Distributed Model Predictive Load Frequency Control of Multi-area Power System with DFIGs

Reliable load frequency control(LFC) is crucial to the operation and design of modern electric power systems. Considering the LFC problem of a four-area interconnected power system with wind turbines, this paper presents a distributed model predictive control(DMPC) based on coordination scheme.The proposed algorithm solves a series of local optimization problems to minimize a performance objective for each control area. The generation rate constraints(GRCs), load disturbance changes, and the wind speed constraints are considered. Furthermore, the DMPC algorithm may reduce the impact of the randomness and intermittence of wind turbine effectively. A performance comparison between the proposed controller with and without the participation of the wind turbines is carried out. Analysis and simulation results show possible improvements on closed–loop performance, and computational burden with the physical constraints.

A Continuous Fault Ride-through Scheme for DFIGs Under Commutation Failures in LCC-HVDC Transmission Systems

Experimental and theoretical studies have confirmed that,relative to a one-shot voltage fault,a doubly-fed induction generator(DFIG)will suffer a greater transient impact during continuous voltage faults.This paper presents the design and application of an effective scheme for DFIGs when a commutation failure(CF)occurs in a line-commutated converter based high-voltage direct current(LCC-HVDC)transmission system.First,transient demagnetization control without filters is proposed to offset the electromotive force(EMF)induced by the natural flux and other low-frequency flux components.Then,a rotor-side integrated impedance circuit is designed to limit the rotor overcurrent to ensure that the rotor-side converter(RSC)is controllable.Furthermore,coordinated control of the demagnetization and segmented reactive currents is implemented in the RSC.Comparative studies have shown that the proposed scheme can limit rotor fault currents and effectively improve the continuous fault ride-through capability of DFIGs.

适应并离网模式切换的DFIG电磁暂态仿真建模

目前,常用的商业仿真软件中的双馈式感应发电机(double-fed Induction Generator,DFIG)模型通常不支持离网运行模式,难以实现风力发电系统由离网到并网的全过程电磁暂态仿真,对电力系统仿真分析工作带来极大不便。研究适应并离网模式切换的DFIG电磁暂态仿真建模方法。基于模块化思想分别对风力机、异步电机、变流器以及断路器进行建模,在系统组装形成DFIG风力发电系统电磁暂态仿真模型。提出了适应并离网模式切换的DFIG受控源等效电路模型和离散时间域实现方法,通过设计状态变量矩阵元素的截取和扩充算法实现了模型在并离网模式下的无缝衔接。在MATLAB/Simulink中搭建DFIG发电系统模型对所提方法和模型进行验证,仿真结果表明,所提建模方法能够很好地支持DFIG离网运行模式,实现DFIG由离网到并网的全过程电磁暂态仿真。

考虑源荷特性的双馈风电机组动态减载协同调频控制

为使双馈风电机组(doubly fed induction generator,DFIG)的有功输出能响应电网频率的变化,有效参与电网频率调节,提高风电系统的稳定性,文中分析了虚拟惯性控制、下垂控制以及减载控制的基本原理,探究了相关参数对风电并网系统频率特性的影响。在此基础上,提出一种考虑系统源荷特性的动态减载协同调频控制策略。该策略结合源荷扰动情况实现减载率修正,通过MATLAB/SIMULINK仿真验证表明:当负荷波动时,该控制策略相较于固定减载调频使含风电系统在不同风速下的频率稳定性有了显著提高。

一种多地DFIG联动控制的多馈入受端系统连续换相失败抑制策略

我国沿海负荷中心呈现直流多馈入的结构特征,海上风电的大规模开发使得系统在非正常工况下容易诱发次同步振荡,电压的快速波动可能会造成连续换相。针对该现象进行机理分析,通过优化双馈风电机组(doubly fed induction generator,DFIG)的控制方法,有效抑制了次同步振荡。同时,针对不同区域风电场调节能力差异,通过关联各直流落点的多馈入交互作用因子(multi-infeed interaction factor,MIIF),充分调动不同区域风电场的补偿能力抬升系统电压。该方法有效地抑制直流多馈入系统的异地连续换相失败。最后,通过PSCAD仿真对比验证所提策略的有效性。

