Control of a wind farm with an internal direct current collection grid

For a high‐voltage, direct current connected wind farm, an internal direct current (DC) collection grid is a possible further development that can reduce the weight of the transformers significantly, with average losses for the DC system of 3%. For the internal DC grid, the DC/DC converters control the power flow and thereby also the voltages. In this paper, the control of the DC/DC converters in the wind farm is investigated in detail. The control strategy is presented, and suitable time constants are chosen depending on the switching frequency of the converters. Also, the required capacitor size to keep the voltage variations within 5% of the rated value in the case without communication within the wind farm is derived. It is shown that the control is stable and can handle faults on the external grid without any communication within the wind farm. Copyright © 2011 John Wiley & Sons, Ltd.     

Validation of a double fed induction generator wind turbine model and wind farm verification following the Spanish grid code

Wind turbine manufacturers are required by transmission system operators for fault ride‐through capability as the penetration of wind energy in the electrical systems grows. For this reason, testing and modeling of wind turbines and wind farms are required by the national grid codes to verify the fulfillment of this capability. Therefore, wind turbine models are required to simulate the evolution of voltage, current, reactive and active power during faults. The simulation results obtained from these wind turbine models are used for verification, validation and certification against the real wind turbines measurement results, although evolution of electrical variables during the fault and its clearance is not easy to fulfill. The purpose of this paper is to show the different stages involved in the fulfillment of the procedure of operation for fault ride‐through capability of the Spanish national grid code (PO 12.3) and the ‘procedure for verification, validation and certification of the requirements of the PO 12.3 on the response of wind farms in the event of voltage dips’. The process has been applied to a wind farm composed of Gamesa G52 wind turbines, and the results obtained are presented. Copyright © 2011 John Wiley & Sons, Ltd.     

电网不同故障下DFIG特性分析及保护措施建议

目前,国内外对DFIG的研究主要侧重于风力发电机组控制策略方面,而对于不同电网故障情况下DFIG的运行特性分析较少。鉴于此,在DIgSILENT／PowerFactory下建立TDFIG模型,利用含风电场的WSCC三机九节点仿真系统,进行了电网不同故障情况下的一系列仿真,重点分析了电网不同故障情况下DFIG的运行特性,研究了风电场与电网之间的交互影响及相应的保护措施,为大规模风电接入电网的运行控制提供依据。     

A review of grid code technical requirements for wind farms

This paper provides an overview of grid code technical requirements regarding the connection of large wind farms to the electric power systems. The grid codes examined are generally compiled by transmission system operators (TSOs) of countries or regions with high wind penetration and therefore incorporate the accumulated experience after several years of system operation at significant wind penetration levels. The paper focuses on the most important technical requirements for wind farms, included in most grid codes, such as active and reactive power regulation, voltage and frequency operating limits and wind farm behaviour during grid disturbances. The paper also includes a review of modern wind turbine technologies, regarding their capability of satisfying the requirements set by the codes, demonstrating that recent developments in wind turbine technology provide wind farms with stability and regulation capabilities directly comparable to those of conventional generating plants.     

基于SOM网的风电变流器故障诊断

我国新疆、甘肃、宁夏、内蒙、浙江、黑龙江、江苏、广东等都在大规模建设风电场,这些风电场建成后,其故障维护就有了很大市场.以新疆风电场为基础,尝试开发用于风力机故障智能诊断的系统.首先介绍了风力机及其变频器系统的结构,分析了变频器的故障机理.使用SOM神经网络对风机变流器进行了诊断,用数据验证了诊断结果.把传统的电力电子设备故障诊断技术与新疆风力机变频器的故障诊断相结合,为风电大面积推广应用产生了积极作用.     

Operation of offshore wind farms connected with DRU-HVDC transmission systems with special consideration of faults

The diode rectifier unit (DRU)-based high-voltage DC (DRU-HVDC) system is a promising solution for offshore wind energy transmission thanks to its compact design, high efficiency, and strong reliability. Herein we investigate the feasibility of the DRU-HVDC system considering onshore and offshore AC grid faults, DC cable faults, and internal DRU faults. To ensure safe operation during the faults, the wind turbine (WT) converters are designed to operate in either current-limiting or voltage-limiting mode to limit potential excessive overcurrent or overvoltage. Strategies for providing fault currents using WT converters during offshore AC faults to enable offshore overcurrent and differential fault protection are investigated. The DRU-HVDC system is robust against various faults, and it can automatically restore power transmission after fault isolation. Simulation results confirm the system performance under various fault conditions.     

