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。

机械式直流断路器性能分析及谐振换流方法

机械式直流断路器(DCCB)是直流开断的主流方案之一。然而,远海风电构网型柔性直流系统对机械式DCCB提出轻型化、紧凑化的要求。性能分析表明,目前的机械式DCCB存在预充电系统耐压要求高、预充电容的容量高、预充能量损失率高、换流时间长、小电流熄弧不稳定等问题。为此,提出一种谐振型机械式DCCB(RM-DCCB),通过控制晶闸管的导通时刻并利用其过零自关断特性,逐次在换流支路注入能量,产生了幅值递增的谐振电流。研究了RM-DCCB的换流特性、数学模型和参数设计方法。通过PSCAD/EMTDC仿真验证了RM-DCCB的开断可行性和参数设计的正确性。研究表明,RM-DCCB是一种具备综合技术优势的换流新方法。

5kV非对称GCT的特性分析与设计

简述了非对称门极换流晶闸管(A-GCT)的结构特点及其工作原理,分析了A-GCT结构参数之间的相互制约关系,给出了高压A-GCT的设计参考。并借助ISE软件对A-GCT的阻断特性、导通特性和开关特性进行了模拟。提取了5kV的A-GCT纵向结构设计参数。

一次复杂的直流线路故障分析

在某直流输电系统发生的一次线路故障中,因连续雷击的影响,造成直流线路过电压保护、阀组差动保护、交直流碰线保护、桥差保护、50 Hz保护等多个保护动作。文中根据相关直流控制保护功能的原理,结合故障录波,对该事故的过程进行了详细分析,并深入分析了产生直流线路过电压的原因及影响。提出了重视换流阀及其控制系统与直流保护的配合、适当改进断开中性母线开关动作逻辑等建议。

Effects of Shape and Quantity of Helical Baffle on the Shell-side Heat Transfer and Flow Performance of Heat Exchangers

Shape and quantity of helical baffles have great impact on the shell-side performance of helical baffled heat exchangers （HBHE）. In this work, three physical models of HBHE with baffles of different shape （trisection, quadrant and sextant sector） were investigated. Numerical simulations were performed on HBHE at three helix an- gles （10°, 25° and 40°） by the software ANSYS CFX. Analyses of numerical results indicate that the sextant HBHE shows relatively better fluid flow performance because the leakage flow in the triangle area is evidently reduced and the fluid streamline appears much closer to an ideal spiral flow, while the trisection and quadrant HBHE show more scattered and disordered streamline distributions. The convective heat transfer coefficient and pressure drop in three types of HBHE were presented. Further investigations on the shell side performance with different helical baf- fles were implemented by the field synergy theory. Both theoretical and numerical analyses gave support on the re- lations between helical baffle shape and shell-side performance. This paper may provide useful reference for the selection of baffle shade and auantitv in HBHE.

Experimental Study of Fouling on Heat Transfer Surface During Forced Convective Heat Transfer

An experimental study was conducted to investigate the fouling process of calcium carbonate on the heat transfer surface, during forced convective heat transfer. The dynamic monitoring apparatus of fouling resistance was set up for the present experiments. The fouling behavio（s were examined under different factors including fluid velocity, hardness,alkalinity, solution temperature, and wall temperature. Asymptotic fouling curves varying with time were obtained. The fouling rate and asymptotic fouling resistance increased and the induction periods were shortened with the fluid velocity decreasing, hardness andalkalinity increasing, and solution temperature and heat transfer surface temperature increasing. Thecomponents of fouling that formed on the heat transfer surface included crystallization fouling and particulate fouling. The thermal performance parameter of fouling,ρfhf, varied from 380 to 2600 kg·W·（m^4·K）^-1, increasing with growing velocity and decreasing solution temperature, hardness or alkalinity. Furthermore, the thermal conductivity of fouling, λf, varied from 1.7 to 2.2 W·（m·K）^-1 .

