Review of the Analysis and Suppression for High-frequency Oscillations of the Grid-connected Wind Power Generation System

High-frequency oscillation(HFO)of gridconnected wind power generation systems(WPGS)is one of the most critical issues in recent years that threaten the safe access of WPGS to the grid.Ensuring the WPGS can damp HFO is becoming more and more vital for the development of wind power.The HFO phenomenon of wind turbines under different scenarios usually has different mechanisms.Hence,engineers need to acquire the working mechanisms of the different HFO damping technologies and select the appropriate one to ensure the effective implementation of oscillation damping in practical engineering.This paper introduces the general assumptions of WPGS when analyzing HFO,systematically summarizes the reasons for the occurrence of HFO in different scenarios,deeply analyses the key points and difficulties of HFO damping under different scenarios,and then compares the technical performances of various types of HFO suppression methods to provide adequate references for engineers in the application of technology.Finally,this paper discusses possible future research difficulties in the problem of HFO,as well as the possible future trends in the demand for HFO damping.

Identification and Analysis of High-Frequency Oscillations in the Eyewalls of Tropical Cyclones

High-frequency oscillations, with periods of about 2 hours, are first identified by applying wavelet analysis to observed minutely wind speeds around the eye and eyewall of tropical cyclones（TCs）. Analysis of a model simulation of Typhoon Hagupit（2008） shows that the oscillations also occur in the TC intensity, vertical motion, convergence activity and air density around the eyewall. Sequences of oscillations in these variables follow a certain order.

Effect of Nonlinearities in Turbine Governor System on Torsional Oscillation Stability

There are several different nonlinearities in the turbine governor system, such as insensitivity, saturation of servo system, and frequency modulation dead band, which reduce the interaction between the governor system and torsional oscillation, and thereby improve the torsional oscillation stability of shafts. This paper suggests intentional incorporation of appropriate nonlinearities in the governor system to improve the torsional oscillation stability of shafts.

TORSIONAL OSCILLATION OF RIGID DISK IN INFINITE TRANSVERSELY ISOTROPIC ELASTIC CYLINDER

In the present analysis torsional oscillation of a rigid disk in an infinite transversely isotropic elastic cylinder is considered. The effects of anisotropy in the stress intensity factor are shown graphically.

Variations in High-frequency Oscillations of Tropical Cyclones over the Western North Pacific

Variations in the high-frequency oscillations of tropical cyclones （TCs） over the western North Pacific （WNP） are studied in numerical model simulations. Power spectrum analysis of maximum wind speeds at 10 m （MWS10） from an ensemble of 15 simulated TCs shows that oscillations are significant for all TCs. The magnitudes of oscillations in MWS10 are similar in the WNP and South China Sea （SCS）; however, the mean of the averaged significant periods in the SCS （1.93 h） is shorter than that in the open water of the WNP （2.83 h）. The shorter period in the SCS is examined through an ensemble of simulations, and a case simulation as well as a sensitivity experiment in which the continent is replaced by ocean for Typhoon Hagupit （2008）. The analysis of the convergence efficiency within the boundary layer suggests that the shorter periods in the SCS are possibly due to the stronger terrain effect, which intensifies convergence through greater friction. The enhanced convergence strengthens the disturbance of the gradient and thermal wind balances, and then contributes to the shorter oscillation periods in the SCS.

PROPOSALS CONCERNING THE RESEARCH WORK OF TURBOGENERATOR TORSIONAL OSCILLATIONS IN CHINA

In this paper,proposals in the following arcas of turbogenerator torsionaloscillation(TTO)researches in China are presented:* Concerning the focal points of TTO studies;* Concerning the modelling and data acquisition of TTO;* Concerning the computation and analysis of TTO transients;* Concerning the monitoring and analyzing devices of TTO;* Concerning the countermeasures against TTO.It is concluded that researches centered on torsional oscillations caused by system dis-turbances and switching events should begin the carlier the better.

