Lumped modelling of open-circuit turbogenerator operationalparameters

Solid-rotor turbine-generators have commonly been derived from the fitting of curves to terminal characteristics obtained by measurement or from finite element simulations. The paper presents a new method to represent the open-circuit operational inductances of a large turbogenerator of 150 MVA. The method differs from current estimation techniques because the lumped model and its parameters are determined simultaneously. It is based on a finite-element electromagnetic linear analysis of the solid-rotor machine cross section in the frequency domain. Network theory is used to characterise the quadrature-axis model where the problems of model structure and parameter determination are treated. A q-axis equivalent circuit with one damper winding and frequency dependent parameters is found. In addition, an analysis of the machine d-axis is presented. It is based in an electromagnetic study of the “black box” two-port network, where the open-circuit operational inductances are represented by frequency dependent parameters     

A continually online trained neurocontroller for excitation andturbine control of a turbogenerator

The increasing complexity of the modern power grid highlights the need for advanced modeling and control techniques for effective control of turbogenerators. This paper presents the design of a continually online trained (COT) artificial neural network (ANN) based controller for a turbogenerator connected to an infinite bus through a transmission line. Two COT ANNs are used for the implementation; one ANN, the neuroidentifier, to identify the complex nonlinear dynamics of the power system and the other ANN, the neurocontroller, to control the turbogenerator. The neurocontroller replaces the conventional automatic voltage regulator (AVR) and turbine governor. Simulation and practical implementation results are presented to show that COT neurocontrollers can control turbogenerators under steady state as well as transient conditions     

Effect of excitation capacitors on transient performance ofreluctance generators

The transient responses of a reluctance generator connected to an infinite power system excited by a bank of terminal capacitances are compared to those when load excitation is used. A mathematical model is developed to simulate the machine with its terminal capacitor. With the aid of a least-square-error method, this model is used to optimize the machine parameters. The capacitance excitation requirements for different load conditions are then computed using a steady-state model. The comparison of the transient responses shows that the terminal-capacitor excitation method has several advantages over the load excitation method. It reduces the first rotor swing and gives more damping to the subsequent rotor oscillations. It also increases the critical fault-clearing time and hence the transient stability limits. In addition, it suppresses all power frequency torque oscillations, which are quite pronounced when load excitation is used     

Characterising the d-axis machine model of a turbogenerator usingfinite elements

An analysis of a long established fundamental assumption is presented. The assumption that superposition is valid in frequency response derived models is shown to be wrong, because eddy current losses in the solid rotor cannot be superimposed in the machine direct-axis. This implies that network theory is not valid in characterising the d-axis machine model. A machine model structure with one damper winding in the d-axis is derived from finite element analysis. Unequal mutual inductances in the machine d-axis are determined and hence the so-called differential leakage inductances are found and they are frequency dependent. The study is made on a 150 MVA turbine generator by simulating the standstill frequency response test with finite elements     

Hydrogen and oxygen from water —IV. Control of an effusional separator during a solar intensity transient

An example performance map of an archetypal reactor for producing hydrogen and oxygen from water in a one-step thermochemical process is presented. We show that readjustment of the device in response to a change in incident solar intensity may be achieved, without changing the reactor temperature, Knudsen number, or feed rate, by simply changing the downstream pressure, with very little effect on the thermal efficiency.     

Comparison of minority carrier diffusion length measurements in silicon solar cells by the photo-induced open-circuit voltage decay (OCVD) with different excitation sources

The photoinduced open-circuit voltage decay technique was used to investigate the minority carrier lifetime in crystalline and polycrystalline silicon solar cells. This convenient investigation technique allows a fast determination of the diffusion length of minority carriers in semiconductor materials and is an important technique to predict the solar cell performance. The decay curves were obtained with different excitation sources, a xenon stroboscope lamp and a Nd:YAG laser, and the results were compared.     

交流同步起动发电机在变频起动中机械特性的分析与计算

立足于铁道部正在研制的第一台交流传动内燃调车机车,在已设计的同步起动发电机参数的基础上,结合变频起动装置的工作原理,分析并计算该起动发电机在起动过程中的机械特性。     

A simple solution for transient heat transfer during heating of a slab product

A theoretical and experimental study of transient heat transfer in the heating of an individual slab product, subjected to an air flow at a temperature of 50°C and a velocity of 1 m/s, is presented. Experimental temperature measurements at the centre of the slab product were made, and the experimental heat-transfer rates were derived from the temperature data. A simplified analytical technique, using the boundary condition of the third kind in transient heat transfer, was used to predict the theoretical heat transfer rates for two cases, the first considering that the heat transfer coefficient is a convective heat transfer coefficient, and the second considering that heat transfer coefficient is the sum of the convective and radiative heat transfer coefficients. The experimental heat-transfer rates were compared with the predictions for two cases, and a very good agreement was obtained.     

