Analysis of low voltage ride-through capability and optimal control strategy of doubly-fed wind farms under symmetrical fault

Given the“carbon neutralization and carbon peak”policy,enhancing the low voltage ride-through(LVRT)capability of wind farms has become a current demand to ensure the safe and stable operation of power systems in the context of a possible severe threat of large-scale disconnection caused by wind farms.Currently,research on the LVRT of wind farms mainly focuses on suppressing rotor current and providing reactive current support,while the impact of active current output on LVRT performance has not been thoroughly discussed.This paper studies and reveals the relation-ship between the limit of reactive current output and the depth of voltage drop during LVRT for doubly-fed induction generator(DFIG)based wind farms.Specifically,the reactive current output limit of the grid-side converter is inde-pendent of the depth of voltage drop,and its limit is the maximum current allowed by the converter,while the reac-tive current output limit of the DFIG stator is a linear function of the depth of voltage drop.An optimized scheme for allocating reactive current among the STATCOM,DFIG stator,and grid-side converter is proposed.The scheme maximizes the output of active current while satisfying the standard requirements for reactive current output.Com-pared to traditional schemes,the proposed LVRT optimization strategy can output more active power during the LVRT period,effectively suppressing the rate of rotor speed increase,and improving the LVRT performance and fault recov-ery capability of wind farms.Simulation results verify the effectiveness of the proposed scheme.

逆变型分布式电源接入对小电阻接地系统馈线零序电流保护的影响

逆变型分布式电源(IIDG)的接入改变了小电阻接地系统的接地故障特性,不仅使传统零序电流的计算方法不再适用,而且对馈线零序电流保护造成了不利的影响。通过分析IIDG的故障电流特性,建立了具有低电压穿越能力的PQ控制型IIDG故障等值模型;在含IIDG小电阻接地系统接地故障分析建模的基础上,提出了零序电流迭代求解算法,并利用MATLAB编写相应的求解程序,将计算结果与电力系统实时仿真系统(RTDS)的仿真结果进行比较,验证了所提算法的有效性和准确性;基于所建立的故障分析模型,揭示了IIDG的接入对小电阻接地系统馈线零序电流保护的影响机理,利用所提零序电流迭代求解算法求解不同故障情况下的馈线零序电流值,验证了理论分析的正确性。

基于撬棒并联动态电阻的自适应双馈风力发电机低电压穿越

当电网故障引起电压跌落时,为防止大装机容量风电场的风机脱网,双馈风力发电机(DFIG)多采用Crowbar电路来实现低电压穿越(LVRT)。传统Crowbar电路采用固定阻值的电阻,很难兼顾对转子电流和直流母线电压的抑制以及对Crowbar的投入工作时间的控制。针对传统Crowbar的不足提出了一种基于Crowbar并联动态电阻的双馈风力发电机低电压穿越方案,制定了该方案的自适应控制策略以及其阻值的整定方法。仿真分析不同跌落深度下所提方案的LVRT特性,并与改变IGBT的导通脉宽的变电阻Crowbar方案进行了比较,结果表明带并联动态电阻Crowbar方案的LVRT效果较好,不仅兼顾了对转子过电流和直流母线过电压的抑制,而且在电压深度跌落时可缩短Crowbar的投入时间,有利于系统电压的恢复。

An Integrated Reactive Power Control Strategy for Improving Low Voltage Ride-through Capability

