Reinforced Control and Operation of DFIG-Based Wind-Power-Generation System Under Unbalanced Grid Voltage Conditions

This paper proposes an enhanced control and operation of a doubly fed induction generator (DFIG) based wind power generation system under unbalanced grid voltage conditions. System behaviors of grid-side and rotor-side converters (GSCs and RSCs) are described. The RSC is controlled to eliminate the electromagnetic torque oscillation at double the grid frequency. Meanwhile, three selective control targets for the GSC, i.e., reducing the pulsations in the total active or reactive power, or unbalanced current outputs from the overall system, are identified and analyzed during voltage imbalance. A new current-control scheme is presented for the GSC and RSC without involving the decomposing of positive and negative sequence currents. The controller consists of a proportional (P) regulator and a resonant (R) one tuned at the grid frequency, which is implemented in the stator stationary reference frame. Finally, simulation studies are carried out on a 1.5-MW wind-turbine-driven DFIG system. The validity of the presented current controller and the feasibility of the proposed control targets are all confirmed by the simulated results.      

Direct power control of wind‐turbine‐driven DFIG during transient grid voltage unbalance

In this paper, a direct power control (DPC) of a wind‐turbine‐driven doubly fed induction generators (DFIGs) under unbalanced network voltage conditions is studied. Variations of the stator output active, reactive and electromagnetic powers are fully deduced in the presence of negative sequence supply voltage. The rotor side converter is controlled on the basis of DPC to eliminate the electromagnetic torque oscillations at double supply frequency under unbalanced stator supply. The rotor voltage references estimation requires only simple calculations without any integral operation. The proposed control scheme removes rotor current regulators and the decomposition processing of positive and negative sequence rotor currents. The performance of the proposed and conventional DPC schemes is compared under the same operating conditions. Simulation results using Matlab/Simulink are carried out for a 1.5 MW DFIG wind generation system to show the validity of the proposed scheme during unbalanced voltage supply. Copyright © 2013 John Wiley & Sons, Ltd.     

A versatile method for computation of power pulsations in DFIG under grid imperfections

In the context of a Doubly Fed Induction Generator (DFIG) connected to the utility grid under unbalanced voltage conditions, the controller design needs to ensure additional challenges such as restricting the Total Harmonic Distortion (THD) in grid current, minimizing the pulsations in generated power, torque, dc link voltage etc. apart from facilitating the generator power control. Thus the schemes for generating reference currents for rotor converters need to incorporate a measure of power pulsations to what is required for steady state power flow control. This paper proposes a versatile scheme for computing the power pulsations in a DFIG connected to grid under unbalance voltage conditions. The active and reactive power oscillations are computed in a simple and straight forward manner using the measured stator voltage and currents in the positive d-q frame without using flux estimation. The scheme is free from flux integration or differentiation, rotor position computation and independent of machine parameters. Further, the worst case error in computation is bound within 3% considering 30% voltage dip, 7% of harmonics, ±10°phase jump or ±10% dc offset in the grid voltage. The effectiveness of the scheme is validated through PSCAD/EMTDC simulations and experimental results for a 2.3 kW DFIG test setup.     

不平衡电压下双馈变速风电机组的控制策略

提出了不平衡电网电压下双馈发电机的控制策略,并建立了双馈发电机在正、反旋转坐标系下的数学模型。在此基础上推导和分析了电网电压不平衡条件下双馈发电机输出的瞬时有功、无功功率的组成。提出了4种可供选择的不平衡电压控制方案,并给出了不同控制目标下转子的正、负序电流目标值的计算原则。通过MATLAB/SIMULINK仿真验证了控制方案的有效性。     

DFIG-DVR系统在不平衡电网电压下的运行特性

针对双馈风电机组(DFIG)在电网电压不平衡时,二倍频扰动分量会造成定转子过电流、功率脉动、转矩脉动等一系列电气和机械的问题,提出了新型DFIG-DVR系统,即串联DVR始终维持DFIG定子端电压恒定,从根源上隔离电网不平衡故障的影响,从而在整个故障运行过程中,DFIG仍可以实现转子侧变换器功率解耦控制和网侧变换器维持直流电压恒定的目标。采用PSCAD/EMTDC建立DFIG-DVR系统模型,对比分析了电网电压不平衡时DVR的不投切与投切对DFIG的影响。结果表明,在电网电压不平衡条件下,所提控制方案可以实现DFIG的平衡运行。     

Compensation of network voltage unbalance using doubly fed induction generator-based wind farms

