A Fault Tolerant Doubly Fed Induction Generator Wind Turbine Using a Parallel Grid Side Rectifier and Series Grid Side Converter

With steadily increasing wind turbine penetration, regulatory standards for grid interconnection have evolved to require that wind generation systems ride-through disturbances such as faults and support the grid during such events. Conventional modifications to the doubly fed induction generation (DFIG) architecture for providing ride-through result in compromised control of the turbine shaft and grid current during fault events. A DFIG architecture in which the grid side converter is connected in series as opposed to parallel with the grid connection has shown improved low voltage ride through but poor power processing capabilities. In this paper, a unified DFIG wind turbine architecture which employs a parallel grid side rectifier and series grid side converter is presented. The combination of these two converters enables unencumbered power processing and robust voltage disturbance ride through. A dynamic model and control structure for this architecture is developed. The operation of the system is illustrated using computer simulations.     

A Dynamic LVRT Solution for Doubly Fed Induction Generators

Doubly fed induction generators have become the most common type of wind turbine generators. However, this type of generator is susceptible to grid-side low voltage and short circuits due to existence of a power electronics converter on the rotor side. When a short circuit or voltage sag happens on the grid side, the rotor current of the generator tends to rise, which could cause damage to the rotor converter. Design and implementation of a series converter on the stator side is presented in this paper to limit the current rise in the rotor. This system includes an active ac/dc inverter, three series transformers, and a dc-bus capacitor. To lower the rating of the components and make the system viable for practical solutions, an exponential decaying sinusoidal voltage, instead of a pure sinusoidal voltage, is applied by the converter during short circuit.      

An Improved Control Strategy of Limiting the DC-Link Voltage Fluctuation for a Doubly Fed Induction Wind Generator

The paper presents to develop a new control strategy of limiting the dc-link voltage fluctuation for a back-to-back pulsewidth modulation converter in a doubly fed induction generator (DFIG) for wind turbine systems. The reasons of dc-link voltage fluctuation are analyzed. An improved control strategy with the instantaneous rotor power feedback is proposed to limit the fluctuation range of the dc-link voltage. An experimental rig is set up to valid the proposed strategy, and the dynamic performances of the DFIG are compared with the traditional control method under a constant grid voltage. Furthermore, the capabilities of keeping the dc-link voltage stable are also compared in the ride-through control of DFIG during a three-phase grid fault, by using a developed 2 MW DFIG wind power system model. Both the experimental and simulation results have shown that the proposed control strategy is more effective, and the fluctuation of the dc-link voltage may be successfully limited in a small range under a constant grid voltage and a non-serious grid voltage dip.     

Frequency Control Design for Offshore Wind Farm Grid With LCC-HVDC Link Connection

This paper considers the formal design for the grid frequency control in an offshore wind farm connected with line-commutated converter high voltage dc (HVDC) link. The control paradigm is based on using the grid frequency control to regulate the HVDC rectifier firing angle or dc-link current and hence control the power flow in the system. The dynamic behaviors of the system are verified by comparing the response from derived transfer functions and PSCAD simulations; hence the grid frequency controllers are designed. The control system performance has been validated by simulations of normal operation and fault regimes. The work provides a good basis for wider research investigation into wind farm operation.     

Two-stage single-phase grid-connected photovoltaic system with reduced complexity

This article presents a grid-connected photovoltaic (PV) system using the classical DC–DC buck converter, which is responsible for stepping down the resulting voltage from several series-connected panels. Besides, the structure provides high power factor operation by injecting a quasi-sinusoidal current into the grid, with near no displacement in relation to the line voltage at the point of common coupling among the PV system and the loads. A CSI employing thyristors is cascaded with the DC–DC stage so that AC voltage results. The inverter output voltage level is adjusted by using a low-frequency transformer, which also provides galvanic isolation. The proposed system is described as mathematical approach and design guidelines are presented, providing an overview of the topology. An experimental prototype is also implemented, and relevant results to validate the proposal are discussed.     

