Advanced control of PWM converter with variable-speed induction generator

This paper describes simple control structures for the vector-controlled stand-alone induction generator (IG) used to operate under variable speeds. Different control principles, indirect vector control and deadbeat current control, are developed for a voltage source pulsewidth-modulation (PWM) converter and the three-phase variable-speed squirrel-cage IG to regulate dc link and generator voltages with the newly designed phase-locked loop circuit. The required reactive power for the variable-speed IG is supplied by means of the PWM converter and a capacitor bank to buildup the voltage of the IG without the need for a battery, to reduce the rating of the PWM converter with the need for only three sensors, and to eliminate the harmonics generated by the PWM converter. These proposed schemes can be used efficiently for variable-speed wind energy conversion systems. The measurements of the IG systems at various speeds and loads are given and show that these systems are capable of good ac and dc voltage regulations.     

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。     

A 4-kW 42-V induction-machine-based automotive power generation system with a diode bridge rectifier and a PWM inverter

The electrical load demand in automobiles has been increasing steadily due to the usage of several subsystems to improve engine performance, passenger comfort, and safety. The current production Lundell alternator is not able to meet the future growing power demand due to its inherent design limitations. Therefore, an efficient, high-power generation system is needed to meet the growing electric power demand in automobiles. The trend to adopt the 42-V power system in automobiles allows one to add more subsystems in an efficient way. In this paper, a three-phase 42-V 4-kW induction-machine-based automotive power generation scheme is proposed to meet the future electrical power demand in automobiles. This scheme uses a low-cost diode bridge rectifier directly connected to the induction machine to transfer active power to the battery and the load. The excitation to the machine is supplied by means of a low-power pulsewidth-modulation (PWM) inverter to control the output voltage of the generator connected to the diode bridge rectifier. This paper presents a new control methodology to regulate the output voltage of an induction generator directly connected to a diode bridge rectifier by controlling the auxiliary PWM inverter. The simulated performance results of a 4-kW 42-V induction generator scheme at various speeds and loads are presented.     

A Three-Phase Inverter with Reactive Power Control

A new type of self-commutated inverter for fixed or moderately variable frequency has been developed. The inverter is characterized by an extremely uncomplicated main circuit. In its basic form the inverter contains two converter circuits: a principal converter circuit and an auxiliary converter circuit. The principal converter circuit transfers power from the input dc side to the output ac side, and the auxiliary converter circuit generates an inductive current to balance the reactive current of a three-phase capacitor on the ac side. This capacitor has the combined function of a phase compensator, a filter capacitor, and is also the source of the commutating voltage. Both converter circuits are of the line commutated type, meaning that at power frequencies normal converter thyristors can be employed. This makes it possible to build high-power inverters without series or parallel connected thyristors. All filter reactors are smoothing reactors placed on the dc side of the converter circuits. Thus the inverter has a very good efficiency even at the higher frequencies. The ability of a converter circuit to generate a negative sequence current when unsymmetrically controlled makes the inverter insensitive to unbalanced loads. The transient behavior of the inverter is similar to that obtained from a conventional self-commutated inverter with an output filter.     

宽风速运行的定子双绕组感应电机风力发电系统拓扑及控制策略

介绍了一种适用于宽风速范围运行的新型定子双绕组感应发电机风力发电系统,其拓扑特点是将控制侧励磁变换器的直流母线经二极管与功率侧直流母线相连。在低风速区,利用电压泵升原理将励磁变换器的直流母线电压泵升至指令值,电能由控制侧直流母线端输出,拓宽了系统在低风速下的风能利用能力;在高风速区,功率绕组侧输出电压可上升至指令值,并联二极管断开,电能由功率侧整流桥的直流母线输出,控制侧变换器调节系统励磁无功保持输出电压恒定。该发电系统拓扑及控制方法实现了系统在宽风速范围内均能输出恒定的高压直流,可简化后级并网逆变器的结构和控制。在一台额定37 kW/DC 600V的样机上进行了实验研究,结果表明该风力发电系统的可行性和控制策略的有效性。     

Model and Performance of Current Sensor Observers for a Doubly Fed Induction Generator

