回路径。
3）双极没有专用的返回路径单极
操作：
计划没有为单极电极或专用金属返回路径操作会给最低的初始成本;单极操作可能通过旁路开关转换器极停机期间,但不是在直流导线故障。
图4.不同的直流方案。
短双停机将遵循一个转换器极停机搭桥手术前可以建立。图4显示了不同直流方案[29]。
D .多端直流输电系统
在这种配置，有超过两套交换器。与12脉冲交换器的一个多端CSC-HVDC系统每根杆在图5.在这种情况下显示，交换器1，并且3可能经营作为整流器，而交换器2经营作为变换器。工作按另一顺序，交换器2can经营作为整流器和交换器1和3作为变换器。通过机械交换一台特定交换器的连接，其他组合可以达到[77]。
图5.多端CSC-HVDC系统并联。
结束语
比较评估,研究、应用程序不同的方案,并对直流与高压交流输电系统,介绍了两份纸的一部分。
High Voltage Direct Current Transmission -A Review, Part I
Mohamed H. Okba1, Mohamed H. Saied2, M Z. Mostafa3, and T. M Abdel- Moneim3
1 M.Sc. Candidate, Electrical Engineering Dept., Testing, Measurement, and Protection Sect., Egyptian Electricity Transmission Co., Al Behira, EGYPT, Eng. Okba86@Gmail. com
2 Ph.D., Member, IEEE, GM, Electrical Engineering Dept., Abu-Qir Fertilizers & Chemical Industries Co., Alexandria, EGYPT,MohammedSaied((i)Gmail. com
3 Full-Prof., Member, IEEE, Electrical Engineering Dept., Faculty of Engineering, Alexandria University, Alexandria, EGYPT
Abstract-Major milestones in the development of high voltage direct current (HVDC) technologies and concepts were achieved in 1950s. Thanks to the high power thyristor switches (1960-70s), the HVDC technologies reached a significant degree of maturity in 1980s. The classical HVDC uses thyristor-based current-sourced line-com mutated converter (LCC) technology.The advent of power semiconductor switches in 1980-90s, with turn on-off capabilities especially the IGBTs and IGCTs, and the on-going progress in this field, have introduced the conventional (two-level) voltage-source converter (VSC) technology and its variety of configurations, multi-level and multi-module VSCs,also as viable converter technologies for power system applications.
The DC system is experiencing significant degree of reemergence due to its potential to either directly address, or to facilitate resolving a large number of existing and anticipated interconnected AC power system steady-state and dynamic issues.
HVDC technology made possible to transfer bulk power over long distances. In part I of this two-parts paper, comparative evaluations, studies, and review of HVDC versus HV AC transmission systems, are presented. Applications, different schemes of HVDC systems are also outlined.
Index Terms- HVDC converters, HVDC converter technologies, Hierarchal Level, HVDC system components,HVDC schemes, HVDC transmission.
transmission line was constructed with DC. Despite the initial supremacy of the DC, the alternating current (AC) supplanted the DC for greater uses. This is because of the availability of the transformers, poly-phase circuits, and the induction motors in the 1880s and 1890s [1]-[2].The ever increasing penetration of the power electronics technologies into power systems is mainly due to the continuous progress of the high-voltage high-power fully-controlled semiconductors [3]-[14].
Transformers are very simple machines and easy to be used to change the voltage levels for transmission, distribution, and stepping down of electric power. Induction motors are the workhorse of the industry and work only with AC. That is why AC has become very useful for the commercial and domestic loads. For long transmission, DC is more favorable than AC because of its economical, technical, and environmental advantages. In general, high voltage direct current (HYDC)transmission systems can be classified in several ways; on the basis of cost, flexibility, and operational requirements.
The simplest HVDC scheme is the back-to-back interconnection, where it has two converters on the same site and has no transmission lines. These types of connections are used as inter-ties between two different AC transmission systems.
