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     

Matching between straight‐wing nonarticulated vertical axis wind turbine and a new wind turbine generator

In the current wind turbine generation system, there are substantial problems such as the fact that the maximum power of the wind turbine cannot be obtained in the presence of fluctuating wind speed, as well as high cost and low annual net electricity production (due to mismatch between generators and wind turbines). A new wind turbine generator optimized for the wind turbine output is presented in order to solve such problems. This wind turbine generator consists of a permanent magnet generator, a reactor, and a rectifier, and uses neither a control circuit which requires standby electricity nor a PWM converter having a switching element. By selecting the most appropriate combination of a permanent magnet generator with multiple windings and a reactor connected in series with each winding, the maximum output of the wind turbine can be obtained without using a control circuit. The new wind turbine generator was directly coupled with a straight‐wing nonarticulated vertical‐axis wind turbine (SW‐VAWT), and matching of the generator with the wind turbine was examined in field tests. The test results and review confirm that the new wind turbine generator is highly matched with the wind turbine in the presence of fluctuating wind speed. © 2010 Wiley Periodicals, Inc. Electr Eng Jpn, 174(2): 26–35, 2011; Published online in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/eej.21036     

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     

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.     

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.     

Direct measurement of lightning current through a wind turbine generator structure

The development of a wind turbine generator system in Japan is regarded as a promising source of clean energy, and its popularization is rapidly increasing. In Japan, there are many wind turbine generator systems installed on the Sea of Japan coast, with large wind capacity. However, since the Sea of Japan coast is a region with abundant winter lightning, a phenomenon peculiar to Japan, the wind turbine generator systems are subject to damage from winter lightning. Thus, it is important to observe the lightning currents striking wind turbine generator systems in order to provide lightning protection. The authors therefore installed currents sensors (Rogowski coils) on wind turbine generators installed in Hokkaido and Kagoshima Prefectures, and performed waveform observation of lightning stroke current in the winter of 2002 and the summer of 2003. These observations yielded a few current waveforms for lightning striking wind turbine generators. This paper describes the investigation and presents the observation results. © 2006 Wiley Periodicals, Inc. Electr Eng Jpn, 157(4): 40– 47, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20250     

A new electro‐static micro‐generator for energy harvesting from diaphragm muscle

In this paper, we propose a new energy scavenging system for powering bio‐implantable devices. Different available energy sources and micro‐generators are examined. The operation of electrostatic generators will be examined in detail, and their design issues are discussed. The proposed generator is analyzed, and a closed form formula is developed for the capacitor voltages. Also, an equation is obtained for the output power for the proposed circuit. It will be shown that the diaphragm muscle in conjunction with the electrostatic micro‐generator can be used to extract energy from human body. The diaphragm muscle has a continuous movement with a relatively fixed frequency. There is enough space around this muscle to place the micro‐generator. Using the proposed system, it will be shown that it is possible to produce 230 mW power in a 125 mm³ volume from the diaphragm muscle. Copyright © 2017 John Wiley & Sons, Ltd.     

A study of generator system selection for large wind turbine generator system

This paper focuses on selection of wind turbine generation systems that include generators, converters, and gears. We study three systems: a permanent magnet generator (PMG) system, a doubly‐fed generator (DFG) system, and a synchronous generator (SYG) system in terms of the system efficiencies and running costs. The system efficiencies and running costs are calculated by considering the relationship between wind power and wind conditions. According to these results, the one‐step gear PMG system is the best choice for a large wind turbine system. © 2007 Wiley Periodicals, Inc. Electr Eng Jpn, 161(1): 51–57, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20506     

Influence of quadrature‐axis reactance of large turbine generator on operation

Two‐pole large generators are extensively used in power utilities. Growing demand for electric power in recent years has led to substantial increases in the capacity of generators. In line with this trend, our company has manufactured two‐pole turbine generators in the 900,000‐kVA to 1,200,000‐kVA classes. As the capacity of generators becomes larger, equivalent circuits are increasingly expected to improve their precision in estimating operation characteristics. The Park model has been widely used as an equivalent circuit model. The Park model with quadrature axis transient reactance $x_{q}^{\prime}$ is claimed to be useful for accurately estimating the load rejection characteristics of turbine generators. However, it is difficult to measure $x_{q}^{\prime}$, few studies based on measured data have appeared. As one of the few examples, we have conducted a measured data on a 500,000‐kVA‐class generator. Recently, the authors obtained test data on $x_{q}^{\prime}$ of a 900,000‐kVA‐class generator. In this study, based on these data, we studied the effect of $x_{q}^{\prime}$ using the finite element method and a transient analysis program (EMTP‐ATP). © 2011 Wiley Periodicals, Inc. Electr Eng Jpn, 177(3): 17–27, 2011; Published online in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/eej.21099     

On‐line partial discharge detection of turbine generator using gigahertz‐band spatial phase difference method

Partial discharge (PD) within voids in solid insulating systems is a symptom and/or a direct cause of a deterioration of stator windings of a turbine generator. The bandwidth of the electromagnetic waves emitted from PD is very broad and ranges to the gigahertz. We developed a new system based on the spatial phase difference method for detecting microwaves (GHz) emitted from PD. In this paper we applied this system to two off‐line turbine generators in the overhaul and an on‐line turbine generator which has two built‐in antennas. On‐line PD detection of the turbine generator is very useful for recognizing abnormal and/or deteriorated stator winding insulation without a machine outage. It was found that the detection system was able to detect microwaves emitted from PD in an operating turbine generator. © 2000 Scripta Technica, Electr Eng Jpn, 131(4): 38‐43, 2000     