An Experimental Study on the Flow Characteristics of OilWater Two-Phase Flow in Horizontal Straight Pipes

The flow patterns and their transitions of oil-water two-phase flow in horizontal pipes were studied. The experiments were conducted in two kinds of horizontal tubes, made of plexiglas pipe and stainless steel pipe with 40 mm ID respectively. No. 46 mechanical oil and tap water were used as working fluids. The superflcial velocity ranges of oil and water were: 0.04-1.2m·s-1 and 0.04-2.2m·s-1, respectively. The flow patterns were identified by visualization and by transient fluctuation signals of differential pressure drop. The flow patterns were defined according to the relative distribution of oil and water phases in the pipes. Flow pattern maps were obtained for both pipelines. In addition, semi-theoretical transition criteria for the flow patterns were proposed, and the proposed transitional criteria are in reasonable agreement with available data in liquid-liquid systems.

Experiment on Effect of Tip Clearance Leakage Flow on Heat Transfer of Turbine Outer Ring

The cascade model was tested using transient liquid crystal temperature measurement technology.The effects of main flow Reynolds number,blowing ratio and tip clearance height on the convective heat transfer coefficient of the turbine outer ring were studied.Two feature lines were marked on the turbine outer ring corresponding to the position of the blade.The conclusions are as follows:The tip clearance leakage flow has a great influence on the convective heat transfer coefficient of the turbine outer ring.When the clearance height and the blowing ratio are kept constant,gradually increasing the main flow Reynolds number will result in an increase in the convective heat transfer coefficient of the turbine outer ring.When the clearance height and the main flow Reynolds number are kept constant and the blowing ratio is gradually increased,the convective heat transfer coefficient of the turbine outer ring is almost constant.The heat transfer coefficient of the turbine outer ring surface is little affected by the blowing ratio;The clearance height has great influence on the heat transfer characteristics of the turbine outer ring.Under the typical working condition in this paper,when the tip clearance height ratio is 1.6%,the convective heat transfer coefficient of the outer surface of the turbine is the highest.

Sediment Movement in Periodic Alternating Current

The present paper summarizes the results of previous studies, including the structure and principle of the rotary ring flume for researching the fine sediment movement, the mechanism of the flume, method of eliminating the influence of the centrifugal force and sediment movement experiments with periodic alternating current. Also included are the experiment-based relationship among sediment concentration, bed shear stress and silt carrying capacity, a proposed erosion-deposition function and bed erosion-deposition calculation together with the results of verification.

Characteristics of radiation and convection heat transfer in indirect near-infrared-ray heating chamber

Numerical study was performed to evaluate the characteristics of combined heat transfer of radiation, conduction and convection in indirect near infrared ray （N/R） heating chamber. The effects of important design parameters such as the shape of heat absorbing cylinder and heat releasing fin on the pressure drop and heat transfer coefficient were analyzed with different Reynolds numbers. The Reynolds numbers were varied from 103 to 3x106, which was defined based on the hydraulic diameter of the heat absorbing cylinder. Analyses were performed to obtain the inner and outer flow and the temperature distributions in the heat absorbing cylinder and the rates of radiation heat transfer and convection heat transfer. As the Reynolds number increases, the convection heat transfer rate is increased while the radiation heat transfer rate is decreased. The average convection heat transfer rate follows a power rule of the Reynolds number. Addition of three-dimensional heat releasing fin to the outside of the heat absorbing cylinder enhances the convection heat transfer.