The mono-polar link connects two converter stations by a single conductor line and the earth or the sea is used as the returned path. The most common HYDC links are bipolar,where two converter stations are connected with bipolar conductors (±), and each conductor has its own ground return.The multi-terminal HYDC transmission systems have morethan two converter stations, which could be connected is seriesor parallel [15]. II. RELIABILITY AND CONTROLLABILITY EV ALUATIONS OF
TRANSMISSION SYSTEMS
Modern power systems are very complex technical structures. They consist of large number of interconnected subsystems and components each of which interact with, and influence, the overall systems reliability. One defmition of reliability is the ability of a component or a system to perform required functions under stated conditions for a stated period of time [16]. Reliability assessments of electrical systems are performed in order to determine where and when new investments, maintenance planning, and operation are going to be made. Power system reliability is often divided by the two functional aspects of system adequacy and security. Adequacy is the ability of the power system to supply the aggregate electric power and energy requirements of the customer at all times, taking into account scheduled and unscheduled outages of system components. Security is the ability of the power system to withstand sudden disturbances such as electric short circuits or non-anticipated loss of system components [16].
A reliability model that includes the whole complexity of the entire electrical power system would be impossible to implement. The analysis would be far too complex and the results would be very difficult to interpret. Instead it is preferable to separate the system into three hierarchal levels(HL): generation(HLl), generation and transmission(HL2),and distribution(HL3). Each level can then be modeled and evaluated individually [16]. A study of HL2 is also referred to as a composite system reliability assessment and this can include both adequacy and security analysis. Reliability assessments of HYDC systems can be modeled and evaluated separately and then included into HL2 to evaluate the effect of the overall system reliability. In reliability assessments of such HVDC systems, it is of great importance to know the technicalities of the system, in order to model it. The next section describes the HVDC systems details.
The IEEE Standard is a guide for the evaluation of the HVDC converter stations reliability [17]. It promotes the basic concepts of reliability, availability, and maintainability (RAM) in all phases of the HVDC station's life cycle. The intention of introducing these concepts of RAM in HVDC projects is to provide help in: i)
system as a single system.
On the other hand, the controllability of HVDC links offers firm transmission capacity without limitation due to network congestion or loop flow on parallel paths. Controllability allows the HVDC to 'leap-frog' mUltiple 'choke-points' or bypass sequential path limits in the AC network. Therefore, the utilization of HVDC links is usually higher than that for extra high voltage ( EHV) AC transmission lowering the transmission cost per MWh. By eliminating loop flow,controllability frees up parallel transmission capacity for its intended purpose of serving intermediate load and providing an outlet for local generation [29].
III. AC VERSUS DC TRANSMISSION
As the rapid development of renewable energy generation,like wind and solar power generation, and high electrical power generated at long-distances, it is urgent to feed these distributed energy back to power grid through an economic and environmental way. Actually, AC is very familiar for
industrial and domestic loads, but it has some limitations for long transmission lines. Moreover, as the city power load is increasing, the capacity of grid need to be expanded, despite that the overhead AC lines have already occupied much transmission space. In a word, a new transmission approach is needed to solve these and other problems, the DC transmission, which is being used in several projects [30],[31]-[32], and [33]-[34]. Switching surges, for example, are the serious transient over voltages for the high voltage transmission lines. In case of AC transmission the peak values are 2 to 3 times normal crest voltage, where for DC transmission it is 1.7 times normal voltage. In addition to, the HVDC transmission has less corona and radio interferences than that of HV AC transmission line [35]-[37]. In the following section, comparisons of the HVDC with the conventional AC transmission systems are carried out.
A. Transmission Costs Comparison
The cost of any AC or DC transmission lines usually includes the cost of main components, such as; right-of-way ( ROW), which is the amount of landscape that might be occupied during installations of towers, conductors, insulators,terminal equipment, in addition to the operational costs such as losses of transmission lines. For given operational constraints of both AC and DC lines, DC lines has the ability to carry as much power with two conductors as AC lines with three
conductors of the same size. Moreover, DC lines require fewer infrastructures than AC lines, which will consequently reduce the cost of DC lines' installation.
1) Economic Considerations:
For a given transmission task, feasibility studies are carried out before the fmal decision of implementing of a HY AC or HVDC system. Whenever long distance transmission is discussed, the concept of "break-even distance" arises. This is where the savings in HVDC line costs offsets the higher converter station costs. Fig.l.a shows typical cost comparison curves between AC and DC transmissions, considering:
? Terminal station costs,
? Line costs, and
? Capitalized value of losses.
The DC curve is not as steep as the AC curve because of considerably lower line costs per kilometer. For long AC lines,the cost of intermediate reactive power compensation has to be taken into account. The break-even distance is in the range of 500 to 800 km depending on a number of other factors, like country-specific cost elements, interest rates for project fmancing, loss evaluation, cost of right-of-way, etc. [38]-[42].
