Assessing transient response of DFIG based wind turbines during voltage dips regarding main flux saturation and rotor deep-bar effect

With increasing wind power penetration, transient responses of doubly-fed-induction-generator (DFIG) based wind turbines gain attentive focus. Accurate prediction of transient performance of DFIG under grid faults is required with increasing wind power penetration. Taking into account the main flux saturation and deep-bar effect, this paper concentrates on transient responses and stability of the DFIG system under symmetrical grid faults. Their roles played in the enhancement of system transient stability are clarified. The analyses proposed contribute greatly to proper selection, design and coordination of protection devices and control strategies as well as stability studies.     

How does market power affect the impact of large scale wind investment in 'energy only' wholesale electricity markets?

In the short run, it is well known that increasing wind penetration is likely to reduce spot market electricity prices due to the merit order effect. The long run effect is less clear because there will be a change in new capacity investment in response to the wind penetration. In this paper we examine the interaction between capacity investment, wind penetration and market power by first using a least-cost generation expansion model to simulate capacity investment with increasing amounts of wind generation, and then using a computer agent-based model to predict electricity prices in the presence of market power. We find the degree to which firms are able to exercise market power depends critically on the ratio of capacity to peak demand. For our preferred long run generation scenario we show market power increases for some periods as wind penetration increases however the merit order counteracts this with the results that prices overall remain flat. Returns to peakers increase significantly as wind penetration increases. The market power in turn leads to inefficient dispatch which is exacerbated with large amounts of wind generation.     

基于静态安全约束的风电场穿透功率极限计算

风电场穿透功率极限的计算是风电场规划阶段的重要内容.以电压稳定为约束条件,分别从静态稳态分析和动态分析两方面进行考虑,确定了既能满足静态稳态运行要求,又动态不失稳的风电场穿透功率极限.静态安全约束指标是线路潮流不过载、节点电压不越限(波动范围5%).暂态稳定的判据是系统中发电机最大功角差第一摆和第二摆衰减震荡,中枢节点电压恢复稳定,各节点电压偏差在额定值的5%之内.     

System operation with high wind penetration

The European Union has committed to reduce the equivalent carbon dioxide emissions by 8% of the 1990 level by the end of 2012. To meet the objective, the member states have financially encouraged the development of renewable energy especially wind power. Locally, this results in some of the highest wind power penetration levels in the world. This paper discusses the transmission challenges of Denmark, Spain, Germany and Ireland. With increasing wind capacity, the transmission system operators (TSOs) became concerned about the impact of high levels of wind generation on system stability. The integration of wind power has been hampered by the lack of suitable dynamic models for use in transient stability programs. The number of different turbine technologies used increased the complexity of the modeling problems.     

大规模风电场对电力系统稳定性影响的研究

风电机组由于其自身特点,风电机组与传统发电机组有不同的稳态和暂态特性,大规模风力发电接入电网后,电网的电压稳定性、暂态稳定性及频率稳定性都会发生变化。主要针对基于普通异步感应电机和基于双馈式感应电机风电机组的风电场对电网稳定性影响进行深入研究,使得对风电场接入电网后,给电网稳定性带来的问题有更全面、更深入的认识,有利于我国风力发电快速、健康发展。     

风电接入对安庆地区电网的影响

为研究风电接入对安庆电网的影响,根据风功率的转换特性及双馈感应电机的运行特性,建立了风电场的风功率模型及双馈感应电机的动态模型,利用中国版BPA软件分析了风电接入系统后对电网静态电压稳定性、网损、短路电流及电网暂态稳定性的影响。结果表明,风电接入后电网电压满足正常运行要求,有利于网损的减小,但增大了短路点的短路电流;此外,由于风电容量较小,风电接入对电网暂态稳定性影响不大。可见,风电接入后安庆电网能安全经济运行。     

基于串联动态制动电阻的异步风电机组暂态稳定性研究

以并网笼型异步风电机组为例,分析了利用串联动态制动电阻提高并网异步风电机组在电网故障下暂态稳定性的作用机理以及效果.建立了并网异步风电机组的数学模型,基于Matlab/Simulink仿真平台,对比分析了采用串联动态制动电阻、并联动态制动电阻以及无功补偿装置的作用效果.仿真结果表明,采用串联动态制动电阻可以有效改善并网异步风力发电机组的暂态稳定性;同时,采用串联动态制动电阻和无功补偿装置,可显著提高机组的暂态稳定性,减少对无功补偿的需求,降低风电场的运行成本.     

