风力发电中英文翻译
风力发电中英文翻译 Prepared on 24 November 2020
风力发电对电力系统的影响
简奥斯丁，费力克斯
（电力系统及发电设备控制和仿真国家重点实验室，纽约市曼哈顿区）
摘要
风力发电依赖于气象条件，并逐渐以大型风电场的形式并入电网，给电网带来各种影响。电网并未专门设计用来接入风电，因此如果要保持现有的电力供应标准，不可避免地需要进行一些相应的调整。讨论了在风电场并网时遇到的各种问题。由于风力发电具有大容量、动态和随机的特性，它给电力系统的有功/无功潮流、电压、系统稳定性、电能质量、短路容量、频率和保护等方面带来影响。针对这些问题提出了相应的解决建议和措施，以及更好利用风力发电。
关键词：风力发电；电力系统；影响；风电场
1.引言
人们普遍接受，可再生能源发电是未来电力的供应。由于电力需求快速增长，对以化石燃料为基础的发电是不可持续的。正相反，风力发电作为一种有前途的可再生能源受到了很多关注。当由于工业的发展和在世界大部分地区的经济增长而发电的消费需求一直稳步增长时，它有减少排放和降低不可替代的燃料储备消耗的潜力。
当大型风电场（几百兆瓦）是一个主流时，风力发电越来越更受欢迎。2006年间，世界风能装机容量从2005年的59091兆瓦达到74223兆瓦。在2006年极大的生长表明，决策者开始重视的风能发展能够带来的好处。由于到2020年12%的供电来于1250GW的安装风电装机，将节约累积吨二氧化碳[1]。
大型风电场的电力系统具有很高的容量，动态随机性能，这将会挑战系统的安全性和可靠性。而提供电力系统清洁能源的同时，风农场也会带来一些对电力系统不利的因素。风力发电的扩展和风电在电力系统的比重增加，影响将很可能成为风力集成的技术性壁垒。因此，应该探讨其影响和提出克服这些问题的对策。
2.风力发电发展现状
从全球风能委员会（GWEC）的报告中，拥有最高装机容量总数的国家是德国（20621兆瓦），西班牙（11615兆瓦），美国（11603兆瓦），印度（6270兆瓦）和丹麦（3136兆瓦）。世界范围内十三个国家现在可以算是达到1000兆瓦的风力发电
能力，法国和加拿大在2006达到这一阈值。如图1所示，直到2006年12月世界累计装机容量前10名[2]。
图1 到2006年12月世界累计装机容量
中国开始发展风电很晚。只有在90年代它才走向市场化的发展和规模建设。这些年新增累积装机容量如图2显示。单一机组容量从100千瓦，200千瓦，300千瓦600千瓦，750千瓦，1500千瓦逐步增加。
图2 在中国累计和新增加安装的风力发电能力
在2006年中国通过安装风能的1347兆瓦，增加了一倍以上的总量容量，比去年的数值增长了70%。这给中国带来多达2604兆瓦的能力，使中国成为世界第六个最大的市场。中国市场在2006年大幅增长，这预计将继续增长并加快增长。根据经批准的和在建设中的项目，在2007年将安装超过1500兆瓦。到2010年底在中国的风电目标为5000兆瓦[3]。
3.风力发电项目的特点
从风能的角度来看，风能资源的最显特点是其变化性。风电场输出的随机变化主要根源于风速的波动和方向。无论是地理性和时间性，风是很易变的。此外，无论是在空间和时间上，这种变化性持续的范围非常广泛。
图3.布鲁克海文国家实验室工作的基础上的农场风谱图由于时间和高度的功能，风速不断变化。风变化的时间尺度显示在图3的风力频谱图上[4]。在一秒到分钟的范围阵风引起动荡的高峰。每日的峰值取决于每天的风速变化和天气高峰取决于天气变化，通常因每天或每周而异，但也包括季节性周期。
从电力系统的角度来看，湍流高峰可能会影响风力发电的电能质量。然而，昼夜和天气的高峰，可能会影响长期的电力系统的平衡，在这样的系统中风速预测起着显着作用。
另一个重要问题是风能资源的长期变化。应知道加速到中心高度的风来计算风电场的输出。大量风速测量表明，风速在一年中大多数是柔和的，介于0和25米/秒的概率是相当大的；年均风速受制于威布尔分布[5]，如公式（1）。
f(x)=k
c (v
c
)
k?1
exp[?(v
c
)
k
] (1)
其中：V是平均风速；k为形状参数；c是尺度参数。
风力发电机的输出之间的关系PW和风速集线器V的高度可以近似表示为风力发电机的输出与风速或分段函数的曲线，如公式（2）。
P W={
P R
V R3?V CI3
P R
v3?V CI3
V R3?V CI3
P R
(V≤V CI or V≥V CO)
( V CI≤V≤V R )
( V≥V R )
(2)
其中：PW是额定功率的风力发电机组的输出；V是风速达枢纽的高度VCI是停机风速；VCO被切出风速；VR被评为风速。
4.风力发电对电力系统的影响
在电力系统中风力发电面临大型风电场对电网一体化的基本技术限制。风力发电对电力系统的影响包括有效功和无效功，电压，系统稳定性，电能质量，短路容量和基础设施的特点由于高容量的风力发电的动态和随机性能。在技术上，它通过以下方式影响和必须详细研究：