The spectral response of the open-circuit voltage: a new characterization tool for solar cells

This paper explores the possibility of measuring the open-circuit voltage of a solar cell as a function of wavelength as a tool for device characterization. Theoretical calculations and computer simulations show that the spectral response of the open-circuit voltage exhibits a similar dependence to the spectral response of the short-circuit current. Experimental studies on silicon solar cells confirmed the strong spectral dependence of the open-circuit voltage. The spectral measurements have been performed using a quasi-steady-state open-circuit voltage method, which also allows to determine the spectral response of the maximum power voltage. The advantages of this new technique over conventional spectral response measurements include its applicability directly after junction formation and the simplicity of the experimental apparatus.     

Characteristics of transient heat transfer during quenching of a vertical hot surface with a falling liquid film

An experimental study has been conducted to elucidate characteristics of transient heat transfer during quenching of a vertical hot surface with a falling liquid film. The experiment was done at atmospheric pressure for the following conditions: an initial surface temperature from 200 to 400^°C, a subcooling of 20– 80 K, average velocity of 0.52– 1.24 m/s, and the block material is copper and carbon steel. The surface temperature and heat flux are estimated from the measured temperatures in the block during the quench by a two‐dimensional inverse solution. It follows that as the position of wetting advances downward, the position at which the heat flux becomes a maximum also advances downward. The time at which the position of maximum heat flux begins to move is one of the most important parameters and can be predicted by a proposed correlation. In addition, it is revealed that the maximum heat flux for copper depends on the length to which it occurs from the leading edge. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(6): 345– 360, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20167     

Simulation of transient convective burning of an n-octane droplet using a four-step reduced mechanism

The transient burning of an n-octane fuel droplet in a hot gas stream is numerically studied using a four-step reduced mechanism, with considerations of droplet surface regression, deceleration due to the drag of the droplet, internal circulation inside the droplet, variable properties, non-uniform surface temperature, and the effect of surface tension. Two different types of the four-step mechanism are examined and found almost identical. The four-step mechanism has earlier instant of the wake-to-envelope transition than the one-step mechanism at low ambient temperature, but this difference between the two mechanisms diminishes when the ambient temperature is increased. The four-step mechanism has smaller mass burning rate for a wake flame but greater mass burning rate for an envelope flame than the one-step mechanism. The two mechanisms have small differences in the critical initial Damkohler number. Lower ambient temperature yields later wake-to-envelope transition and smaller mass burning rate. Higher ambient pressure has greater overall mass burning rate because of greater gas density and thus greater concentrations of reactants for a major part of the lifetime. Greater ambient mass fraction of oxygen yields faster oxidation kinetics and greater Damkohler number. As the ambient mass fraction of oxygen increases, the instant of wake-to-envelope transition advances for an initial wake flame, and finally the initial flame becomes an envelope flame when the ambient mass fraction of oxygen exceeds some critical value. A correlation is developed for the critical initial Damkohler number in terms of the ambient temperature, ambient pressure, and ambient mass fraction of oxygen.     

Molecular dynamics calculation of heat dissipation during sliding friction

A novel method of calculation of heat dissipation during sliding between metals was presented by using molecular dynamics. Temperature distribution in the regions near the contact interface was calculated. The calculation results show that plastic deformation in the near-surface regions accounts for most of the friction heat and the temperature increase. Friction heat was built up in the regions subject to plastic deformation. In the case that no plastic deformation occurred, elastic waves contributed mainly to the energy dissipation so that no large heat buildup could take place in the vicinity of the contact regions.     

The transient response of an evaporator fed through an electronic expansion valve

The primary task of an expansion valve in a refrigerating machine is to control the mass flowrate into the evaporator to obtain optimal operation without ‘hunting’ under given conditions. A stepper-motor-controlled expansion valve and an evaporator have been studied. The tests were carried out on a fixed refrigerating machine of cooling capacity less than 6 kW, with constant condensation conditions and variable evaporating temperature (−20 to +10°C) and compressor speed (1000 to 3000 r.p.m). Two control algorithms (proportional/derivative and qualitative optimal regulation) have been developed for opening and closing the valve with the stepper motor. The control parameters depend on both the expansion valve and the evaporator transfer functions. In steady-state conditions, the system is stable with a superheat equal to the set value. Under transient conditions, with step excitations of 300 and 1000 r.p.m as well as for cold-start of the machine, the control algorithms are adequate for regulating the refrigerant flowrate into the evaporator. © 1997 John Wiley & Sons, Ltd.     

