ESTIMATION OF ROTOR VOLTAGE VECTOR ON THE DOUBLE EXCITED INDUCTION MACHINE USED IN WECS

ABSTRACT

The aim of this work is to estimate and control the rotor voltage of a double excited induction machine DEIM that is necessary to operate it as a generator in wind energy conversion systems (WECS), with speed range extending from the sub synchronous range to double the synchronous speeds. For efficient power generation, the DEIM is to be operated at its rated electric torque which is realised by having both rotor and stator currents magnitudes constant throughout the whole operating range. Also the angle between the rotor current and stator current is kept constant irrespective of the rotor speed.     

CORRESPONDENCE: VARIABLE-SPEED,CONSTANT-FREQUENCY HYSTERESIS GENERATOR

ABSTRACT

The hysteresis generator has been mentioned in the literature [1,2], but analytical studies seem to be absent. The supersynchronously operated hysteresis machine offers promise of a simple infinite bus generator for use with a prime mover characterized by either speed fluctuation or poor speed regulation.

In this note, a previously presented asynchronous equivalent circuit model [3] and example machine data [4] are utilized to predict asynchronous hysteresis generator performance for speeds from 1.0 to 1.5 per unit. The example hysteresis generator is seen to be a near constant output power device over a wide range of supersynchronous speeds when operated on an infinite bus.     

ANALYSIS OF A SERIES-DELTA CONNECTED TANDEM INDUCTION MOTOR

Abstract

A tandem induction motor comprised of two independently housed stator windings, one housing stationary and the other able to be physically rotated, and a squirrel-cage rotor common to both stators is described. When the motor is fed from a single source, altering the position of the moveable stator changes shaft speed in a similar manner to operating a normal induction motor at variable terminal voltage

This paper presents an equivalent circuit model of the tandem induction motor for series-delta connected stator windings. Using the equivalent circuit stator current, power factor, and torque are calculated and then compared to laboratory test results obtained from a 3 kW tandem motor test rig     

PERFORMANCE ANALYSIS OF SELF-EXCITED INDUCTION AND RELUCTANCE GENERATORS

ABSTRACT

The paper presents a suggested method for ac power generation from a constant speed drive using capacitor excited induction and reluctance machines. The proposed arrangement is adapted to use one stator frame and two rotor types, namely, salient poles reluctance and squirrel-cage induction rotors. Operational and steady-state equivalent circuits for both types are developed and used to predict the dynamic and steadystate behaviours under load conditions. This analysis depends mainly upon the computation of the power angle for a given machine parameters, capacitor ratings, and load conditions at a constant speed. Further, stability limits of both generators are investigated by developing the active-reactive power (P-Q) diagram for each generator. Conditions required to achieve self-excitation are also given. Performances of the two generator types are evaluated theoretically and experimentally.     

Wind turbine emulator based on blade element momentum theory for variable‐speed wind power generation system

This paper proposes a wind turbine emulator (WTE) based on the blade momentum theory, and tests the variable‐speed wind power generation system using a pulse‐width modulation (PWM) converter to verify the accuracy of the emulator. The behavior of the wind turbine for natural wind is reproduced by the WTE based on the proposed theory. The variable‐speed wind power generation system employs a vector control system to control the torque and speed of the permanent magnet synchronous generator in the converter side. The windmill rotational speed is controlled to maximize the efficiency of the wind turbine against wind velocity. And the active power and reactive power are controlled in the inverter side, and the generated power is sent to the grid while controlling the DC link voltage to be constant at the same time. The behaviors of the WTE are compared with the simulation results and experimental results using a real wind turbine. These experimental and simulation results show that the test bench with the proposed WTE has sufficient performance and accuracy to verify variable‐speed wind generator systems. © 2015 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

无刷双馈风力发电机的控制策略与实现

为了探讨变速恒频(VSCF)控制的无刷双馈发电机(BDFM)风电系统的有效控制策略,在分析BDFM的结构特点和工作原理基础上,给出VSCF发电模式的控制方法,构建了转子转速约束下的电机d-q坐标系数学模型,推导出变速恒频运行模式的功率控制策略.根据BDFM的数学模型及功率控制策略,设计了电机侧与网侧均包含可双向变换的变频器主电路拓朴,实现了风力发电机的四象限平滑运行和VSCF的功率控制策略,并对BDFM发电系统的VSCF运行模式进行了仿真试验.仿真结果验证了本控制策略能够实现变速恒频发电,电机空载启动到1 050 r/min的响应时间为0.2 s,能对有功功率和无功功率解耦控制.     

