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.

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.

A Novel Grid Synchronization Control of Doubly-fed Induction Generator

The dynamic performance of doubly-fed induction generator(DFIG) before and after connection is analyzed based on corresponding mathematical models and transfer functions in decoupled vector control.The parameter tuning methods of rotor current regulator before and after connection are given.To reach same dynamic performance the parameters should take different values and be switched before and after connection.However on one hand the closing moment of stator contactor is difficult to get as the feedback signal is usually twenty millisecond delay or so.The delay in parameter switching will affect rotor current and torque dynamics during the delayed period after connection. On the other hand parameter switching is troublesome.Hence a synchronization control strategy without parameter switching is proposed and analyzed in detail,which has linear rising exciting current to avoid current overshooting. The dynamic performance of the proposed strategy is analyzed in frequency domain and implemented on a DFIG experimental platform subsequently.The proposed synchronization strategy is validated by experimental results.

Reactive Current Allocation and Control Strategies Improvement of Low Voltage Ride Though for Doubly Fed Induction Wind Turbine Generation System

为满足风电机组低电压穿越（low voltage ride through,LVRT）的测试要求及其电气模型一致性评估需要,提出考虑向电网注入无功电流的双馈风电机组LVRT的控制策略。在阐述风电机组LVRT测试要求及控制原理的基础上,推导双馈发电机（doubly fed induction generator,DFIG）定子侧及网侧变流器输出无功电流极限表达式,研究电网电压跌落深度和发电机总输出有功功率对其无功电流极限值的影响规律,进而提出DFIG在LVRT期间的无功电流分配算法和改进的有功、无功功率控制策略。最后,以某实际2 MW双馈风电机组为例,分别对风速为5和12 m/s、电网电压对称跌落至20%和50%工况下的LVRT运行性能进行仿真比较和样机测试。与传统LVRT控制方法的对比表明,所提改进控制策略能更好地满足风电机组LVRT的测试要求。样机测试结果进一步证明了改进控制策略和仿真模型的有效性。

Steady State Analysis of a Doubly Fed Induction Generator

In this paper, we present the steady state analysis of a double-fed induction generator (DFIG) adopted for wind power generation. The three-phase induction machine connected to the network, to work as a generator for wind farms, is excited on the rotor circuit by a slip-frequency current injected to the rotor, from an exciter mounted on the same shaft of the machine. The resulting rotating magnetic field rotates at synchronous speed;as such the generated power has a constant frequency independent of the shaft speed. Effects of the excitation voltage magnitude and phase angle on the active and reactive power are studied, when the machine runs at constant speed. It has been shown that by controlling the excitation voltage magnitude and phase angle would control the mode of operation of the machine;motor mode or generator mode. Furthermore, the effects of the shaft speed on the active and reactive power at constant excitation voltage magnitude and constant phase angle are also investigated.

Difference between grid connections of large-scale wind power and conventional synchronous generation

In China, regions with abundant wind energy resources are generally located at the end of power grids. The power grid architecture in these regions is typically not sufficiently strong, and the energy structure is relatively simple. Thus, connecting large-capacity wind power units complicates the peak load regulation and stable operation of the power grids in these regions. Most wind turbines use power electronic converter technology, which affects the safety and stability of the power grid differently compared with conventional synchronous generators. Furthermore, fluctuations in wind power cause fluctuations in the output of wind farms, making it difficult to create and implement suitable power generation plans for wind farms. The generation technology and grid connection scheme for wind power and conventional thermal power generation differ considerably. Moreover, the active and reactive power control abilities of wind turbines are weaker than those of thermal power units, necessitating additional equipment to control wind turbines. Hence, to address the aforementioned issues with large-scale wind power generation, this study analyzes the differences between the grid connection and collection strategies for wind power bases and thermal power plants. Based on this analysis, the differences in the power control modes of wind power and thermal power are further investigated. Finally, the stability of different control modes is analyzed through simulation. The findings can be beneficial for the planning and development of large-scale wind power generation farms.