Fig. l.b shows the power capacity versus distances for both AC and DC systems.
1) Environmental1ssues:
[40]-[41]. This reduces the visual impact and saves land compensation for new projects. It is also possible to increase the power transmission capacity for
existing rights of way.
(a)
(b)
Fig. I. Comparison between AC and DC systems, (a) Cost comparison curves,
(b) Power capacity versus distances.
Tower structures of DC and AC overhead transmission lines are shown in Fig. 2.There are some environmental issues must be considered for the converter stations. These issues are focused in [43]-[45]. The use of ground or sea return paths in monopolar operation, electromagnetic compatibility, visual impact, and audible noise are explained in [46]-[48]. In [49],an overview of the engineering methods, tools, and design solutions is introduced. Verification methods used in HVDC converter stations design considering acoustic requirements are also explained.
IV. VOLTAGE STABILITY OF HVDC VERSUS HV AC
INTERCONNECTIONS
Long transmission lines are required to deliver the power to the major load centers or the nearest connection point of the existing transmission network. For long transmission of bulk power several technical and economic issues have to be considered before an optimal decision can be made. V oltage stability in general is one of the main technical issues to be considered [50]-[51]. Several methods, used to obtain the stability margin of a HVDC system, are well presented in the literature [51]-[59]. The most common voltage stability indices used for HVDC systems are maximum available power( MAP), critical effective short circuit ratio ( CESCR) and voltage stability factor (VSF).
stability equations are also derived taking into account load characteristics and system parameters. These methods are applied in [53] to determine the most unfavorable load characteristics with respect to degrading power/voltage stability margins. This is done by analyzing the impact of load characteristic on maximum power instability( dP/dI) and MAP of the HYDC system. The Short Circuit Ratio ( SCR) or CESCR are also considered as stability factors for an HYDC system, but only appropriate to evaluate the impacts of AC system on the stability margin of HVDC [60].
Authors in [59] introduced a new index ( dQt/eig_min) for voltage stability analysis of AC/DC systems. This index is used to classify the system into soft and non-soft modal systems. The latter is defmed as the system with constant dQt/eig_ min for all the SCRs and vice versa for the former.
This index also serves as a basis to decide the type of reactive power compensation and HVDC control strategy.
DCtower：
Fig.2. Typical transmission line structures for approximately 1000 MW.
While the above mentioned indices can be used to compare voltage stability margins between HVDC systems, they are not applicable for HV AC and HVDC comparison. In [56], authors extend the conventional point of collapse (PoC) method developed for AC systems to detennination of saddle-node bifurcation in systems including HVDC links. In [57], a comparison of the performance of the PoC and continuation methods for large AC/DC systems is presented. The proposed continuation method is applied in the two free software packages for stability studies; (UWPflow) and ( PSAT) [61][63].
A nonlinear programming approach for estimating the voltage stability in AC/DC systems based on the above mentioned algorithms is presented in [59] where PoCs are found by solving an optimization problem for several test systems. However, more in-depth analytical explanation is
required, and control issues of HVDC systems need to be considered. Inappropriate control schemes of firing, extinction,and overlap angles results in commutation failure or singularity in the Jacobian matrix. Therefore, PoC based on this method is not reliable to be used in the comparison of voltage stability of HVDC and HV AC systems. The dVac/dq factor at a particular bus is a commonly used voltage stability index in both AC and DC systems [51], [54]-[55]. However, it has never been used for comparison purposes between HV AC and HVDC systems.
V. ADV ANTAGES AND DISADV ANTAGES OF HVDC
Although the rationale for selection of HVDC is often economic, there may be other reasons for its selection. In many cases more AC lines are needed to deliver the same power over the same distance due to system stability limitations.Furthermore, the long distance AC lines usually require
long cable circuits, bypass network congestion, share utility rights-of-way without degradation of reliability, and mitigate environmental concerns. In all of these applications, HVDC nicely complements the AC transmission system. The following points highlight different advantages and disadvantages of the HVDC systems [29].
A. Advantages
1) Greater power per conductor.
2) Simpler line construction and smaller transmission towers.
3) A bipolar HVDC line uses only two insulated sets of conductors, rather than three.
4) Narrower right-of-way.
5) Require only one-third the insulated sets of conductors as a double circuit AC line.
6) Approximate savings of 30% in line construction.
7) Ground return can be used.
