An online technique for monitoring the insulation condition of AC machine stator windings

A novel online technique for monitoring the insulation condition of ac machine stator windings is proposed in this paper. The concept is to measure the differential leakage currents of each phase winding from the terminal box in a noninvasive manner to assess the insulation condition during motor operation. The conventional differential CTs used for phase fault protection can be replaced with high performance current sensors to measure the leakage current with higher accuracy. Indicators for insulation condition such as the capacitance and dissipation factor are calculated based on the measurements to provide a low cost solution for online insulation condition assessment. A simplified online insulation system model is derived for analysis and interpretation of the measured data. Experimental results on a 15-hp induction motor under simulated insulation degradation conditions show that the proposed technique is a very sensitive method capable of detecting incipient signs of insulation degradation.     

A wideband lumped circuit model of the terminal and internal electromagnetic response of rotating machine windings with a coaxial insulation system

The polyphase model can be used to simulate the terminal and internal electromagnetic response of, for example, Powerformer/spl trade/: a new power generator. The circuit parameters are based on geometrical and material data. The slot leakage is modeled by means of a reluctance circuit, which is coupled to the electric circuit by means of winding templates. The capacitive current and its losses in the outer semiconducting layer of the cable are modeled with a simplified version of an RC model that has been used previously for other coaxially insulated windings. The eddy current losses were neglected; however, it is presented how they can be included in the model. The simulated frequency and transient response of the lumped circuit is compared with measurements on an 11-MVA/45-kV, 600-r/min hydro Powerformer. With exception of the damping, the agreement is good, qualitatively, up to 100 kHz.     

Assessment of the WWER-1000 reactor condition

The current state of the materials (steel, welds, anticorrosive cladding metal) of WWER-1000 RPVs has been examined, taking into consideration special features of their manufacture and operation for 30 years. The possibility of operating these reactors after their design service life has been estimated, allowing for the degradation of properties due to thermal and radiation influence, as well as damage from cyclic loads. Life extension has been shown to be possible but will require future analysis of the state of the material at the end of the design life.     

A programmable dynamometer for testing rotating machinery using athree-phase induction machine

A programmable dynamometer is proposed where the user can define the steady-state and inertial torque-speed characteristic of the load. Rotating machinery can be tested on this dynamometer to determine their transient and steady-state characteristics with a practical load. The user inputs the desired load characteristics by selecting polynomial and inertia coefficients. The system uses proportional-integral (PI) controllers to control a three-phase induction motor using indirect field orientation. Simulations show that the dynamometer offers excellent tracking of the desired torque-speed characteristic and simplicity of use     

Impulse voltage withstand capability of rotating machine insulationas determined from model specimens

An evaluation of present-day turn and groundwall insulation is discussed. The surge breakdown voltage of unaged insulated specimens was evaluated at two rates of surge voltage rise; the values of breakdown voltage were compared to those obtained by direct voltage and 60 Hz alternating voltage. The insulations were then aged by voltage from conductor to ground and periodically evaluated for surge breakdown values. Finally, the specimens were tested for surge voltage endurance, and this endurance was compared to alternating voltage endurance     

Double-ring high-frequency common-mode switching oscillation current sensor for inverter-fed machine winding insulation monitoring

Insulation failure significantly contributes to the unpredictable shutdown of power equipment. Compared to the partial discharge and high-frequency(HF) injection methods, the HF common-mode(CM) leakage current method offers a non-intrusive and highly sensitive alternative. However, the detection of HF CM currents is susceptible to interference from differential-mode(DM) currents, which exhibit high-amplitude and multifrequency components during normal operation. To address this challenge, this p...     

Practical application of inverse heat conduction for wall condition estimation on a rotating cylinder

The solution of the linear, inverse, transient heat conduction problem (IHCP) in a cylindrical geometry is analysed. The rotating cylinder under investigation is experiencing boiling convection induced by the impingement of a water jet. The initial temperature is known, additional temperature measurements in time are taken with sensors positioned at a constant radius within the solid material, and the estimation of the wall heat flux at the external radius is sought. First, simulated temperature measurements inside the cylinder are processed in order to be used to estimate the wall heat flux. When noise is present in the data, some of the simulated results obtained using the least squares method exhibit oscillatory behavior, but these large oscillations are substantially reduced by the implementation of a regularization technique. Real experimental data are also used for the wall condition estimation and for the subsequent building of local boiling curves are plotted and discussed. The question of the possible effect of a temperature dependent conductivity on the reconstructed wall condition is also considered.     

Assessment of Arrhenius activation energy in stretched flow of nanofluid over a rotating disc

The study of shape effects in nanofluids plays a vital role in the fluid flow as well as heat propagation. Hence, the different shape factors of molybdenum disulfide MoS₂ on a stretchable disc with rotation under the effect of the magnetic field, chemical response, and activation energy are examined. The set of partial differential equations is converted into ordinary differential equations (ODEs) by using von Karman's transformations, and the obtained ODEs are solved by using Runge-Kutta-Fehlberg 45 and shooting techniques. The numerical computations are made by utilizing well-known maple 17 software. The different physical parameter effects on velocity, temperature, and concentration curves, as well as skin friction and Sherwood numbers, are analyzed through graphs. It is noted that the larger values of solid volume fraction enhances the drag coefficient and reduces the rate of heat transfer. It is further observed that the curve of a viscous fluid is always lesser than curve of a nanofluid.     

