三峡大学科技学院
毕业设计（论文）
译文
译文题目 高压直流输电系统发展论述
英文题目
学生姓名： 学号：
专业： 电气工程及其自动化 班级：
指导教师： 王彦海
评阅教师： 王彦海
完成日期 二○一五 年 十二 月 二十七 日
摘要：高压直流（HVDC ）技术和概念的发展里程碑是在20世纪50年代。由于采用了高功率晶闸管开关（1960-70s ），直流输电技术在20世纪80年代达到了一个显著的成熟度。经典的HVDC 使用基于晶闸管的电流的整流转换器（LCC ）技术。功率的半导体开关出现在1980-90s ，带转向通断能力尤其是IGBT 和IGCT 的，并在正在进行的进展此领域中，介绍了传统的（二级）电压源转换器（VSC ）技术和其各种构造，多层次，多模块VSCS ，也作为可行转换器技术电力系统的应用程序。
直流系统由于其潜力重新出现，由于其潜在要么直接处理，或便于解决了大量的现有的和预期的互联交流电力系统的稳态和动态的问题。高压直流输电技术得以实现长距离传输大容量电力。HVDC 技术是长距离传输成为可能。比较评估，研究和审查直流输电与高压交流输电系统。介绍，应用，高压直流系统的不同的方案的概述。
关键词：直流转换器，直流输电换流技术，层次水平，直流输电系统组成，高压直流输电方案，特高压直流输电。
一．介绍
世界上第一台发电机是直流电发电机而导致第一个输电线路也是直流。尽管当时直流电至高无上，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1880-1890年代的普及。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。
变压器是一个简单的机械装置并被广泛被用来改变电压等级，输电，配电，以及电平下降。磁感应电动机是电产业的初始并且仅与交流电一起使用。这就是为什么交流电在商业上与国内负荷上非常有用的原因。在长距离传输上直流电在经济性、技术性和环境上比交流电更有优势。一般情况下，高压直流（HVDC ）传输系统的优势可以分为成本、灵活性和操作要求的基础三个方面。
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。这些类型的连接是用的两种不同的交流输电系统之间的相互关系。
复返链接是连接两个换流站，由单一导体线和大地或海洋用作返回路径。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。
二．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。可靠性的定义是一个组件或系统，以规定的条件下在既定期间内执行所需的功能的能力。电气系统的可靠性评估是为了确定投资，维护计划和作业是否进行以及何时进行。
电力系统可靠性通常是由系统的充足性和安全性两方面的功能划分。电力系统在任何时候要提供给客户的电力需求，并要考虑到系统部件的定期和不定期的断电的能力。安全性是指电力系统承受突然干扰，如电短路或系统组件的非预期损失的能力。
一个要包括整个电力系统复杂性的可靠性模型不可能实现。分析来说过于复杂，并且结果很难解释。如果将单独的系统分成三个阶段(HL)：发电(HL1)，发电和输电(HL2)，配电(HL3)。每一阶段就可以单独的建模和评估。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，然后列入第二阶段评估系统整体可靠性的影响。直流输电系统的可靠性评估是一个非常重要的指导系统以及模型。
电气与电子工程师协会的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①帮助改善电站服务的可靠性、可用性、可维护性②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③减少损耗④减少多余设计⑤提升高压直流
输电系统整流器的规格。 从另一方面讲，高压直流输电环节的可控性提供了坚实的传输容量。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, 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，并进行传输容量。
三．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，直流输电是通过紧接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，交流电是非常熟悉工业和家用负载的，但它在长距离传输中有一定的局限性。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，但是直流输电作为一个新的传输方法是解决这些问题和其他问题的新方法，它是在几个项目被使用。
例如，开关操作中，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，而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，而直流输电是正常电压的1.7倍。此外，特高压直流输电相比高压交流输电线路具有较小的电晕。
A. 传输损耗比较
所有交流或直流的输电和送电线费用通常包括主要以下部分，例如在塔的设施期间，相当数量地区也许被风景占领，架设指挥塔，绝缘体，终端设备的费用。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，必须给予直流线路尽可能多的权力且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。
1）经济因素
对于一项特定传输任务，在最后决定实施被执行高压交流输电系统或高压直流输电系统前都要进行可行性研究。每当长距离传输时所讨论的，“收支平衡距离”的概念就产生了。这就是直流线路成本的节约，距离越长抵消换流站的成本就越高。
2）环境问题
直流输电系统基本上与环境友好，因为利用现有的发电厂是改进能源传输更一个更高效的方式。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。