（1）有功和无功流
风力发电是一个间歇性和随机的电源，将功率流复杂化。由于为了捕获更多的风能能源，许多风电场建成远离负荷中心，总有传输风力发电一些的障碍。当引进额外的风力发电时一些传输或配电线路和其他电气设备可能过载。因此，应确保互相连接传输或配电线路不过载。有功和无功要求，都应予以调查。无功功率应不仅在PCC中产生，但也通过整个网络产生，并应本地补偿[6]。
用于常规发电机的分析的方法是确定的，而忽略了不确定性的风速和负荷预测。因此，概率性的方法是比较适合风力发电的。约束以概率形式描述，并且预期参数值，如电压和功率，可以被计算。
（2）电压调节
一旦风电场已经确定了其地点，连接到电网的点必须确定。对于小型风力发电场，可以在低电压下连接，从而节省了开关设备、电缆和变压器的成本。如果拟议的发展规模太大导致不可以与当地分布电压的连接，进而不能满足较高的电压传输网络的需要[7]。
在电力系统中随着风力发电安装容量的增加，风力发电的变化引起电压变化，特别是如果并入电网，这可能不是专门设计用于迎合重要和可能快速变化的负载，这是由风力发电变化引起的。因此，需要采取监管措施，使电压保持在指定的范围内。然而，为了控制电压，这可能导致增加对无功功率的辅助服务[8]。
（3）系统的稳定性
在风力发电的电力系统中，电压稳定和频率的稳定性都受到风功率集成影响，这不仅是因为风力发电的加入将改变流量分布，也因为风力发电机与传统的同步机无论是在稳态或瞬态状态时相比表现不同[9]。
对于目前的风力发电场，当发生干扰时，保护操作通常是切断风电场之间的连接电网。因此，在这种时刻的暂态稳定是非常重要的，尤其是当大型风电场的有机结合时最为重要。然而，由于电网结构，风也可能使电源集成系统的瞬态稳定性差。因此，不同的电力系统，暂态稳定性应分别进行分析。
固定速度的风力涡轮机输出有功功率时，它吸收无功功率。“风电场无功功率的整体需求是相当大，从而导致减少在PCC附近地区的电压稳定。与此相反，双馈变速风力发电机组对无功功率有一定的控制能力。根据不同的操作和控制计划，这种风力发电机组可以吸收或输出无功功率控制电压，有利于电压稳定。电压稳定也与短路容量相关，传输的PCC行比R / X和在风力发电场使用的无功补偿方法有关。
（4）电能质量
风力发电的波动和相关电源（AC或DC）的传输、供电质量有直接的影响。结果，大量的电压波动，可能会导致电压在调控范围外变化，以及违反闪烁和其他电源的质量标准。在连续的运行和开关操作，风力发电机组，引起电压波动和闪烁，这些因素是风力发电影响电网电能质量的主要因素。对于变速风力涡轮机和恒定频率，转换器造成的谐波问题，也应考虑。
风力涡轮机对电网干扰有不同的原因，其中大多原因是风力机本体。有关参数列于表1[10]。平均发电量，湍流强度及风切变与气象和地理条件因素相关。所有其他的原因不仅归咎于电器元件的特点，如发电机，变压器等，也是转子和传动系统的空气动力学和机械性能的原因。涡轮形式（即变量与主要固定的速度档位与节距调节）对风力涡轮机和风力发电场的电能质量特性有重要性。
闪烁是由风力发电机组的有功功率或无功功率的的波动造成的。固定速度的风力发电机闪烁的主要原因是塔的尾流。而变速风力发电机，平滑了快速功率波动，塔的尾流不影响输出功率。因此，变速风力发电机组的闪烁一般比定速闪烁风力发电机低。
表1.风力发电机和风力发电厂对电网造成的影响
参数原因
电压升高电能生产
开关操作
塔影效应电压波动和闪烁叶片调节误
差
偏航误差
风切变
风速波动
谐波变频器
晶闸管控制器电压峰值和谷值开关操作（5）短路容量
往往是大多数的风力发电场远离负荷中心建造，这意味着他们之间和其他间的电力系统的电气之间的距离，是相当远的。有一常理说，长电距离，使电压变化较大，但短路问题少[11]。
然而，风力发电场将能够给未来的电力系统运行的短路电流计算带来越来越重要的影响。原因是双重的。一个是上述的事实，风力发电网站通常是远离的传统的电力中心。这意味着短路电流的分布可能产生了很大的变化，导致一个完全不同的短路容量地图。其他事实的原因是，今天，越来越多的风力发电，特别是以所谓的大型风力发电场（数百兆瓦）的形式。在风电场大量的个别单位连接在一起，总代能力将大大上升。
风电场对相邻节点短路能力有很大影响，然而对远离PCC节点的影响不大[9]。因此，当具有大容量的风场并入电网时，相邻变压器和交换机的容量可能需要增加。应该进一步研究的是：如何判断风力发电对现有网络上的电气设备短路电流额定值的影响。
（6）频率调整
为了在规定的标准范围内控制电力系统频率，要求一些发电厂向电网公司提供频率控制配套服务。然而，风力发电量总额的增加，其变化频率输出是一个很重要的影响[8]。
（7）保护
电流在风电场和电网之间的流动是双向的，这是在保护的设计和配置应予以考虑的。无论风力发电机采用何种发电机，风电场的整合将增加电网故障水平，进而影响原有的电网保护装置继电器的设置。这可能需要增加新的保护装置或修改原有保护设