Experimental investigation on the heat and water transfer enhancement in a membrane-based air-to-air humidifier at insulation condition

In the present study, a membrane-based air-to-air planar humidifier (MAPH) with baffle-blocked flow channels and a common MAPH are fabricated, tested and compared. These MAPHs are well thermal insulated from their surroundings. Polyoxymethylene (POM) plates with some unique properties such as large tensile and flexural strength, high chemical resistance and high stiffness are used to create channels at dry and humid sides of MAPHs. The obtained findings revealed that the higher heat and water transfer rates and smaller dew point approach temperature (DPAT) in entire tested flow rates occurs in baffle-blocked MAPH. To evaluate the MAPH performance with considering the pressure drop, a dimensionless parameter, performance evaluation criteria (PEC), is introduced. At flow rates less than 1 m³/h, PEC is less than 1, indicating a decline in MAPH performance with considering the pressure drop. In baffle-blocked MAPH using water trap in the inlet of dry side leads to the performance deterioration. Additionally, the increased relative humidity (RH) of humid side inlet causes an increase in DPAT, consequently, the performance deterioration.     

Assessment of the WWER-440/V-213 reactor condition

The current state of materials (steel, welds, anticorrosive cladding metal) of WWER-440/V-213 RPVs has been evaluated with respect to the peculiarities of their manufacturing process and operation for 30 years. The possibility of operating these reactors after the design service life has been assessed considering the degradation of properties due to thermal and radiation embrittlement, as well as damage from cyclic loads. Taking into account that the main results of operating effects were obtained in research laboratories it is necessary first of all to continue the investigations in the commercial reactors. In future, it will be possible to extend the service life of this type of reactors. For example, the positive results have been already obtained for unit No. 3 Kola NPP reactor.     

室内空调条件下孝感市建筑物屋顶、墙体的隔热节能指标的选取

根据建筑物热工气候分区,对夏热冬冷地区的孝感市运用“建筑热环境与建筑节能设计标准相关控制法”中所提出的简化公式及孝感市的气象参数,计算出了室内空调条件下建筑物屋顶、墙体的隔热控制和节能控制指标,为我国夏热冬冷地区及其他地区建筑的隔热和节能控制设计提供参考依据。     

提高真空绝热板绝热效果的探索

真空绝热板（Vacuum Insulated Panel）的绝热效果不仅和板内的真空度有关,而且和芯层材料有关,文章对真空绝热板的绝热机理进行了分析,并对影响真空绝热板导热系数的众多因素加以论述.从影响因素入手,提出了降低VIP导热系数的方法.     

Significance of thermal radiation on dusty fluid flow over a stretching rotating disk with convective boundary condition

This paper explores the flow of dusty fluid over a stretching rotating disk with thermal radiation. Further, the convective boundary condition is considered in this modeling. The described governing equations are reduced to ordinary differential equations by using apt similarity transformations and then they are numerically solved using Runge–Kutta–Fehlberg-45 scheme. To gain a clear understanding of the current boundary layer flow problem, the graphical results of the velocity and thermal profiles, shear stresses at the disk, and Nusselt number are drawn. Results reveal that the increase in the value of the porosity parameter reduces the velocity of both particle and fluid phases. The increase in the value of the Biot number improves the temperature gradient of both particle and fluid phases. The rise in the value of the radiation parameter advances the heat transference of both phases. The rise in the value of the Biot number improves the rate of heat transfer. Finally, increasing the value of the radiation parameter improves the rate of heat transfer.     

真空绝热板绝热特性研究

真空绝热板的绝热效果取决于多方面因素,分析了真空绝热板的隔热机理及影响其绝热性能的因素,并提出了提高真空绝热板绝热效果的途径。     

CFD modeling of two-stage ignition in a rapid compression machine: Assessment of zero-dimensional approach

In modeling rapid compression machine (RCM) experiments, zero-dimensional approach is commonly used along with an associated heat loss model. The adequacy of such approach has not been validated for hydrocarbon fuels. The existence of multi-dimensional effects inside an RCM due to the boundary layer, roll-up vortex, non-uniform heat release, and piston crevice could result in deviation from the zero-dimensional assumption, particularly for hydrocarbons exhibiting two-stage ignition and strong thermokinetic interactions. The objective of this investigation is to assess the adequacy of zero-dimensional approach in modeling RCM experiments under conditions of two-stage ignition and negative temperature coefficient (NTC) response. Computational fluid dynamics simulations are conducted for n-heptane ignition in an RCM and the validity of zero-dimensional approach is assessed through comparisons over the entire NTC region. Results show that the zero-dimensional model based on the approach of ‘adiabatic volume expansion’ performs very well in adequately predicting the first-stage ignition delays, although quantitative discrepancy for the prediction of the total ignition delays and pressure rise in the first-stage ignition is noted even when the roll-up vortex is suppressed and a well-defined homogeneous core is retained within an RCM. Furthermore, the discrepancy is pressure dependent and decreases as compressed pressure is increased. Also, as ignition response becomes single-stage at higher compressed temperatures, discrepancy from the zero-dimensional simulations reduces. Despite of some quantitative discrepancy, the zero-dimensional modeling approach is deemed satisfactory from the viewpoint of the ignition delay simulation.     

