Analytical Model and Topology Optimization of Doubly-fed Induction Generator

As the core component of energy conversion for large wind turbines,the output performance of doubly-fed induction generators (DFIGs) plays a decisive role in the power quality of wind turbines.To realize the fast and accurate design optimization of DFIGs,this paper proposes a novel hybriddriven surrogate-assisted optimization method.It firstly establishes an accurate subdomain model of DFIGs to analytically predict performance indexes.Furthermore,taking the inexpensive analytical dataset produced by the subdomain model as the source domain and the expensive finite element analysis dataset as the target domain,a high-precision surrogate model is trained in a transfer learning way and used for the subsequent multi-objective optimization process.Based on this model,taking the total harmonic distortion of electromotive force,cogging torque,and iron loss as objectives,and the slot and inner/outer diameters as parameters for optimizing the topology,achieve a rapid and accurate electromagnetic design for DFIGs.Finally,experiments are carried out on a 3MW DFIG to validate the effectiveness of the proposed method.

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.

Coordinated Rotor-Side Control Strategy for Doubly-FedWind Turbine under Symmetrical and Asymmetrical Grid Faults

In order to solve the problems of rotor overvoltage,overcurrent and DC side voltage rise caused by grid voltage drops,a coordinated control strategy based on symmetrical and asymmetrical low voltage ride through of rotor side converter of the doubly-fed generator is proposed.When the power grid voltage drops symmetrically,the generator approximate equation under steady-state conditions is no longer applicable.Considering the dynamic process of stator current excitation,according to the change of stator flux and the depth of voltage drop,the system can dynamically provide reactive power support for parallel nodes and suppress the rise of DC side voltage and rotor over-current.When the grid voltage drops asymmetrically,the positive and negative sequence components are separated in the rotating coordinate system.The doubly fed generator model is established to suppress the rotor positive sequence current and negative sequence current respectively.At the same time,the output voltage limit of the converter is discussed,and the reference value is adjusted within the allowable output voltage range.In order to adapt to the occurrence of different types of power grid faults and complex operating conditions,a fast switching module of fault type detection and rotor control mode is designed to detect the type of power grid faults and voltage drop depth in real time and switch the rotor side control mode dynamically.Finally,the simulation model of the doubly fed wind turbine is constructed in Matlab/Simulink.The simulation results verify that the proposed control strategy can improve the low-voltage ride through performance of the system when dealing with the symmetrical and asymmetric voltage drop of the power grid and identify the power grid fault type and provide the correct control strategy.

Research on Control Strategy of Grid-connected Brushless Doubly-fed Wind Power System Based on Virtual Synchronous Generator Control

The brushless doubly-fed wind power system based on conventional power control strategies lacks ‘inertia’ and the ability to support grid,which leads to the decline of grid stability.Therefore,a control strategy of brushless doubly-fed reluctance generator(BDFRG) based on virtual synchronous generator(VSG) control is proposed to solve the problem in this paper.The output characteristics of BDFRG based on VSG are similar to a synchronous generator(SG),which can support the grid frequency and increase the system ‘inertia’.According to the mathematical model of BDFRG,the inner loop voltage source control of BDFRG is derived.In addition,the specific structure and parameter selection principle of outer loop VSG control are expounded.The voltage source control inner loop of BDFRG is combined with the VSG control outer loop to establish the overall architecture of BDFRG-VSG control strategy.Finally,the effectiveness and feasibility of the proposed strategy are verified in the simulation.

Parameter Deviation Effect Study of the Power Generation Unit on a Doubly-Fed Induction Machine-based Shipboard Propulsion System

To lower the difficulty of fault protection,a doubly-fed induction machine based shipboard propulsion system(DFIM-SPS)that is partially power decoupled is presented.In such an intrinsically safe SPS architecture,a synchronous generator(SG)is employed for power generation,and the accuracy of the parameters of power generation unit(PGU)plays an important role in SPS stable operation.In this paper,the PGU parameter deviations are studied to evaluate the effects on system performance.The models of salient-pole SG,type DC1A excitation system(EXS)and DFIM are illustrated first.Besides,the corresponding control scheme is explained.For the 16 important parameters of PGU,up to 40%of parameter deviations are applied to implement parameter sensitivity analysis.Then,simulation studies are carried out to evaluate the parameter deviation effects on system performance in detail.By defining three parameter deviation effect indicators(PDEIs),the effects on the PGU output variables,which are the terminal voltage and output active power,are studied.Moreover,the increasing rates of PDEIs with different degrees of parameter deviations for the key parameters are analyzed.Furthermore,the overall system performance is investigated for the two most influential PGU parameters.This paper provides some vital clues on SG and EXS parameter identification for DFIM-SPS.

