Electromechanical Transient Modeling Analysis of Large-Scale New Energy Grid Connection

The synchronous virtual machine uses inverter power to imitate the performance of the conventional synchronous machine.It also has the same inertia,damping,frequency,voltage regulation,and other external performance as the generator.It is the key technology to realize new energy grid connections’stable and reliable operation.This project studies a dynamic simulation model of an extensive new energy power system based on the virtual synchronous motor.A new energy storage method is proposed.The mathematical energy storage model is established by combining the fixed rotor model of a synchronous virtual machine with the charge-discharge power,state of charge,operation efficiency,dead zone,and inverter constraint.The rapid conversion of energy storage devices absorbs the excess instantaneous kinetic energy caused by interference.The branch transient of the critical cut set in the system can be confined to a limited area.Thus,the virtual synchronizer’s kinetic and potential energy can be efficiently converted into an instantaneous state.The simulation of power system analysis software package(PSASP)verifies the correctness of the theory and algorithm in this paper.This paper provides a theoretical basis for improving the transient stability of new energy-connected power grids.

Transient Model for Shafting Vibration of Hydro Turbine Generating Sets

The shafting vibration is closely related to the rotational angular speed.The angular speed of hydro turbine generating sets(HTGS)is rapidly change in fault transient,it maybe reduce the shafting damage.By means of energy analysis,the differential equation of shafting vibration for the HTGS is derived,in which include the equation of generator rotor and hydro turbine runner,it can be applied to transient analysis.Shafting model is transformed into first order differential equation groups,and is combined with the motion equation of HTGS to build integrated model.Various additional forces of shafting are taken as input inspire in proposed model,the generality of model is good.At last,the shafting vibration in emergency stop transient is simulated.

Comparative Study on Electromagnetic and Electrome-chanical Transient Model for Grid-connected Photovoltaic Power System

With the development of new energy technology, there are increasing applications of grid-connected photovoltaic power generation system. However, there is little research on development of electromechanical model of large scale photovoltaic power station. The computational speed will be very slow if electromagnetic transient model is used for stability study because of its complexity. Therefore, study on electromechanical transient model of grid-connected photovoltaic power generation system is of great meaning. In this paper, electromagnetic transient model of photovoltaic power generation system is introduced first, and then a general electromechanical transient model is proposed. These two kinds of simulation model are set up in PSCAD. By comparing the simulation results of two models, the correctness and validity of the electromechanical transient model is verified. It provides reference model for efficient simulation and modeling of grid-connected photovoltaic power station in large-scale power systems.

Mitigation of Transients in Capacitor Coupled Substations Using Traditional RLC Filter Techniques

This article presents an extensive examination and modeling of Capacitor Coupled Substations (CCS), noting some of their inherent constraints. The underlying implementation of a CCS is to supply electricity directly from high-voltage (HV) transmission lines to low-voltage (LV) consumers through coupling capacitors and is said to be cost-effective as compared to conventional distribution networks. However, the functionality of such substations is susceptible to various transient phenomena, including ferroresonance and overvoltage occurrences. To address these challenges, the study uses simulations to evaluate the effectiveness of conventional resistor-inductor-capacitor (RLC) filter in mitigating hazardous overvoltage resulting from transients. The proposed methodology entails using standard RLC filter to suppress transients and its associated overvoltage risks. Through a series of MATLAB/Simulink simulations, the research emphasizes the practical effectiveness of this technique. The study examines the impact of transients under varied operational scenarios, including no-load switching conditions, temporary short-circuits, and load on/off events. The primary aim of the article is to assess the viability of using an established technology to manage system instabilities upon the energization of a CCS under no-load circumstances or in case of a short-circuit fault occurring on the primary side of the CCS distribution transformer. The findings underscore the effectiveness of conventional RLC filters in suppressing transients induced by the CCS no-load switching.