Aggregated dynamic model for wind farms with doubly fed induction generator wind turbines

As a result of increasing wind farms penetration in power systems, the wind farms begin to influence power system, and thus the modelling of wind farms has become an interesting research topic. Nowadays, doubly fed induction generator based on wind turbine is the most widely used technology for wind farms due to its main advantages such as high-energy efficiency and controllability, and improved power quality. When the impact of a wind farm on power systems is studied, the behavior of the wind farm at the point common coupling to grid can be represented by an equivalent model derived from the aggregation of wind turbines into an equivalent wind turbine, instead of the complete model including the modelling of all the wind turbines. In this paper, a new equivalent model of wind farms with doubly fed induction generator wind turbines is proposed to represent the collective response of the wind farm by one single equivalent wind turbine, even although the aggregated wind turbines operate receiving different incoming winds. The effectiveness of the equivalent model to represent the collective response of the wind farm is demonstrated by comparing the simulation results of equivalent and complete models both during normal operation and grid disturbances.     

Multi-pole permanent magnet synchronous generator wind turbines' grid support capability in uninterrupted operation during grid faults

Emphasis in this paper is on the fault ride-through and grid support capabilities of multi-pole permanent magnet synchronous generator (PMSG) wind turbines with a full-scale frequency converter. These wind turbines are announced to be very attractive, especially for large offshore wind farms. A control strategy is presented, which enhances the fault ride-through and voltage support capability of such wind turbines during grid faults. Its design has special focus on power converters' protection and voltage control aspects. The performance of the presented control strategy is assessed and discussed by means of simulations with the use of a transmission power system generic model developed and delivered by the Danish Transmission System Operator Energinet.dk. The simulation results show how a PMSG wind farm equipped with an additional voltage control can help a nearby active stall wind farm to ride through a grid fault, without implementation of any additional ride-through control strategy in the active stall wind farm.     

Transient stability of a fixed speed wind farm

A typical fixed speed wind farm connected to a simple grid is modelled. Using this model, a three-phase fault is applied close to the wind farm, and cleared by disconnecting the affected line. The effect of several electric, mechanical and operational parameters on the critical fault-clearing time of this base case is evaluated and discussed. The studied parameters are the short-circuit power at the connection bus, the reactive power compensation, the distance to the fault, the rotor inertia, the hub-generator resonant frequency, the wind speed and the power output. For each parameter, the relationship between its value and the critical fault-clearing time is shown graphically. The results help to understand the transient stability phenomena in fixed speed wind farms, and could help to design fixed speed wind farms attending to transient stability requirements.     

SVC对并网型风电场运行性能的影响分析

建立了风力发电机组和静止无功补偿器SVC的数学模型,并利用MATLAB/Simulink软件搭建了风电场接入电网后的仿真模型,针对风电系统中经常出现的联络线短路故障和风电场风速扰动,通过仿真计算表明,SVC不仅可以在常见的扰动下有效地提高风电场的稳定性,而且能够在快速的风速扰动下平滑风电场的有功功率输出,降低风电场对电网的冲击。     

Coordinate control strategy for stability operation of offshore wind farm integrated with Diode-rectifier HVDC

Due to low investment cost and high reliability, a new scheme called DR-HVDC (Diode Rectifier based HVDC) transmission was recently proposed for grid integration of large offshore wind farms. However, in this scheme, the application of conventional control strategies for stability operation face several challenges due to the uncontrollability of the DR. In this paper, a coordinated control strategy of offshore wind farms using the DR-HVDC transmission technology to connect with the onshore grid, is investigated. A novel coordinated control strategy for DR-HVDC is proposed based on the analysis of the DC current control ability of the full-bridge-based modular multilevel converter (FB-MMC) at the onshore station and the input and output characteristics of the diode rectifier at the offshore. Considering the characteristics of operation stability and decoupling between reactive power and active power, a simplified design based on double-loop droop control for offshore AC voltage is proposed after power flow and voltage–current (I–V) characteristics of the offshore wind farm being analyzed. Furthermore, the impact of onshore AC fault to offshore wind farm is analyzed, and a fast fault detection and protection strategy without relying on communication is proposed. Case studies carried out by PSCAD/EMTDC verify the effectiveness of the proposed control strategy for the start up, power fluctuation, and onshore and offshore fault conditions.     