Robust Position Observer for Sensorless Direct Voltage Control of Stand-Alone Ship Shaft Brushless Doubly-Fed Induction Generators

The aim of this paper is to investigate an adaptive sensorless direct voltage control(DVC)strategy for the stand-alone ship shaft brushless doubly-fed induction generators(BDFIGs).The proposed new rotor position observer using the space vector flux relations of BDFIG may achieve the desired voltage control of the power winding(PW)in terms of magnitude and frequency,without any speed/position sensors.The proposed algorithm does not require any additional observers for obtaining the generator speed.The proposed technique can directly achieve the desired DVC based on the estimated rotor position,which may reduce the overall system cost.The stability analysis of the proposed observer is investigated and confirmed with the concept of quadratic Lyapunov function and using the multi-model representation.In addition,the sensitivity analysis of the presented method is confirmed under different issues of parameter uncertainties.Comprehensive results from both simulation and experiments are realized with a prototype wound-rotor BDFIG,which demonstrate the capability and efficacy of the proposed sensorless DVC strategy with good transient behavior under different operating conditions.Furthermore,the analysis confirms the robustness of the proposed observer via the machine parameter changes.

A novel transient rotor current control scheme of a doubly-fed induction generator equipped with superconducting magnetic energy storage for voltage and frequency support

A novel transient rotor current control scheme is proposed in this paper for a doubly-fed induction generator（DFIG）equipped with a superconducting magnetic energy storage（SMES） device to enhance its transient voltage and frequency support capacity during grid faults. The SMES connected to the DC-link capacitor of the DFIG is controlled to regulate the transient dc-link voltage so that the whole capacity of the grid side converter（GSC） is dedicated to injecting reactive power to the grid for the transient voltage support. However, the rotor-side converter（RSC） has different control tasks for different periods of the grid fault. Firstly, for Period I, the RSC injects the demagnetizing current to ensure the controllability of the rotor voltage. Then, since the dc stator flux degenerates rapidly in Period II, the required demagnetizing current is low in Period II and the RSC uses the spare capacity to additionally generate the reactive（priority） and active current so that the transient voltage capability is corroborated and the DFIG also positively responds to the system frequency dynamic at the earliest time. Finally, a small amount of demagnetizing current is provided after the fault clearance. Most of the RSC capacity is used to inject the active current to further support the frequency recovery of the system. Simulations are carried out on a simple power system with a wind farm. Comparisons with other commonly used control methods are performed to validate the proposed control method.

Suppression of thrust fluctuation of doubly-fed linear motor

Because the mover and stator of a doubly-fed linear motor are both equipped with three-phase windings, the motor enables contactless energy to be transferred from the stator to the mover. Thus, when a doubly-fed linear motor is used in the linear driving system of trains, neither an overhead line nor a third rail is needed. However, the doubly- fed motor will lead to more severe thrust fluctuation than the unilateral motors. In order to overcome this shortcoming, the thrust fluctuation of the doubly-fed motor should be analyzed. In this paper, an expression for motor thrust is derived. It points out that the slot effect caused by the slotted structure is the main reason for the increase of thrust fluctuation. The method of adopting unequal pole pitch and unequal slot numbers per pole per phase of stator and mover, is proposed to weaken the slot effect when designing the motor slot parameters. Three different motor models are simulated by ANSOFT. The proposed method results in a motor thrust fluctuation coefficient of 8.4%, compared to 49.4% for the conventional method. Therefore, the motor＇s thrust fluctuation is effectively suppressed by the proposed method.