双馈风力发电系统低电压穿越的建模与控制

为提高双馈感应风力发电机组并网的运行稳定性,研究电网故障下双馈风电机组的运行特性,使发电机在电网故障时仍能保持不间断的运行。本文主要研究了基于Orowbar保护控制的低电压穿越运行的控制策略,通过Orowbar保护电路来限制电压跌落时转子回路的最大电流,并通过仿真分析了电压跌落的程度和旁路电阻的取值对控制的影响。仿真结果验证了在电网电压骤降下Crowbar保护电路的有效性,可实现双馈风力发电机不间断的运行。     

VSC-HVDC抑制并网PCC点电压波动浅析

大型风电场接入电网可能引起电压闪变和波动、电压暂降、电压偏差以及谐波等电能质量问题。通常对风电场进行无功补偿可以抑制系统电压波动,而轻型直流输电则可以起到无功补偿的作用,抑制并网PCC点电压波动、改善风电场的并网性能。     

三相故障时双馈风力发电机并网稳定性的改进

为使风电并网后的能够稳定运行,从双馈风电机组的自身特点为角度分析双馈风力发电并网对电网系统稳定性的影响,以Matlab/Simulink为平台搭建风电并网的模型,模拟仿真双馈风机在三相故障时加入两种无功补偿装置后的情况,比较得到最优的无功补偿装置以改进风电在并网时的稳定性。结果表明,静止同步补偿器在提升风电并网电力系统稳定性比SVC静态无功补偿器方面有更好的效果。     

Solid State Voltage and Frequency Controller for a Stand Alone Wind Power Generating System

This paper deals with the voltage and frequency controller of a wind turbine driven isolated asynchronous generator. The proposed voltage and frequency controller consists of an insulated gate bipolar junction transistor based voltage source converter along-with battery energy storage system at its dc link. The proposed controller is having bidirectional active and reactive powers flow capability by which it controls the system voltage and frequency with variation of consumer loads and the speed of the wind turbine. It is also having capability of harmonic elimination and load balancing. The proposed electro-mechanical system along with its controller is modeled and simulated in MATLAB using Simulink and power system block-set toolboxes. Performance of the proposed controller is presented to demonstrate voltage and frequency control of a wind turbine driven isolated asynchronous generator along with harmonic elimination and load balancing.     

Control of the Reactive Power Supplied by a WECS Based on an Induction Generator Fed by a Matrix Converter

In this paper, a new control system to regulate the reactive power supplied by a variable-speed wind energy conversion system (WECS), based on an induction generator fed by a matrix converter (MC), is presented. The control system discussed in this paper is based on an input current observer, implemented using an estimation of the modulation matrix, and a nonlinear control loop that regulates the displacement angle at the MC input. The reactive power capability of the proposed system is also investigated. The work presented in this paper demonstrates that, for the proposed WECS, the maximum reactive power supplied to the grid is about 40% of the nominal value. Experimental results obtained from an experimental prototype are presented in this paper. The performance of the system using a wind turbine emulator and typical wind profiles is discussed in this paper.      

电网故障时双馈风力发电机励磁改进控制策略

双馈风力发电机（DFIG）在风能捕获过程中,传统的定子磁链定向控制都假定电网上的电压恒定不变,定子磁链也恒定不变,没有考虑定子励磁电流的动态响应过程,这个误差在电网故障时会对系统产生极不稳定的作用。为此应考虑该响应过程,对控制檳型进行修正,提出一种应对电网电压跌落时的励磁改迸策略,即在建立转子侧变换器控制模型时,把反映励磁电流过渡过程的定子电压变化量考虑进来。通过推导定子磁场定向下的转子电压方程,搭建定子磁链定向矢量控制模型,经过仿真结果比较,说明在加入励磁补偿电压项后,风电系统在应对电网电压故障时的电流控制性能得到明显改善,有效提升了系统的低电压穿越能力。     