Abstract—This article presents a stator and rotor current observer for a doubly fed induction generator. First, the dynamic models of the wind turbine drive train are presented, and the vector control strategies of a doubly fed induction generator for the rotor-side and grid-side converters are described. A stator and rotor current observer model, which is based on the state–space models of doubly fed induction generators, is then derived by using the stator and rotor voltage signals as inputs. To demonstrate the effectiveness of the proposed current observer, its dynamic performance is simulated using a MATLAB/Simulink software platform under the conditions of active power change of doubly fed induction generators and grid voltage dip fault. Furthermore, the robustness of the proposed current observer is investigated when the doubly fed induction generator rotor resistance is changed. Results show that the proposed observer has good coherence and robustness with the current sensor output when the doubly fed induction generator is in dynamic and transient responses. Compared with the referenced bilinear observer, the proposed observer has better fault-tolerant ability when the fault in the observed current sensor occurs.     

Performance of a grid-connected wind generation system with a robust susceptance controller

Wind turbine driven induction generators are vulnerable to transient disturbances like wind gusts and low voltages on the system. The fixed capacitor at the generator terminal or the limited support from the grid may not be able to provide the requisite reactive power under these abnormal conditions. This paper presents a susceptance control strategy for a variable speed wound-rotor induction generator which can cater for the reactive power requirement. The susceptance is adjusted through a robust feedback controller included in the terminal voltage driven automatic excitation control circuit. The fixed parameter robust controller design is carried out in frequency domain using multiplicative uncertainty modeling and H_∞ norms. The robustness of the controller has been evaluated through optimally tuned PID controllers. Simulation results show that the robust controller can effectively restore normal operation following emergencies like sudden load changes, wind gusts and low voltage conditions. The proposed robust controller has been shown to have adequate fault ride through capabilities in order to be able to meet connection requirements defined by transmission system operators.     

基于DFIG风电并网的VSC-HVDC电压稳定性分析

研究了基于双馈感应电机(doubly-fed induction generator,DFIG)的电压源高压直流输电(voltage source conveyor-high voltage direct current,VSC-HVDC)系统的风电并网技术.为解决风电并网运行存在的电压稳定性的问题,通过对风电机组无功...     

含风力发电机组的配电网潮流计算

将每一台风力发电机组的有功出力视为风速的函数,将其无功出力视为有功功率和节点电压的函数,建立了基于异步发电机等值电路的风力发电机组P-Q(V)稳态模型,提出了确定并联电容器组初始安装容量、安装组数和实时投切组数的方法。将运用C 语言编制的程序嵌入已有的前推回推法潮流计算程序,实现了考虑风力发电机组的配电网潮流计算。采用改造后计及分布式电源的配电网潮流计算程序对33节点系统进行仿真计算,结果表明了该方法和程序的有效性和实用性。     

Speed control of grid-connected switched reluctance generator driven by variable speed wind turbine using adaptive neural network controller

In wind energy conversion system, variable speed operation is becoming popular nowadays, where conventional synchronous generators, permanent magnet synchronous generators, and doubly fed induction generators are commercially used as wind generators. Along with the existing and classical solutions of the aforementioned machines used in wind power applications, the switched reluctance generator (SRG) can also be considered as a wind generator due to its inherent characteristics such as simple construction, robustness, low manufacturing cost, etc. This paper presents a novel speed control of switched reluctance generator by using adaptive neural network (ANN) controller. The SRG is driven by variable speed wind turbine and it is connected to the grid through an asymmetric half bridge converter, DC-link, and DC-AC inverter system. Speed control is very important for variable speed operation of SRG to ensure maximum power delivery to the grid for any particular wind speed. Detailed modeling and control strategies of SRG as well as other individual components including wind turbine, converter, and inverter systems are presented. The effectiveness of the proposed system is verified with simulation results using the real wind speed data measured at Hokkaido Island, Japan. The dynamic simulation study is carried out using PSCAD/EMTDC.     