On the wind power rejection in the islands of Crete and Rhodes

Crete and Rhodes represent the two biggest isolated power systems in Greece. The energy production in both islands is based on thermal power plants. The annual wind energy rejection percentage is calculated for Crete and Rhodes in this paper. The rejected wind energy is defined as the electric energy produced by the wind turbines and not absorbed by the utility network, mainly due to power production system's stability and dynamic security reasons. A parametric calculation of the annual wind energy rejection percentage, in terms of the installed wind power, the power demand and the maximum allowed wind power instant penetration percentage, is accomplished. The methodology takes into account (i) the wind power penetration probability, restricted by the thermal generators technical minima and the maximum allowed wind power instant penetration percentage over the instant power demand; and (ii) the wind power production probability, derived by the islands' wind potential. The present paper indicates that isolated power systems which are based on thermal power plants have a limited wind power installation capacity—in order to achieve and maintain an adequate level of system stability. For a maximum wind power instant penetration percentage of 30% of the power demand, in order to ensure an annual wind energy rejection percentage less than 10%, the total installed wind power should not exceed the 40% of the mean annual power demand. The results of this paper are applicable to medium and great size isolated power systems, with particular features: (i) the power production is based on thermal power plants; (ii) the power demand exhibits intensive seasonal variations and is uncorrelated to the wind data; (iii) the mean annual power demand is greater than 10MW; and (iv) a high wind potential, presenting mean annual wind velocity values greater than 7·5ms^?1, is recorded. Copyright © 2007 John Wiley &Sons, Ltd.     

Coordinated control of TCPS and SMES for frequency regulation of interconnected restructured power systems with dynamic participation from DFIG based wind farm

Among the several wind generation technologies, variable-speed wind turbines utilizing doubly fed induction generators (DFIG) are gaining momentum in the power industry. Increased penetration of these wind turbine generators displaces conventional synchronous generators which results in erosion of system frequency. With this assertion, the paper analyzes the dynamic participation of DFIG for frequency control of an interconnected two-area power system in restructured competitive electricity market. Frequency control support function responding proportionally to frequency deviation is proposed to take out the kinetic energy of wind turbine for improving the frequency response of the system. Impacts of varying wind penetration in the system and varying active power support from DFIG on frequency control have been investigated. The presence of thyristor controlled phase shifter (TCPS) in series with the tie-line and Superconducting Magnetic Energy Storage (SMES) at the terminal of one area in conjunction with dynamic active power support from DFIG results in optimal transient performance for PoolCo transactions. Integral gains of AGC loop and parameters of TCPS and SMES are optimized through craziness-based particle swarm optimization (CRPSO) in order to have optimal transient responses of area frequencies, tie-line power deviation and DFIG parameters.     

Analysis of a pumped storage system to increase the penetration level of renewable energy in isolated power systems. Gran Canaria: A case study

A significant number of islands have been forced to restrict the penetration level of renewable energy sources (RES) in their conventional electrical power systems. These limitations attempt to prevent problems that might affect the stability and security of the electrical system. Restrictions that may apply to the penetration of wind energy can also be an obstacle when meeting European Union renewable energy objectives. As a partial solution to the problem, this paper proposes the installation of a properly managed, wind-powered, pumped hydro energy storage system (PHES) on the island of Gran Canaria (Canary Islands). Results from a dynamic model of the island’s power system show that the installation of a pumped storage system is fully supported in all circumstances. They also show that the level of wind penetration in the network can be increased. These results have been obtained assuming that two of the largest existing reservoirs on the island (with a difference in altitude of 281 m and a capacity of aprox. 5,000,000 m³ each) are used as storage reservoirs with three 54 MW generators. Likewise, the ability of such facilities to contribute to the stability of the system is shown. This type of installation can reduce fossil fuel consumption, reducing CO₂ emissions. Moreover, not only can the PHES improve wind penetration level, but it also allows the number of wind farms installed to be increased. Regions with geographically suitable sites and energy problems similar to those on the Canary Islands are encouraged to analyze the technical and economic feasibility of installing similar power systems to the one in this paper. Such systems have an enormous, unexplored potential within the general guiding framework of policies promoting clean, renewable energy.     