四. 直流输电的优点和高压直流输电的后发劣势
虽然选择直流输电的理由通常是经济但可能还有其他原因。 在许多情况下，同一距离下由于系统稳定的局限性需要更多的交流线路提供相同的权力。此外，长途线路通常需要中间交换站和无功补偿器，这增加了交流输电变电站成本。 直流输电可能是互连两个异步网络唯一可行的方式。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) 在线路施工节省大约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) 线路不需要无功补偿。
14) 同步操作不是必需的。
15) 互连不同频率的交流系统。
16) 不会产生交流系统的短路电流。
17) 其可控性允许直流“超越”多“薄弱点”。
18) 直流地下或海底电缆没有物理限制限制距离或功率电平
19) 可用于共享行与其他实用程序
20) 直流地下或海底电缆大大节省安装电缆和损失成本
B.缺点
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）转换器产生谐波，所以需要过滤器。
5) 盈亏平衡距离影响通行权的成本和线路建设。
五. 高压直流输电系统的应用
A. 远距离大容量输电
高压直流输电系统通常提供了更经济的方式而替代交流输电，一般用在在长的距离，大容量电力输送的清洁远程资源，如水电开发，坑口电厂，太阳能，大型风力发电场，或大热岩地热产生的高电能。传输与使用较少的高压直流输电线路比交流输电更合适。
B. 电缆传输
不像在A 线缆的情况下，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，没有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，交流输电在有缆绳容量的情况下由于当前的费用有它易反应的组分，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补偿对地下缆绳以增加的费用。
C.异步关系
随着高压直流输电系统，互连异步网络之间可以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情况下互连，同时提供了两个系统之间的缓冲区。通常，这些互连使用到后端转换器没有传输线。 异步直流环节在一个网络中断传播中有效地采取行动从而传递到另一个网络级。 这让更高的功率传输是可以实现的，并在弱电系统的应用中采用电容整流转换器提高了电压稳定性。有了动态电压支撑和改善电压稳定性，而不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。因为没有最小功率或电流限制其反向功率方向可不受任何限制。
D.离岸的传送
自励式，动态电压控制，以及启动能力，允许VSC 转换器和孤立的岛屿上负载，或海上钻井和生产平台的长途海底电缆隔离。 VSC转换器可以在变量频率下更有效地推动大型压缩机或泵使用高压电机负载。大型远程风力发电阵列需要收集器系统中，无功功率支持的渠道传播，其传播对于风力发电必须经常穿越风景或环境敏感地区的水域。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。
E. 对大市区的功率传输
大城市的电源取决于地方一代的力量进口能力。若当地一代比较陈旧其效率就不及位于远程的新单位。空气质量法规可能限制这些老单位的可用性。由于通行权的限制和土地使用的限制新的传输在大城
市很难完成。协定基于VSC 的地下传输电路可以被安置带来现有的两用优先权，以及提供电压支持允许了更加经济的电源和不用妥协的可靠性。接收终端像给予力量，提供电压规则和动力的虚拟的发电机一样成为有反应的电力储备。电站结构紧凑，主要选址在市区并安置在室内在进行下比较容易。此外， VSC提供的动态电压支持会媲美交流输电的输电能力。
这些应用可以被总结如下：
1) 通过长途架空线传输进行大能量输电。
2) 通过海底电缆传输大部分能量。
3) 在背靠背直流链接下快速和精确地控制能量流，创建一个积极的机电振荡阻尼，通过调节发射功率提高网络的稳定性。
4) 使用异步背靠背直流链接连接两个不同频率的交流系统，没有对系统频率或相位角度的约束。
5) 多端直流链接为用于为广大地区提供必要的战略和政治关系的潜在合作伙伴。
6) 当消费者很远时为其提供可新的能源，例如水力发电，矿嘴、太阳，风力场或者热石地热能。
7) 脉冲宽度调制可用于基于晶闸管常规高压直流VSC 的HVDC 技术。这种技术非常适用于风电连接到电网。
8) 在不增加短路功率，无功功率没有的情况下连接两个交流系统到直流链路传输。
原文：
High Voltage Direct Current Transmission
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 andIGCTs, 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 DCsystem 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.
Index Terms： HVDC converters, HVDC converter technologies, Hierarchal Level, HVDC system components,HVDC schemes, HVDC transmission.
I. INTRODUCTION
The first electric generator was the direct current ( DC)generator, and hence, the first electric power
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 .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 .
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 .