Thermal hysteresis in fibrous insulation

Thermal hysteresis, characterized by both the difference between the adsorption and desorption isotherms and the difference between heat of adsorption and heat of desorption, is investigated for typical fiberglass insulation. The heats of sorption are measured, as a function of equilibrium adsorbate mass, using an apparatus based on the energy balance principle. A numerical simulation, employing a local-volume-averaging formulation and measured heats of sorption, is performed to examine the effect of thermal hysteresis on simultaneous heat and mass transfer and moisture/frost accumulation in a transient process for a fiberglass insulation slab. The results show that the thermal hysteresis effect is pronounced regardless of whether an insulation slab is initially dry or wet. Neglecting hysteresis effects in simulating a dynamic transport process can lead to errors in the instantaneous heat flux of up to 51% for an initially dry fiberglass slab and 23% for an initially moist slab.     

PCM thermal insulation in buildings

This paper presents the results of a numerical and experimental study of phase change material (PCM) filled walls and roofs under real operational conditions to achieve passive thermal comfort. The numerical part of the study was based on a one-dimensional model for the phase change problem controlled by pure conduction. Real radiation data was used to determine the external face temperature. The numerical treatment was based upon using finite difference approximations and the ADI scheme. The results obtained were compared with field measurements. The experimental set-up consisted of a small room with movable roof and side wall. The roof was constructed in the traditional way but with the phase change material enclosed. Thermocouples were distributed across the cross section of the roof. Another roof, identical but without the PCM, was also used during comparative tests. The movable wall was also constructed as is done traditionally but with the PCM enclosed. Again, thermocouples were distributed across the wall thickness to enable measurement of the local temperatures. Another wall, identical but without the PCM, was also used during comparative tests. The PCM used in the numerical and experimental tests was composed of a mixture of two commercial grades of glycol in order to obtain the required fusion temperature range. Comparison between the simulation results and the experiments indicated good agreement. Field tests also indicated that the PCM used was adequate and that the concept was effective in maintaining the indoor temperature very close to the established comfort limits. Further economical analysis indicated that the concept could effectively help in reducing the electric energy consumption and improving the energy demand pattern. © 1997 by John Wiley & Sons, Ltd.     

Assessment of diffuse solar energy under general sky condition using artificial neural network

In this paper, artificial neural network (ANN) models are developed for estimating monthly mean hourly and daily diffuse solar radiation. Solar radiation data from 10 Indian stations, having different climatic conditions, all over India have been used for training and testing the ANN model. The coefficient of determination (R²) for all the stations are higher than 0.85, indicating strong correlation between diffuse solar radiation and selected input parameters. The feedforward back-propagation algorithm is used in this analysis. Results of ANN models have been compared with the measured data on the basis of percentage root-mean-square error (RMSE) and mean bias error (MBE). It is found that maximum value of RMSE in ANN model is 8.8% (Vishakhapatnam, September) in the prediction of hourly diffuse solar radiation. However, for other stations same error is less than 5.1%. The computation of monthly mean daily diffuse solar radiation is also carried out and the results so obtained have been compared with those of other empirical models. The ANN model shows the maximum RMSE of 4.5% for daily diffuse radiation, while for other empirical models the same error is 37.4%. This shows that ANN model is more accurate and versatile as compared to other models to predict hourly and daily diffuse solar radiation.     

Economics of thermal insulation

Thermal insulation can be seen, from current option-costing analyses, to be usually an excellent financial investment. Considerations of capital and revenue costs lead to simple design procedures for predicting the optimal economic thicknesses of insulants for plane walls and pipes, respectively. An energy utilisation inefficiency factor is proposed, together with a new profession of engineer—the energy accountant.     

Assessment of the local thermal non-equilibrium condition in developing forced convection flows through fluid-saturated porous tubes

The present investigation is concerned with the numerical simulation of forced convection heat transfer flows in a tube filled with a fluid-saturated porous medium. Steady state incompressible flows with isothermal tube walls are considered along with a uniform inlet approach velocity and temperature conditions. In addition, the generalized form of the momentum equation is considered by accounting for the solid boundary and the Forchheimer quadratic inertial effects without invoking the boundary layer approximations. Moreover, the energy transport is simulated using the two-equation model, which accounts separately for the local fluid and solid temperatures. The numerical solution is obtained through the application of the finite volume method. The validity of the local thermal equilibrium (LTE) was tested over a wide domain of the employed dimensionless parameters, namely; the Peclet number, Biot-like number, effective fluid-to-solid thermal conductivity ratio, Reynolds number, Forchheimer dimensionless coefficient and Darcy number. The validity of the LTE condition was examined for the full tube length and upon excluding the first tube diameter length.