Probabilistic Load Flow Algorithm with the Power Performance of Double-Fed Induction Generators

Probabilistic load flow(PLF)algorithm has been regained attention,because the large-scale wind power integration into the grid has increased the uncertainty of the stable and safe operation of the power system.The PLF algorithm is improved with introducing the power performance of double-fed induction generators(DFIGs)for wind turbines(WTs)under the constant power factor control and the constant voltage control in this paper.Firstly,the conventional Jacobian matrix of the alternating current(AC)load flow model is modified,and the probability distributions of the active and reactive powers of the DFIGs are derived by combining the power performance of the DFIGs and the Weibull distribution of wind speed.Then,the cumulants of the state variables in power grid are obtained by improved PLF model and more accurate power probability distributions.In order to generate the probability density function(PDF)of the nodal voltage,Gram-Charlier,Edgeworth and Cornish-Fisher expansions based on the cumulants are applied.Finally,the effectiveness and accuracy of the improved PLF algorithm is demonstrated in the IEEE 14-RTS system with wind power integration,compared with the results of Monte Carlo(MC)simulation using deterministic load flow calculation.

An analytic electromagnetic calculation method for performance evolution of doubly fed induction generators for wind turbines

An analytic electromagnetic calculation method for doubly fed induction generator(DFIG) in wind turbine system was presented. Based on the operation principles, steady state equivalent circuit and basic equations of DFIG, the modeling for electromagnetic calculation of DFIG was proposed. The electromagnetic calculation of DFIG was divided into three steps: the magnetic flux calculation, parameters derivation and performance checks. For each step, the detailed numeric calculation formulas were all derived. Combining the calculation formulas, the whole electromagnetic calculation procedure was established, which consisted of three iterative calculation loops, including magnetic saturation coefficient, electromotive force and total output power. All of the electromagnetic and performance data of DIFG can be calculated conveniently by the established calculation procedure, which can be used to evaluate the new designed machine. A 1.5 MW DFIG designed by the proposed procedure was built, for which the whole type tests including no-load test, load test and temperature rising test were carried out. The test results have shown that the DFIG satisfies technical requirements and the test data fit well with the calculation results which prove the correctness of the presented calculation method.

Effects of Wind Turbines Equipped with Doubly-fed Induction Generators on Distance Protection

Nowadays wind energy is the fastest growing renewable energy resource in the world.The problems of integrating wind farms are caused by changes of wind speed during a day.Moreover,the behaviors of wind turbines equipped with doubly-fed induction generators differ fundamentally from synchronous generators.Therefore,more considerations are needed to analyze the performances of the distance protection relays.The protection of a wind farm with distance relay is inspected.By changing the conditions of the wind farm,the characteristics of the distance relay are studied.

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.

Dynamic Couplingand Cooperative Control for Multi-paralleled Doubly Fed Induction Generator Wind Farms during Symmetrical Low Voltage Ride-through in a Weak Grid

In multi-fed grid-connected systems,there are complex dynamic interactions between different pieces of equipment.Particularly in situations of weak-grid faults,the dynamic coupling between equipment becomes more pronounced.This may cause the system to experience small-signal instability during the fault steady-state.In this paper,multi-paralleled doubly fed induction generator(DFIG)-based wind farms(WFs)are taken as an example to study the dynamic coupling within a multi-fed system during fault steady-state of symmetrical low voltage ride-through(LVRT)in a weak grid.The analysis reveals that the dynamic coupling between WFs will introduce a damping shift to each WF.This inevitably affects the system’s dynamic stability and brings the risk of small-signal instability during fault steady-state in LVRT scenarios.Increasing the distance to fault location and fault severity will exacerbate the dynamic coupling between WFs.Because of the dynamic coupling,adjusting the control state of one WF will affect the stability of the remaining WFs in the system.Hence,a cooperative control strategy for multi-paralleled DFIG WFs is proposed to improve dynamic stability during LVRT.The analysis and the effectiveness of the proposed control strategy are verified by modal analysis and simu-lation.

Dynamic modelling and robust current control of wind-turbine driven DFIG during external AC voltage dip

Doubly-Fed Induction Generator （DFIG）, with vector control applied, is widely used in Variable-Speed Constant- Frequency （VSCF） wind energy generation system and shows good performance in maximum wind energy capture. But in two traditional vector control schemes, the equivalent stator magnetizing current is considered invariant in order to simplify the rotor current inner-loop controller. The two schemes can perform very well when the grid is in normal condition. However, when grid disturbance such as grid voltage dip or swell fault occurs, the control performance worsens, the rotor over current occurs and the Fault Ride-Through （FRT） capability of the DFIG wind energy generation system gets seriously deteriorated. An accurate DFIG model was used to deeply investigate the deficiency of the traditional vector control. The improved control schemes of two typical traditional vector control schemes used in DFIG were proposed, and simulation study of the proposed and traditional control schemes, with robust rotor current control using Internal Model Control （IMC） method, was carded out. The validity of the proposed modified schemes to control the rotor current and to improve the FRT capability of the DFIG wind energy generation system was proved by the comparison study.