Fault ride through of fully rated converter wind turbines with AC and DC transmission

Fault ride through of fully rated converter wind turbines in an offshore wind farm connected to onshore network via either high voltage AC (HVAC) or high voltage DC (HVDC) transmission is described. Control of the generators and the grid side converters is shown using vector control techniques. A de-loading scheme was used to protect the wind turbine DC link capacitors from over voltage. How de-loading of each generator aids the fault ride through of the wind farm connected through HVAC transmission is demonstrated. The voltage recovery of the AC network during the fault was enhanced by increasing the reactive power current of the wind turbine grid side converter. A practical fault ride through protection scheme for a wind farm connected through an HVDC link is to employ a chopper circuit on the HVDC link. Two alternatives to this approach are also discussed. The first involves de-loading the wind farm on detection of the fault, which requires communication of the fault condition to each wind turbine of the wind farm. The second scheme avoids this complex communication requirement by transferring the fault condition via control of the HVDC link to the offshore converter. The fault performances of the three schemes are simulated and the results were used to assess their respective capabilities.     

Coordinated Reactive Power Control of a Large Wind Farm and a STATCOM Using Heuristic Dynamic Programming

A novel interface neurocontroller (INC) is proposed for the coordinated reactive power control between a large wind farm equipped with doubly fed induction generators (DFIGs) and a static synchronous compensator (STATCOM). The heuristic dynamic programming (HDP) technique and radial basis function neural networks (RBFNNs) are used to design this INC. It effectively reduces the level of voltage sags as well as the over-currents in the DFIG rotor circuit during grid faults, and therefore, significantly enhances the fault ride-through capability of the wind farm. The INC also acts as a coordinated external damping controller for the wind farm and the STATCOM, and therefore, improves power oscillation damping of the system after grid faults. Simulation studies are carried out in PSCAD/EMTDC and the results are presented to verify the proposed INC.      

超导储能对并网型风电场运行性能影响的仿真分析

采用超导储能(SMES)可以改善风电场并网运行的稳定性,针对风电系统中出现的联络线短路故障和风电场的风速扰动,提出利用超导储能安装点的电压偏差信号作为超导储能有功控制器的控制策略。为了验证这种策略的有效性,建立了风电机组和超导储能装置的数学模型,并利用MATLAB/Simulink软件搭建了风电场接入电网后的仿真模型。仿真结果表明,采用该控制策略不仅可以在网络故障后有效地提高风电场的稳定性,而且能够在快速的风速扰动下平滑风电场的功率输出,降低风电场对电网的冲击。     

适用于短路故障分析的风电场动态等值建模方法

针对风电场集电网络等值较为粗略、对非对称故障时等值模型精确度关注较少的情况,提出一种适用于短路故障分析的风电场动态等值建模方法。首先,依据系统潮流计算结果对集电网络进行等值解耦计算,并采用基于模块度的K均值算法对各风力机端电压进行聚类分析,实现风电机群内的多机聚合。其次,考虑到集电网络分布电容的影响,对聚合后风电场设置集中无功补偿电容。随后,为保证非对称故障过程中等值模型的有效性,提出风电场零序网络的等值方法。最后,基于时域仿真软件PSCAD/EMTDC下搭建的电磁暂态仿真模型,验证了所提动态等值方法的有效性。     

Review of international grid codes for wind power integration: Diversity,technology and a case for global standard

This paper presents a comprehensive study on the latest grid code regulations enforced by transmission system operators on large wind power plants (WPPs). First, the most common requirements included in the majority of international grid codes are compared; namely, low and high voltage ride-through capabilities, active and reactive power responses during and after faults, extended range of voltage–frequency variations, active power (frequency) control facility, and reactive power (voltage) regulation support. The paper also presents a discussion on the global harmonization of international grid codes as well as future trends expected in the regulations. Finally, the evolution of different wind generator technologies to fulfill various grid code requirements is investigated. The presented study will assist system operators to establish their connection requirements for the first time or to compare their existing regulations with other operators. It also enables wind turbine manufacturers and wind farm developers to obtain a more precise understanding from the latest international requirements imposed on modern wind farms.     