Novel Application-oriented Transient Fuel Model of a Port Fuel Injection S. I. Engine

Most researches on transient fuel control of port fuel injection S.I. engine are carried out from the perspective of advanced mathematical theories. When it comes to practical control, there exist many limitations although they are more intelligent. In order to overcome the fuel wetting effect of PFI engine, the application-oriented transient fuel control is studied by analyzing the key parameters which are closely related with the engine transient characteristics. Both validity and simplicity are taken into consideration. Based on the fuel wall-wetting theory and popular fuel compensation strategy, short-term transient fuel（STF） and long-term transient fuel（LTF）, as well as their individual decay approaches, are introduced. STF is to compensate the drastic fuel film loss caused by sudden throttle change, while the function of LTF is to compensate the fuel film loss by manifold air pressure（p） fluctuation. Each of them has their respective pros and cons. The engine fuel mass and air mass are also calculated for air-fuel ratio（AFR） according to ideal gas state equation and empirical equations. The vehicle acceleration test is designed for model validation. The engine experiences several mild and heavy accelerations corresponding to the gear change during vehicle acceleration. STF and LTF control are triggered reliably. The engine transient fuel control simulation adopts the same inputs as the test to ensure consistency. The logged test data are used to check the model output. The results show that the maximum fuel pulse width（FPW） error reaches 2 ms, and it only occurs under engine heavy acceleration condition. The average FPW error is 0.57 ms. The results of simulation and test are close overall, which indicates the accuracy of steady and transient fuel. The proposed research provides an efficient approach not only suitable for practical engineering application, but also for AFR prediction, fuel consumption calculation, and further studies on emission control.

Application of transient burning rate model of solid propellant in electrothermal-chemical launch simulation

A 30 mm electrothermal-chemical(ETC) gun experimental system is employed to research the burning rate characteristics of 4/7 high-nitrogen solid propellant. Enhanced gas generation rates(EGGR) of propellants during and after electrical discharges are verified in the experiments. A modified 0D internal ballistic model is established to simulate the ETC launch. According to the measured pressure and electrical parameters, a transient burning rate law including the influence of EGGR coefficient by electric power and pressure gradient(dp/dt) is added into the model. The EGGR coefficient of 4/7 high-nitrogen solid propellant is equal to 0.005 MW-1. Both simulated breech pressure and projectile muzzle velocity accord with the experimental results well. Compared with Woodley's modified burning rate law, the breech pressure curves acquired by the transient burning rate law are more consistent with test results. Based on the parameters calculated in the model, the relationship among propellant burning rate, pressure gradient(dp/dt) and electric power is analyzed. Depending on the transient burning rate law and experimental data, the burning of solid propellant under the condition of plasma is described more accurately.

Hybrid graded element model for transient heat conduction in functionally graded materials

This paper presents a hybrid graded element model for the transient heat conduction problem in functionally graded materials （FGMs）. First, a Laplace transform approach is used to handle the time variable. Then, a fundamental solution in Laplace space for FGMs is constructed. Next, a hybrid graded element is formulated based on the obtained fundamental solution and a frame field. As a result, the graded properties of FGMs are naturally reflected by using the fundamental solution to interpolate the intra-element field. Further, Stefest＇s algorithm is employed to convert the results in Laplace space back into the time-space domain. Finally, the performance of the proposed method is assessed by several benchmark examples. The results demonstrate well the efficiency and accuracy of the proposed method.

Middle cerebral artery occlusion methods in rat versus mouse model of transient focal cerebral ischemic stroke

Experimental stroke research commonly employs focal cerebral ischemic rat models （Bederson et al., 1986a; Longa et al., 1989）. In human patients, ischemic stroke typically results from thrombotic or embolic occlusion of a major cerebral artery, usually the mid- dle cerebral artery （MCA）. Experimental focal cerebral ischemia models have been employed to mimic human stroke （Durukan and Tatlisumak, 2007）. Rodent models of focal cerebral ischemia that do not require craniotomy have been developed using intraluminal suture occlusion of the MCA （MCA occlusion, MCAO） （Rosamond et al., 2008）. Furthermore, mouse MCAO models have been wide- ly used and extended to genetic studies of cell death or recovery mechanisms （Liu and McCullough, 2011）. Genetically engineered mouse stroke models are particularly useful for evaluation of isch- emic pathophysiology and the design of new prophylactic, neuro- protective, and therapeutic agents and interventions （Armstead et al., 2010）. During the past two decades, MCAO surgical techniques have been developed that do not reveal surgical techniques for mouse MCAO model engineering. Therefore, we compared MCAO surgical methods in rats and mice.

