谐波对换流变压器损耗的影响研究

高压直流输电的换流变压器是一种高度非线性的谐波源,运行产生的谐波会影响其负载损耗。采用IEC61378-2-2001和IEEE1158-1991负载损耗算法,对换流变压器的负载损耗进行了实例计算,分析了谐波对换流变压器负载损耗的影响。研究结果表明,谐波对换流变压器损耗的影响程度随谐波次数的不同而变化,高次谐波的影响很小,低次谐波的影响较大。该结论可为谐波控制研究提供参考。     

桩周土开挖条件下桩基础承载特性数值分析

运用桩基础支撑上部既有建筑物地下增层的过程中,随着桩周土减少和桩基础的暴露,桩的承载特性会出现明显变化.文章运用ANSYS有限元程序分析了桩周土开挖前、后以及不同开挖深度等工况下,桩基础承载特性的变化规律.结果表明：随着桩周土开挖深度增大,被开挖段的桩身侧阻力消失,需要增加未开挖部分桩身侧阻力、桩端阻力来增强桩身轴力.同时,也需增大桩基沉降量和桩土间的滑移量.     

微电网中光伏并网发电系统的建模仿真

微电网技术的发展能有效地解决分布式发电的接入问题,有利于新能源的开发.太阳能作为最具有利用价值的清洁能源之一,并网光伏发电系统的应用前景越来越广阔.通过利用Mat-lab\Simulink软件,开发出了适用于工程计算的光伏电池模型,基于扰动观察法建立了最大功率跟踪模型,结合空间矢量调制(SVPWM)算法设计出了电压外环-电流内环双环控制结构的逆变器并网控制策略,在此基础上搭建了完整的光伏并网发电系统运行与控制模型.仿真结果表明建立的模型能较好的反映三相光伏并网系统运行特性,为微电网的进一步研究提供了基础.     

微电网运行的稳定控制研究

稳定运行是微电网的首要目标,针对微电网运行过程中功率不平衡的问题,搭建北京建筑大学微网实验平台,选用beckhoff的TwinCATPLC作为主控制,编写控制方案以调节储能、改变负荷,实现了微电网系统的稳定运行.试验结果表明,编写的控制方案能较好地模拟微网的不同状态,使微电网发出功率平衡,且可以实现不同状态下微网的平滑过渡.     

杂合抗菌肽MgJ抗菌活性的初步研究

采用琼脂孔穴扩散法和微量稀释法,对人工合成的杂合抗菌肽抗菌肽MgJ的抗菌活性进行研究,结果表明,抗菌肽MgJ具有较强的抑制大肠杆菌与金黄色葡萄球菌的活性,对金黄色葡萄球菌的最低抑菌浓度(MIC)为10μg/ml,对大肠杆菌的最低抑菌浓度(MIC)为5μg/ml.     

基于模糊评价法的毕业设计成绩评定体系研究

毕业设计成绩评定是一个多因素综合评价问题,采用模糊评价法能避免评价过程中主观性因素对结果的影响,本文提出了毕业设计模糊评价体系,给出了综合评判义性失效时的解决办法,讨论了各项指标分别评定的优越性．     

酸气回注方案关键技术研究

酸气回注是一种安全环保的酸气处理方式,其原理是将酸气压缩后通过井筒回注到已废弃的产层或水层中.基于酸气不同组分的相平衡机理进行酸气回注方案设计,分析不同相态的输送方式,结合酸气脱水、增压、管线腐蚀和水合物防止等关键工艺技术,优选合理的回注工艺参数.     

A modern and robust methodology for modeling anisotropic creep characteristics of Ni-based DS and SC superalloys

According to more recent work,the Wilshire equations have shown good prediction accuracy in a wide range of materials and stress-temperature conditions,particularly in extrapolation of short term results to long term predictions.In the current paper,this methodology was further developed for modeling anisotropic creep characteristics(i.e.minimum creep strain εmin ,stress rupture life tf and time to a specified strain tε)of four typical Ni-based directionally solidified(DS)and single crystal(SC)superalloys,where a simple orientation factor related to the ultimate tensile strength(UTS)was introduced.The application of these simplistic approaches showed that the anisotropic creep characteristics in a wide range of stress-temperature conditions can be accurately simulated.Meanwhile,during the application of the modified Wilshire equations,break points occurring at the specified stress levels agree well with the transition of creep deformation mechanisms occurring in different stress regions,which provides confidence for using this method.     

A miniature liquid helium temperature JT cryocooler for space application

Along with the development of space science and technology,miniature liquid helium temperature long life cryocooler is a focus subject in cryogenic study.Since it is the precondition of space detection researches,institutions of space in many countries do the research on it.In this article,we designed a compound cooling system.A three-stage high frequency thermal coupled pulse tube cryocooler was used to precool a Joule-Thomson(JT)cryocooler.This system has no moving parts at low temperatures and is hence suitable for space operation.Liquid helium temperature was successfully achieved in both open loop and closed cycle experiments.In the closed cycle experiment,when 473 W electric power was inputted,the cooling system reached a no-load temperature of 4.4 K,and a cooling capacity of 11.6 mW was provided at 4.54 K.It is the first miniature liquid helium temperature JT cryocooler in China and the research achievement paves a way for the space application of ultra-long wave infrared detection and THz technologies.     

Large eddy simulation and proper orthogonal decomposition analysis of turbulent flows in a direct injection spark ignition engine: Cyclic variation and effect of valve lift

Large eddy simulation(LES)is used to calculate the in-cylinder turbulent flow field in a direct injection spark ignition(DISI)engine.The computations are carried out for three different maximum valve lifts(MVL)and throughout 100 consecutive engine cycles.The simulated results as well as corresponding particle image velocimetry(PIV)measurement database are analyzed by the proper orthogonal decomposition(POD)method.Through a new developed POD quadruple decomposition the instantaneous in-cylinder flow fields are decomposed into four parts,named mean field,coherent field,transition field and turbulent field,respectively.Then the in-cylinder turbulent flow characteristics and cycle-to-cycle variations(CCV)are studied separately upon the four part flow fields.Results indicate that each part exhibits its specific characteristics and has close connection with others.The mean part contains more than 50%of the total kinetic energy and the energy cascade phenomenon occurs among the four part fields;the coherent field part possesses the highest CCV level which dominates CCV of the bulk flow.In addition,it is observed that a change in MVL affects significantly the in-cylinder flow behavior including CCV,especially for the coherent part.Furthermore,the POD analysis demonstrates that at least 25 sample cycles for the mean velocity and 50 sample cycles for the RMS velocity are necessary for obtaining converged and correct results in CCV.