风电场的主接线、并网和运行方式分析

"十一五"期间,上海南汇地区先后建造了35kV南汇风电场、110kV东海大桥风电场和35kV临港新城风电场。通过对南汇地区3个风电场的建设回顾,比较了风电场的主接线形式和运行方式,分析了风电场的并网操作和低电压穿越等问题,并提出了有待改进的技术措施和相关建议。     

Co‐ordinated voltage control of DFIG wind turbines in uninterrupted operation during grid faults

Emphasis in this article is on the design of a co‐ordinated voltage control strategy for doubly fed induction generator (DFIG) wind turbines that enhances their capability to provide grid support during grid faults. In contrast to its very good performance in normal operation, the DFIG wind turbine concept is quite sensitive to grid faults and requires special power converter protection. The fault ride‐through and grid support capabilities of the DFIG address therefore primarily the design of DFIG wind turbine control with special focus on power converter protection and voltage control issues. A voltage control strategy is designed and implemented in this article, based on the idea that both converters of the DFIG (i.e. rotor‐side converter and grid‐side converter) participate in the grid voltage control in a co‐ordinated manner. By default the grid voltage is controlled by the rotor‐side converter as long as it is not blocked by the protection system, otherwise the grid‐side converter takes over the voltage control. Moreover, the article presents a DFIG wind farm model equipped with a grid fault protection system and the described co‐ordinated voltage control. The whole DFIG wind farm model is implemented in the power system simulation toolbox PowerFactory DIgSILENT. The DFIG wind farm ride‐through capability and contribution to voltage control in the power system are assessed and discussed by means of simulations with the use of a transmission power system generic model developed and delivered by the Danish Transmission System Operator Energinet.dk. The simulation results show how a DFIG wind farm equipped with voltage control can help a nearby active stall wind farm to ride through a grid fault, without implementation of any additional ride‐through control strategy in the active stall wind farm. Copyright © 2006 John Wiley &Sons, Ltd.     

Application analysis of a superconducting fault current limiter-magnetic energy storage system for the wind farm

风力发电所面临的两大重要问题是低电压穿越能力弱和功率输出不稳定。为了同时解决这两个问题,我们提出了超导限流-储能系统,并进行了单机系统的仿真研究,证实了该方案的有效性。然而对于风电场的应用,目前尚无研究。本文将超导限流-储能系统的应用扩展到风电场,分析了其提高低电压穿越能力和稳定有功功率输出的机理,并进行了仿真研究。从仿真结果来看,超导限流-储能系统能够同时提高风电场所有风机的低电压穿越能力,并能有效地平滑整个风电场的有功输出功率。考虑不同风机的互补效应,将该系统应用于风力发电场与直接应用于单台风机相比,其储能量和功率输出的要求可以大大降低,从而可以有效地减少系统总成本,因而具有更好的应用前景。     

Impact of fault ride‐through requirements on fixed‐speed wind turbine structural loads

The emphasis in this article is on the impact of fault ride‐through requirements on wind turbines structural loads. Nowadays, this aspect is a matter of high priority as wind turbines are required more and more to act as active components in the grid, i.e. to support the grid even during grid faults. This article proposes a computer approach for the quantification of the wind turbines structural loads caused by the fault ride‐through grid requirements. This approach, exemplified for the case of a 2MW active stall wind turbine, relies on the combination of knowledge from complimentary simulation tools, which have expertise in different specialized wind turbines design areas. Two complimentary simulation tools are considered i.e. the detailed power system simulation tool PowerFactory from DIgSILENT and the advanced aeroelastic computer code HAWC2, in order to assess of the dynamic response of wind turbines to grid faults. These two tools are coupled sequently in an offline approach, in order to achieve a thorough insight both into the structural as well as the electrical wind turbine response during grid faults. The impact of grid requirements on wind turbines structural loads is quantified by performing a rainflow and a statistical analysis for fatigue and ultimate structural loads, respectively. Two cases are compared i.e. one where the turbine is immediately disconnected from the grid when a grid fault occurs and one where the turbine is equipped with a fault ride‐through controller and therefore it is able to remain connected to the grid during the grid fault. Copyright copy; 2010 John Wiley & Sons, Ltd.