A transient single particle model under FCI conditions

The paper is focused on the coupling effect between film boiling heat transfer and evaporation drag around a hot-particle in cold liquid. Based on the continuity, momentum and energy equations of the vapor film, a transient two-dimensional single particle model has been established. This paper contains a detailed description of HPMC (High-temperature Particle Moving in Coolant) model for studying some aspects of the premixing stage of fuel-coolant interactions (FCIs). The transient process of high-temperature particles moving in coolant can be simu-lated. Comparisons between the experiment results and the calculations using HPMC model demonstrate that HPMC model achieves a good agreement in predicting the time-varying characteristic of high-temperature spheres moving in coolant.

SELECTION OF TRANSMISSION LINE MODELS USED IN POWER SYSTEM TRANSIENT SIMULATIONS

A method used for determining the number of equivalent π sections oftransmission line model according to the frequency range of interest and the model accura-cy defined herein is proposed.Factors influencing the discrepancies between continuous ordistributed parameter and multiple π or lumped parameter models are discussed.Generalconclusions concerning the π section lengths of line models used in transient stability,faulttransient and switching over-voltage studies are drawn.Time-domain simulation resultsconfirm the effectiveness of this method.

Transient simulation of a pump-turbine with misaligned guide vanes during turbine model start-up

Experimental studies of a model pump-turbine S-curve characteristics and its improvement by misaligned guide vanes （MGV） were extended to prototype pump turbine through 3-D transient flow simulations. The unsteady Reynolds-averaged Navier-Stokes equations with the SST turbulence model were used to model the transient flow within the entire flow passage of a reversible pump-turbine with and without misaligned guide vanes during turbine model start-up. The unstable S-curve and its improvement by using misaligned guide vane were verified by model test and simulation. The transient flow calculations were used to clarify the variations of pressure pulse and internal flow behavior in the entire flow passage. The use of misaligned guide vanes can eliminate the S-curve characteristics of a pump-turbine, and can significantly increase the pressure pulse amplitude in the entire flow passage and the runner radial forces during start-up. The MGV only decreased the pulse amplitude on the guide vane suction side when the rotating speed was less than 50% rated speed. The hydraulic reason is that the MGV dramatically changed the flow patterns inside the entire flow passage, and destroyed the symmetry of the flow distribution inside the guide vane and runner.

Modeling of Propagation and Transformation of Transient Nonlinear Waves on A Current

A novel theoretical approach is applied to predict the propagation and transformation of transient nonlinear waves on a current. The problem was solved by applying an eigenfunction expansion method and the derived semi-analytical solution was employed to study the transformation of wave profile and the evolution of wave spectrum arising from the nonlinear interactions of wave components in a wave train which may lead to the formation of very large waves. The results show that the propagation of wave trains is significantly affected by a current. A relatively small current may substantially affect wave train components and the wave train shape. This is observed for both opposing and following current. The results demonstrate that the application of the nonlinear model has a substantial effect on the shape of a wave spectrum. A train of originally linear and very narrow-banded waves changes its one-peak spectrum to a multi-peak one in a fairly short distance from an initial position. The discrepancies between the wave trains predicted by applying the linear and nonlinear models increase with the increasing wavelength and become significant in shallow water even for waves with low steepness. Laboratory experiments were conducted in a wave flume to verify theoretical results. The free-surface elevations recorded by a system of wave gauges are compared with the results provided by the nonlinear model. Additional verification was achieved by applying a Fourier analysis and comparing wave amplitude spectra obtained from theoretical results with experimental data. A reasonable agreement between theoretical results and experimental data is observed for both amplitudes and phases. The model predicts fairly well multi-peak spectra, including wave spectra with significant nonlinear wave components.