A proposal of a power distortion compensator using a matrix converter

Recently, renewable resource supplies, such as fuel cells, photovoltaic cells, wind power, and engine generators for distributed power systems have been studied intensely. Conventional compensators with switching devices are constructed based on a voltage source inverter using six arms. Therefore, conventional power quality compensators require a large electrolytic capacitor in the DC link part of the equipment. The use of a large capacitor hinders downsizing efforts and the lowering of equipment costs. Direct converters, which do not have a large electrolytic capacitor and an initial charge circuit, can be used to realize downsizing and lowering of equipment costs, when compared with conventional converters. This paper proposes a new application of a matrix converter to a PM generator for power quality compensation, such as reactive power compensation, harmonic current, and power interruption. The novel point of this work is that the matrix converter provides reactive power with harmonic current. Simulated and experimental results confirm that the matrix converter can maintain high performance the same as a conventional active filter and an uninterruptible power supply (UPS). © 2010 Wiley Periodicals, Inc. Electr Eng Jpn, 171(4): 37–44, 2010; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20972     

An Energy-efficient Switching Scheme with Reduced Switching Transients for a Wind-driven Induction Generator

This article presents a scheme for improving the power output of grid-connected induction generator commonly used in wind energy conversion systems. Generally, the stator of the induction generator is connected in a star with a line voltage of √3 times the rated winding voltage to reduce the line current and, hence, conductor size. To extend the generating operation over a wider speed range, delta-star switchable stator windings are also in vogue. In such cases, the stator is star connected in the lower speed range and switched to a delta connection above a threshold speed. In this study, a new switching scheme is proposed wherein the stator coils are always connected in a star, while the stator is connected to different voltages in low- and high-speed conditions. At low wind speeds, nominal winding voltage is applied to the stator, whereas at higher speeds, the stator applied voltage is √3 times higher than the rated winding voltage. The efficacy of the scheme is demonstrated experimentally with a suitable microcontroller-based switching arrangement. Typical results indicate an increase in output with reduced switching transients. A case study on a 3-Φ, 50-kW induction generator is presented to emphasize the performance improvement with the proposed scheme.     

Load Independent AC/DC Power Supply for Higher Frequencies with Sine-Wave Output

For ac voltage power supplies with a dc voltage input, a series inverter can be used. The advantage of sine-wave output and low switching losses is in contrast to the poor regulation of the output voltage and the load dependence of most series inverter circuits. An improved series inverter with a divided resonance capacitor is described. This inverter can operate from no load to full load and for inductive load as well as for resistive or capacitive load. It provides excellent output voltage regulation for all kinds of loads. A steady- state analysis of the inverter for reactive load is given as well as the optimal design for the divided capacitor. A second circuit formed from two of the above inverters provides a controllable output voltage independent of the input voltage and the load and thus overcomes a disadvantage of many series inverters. Both circuits were tested in experimental setups for an output frequency of 400 Hz, and the results are discussed.     

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.     

多场景下含风电机组的配电网无功优化的研究

研究了多场景下含风电机组的配电网无功优化问题。利用概率统计的思想解决了风电机组有功输出的不确定性问题,根据转子侧最大电流限制条件确立了风电机组无功输出范围。结合传统的电容器无功补偿方法,将风电机组作为连续可调无功源参与到配电网的无功优化。建立了以系统网损最小和节点电压越限惩罚为目标的无功优化模型。算例表明不同场景下的风电机组参与配电网无功优化可有效地降低系统的网损,提高各节点电压,同时,增强配电系统受风速影响的适应性。     

A developed control strategy for mitigating wind power generation transients using superconducting magnetic energy storage with reactive power support

The fast variations of wind speed during extreme wind gusts result in fluctuations in both generated power and the voltage of power systems connected to wind energy conversion system (WECS). This paper presents a control strategy which has been tested out using two scenarios of wind gusts. The strategy is based on active and reactive powers controls of superconducting magnetic energy storage (SMES). The WECS includes squirrel cage induction generator (SCIG) with shunt connected capacitor bank to improve the power factor. The SMES system consists of step down transformer, power conditioning unit, DC–DC chopper, and large inductance superconducting coil. The WECS and SMES are connected at the point of common coupling (PCC). Fuzzy logic controller (FLC) is used with the DC–DC chopper to control the power transfer between the grid and SMES coil. The FLC is designed so that the SMES can absorb/deliver active power from/to the power system. Moreover, reactive power is controlled to regulate the voltage profile of PCC. Two inputs are applied to the FLC; the wind speed and SMES current to control the amount active and reactive power generated by SMES. The proposed strategy is simulated in MATLAB/Simulink®. The proposed control strategy of SMES is robust, as it successfully controlled the PCC voltage, active and reactive powers during normal wind speeds and for different scenarios of wind gusts. The PCC voltage was regulated at 1.0 pu for the two studied scenarios of wind gusts. The fluctuation ranges of real power delivered to the grid were decreased by 53.1% for Scenario #1 and 56.53% for Scenario #2. The average reactive power supplied by the grid to the wind farm were decreased by 27.45% for Scenario #1 and 31.13% for Scenario #2.     