II. RELIABILITY AND CONTROLLABILITY EVALUATIONS OF T RANSMISSION 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 . Reliability assessments of electrical systems are performed in order to deter
mine 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 .
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 . 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 . 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) Improving RAM for stations in service, ii) Calculating and comparing RAM for different HVDC designs,iii) Reducing costs, iv) Reducing spare parts, and v) Improving HVDC converter specifications , several researches have been published covering the area of assessing the reliability of the HVDC 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 .
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. 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 . 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 , 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 condu
ctors 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.
2) Environmental1ssues:
A HVDC transmission system is basically environment friendly, because the improved energy transmission possibilities contribute to a more efficient utilization of existing power plants. The land coverage and the associated right-of-way cost for a HYDC overhead transmission line is not as high as that of an AC line. 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.
IV. ADVANTAGES AND DISADVANTAGES 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
intermediate switching stations and reactive power compensation. This can increase the substation costs for AC transmission to the point where it is comparable to that for HVDC transmission [29].
HVDC may be the only feasible way to interconnect two asynchronous networks, reduce fault currents, utilize
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) May interconnect AC systems of different frequencies.
16) Controllability allows the HVDC to 'leap-frog' multiple 'choke-points' .
17) Low short-circuit current on D.C line.
18) No physical restriction limiting the distance or power level for HVDC underground or submarine cables
19) Can be used on shared ROW with other utilities
20) Considerable savings in installed cable and losses costs for underground or submarine cable systems
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. .
V. APPLICATIONS OF HVDC TRANSMISSION SYSTEMS
A. Long Distance Bulk Power Transmission
As shown above, HVDC transmission systems often provide a more economical alternative to AC transmission for exploiting the high electrical power generated at long-distances and bulk-power delivery from clean remote resources, such as;hydroelectric developments, mine-mouth power plants, solar,large-scale wind farms ,or major hot-rock geothermal energy.This ransmission is established using fewer lines with 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 HVD
C 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.
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 . 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 .
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. Tran
smission 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 .
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 existing dual-use rights-of-way to bring in power, as well as to provide voltage support allowing a more economical power supply without compromising reliability. The receiving terminal acts like a virtual generator delivering power and 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 .
These applications can be summarized as follows:
1) 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.
三峡大学科技学院
毕业设计（论文）
译文
译文题目 高压直流输电系统发展论述
英文题目
学生姓名： 学号：
专业： 电气工程及其自动化 班级：
指导教师： 王彦海
评阅教师： 王彦海
完成日期 二○一五 年 十二 月 二十七 日
摘要：高压直流（HVDC ）技术和概念的发展里程碑是在20世纪50年代。由于采用了高功率晶闸管开关（1960-70s ），直流输电技术在20世纪80年代达到了一个显著的成熟度。经典的HVDC 使用基于晶闸管的电流的整流转换器（LCC ）技术。功率的半导体开关出现在1980-90s ，带转向通断能力尤其是IGBT 和IGCT 的，并在正在进行的进展此领域中，介绍了传统的（二级）电压源转换器（VSC ）技术和其各种构造，多层次，多模块VSCS ，也作为可行转换器技术电力系统的应用程序。
直流系统由于其潜力重新出现，由于其潜在要么直接处理，或便于解决了大量的现有的和预期的互联交流电力系统的稳态和动态的问题。高压直流输电技术得以实现长距离传输大容量电力。HVDC 技术是长距离传输成为可能。比较评估，研究和审查直流输电与高压交流输电系统。介绍，应用，高压直流系统的不同的方案的概述。
关键词：直流转换器，直流输电换流技术，层次水平，直流输电系统组成，高压直流输电方案，特高压直流输电。