Dynamic modeling and direct power control of wind turbine driven DFIG under unbalanced network voltage conditions

This paper proposes an analysis and a direct power control (DPC) design of a wind turbine driven doubly-fed induction generator (DFIG) under unbalanced network voltage conditions. A DFIG model described in the positive and negative synchronous reference frames is presented. Variations of the stator output active and reactive powers are fully deduced in the presence of negative sequence supply voltage and rotor flux. An enhanced DPC scheme is proposed to eliminate stator active power oscillation during network unbalance. The proposed control scheme removes rotor current regulators and the decomposition processing of positive and negative sequence rotor currents. Simulation results using PSCAD/EMTDC are presented on a 2-MW DFIG wind power generation system to validate the feasibility of the proposed control scheme under balanced and unbalanced network conditions.

Analysis of Short-circuit Characteristics and Calculation of Steady-state Short-circuit Current for DFIG Wind Turbine

Large-scale doubly-fed induction generator(DFIG)wind turbines are connected to the grid and required to remain grid-connection during faults,the short-circuit current contributed by the generation has become a significant issue.However,the traditional calculation methods aiming at synchronous generators cannot be directly applied to the DFIG wind turbines.A new method is needed to calculate the short-circuit current required by the planning,protection and control of the power grid.The short-circuit transition of DFIG under symmetrical and asymmetric short-circuit conditions are mathematically deduced,and the short-circuit characteristics of DFIG are analyzed.A new method is proposed to calculate the steady-state short-circuit current of DFIG based on the derived expressions.The time-domain simulations are conducted to verify the accuracy of the proposed method.

PI-MPC Frequency Control of Power System in the Presence of DFIG Wind Turbines

For the recent expansion of renewable energy applications, Wind Energy System (WES) is receiving much interest all over the world. However, area load change and abnormal conditions lead to mismatches in frequency and scheduled power interchanges between areas. These mismatches have to be corrected by the LFC system. This paper, therefore, proposes a new robust frequency control technique involving the combination of conventional Proportional-Integral (PI) and Model Predictive Control (MPC) controllers in the presence of wind turbines (WT). The PI-MPC technique has been designed such that the effect of the uncertainty due to governor and turbine parameters variation and load disturbance is reduced. A frequency response dynamic model of a single-area power system with an aggregated generator unit is introduced, and physical constraints of the governors and turbines are considered. The proposed technique is tested on the single-area power system, for enhancement of the network frequency quality. The validity of the proposed method is evaluated by computer simulation analyses using Matlab Simulink. The results show that, with the proposed PI-MPC combination technique, the overall closed loop system performance demonstrated robustness regardless of the presence of uncertainties due to variations of the parameters of governors and turbines, and loads disturbances. A performance comparison between the proposed control scheme, the classical PI control scheme and the MPC is carried out confirming the superiority of the proposed technique in presence of doubly fed induction generator (DFIG) WT.

Analysis and Simulation of a Crowbar Protection for DFIG Wind Application during Power Systems Disturbances

The performance of a 1.5 MVA wind-power Doubly Fed Induction Generator （DFIG） under network fault is studied using simulator developed in MATLAB-SIMULINK. This paper investigates a new control method able to improve the fault-ride through capability of DFIG. In such generators the appearance of severe voltage sags at the coupling point make highlight important over currents at the rotor/stator windings, making the use of crowbar protection device necessary and inevitable in order to protect the machine as well as the rotor side power converter. The simulator consists of the DFIG analytical model, power transformer model and the detailed frequency converter model including crowbar protection device. Simulation results are carried out to show the transient behavior of the DFIG when a sudden voltage dip is introduced with and without the crowbar implementation.

High Frequency Charging Techniques—Grid Connected Power Generation Using Switched Reluctance Generator

Power generation becomes the need of developed, developing and under developed countries to meet their increasing power requirements. When affordability increases their requirement of power increases, this happens when increased per capita consumption. The existing power scenario states that highest power is produced using firing of coals called thermal energy. A high efficiency Switched Reluctance Generator (SRG) based high frequency switching scheme to enhance the output for grid connectivity is designed, fabricated and evaluated. This proposed method generates the output for the low wind speed. It provides output at low speed because of multi-level DC-DC converter and storage system. It is an efficient solution for low wind power generation. The real time readings and results are discussed.