Viscosity Transient Phenomenon during Drop Impact Testing and Its Simple Dynamics Model

Most soft materials behave as if they were hardened when subjected to an impact force. The strain rate dependence of viscosity resistance is the reason for this behavior. The authors carried out drop impact tests on several types of soft materials under the condition of a flat frontal impact. The impact force waveform of soft materials was found to consist of a thorn-shaped waveform and a succeeding mountain-shaped waveform. Based on our experimental observations, we believe that a large viscosity resistance is rapidly changed to a small resistance in the course of the impact. In the present study, the cause of this distinct waveform is discussed based on a dynamics model. The study applies a standard linear solid (SLS) model in which the viscosity transient phenomenon is considered is applied. Three types of impact force waveforms of actual soft materials are simulated using the SLS model. Some features of the impact force waveform of soft materials can be explained using the SLS model.

A NUMERICAL PRESSURE TRANSIENT MODEL FOR MULTILAYERED GAS RESERVOIRS WITH PSEUDOSTEADY STATE FORMATION CROSSFLOW

Reservoir deposition occurs over geologic periods of time. Although reservoirs are assumed to be homogenous for simplicity of analysis, most reservoirs are heterogeneous in nature. Some common forms of hetergeneity are the presence of layers and the presence of different zones of fluids and/or rock in the formation. A modified semi-permeable model for multi-layered gas reservoirs with pseudo-steady state interlayer crossflow was developed. The model accounted for the effect of skin and wellbore storage, considers all layers open to a single well, which flows at constant total rate. This new numerical solution was proved to be computationally very efficient, and it has been validated by comparing the results with those of some simple, well known models in the well testing literature. The effects of the reservoir parameters such as permeability, vertical permeability, skin, wellbore storage on the wellbore response, pressure and layer production rate were investigated. Numerical solutions of the problem for the modified semi-permeable model were used to find the structure of crossflow in typical cases.

Transient Thermal Modeling in Laser Welding of Metallic/Nonmetallic Joints Using SolidWorks^(█) Software

The purpose of this research is to develop a SolidWorks? model for transient temperature field of laser welding of PMMA/SS 304 materials for application in fabrication of the ultrasonic back-plate, with a view of optimizing the experimental conditions. The study is carried out on these materials because of the increasing application of both metals and non-metals. The work focuses specifically on these materials because they have been experimentally studied previously and as such, this study can be accepted as an assessment into feasibility of using SolidWorks? model to study the temperature field of the laser welding processes of metals and non-metals. The results of the SolidWorks? transient thermal model show that there is a concentration of high temperatures at the point of contact. It also shows that temperature decreases as we move in (between laser and the top face) to the thickness of the part. Additionally the maximum temperature occurs at the last point of the welding;this may be due to the accumulation of the temperature before arriving at the end. These findings are comparable to the previous simulated and experimental results on temperature field during laser welding of PMMA/SS 304 materials. However, SolidWorks? is shown to present a challenge in modeling a moving source of laser power.

NUMERICAL MODEL OF THERMAL TRANSIENT EFFECT IN FIBER OPTIC GYRO

The sensitivity of the interferometric fiber optic gyro in the presence of time varying thermal gradients plays a key role in its performance. It is well known that this sensitivity is due to the difference of index changes between the points symmetrical with respect to the middle of the coil. In order to reduce this sensitivity, different winding patterns, such as quadrupolar winding, were introduced to keep the thermal environment of the symmetrical points. In this paper, a numerical model of the transient temperature distribution in the gyro was established. The temperature gradient of the coil was solved in conjugation with the nature convection heat transfer in the aperture between the coil and the case. Effects of the winding pattern and the design of its case were investigated to optimize the design of the interferometric fiber optic gyro.

采用改进MPC提高储能系统电压暂态特性研究

储能系统通过换流器实现并网运行,常规换流器因为其开关频率低,在采样、计算环节存在延时导致储能系统暂态特性差,甚至引起整个电网不稳定。本文采用模型预测控制MPC(model predictive control)来实现储能系统功率快速响应,避免延时影响。引入功率权重价值函数在有功、无功MPC控制中,计算储能换流器输出电压最优值。针对因为滤波电感参数偏移导致的MPC模型不准确问题,本文采用电感误差补偿控制来提高模型精度。通过Matlab/Simulink仿真和实验来验证本文所提方案可提高储能系统暂态特性,有效消除误差对MPC控制性能的影响。