一．介绍
世界上第一台发电机是直流电发电机而导致第一个输电线路也是直流。尽管当时直流电至高无上，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1880-1890年代的普及。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。
变压器是一个简单的机械装置并被广泛被用来改变电压等级，输电，配电，以及电平下降。磁感应电动机是电产业的初始并且仅与交流电一起使用。这就是为什么交流电在商业上与国内负荷上非常有用的原因。在长距离传输上直流电在经济性、技术性和环境上比交流电更有优势。一般情况下，高压直流（HVDC ）传输系统的优势可以分为成本、灵活性和操作要求的基础三个方面。
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。这些类型的连接是用的两种不同的交流输电系统之间的相互关系。
复返链接是连接两个换流站，由单一导体线和大地或海洋用作返回路径。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。
二．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。可靠性的定义是一个组件或系统，以规定的条件下在既定期间内执行所需的功能的能力。电气系统的可靠性评估是为了确定投资，维护计划和作业是否进行以及何时进行。
电力系统可靠性通常是由系统的充足性和安全性两方面的功能划分。电力系统在任何时候要提供给客户的电力需求，并要考虑到系统部件的定期和不定期的断电的能力。安全性是指电力系统承受突然干扰，如电短路或系统组件的非预期损失的能力。
一个要包括整个电力系统复杂性的可靠性模型不可能实现。分析来说过于复杂，并且结果很难解释。如果将单独的系统分成三个阶段(HL)：发电(HL1)，发电和输电(HL2)，配电(HL3)。每一阶段就可以单独的建模和评估。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，然后列入第二阶段评估系统整体可靠性的影响。直流输电系统的可靠性评估是一个非常重要的指导系统以及模型。
电气与电子工程师协会的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①帮助改善电站服务的可靠性、可用性、可维护性②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③减少损耗④减少多余设计⑤提升高压直流
输电系统整流器的规格。 从另一方面讲，高压直流输电环节的可控性提供了坚实的传输容量。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, 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，并进行传输容量。
三．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，直流输电是通过紧接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，交流电是非常熟悉工业和家用负载的，但它在长距离传输中有一定的局限性。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，但是直流输电作为一个新的传输方法是解决这些问题和其他问题的新方法，它是在几个项目被使用。
例如，开关操作中，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，而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，而直流输电是正常电压的1.7倍。此外，特高压直流输电相比高压交流输电线路具有较小的电晕。
A. 传输损耗比较
所有交流或直流的输电和送电线费用通常包括主要以下部分，例如在塔的设施期间，相当数量地区也许被风景占领，架设指挥塔，绝缘体，终端设备的费用。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，必须给予直流线路尽可能多的权力且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。
1）经济因素
对于一项特定传输任务，在最后决定实施被执行高压交流输电系统或高压直流输电系统前都要进行可行性研究。每当长距离传输时所讨论的，“收支平衡距离”的概念就产生了。这就是直流线路成本的节约，距离越长抵消换流站的成本就越高。
2）环境问题
直流输电系统基本上与环境友好，因为利用现有的发电厂是改进能源传输更一个更高效的方式。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。
四. 直流输电的优点和高压直流输电的后发劣势
虽然选择直流输电的理由通常是经济但可能还有其他原因。 在许多情况下，同一距离下由于系统稳定的局限性需要更多的交流线路提供相同的权力。此外，长途线路通常需要中间交换站和无功补偿器，这增加了交流输电变电站成本。 直流输电可能是互连两个异步网络唯一可行的方式。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) 在线路施工节省大约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) 线路不需要无功补偿。
14) 同步操作不是必需的。
15) 互连不同频率的交流系统。
16) 不会产生交流系统的短路电流。
17) 其可控性允许直流“超越”多“薄弱点”。
18) 直流地下或海底电缆没有物理限制限制距离或功率电平
19) 可用于共享行与其他实用程序
20) 直流地下或海底电缆大大节省安装电缆和损失成本
B.缺点
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）转换器产生谐波，所以需要过滤器。
5) 盈亏平衡距离影响通行权的成本和线路建设。
五. 高压直流输电系统的应用
A. 远距离大容量输电
高压直流输电系统通常提供了更经济的方式而替代交流输电，一般用在在长的距离，大容量电力输送的清洁远程资源，如水电开发，坑口电厂，太阳能，大型风力发电场，或大热岩地热产生的高电能。传输与使用较少的高压直流输电线路比交流输电更合适。
B. 电缆传输
不像在A 线缆的情况下，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，没有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，交流输电在有缆绳容量的情况下由于当前的费用有它易反应的组分，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补偿对地下缆绳以增加的费用。
C.异步关系
随着高压直流输电系统，互连异步网络之间可以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情况下互连，同时提供了两个系统之间的缓冲区。通常，这些互连使用到后端转换器没有传输线。 异步直流环节在一个网络中断传播中有效地采取行动从而传递到另一个网络级。 这让更高的功率传输是可以实现的，并在弱电系统的应用中采用电容整流转换器提高了电压稳定性。有了动态电压支撑和改善电压稳定性，而不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。因为没有最小功率或电流限制其反向功率方向可不受任何限制。
D.离岸的传送
自励式，动态电压控制，以及启动能力，允许VSC 转换器和孤立的岛屿上负载，或海上钻井和生产平台的长途海底电缆隔离。 VSC转换器可以在变量频率下更有效地推动大型压缩机或泵使用高压电机负载。大型远程风力发电阵列需要收集器系统中，无功功率支持的渠道传播，其传播对于风力发电必须经常穿越风景或环境敏感地区的水域。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。
E. 对大市区的功率传输
大城市的电源取决于地方一代的力量进口能力。若当地一代比较陈旧其效率就不及位于远程的新单位。空气质量法规可能限制这些老单位的可用性。由于通行权的限制和土地使用的限制新的传输在大城
市很难完成。协定基于VSC 的地下传输电路可以被安置带来现有的两用优先权，以及提供电压支持允许了更加经济的电源和不用妥协的可靠性。接收终端像给予力量，提供电压规则和动力的虚拟的发电机一样成为有反应的电力储备。电站结构紧凑，主要选址在市区并安置在室内在进行下比较容易。此外， VSC提供的动态电压支持会媲美交流输电的输电能力。
这些应用可以被总结如下：
1) 通过长途架空线传输进行大能量输电。
2) 通过海底电缆传输大部分能量。
3) 在背靠背直流链接下快速和精确地控制能量流，创建一个积极的机电振荡阻尼，通过调节发射功率提高网络的稳定性。
4) 使用异步背靠背直流链接连接两个不同频率的交流系统，没有对系统频率或相位角度的约束。
5) 多端直流链接为用于为广大地区提供必要的战略和政治关系的潜在合作伙伴。
6) 当消费者很远时为其提供可新的能源，例如水力发电，矿嘴、太阳，风力场或者热石地热能。
7) 脉冲宽度调制可用于基于晶闸管常规高压直流VSC 的HVDC 技术。这种技术非常适用于风电连接到电网。
8) 在不增加短路功率，无功功率没有的情况下连接两个交流系统到直流链路传输。
原文：
High Voltage Direct Current Transmission
