考虑撬棒和直流卸荷保护的双馈电机电磁暂态过程分析

故障期间,撬棒（crowbar）保护和直流卸荷（chopper）保护的工作状态对双馈风电机组（DFIG）的暂态过程有重要影响。为此,针对三相故障不同严重程度下的双馈风力发电系统,综合考虑撬棒保护和直流卸荷保护的协调控制方式,通过分析转子电流、直流母线电压与机端电压跌落的关系,新建统一的保护动作分析判据;在此基础上,分析撬棒保护和直流卸荷保护在不同电压跌落深度下的状态,并建立统一的双馈电机故障后暂态等值模型。最后基于PSCAD/EMTDC提供的1 MW双馈电机标准模型,验证了所建DFIG暂态等值模型的准确性。     

变阻值Crowbar电路及其在提高双馈式风电机组低电压穿越能力中的应用

双馈风力发电机的低电压穿越能力较差,Crowbar技术是提高双馈风力发电机低电压穿越能力的有效手段。分析了DFIG机端短路时Crowbar阻值对转子电流和暂态过程的影响,指出传统Crowbar电路采用固定的阻值,无法兼顾低电压穿越过程中各阶段对该阻值的不同要求。为此提出了一种变阻值Crowbar的电路,采用这种电路只要控制脉宽就可以改变Crowbar电路的等效电阻,在电网发生地电压故障后,可以根据保护过程不同阶段的特点及时调整Crowbar电路电阻,提高双馈风力发电机的低电压穿越能力。为了验证调整效果对新设计的Crowbar电路的调整效果进行了仿真。仿真结果表明,变阻值Crowbar能够通过控制脉宽实现对Crowbar等效电阻的有效控制。     

Fault ride-through capability of DFIG wind turbines

This paper concentrates on the fault ride-through capability of doubly fed induction generator (DFIG) wind turbines. The main attention in the paper is, therefore, drawn to the control of the DFIG wind turbine and of its power converter and to the ability to protect itself without disconnection during grid faults. The paper provides also an overview on the interaction between variable-speed DFIG wind turbines and the power system subjected to disturbances, such as short circuit faults. The dynamic model of DFIG wind turbine includes models for both mechanical components as well as for all electrical components, controllers and for the protection device of DFIG necessary during grid faults. The viewpoint of the paper is to carry out different simulations to provide insight and understanding of the grid fault impact on both DFIG wind turbines and on the power system itself. The dynamic behaviour of DFIG wind turbines during grid faults is simulated and assessed by using a transmission power system generic model developed and delivered by the Danish Transmission System Operator Energinet.dk in the power system simulation toolbox PowerFactory DIgSILENT. The data for the wind turbines are not linked to a specific manufacturer, but are representative for the turbine and generator type used in variable-speed DFIG wind turbines with pitch control.     

Parameterization of a synchronous generator to represent a full‐scale converter generator for fault studies

This paper addresses the representation of the full‐scale converter generator technology by equivalent operational impedances valid for short‐circuit current calculations. Modern wind power plants are required and designed to ride through faults in the network, subjected to fault clearing. Accurate knowledge of the wind turbine short‐circuit current contribution is needed for component sizing and protection relay settings during faults within the wind power plant collector system or in the external networks. The industry standard employs software packages where generators are represented by their equivalent synchronous generator operational impedances, calculated according to standards or for electromagnetic simulation models, when studying fault currents and protection settings for wind power plant installations. Hence, it is of importance to represent non‐synchronous wind generators also by an equivalent synchronous generator. The effectiveness of the proposed method has been validated by simulation and site measurements. Copyright © 2013 John Wiley & Sons, Ltd.     

双馈感应式风力发电系统低电压运行特性研究

双馈感应发电机（DFIG）具有有功、无功功率独立调节能力及励磁变频器所需容量小等优点,在风力发电系统中得到越来越广泛的应用。但正是励磁变频器的过流能力限制使得其对电网故障非常敏感,电网故障下DFIG风电机组的控制能力受到限制。当前国外大多数风电并网标准都要求风力发电机在电网电压跌落的情况下不能从电网中解列,以便在故障后电网恢复过程中提供功率支持,避免发生后续更为严重的电网故障,这即是对风电机组低电压穿越能力的要求。为了保护变流器和对电网提供支撑,需要研制一种能够在电网故障发生时为故障电流进行旁路的设备——Crowbar电路。针对Crowbar的电流旁路装置进行了研究,说明Crowbar电路具有抑制转子浪涌电流和保护直流母线的作用,并在小功率平台上进行了试验,证明了这种设备对于提高DFIG系统的LVRT能力具有重要的作用。     

Parameterization of a synchronous generator to represent a doubly fed induction generator with chopper protection for fault studies

This paper addresses the representation of the wound rotor asynchronous generators by an equivalent synchronous generator, valid for short circuit current calculations. Modern wind power plants are required and designed to ride through faults in the network, subjected to fault clearing. Accurate knowledge of the wind turbine short circuit current contribution is needed for component sizing and protection relay settings during faults within the wind power plant collector system or in the external networks. When studying fault currents and protection settings for wind power installations, the industry standard is to employ software packages where generators are represented by their equivalent synchronous generator operational impedances. Hence, it is of importance to represent non‐synchronous wind generators by an equivalent synchronous generator. Copyright © 2010 John Wiley & Sons, Ltd.     