基于模型数据混合驱动的大规模双馈风电场并网数字孪生建模

当前双馈风机详细模型具有多时间尺度动态和复杂控制环节,导致建模困难、大规模双馈风电场并网详细仿真建模所需算力庞大的问题。实际风机并网外特性无法实时反馈到仿真系统中,同时风电场功率输出受到多方面如风速、风向、老化等影响,使仿真系统无法精确拟合真实电力系统。针对这一问题,该文构建大规模数字孪生双馈风电场并网的电力系统模数混合驱动模型,实现了风机与电网互动反馈的实时更新,降低了算力需求,同时与风机物理实体实时匹配的虚拟模型具备较高的输出精度,满足精细化仿真需求。文中每台双馈风机采用长短期记忆网络构建数据驱动模型,风电场架构及所并电网选取常用的物理仿真模型,以数字孪生技术实现风机物理模型维护、虚拟数据模型更新的双向反馈。算例通过真实量测数据测试,结果表明在不同工况下该双馈风电场并网数字孪生模型较传统模型具备更优秀的拟合能力与精准度,同时实现了虚拟模型的实时更新能力。该模型有助于在大规模风电场并网中探寻具体某台风机故障、控制方式变化、风速波动等情况,研究风机之间相互影响、电网出现扰动时对风电场的影响。

Voltage phase angle jump characteristic of DFIGs in case of weak grid connection and grid fault

In the condition of connecting large scale doubly-fed induction generators (DFIGs) into weak grid,the closely coupled interactions between wind generators and power grid becomes more severe.Some new fault characteristics including voltage phase angle jump will emerge,which will influence the power quality of power system.However,there are very few studies focusing on the mechanism of voltage phase angle jump under grid fault in a weak grid with wind turbine integration.This paper focuses on the scientific issues and carries out mechanism studies from different aspects,including mathematical deduction,field data analysis and time domain simulation.Based on the analysis of transientcharacteristics of DFIGs during the grid fault,this paper points out that the change of terminal voltage phase angle in DFIGs is an electromagnetism transition process,which is different from conventional synchronous generator.Moreover,the impact on transient characteristics of voltage phase angle are revealed in terms of fault ride through(FRT) control strategies,control parameters of current inner-loop of rotor-side converter and grid strength.

Transient characteristics and adaptive fault ride through control strategy of DFIGs considering voltage phase angle jump

Wind power in China has experienced fast development in recent years. However, areas rich in wind power resources are often far away from loads centers,which leads to weak connection between wind turbines and power grid. When a grid fault occurs, new transient characteristics in weak grid integrated with doubly-fed induction generators(DFIGs) may present, such as voltage phase angle jump. Current control strategies for wind turbine with strong grid connection are hard to be adapted under weak gird connection. This paper explores the transient characteristics of DFIGs under voltage phase angle jump through analyzing the operation and control characteristics of DFIGs connected into weak grid when the voltage phase angle jumps. Fault ride through(FRT) control strategy of DFIGs based on adaptive phase-locked loop is proposed to adapt weak grid condition. The reference frame of the proposed strategy will be changed in real-time to track the operation condition of DFIGs according to the terminal voltage, and different phase tracking method is adopted during the grid fault. Field data analysis and time domain simulation are carried out. The results show that voltage phase angle jumps when a grid fault occurs, which weakens the FRT capability of DFIGs, and the proposed FRT control strategy can optimize transient characteristics of DFIGs, and improve the FRT capability of DFIGs.

双馈风力发电机的无功功率SMC控制

为了在DFIG系统启动时获得更好的功率调节性能,在DFIG机侧引入滑模控制器(SMC)。在无功功率调节电压环节加入基于指数趋近律的动态滑模变结构控制器,仿真结果显示在加入SMC控制器后,有功功率及无功功率的启动调节时间约缩短了1 s,系统稳定性也有所提升,表明该方法有效、合理,适用于工程。

Power System Transient Stability Analysis with Integration of DFIGs Based on Center of Inertia