8) Each conductor can be operated as an independent circuit.
9) No charging current at steady state.
10) No Skin effect.
11) Lower line losses.
12) Line power factor is always unity.
13) Line does not require reactive compensation.
14) Synchronous operation is not required.
15) Distances are not limited by stability.
16) May interconnect AC systems of different frequencies.
17) Low short-circuit current on D.C line.
18) Does not contribute to short-circuit current of an AC system.
19) Controllability allows the HVDC to 'leap-frog' multiple 'choke-points' .
20) No physical restriction limiting the distance or power level for HVDC underground or submarine cables
21) Can be used on shared ROW with other utilities
22) Considerable savings in installed cable and losses costs for underground or submarine cable systems [29].
B. Disadvantages
1) Converters are expensive.
2) Converters require much reactive power.
3) Multi-terminal or network operation is not easy.
4) Converters generate harmonics and hence, require filters.
5) Break-even distance is influenced by the costs of right-of-way and line construction with a typical value of 500 km[38]-[40].
VI. ApPLICATIONS OF HVDC TRANSMISSION SYSTEMS
HVDC has gradually become a mature technology for AC system interconnection since the commissioning of the first commercial project between Mainland Sweden to Gotland island in 1954 [30]. The applications of HVDC technology are justified by some special conditions where HVDC is the most feasible or may be the only solution. Such applications include bulk power transmission over long distances, sub-marine cable transmission, and asynchronous systems inter-connection [64].HVDC transmission applications can be broken down to the following different basic categories [29], [37] AND [64].
A. Long Distance Bulk Power Transmission
As shown above, HVDC transmission systems often provide a more economical alternative to AC
HVDC than with AC transmission.
B. Cable Transmission
Unlike the case for AC cables, there is no physical restriction limiting the distance or power level for HVDC underground or submarine cables. Underground cables can be used on shared ROW with other utilities, without impacting reliability concerns over use of common corridors. Saving
advantages of underground and submarine cable systems 'have been shown previously, knowing that depending on the power level to be transmitted; these savings can offset the higher converter station costs at distances of 40 km or more.
On the other hand, for AC transmission over a distance,there is a drop-off in cable capacity due to its reactive component of charging current, since cables have higher capacitances and lower inductances than AC overhead lines.Although this can be compensated by intermediate shunt compensation for underground cables at increased expense,it is not practical to do so for submarine cables [65]-[66].
C. Asynchronous Ties
With HVDC transmission systems, interconnections can be made between asynchronous networks for more economic or reliable system operation. The asynchronous interconnection allows interconnections of mutual benefit while providing a buffer between the two systems. Often these interconnections use back-to-back converters with no transmission line [67].
Asynchronous HVDC links effectively act against propagation of cascading outages in one network from passing to another network.
Higher power transfers can be achieved, with improved voltage stability in weak system applications, using capacitor commutated converters. The dynamic voltage support and improved voltage stability offered by voltage source converter( VSC) based converters permits even higher power transfers without as much need for AC system reinforcement. VSC converters do not suffer commutation failures, allowing fast recoveries from nearby AC faults. Economic power schedules
which reverse power direction can be made without any restrictions since there is no minimum power or current restrictions [68].
D. Offihore Transmission
Self-commutation, dynamic voltage control, and black-start capability allow compact VSC HVDC transmission to serve isolated and orphaned loads on islands, or offshore drilling and production platforms over long distance submarine cables.This capability can eliminate the need for running uneconomic or expensive local generation or provide an outlet for offshore generation such as that from wind.
The VSC converters can operate at variable frequency to more efficiently drive large compressor or pumping loads using high voltage motors. Large remote wind generation arrays require a collector system, reactive power support, and outlet transmission. Transmission for wind generation must often traverse scenic or environmentally sensitive areas or bodies of water. Many of the better wind sites with higher capacity factors are located offshore. VSC based HVDC transmission not only allows efficient use of long distance land or submarine cables but also provides reactive support to the wind generation complex and interconnection point
[29].
E. Power Delivery to Large Urban Areas
Power supply for large cities depends on local generation and power import capability. Local generation is often older and less efficient than newer units located remotely. Air quality regulations may limit the availability of these older units. New transmission into large cities is difficult to site due to right-of-way limitations and land use constraints. Compact VSC-based underground transmission circuits can be placed on
supplying voltage regulation and dynamic reactive power reserve. Stations are compact and housed mainly indoors making siting in urban areas somewhat easier. Furthermore,the dynamic voltage support offered by the VSC can often increase the capability of the adjacent AC transmission [29].