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.     

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.     

Effectiveness evaluation of passive resistive element placement on a fault ride‐through enhancement in a DFIG‐based wind energy conversion system

Over the last few decades, doubly fed induction generators (DFIG) have become one of the most successful and preferred types of wind energy generators (WEG). The DFIG has the advantages of a wide range of speed operations, a high efficiency, and partial rated converters. However, direct coupling of the stator with the grid makes the system more prone to grid disturbances. The consequences of grid disturbances, such as a rotor overcurrent, stator overcurrent, electromagnetic torque oscillations, and direct current (DC) link overvoltage, are the predominant considerations that affect the rotor circuit, stator circuit, mechanical components, and DC‐link capacitor of the DFIG, respectively. These uncertainties affect the operation of the generator and may lead to the generator to be shutdown. In this paper, a novel position for the placement of a passive resistive element (PRE) is illustrated. This position of the PRE placement is compared with all other possible locations for the PRE. The different locations for PRE placement are the stator side, rotor side, across the DC‐link capacitor, and between the rotor side converter (RSC) and grid side converter (GSC). This paper aims to determine a cost‐effective solution among all possible locations of the PRE placement. The novel position of the PRE, ie, between the RSC and GSC, is the best position among the other possible locations of the PRE, when the performance, cost, and loss are taken into consideration. The effectiveness of the PRE is further compared with the resistive‐type superconducting fault current limiter (R‐SFCL). The PRE performs better and has a lower cost than the R‐SFCL.     

双馈电机的Crowbar参数整定及保护特性研究

为了防止变速恒频双馈风力发电机短路电流对转子侧变换器造成损坏,常采用Crowbar保护电路。建立风力发电机短路电磁过渡过程的数学模型,通过对数学模型进行拉普拉斯变换及其反变换,求取短路电流的表达式,并求出最不利的情况下电流的最大值,在该值下对Crowbar电路的参数进行整定及保护。为了验证所整定的Crowbar电阻的精确性,分别以3 MW,2.75 MW,660 kW发电机为例进行分析,仿真结果表明,在该数学模型下,所整定的电阻提高了对Crowbar电路的旁路电阻整定的准确性,不仅能对转子侧变换器进行保护,同时也能对发电机和机械结构起到保护作用。     

Comparison of simulators for variable‐speed wind turbine transient analysis

This paper presents a comparison of three variable‐speed wind turbine simulators used for a 2 MW wind turbine short‐term transient behaviour study during a symmetrical network disturbance. The simulator with doubly fed induction generator (DFIG) analytical model, the simulator with a finite element method (FEM) DFIG model and the wind turbine simulator with an analytical model of DFIG are compared. The comparison of the simulation results shows the influence of the different modelling approaches on the short‐term transient simulation accuracy. Copyright © 2005 John Wiley & Sons, Ltd.     

电压跌落时带有Crowbar电路的双馈感应发电机的瞬态分析

为了研究双馈感应发电机对电网电压跌落的适应能力,以及其实现低电压穿越的功能,文章通过将由向量法求出的瞬态电流与由等效电路法求出的稳态电流进行叠加而得出的定子、转子故障电流的近似解析式,来分析在定子端三相对称电压跌落、转子侧变换器断开、投入Crowbar电路情况下的双馈感应发电机内部的电磁关系变化过程。此外,在理论分析的基础上,文中建立了2　MW双馈感应发电机的PSCAD模型,且在7.5 kW双馈风力发电测试平台上进行了实验验证。仿真和实验结果表明,这种通过瞬态电流和稳态电流进行叠加的方法而求得的双馈感应发电机故障电流的近似解析表达式可以准确地反映出双馈感应发电机磁链和电流的瞬态变化。     

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.     

大型变速恒频风力发电机组建模与仿真

基于SIMULND技术,利用双馈感应发电机、风力机、电压型变流器、电压型逆变器的数学模型建立了相应的仿真模型;并对定子侧直接与50Hz电网连接,转子侧连接AC/DC/AC变流器的典型变速恒频风力发电系统进行了仿真验证.几个单独的风力发电系统子系统可以通过级联的方式构成大型风力发电场来模拟整个电网的运行情况.仿真结果表明,DFIG仿真模型的正确性和验证大型风电场的有效性.     