应用于DFIG风电场的VSC-HVDC控制策略

随着风力发电的快速发展,如何连接风电场和电网成为至关重要的问题。研究了基于双馈感应电机(DFIG)的风电场连接方式及控制策略,通过与交流输电和电流源型高压直流输电(CSC- HVDC)的比较,说明电压源型高压直流输电(VSC-HVDC)技术在连接风电场与电网方面具有独特优势。在换流站的dq解耦控制基础上,对DFIG与VSC-HVDC的换流站的控制设计进行了研究。利用PSCAD/EMTDC平台对该方案进行了仿真研究。仿真结果表明,该方案可以实现一个换流站连接多台DFIG的结构,并具有良好的控制效果。

基于量测信号扰动的DFIG变流器控制参数辨识方法

双馈感应发电机(DFIG)的变流控制器模型是整个机组模型中的关键部分。变流控制器的参数众多且内外环比例—积分(PI)控制器之间存在级联,常规辨识方法或遇到参数可辨识性问题,或工程实现难度较大。文中提出一种以"分步辨识、交互迭代"为特点,基于二次侧量测信号扰动的控制器参数辨识方法。该方法分别在变流控制器中各PI控制器输入端所用的量测信号上施加扰动,从而突出某个PI控制器在特定量测信号扰动下的作用,并将该PI控制器的可测量控制目标作为观测变量,进而重点辨识其参数。对于存在级联的两个PI控制器,先对后一级PI控制器所用量测信号施加扰动,同时屏蔽前一级PI控制器所用量测信号的变化,从而可实现两个级联PI控制器参数的解耦辨识。该方法可以使变流控制器中每个PI控制器的两个参数在特定的量测信号扰动下被单独辨识,有效保证了参数的可辨识性;而对于全部待辨识参数,可通过多轮交互迭代辨识进一步提高整体辨识精度。

配置STATCOM的DFIG风电场在不对称电网故障下的控制策略

该文研究在不对称电网故障穿越的第2阶段,双馈式(doubly-fed induction generator,DFIG)风电场如何实现"自身不脱网安全运行并同时满足并网导则无功支撑要求"的问题。由于受限于变流器的容量,DFIG机组无法保证在所有故障情况下实现既定的控制目标。为此,通过引入最优化的方法,分析了变流器容量限制条件下DFIG机组的可控性问题,得到了以电网电压不平衡度所量化表示的可控区,指出为确保在不对称跌落故障较严重时能满足并网导则要求,有必要在DFIG风电场层面配置静止同步补偿器(staticsynchronous compensator,STATCOM)以降低场内DFIG机组的机端电压不平衡度。最后,设计了STATCOM所需容量最小的电压补偿控制策略,并给出了相应的容量匹配方法。仿真结果验证了该文提出的在不对称低电压穿越(lowvoltage ride-through,LVRT)第2阶段DFIG机组可控区的正确性、STATCOM辅助控制策略的有效性以及相应容量匹配方法的可行性。

采用串联网侧变换器的DFIG风电系统低电压穿越控制

双馈感应发电机(DFIG)因其对电网故障的敏感性,其低电压穿越运行控制技术备受关注。文中针对定子侧加装串联网侧变换器的新型DFIG风电系统,详细研究了其低电压穿越运行特性,提出了一种适用于新拓扑下发电系统的运行控制方案:通过控制串联网侧变换器抑制定子磁链暂态直流分量及电网电压负序分量对发电机转子侧的不良影响;通过控制转子侧变换器进一步限制故障时发电机的定、转子电流,从而实现发电系统的低电压穿越运行。仿真结果表明:采用所提出的控制方案,可实现电网故障时DFIG风电系统的零电压穿越运行;采用新拓扑的DFIG风电系统具有优良的电网对称及非对称短路故障穿越能力。

不平衡电网电压下基于串联网侧变换器的DFIG控制策略

电网电压不平衡会导致双馈感应发电机组(DFIG)定、转子电流出现较大不平衡,使发电机功率和电磁转矩发生振荡,从而恶化机组运行状况。分析了串联网侧变换器抑制不平衡电网电压对DFIG系统影响的机理,利用并联网侧变换器的控制及静止坐标系下的比例谐振控制器,提出了基于串联网侧变换器的DFIG在不平衡电网电压条件下的控制策略;在实现DFIG电磁转矩、直流母线电压及系统总输出有功功率无2倍频波动的同时,使DFIG定、转子三相电流平衡。所述方法具有不改变转子侧变换器的控制策略、无需求解复杂高阶矩阵的特点。对一台基于串联网侧变换器的2MW DFIG系统进行了仿真,验证了所提出控制策略的正确性和有效性。

DFIG风电场并网引发多机电力系统次同步振荡开环模式分析方法

针对双馈感应发电机（doubly-fed induction generator,DFIG）风电场并网引发电力系统次同步振荡问题,基于开环模式谐振理论,提出一种研究多机电力系统中风电场并网引发次同步振荡的开环模式分析方法。当DFIG风电场的开环次同步振荡模式与其余电力系统的开环次同步振荡模式相近时,由于发生开环模式谐振,相应的两个闭环次同步模式中的一个的模式阻尼会降低。该开环模式分析方法可以辨识相近的开环次同步振荡模式,并根据推导出的开环模式指标估计开环模式谐振对系统稳定性的影响。同时,该方法可以有效识别参与开环模式谐振的模式与同步机、并网风电场之间的相关性,进而通过采取措施,消除DFIG风电场并网引发系统发生次同步振荡的风险。该文通过改进IEEE第一标准测试系统和多机算例系统,验证了开环模式分析方法的正确性和有效性。