As power systems experience increased wind penetration,an effective analysis and assessment of the influence of wind energy on power system transient stability is required.This paper presents a novel center of inertia(COI)approach to understand how integrated doubly fed induction generators(DFIGs)affect the transient dynamics of a power system.Under the COI coordinate,the influence of integrated DFIGs is separated into the COI related and individual synchronous generator related parts.Key factors that affect the COI’s dynamic motion as well as the rotor dynamics of each individual synchronous generator with respect to the DFIG integration are investigated.To further validate the analysis,comparative simulations of three different scenarios with varying DFIG capacities,access locations,and the replacement of synchronous generators are conducted.The results show that the dynamics of the COI and the individual generators are affected by the integrated DFIGs via different mechanisms,and are sensitive to different variables in the DFIG’s integration condition.

Wind speed equalization-based incoming wind classification by aggregating DFIGs

With the development of wind energy,it is necessary to develop equivalent models to represent dynamic behaviors of wind farms in power systems.The equivalent wind method is investigated for the aggregation of doubly-fed induction generator wind turbines.The detailed procedures for the calculation of equivalent wind are analyzed.The necessity of classifying incoming winds is shown.To improve the performances of the method,incoming winds are classified according to mean wind speeds and positive/negative semi-variances of wind speeds,and a group of turbines with similar incoming winds are aggregated together.The effectiveness of the method is verified through simulations in MATLAB/Simulink.

Optimized and coordinated model predictive control scheme for DFIGs with DC-based converter system

This paper proposes an optimized and coordinated model predictive control(MPC) scheme for doublyfed induction generators(DFIGs) with DC-based converter system to improve the efficiency and dynamic performance in DC grids. In this configuration, the stator and rotor of the DFIG are connected to the DC bus via voltage source converters, namely, a rotor side converter(RSC) and a stator side converter(SSC). Optimized trajectories for rotorflux and stator current are proposed to minimize Joule losses of the DFIG, which is particularly advantageous at low and moderate torque. The coordinated MPC scheme is applied to overcome the weaknesses of the field-oriented control technique in the rotor flux-oriented frame, which makes the rotor flux stable and the stator current track its reference closely and quickly. Lastly, simulations and experiments are carried out to validate the feasibility of the control scheme and to analyze the steady-state and dynamic performance of the DFIG.

Improvement to observability measures of LFO modes in power systems with DFIGs

Observation of the low-frequency oscillation (LFO) modes in power systems is important to design the damping scheme. The state equations of the power system with the doubly-fed induction generators (DFIGs) are derived to find the LFO modes related to the synchronous generator (SGs) and the DFIGs. The definition of the observability measure is improved to consider the initial output and the attenuation speed of the modes. The sensitivities of the observability measures to the control parameters are derived. The numerical results from the small and large-disturbance validate the LFO modes caused by the DFIGs, and different observability measures are compared. Adjustment of the control parameters is chosen based on the sensitivity model to improve the observability and damping ratio of the LFO mode, and the stability of the wind power system.

The Mechanism of DFIGs Grouping Tripped Off from Power Grid

With more and more big wind farms integrating to grid,the faults of doubly fed induction generators(DFIGs)grouping tripped off(GTO)have occurred and bring serious power impacts on the grid.Usually,it was considered that the fault of DFIGs GTO is only related to the low/high voltage disturbances in the grid and manipulated as an electromagnetic transients.But,we discover that the fault of DFIGs GTO are also related to operating condition of DFIGs and schemes of crowbar protection equipped with DFIGs,and it may be extended to an electromechanical transient in some situations.Based on the investigation of the factors related to DFIGs GTO,such as degree of voltage disturbances,operating conditions of DFIGs and schemes of crowbar protection,three types of DFIGs GTOs are classified and corresponding mechanisms are revealed.The calculation methods to confirm the mechanism are established.The effectiveness of the proposed methods are verified by simulating some cases of DFISs GTO.