Among all renewable energies,wind power is rapidly growing,whereby it has the most participation to supply power.Doubly fed induction generator(DFIG)is the most popular wind turbine,as it can play a very significant role to enhance low voltage ride through(LVRT)capability.Ancillary services such as voltage control and reactive power capability are the main topics in wind power control systems that should be handled profoundly and carefully.The lack of reactive power during fault period can result in instability in generators and/or disconnection of the wind turbine from the power system.The main aims of this study are to illustrate the most effective approaches subject to improve the efficiency,stability,and reliability of wind power plant associated with LVRT capability enhancement.This effectiveness and efficiency are demonstrated by,firstly,comparison between all types of wind turbines,focusing on the ancillary services,after the existing advanced control strategies.According to the literature,there is a consensus that modifying converter-based control topology is the most effective approach to enhance LVRT capability in DFIG-based wind turbine(WT).Therefore,an advanced integrated control strategy is designed by considering the effect of the rotor side converter(RSC)and the grid side converter(GSC).A model of the wind power plant is presented based on the control objectives.MATLAB/Simulink is also used to illustrate the effectiveness of the designed algorithm.

A Review of State-of-the-art Flexible Power Point Tracking Algorithms in Photovoltaic Systems for Grid Support:Classification and Application

To maximize conversion efficiency,photovoltaic(PV)systems generally operate in the maximum power point tracking(MPPT)mode.However,due to the increasing penetra tion level of PV systems,there is a need for more developed control functions in terms of frequency support services and voltage control to maintain the reliability and stability of the power grid.Therefore,flexible active power control is a manda tory task for grid-connected PV systems to meet part of the grid requirements.Hence,a significant number of flexible pow er point tracking(FPPT)algorithms have been introduced in the existing literature.The purpose of such algorithms is to real ize a cost-effective method to provide grid support functional ities while minimizing the reliance on energy storage systems.This paper provides a comprehensive overview of grid support functionalities that can be obtained with the FPPT control of PV systems such as frequency support and volt-var control.Each of these grid support functionalities necessitates PV sys tems to operate under one of the three control strategies,which can be provided with FPPT algorithms.The three control strate gies are classified as:①constant power generation control(CP GC),②power reserve control(PRC),and③power ramp rate control(PRRC).A detailed discussion on available FPPT algo rithms for each control strategy is also provided.This paper can serve as a comprehensive review of the state-of-the-art FPPT algorithms that can equip PV systems with various grid support functionalities.

Modern Control Strategies of Doubly-Fed Induction Generator Based Wind Turbine System

A doubly-fed induction generator(DFIG)based configuration is still preferred by wind turbine manufacturers due to the cost-effective power converter and independent control of the active power and reactive power.To cope with stricter grid codes(e.g.reactive power compensation,low voltage ride-through operation,as well as steady and safe operation during long-term distorted grid),control strategies are continuously evolving.This paper starts with a control strategy using the combined reactive power compensation from both the back-to-back power converters for their optimized lifetime distribution under normal grid conditions.Afterwards,an advanced demagnetizing control is proposed to keep the minimum thermal stress of the rotor-side converter in the case of the short-term grid fault.A modularized control strategy of the DFIG system under unbalanced and distorted grid voltage is discussed,with the control targets of the smooth active and reactive power or the balanced and sinusoidal current of the rotor-side converter and the grid-side converte。Finally,a bandwidth based repetitive controller is evaluated to improve the DFIG system's robustness against grid frequency deviation.

Susceptibility of Large Wind Power Plants to Voltage Disturbances-Recommendations to Stakeholders

Sufficient fault ride-through(FRT)of large wind power plants(WPPs)is essential for the operation security of transmission system.The majority of studies on FRT do not include all disturbances originating in the transmission system or the disturbances irrelevant to the operation security.Based on the knowledge of power quality,this paper provides a guide to stakeholders in different aspects of FRT for wind turbines(WTs)and WPPs.This paper details the characteristics of the most common disturbances originated in the transmission system,how they propagate to the WT terminals,and how they impact the dynamic behavior of a large WPP.This paper shows that the details of the voltage disturbances,not only in the transmission system,but also at the WT terminals,should be taken into consideration.Moreover,a detailed representation or characterization of voltage dips is important for FRT studies,despite that the simplified models used in the literature are insufficient.This paper strongly recommends that distinct events and additional characteristics such as the phase-angle jump and oscillations in the transition segments should be considered in FRT analysis.