A control strategy for compensating AC network voltage unbalance using doubly fed induction generator (DFIG)-based wind farms is presented. A complete DFIG dynamic model containing both the rotor and grid side converters is used to accurately describe the average and ripple components of active/reactive power, electromagnetic torque and DC bus voltage, under unbalanced conditions. The principle of using DFIG systems to compensate grid voltage unbalance by injecting negative sequence current into the AC system is described. The injected negative sequence current can be provided by either the grid side or the rotor side converters. Various methods for coordinating these two converters are discussed and their respective impacts on power and torque oscillations are described. The validity of the proposed control strategy is demonstrated by simulations on a 30 MW DFIG-based wind farm using Matlab/Simulink during 2 and 4% voltage unbalances. The proposed compensation strategy can not only ensure reliable operation of the wind generators by restricting torque, DC link voltage and power oscillations, but also enable DFIG-based wind farms to contribute to rebalancing the connected network.     

VSC-based direct torque and reactive power control of doubly fed induction generator

This paper proposes a novel direct torque and reactive power control (DTC) for grid-connected doubly fed induction generators (DFIGs) in the wind power generation applications. The proposed DTC strategy employs a variable structure control (VSC) scheme to calculate the required rotor control voltage directly and to eliminate the instantaneous errors of active and reactive powers without involving any synchronous coordinate transformations, which essentially enhances the transient performance. Constant switching frequency is achieved as well by using space vector modulation (SVM), which eases the designs of power converter and ac harmonic filters. Simulated results on a 2 MW grid-connected DFIG system are presented and compared with those of the classic voltage-oriented vector control (VC) and traditional look-up-table (LUT) direct power control (DPC). The proposed VSC DTC maintains enhanced transient performance similar to the LUT DPC and keeps the steady-state harmonic spectra at the identical level as the VC strategy when the network is strictly balanced. Besides, the VSC DTC strategy is capable of fully eliminating the double-frequency pulsations in both the electromagnetic torque and the stator reactive power during network voltage unbalance.     

Dynamic modelling and control of fully rated converter wind turbines

Today, many countries are integrating large amount of wind energy into the grid and many more are expected to follow. The expected increase of wind energy integration is therefore a concern particularly to transmission grid operators. Based on the past experience, some of the relevant concerns when connecting significant amount of wind energy into the existing grid are: fault ride through requirement to keep wind turbines on the grid during faults and wind turbines have to provide ancillary services like voltage and frequency control with particular regard to island operation.While there are still a number of wind turbines based on fixed speed induction generators (FSIG) currently running, majority of wind turbines that are planned to be erected are of variable speed configurations. The reason for this is that FSIG are not capable of addressing the concern mentioned above. Thus, existing researches in wind turbines are now widely directed into variable speed configurations. This is because apart from optimum energy capture and reduction of mechanical stress, preference of these types is also due to the fact that it can support the network such as its reactive power and frequency regulation. Variable wind turbines are doubly fed induction generator wind turbines and full converters wind turbines which are based on synchronous or induction generators.This paper describes the steady state and dynamic models and control strategies of wind turbine generators. The dynamic models are presented in the dq frame of reference. Different control strategies in the generator side converter and in the grid side converter for fault ride through requirement and active power/frequency and reactive/voltage control are presented for variable speed wind turbines.     

Flicker study on variable speed wind turbines with doubly fed induction generators

Grid connected wind turbines may produce flicker during continuous operation. This paper presents a simulation model of a MW-level variable speed wind turbine with a doubly fed induction generator developed in the simulation tool of PSCAD/EMTDC. Flicker emission of variable speed wind turbines with doubly fed induction generators is investigated during continuous operation, and the dependence of flicker emission on mean wind speed, wind turbulence intensity, short circuit capacity of grid and grid impedance angle are analyzed. A comparison is done with the fixed speed wind turbine, which leads to a conclusion that the factors mentioned above have different influences on flicker emission compared with that in the case of the fixed speed wind turbine. Flicker mitigation is realized by output reactive power control of the variable speed wind turbine with doubly fed induction generator. Simulation results show the wind turbine output reactive power control provides an effective means for flicker mitigation regardless of mean wind speed, turbulence intensity and short circuit capacity ratio.     

Doubly-fed induction generator drive based WECS using fuzzy logic controller

The purpose of this paper is to improve the control performance of the variable speed, constant frequency doubly-fed induction generator in the wind turbine generation system by using fuzzy logic controllers. The control of the rotor-side converter is realized by stator flux oriented control, whereas the control of the grid-side converter is performed by a control strategy based on grid voltage orientation to maintain the DC-link voltage stability. An intelligent fuzzy inference system is proposed as an alternative of the conventional proportional and integral (PI) controller to overcome any disturbance, such as fast wind speed variation, short grid voltage fault, parameter variations and so on. Five fuzzy logic controllers are used in the rotor side converter (RSC) for maximum power point tracking (MPPT) algorithm, active and reactive power control loops, and another two fuzzy logic controllers for direct and quadratic rotor currents components control loops. The performances have been tested on 1.5 MW doubly-fed induction generator (DFIG) in a Matlab/Simulink software environment.     