Performance Analysis of Nested-loop Secondary Linear Doubly-fed Machine Considering End Effects

Nested-loop secondary linear doubly-fed machine(NLS-LDFM) is a novel linear machine evolved from rotary brushless doubly-fed induction machine, which has a good application prospect in linear metro. In order to analyze the performance of NLS-LDFM, the mechanism and action rules of end effects are investigated in this paper. Firstly, the mechanism of static and dynamic end effects is analyzed in aspect of direct coupling, winding asymmetry and transient secondary current. Furthermore, based on the winding theory for short primary linear machines, the machine parameters are established qualitatively considering pulsating magnetic field of NLS-LDFM. Finally, the NLS-LDFM performance analysis is supplemented by the finite element algorithm(FEA) simulation and experiments under different operating conditions.

Brushless Doubly-Fed Induction Machine as a Variable Frequency Transformer

VFT （variable frequency transformer） has been recently used as art alternative to HVDC （high voltage direct current） to control power flow between asynchronous networks. VFT consumes less reactive power than a back-to-back HVDC system, provides faster initial transient recovery, and has better natural damping capability. VFT is simply a DFIM （doubly-fed induction machine） where the machine torque controls the power flow from stator to rotor and vice versa. The main disadvantage of this VFT is the slip rings and brushes required for the rotor circuit, especially in bulk power transmission. The BDFM （brushless doubly-fed machine） with nested cage rotor machine is proved to be a comparable alternative to conventional DFIM in many applications with the advantage that all windings being in the stator frame with fixed output terminals. In this paper, the BDFM is used as a BVFT （brushless variable frequency transformer）. A prototype machine is designed and simulated to verify the system validity.

Step-Loading Characteristics of Gas Engine Cogeneration System Using Doubly-Fed Induction Generator in Stand-Alone Operation

Application of a DFIG （doubly-fed induction generator）, which is one of adjustable speed generators, to a gas engine cogeneration system has been investigated. To operate during a blackout as an emergency power supply is one of important roles for the gas engine eogeneration system. In the case of conventional constant speed of synchronous generator, the amount of the allowed step load is limited to around 30% of the rated power. On the other hand, DFIG is expected to increase the amount of step load during the stand-alone operation. In this paper, it has been demonstrated that an increase in the gas engine speed resulted in an increase in the maximum amount of step load using experimental equipment with a real gas engine. It has been concluded that the proposed system can improve the performance of an emergency power supply at step-loading.

A novel control strategy of a variable-speed doubly-fed-induction-generator-based wind energy conversion system

This paper aims to address the issue of control of a variable-speed wind turbine based on doubly-fed induction generators. In this work,an effort is made to extract the maximum efficiency from a doubly-fed induction generator-based variable-speed wind turbine by controlling the rotor current. In the first step, a maximum power point tracking technique is used to extract the maximum power from theturbine. Then a stator-flux-oriented vector control strategy is employed to control the rotor-side current. Subsequently, a grid voltagevector-oriented control strategy is used to control the grid-side system of the grid-connected generator. Considering the nonlinearityand parameter uncertainty of the system, an active disturbance rejection controller with a sliding-mode-based extended-state observeris developed for the above-mentioned control strategies. Furthermore, the stability of the controller is tested and the performance of thecontroller is compared with the classical proportional-integral controller based on disturbance rejection, robustness and tracking capability in a highly non-linear wind speed variation scenario. Modelling, control and comparison are conducted in the MATLAB®/Simulink®environment. Finally, a real-time hardware set-up is presented using the dSPACE ds-1104 R&D processing board to validate the controlscheme. From the result of the experiments, it is seen that the proposed controller takes 10-15 control cycles to settle to its steady-statevalues, depending on the control loop, whereas the conventional proportional-integral controller takes 60-75 control cycles. As a result,the settling time for the proposed control scheme is shorter than that of the proportional-integral controller.

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.