Mixed convection characteristics in lid-driven cavity containing heated triangular block

This research work numerically analyzes 2D,steady state,mixed convective heat transfer for Newtonian fluids in lid driven square enclosure with centered triangular block（blockage—10%or 30%）maintained either at the constant wall temperature or constant heat flux thermal conditions.The fluid flow in the enclosure is initiated by top moving wall in＋x-direction,while all other walls are stationary.The top and bottom walls are thermally insulated.In particular,the governing field equations are solved for range of governing parameters such as,Reynolds number（1–1000）,Prandtl number（1–100）,and Grashof number

3D numerical study on flow structure and heat transfer in a circular tube with V-baffles

A 3D numerical investigation has been carried out to examine periodic laminar flow and heat transfer characteristics in a circular tube with 45°V-baffles with isothermal wall.The computations are based on the finite volume method(FVM),and the SIMPLE algorithm has been implemented.The fluid flow and heat transfer characteristics are presented for Reynolds numbers ranging from 100 to 2000.To generate main longitudinal vortex flows through the tested section,V-baffles with an attack angle of 45°are mounted in tandem and in-line arrangement on the opposite positions of the circular tube.Effects of tube blockage ratio,flow direction on heat transfer and pressure drop in the tube are studied.It is apparent that a pair of longitudinal twisted vortices(P-vortex)created by a V-baffle can induce impingement on a wall of the inter-baffle cavity and lead a drastic increase in heat transfer rate at tube wall.In addition,the larger blockage ratio results in the higher Nusselt number and friction factor values.The computational results show that the optimum thermal enhancement factor is around 3.20 at baffle height of B=0.20 and B=0.25 times of the tube diameter for the V-upstream and V-downstream,respectively.

Fault location for parallel transmission lines using one-terminal current traveling waves

With proper phase module transformation,parallel lines can be decomposed to the same directional net and the reverse directional net. The propagation characteristics of traveling waves in the reverse directional net were analyzed,and the refraction coefficient at the fault point for a single phase fault was derived. In addition,the module selection was discussed. Simulation results show that satisfying accuracy can be achieved with the proposed method. Moreover,it is immune to fault types,fault resistances,and outside system parameters.

Numerical investigation on flow and convective heat transfer of aviation kerosene at supercritical conditions

In this paper,characteristics of flow and convective heat transfer of China RP-3 kerosene in straight circular pipe were numerically studied.Navier-Stokes equations were solved using RNG k-turbulence model with low Reynolds number correction.The thermophysical and transport properties of the China RP-3 kerosene were calculated with a 10-species surrogate and the extended corresponding state method(ECS) combined with Benedict-Webb-Rubin equation.The independence of grids was first studied and the numerical results were then compared with experimental data for validation.Under flow conditions given in the paper,the results show that deterioration of convective heat transfer occurs when the wall temperature is slightly higher than the pseudo-critical temperature of kerosene for cases with wall heat flux of 1.2 and 0.8 MW/m 2.The degree of the heat transfer deterioration is weakened as the heat flux decreases.The deterioration,however,does not happen when the heat flux on the pipe wall is reduced to 0.5 MW/m 2.Based on the analysis of the near-wall turbulent properties,it is found that the heat transfer deterioration and then the enhancement are attributed partly to the change in the turbulent kinetic energy in the vicinity of pipe wall.The conventional heat transfer relations such as Sieder-Tate and Gnielinski formulas can be used for the estimation of kerosene heat convection under subcritical conditions,but they are not capable of predicting the phenomenon of heat transfer deterioration.The modified Bae-Kim formula can describe the heat transfer deterioration.In addition,the frictional drag would increase dramatically when the fuel transforms to the supercritical state.

On Identical Problems of Natural Convection in Enclosures and Applications of the Identity Character

In this paper, a concept of 'identical problems' of natural convection in enclosures is presented. A pair of problems of natural convection in enclosures is said to be identical if they look different in appearance but with appropriate selection of coordinates and appropriate definition of dimensionless variables for each problem, they will have identical geometric configurations and identical dimensionless governing equations and related boundary conditions. The identical characteristics of a pair of problems of natural convection in enclosures with an internal isolated island are demonstrated via dimensionless mathematical formulation and flow visualization results. Numerical computations are performed and the predicted streamlines agree with the flow visualization results. A number of other possible pairs of identical problems are also presented. Applications of the identical character are provided and assumptions under which the identical problems may exist are discussed.