适合于变速恒频双馈感应发电机的Crowbar对比分析

电网运行新规则要求变速恒频双馈感应发电机在电网电压跌落的情况下仍与电网相连接，为做到这一点需要安装低压旁路系统。综合分析了低压带来的负面影响以及低压旁路的具体实现方法；对比了多种Crowbar电路各自的优缺点；最后介绍了变速恒频双馈发电机相关保护控制策略和新型旁路系统。     

基于Crowbar保护的双馈感应发电机组的低电压穿越研究

分析了双馈感应发电机组在电网电压跌落时Crowbar阻值变化对电网的影响。根据双馈感应发电机(Doubly-FedInduction Generator,DFIG)的数学模型,推导出在发电机机端发生对称故障时,定子、转子电流的表达式,通过故障期间的最大转子电流,给出Crowbar阻值估算值。在EPSCAD/EMTDC软件中仿真分析不同的Crowbar阻值对系统的影响,验证公式推导的正确性,并通过仿真试验确定合理的Crowbar切除出时间。     

A novel line drop secondary voltage control algorithm for variable speed wind turbines

This paper addresses the design and implementation of the line drop secondary voltage control (LDSVC) for the doubly fed induction generator‐wind turbine (DFIG‐WT) complemented with reactive power allocation algorithm to achieve more efficient voltage regulation, reactive power compensation and to enhance the transient stability margin of the electric power system. The LDSVC is used to generate the local voltage reference, providing an improvement for overall voltage profile. The paper presents the influence of the integration of variable speed wind turbines‐based doubly fed induction generator (DFIG) while employing LDSVC for increasing the transient stability margin. This paper proposes an improved voltage control scheme, based on a secondary voltage controller complemented with an automatic gain controller (AGC). The scheme is applied to a wind energy system incorporating DFIG‐based wind turbines. The controller structure is developed and the performance of the self‐tuning AGC scheme is developed and analysed. The proposed controller is tested in response to system contingencies for different short circuit ratios. The performance of the secondary voltage control without and with AGC is verified. The influence of the AGC in improving the transient response and damping of voltage oscillations is verified. Copyright © 2010 John Wiley & Sons, Ltd.     

Model‐based control addition to prescribe DFIG wind turbine fast frequency response

This paper investigates the physical capability of double‐fed induction generator (DFIG) wind turbines for inertial support of frequency response. Frequency stability is modeled using the DFIG electromechanical and generator controller dynamics, and a destabilizing effect is demonstrated in low‐inertia systems. To improve response, a synchronous reference frame DFIG controller is proposed that acts by following low‐frequency grid dynamics and adds a fast acting proportional plus integral (PI)‐controlled frequency‐responsive component to existing qd current commands. The proposed controller is derived in a straightforward manner using only the DFIG dynamic equations and is designed using pole/zero placement techniques. Laboratory experiments using a micro‐scale DFIG wind turbine with hub‐emulating flywheel prove better capability for transient frequency regulation even under extreme load change. The result is a DFIG controller that balances the appearance of transients in electrical and mechanical systems. Value is achieved in providing immediate continuous inertial response to support load change. The proposed frequency response can improve the use of existing physical inertia from wind turbines.     

并网双馈风电机组低电压穿越能力研究

详细分析了双馈风电机组LVRT功能的实现原理,并在电力系统仿真分析软件PSASP中建立双馈风电机组的LVRT功能模型,采用地理接线图直观地表示风电场外部系统发生短路故障瞬间对风电机组端电压的影响．并以我国某地区电网为例来分析在风电场接入方式不同的情况下系统短路故障对风电机组的影响。根据仿真结果给出风电机组LVRT能力的最低电压限值要求。最后提出了利用串联制动电阻来提高风电机组的LVRT能力的新方法。分析结果表明,串联制动电阻能够可观地提高风电机组的低电压穿越能力。具有较高LVRT能力的风电机组。可以节省一定的投资费用,在一定程度上降低了风电的上网电价。     

Dynamic contribution of variable-speed wind energy conversion system in system frequency regulation

Frequency regulation in a generation mix having large wind power penetration is a critical issue, as wind units isolate from the grid during disturbances with advanced power electronics controllers and reduce equivalent system inertia. Thus, it is important that wind turbines also contribute to system frequency control. This paper examines the dynamic contribution of doubly fed induction generator (DFIG)-based wind turbine in system frequency regulation. The modified inertial support scheme is proposed which helps the DFIG to provide the short term transient active power support to the grid during transients and arrests the fall in frequency. The frequency deviation is considered by the controller to provide the inertial control. An additional reference power output is used which helps the DFIG to release kinetic energy stored in rotating masses of the turbine. The optimal speed control parameters have been used for the DFIG to increases its participation in frequency control. The simulations carried out in a two-area interconnected power system demonstrate the contribution of the DFIG in load frequency control.