基于无源控制的DFIG并网次同步控制相互作用抑制策略研究

为抑制双馈感应发电机(doubly-fed induction generator,DFIG)风电场与串补输电线路间次同步控制相互作用(sub-synchronous control interaction,SSCI),提出基于改进无源控制的SSCI抑制策略。首先,分析了DFIG并网中SSCI发生机理,发现转子侧变流器双环PI控制对SSCI影响较大,可对PI控制进行改进以抑制SSCI。其次,基于DFIG欧拉-拉格朗日(Euler-Lagrange,EL)模型和无源理论,设计转子侧无源控制器,通过计算稳定状态点,注入阻尼抑制SSCI。为提高抑制能力,采用带通滤波器改进无源控制,通过保持输入信号稳定提升控制器性能。最后,给出小干扰分析和时域仿真分析结果,并与PI控制和常规无源控制对比,验证所提策略在串补度变化和风速变化时的抑制效果。

Modeling and small-signal stability analysis of doubly-fed induction generator integrated system

Owing to their stability,doubly-fed induction generator(DFIG)integrated systems have gained considerable interest and are the most widely implemented type of wind turbines and due to the increasing escalation of the wind generation penetration rate in power systems.In this study,we investigate a DFIG integrated system comprising four modules:(1)a wind turbine that considers the maximum power point tracking and pitch-angle control,(2)induction generator,(3)rotor/grid-side converter with the corresponding control strategy,and(4)AC power grid.The detailed small-signal modeling of the entire system is performed by linearizing the dynamic characteristic equation at the steady-state value.Furthermore,a dichotomy method is proposed based on the maximum eigenvalue real part function to obtain the critical value of the parameters.Root-locus analysis is employed to analyze the impact of changes in the phase-locked loop,short-circuit ratio,and blade inertia on the system stability.Lastly,the accuracy of the small-signal model and the real and imaginary parts of the calculated dominant poles in the theoretical analysis are verified using PSCAD/EMTDC.

基于MPC算法的风电参与频率调控策略

为发挥风电辅助调频作用及快速响应优势,更好改善电网频率特性,文章提出了一种基于模型预测控制(Model Predictive Control,MPC)算法的风电参与频率调控策略。首先,在传统综合惯性控制中,引入了MPC算法,实现对惯性控制参数的实时更新,设计了MPC优化目标函数;然后,建立了控制信号的选择规则,在预测模型的滚动优化过程中对多步控制信号进行动态调整,形成考虑延时和丢包的延迟补偿机制;最后,在不同工况下进行仿真,与基于传统惯性控制的风电辅助调频方法相比,文章所提MPC策略提高了双馈异步风力发电机(Doubly Fed Induction Generator,DFIG)一次调频对通信延时的鲁棒性,提高了电力系统的安全性,同时减小了系统超调量和稳态偏差,验证了所提策略的有效性。

双馈异步风电机组机侧转速控制器的设计与仿真研究

双馈异步风力发电机组(DFIG)机侧(即转子侧)变流器作为其电控系统的核心控制部件,主要负责双馈感应电机的转速控制和发电机无功调节任务,但由于其具有非线性、强耦合等复杂特性,导致变流器的控制器设计十分困难。针对上述情况,提供一种DFIG转子侧变流器控制策略设计方法和控制参数优化方法,可通过调节转子侧电流大小实现双馈感应电机转速、无功的无静差调节;并以1.5 MW DFIG实际参数为模型,利用Simulink仿真软件对该控制策略进行仿真验证。研究结果表明:利用PI控制器可实现DFIG转速-转矩控制,发电机转子侧电流理论上可实现无静差跟踪。

基于负序电流抑制策略的DFIG系统

主要针对双馈异步风力发电机(Doubly Fed Induction Generator,DFIG)系统在不平衡电网电压条件下的网侧变换器进行研究。基于不平衡电网电压条件,发现在传统的控制策略下,网侧逆变器会向电网输送含有二倍频的有功和无功功率,且输送过来的二倍频会产生一定波动,这些波动可以直接影响三相电网的电压和电流值,从而使电网的电流值发生畸变。因此在不平衡电网电压的工况下,提出了DFIG系统的负序电流抑制策略,以抑制电网电压不平衡时产生的负序电流。

基于静止同步补偿器附加阻尼控制抑制次同步振荡的方法研究

本文设计了一种基于滤波器、增益和相移组合的阻尼控制器,能够为多种系统条件提供正阻尼,从而覆盖一个宽频带的振荡频率。时域仿真证明了该方案在不同振荡频率下抑制SSCI的有效性。