±1100 kV换流站高频干扰源模型的研究

换流器作为换流站最关键的部件,是产生载波噪声干扰的主要设备,对其进行详细的建模以得到准确的高频干扰水平,能为即将投运的±1 100 kV特高压直流工程高频滤波器的优化研究打下坚实的基础。在对换流阀的高频特性进行深入分析的基础上,首先提出了3种换流阀高频模型,并通过与已投运的相似设计的HVDC工程换流阀组产生的高频噪声水平进行比较,确定换流阀模型2来反应其实际的高频特性;其后,利用PSCAD仿真平台搭建准东—皖南特高压直流输电工程的模型,通过仿真计算得到整流站与逆变站换流站平波电抗器前电压波形,并通过傅立叶变换得出其暂态过程的高频噪声水平。考虑到换流阀高频模型实际应用复杂,根据所测得的噪声水平提出用公式表示其频谱特性,并根据换流器内部电压的分布进一步提出分压模型,得出将公式表示的噪声源分压模型作为激励源用于换流站高频特性的后续研究更符合实际。

IGCT的换流机理

提出一种独特的IGCT换流等效电路,建立相应的数学模型,并借助Mathematic软件对 该模型进行求解;分析了主要参量与换流时间的关系,在此基础上,描述换流后的关断过程,阐明 IGCT的换流机理。

一种混合式高压直流断路器的换流特性研究

高压直流电网发生短路故障时,为了系统安全稳定运行,直流断路器应在很短的时间内切除故障线路,而电流转移过程是断路器成功切断故障电流的关键。文中对级联全桥混合式高压直流断路器换流过程进行分析,着重研究了故障电流分断过程中断路器的3次换流过程,并建立换流过程的等效模型。在PSCAD/EMTDC中仿真分析了3次换流过程的主要影响因素及其影响规律,得到混合式断路器的换流特性结论,为研制开断时间更短、开断容量更高的直流混合式断路器提供参考。

交直流混联电网LCC-HVDC换流器建模方法综述

交直流电网混联,大规模电力跨区输送成为我国电力系统的主要特点。电网换相换流器型直流输电(line commutated converter high voltage direct current,LCC-HVDC)是我国交直流混联电网的主要组成部分,为实现交直流混联电网快速、准确仿真,该文对现有的LCC-HVDC换流器建模方法进行了分析与总结,对其优缺点进行评述,并根据作者观点,提出可进一步研究的内容:在仿真规模较大的交直流混联电网时,可用开关函数对LCC-HVDC进行建模,但模型准确度需要提升;多条LCC-HVDC输电线路的仿真可使用换流器级别模型与换流阀级别模型进行组合仿真;不同精细程度模型之间的数据接口要进行优化设计。

传统直流换流阀动态关断模型及对换相失败评估准确性的影响

传统高压直流输电系统逆变侧发生交流故障时,换流阀晶闸管的关断特性会随正向电流、电流过零点下降率和结温等外部条件的改变而动态变化,而现有研究多将晶闸管视为理想器件或仅保留一定的关断裕度,很可能会导致换相失败判别结果不准确等问题。为掌握晶闸管关断特性的动态变化对换相失败评估准确性的影响,首先从理论上分析晶闸管关断特性影响换相过程的机理;然后考虑多维因素综合作用下的晶闸管关断特性,提出并建立换流阀动态关断模型,通过与SABER中晶闸管详细物理模型对比验证关断模型的正确性;最后在PSCAD/EMTDC中分别对比采用换流阀动态关断模型和采用传统模型的基于电网换相换流器的高压直流输电(line commutated converter based high voltage direct current,LCC-HVDC)在单相和三相故障情况下的换相失败判别结果。结果表明,采用传统模型可能会在未发生换相失败时判断为已发生换相失败,对系统提出更高的关断要求,还可能会导致换相失败发生时刻判断不准确的问题;采用所提出的换流阀动态关断模型,能够使换相失败的判别结果更加准确。