备的继电器的设置。尤其是如果风电场连接到分配网络，断路器可能在风电场装机容量增加时产生超负荷[8]。
5.减轻风力发电的影响的对策
无功补偿设备的应用，如静止无功补偿（SVC）和静止同步补偿器（STATCOM）在风力发电中减轻其对电力系统的影响起着重要作用。为了保持电压等级，电网公司可以提供额外的或升级的电压控制设施。无功补偿设备应该安装在风电场升压变电站，这具有快速响应特性，并且可不断调节，如在SVC和STATCOM等。为了减少风力发电造成的电压波动和闪烁，既需要速度控制应加以改善，以便和俯仰角控制最大限度地减少了风力发电机的输出波动，而风力发电机的输出最大化。同时，如在风场安装辅助设备SVC和储能装置也可以减轻电压波动和闪烁。在大多数情况下，快速作用无功补偿设备，包括SVC和STATCOM，应被纳入为提高网络的暂态稳定的设备之中。
从风力发电方面，它可以通过不断的功率因数控制或恒压控制提高电力系统的电压稳定增加风力发电的渗透。从电网方面，这对加强和改变目前的网络具有重要意义。电压源换流器系统（VSC）为基础的高压直流输电（VSC-HVDC系统）是一个不需
要任何额外赔偿的传输系统，因为这是转换器的控制固有的[13]。因此，它将是一个很好的工具，它使风力发电成一个网络，即使在一个弱网络中，无需提高点短路比，也能实现。VSC-HVDC的有功功率控制能力，然后是一个完美的处理有源功率/频率控制
的工具。它有能力以一个很好的方式处理风电并足以快速反应抵消电压变化，它可以提高系统的稳定性和电能质量。
6.结论
距今25年，风能已经经过很长的时间，它很可能会在未来20年继续推进。有许多关于整合风力发电系统的运作和发展的问题。虽然风力发电取代了产生相当数量能量的传统植物，关注点都集中在了风力发电和电网之间的相互作用上。本文提供了一个概览风力发电对电力系统的影响和相应的对策建议来处理这些问题，为了适应风在电力系统的发电。
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附件2：外文原文
Influence Research of Wind Power Generation on Power Systems
Jane Austen，Kurt Felix
（State Key Lab of Control and Simulation of Power Systems and Generation Equipments，Manhattan District，New York，United States）
Abstract
Wind power generation is always weather dependent and has the trend of being integrated to power systems as the form of large-scale wind farms, which influences on power systems. Since the power network was not designed specifically to accommodate this type of generation, there are inevitably some points at which modifications must be executed if existing standards of electricity supply are to be maintained. This paper discusses in general terms the problems which are encountered by the developers of wind power generation projects and by utility grids when dealing with projects to integrate wind farms to power systems. The influence includes active and reactive power flow, voltage, system stability, power quality, short-circuit capacity, system reserve, frequency and protection due to the characteristics of high-capacity, dynamic and stochastic performance of wind power generation. Corresponding countermeasures to handle these issues are recommended in order to accommodate wind power generation in power systems.