These applications can be summarized as follows:
I) Power transmission of bulk energy through long distance overhead lines.
2) Power transmission of bulk energy through sea cables.
3) Fast and precise control of energy flow over back-to-back HVDC links, creating a positive damping of electromechanical oscillations, and enhancing the network stability, by modulating the transmitted power.
4) Linking two AC systems with different frequencies using asynchronous back-to-back HVDC links, which have no constraints with respect to systems' frequencies or phase angles.
5) Multi-terminal HVDC links are used to offer necessary strategically and political connections in the traversed areas of the potential partners, when power is to be transmitted from remote generation locations, across different countries, or different areas within one country.
6) Link renewable energy sources, such as hydroelectric,mine-mouth, solar, wind farms, or hot-rock geothermal power, when are located far away from the consumers.
7) Pulse-Width Modulation ( PWM) can be used for the VSC based HVDC technology as opposed to the thyristor based conventional HVDC. This technology is well suited for wind power connection to the grid.
8) Connecting two AC systems without increasing the short circuit power, that the reactive power does not get transmitted over a DC links. This technique is useful in generator connections, various applications of an HVDC system shown in Fig. 3.
Fig. 3. Various applications of HVDC systems.
VII. DIFFERENT HVDC SCHEMES [69]-[73]
A. Back-To-Back Converters
The "Back-to-back" indicates that the rectifier and inverter are located in the same station. Back-to-back converters are mainly used for power transmission between adjacent AC grids which cannot be synchronized. They can also be used within a meshed grid in order to achieve a defined power flow.
constraints prevent the use of electrodes. In such cases, a metallic return path is used in spite of increased cost and losses.
C. Bipolar Long-Distance Transmissions
A bipolar is a combination of two independent poles in such a way that a common low voltage return path, if available, will only carry a small unbalance current during normal operation. This configuration is used if the required transmission capacity exceeds that of a single pole. It is also used if requirement to higher energy availability or lower load rejection power makes it necessary to split the capacity on two poles. During maintenance or outages of one pole, it is still possible to transmit part of the power. More than 50% of the transmission capacity can be utilized, limited by the actual overload capacity of the remaining pole, while require only one-third the insulated sets of conductors compared to a double-circuit AC line.
Other advantages of a bipolar solution over a solution with two monopoles are reduced cost, due to one common or no return path, and lower losses. In [74]-[76] the bipolar HVDC system configuration has been modeled. The reliability models in these three papers are similar to each other but the objectives in the papers differ.
1) Bipolar With Ground Return Path:
This is a commonly used configuration for a bipolar transmission system. The solution provides a high degree of flexibility with respect to operation with reduced capacity during contingencies or maintenance, upon a single-pole fault,the current of the sound pole will be taken over by the ground return path and the faulty pole will be isolated. Following a pole outage caused by the converter, the current can be commutated from the ground return path into a metallic return path provided by the HVDC conductor of the faulty pole.
2) Bipolar With Dedicated Metallic Return Path For
Monopolar Operation:
If there are restrictions even to temporary use of electrodes,or if the transmission distance is relatively short, a dedicated LVDC metallic return conductor can be considered as an alternative to a ground return path with electrodes.
3) Bipolar Without Dedicated Return Path For Monopolar
Operation:A scheme without electrodes or a dedicated metallic return path for monopolar operation will give the lowest initial cost;Monopolar operation is possible by means of bypass switches during a converter pole outage, but not during an HVDC conductor outage.
4 shows different HVDC Schemes [29].
D. Multi-terminal HVDC System
In this configuration, there are more than two sets of converters. A multi-terminal CSC-HVDC system with 12-pulse converters per pole is shown in Fig. 5. In this case,converters 1 and 3 can operate as rectifiers, while converter 2 operates as an inverter. Working in the other order, converter 2can operate as a rectifier and converters 1 and 3 as inverters.By mechanically switching the connections of a given converter, other combinations can be achieved [77].
Fig. 5. Multi-terminal CSC-HVDC system- parallel connection.
VIII. CONCLUSION
Comparative evaluations, studies, applications, different schemes, and review of HVDC versus HV AC transmission systems, are presented in this part of the two-parts paper.