A note for the mechanism of high-frequency oscillation instability resulted from absorbing boundary conditions

In this paper the explanation of the mechanism of high-frequency oscillation instability resulted from absorbing boundary conditions is further improved. And we analytically prove the proposition that for one dimensional discrete model of elastic wave motion, the module of reflection factor will be greater than 1 in high frequency band when artificial wave velocity is greater than 1.5 times the ratio of discrete space step to discrete time step. Based on the proof, the frequency band in which instability occurs is discussed in detail, showing such high-frequency waves are meaningless for the numerical simulation of wave motion.

COMPARISON OF HIGH-FREQUENCY OSCILLATION VEN-TILATION WITH CONVENTIONAL MANDATORY VENTILATION IN ANIMAL ARDS MODEL

To compare effect of high-frequency oscillation ventilation （HFOV） and conventional mandatory ventilation （CMV） on lung injury development in rabbit with acute respiratory distress syndrome （ ARDS）. Methods Animals that underwent saline lung lavage to produce lung injury were randomized to one of the two treatment groups （ HFOV or CMV, n =6）. PaCO2 was maintained between 35 -45mmHg and arterial oxygen saturation （ SaO2 ） was maintain 〉 88% by adjusting corresponding ventilator parameters. Ventilation period was 6h. Lung fluids were aspirated before and at the end of ventilation for cell analysis. Then the animals were euthanized, lung tissue was removed for wet/dry weight measurement, light and electron microscopic examination. Besults The difference of artery blood gas analyses（pH, PaO2, PaCO2 ） between HFOV and CMV was insignificant. The difference between HFOV and CMV in cytological examination of lung fluids, wet/dry weight measurement was also insignificant. But compared with CMV, HFOV not only reduced the area of lung injury, but also reduced lung injury score in light and electron microscopic examination. Conclusion When same artery blood gas analysis was obtained, HFOV significantly reduced lung injury development in ARDS animal than CMV. As a lung protection strategy, HFOV can be used in the treatment of ARDS.

Torsional Vibration of Roller Oscillating Tooth Gear Drive

Torsional vibration of roller oscillating tooth gear drive (ROTGD) is studied in this paper. On the basis of conservation law for kinetic energy and potential energy, the mathematical expressions are developed which describe transformation of moment of inertia of inertial components into input shaft. Also, the formula is derived which expresses transformation of contact stiffness of elastic components into input shaft torsional stiffness. Besides, torsional vibration model of ROTGD is presented by using the transfer matrix method, and natural frequencies and vibration mode shapes are determined. Eventually, an example is given.

Overview of Sub-synchronous Oscillation in Wind Power System

Nowadays with the improvement in the degree of emphasis on new energy, the wind power system has developed more and more rapidly over the world. Usually the wind plants are located in the remote areas which are far from the load centers. Generally series compensated AC transmission and high voltage DC transmission are made use of to improve the transmission capacity as two main effective ways which can solve the problem of large scale wind power transmission. The paper describes the three kinds of impact varieties and impact mechanisms in the sub-synchronous oscillation phenomena of wind power system based on doubly fed induction generator (DFIG) wind generators. At last, we point out the important problem that should be stressed in the wind power system.

Simulation of wind-induced near-inertial oscillations in a mixed layer near the east coast of Korea in the East/Japan Sea

Using a simple damped slab model, it was possible to show that a local wind induced 88% （15 of 17） of the near-inertial oscillations （NIO） observed in the mixed layer near the east coast of Korea from 1999 to 2004. The model, however, overestimated the energy level in about two-thirds of the simulated cases, because the slab model was forced with winds whose characteristic period was shorter than the damping time scale of the model at 1.5 d. At the observation site, due to typhoons and orographic effects, high-frequency wind forcing is quite common, as is the overestimation of the energy level in the slab model results. In short, a simple slab model with a damping time-scale of about 1.5 d would be enough to show that the local wind was the main energy source of the near-inertial energy in this area, but the model could not be used to accurately estimate the amount of the work done by the wind to the mixed layer.