一种基于自适应峰值电流检测的模式切换电路北大核心

在降压转换器中，为了在不同的负载情况下获得高效率，常采用的方法是在重载时使用脉冲宽度调制(PWM),在轻载时使用脉冲频率调制(PFM),因此需要模式切换信号去控制整个降压转换器的工作状态，同时模式切换信号也可以用于自适应改变功率级电路中的功率管栅宽，减小功率管的栅极电容，提高整体电路的效率。文章设计了一个自适应峰值电流模式切换电路，用于产生模式切换信号，其原理是监控峰值电流的变化，产生峰值电压，将峰值电压与参考电压进行比较，得到模式切换信号，以决定降压转换器是采用PFM模式还是PWM模式。仿真结果表明，在负载电流0.5～500 mA范围内，该电路可以在两种调制模式之间平稳切换，其峰值效率可提升到94%以上。     

双馈风电机组低电压穿越问题研究

本文介绍了储能Crowbar和电网电压跌落时的无功需求,在此基础上本文提出了一种新的网侧变流器故障时无功控制策略,仿真验证了控制策略的有效性。接着提出了双馈电机风力发电系统低电压穿越的控制逻辑,在双馈电机风力发电系统仿真平台上运用Matlab/simulink,采用储能Crowbar和故障时无功控制策略以及叶尖速比控制等策略实现了双馈电机风力发电系统的低电压穿越。     

An Integrated Current Source Inverter With Reactive and Harmonic Power Compensators

An integrated current source converter system is presented based on an assembly of a thyristor-based current source inverter (CSI) in parallel with an insulated-gate-bipolar-transistor-based voltage source inverter (VSI) along with passive capacitors for high-power induction motor drive applications. The proposed configuration installs the VSI and the capacitor in such a way that both provide reactive power for generating the leading power factor required to accomplish natural commutations of the CSI. Based on the collaborative operation of the VSI and the parallel capacitor, the proposed system can be designed with a compromise between the VSI power capacity and the capacitor size. In addition, the VSI compensates harmonic current components from the thyristor-based CSI, while the capacitor filters out the voltage spikes during commutation of the thyristors. As a result, sinusoidal motor currents with improved harmonic spectrum can be drawn from this system. The proposed system utilizes the high-power capability of the thyristor-based CSI to supply high real power, while the VSI with easy controllability regulates the induction motor. Theoretical analyses based on mathematical modeling are presented in detail for the relationship between the inverter rating and the capacitor size, design considerations of the capacitor size, and the loss performances.      

电网电压骤降情况下双馈风力发电机低电压穿越技术分析

根据电网规程要求．在电网故障情况下风力发电机组应能保持与电网的连接并向电网不问断供电。文中建立了在电网故障情况下的双馈感应发电机（DFIC）的数学模型,然后分析了实现低电雁穿越（LVRT）的各种改进控制策略和技术方法。     

Extending the Life of Gear Box in Wind Generators by Smoothing Transient Torque With STATCOM

Gearboxes for wind turbines must ensure high reliability over a period of 20 years, withstanding cumulative and transient loads. One main challenge to this is represented by electromagnetic torque transients caused by grid faults and disturbances, which will result in significant stresses and fatigue of the gearbox. Possibilities for limiting the torque transients in fixed-speed wind generators have not been previously reported. This paper presents a technique by which the transient torques during recovery after a grid fault can be smoothed in a wind farm with induction generators directly connected to the grid. A model-based control technique using the quasi-stationary equivalent circuit of the system is suggested for controlling the torque with a static synchronous compensator (STATCOM). The basis of the approach consists of controlling the induction generator terminal voltage by the injection/absorption of reactive current using the STATCOM. By controlling the terminal voltage as a function of the generator speed during the recovery process, the electromagnetic torque of the generator is indirectly controlled, in order to reduce the drive train mechanical stresses caused by the characteristics of the induction machine when decelerating through the maximum torque region. The control concept is shown by time-domain simulations, where the smoothing effect of the proposed technique on a wind turbine is seen during the recovery after a three-phase-to-ground-fault condition. The influence of the shaft stiffness in a multimass drive train model is discussed, and the performance of the control concept in the case of parallel connection of several turbines is investigated to discuss the applicability in a wind farm.      