Behavior an Induction Generator without and with a Voltage Regulator

During the isolated use of a wind system, the output voltage of the self-excited induction generator depends on the variation characteristic of its parameters: the excitation condensers, the drive speed and the load. Therefore, the regulation of the tension appears to be of great interest. We focused on the use of an analogical regulator of tension, with the aim of controlling the tension at the exit of the self-excited induction generator. So we modelled, implanted and simulated a wind system (Self-excited induction generator, converters (AC/DC, DC/DC) and load it) in the Orcad/Pspice environment. In the first time the behaviour of the asynchronous generator was analyzed when the load, the excitation capacitor and the drive speed vary in the absence of any form of regulation. This analysis was conducted with the aim of defining the limits of the machine exploitation. In the second time the functioning mode is controlled by an analogical control of tension. The results of simulation show the good performances of the system during the application of the proposed voltage regulator.

New energy power generation-wind power generation

This paper introduced the status quo of wind power and wind power generation technology. Focusing on the introduction of wind power generating system ibrational self-consistent field（VSCF）, program implementation included Alternating Current （AC）-Direct Current （DC）-AC conversion system, magnetic field modulation generator system, doubly-fed generator system etc. Among these, doubly-fed generator system is the trend. Where to build the wind farm is very important, so a perfect site is needed. Wind power generation will have a bright future. As long as the wind power can be linked to the grid in large scale.

Analysis on Effect of Parameters of Different Wind Generator on Power Grid Transient Stability

To analyze the factors which affecting transient stability of power system, the dynamic model of doubly-fed induction generator and direct-drive PM synchronous generator has been built using PSCAD. Impact of different wind farm integration on grid typically in China has been presented. The influence of the variations of transient reactance, negative sequence reactance and rotary inertia on critical clearing time of power system transient stability is analyzed by time-domain simulation. Mixture operation of DFIG and PMSG to optimize the stability of system has been analyzed firstly. The digital simulation results show that doubly-fed induction wind turbines is a better choice to meet the requirement of system instability due to large wind farm integration in comparison with direct-drive PM synchronous wind turbines. With a rather large rotary inertia, the proper ratio of direct-drive PM synchronous wind turbines used in wind farm could be comprehensive planning by optimized the stability of system. Analysis of this paper should be provided as academic reference for improving design of wind farm system.

Comparative Performance of Fixed-Speed and Variable-Speed Wind Turbine Generator Systems

In the early development of the wind energy, the majority of the wind turbines have been operated at constant speed. Subsequently, the number of variable-speed wind turbines installed in wind farms has increased. In this paper, a comparative performance of fixed and variable speed wind generators with Pitch angle control has been presented. The first is based on a squirrel cage Induction Generator （IG） of 315 kW rated power, connected directly to the grid. The second incorporated a Permanent Magnet Synchronous Generator （PMSG） of 750 kW rated power. The performances of each studied wind generator are evaluated by simulation works and variable speed operation is highlighted as preferred mode of operation.

A Fuzzy Logical MPPT Control Strategy for PMSG Wind Generation Systems

Making full use of wind power is one of the main purposes of the wind turbine generator control. Conventional hill climbing search （HCS） method can realize the maximum power point tracking （MPPT）. However, the step size of HCS method is constant so that it cannot consider both steady-state response and dynamic response. A fuzzy logical control （FLC） algorithm is proposed to solve this problem in this paper, which can track the maximum power point （MPP） quickly and smoothly. To evaluate MPPT algorithms, four performance indices are also proposed in this paper. They are the energy captured by wind turbine, the maximum power-point tracking time when wind speed changes slowly, the fluctuation magnitude of real power during steady state, and the energy captured by wind turbine when wind speed changes fast. Three cases are designed and simulated in MATLAB/Simulink respectively. The comparison of the three MPPT strategies concludes that the proposed fuzzy logical control algorithm is more superior to the conventional HCS algorithms.

Coordination between Fault-Ride-Through Capability and Over-current Protection of DFIG Generators for Wind Farms

Fault Ride-Through （FRT） capabilities set up according to the grid codes may affect the performance of related protective elements during fault periods. Therefore, in this paper the coordination between the FRT capability and over-current protection of DFIG Wind Generators in MV networks is investigated. Simulation test cases using MATLAB-Simulink are implemented on a 365-MW wind farm in AL-Zaafarana, Egypt. The simulation results show the influence of the FRT capability on the protective relaying coordination in wind farms, showing that the FRT may work in situations where is were expected not to work, and then disabling the over-current protection, which should have worked in this situation.

