Performance evaluation of 6000‐A‐class superconducting field windings for 600‐MW‐class superconducting generators

Superconducting generators have many advantages such as increasing generator efficiency and improving power system stability. In Japan, a national project has been conducted since 2000 which is aimed at the development of fundamental technologies required for high‐output‐density and large‐capacity superconducting generators. This paper describes the results of this project, focusing on 6000‐A‐class field winding development. Copyright © 2004 Wiley Periodicals, Inc. A superconducting generator with a high output density and a large capacity has inherent factors that decrease superconducting stability. These are: (1) increase in the magnetic field in the winding which is caused by the increase in winding current density and (2) difficulty in fabricating windings which increases as a conductor diameter becomes larger. To secure the stability, we adopted a higher‐copper‐content conductor and a design that increases winding fixing pressure, along with devising a winding method that accommodates larger conductor diameter. These improvements were applied to a partial model of a 600‐MW field winding. Test results of the model showed good stability, indicating that design and fabrication technique for a 6000‐A‐class superconducting field winding has been successfully evaluated. © 2007 Wiley Periodicals, Inc. Electr Eng Jpn, 159(2): 7– 18, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20332     

Development of a 70‐MW‐class quick response excitation superconducting generator

A superconducting generator is expected to be a next‐generation machine due to its many advantages over a conventional generator. Super‐GM has been promoting R&D of 70‐MW‐class superconducting model generators to verify the basic technologies required for the design and manufacture of a 200‐MW‐class superconducting pilot generator. Verification tests of the third model generator, quick response excitation type C generator, were finished in June 1999. The excellent characteristics as a superconducting generator were verified through the series of tests. We report main test results such as “open and short‐circuit test,” “loaded test,” “quick excitation test,” “negative‐phase sequence overcurrent test,” and “sudden short circuit test.” © 2002 Wiley Periodicals, Inc. Electr Eng Jpn, 140(1): 22–29, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.10029     

Performance evaluation of squirrel‐cage damper structure for superconducting generators

Superconducting generators have many advantages such as increasing generation efficiency and improving power system stability. In Japan, 70‐MW‐class superconducting generators have been developed through the national project “Electric Power Application of Superconducting Technologies (1988–1999).” In this paper, performance evaluation of the rotor damper structure for the 70‐MW‐class model generator is discussed. Double‐layered damper structure, which consists of a squirrel‐cage warm damper and a single‐layered cold damper, was set as the target. As the first step, basic electromagnetic and thermal performances were obtained. Consequently, it was proven that the damper structure had the performances to satisfy the requirements of the superconducting generators. Performances obtained from the experimental model, design of the 70‐MW model rotor, and evaluation by the verification tests are described in this paper. © 2005 Wiley Periodicals, Inc. Electr Eng Jpn, 151(1): 19–33, 2005; Published online in Wiley InterScience ( www.interscience. wiley.com ). DOI 10.1002/eej.20025     

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.     

A study of AC link and DC link method for wind power generation connected to electric power system

In this paper, grid‐connection of wind power generators was evaluated from the viewpoint of frequency fluctuation. Wind power generation is a power generation method that depends on natural energy, and there is some concern that it may exert a negative influence on electric power quality. As a result, it is necessary to maintain high electric power quality when wind power generation is connected to the grid. The AC link method, the AC‐DC link method, and the DC link method are alternatives for grid connection of wind power generators. The model system was constructed with the use of a synchronous generator and an induction generator as wind power generator, and verification experiments were performed. Verification experiments using the various grid‐connection methods for each generator were conducted. The steadiest frequency characteristic was obtained in the DC link method. In particular, the stability level was highest in the wind power generation system using the DC link method with a wound‐rotor induction generator. Generally, induction generators are grid‐connected by the AC link method. Therefore, grid‐connection of induction generators by the DC link method appears effective from the viewpoint of frequency fluctuation. © 2011 Wiley Periodicals, Inc. Electr Eng Jpn, 178(3): 21–30, 2012; Published online in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/eej.21196     

State criterion of wind turbine generator operation using tower shadow effect (Part 2)

In order to obtain wind energy effectively, the pole‐change‐type induction generators are used as the wind turbine generators. Otherwise, the pole‐change‐type induction generator causes the voltage dips at pole changing time. To maintain the power quality, it is important to know the state change of the generator operation. Therefore, the authors have studied a state criterion of generator using the tower shadow effect, which is the active power oscillation caused by a rotation torque drop when the tower and the turbine blade overlap each other. In this paper, an improved identification method of oscillation frequency, which is the criterion of wind turbine generator operation, is proposed. The proposed method is applied to measured data and good results are obtained. © 2007 Wiley Periodicals, Inc. Electr Eng Jpn, 162(1): 25–31, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20395     