Keywords: Wind power generation；Power system；Influence；Wind farms
1. Introduction
There is widespread acceptance that renewable generation is the future of electricity supply. Generation based on fossil fuels is not sustainable as power electricity is being consumed rapidly. On the contrary, wind power has attracted much attention as a promising renewable energy resource. It has potential benefits in curbing emissions and reducing the consumption of irreplaceable fuel reserves when the demand for power electricity has been steadily growing due to the industrial developments and the growth of the economy in most parts of the world.
Wind power generation is becoming more and more popular while the large-scale wind farm (hundreds of megawatts) is the mainstream one. During 2006, the world’s installed wind capacity reached 74 223 MW, up from 59 091 MW in 2005,which include wind energy developments in more than 70 countries around the world. The tremendous growth in 2006 shows that decision makers are starting to take seriously the benefits that wind energy development can bring.
There are no technical, economic or resource barriers to supplying 12% of the world’s electricity needs with wind power alone by 2020, and this against the challenging backdrop
of a projected two thirds increase of electricity demand by that date. The report is a crucial tool in the race to cut greenhouse gas emissions as 12% electricity from a total of 1 250 GW of wind power installed by 2020 will save a cumulative 10771 million tons of CO2[1].
Large-scale wind farms connected to power systems have characteristics of high capacity, dynamic and stochastic performance, which challenges system security and reliability. While providing the clean power for power systems, wind farms will also bring about some unfavorable influence on power systems. With the expansion of wind power generation and the increase of wind power ratio in a power system, the influence will likely become the technical barriers for wind power integration. Therefore, the influence should be discussed and the countermeasures to overcome these issues should be proposed.
According to the issues mentioned above, this paper discusses in general terms the problems which are encountered by the developers of wind power generation projects and by utility grids when dealing with projects to integrate wind farms to power systems. Due to the characteristics of high-capacity, dynamic and stochastic performance of wind power generation, the influence includes active and reactive power flow, voltage, system stability, power quality, short-circuit capacity, system reserve, frequency and protection. After that, corresponding countermeasures to handle these problems are recommended in order to accommodate wind power generation in power systems.
2. Development situation of wind power generation
From the report of the Global Wind Energy Council (GWEC), the countries with the highest total installed capacity are Germany (20 621 MW), Spain (11 615MW), the USA (11603MW), India(6270 MW) and Denmark (3 136 MW). Thirteen countries around the world can now be counted among those with over 1000 MW of wind capacity, with France and Canada reaching this threshold in 2006. shows the top 10 cumulative installed capacity of the world until December, 2006[2].
Fig. 1 Top 10 cumulative installed capacity of the world until December,2006 China started to develop wind power very late. It stepped into the stage of commercialized development and scale construction only in 1990s. Accumulated and newly added installed generating capacity over the years is shown in single-unit capacity increased from 100 kW, 200 kW, and 300 kW to 600 kW, 750 kW, and 1500 kW step by step.