Development of a Full Parameterized FE-modeling Tool for Efficient Vibration Investigations on End Windings of Turbo- and Hydro-Generators

End windings of generators are excited to vibrations due to electromagnetic forces which can cause severe damage and noise. To avoid this, it is important to predict the natural frequencies and modes of the end windings with finite elements. Due to the complex structure and unknown boundary conditions, the conventionally calculation of stator end windings has been very difficult and time consuming up to now. This paper describes the development of a full parameterized modeling tool, which allows a quick calculation of natural frequencies during the design phase of the generator. To keep the computing time low, it is important to find a way to get exact calculation results without detailed modeling of all pans. Additionally, special attention was paid to the active part, which has been replaced by spring-damper elements, and the determination of their stiffness via experimental modal analysis combined with finite element calculations.

Hybrid PV/Wind/Diesel Based Distributed Generation for an Off-Grid Rural Village in Afghanistan

Afghanistan has a tremendous resource potential of renewable energy especially solar and the wind. Therefore, utilization of these resources has a special rule for the remote areas where access to the electrical grid or secure power supply is a dream for most of the people. This paper presents a feasibility and usefulness of hybrid power generation based on PV/wind/diesel generator for an off-grid rural village that feeds the load at a rate of average 7.9 kWh/day with 1.32 kW peak load. GsT （geospatial toolkit） is used to obtain the solar and wind data of the site. Windographer software is used to analyze the wind resource data of the site. HOMER Pro software package is used to select the suitable and reliable hybrid generation system and calculate the optimal capacities and costs of the components. Through the study, it is found that this state of the art adaptation could provide vast opportunities for off-grid rural communities such as in Afghanistan where enough high penetration of renewable energy is available.

Analysis and Control of a Three-Phase Switched Reluctance Wind Generator

This paper deals with various aspects of modeling, simulation, analysis, and control ofa SRG （switched reluctance generator） in different modes of operation. Excitation method of such a generator is shortly described. The output power of the SRG is controlled so that it can track the maximum output power of a wind turbine drive applications. Also, the output generator voltage is stabilized under either load or wind speed variations. Basic operational characteristics of a three-phase 6/4 SRG are developed through experimental observation. Meanwhile, the real-time operation was implemented in a DSP （digital signal processor） environment. The simulation of the complete system model has been obtained using the Matlab/Simulink software. A good agreement between experimental and simulation results has been observed which supports the validity of the proposed analysis.

Robust Optimal Framework for Doubly Fed Induction Generator with Uncertain Dynamics

A robust optimal framework is designed herein to mitigate the oscillatory dynamics in a doubly fed induction generator(DFIG)even in the presence of network disturbances and input variation.To address uncertain dynamics,herein,a novel transformation formula is developed for a wind energy conversion system.An unscented Kalman filter is applied to estimate the un-measured internal states of the wind energy conversion system using terminal measurements.The detailed convergence and stability analyses of the presented framework are investigated to validate its effectiveness.Additionally,comparative modal analyses are carried out to demonstrate the improvement in the damping of critical low-frequency oscillatory modes using the presented framework.The simulation results demonstrate satisfactory performance under various operating scenarios,such as increasing and decreasing wind speed and varying the terminal voltage.The comparative performance is demonstrated to validate the effectiveness of the presented framework over that of the state-of-the-art frameworks.Index Terms—Optimal control,power system,renew-able energy sources,wind energy conversion system,doubly fed induction generator.

Application of fuzzy logic control algorithm as stator power controller of a grid-connected doubly-fed induction generator

This paper discusses the power outputs control of a grid-connected doubly-fed induction generator （DFIG） for a wind power generation systems. The DFIG structure control has a six diode rectifier and a PWM IGBT converter in order to control the power outputs of the DFIG driven by wind turbine. So, to supply commercially the electrical power to the grid without any problems related to power quality, the active and reactive powers （Ps, Qs） at the stator side of the DFIG are strictly controlled at a required level, which, in this paper, is realized with an optimized fuzzy logic controller based on the grid flux oriented control, which gives an optimal operation of the DFIG in sub-synchronous region, and the control of the stator power flow with the possibility of keeping stator power factor at a unity.

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.

Simulation of grid tracking control of power system connection and maximum power brushless doubly-fed generator in point wind

In this paper, based on the analysis of the mathematical model in a common synchronous reference frame of the brushless doubly-fed generator （BDFG）, the grid connection strategy and maximum energy extraction control were both analyzed. Besides, the transient simula- tion of no-load model and generation model of the BDFG have been developed on the MATLAB/Simulink platform. The test results during cutting-in grid confirmed the good dynamic performance of grid synchronization and effective power control approach for the BDFG-based variable speed wind turbines.