Double-fed induction generator control for variable speed wind power generation

Vector control of a doubly fed induction generator drive for variable speed wind power generation is described. A wound rotor induction machine with back-to-back three phase power converter bridges between its rotor and the grid forms the electrical system. The control scheme uses stator flux-oriented control for the rotor side converter bridge control and grid voltage vector control for the grid side converter bridge. A complete simulation model is developed for the control of the active and reactive powers of the doubly fed generator under variable speed operation. Several studies are performed to test its operation under different wind conditions. A laboratory test setup consisting of a wound rotor induction machine driven by a variable speed dc motor is used to validate the software simulations.     

Use of multifield induction machine to determine the characteristics of self-excited generator systems at variable speed,constant voltage and constant frequency

This paper considers a generator system using variable energy such as wind power. The variable energy influences the voltage and frequency of a generator system. However, the load requires constant voltage and frequency. Therefore, this paper proposed that a generator system have the following good points:

• (1) This generator is a brushless system which makes use of the multifield induction machine;
• (2) Frequency converter of small capacity which is used for constant voltage and frequency in load at variable speed;
• (3) The PWM inverter can supply reactive power to the self-excited generator; and
• (4) Voltage wave of load is sine wave by LC filter of the frequency converter.

The multifield induction machines of this generator system have 4 poles and 8 poles similar to stator winding. The rotor of the multifield machine is especially cage type. This rotor can transform among the 4-pole field and 8-pole field on the stator. In this paper, an experimental study of the brushless, self-excited, constant voltage, constant frequency, variable speed and sine wave generator system by multifield induction machine is described. The component of this generator system, operation of multifield induction machine, characteristics of generator system, and the effect of LC filter in frequency converter are described.     

风电参与一次调频的小扰动稳定性分析

为了分析参与电网一次调频的风电系统对电力系统稳定性的影响,建立基于直驱永磁型风电系统的单机无穷大系统小扰动稳定性分析数学模型.根据风速的变化,在额定风速以下采用最大功率点追踪控制,以实现风电系统输出功率最大:额定风速以上采用恒功率控制,保证风电系统安全运行.研究考虑负荷动态模型时风速变化对系统特征值的影响,结果表明,在整个风速变化区问系统均能保持小扰动稳定.在理论分析的基础上进行时域仿真,来验证理论分析的正确性.仿真结果表明,风电系统能够参与电网一次调频,可有效改善电网的频率特性,为直驱型风电安全稳定并网运行提供了可借鉴的理论依据.     

直驱永磁同步风力发电机的最佳风能跟踪控制

与双馈交流励磁风力发电系统相比,直驱永磁同步风力发电系统具有结构简单、发电效率及运行可靠性高等优点。文章采用双脉宽调制(pulse-width modulation,PWM)变换器作为直驱永磁同步发电机的并网电路,根据风力机和发电机的运行特性提出了一种基于最佳功率给定的发电机最大风能跟踪控制策略。建立了基于双PWM变换器的直驱永磁同步风力发电实验系统,实验结果验证了所提出控制策略的正确性。该发电系统可实现最大风能跟踪控制、并网有功和无功功率独立控制以及变速恒频发电运行。     