China doubled more than its total installed capacity by installing 1 347 MW of wind energy in 2006, a 70% increase from last year’s figure. This brings China up to 2 604 MW of capacity, making it the sixth largest market world wide. the Chinese market has grown substantially in 2006, and this growth is expected to continue and speed up. According to the list of approved projects and those under construction, more than 1 500 MW will be installed in 2007. The goal for wind power in China by the end of 2010 is 5000 MW[3].
Fig. 2 Accumulative and newly-added installed capacity of wind power in China
3. Characteristics of wind power generation
From the point of view of wind energy, the most striking characteristic of the wind resource is its variability. The stochastic variation of wind farms outputs root mainly in fluctuation of the wind speeds and directions. The wind is highly variable, both geographically and temporally. Furthermore this variability persists over a very wide range of scales, both in space and time.
Fig. 3 Wind spectrum farm Brookhaven based on work by van der Hoven The wind speed varies continuously as a function of time and height. The time scales of wind variations are presented in as a wind frequency spectrum[4]. The turbulent peak is caused by gusts in the sub second to minute range. The diurnal peak depends on daily wind speed variations and the synoptic peak depends on changing weather patterns, which typically vary daily to weekly but include also seasonal cycles.
From a power system perspective, the turbulent peak may affect the power quality of wind power generation. The influence of turbulences on power quality depends very much on the turbine technology applied. Variable-speed wind turbines, for instance, may absorb short-term power variations by the immediate storage of energy in the rotating masses of wind turbine drive trains. That means that the power output is smoother than strongly grid-coupled turbines, fixed-speed wind turbines. Diurnal and synoptic peaks, however, may affect the long-term balancing of power system, in which wind speed forecasts plays a significant role.
Another important issue is the long-term variations of the wind resources. The wind speed up to the height of the hub should be known to calculate the wind farm output. A number of measurements of wind speeds show that wind speeds are mostly mild in a year; their probabilities between 0 and 25m/s are considerable; most of the average annual wind speeds subject to the Wei bull distribution[5], as in formula(1).
f(x)=k
c (v
c
)
k?1
exp[?(v
c
)
k
] (1)
Where: v is average wind speed; k is shape parameter; c is scale parameter.
The relationship between the wind turbine output Pw and the wind speed up to the height of the hub v can be expressed approximately as the curve of wind turbine’s outputs vs. wind speed or a subsection function, as in formula (2).
P W={
P R
V R3?V CI3
P R
v3?V CI3
V R3?V CI3
P R
(V≤V CI or V≥V CO)
( V CI≤V≤V R )
( V≥V R )
(2)
Where: P w is rated output of the wind turbine; v is wind speed up to the height of the hub; V CI is cut-in wind speed; V CO is cut-out wind speed; V R is rated wind speed.
4. Influence of wind power generation on power systems
High penetration of wind power in the power systems faces fundamental technical limits with regard to the integration of large-scale wind farms to the grid. The influence of wind power generation on power systems includes active and reactive power flow, voltage, system stability, power quality, short-circuit capacity, system reserve and infrastructure due to the characteristics of high-capacity, dynamic and stochastic performance of wind power generation. Technically, it influences the gird in the following ways and has to be studied in detail:
（1）Active and Reactive Power Flow
Wind power is a kind of intermittent and stochastic power source, which will complicate the power flow. Because many wind farms are built far away from load centers in order to capture more wind energy, there is always some obstacle of transmitting wind power. Some transmission or distribution lines and other electrical equipments may be over-loaded when the additional wind power generation is introduced. So it should be ensured that the interconnecting transmission or distribution lines will not be over-loaded. Both active and reactive power requirements should be investigated. Reactive power should be generated not only at PCC, but also throughout the network, and should be compensated locally[6].
The methods utilized for analysis of conventional generators are certain and ignore the uncertainty of wind speed and load forecasts. Therefore, the probabilistic method is more suitable for wind power generation. This model is based on the wind speed distribution, such
as formula (1). The constraints are described by probabilistic forms and the expected values of parameters, such as voltages and powers can be computed.
（2）Voltage Regulation
Once a wind farm has identified its site, the point at which connection to the grid must be identified. Small wind farm can connect at lower voltage, thereby saving on switchgear, cable and transformer costs. If the size of the proposed development is too large to be connected at the local distribution voltage, access to the transmission network at a higher voltage is required[7].