恒速风电机组对电网暂态电压稳定性的影响

基于恒速风电机组的结构,给出风电机组各部分的数学模型,利用仿真软件DIgSILENT/PowerFactory建立恒速风电机组的动态模型,并利用该模型在WSCC 3机9节点算例中仿真恒速风电机组并网运行时因风速变化引起的电网电压波动,通过极限切除时间分析恒速风电机组并网对电网暂态电压稳定性的影响,并与接入相同容量的同步机组进行比较。结果表明,恒速风电机组的接入降低了电网的暂态电压稳定性,且接入恒速风电机组装机容量越大,电网的暂态电压稳定性越差。由此提出改善恒速风电机组并网后电网暂态电压稳定性的建议。     

风电高渗透虚拟同步电力系统的功角暂态稳定性分析

首先,建立互联系统接入变速风电机组后的动态模型,推导虚拟同步电力系统的暂态能量函数,分析表明在不同振荡阶段,惯性控制对暂态能量转换过程的影响不同;其次,提出两区域协同变惯性控制策略,改善含虚拟惯性互联系统的功角稳定性。最后,搭建风电高渗透的四机两区仿真系统,验证所提控制策略能够兼顾频率调节功能,并改善系统的动态稳定性。     

双馈风力发电机组等效模型对机组暂态稳定性影响

为研究并网双馈风力发电机组的机电耦合作用对机组暂态性能的影响,采用等效集中质量法,在Mat-lab/Simulink环境下,提出一种基于叶片弯曲柔性和传动轴扭转柔性的风力发电机组3质量块等效模型。结合双馈发电机控制策略,分别以额定功率750 kW和3 MW的风力发电机组为例,对其在电网电压跌落和机械扰动两种情况下进行暂态稳定性仿真,并将仿真结果和传统的1质量块、2质量块等效模型的仿真结果进行比较分析。结果表明:随着单机容量的增加,叶片的柔性对机组暂态稳定性的影响逐渐增大,建立3质量块等效模型对研究大容量双馈风力发电机组暂态稳定性是必要和有效的。     

Maximum wind energy contribution in autonomous electrical grids based on thermal power stations

Greek islands cover their continuously increasing electricity demand on the basis of small autonomous thermal power stations. This electrification solution is related with increased operational cost and power insufficiency, especially during summer. On the other hand, the stochastic behaviour of the wind and the important fluctuations of daily and seasonal electricity load in almost all Greek islands pose a substantial penetration limit for the exploitation of the high wind potential of the area. In this context, the present study is concentrated on developing an integrated methodology which can estimate the maximum wind energy contribution to the existing autonomous electrical grids, using the appropriate stochastic analysis. For this purpose one takes into account the electrical demand probability density profile of every island under investigation as well as the operational characteristics of the corresponding thermal power stations. Special attention is paid in order to protect the existing internal combustion engines from unsafe operation below their technical minima as well as to preserve the local system active power reserve and the corresponding dynamic stability. In order to increase the reliability of the results obtained, one may use extensive information for several years. Finally, the proposed study is integrated with an appropriate parametrical analysis, investigating the impact of the main parameters variation on the expected maximum wind energy contribution.     

弱电网条件下STATCOM对风电场电压稳定性的影响

为研究静止同步补偿器(STATCOM)对连接到弱电网风电场电压稳定性的影响,基于双馈型风机和STATCOM数学模型,通过数学分析研究了不同电网强度下连接到弱电网风电场的电压静态稳定问题,通过系统仿真研究了STATCOM对风电场静态和动态行为的影响。结果表明,弱电网条件下风电场的电压静态稳定极限和功率传输极限会随着电网强度的减弱而降低;STATCOM的应用减小了弱电网条件下风电场电压的波动幅度,提高了电压稳定极限和功率传输能力,且有利于实现风机的故障穿越。     

An integrated reservoir-power system model for evaluating the impacts of wind integration on hydropower resources