Development of a novel wind turbine simulator for wind energy conversion systems using an inverter-controlled induction motor

A wind turbine simulator for wind energy conversion systems has been developed with a view to design, evaluate, and test of actual wind turbine drive trains including generators, transmissions, power-electronic converters and controllers. The simulator consists of a 10-hp induction motor (IM) which drives a generator and is driven by a 10-kW variable speed drive inverter and real-time control software. In this simulator, a microcontroller, a PC interfaced to LAB Windows I/O board, and an IGBT inverter-controlled induction motor are used instead of a real wind turbine to supply shaft torque. A control program based on C language is developed that obtains wind profiles and, by using turbine characteristics and rotation speed of IM, calculates the theoretical shaft torque of a real wind turbine. Comparing with this torque value, the shaft torque of the IM is regulated accordingly by controlling stator current demand and frequency demand of the inverter. In this way, the inverter driven induction motor acts like a real wind turbine to the energy conversion system. The drive is controlled using the measured shaft torque directly, instead of estimating it as conventional drives do. The experimental results of the proposed simulator show that this scheme is viable and accurate. This paper reports the operating principles, theoretical analyses, and test results of this wind turbine simulator.     

A complete modeling and simulation of DFIG based wind turbine system using fuzzy logic control

The current paper talks about the variable speed wind turbine generation system (WTGS). So, the WTGS is equipped with a doubly-fed induction generator (DFIG) and two bidirectional converters in the rotor open circuit. A vector control (VC) of the rotor side converter (RSC) offers independent regulation of the stator active and reactive power and the optimal rotational speed tracking in the power maximization operating mode. A VC scheme for the grid-side converter (GSC) allows an independent regulation of the active and reactive power to exchange with the grid and sinusoidal supply currents and keeps the DC-link voltage constant. A fuzzy inference system (FIS) is adopted as an alternative of the conventional proportional and integral (PI) controller to reject some uncertainties or disturbance. The performances have been verified using the Matlab/Simulink software.     

A method of tracking the peak power points for a variable speed wind energy conversion system

In this paper, a method of tracking the peak power in a wind energy conversion system (WECS) is proposed, which is independent of the turbine parameters and air density. The algorithm searches for the peak power by varying the speed in the desired direction. The generator is operated in the speed control mode with the speed reference being dynamically modified in accordance with the magnitude and direction of change of active power. The peak power points in the P-/spl omega/ curve correspond to dP/d/spl omega/=0. This fact is made use of in the optimum point search algorithm. The generator considered is a wound rotor induction machine whose stator is connected directly to the grid and the rotor is fed through back-to-back pulse-width-modulation (PWM) converters. Stator flux-oriented vector control is applied to control the active and reactive current loops independently. The turbine characteristics are generated by a DC motor fed from a commercial DC drive. All of the control loops are executed by a single-chip digital signal processor (DSP) controller TMS320F240. Experimental results show that the performance of the control algorithm compares well with the conventional torque control method.     

Coordinated control for unbalanced operation of stand‐alone doubly fed induction generator

This paper proposes a coordinated control scheme of a stand‐alone doubly fed induction generator (DFIG)‐based wind energy conversion system to improve the operation performance under unbalanced load conditions. To provide excellent voltage profile for load, a direct stator flux control scheme based on auto‐disturbance rejection control (ADRC) is applied, and less current sensors are required. Due to the virtues of ADRC, the controller has good disturbance rejection capability and is robust to parameter variation. In the case of unbalanced loads, the electromagnetic torque pulsations at double synchronous frequency will exist. To eliminate the undesired effect, the stator‐side converter (SSC) is used to provide the negative sequence current components for the unbalanced load. Usually, proportional integral controllers in a synchronous reference frame are used to control SSC. To simplify the algorithm, an improved proportional resonant (PR) control is proposed and used in the current loop without involving positive and negative sequence decomposition. The improved PR provides more degree of freedom which could be used to improve the performance. The effectiveness of the proposed control scheme has been validated by the simulation and experimental results. Copyright © 2012 John Wiley & Sons, Ltd.     