A feedback loop regulated bootstrap driver circuit with improved drive capability for high voltage buck DC–DC converter

A novel bootstrap driver circuit applied to high voltage buck DC–DC converter is proposed. The gate driver voltage of the high side switch is regulated by a feedback loop to obtain accurate and stable bootstrapped voltage. The charging current of bootstrap capacitor is provided by the input power of the DC–DC converter directly instead of internal low voltage power source, so larger driver capability of the proposed circuit can be achieved. The bootstrap driver circuit starts to charge the bootstrap capacitor before the switch node SW drop to zero voltage at high-side switch off-time. Thus inadequate bootstrap voltage is avoided. The proposed circuit has been implemented in a high voltage buck DC–DC converter with 0.6 µm 40 V CDMOS process. The experimental results show that the bootstrap driver circuit provides 5 V stable bootstrap voltage with higher drive capability to drive high side switch. The proposed circuit is suitable for high voltage, large current buck DC–DC converter.     

Flexible control of small wind turbines with grid failure detection operating in stand-alone and grid-connected mode

This paper presents the development and test of a flexible control strategy for an 11-kW wind turbine with a back-to-back power converter capable of working in both stand-alone and grid-connection mode. The stand-alone control is featured with a complex output voltage controller capable of handling nonlinear load and excess or deficit of generated power. Grid-connection mode with current control is also enabled for the case of isolated local grid involving other dispersed power generators such as other wind turbines or diesel generators. A novel automatic mode switch method based on a phase-locked loop controller is developed in order to detect the grid failure or recovery and switch the operation mode accordingly. A flexible digital signal processor (DSP) system that allows user-friendly code development and online tuning is used to implement and test the different control strategies. The back-to-back power conversion configuration is chosen where the generator converter uses a built-in standard flux vector control to control the speed of the turbine shaft while the grid-side converter uses a standard pulse-width modulation active rectifier control strategy implemented in a DSP controller. The design of the longitudinal conversion loss filter and of the involved PI-controllers are described in detail. Test results show the proposed methods works properly.     

Feedback Linearization Of Direct-Drive Synchronous Wind-Turbines Via a Sliding Mode Approach

As the grid code specifies, wind turbines have to remain connected to the grid at voltage levels far below the nominal values. The improvement of wind turbine performance under such conditions has become a problem of general concern. However, this performance usually relies on conventional linear controllers that operate at network faults far off the nominal point for which they were designed. As a consequence, wind turbines should operate with increasing converter currents, which may result in converter damage. This paper proposes a nonlinear controller for converter-based wind turbines that ensures that the currents are maintained within the design limits. The controller is based on feedback linearization theory and is applied to the system through a sliding mode approach. This controller is robust against system perturbations and uncertainties, and overcomes the usually complex implementation that is associated with feedback linearization controllers.     

Vector Control of Front-End Converters for Variable-Speed Wind–Diesel Systems

This paper presents a novel power-balance control method for a wind–diesel generation feeding an isolated grid. The system is based on a variable-speed wind energy conversion system (WECS) connected to an ac load using a power converter. An energy storage system (ESS), connected to the ac load using an additional converter, is used to balance the power generated by the WECS with the load. In this paper, the vector control systems for both interfacing power converters are discussed; the control uses the WECS converter to regulate the ac load voltage and the ESS converter to regulate the power flow to achieve a power balance. A small signal model is used to design the control systems. Finally, the proposed control is implemented in a 2-kW experimental prototype and the experimental results are fully analyzed and discussed in the paper.