直流熔断器并联燃弧不均机理研究及优化设计

针对直流熔断器分断低过载电流时易出现并联熔体燃弧不均的问题,基于MATLAB自编程仿真和实验验证,对并联燃弧不均的原因进行探究并提出改进措施。通过电场、热场和电路的耦合求解,获得燃弧过程各并联狭颈的电流及温度,并解释了并联狭颈燃弧不均的原因。利用建立的数学模型计算并分析电流、狭颈尺寸差对并联燃弧不均现象的影响,得出不同加工误差下的临界均匀燃弧电流密度;提出采用并联均流桥的设计改善并联燃弧不均现象。改进设计额定2000 V/2000 A的熔断器并进行2500 A低过载电流开断实验,实验结果验证了该设计方案的可行性和有效性。

基于单晶闸管级电路拓扑的串联晶闸管反向恢复过程建模及换相过冲电压研究

高压直流换流阀通常以串联晶闸管作为开关器件,在换相过程结束后会产生换相过冲电压,其大小是换流站绝缘水平设计的重要参考,也对晶闸管的耐压性能提出了要求。尽管晶闸管反向恢复电荷分散性可能对换相过冲电压,尤其是晶闸管反向恢复电压,造成影响,但鲜有文献对其进行详细分析。首先对比了3种常见的独立晶闸管反向恢复模型,并对其应用于串联晶闸管建模进行了适应性分析。然后,提出了反向恢复过程中的串联晶闸管等效模型,并基于此建立了换流阀关断模型。随后,通过仿真计算验证了所提出的等效模型的有效性,并将其应用于换相过冲电压分析。结果表明:采用均匀模型对换流阀电压应力进行估算存在合理性,但是由经验公式推导晶闸管级反向恢复电压可能引入较大误差;采用1:n-1分布方式模拟晶闸管反向恢复电荷分散性会引入过大的安全裕度,可能会降低换流阀的经济性。

Analysis of interaction between surface and sewer pipe system based on computational fluid dynamics

To verify the accuracy of weir and orifice formula and analyze the hydraulic characteristics of exchange flow in a manhole,a three-dimensional numerical model was proposed to assess the exchange flow rate between the surface and sewer pipe systems based on the real-world scale model.The hydrodynamic model is based on the three-dimensional Navier-Stokes equations including the standard k-εmodel for turbulence processes,and the volume of fluid(VOF)method for capturing the free surface.The results of the computational fluid dynamics(CFD)simulation are compared with the conventional overflow equations,showing that the weir and orifice formula is appropriate to determine the exchange flow rate between two systems in this specific study case.Streamlines and velocity contours at the center profile under both the inflow and surcharge conditions show that the exchange flow is directly related to the water level on the surface and the junction area between the manhole and right pipe.The results demonstrate the potential application of CFD in analyzing the interaction of urban flood flows,which can provide much clearer details of the interaction process.

A 10bit 2GHz CMOS D/A Converter for High-Speed System Applications

This paper presents a 2GS/s 10bit CMOS digital-to-analog converter （DAC） that consists of two unit current-cell matrixes for 6MSBs and 4LSBs, respectively, trading off between the precision and size of the chip. Current mode logic （CML） is used to ensure high speed,and a double centro-symmetric current matrix is designed by the Q^2 random walk strategy in order to ensure the linearity of the DAC. The DAC occupies 2.2mm × 2.2mm of die area and consumes 790mW with a single 3.3V power supply.

Adaptive controller design based on input-output signal selection for voltage source converter high voltage direct current systems to improve power system stability

An input-output signal selection based on Phillips-Heffron model of a parallel high voltage alternative current/high voltage direct current(HVAC/HVDC) power system is presented to study power system stability. It is well known that appropriate coupling of inputs-outputs signals in the multivariable HVDC-HVAC system can improve the performance of designed supplemetary controller. In this work, different analysis techniques are used to measure controllability and observability of electromechanical oscillation mode. Also inputs–outputs interactions are considered and suggestions are drawn to select the best signal pair through the system inputs-outputs. In addition, a supplementary online adaptive controller for nonlinear HVDC to damp low frequency oscillations in a weakly connected system is proposed. The results obtained using MATLAB software show that the best output-input for damping controller design is rotor speed deviation as out put and phase angle of rectifier as in put. Also response of system equipped with adaptive damping controller based on HVDC system has appropriate performance when it is faced with faults and disturbance.