Study and Mitigation of Subsynchronous Oscillations with SSC Based SSSC

This paper proposes a powerful subsynchronous component based (SSC) controller to mitigate the subsynchronous resonance (SSR) with statics synchronous series compensator (SSSC). The mitigation of SSR is achieved by increasing the network damping at those frequencies which are close to the torsional frequency of the turbine-generator shaft. The increase of network damping is done by the extraction of subsynchronous component of voltage and current from the measured signal of the system. From the knowledge of subsynchronous components, a series voltage is injected by SSSC into the transmission line to make the subsynchronous current to zero which is the main cause of turbine oscillations. To analyze the effectiveness of the proposed control scheme, IEEE first benchmark model has taken. The results show the accuracy of the proposed control scheme to mitigate the Torque amplification of SSR.

Prognostic value of high-frequency oscillations combined with multimodal imaging methods for epilepsy surgery

Background:The combination of high-frequency oscillations(HFOs)with single-mode imaging methods has been proved useful in identifying epileptogenic zones,whereas few studies have examined HFOs combined with multimodal imaging methods.The aim of this study was to evaluate the prognostic value of ripples,an HFO subtype with a frequency of 80 to 200 Hz is combined with multimodal imaging methods in predicting epilepsy surgery outcome.Methods:HFOs were analyzed in 21 consecutive medically refractory epilepsy patients who underwent epilepsy surgery.All patients underwent positron emission tomography(PET)and deep electrode implantation for stereo-electroencephalography(SEEG);11 patients underwent magnetoencephalography(MEG).Sensitivity,specificity,positive predictive value,negative predictive value,and accuracy in predicting surgical outcome were calculated for ripples combined with PET,MEG,both PET and MEG,and PET combined with MEG.Kaplan-Meier survival analyses were conducted in each group to estimate prognostic value.Results:The study included 13 men and 8 women.Accuracy for ripples,PET,and MEG alone in predicting surgical outcome was 42.9%,42.9%,and 81.8%,respectively.Accuracy for ripples combined with PET and MEG was the highest.Resection of regions identified by ripples,MEG dipoles,and combined PET findings was significantly associated with better surgical outcome(P<0.05).Conclusions:Intracranial electrodes are essential to detect regions which generate ripples and to remove these areas which indicate good surgical outcome for medically intractable epilepsy.With the assistance of presurgical noninvasive imaging examinations,PET and MEG,for example,the SEEG electrodes would identify epileptogenic regions more effectively.

Determination of the Unperturbed Period of a Torsion Pendulum in the Time-of-Swing Method

A symmetric disc-shaped torsion pendulum was designed to study the nonlinear effects of the torsion fiber. It was showed that the period of the pendulum decreases with the amplitude of the oscillation. Determination of the unperturbed period through a fit to measured data of the period variation with amplitude shows that this systematic error in the measurement of Newtonian gravitational constant G by the time-of-swing method can be reduced by an order of the magnitude at least.

Expert consensus on clinical applications of high-frequency oscillations in epilepsy

Studies in animal models of epilepsy and pre-surgical patients have unanimously found a strong correlation between high-frequency oscillations(HFOs,>80 Hz)and the epileptogenic zone,suggesting that HFOs can be a potential biomarker of epileptogenicity and epileptogenesis.This consensus includes the definition and standard detection techniques of HFOs,the localizing value of pathological HFOs for epileptic foci,and different ways to distinguish physiological from epileptic HFOs.The latest clinical applications of HFOs in epilepsy and the related findings are also discussed.HFOs will advance our understanding of the pathophysiology of epilepsy.

High-frequency oscillations detected by electroencephalography as biomarkers to evaluate treatment outcome,mirror pathological severity and predict susceptibility to epilepsy

High-frequency oscillations(HFOs)in the electroencephalography(EEG)have been extensively investigated as a potential biomarker of epileptogenic zones.The understanding of the role of HFOs in epilepsy has been advanced considerably over the past decade,and the use of scalp EEG facilitates recordings of HFOs.HFOs were initially applied in large scale in epilepsy surgery and are now being utilized in other applications.In this review,we summarize applications of HFOs in 3 subtopics:(1)HFOs as biomarkers to evaluate epilepsy treatment outcome;(2)HFOs as biomarkers to measure seizure propensity;(3)HFOs as biomarkers to reflect the pathological severity of epilepsy.Nevertheless,knowledge regarding the above clinical applications of HFOs remains limited at present.Further validation through prospective studies is required for its reliable application in the clinical management of individual epileptic patients.