Maximum output power control system of variable‐speed small wind generators

This paper proposes a maximum output power control system for variable‐speed small wind generators. The proposed control system adjusts the rotational speed of a single‐phase AC generator to the optimum rotational speed, which yields the maximum output power according to the natural wind speed. Since this adjustment is performed on‐line in order to adapt to variations in wind speed, the rotational speed of the single‐phase AC generator is adjusted by controlling the generated current flowing in an FET (field‐effect transistor) device, serving as the generated power brake, which is linked directly to the single‐phase AC generator. In order to reduce heat loss from the FET device, a PWM (pulse width modulation) controller is introduced. An experimental model of the proposed control system was built and tested, and the validity and practicality of the proposed control system were confirmed by the experimental results. © 2008 Wiley Periodicals, Inc. Electr Eng Jpn, 165(1): 9–17, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20692     

Transient stability simulation of wind generator expressed by two‐mass model

Recently, wind power generation is increasing worldwide. In wind power stations, induction machines are mostly used as generators. Since induction generators have a stability problem similar to the transient stability of synchronous machines, it is important to analyze the transient stability of power systems including wind generators. Although there have been some reports analyzing the transient stability problem, wind turbine and wind generator are, in most cases, modeled as a one‐mass shaft system having total inertia constant. This paper presents simulation analyses of transient stability of power system including induction generator which is expressed by a two‐mass shaft model and analyzes an effect of shaft system modeling on the transient stability characteristics. Simulations are performed by PSCAD/EMTDC in this study. © 2007 Wiley Periodicals, Inc. Electr Eng Jpn, 162(3): 27–37, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20394     

双馈电机在3MW齿轮箱试验台中的应用

在分析同步电机、双馈电机的特性基础上,提出了一种采用双馈电机对拖的方式来实现3 MW齿轮箱的试验方法.采用2台双馈电机和1台变频电机进行控制,变频电机控制试验台的速度以及补偿试验台的功率损耗,分别控制双馈电机转子的电流和频率来达到控制负载功率的目的.该方法具有良好的控制精度、高效节能,投资成本低.     

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.     

Power smoothing and MPPT for grid‐connected wind power generation with doubly fed induction generator

Recently, doubly fed induction generators (DFIG) and synchronous generators have been applied mostly to wind power generation, and variable speed control and power factor control have been implemented for high efficiency of wind energy capture and for high quality of power system voltage. In variable speed control, the wind speed or generator speed is used for maximum power point tracking. However, the properties of wind generation power fluctuations due to wind speed variation have not yet been investigated for those forms of control. The authors discuss power smoothing by these forms of control for DFIG interconnected to a 6.6‐kV distribution line. The performance is verified by means of the power PSCAD/EMTDC system simulation software for actual wind speed data and is investigated by using an approximate equation for wind generation power fluctuations as a result of wind speed variation. © 2011 Wiley Periodicals, Inc. Electr Eng Jpn, 177(2): 10–18, 2011; Published online in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/eej.20958     

Magnetic field analysis of a 70‐MW‐class superconducting generator by the 3D finite element method

The three‐dimensional magnetic field and eddy current distribution of the 70‐MW‐class superconducting generator was calculated by a finite element method. The condition for the exciting armature coil was calculated by the jω‐method. In the magnetic flux distribution of the armature winding bore, the value obtained by analysis had good agreement with the measurement. An analysis that let the rotor move by the synchronizing speed was also done. The armature voltage for the nonload condition and the armature current for the three‐phase short‐circuit condition obtained by calculations had good agreement with the measurements. The validity of the analytical model for three‐dimensional magnetic field analysis of superconducting generators was confirmed from these results. In addition, the synchronous reactances were calculated using these results with eddy current in the facing. It was found that the facing had the effect of decreasing synchronous reactance by about 5%. © 2000 Scripta Technica, Electr Eng Jpn, 134(2): 53–60, 2001     

Development of voltage sag compensator and UPS using a flywheel induction motor and an engine generator

Flywheel energy storage systems are attracting attention as uninterruptible power supplies (UPS) from the viewpoint of environmental friendliness and high durability performance. Using a low‐speed, heavy flywheel and a low‐cost squirrel‐cage induction motor/generator, two applications are proposed: (1) an 11‐kW voltage sag compensator using a capacitor self‐excited induction generator without semiconductor converters and (2) a UPS composed of the flywheel system and an engine generator. Based on experimental results, an ideal voltage sag compensator and UPS are realized by the low‐technology flywheel system. © 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 167(1): 74– 81, 2009; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20784     