Pressure fluctuation and its influencing factors in circulating water pump

In order to investigate the effect of sampling frequency and time on pressure fluctuations, the three-dimensional unsteady numerical simulations were conducted in a circulating water pump. Through comparison of turbulence models with hydraulic performance experiment, SST k-co model was confirmed to study the rational determination of sampling frequency and time better. The Fast Fourier Transform （FFT） technology was then adopted to process those fluctuating pressure signals obtained. On these bases, the characteristics of pressure fluctuations acting on the tongue were discussed. It is found that aliasing errors decrease at higher sampling frequency of 17 640 Hz, but not at a lower sampling frequency of 1 764 Hz. Correspondingly, an output frequency range ten-times wider is obtained at 17 640 Hz. Compared with 8R, when the sampling time is shorter, the amplitudes may be overvalued, and the frequencies and amplitudes of low-frequency fluctuations can not be well predicted. The frequencies at the tongue are in good agreement with the values calculated by formula and the frequency compositions less than the blade passing frequency are accurately predicted.

Average Cycle Period in Asymmetrical Flashing Ratchet

The directed motion of a Brownian particle in a Bashing potential with various transition probabilities and waiting times in one of two states is studied. An expression for the average cycle period is proposed and the steady current J of the particle is calculated via Langevin simulation. The results show that the optimal cycle period (Tm), which takes the maximum of J, is shifted to a small value when the transition probability A from the potential on to the potential off decreases, the maximal current appears in the case of the average waiting time in the potential on being longer than in the potential off, and the direction of current depends on the ratio of the average times waiting in two states.

Study on productivity of horizontal wells with segmental perforation based on pseudo steady-state flow

Segmental perforation is widely used for horizontal wells. However,the flow of fluid in porous media is a complex problem. Using the Fourier transform,principle of potential superposition,trigonometric function transform,asymptotic analyses,a pressure solution of a pseudo steady-state flow model in 3D circular-boxed media has been established. Comparing with the productivity of vertical wells,an equivalence radius model can be obtained. Based on the model,a method of evaluating the productivity of segmental perforation horizontal well is presented by means of principle of superposition. It shows that the equivalence radius is different for various positions of horizontal wells; the output of both ends of horizontal wells is greater than the others under the same length of perforation interval; it is more important to obtain high productivity by increasing the length of perforation interval than enlarging the spacing between perforation intervals. The result of this research can be used to ascertain the yield of each perforated interval.

Modeling of a Reverse Flow Reactor for Methanol Synthesis Modeling of a Reverse Flow Reactor for Methanol Synthesis

An accurate one-dimensional, heterogeneous model taking account of axial dispersion and heat transfer to the reactor wall, and heat conduction through the reactor wall for methanol synthesis in a bench scale reactor under periodic reversal of flow direction is presented. Adjustable parameters in this model are the effectiveness factors for each of the three reactions occurring in the synthesis and a factor for the bed to wall heat transfer coefficient correlation. Experimental data were used to evaluate these parameters and reasonable values of these parameters were obtained. The model was found to closely predict the reactor performance under a wide range of operating conditions, such as carbon oxide concentrations, volumetric flow rate, and cyclic period.

Average modeling of Single Stage Flyback PFC ＋ Flyback DC／DC converter

With the use of this novel average model for Single Stage Flyback PFC+Flyback DC/DC converter, voltage control mode, peak current control mode and average current control mode can be simulated easily by changing the model's parameters. It can be used to do various analysis not only for small signal and static behavior but also for large signal and dynamic behavior of the converter. By using this average model the simulation speed can be improved by 2 orders of magnitude above that obtained by using the conventional switched model. It can be applied to optimize the trade\|off between high power factor, voltage stress, current stress and good output performance while designing this kind of single stage PFC converter. A 60W single stage power factor corrector was built to verify the proposed model. The modeling principle can be applied to other Single Stage PFC topologies.