A flexible power control strategy for rotor-side converter of DFIG under unbalanced grid voltage sags

Doubly fed induction generator (DFIG) is widely applied in variable-speed wind energy conversion system. The disconnection of a substantial amount of DFIG may arouse the instability problem of power system, thus wind power generators have to remain connected during short-time grid faults. As a result, the voltage sag will lead to overcurrent and overvoltage in the rotor winding of a DFIG, moreover the unbalanced voltage sags will also cause serious fluctuations in its electromagnetic torque and output power. This paper studies the relationship of the stator instantaneous powers with the three-phase stator voltage and rotor current of a DFIG under unbalanced grid voltages. A generalized formula of current reference for the rotor-side converter of DFIG is constructed by introducing continuous adjustment coefficients. Meanwhile, the analytical equations of rotor peak current, stator active and reactive power fluctuations are derived to characterize the operating performance of DFIG. The impacts of adjustment coefficients on DFIG control performance and the feasible region of coefficients restrained by the rotor current constraint are discussed. In consideration of the rotor current limit, the flexible power control strategy for DFIG in unity power factor mode (UPFM) and reactive power supporting mode (RPSM) is presented. The correctness of proposed method is verified by simulation and experiment tests of single-DFIG and multi-DFIG systems.     

A systematic approach to design and operation of a doubly fed induction generator

The doubly fed induction generator (DFIG) is currently one of the most common topologies employed for wind turbine generators (WTGs). The system has the benefit of a back-to-back voltage sourced converter (VSC) of reduced rating, due to its connection to the rotor windings. This paper considers the impact of mechanical and electrical parameters on the kVA requirements of the two VSCs, which together with the dc link capacitor serve as the rotor winding's power supply. This topology is contrasted with alternatives utilizing a diode rectifier-voltage sourced inverter pair and a set of design curves are generated. In addition to steady-state analysis, an operating strategy for reactive power allocation management is proposed. The theoretical considerations are validated with results obtained from representation of the system in an electromagnetic transient program.     

宽风速运行的定子双绕组感应电机风力发电系统励磁电容的优化方案

将控制侧静态励磁控制器(SEC)的直流母线端和功率侧整流桥的输出端通过功率二极管并接起来,可使定子双绕组感应电机(DWIG)风力发电系统在宽风速范围内输出恒定的高压直流。鉴于新拓扑和新系统控制策略的特殊性,本文详细分析了系统在高低风速两种运行状态下的控制绕组电流变化规律,并在此基础上以SEC容量最小为目标,研究了新系统的励磁电容优化方案。方案运用Simulink仿真得到励磁电容变化对控制绕组电流的影响,并依据仿真结果运用循环计算和实验验证相结合的方法,最终得到最优的励磁电容值及最佳的切换转速。实验结果证明了该优化方案的正确性和有效性,新系统在保持SEC容量为电机额定功率1/3的情况下实现了宽风速运行,充分利用了低风速下的风能。     

PWM-VSI inverter-assisted stand-alone dual stator winding induction generator

This paper presents a novel usage of a dual stator winding three-phase induction machine as a stand-alone generator with both controlled output load voltage magnitude and frequency. This generator, with both three-phase power and control windings housed in the stator structure, has the load connected to the power winding and a three-phase pulsewidth modulation (PWM) voltage-source inverter sourcing the control winding. The input to the PWM inverter is either a battery source or a charged DC capacitor. The operational characteristics of these generator schemes with either of the two inverter sources are investigated and shown to have desirable performance. How the load voltage magnitude depends on the various control and design parameters such as rotor speed, compensating capacitance, and load impedance is determined using a detailed mathematical model of the system.