风力发电机并网后的电网电压和功率分析

对用鼠笼式感应发电机发电的恒速风力机和用双馈感应发电机发电的变速风力机的工作原理及其在电网的接入方式、接入风力发电机后的电网电压和功率进行了分析,对不同风电穿透力下电压对风速扰动的响应进行了讨论,并对电网故障时电压变化及风电场低压穿越技术进行了研究。采用理论分析与计算机仿真方法得到了相关结论:不同风力机机型对电网的作用不同;鼠笼式风电机组构成的风电场穿透功率大于10%以后会引起公共连接点处电压偏移超过10%;电网故障后双馈风电机组和鼠笼式风电机组电压恢复能力不同,在风电场加入STATCOM后,可以实现低电压穿越。     

双馈风力发电机变换器的控制方法研究

本文主要研究了双馈感应电机(DFIG)的网侧和转子侧变换器的控制方法。双馈感应电机中转子侧变换器采用定子磁链定向矢量控制技术,实现了风能的最大跟踪以及有功功率和无功功率的解耦控制。网侧变换器采用电网电压定向矢量控制技术,控制了直流母线电压的恒定以及调节了网侧的功率因素。采用PSCAD/EMTDC电力系统仿真软件,构建了包括风速模型、风力机模型、变速恒频双馈电机、定子侧和转子侧变换器在内的双馈风力发电机仿真模型。仿真实验结果表明,所提方法是有效、合理的。     

变速恒频风力发电机组励磁变频器的研究

研制了一套基于TMS320LF2407A和TMS320VC33双DSP的风力发电系统。在分析了该系统控制方法的基础上，建立了交直交变频器网侧电压定向以及转子侧定子磁场定向的矢量控制系统模型。对系统的变速恒频控制、并网控制技术进行了实验研究，结果表明，该系统能够在发电机允许的速度范围内实现软并网，输入输出有功、无功功率可以独立调节，适合于兆瓦级风力发电系统。     

A novel method to improve low‐voltage ride‐through capability of wind turbine generators

With increasing penetration of wind farms, power grids have responded by developing specific grid codes to maintain their stability. One of the main grid codes is the low‐voltage ride‐through (LVRT) capability, which requires the wind generator to remain connected when the grid voltage sags for a certain time period. A wind farm with squirrel cage induction generators suffers this LVRT problem the most because of their direct connection to the grid and reactive power consumption. In this paper, a new method is proposed to solve this problem by shunt‐connecting a motor‐driven mechanical load to the cage wind generator. For driving mechanical loads, the induction motor is most widely used in industries. This paper studies the terminal voltage holding effect of an induction machine during grid voltage sag due to the magnetic flux holding effect and the saturation characteristic. Taking advantage of this effect, the induction motor that is used for driving mechanical load is then proposed to improve the LVRT capability of wind turbine generators. Furthermore, the change of the rotating speed or slip of the induction machine is found to have a great impact on improving the LVRT. By adding some inertia to the motor‐driven mechanical load, an enhanced voltage holding effect, and therefore LVRT improvement, is expected for the wind farm. Both simulation and experimental results prove the effectiveness of the proposed method. © 2013 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

Modeling and control of a variable-speed constant-frequency synchronous generator with brushless exciter

This paper presents the modeling, control, and implementation of a novel variable-speed constant-frequency power generation system for renewable and distributed energy applications. The generation system consists of a wound-rotor generator, a brushless exciter and a low-rating controlled power converter. The main generator is a doubly fed induction machine which is operated as a synchronous generator. The advantages of the proposed system are reduced harmonic injection to power grid, wide speed operation range covering both subsynchronous and super-synchronous speeds, self var support, and increased reliability. It can be directly applied to wind power generators, small-scale hydroelectric generators, stand-alone diesel and gasoline generators, and aerospace and naval power generation systems where a variable speed turbine/engine is employed. An equivalent circuit model of a doubly fed generator was developed incorporating stator and rotor iron losses. Then the control of a standalone generation system is developed based on the mathematical model. Detailed implementation procedure is given. An experimental system and its control were implemented using an embedded real-time digital signal processor. Measurements of the experimental system validated the system design and readiness for prototyping in a relatively large power range.     