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     

Starting method of a superconducting generator in a combined‐cycle power plant

In the combined‐cycle power plant generators are started by using the igniting arrangement up to the ignition rotational speed of the gas turbine. On the other hand, in the case of using the superconducting generator, it is difficult to apply the igniting arrangement used to generate electricity on the combined cycle for the structure as is. We examined the induction motor starting method for the superconducting generator by using the 70‐MW‐class quick‐response excitation superconducting model generator and the VVVF power supply. From the examination, we confirmed the ability to raise the rotational speed from 6 to 360 rpm. Moreover, it was found to be able to start 200‐MW‐class superconducting generators by the induction motor starting method with the analysis. © 2007 Wiley Periodicals, Inc. Electr Eng Jpn, 160(2): 30– 38, 2007; Published online in Wiley InterScience ( www.interscience. wiley.com ). DOI 10.1002/eej.20283     

Study of Low‐Voltage Ride‐Through Performance for Wind Power Generation with Doubly Fed Induction Generator

The introduction of wind power generation is increasing rapidly. The ratio of wind power generation to the total generation capacity is becoming higher and higher. When a phase‐to‐phase fault occurs in the power system, the frequency of the power system is lower due to disconnection of wind power generation with doubly fed induction generators (DFIG). Therefore, the power system might become unstable. This paper describes an LVRT (low‐voltage ride‐through) performance improvement scheme for wind power generation with DFIG. The wind power generator is disconnected from the grid in case of a power system fault. It is made to operate in isolation from the grid by controlling the inverters installed with the generators. After clearance of the power system fault, wind power generation is immediately reconnected to the grid. As a result, instability in the power system disappears. The performance of LVRT is confirmed by using the simulation software PSCAD/EMTDC. The simulation results show excellent results for the three‐phase short‐circuit fault with a voltage dip of 100%. © 2013 Wiley Periodicals, Inc. Electr Eng Jpn, 185(1): 17–26, 2013; Published online in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/eej.22423     

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.     

A fuzzy logic‐controlled SMES for damping shaft torsional oscillations of synchronous generator

This paper presents a fuzzy logic control scheme for the superconducting magnetic energy storage (SMES) based on a PWM voltage source converter and a two‐quadrant chopper using an insulated‐gate‐bipolar‐transistor (IGBT) to dampen turbine‐generator shaft torsional oscillations. Simulation results of balanced faults in a single machine connected to an infinite bus system show that the proposed fuzzy logic‐controlled SMES is effective in damping shaft torsional oscillations of synchronous generators (GENs). © 2006 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     

风电机组综合状态在线监测与远程诊断中心的建立

近年来,风电机组振动状态在线监测系统得到不断地应用和推广,但是仅对机组机械传动部分振动信号进行评估分析,监测手段过于单一,不能全方位捕捉机组故障状态.为此,提出在风电机组振动在线监测技术研究的基础上,进行风电机组叶片应变状态监测、齿轮箱油液在线监测、发电机电气参数监测及风电机组生产运行参数监测技术的研究,研制风力发电机组综合状态在线监测系统,并建立远程诊断中心,将风力发电机组振动在线监测系统全面升级为风力发电机组综合状态在线监测系统,以期为实现风电场少人、无人值守目标奠定坚实的基础.     

An experimental study of lightning overvoltages in wind turbine generation systems using a reduced‐size model

Wind turbine generation systems are built at locations where few tall structures are found nearby so as to obtain good wind conditions, and thus, they are often struck by lightning. To promote wind power generation, lightning‐protection methodologies for such wind turbine generation systems have to be established. This paper presents the result of an experimental study of lightning overvoltages in wind turbine generation systems using a reduced‐size wind turbine model. Overvoltages observed at wavefronts of lightning surges are focused on in this study. In the experiments, lightning strokes to one of the blades and to the nacelle were considered, and voltages and currents at various positions of the wind turbine model were measured. The following points have been deduced from the results: (i) The voltage rise due to the tower footing resistance can cause a significant voltage difference between the tower foot and an incoming conductor led from a distant point. Also, a voltage difference between the bottom of down conductors installed inside the tower and an incoming conductor can be of significance. (ii) The lightning current flowing through the tower body induces voltages in main and control circuits which form loops, and the induced voltages can cause overvoltages and malfunctions. (iii) Traveling‐wave phenomena in a wind turbine generation system for a lightning strike to the tip of a blade and to the nacelle have been clarified from the measured waveforms. This information can be used for developing an EMTP simulation model of wind turbine generation systems. © 2007 Wiley Periodicals, Inc. Electr Eng Jpn, 158(4): 22– 30, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20466     

Influence of wind turbulence on yaw‐control gears in wind turbine

This paper describes the influence of wind turbulence on the yaw‐control gears of the nacelle in a wind power station. The site is located on Tappi Cape in Aomori prefecture, where major wind turbulence occurs due to the strong western wind and the steep slope of the cape. This paper discusses two adjacent wind towers out of a total of eleven and clarifies the influence of wind turbulence on the leeward wind power generator. Measurements showed that the turbulence and the standard deviation frequently exceeded IEC Wind Turbine Standards. Consequently, the torque applied to the yaw‐control gears oscillated with a short time period and its magnitude was frequently greater than 4.8 ton‐m (47 kN‐m), regarded as the metal fatigue limit. By a method in which the output of the wind turbine generator is controlled, the magnitude of the oscillating torque can be reduced below the limit of material fatigue. © 2011 Wiley Periodicals, Inc. Electr Eng Jpn, 176(4): 15–25, 2011; Published online in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/eej.21141     

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。