After failures, if the transient unstability does not occur in power systems, some wind turbines shut down due to their low voltage protections. Then outputs of wind farms decrease, which means that the power system lose reactive loads. Therefore the voltage levels climb up, even beyond the upper limits of wind farms buses.
Capacitors are the common reactive power compensation methods. When voltage levels dropdown, the amount of compensation decreases much. However the reactive power demands increase when the asynchronous machines are utilized in wind farms. So voltage levels drop down more, even beyond the lower limits of wind farms buses.
With the increase of wind power installed capacity in power systems, the variability of wind power generation causes variability of voltage level, particularly if integrated into the grid which might not be specifically designed to cater for the significant and possibly rapid load variations (compared with normal customer load variation) caused by highly variable wind power generation. Therefore, the regulatory measures are needed to maintain the voltage level in a specified range. However, the variability of wind power generation is not a low probability; this could result in an increased requirement for reactive power ancillary services to manage voltage control[8].
（3）System Stability
In the power system with high wind power penetration, the transient stability, voltage stability and frequency stability are all influenced by the wind power integration not only because the injection of wind power will change the power flow distribution, transferred power of each transmission line and total inertia of the whole power system, but also because the wind turbine generators perform differently in either steady-state or transient-state compared with the conventional synchronous machine[9].
For current operation of wind farms, protections usually cut off the connections between wind farms and the grid when great disturbances occur. This is equivalent to arouse new generators tripping disturbance after the great disturbances. So the transient stability in such moment is very crucial, especially when large-scale wind farms are integrated. Compared the variable-speed wind turbine based on the doubly-fed induction generator (DFIG) with the fixed-speed wind turbine based on the induction motor, the former is more robust after short-circuit failures and can strengthen system stability with keeping enough stability margin. However, wind power integration may also make the system transient stability worse due to the grid structure. Therefore, transient stabilities of different power systems should be analyzed respectively.
The fixed-speed wind turbine absorbs the reactive power when outputting the active power. The whole demand of a wind farm for the reactive power is considerable, which lead to the decrease of the voltage stability in the area near PCC. On the contrary, the variable-speed wind turbine based on DFIG has certain ability to control the reactive power. According to different operation and control schemes, this wind turbine can absorb or output the reactive power to control the voltage, which benefits the voltage stability. The voltage stability is also related with the short-circuit capacity of PCC, transmission line ratios of R/X and reactive compensation methods utilized of wind farms.
（4）Power Quality
Fluctuations in the wind power and the associated power transport (AC or DC), have direct consequences to the power supply quality. As a result, large voltage fluctuations may result in voltage variations outside the regulation limits, as well as violations on flicker and other power quality standards. During the continuous operation and switching operation, wind turbine causes voltage fluctuation and flicker, which are main concerns of unfavorable influence of wind power generation on power quality of the grid. For wind turbine of variable-speed and constant-frequency, harmonic issue caused by converters should also be considered.
Tab. 1 Crid interferences caused by wind turbines and wind farms The grid interferences of wind turbines or wind farms have different causes, which are mostly turbine-specific. The relevant parameters are listed in [10]. Average power production, turbulence intensity and wind shear refer to causes that are determined by meteorological and geographical conditions. All the other causes are attributed not only by the characteristics of
the electrical components, such as generators, transformers and so on, but also by the aerodynamic and mechanical behavior of the rotor and drive train. The turbine type . variable versus fixed speed stall versus pitch-regulated) is of major importance to the power quality characteristics of wind turbines and wind farms.
Flicker is caused by the fluctuation of active and/or reactive power of wind turbines. The main reason for flicker in fixed-speed wind turbines is the wake of the tower while for variable-speed wind turbines, fast power fluctuations are smoothed and the wake of the tower does not affect power output. Therefore, the flicker of variable-speed wind turbines is in general lower than the flicker of fixed-speed wind turbines. In wind farms, power fluctuations are smoothed because of the fact wind turbines are correlated.
（5）Short-Circuit Capacity.
The majority of the wind power farms tend to be constructed remote from the load center, which means that electrical distance between them and the other part of the power system is rather long. There is a common sense that long electrical distance makes voltage variation bigger but short circuit problem smaller[11].