Despite the potential for hydroelectric dams to help address challenges related to the variability and unpredictability of wind energy, at present there are few systems-based wind-hydro studies available in the scientific literature. This work represents an attempt to begin filling this gap through the development of a systems-based modeling framework for analysis of wind power integration and its impacts on hydropower resources. The model, which relies entirely on publicly available information, was developed to assess the effects of wind energy on hydroelectric dams in a power system typical of the Southeastern US (i.e., one in which hydropower makes up <10% of total system capacity). However, the model can easily reflect different power mixes; it can also be used to simulate reservoir releases at self-scheduled (profit maximizing) dams or ones operated in coordination with other generators to minimize total system costs. The modeling framework offers flexibility in setting: the level and geographical distribution of installed wind power capacity; reservoir management rules, and static or dynamic fuel prices for power plants. In addition, the model also includes an hourly ‘natural’ flow component designed expressly for the purpose of assessing changes in hourly river flow patterns that may occur as a consequence of wind power integration. Validation of the model shows it can accurately reproduce market price dynamics and dam storage and release patterns under current conditions. We also demonstrate the model's capability in assessing the impact of increased wind market penetration on the volumes of reserves and electricity sold by a hydroelectric dam.     

Analysis and estimation of transient stability for a grid-connected wind turbine with induction generator

Increasing levels of wind energy in modern electrical power system is initiating a need for accurate analysis and estimation of transient stability of wind turbine generation systems. This paper investigates the transient behaviors and possible direct methods for transient stability evaluation of a grid-connected wind turbine with squirrel cage induction generator (SCIG). Firstly, by using an equivalent lump mass method, a three-mass wind turbine equivalent model is proposed considering both the blades and the shaft flexibility of the wind turbine drive train system. Combined with the detailed electromagnetic transient models of a SCIG, the transient behaviors of the wind turbine generation system during a three-phase fault are simulated and compared with the traditional models. Secondly, in order to quickly estimate the transient stability limit of the wind turbine generation system, a direct method based on normal form theory is proposed. The transient models of the wind turbine generation system including the flexible drive train model are derived based on the direct transient stability estimation method. A method of critical clearing time (CCT) calculation is developed for the transient stability estimation of the wind turbine generation system. Finally, the CCT at various initial mechanical torques for different dynamical models are calculated and compared with the trial and error method by simulation, when the SCIG stator terminal is subjected to a three-phase short-circuit fault. The results have shown the proposed method and models are correct and valid.     

Dynamic model of wind energy conversion systems with variable speed synchronous generator and full-size power converter for large-scale power system stability studies

This paper presents a dynamic model for variable speed wind energy conversion systems, equipped with a variable pitch wind turbine, a synchronous electrical generator, and a full power converter, specially developed for its use in power system stability studies involving large networks, with a high number of buses and a high level of wind generation penetration. The validity of the necessary simplifications has been contrasted against a detailed model that allows a thorough insight into the mechanical and electrical behavior of the system, and its interaction with the grid. The developed dynamic model has been implemented in a widely used power system dynamics simulation software, PSS/E, and its performance has been tested in a well-documented test power network.     

Comparative study on transient stability analysis of wind turbine generator system using different drive train models

A huge number of wind generators are going to be connected with the existing network in the near future. Therefore it is necessary to analyse the transient stability of power systems, including wind turbine generator systems (WTGS). It has already been reported that one-mass or lumped model of wind turbine system is insufficient to analyse the transient behaviour of WTGS. It has also been reported that for the precise transient analysis of WTGS, a six-mass drive train model is needed. The reduced order models (three-mass and two-mass) have also been adopted so far for transient behaviour analysis. But the transient stability analysis of using six-mass, three-mass and two-mass drive train models has not been reported sufficiently so far in the literature. The authors have conducted an analysis using these methods. First, a detailed transformation procedure is presented from six-mass drive train model to two-mass model, which can be used in the analysis of transient stability simulation with sufficient accuracy. It is then determined which drive train model is appropriate for transient stability analysis of grid-connected WTGS. The effects of drive train parameters (such as inertia constant, spring constant and damping constant) on stability are examined using the above mentioned types of drive train models. Moreover, different types of symmetrical and asymmetrical faults at different wind generator power levels are considered in the simulation analyses with and without considering damping constants in six-mass, three-mass and two-mass shaft models. Considering the simulation results, it can be concluded that two-mass shaft model is sufficient for the transient stability analysis of WTGS.     

海上风电多端柔性直流并网系统频率支持研究

针对海上风电多端柔性直流(VSC-MTDC)并网系统,重点研究风电VSC-MTDC对岸上电网调频功能.通过建立详细风电场、换流站和电网模型,提出一种适用于海上风电VSC-MTDC并网系统动态频率调节方法,即改进斜率控制.整个海上风电场VSC-MTDC加入改进斜率控制后,可使有功功率在若干岸上换流站之间合理分配,从而确保...