Comparison of the response of doubly fed and fixed-speed induction generator wind turbines to changes in network frequency

Synchronous and fixed-speed induction generators release the kinetic energy of their rotating mass when the power system frequency is reduced. In the case of doubly fed induction generator (DFIG)-based wind turbines, their control system operates to apply a restraining torque to the rotor according to a predetermined curve with respect to the rotor speed. This control system is not based on the power system frequency and there is negligible contribution to the inertia of the power system. A DFIG control system was modified to introduce inertia response to the DFIG wind turbine. Simulations were used to show that with the proposed control system, the DFIG wind turbine can supply considerably greater kinetic energy than a fixed-speed wind turbine.     

Performance of PI controller for control of active and reactive power in DFIG operating in a grid-connected variable speed wind energy conversion system

Due to several factors, wind energy becomes an essential type of electricity generation. The share of this type of energy in the network is becoming increasingly important. The objective of this work is to present the modeling and control strategy of a grid connected wind power generation scheme using a doubly fed induction generator (DFIG) driven by the rotor. This paper is to present the complete modeling and simulation of a wind turbine driven DFIG in the second mode of operating (the wind turbine pitch control is deactivated). It will introduce the vector control, which makes it possible to control independently the active and reactive power exchanged between the stator of the generator and the grid, based on vector control concept (with stator flux or voltage orientation) with classical PI controllers. Various simulation tests are conducted to observe the system behavior and evaluate the performance of the control for some optimization criteria (energy efficiency and the robustness of the control). It is also interesting to play on the quality of electric power by controlling the reactive power exchanged with the grid, which will facilitate making a local correction of power factor.     

DFIG sliding mode control fed by back-to-back PWM converter with DC-link voltage control for variable speed wind turbine

This paper proposes an indirect power control of doubly fed induction generator (DFIG) with the rotor connected to the electric grid through a back-to-back pulse width modulation (PWM) converter for variable speed wind power generation. Appropriate state space model of the DFIG is deduced. An original control strategy based on a variable structure control theory, also called sliding mode control, is applied to achieve the control of the active and reactive power exchanged between the stator of the DFIG and the grid. A proportional-integral-(PI) controller is used to keep the DC-link voltage constant for a back-to-back PWM converter. Simulations are conducted for validation of the digital controller operation using Matlab/Simulink software.     

Low voltage ride-through strategies for doubly fed induction machine pumped storage system under grid faults

Pumped storage units bring stability to the electrical power system, so they must remain connected to the grid even during grid faults. In this paper, the authors propose efficient and simple solutions for a doubly fed induction machine pumped storage (DFIMPS) system during grid faults. In case of balanced grid faults, a control reconfiguration strategy is introduced and a hardware solution is applied in the case of unbalanced grid faults. The reconfiguration strategy consists of a commutation between different control strategies; when a balanced grid voltage fault occurs during pumping mode, the control algorithm switches to the synchronization one but based on the new grid conditions. So the proposed reconfiguration method reduces the negative impacts of grid fault occurrence on the DFIMPS system by cancelling rotor and stator over-currents and decreasing the electromagnetic torque and stator power oscillations. Simulation results carried out on a 4 kW DFIMPS system illustrate the effectiveness of the proposed approach.     

Influence of Tower Shadow and Wind Turbulence on the Performance of Power System Stabilizers for DFIG-Based Wind Farms

The aim of the paper is to demonstrate the way in which mechanical power variations, due to tower shadow and wind turbulence, influence control performance of power system stabilizer (PSS) loops for doubly-fed induction generators (DFIGs). The PSS auxiliary loops are applied on a specific DFIG control scheme, the flux magnitude and angle controller (FMAC). However, since the PSS signal is applied at the output of the basic controller, the PSS performance characteristics displayed are deemed typical for DFIG control schemes in general. The relative capabilities of PSS controllers based on stator power, rotor speed, and network frequency, when the DFIG turbine is subjected to aerodynamic torque variations, are investigated via simulation studies. A two-generator aggregate model of a wind farm is introduced, which enables the influence of tower shadow and wind turbulence on both an individual turbine and on the overall wind farm itself to be assessed.     

基于转矩极限的改进风电机组虚拟惯量控制策略

为解决大规模风电并网带来的系统频率稳定性降低问题,风电机组通过虚拟惯量控制可为系统提供短期频率支撑,然而惯性响应期间风电机组转速收敛缓慢,导致一部分转子动能被无故浪费;转速恢复阶段的有功突变易造成频率二次跌落。为此,提出基于转矩极限的改进风电机组虚拟惯量控制策略,实现在释放较少动能的前提下提供与传统策略相同的频率响应服务;并在频率步入准稳态时,借助时变功率函数开始转速恢复,实现转速快速恢复的同时缓解二次频率跌落。基于EMTP-RV仿真软件搭建包含风电场的电力系统模型,验证了所提策略的有效性。