However, wind power farms will be able to give more and more significant effects on the calculation of short circuit current in future power system operation. The reason is twofold. One is the above stated fact that wind power generation site is usually apart from the conventional electrical power center. It implies that the distribution of short circuit current might make a drastic change, leading to a completely different short circuit capacity map. The other reason is the fact that more and more wind power generation today are particularly in the form of so called large-scale wind farms (hundreds of megawatts). In wind farms a substantial number of individual units are connected together, and the total generation capacity will greatly rise up.
Wind farms have great influence on the short circuit capacities of adjacent nodes while the nodes far apart from PCC are little influenced[9]. Therefore, when wind farms with huge capacities are integrated into the grid, capacities of adjacent transformers and switches may need to be increased. It should be studied further how to determine the influence of wind power generation on the short circuit current ratings of existing electrical equipments on the network.
（6）System Reserve.
Meanwhile, system load forecast will potentially become less accurate as the amount of wind power generation increases, which in turn influence power system operational scheme and unit commitment. In the case of generation reserve forecasts, this could translate into higher reserve level requirements to cover uncertainties in the availability of wind power generation[8].
（7）Frequency Regulation.
In order to control power system frequency within defined standards, grid corporations require some power plants to provide frequency control ancillary services. However, as the total amount of wind power generation increases, variation in its output will have a more significant impact on the frequency[8].
（8）Protection.
As for any proposed expansion of a utility grid, there are problems to be addressed with each application for connection. For connection to the distribution system, the proposed generation must have a relatively low capacity. In general, that part of the network is radial and the protection would be graded to expect fault current to flow outwards from the hulk supply point. Introduction of a generator into the radial network means that fault current can now be supplied which does not flow in the expected direction. A detailed check of the protection settings with the proposed generator in the network model is therefore required. The results of this check may show that the protection will function quite adequately as it is. At the other extreme, it may show that the existing relays cannot protect the network while the new generation and a redesign of the protection is necessary[7].
The power flows between wind farms and the grid are bidirectional, which should be considered during the protections design and configuration. Whatever kind of generators are adopted in wind turbines, integration of a wind farm will increase the fault level of grid and furthermore affect the relay settings of original protection devices of grid. It is probably necessary to add new protection devices and/or modify the relay settings of original protection devices. Particularly if wind farms are connected into distribution networks, overloading of circuit breakers may occur with the increase of installed capacity of wind farm[8].
5. Countermeasures to mitigate wind power generation influence
The applications of the reactive compensation equipments, such as static var compensator (SVC) and static synchronous compensator (STATCOM) play an important role in wind power generation to mitigate its influence on the power system. In order to maintain the voltage level, the grid corporation may provide additional or upgraded voltage control facilities. Reactive compensation equipments should be installed in the step-up substation of wind farm, which has a fast response characteristic and can be regulated continuously, such as SVC and STATCOM, etc. In order to mitigate voltage fluctuation and flicker caused by wind power generation, both speed control and pitch angle control should be improved so as to minimize the fluctuation of wind turbine output while maximizing the output of wind turbine. Meanwhile, installation of auxiliary devices on wind farm such as SVC and energy storage device can also mitigate both voltage fluctuation and flicker. In most cases, fast-acting reactive-power compensation equipment, including SVC and STATCOM, should be included for improving the transient stability of the network.
From the wind power generation side, it can improve the voltage stability of the power system and increase wind power penetration by the constant power factor control or constant voltage control. From the grid side, it is of significance to reinforce and change the current network. Voltage source converter (VSC) based HVDC (VSC-HVDC) is a transmission system that does not require any additional compensation, as this is inherent in the control of the converters[13]. It will therefore be an excellent tool for bringing wind power into a network, even at weak points in a network and without having to improve the short-circuit ratio. The active power control capability of VSC-HVDC could then be a perfect tool for handling active power/frequency control. It is capable of handling wind power and of reacting rapidly enough to counteract voltage variations in an excellent way, which can improve the system stability and power quality.
6. Conclusion
Wind energy has come a long way since the prototypes of just 25 years ago, and it will probably continue to advance over the next twenty years. There are a number of issues associated with integration of wind power in system operation and development. Although penetration of wind power generation may displace significant amount of energy produced by conventional plants, concerns are focused on the interaction between wind power generation and the grid. This paper has provided an overview of the influence of wind power generation
on power systems and recommended corresponding countermeasures to handle these issues
in order to accommodate wind power generation in power systems.
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