百万千瓦级超超临界汽轮发电机试验站拖动系统的方案选择

百万千瓦级超超临界汽轮发电机试验站拖动系统的选择既应满足最大容量等级汽轮发电机的拖动要求,同时也应尽量满足各种不同容量等级的汽轮发电机试验时的能源节约要求。拖动系统不仅要满足汽轮发电机的起动要求,同时也应保证汽轮发电机紧急时刻的安全刹车。拖动系统采用两台异步电动机联合运行方案,6MW异步电机由变频器供电,用于被试汽轮发电机的起动、制动,11MW异步电机挂网运行与6MW异步电机联合拖动,共同承担短路试验的负荷。     

Doubly fed induction generator systems for wind turbines

This article shows that adjustable speed generators for wind turbines are necessary when output power becomes higher than 1 MW. The doubly fed induction generator (DFIG) system presented in this article offers many advantages to reduce cost and has the potential to be built economically at power levels above 1.5 MW, e.g., for off-shore applications. A dynamic model of the DFIG was derived to develop a vector controller to decouple dynamically active and reactive power control. Simulations show excellent response of the DFIG independent of speed. Measurements obtained from 1.5 MW units currently in operation confirm the theoretical results     

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.     

双馈风力发电机组及其低电压穿越技术的研究

双馈风力发电机组是风力发电机型中的主流机型,从双馈电机的基本工作原理出发,建立了双馈发电机的数学模型,并对其在风速发生变化时进行仿真.利用其定子磁链定向对其进行矢量控制,分析了双馈电机转矩和功率独立调节的基本原理.研究了双馈风电机组在电网故障时其运行情况,通过硬件方法使双馈风电机组具有更强的低电压穿越能力.就是在电网出...     

基于超导储能的直驱风电系统功率平滑控制

针对并网运行直驱风力发电系统输出有功功率的波动问题,对变换器直流环节并联超导储能系统的控制方式在计及风速模型的基础上做进一步的分析,并对超导储能系统斩波器提出双闭环加脉冲判断的控制策略,确保超导磁体线圈电流水平,使超导储能系统可以快速、准确地吞吐能量,保证直驱风力发电系统在最大限度捕获风能的同时,向电网输送较为平滑的有功功率。对增加超导储能系统的直驱风力发电系统的建模仿真结果,说明了该方法的正确性和有效性。     

Pitch control based on voltage dip detection for improving the LVRT of wind farm

For the stability of power systems including large‐scale generation of wind power, wind farms are expected to fulfill the requirement with the capability to remain connected to the systems during a momentary voltage dip occurring in power networks. This has prompted many utilities to adopt the low‐voltage ride‐through (LVRT) of wind turbine generators (WTGs) as one of the requirements in interconnection of large wind farms. This paper presents a new method of pitch angle control for fixed‐speed wind turbine (FSWT) to achieve LVRT capability improvement. The FSWT is equipped with directly grid‐coupled squirrel‐cage induction generator and the LVRT behavior of such wind turbine is closely related to the overspeeding of wind turbine rotor during voltage dip. If the turbine rotor speed can be reduced quickly during voltage dip so as not to rise over the maximum speed, then the sudden disconnection of WTG can be avoided. The proposed pitch control system can modify the pitch angle in the short response time by the coordination of protective relay. Then the pitch angle is adjusted by a feedback proportional integral controller based on the measurement of induction generator terminal voltage. Simulation study shows that the application of the proposed pitch control system can improve the LVRT performance of a wind farm equipped with FSWTs. © 2011 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

Sensorless vector controlled converter for variable speed wind generation system using an induction generator

This paper proposed two types of speed sensorless converters that can control rotational speed and electric power. One is based on a slip frequency control system. The other is based on a vector control system. The rotational speed of the wind turbine is estimated with the phase voltage and phase current of the induction generator by the adaptive rotor flux observer. The estimated wind turbine rotational speed ω_r^est is used as the feedback of the speed control loop in the converter control system. Also, the estimated rotor flux ?_2d^est is used for the vector control. The simulation results confirm that both of them perform satisfactorily under the speed sensorless operation. The method based on the vector control system generates more electrical energy than does the method based on the slip frequency control system. © 2007 Wiley Periodicals, Inc. Electr Eng Jpn, 159(4): 62–75, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20323