钻柱的黏滑与高频扭转耦合振动测量与分析

钻井系统的自激扭转振动会导致钻头和地层之间的接触力或切削力相对速度出现下降特征。为减轻这种机制的影响,通过对三轴振动的时域、频域分析,研究了钻柱扭转振动特征。研究结果发现,低频的扭转振动会引发黏滑振动,黏滑振动频率为0.128 Hz,三轴振动和转速会出现周期性波动。钻柱发生高频扭转振动(HFTO)时,三轴加速度都出现了177.2 Hz的主频率。时域分析发现,切向加速度远大于轴向和法向加速度峰值,均方根值也较高,表明切向振动波动较大、能量高,说明此时井下正发生扭转振动。黏滑与HFTO发生耦合时,法向加速度会出现2个主频,即黏滑时的主频和HFTO的主频。高扭转频率会提高扭矩和机械转速导致钻具疲惫。研究结果对描述扭转振动的特征,判断钻井过程是否发生黏滑、HFTO和及时采取消除黏滑振动、缓解钻具疲惫技术措施具有指导作用。

基于能量耗散的电压源型双馈风电机组轴系扭振阻尼特性分析

电压源型双馈风电机组具有建立和支撑电压的能力从而受到了广泛关注,然而采用电压源控制的双馈风电机组(doubly fed induction generators,DFIG)响应电网动态会更易引起轴系扭振问题,甚至恶化扭振阻尼。因此从能量耗散的角度分析了轴系扭振的阻尼特性,并提出了轴系扭振阻尼的量化表征方法。首先,基于轴系的二质量块传动模型构建了轴系的暂态能量函数;在此基础上,提取出轴系的势能和耗散能量,表征轴系振荡过程中能量的积累和耗散效应,并探究了能量耗散与特征值实部之间的联系;分析了不同的虚拟同步机(virtual synchronous generator,VSG)控制参数对轴系扭振阻尼特性的影响;并定义了能量耗散因子用以量化表征轴系扭振的阻尼。最后对提出的结论与方法进行了仿真验证。

钻孔差应变仪观测的苏门答腊大地震激发的地球环型自由振荡

我们率先用钻孔应变仪观测资料研究了地球自由振荡．泰安地震台有差应变和体应变两种钻孔应变观测仪器，都观测到2004年12月26日苏门答腊大地震激发的地球自由振荡．体应变仪观测到地球的球型振荡，而差应变仪观测到地球的环型振荡．两种观测还记录到环型振荡与球型振荡相互的耦合作用．观测也记录到明显的一些振型的谱线分裂现象．用直接观测到的高质量的环型振荡资料求解了0T8-0T19等主要振型的Q值．根据两个方向的剪应变观测数据，求解了环型振荡的最大剪应变方向，发现这个方向是相当稳定的．这种研究表明，高精度钻孔应变观测仪是一种理想的地壳形变观测仪器，可以为地球自由振荡研究提供更丰富的信息．

电力系统的次同步振荡问题

随着我国互联电网规模的快速发展,尤其是(可控)串联补偿装置和高压直流输电的广泛应用,电力系统的次同步振荡问题已经变得比较突出。本文介绍了电力系统次同步振荡问题的起因与危害,回顾了国内外对次同步振荡问题的研究过程,阐述了次同步振荡所研究的感应发电机效应、轴系扭转振荡、暂态扭矩放大以及由其它电气装置(如电力系统稳定器PSS、静止无功补偿器SVC和HVDC等)引起的次同步振荡现象的主要内容,指出了需要进一步关注和研究的问题。