颗粒煤基质尺度计算新方法及应用

煤基质内部富含复杂孔隙,为瓦斯提供了大量的存储空间和运移通道。孔隙内的瓦斯流进裂隙需要经过扩散过程,基质尺度在一定程度上决定了扩散到裂隙的阻力,影响着瓦斯扩散的难易程度。研究以瓦斯扩散与煤基质尺度内在联系为出发点,通过处理颗粒煤解吸瞬态过程数据得到不同解吸时刻煤的双重孔隙结构内的瓦斯浓度与质量交换速率的定量关系,结合时变扩散系数对基质形状因子进行计算,提出了基于瞬态扩散的颗粒煤基质尺度计算方法,并进行了试验验证。结果表明:与较小颗粒煤相比,保存完整基质形态的大颗粒煤初始扩散系数基本不变,因此初始扩散系数的值在一定程度上可以表征基质的破坏程度。基质形状因子随解吸时间的延长而减小,可划分为急降阶段、缓降阶段和稳定阶段,其中稳定阶段基质形状因子能够准确反映扩散后期拟稳态下的基质形态,最适合求解基质尺度。该方法可以反映颗粒煤粉化损伤过程中的基质尺度变化规律,为解释扩散极限粒径的存在提供依据。糯东煤样的3种试验粒径基质尺度随煤颗粒的增大而增大,分别为0.059、0.287、0.457 mm,并且在大粒径范围具有无差性,证明了该方法的准确性。颗粒煤基质尺度可以用来修正K1值的计算参数,使瓦斯损失量计算模型在粉化程度高的煤样中同样具有很好的适用性。

汽车尾气温差发电系统瞬态回收性能分析

为了预测温差发电(thermoelectric generator, TEG)系统的动态特性,基于COMSOL Multiphy-sics建立了用于求解温差发电系统温度场分布的瞬态计算流体力学(computational fluid dynamics, CFD)模型和用于研究温差发电模块瞬态响应特性的分析模型,提出了混合瞬态CFD-分析模型,并经过瞬态试验验证.结果表明:由于热惯性的影响,TEG系统的转化效率会出现一个瞬时的较高值;相较于尾气温度和质量流量的瞬态波动,热电半导体的热端温度和冷端温度会存在时滞;在美国环保局的高速公路燃油经济性测试(highway fuel economy test, HWFET)模式循环工况下,瞬态模型求解得到整个温差发电系统的平均输出功率、平均转化效率分别为35.63 W和3.40%,瞬态模型的输出电压平均误差为6.41%;该模型能够以较高的精度及较短的计算时间预测温差发电系统在瞬态热源激励下的瞬态响应特性.

瞬时弹性成像技术和无创诊断模型诊断原发性胆汁性胆管炎患者肝纤维化效能研究

目的研究瞬时弹性成像技术(TE)和常用的无创诊断模型诊断原发性胆汁性胆管炎(PBC)患者组织学分期的效能。方法2019年1月~2023年8月首都医科大学附属北京佑安医院就诊的PBC患者117例,均接受肝活检、肝脏硬度检测(LSM)和常规实验室检查,计算天冬氨酸氨基转移酶/血小板比值(APRI)、基于4因子的肝纤维化指数(FIB-4)、γ-谷氨酰转肽酶/血小板比值(GPR)、Sheth指数、King’s评分、哥德堡大学肝硬化指数(GUCI)评分、白蛋白-胆红素(ALBI)评分和总胆汁酸-红细胞体积分布宽度(TR)评分。应用ROC曲线分析判断各指标的诊断效能。结果在117例PBC患者中,病理学诊断I期17例,II期59例、III期34例和IV期7例;APRI诊断PBC II期以上的AUC最大,为0.875(0.776~0.974),其敏感性为75.8%,特异性为87.5%,阳性预测值为76.5%和阴性预测值为87.0%,其次是GUCI评分,其AUC=0.871(0.766~0.976),其效能分别为88.7%、75.0%、65.6%和92.4%。结论APRI是一种简便、廉价的非侵入性预测指标,用于判断II以上PBC患者有良好的诊断效能,值得临床扩大验证。