并网型双馈风电机组动态稳定性仿真

介绍了变速恒频双馈风电机组的基本结构,建立了双馈风电机组在d-q坐标系下的动态数学模型。应用Matlab/Simulink搭建了系统仿真模块,并对并网型双馈风电机组的动态稳定性做了仿真试验研究,结果表明:在风电系统中利用变速技术后可以提高风能的利用率,改善电力系统稳定性;变速双馈风电机组具有低电压穿越功能。

集群双馈风电场的分次调压控制

基于集群风电场间无功电压耦合较强、通信条件较差的特点,提出以双馈风电机组为主、电容器/静止无功补偿器(SVC)为辅,不过分依赖风电机组与风电场自动电压控制系统间通信的集群双馈风电场分次调压控制策略。事先根据电网运行状况计算得到不需实时更新的双馈风电机组的PQ曲线,同步补偿无功功率以应对有功出力的变化,并仅在电网运行状况发生大幅变化时更新PQ曲线;在调节效果不够理想或PQ曲线未更新时,再利用电容器/SVC进行二次调压。通过对中国北方某地区集群风电场的仿真分析,验证了所述控制策略可有效减少风电机组连锁脱网的发生机会,并且其控制效果不受风电机组与风电场自动电压控制系统间通信中断的影响。

基于九开关变换器的双馈风电系统运行原理与控制研究

典型的双馈风力发电系统通常采用背靠背双脉宽调制变流器,但该结构共需12个IGBT,所需开关器件多。为减小双馈风电变流器的体积,提高变流器的功率密度,提出利用新型九开关变换器代替背靠背变流器的方案。该方案节省了3个IGBT,通过合适的调制控制策略仍能保证与原系统具有同样的控制性能。结合九开关变换器特有的正弦脉宽调制方式,分析九开关变换器在双馈风力发电系统中的应用特点,把九开关变换器等效为网侧等效变换器和机侧等效变换器,给出系统的控制策略,并通过Matlab/Simulink仿真及实验验证该新型拓扑方案的可行性及控制策略的正确性。理论分析与实验结果均表明,这种新型拓扑结构在双馈风力发电系统中具有较好的应用前景。

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。

基于发电-负荷需求系统评估方法的智能微网风险评估

智能微网融合了多种类型的分布式电源、储能及电力电子装置,这给其安全运行和风险评估带来了挑战。将智能微网看作一个发电-负荷需求系统,采用对发电及负荷分级模型的卷积求法对其进行风险评估。以欧洲典型低压微电网为例进行相关风险指标的计算,量化其风险水平,分析影响其风险水平的因素,并对如何改善该系统的风险水平提出了建议:影响系统风险水平的因素有发电容量及负荷水平;加入适量的柴油发电机组和储能设备可显著改善系统的风险水平。

九开关双馈风力发电系统的恒定开关频率电流滑模控制方法

较之基于背靠背变换器的双馈风电系统,基于九开关变换器（nine-switchconverter,NSC）的双馈风电系统所需器件更少,且可通过对NSC直流母线电压及电流容量在机侧与网侧的动态分配实现低电压穿越（low-voltage-ridethrough,LVRT）,并具备无功补偿能力。对九开关双馈风电系统的控制,目前均沿用了传统矢量控制与NSC特殊调制相结合的方法,控制过程复杂,性能提升有限。为简化控制过程并提升控制性能,该文提出适用于NSC双馈风电系统的恒定开关频率电流滑模控制,该方法直接在三相静止坐标系进行电流滑模闭环控制,生成NSC驱动所需占空比。该文对所提方案进行详细分析,并通过实验将其与传统比例积分控制进行对比。实验结果表明,所提滑模控制具有优于比例积分控制的动态响应和LVRT性能。

Enhanced control of DFIG-used back-to-back PWM VSC under unbalanced grid voltage conditions

This paper presents a unified positive-and negative-sequence dual-dq dynamic model of wind-turbine driven doubly-fed induction generator(DFIG) under unbalanced grid voltage conditions. Strategies for enhanced control and operation of a DFIG-used back-to-back(BTB) PWM voltage source converter(VSC) are proposed. The modified control design for the grid-side converter in the stationary αβ frames diminishes the amplitude of DC-link voltage ripples of twice the grid frequency,and the two proposed control targets for the rotor-side converter are alternatively achieved,which,as a result,improve the fault-ride through(FRT) capability of the DFIG based wind power generation systems during unbalanced network supply. A complete unbalanced control scheme with both grid-and rotor-side converters included is designed. Finally,simulation was carried out on a 1.5 MW wind-turbine driven DFIG system and the validity of the developed unified model and the feasibility of the proposed control strategies are all confirmed by the simulated results.