Numerical Analysis of the Nonlinear Parameterization of Waves in Currents over a Submerged Sill with a Non-Hydrostatic Model

An investigation of the effects of a uniform current strength direction(following or opposing wave propagation) on the nonlinear transformation of irregular waves over a submerged trapezoidal sill is carried out using SWASH,a non-hydrostatic numerical wave model.The nonlinear parameters(i.e.,asymmetry,skewness,and kurtosis) are calculated,and the empirical formulas for these parameters are presented as a function of the local Ursell number based on the present numerical data measured.In the shoaling area of the submerged sill,the nonlinear characteristics of waves are more obvious when waves propagate in the same direction as the currents than when waves propagate in the opposite direction.Whereas nonlinear parameters grow with the strengthening of the following currents over the crest,they tend to decrease as the adverse current velocity increases over the crest area of the submerged sill.

Simulation of Steady-State and Dynamic Behaviour of a Plate Heat Exchanger

The present paper deals with both the steady-state and dynamic simulation of a plate heat exchanger, in counter-flow arrangement. A CFD （computational fluid dynamics） program FLUENT has been used to predict the temperature distribution in steady-state conditions in plate heat exchanger as well as fluid temperatures at exit of flow channels in transient condition. The results are presented for the heat exchanger, which is simulated according to the configuration of the plate heat exchanger used in the experiment. The simulated results obtained by the CFD model have been compared with the experimental data from the literature, which shows that the CFD model developed in this study is capable of predicting the steady-state and transient performance of the plate heat exchangers satisfactorily.

A Straight-bladed Vertical Axis Wind Turbine with a Directed Guide Vane Row-Effect of Guide Vane Geometry on the Performance-

The objective of this study is to show the effect of guide vane geometry on the performance. In order to overcome the disadvantages of vertical axis wind turbine, a straight-bladed vertical axis wind turbine （S-VAWT） with a directed guide vane row has been proposed and tested by the authors. According to previous studies, it was clarified that the performance of the turbine can be improved by means of the directed guide vane row. However, the guide vane geometry of S-VAWT has not been optimized so far. In order to clarify the effect of guide vanegeometry, the effects of setting angle and gap between rotor blade and guide vane on power coefticlent and start- ing characteristic were investigated in the experiments. The experimental study of the proposed wind turbine was carried out by a wind tunnel. The wind tunnel with a diameter of 1.8m is open jet type. The wind velocity is 8 m/s in the experiments. The rotor has three straight blades with a profile of NACA0018 and a chord length of 100 mm, a diameter of 0.6 m and a blade height of 0.7 m. The guide vane row consists of 3 arc plates.

Thermal Performance of a Single-row Fin-and-tube Heat Exchanger

Experiments were carried out to study the heat transfer characteristics of a single-row aluminum fin-and-tube crossflow heat exchanger with an emphasis in the regime of low flow rate of the in-tube fluid. The Chilton-Colburn analogy, in conjunction with the least-squares power-law technique, was used to correlate experimental data. Both air- and water-side heat transfer correlations were developed in the form of the Nusselt numbers as a function of Reynolds and Prandtl numbers. The experimental observations are quantitatively compared to the predictions of correlations available in the published literature. Different transfer mechanisms were found to be operative in the ranges of water-side Reynolds numbers based on the hydraulic diameter. In a range of Reynolds number from 1,200 to 6,000, the water-side thermal resistance accounts for less than ten percent of the overall thermal resistance. The dominant thermal resistance is always on the air-side. On the other hand, the thermal resistance of water-side is nearly equal to that of air-side in a Reynolds number range from 500 to 1,200.

Study on asymmetric interior corner flow in microgravity condition

The capillary flow in asymmetric interior corner consisting of straight vane and curved wall is studied with analytical solution.The concept of equivalent interior corner angle is proposed to convert the asymmetric interior corner model into symmetric interior corner model.Then the governing equations of interior corner flow are established,and based on which the interior corner flow is calculated.This method is used to analyze the capillary flow in cylindrical vane-type surface tension tank with outer vanes.The research can provide beneficial reference to the design of vane-type surface tension tank.