液力变矩型同步风力发电机低电压穿越的控制原理及测试分析

液力变矩型同步风力发电机是一种基于液力变矩器和无刷同步发电机耦合的前端调速式风力发电机组,关键技术在于采用前端调速技术,使发电机转速恒定可以实现同步并网。风机在低速传动链上采用转速模糊闭环控制,高速传动链上采用液力变矩技术闭环控制转矩平衡,使风机在发生低电压穿越时转速与转矩控制的动态响应速度和精准度得到了较大改善。同时,同步发电机励磁系统采用强励模式,迅速提高发电机电动势及时补偿电磁功率的损失,并为电网提供无功能量支撑维持电网稳定。该机型低电压穿越性能经过现场实地认证测试,结果充分验证了其控制策略的有效性和可行性。     

变速恒频双馈风力发电机的励磁控制系统设计及试验研究

双馈电机变速恒频风力发电系统是通过调节转子绕组励磁电流的频率、幅值、相位和相序来实现变速恒频控制的.分析了双馈电机工作原理和数学模型,并设计了基于TMS32OF2812DSP的变速恒频双馈风力发电机的励磁控制试验系统.对电机的并网控制以及电机的自并励启动进行了试验研究,为变速恒频双馈风力发电机励磁控制策略的设计奠定了基础.     

Smoothing of Wind Power Fluctuations for Permanent Magnet Synchronous Generator-Based Wind Energy Conversion System and Fault Ride-through Consideration

Due to the increment of penetration level of wind power generation, output power fluctuation is one of the most important issue's that can destabilize the power system operation. This article mainly deals with the smoothing of the output power fluctuations of a wind energy conversion system based permanent magnet synchronous generator and fault ride-through enhancement during a grid fault. The concerned wind energy conversion system based permanent magnet synchronous generator adopts an AC-DC-AC converter system. The proposed control method limits the wind energy conversion system output power by adjusting the pitch angle of the wind turbine blades when wind speed is above the rated wind speed. In the grid-side converter, a fuzzy logic controller is used to determine the torque reference for which the kinetic energy stored by the inertia of wind turbine can smooth the output power fluctuations of the permanent magnet synchronous generator. Also, the DC-link voltage, controlled by the grid-side inverter, is adjusted in accordance with the output power fluctuations of the permanent magnet synchronous generator using a voltage smoothing index. Moreover, in this aticle, the proposed method ensures that the wind turbine stays operational during grid faults and provides fast restoration once the fault is cleared. To show the effectiveness of the proposed method, simulations under different conditions have been performed by using MATLAB/Simulink® (The Math Works, Natick, MA, USA).     

双馈风力发电机组系统接入与稳定运行仿真

分析了包含大量异步风力发电机组的风电场并网运行后对电力系统静态和动态稳定性的影响。从系统接入和稳定运行的角度研究了双馈风力发电机组及普遍采用的定子磁链定向矢量控制策略在提高风电系统稳定性方面的优势与不足。以实际机组为例在PSCAD/EMTDC平台上建立了仿真模型,结果表明双馈风力发电机组在风速发生变化时不仅能够以变速恒频方式运行并追踪最大风能,且电网电压也比传统鼠笼式风力发电机组更为稳定。在系统发生最严重的三相接地故障时,风电场具有更好的暂态稳定性。     

Wind turbine Generator modeling and Simulation where rotational speed is the controlled variable

To optimize the power produced in a wind turbine, the speed of the turbine should vary with the wind speed. A simple control method is proposed that will allow an induction machine to run a turbine at its maximum power coefficient. Various types of power control strategies have been suggested for application in variable speed wind turbines. The usual strategy is to control the power or the torque acting on the wind turbine shafts. This paper presents an alternative control strategy, where the rotational speed is the controlled variable. The paper describes a model, which is being developed to simulate the interaction between a wind turbine and the power system. The model is intended to simulate the behavior of the wind turbine using induction generators both during transient grid fault events and during normal operation. Sample simulation results for two induction generators (2/0.5 MW) validate the fundamental issues.