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 andIGCTs, 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 DCsystem 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.
Index Terms： HVDC converters, HVDC converter technologies, Hierarchal Level, HVDC system components,HVDC schemes, HVDC transmission.
I. INTRODUCTION
The first electric generator was the direct current ( DC)generator, and hence, the first electric power
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 .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 .
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 .
II. RELIABILITY AND CONTROLLABILITY EVALUATIONS OF T RANSMISSION 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 . Reliability assessments of electrical systems are performed in order to deter
mine 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 .
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 . 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 . 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) Improving RAM for stations in service, ii) Calculating and comparing RAM for different HVDC designs,iii) Reducing costs, iv) Reducing spare parts, and v) Improving HVDC converter specifications , several researches have been published covering the area of assessing the reliability of the HVDC 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 .
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. 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 . 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 , 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 condu
ctors 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.
2) Environmental1ssues:
A HVDC transmission system is basically environment friendly, because the improved energy transmission possibilities contribute to a more efficient utilization of existing power plants. The land coverage and the associated right-of-way cost for a HYDC overhead transmission line is not as high as that of an AC line. 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.
IV. ADVANTAGES AND DISADVANTAGES 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
intermediate switching stations and reactive power compensation. This can increase the substation costs for AC transmission to the point where it is comparable to that for HVDC transmission [29].
HVDC may be the only feasible way to interconnect two asynchronous networks, reduce fault currents, utilize
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) May interconnect AC systems of different frequencies.
16) Controllability allows the HVDC to 'leap-frog' multiple 'choke-points' .
17) Low short-circuit current on D.C line.
18) No physical restriction limiting the distance or power level for HVDC underground or submarine cables
19) Can be used on shared ROW with other utilities
20) Considerable savings in installed cable and losses costs for underground or submarine cable systems
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. .
V. APPLICATIONS OF HVDC TRANSMISSION SYSTEMS
A. Long Distance Bulk Power Transmission
As shown above, HVDC transmission systems often provide a more economical alternative to AC transmission for exploiting the high electrical power generated at long-distances and bulk-power delivery from clean remote resources, such as;hydroelectric developments, mine-mouth power plants, solar,large-scale wind farms ,or major hot-rock geothermal energy.This ransmission is established using fewer lines with 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 HVD
C 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.
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 . 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 .
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. Tran
smission 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 .
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 existing dual-use rights-of-way to bring in power, as well as to provide voltage support allowing a more economical power supply without compromising reliability. The receiving terminal acts like a virtual generator delivering power and 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 .
These applications can be summarized as follows:
1) 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.