A model of the thermal transient state of a wall of a room during the heating by a heating system

After turning on a room heating system (e.g. central heating) a thermal transient phenomenon takes place on the wall–room system, until it reaches a final thermal equilibrium state. The temperature profiles on the wall cross‐section, starting from an initial profile, corresponding to the initial thermal equilibrium state, come gradually through successive intermediate temperature profiles, to a final temperature profile corresponding to the final thermal equilibrium state. These intermediate, nonlinear and time‐dependent temperature profiles characterize the wall thermal transient state and describe the dynamic thermal behaviour of the wall–room system. The mass of the air in the room is negligible, compared to the mass of the surrounding walls, so the dynamic behaviour of the room–wall system is imposed by the corresponding thermal dynamic behaviour of the walls. The influence of this thermal transient state is important for the room heating behaviour because it acts as a thermal flywheel attenuating and smoothing the room temperature variations. In the present work, using the integral method, analytical expressions yielding the temperature profiles, and the duration of the transient state as a function of thermal and structural characteristics have been developed. Conclusions were drawn on the dynamic thermal behaviour of the room–wall system. Copyright © 2000 John Wiley & Sons, Ltd.     

Transient performance of an isolated induction generator underunbalanced excitation capacitors

This paper presents transient performance of a stand-alone self-excited induction generator (SEIG) under unbalanced excitation capacitors. An approach based on a three-phase induction machine model is employed to derive dynamic equations of an isolated SEIG under unbalanced conditions. The neutral points of both a Y-connected excitation capacitor bank and Y-connected stator windings of the SEIG is connected together through a neutral line. Experimental results obtained from a laboratory 1.1 kW induction machine driven by a DC motor are also performed to confirm the feasibility and effectiveness of the proposed method     

Experimental analysis of a transfer function for the transient response of an evaporator in an absorption refrigeration GAX system

The absorption refrigeration systems (ARS) are highly non-linear, therefore presents oscillations on the responses of its components. The physical analysis of these systems is complex, particularly the transitory response. This paper presents the prediction response of an evaporator for an ARS GAX when occurs a modification in the refrigerant mass flow rate at the evaporator inlet section; this is obtained by means of a transfer function in order to determinate the cooling load and improve the stabilization time for different operation conditions. Different control instruments have been installed in different sections of GAX system in order to obtain an operational stability and carried out the theoretical and experimental analysis. The theoretical results show a close tendency with experimental data, errors below ±2.5% are found. The results obtained between the heat load and the refrigerant mass flow rate at the evaporator will be used to design a control algorithm in order to obtain a complete autonomy system.     

Non-linear dynamical analyses of transient surface temperature fluctuations during subcooled pool boiling on a horizontal disk

The class of dynamics in pool boiling on a large-size heater is assessed under subcooled pool boiling conditions. Transient surface temperature measurements are obtained using surface micro-machined K-type thin film thermocouples (TFT) in 10 °C subcooled pool boiling experiments on a 62.23 mm diameter silicon wafer using PF-5060 as the test liquid. Surface temperature data is obtained at each steady state condition to generate the boiling curve. The fraction of false-nearest neighbors, recurrence plots and space–time separation plots are obtained using the TISEAN package. The correlation dimension is then estimated from the re-constructed phase space data using a naïve algorithm. The correlation dimension varies from ～11.2 to 11.5 near onset of nucleate boiling (ONB), to ～7–10 in fully developed nucleate boiling (FDNB) ～7–9 near critical heat flux (CHF) condition, and from ～6.6 to 7.7 in film boiling. False-nearest neighbor estimates and recurrence plots show that nucleate boiling may be dominated by statistical processes near ONB and in partial nucleate boiling (PNB). In contrast, FDNB, CHF and film boiling seem chaotic and governed by deterministic processes.     

A semi-implicit numerical scheme for a transient two-fluid three-field model on an unstructured grid

A three-dimensional (3D) unstructured hydrodynamic solver for transient two-phase flows has been developed for a 3D component of a nuclear system code and a component-scale analysis tool. A two-fluid three-field model is used for the two-phase flows. The three fields represent a continuous liquid, an entrained liquid, and a vapor field. An unstructured grid is adopted for realistic simulations of the flows in a complicated geometry. The semi-implicit ICE (Implicit Continuous-fluid Eulerian) numerical scheme has been adapted for an unstructured non-staggered grid. This paper presents the numerical method and the preliminary results of the calculations. The results show that the modified ICE scheme is robust and predicts the phase changes and the flow transitions due to a boiling and a flashing very well.     

Multidimensional and transient modeling of an alkaline water electrolysis cell

A three-dimensional (3-D) transient numerical model of an alkaline water electrolysis (AWE) cell with potassium hydroxide solution is developed by rigorously accounting for the hydrogen and oxygen evolution reactions and resulting species and charge transport through various AWE components. First, the AWE model is experimentally validated against a polarization curve corresponding to a wide range of currents as high as 2.0 A·cm^?2. In general, the simulation results compare well with the measured data and further reveal the operating characteristics of AWE cells, showing key distributions of solid/electrolyte potentials and multidimensional contours of reactant and product concentrations at various current densities. In particular, the contribution of hydroxide ion (OH^?) diffusion to the ohmic losses through porous electrodes and a porous separator are quantitatively examined at low and high electrolyte flow rates. The present full 3-D AWE model provides a basic understanding of the electrochemical and transport phenomena and can be further applied to practical large-scale AWE cell and stack geometries for grid-scale hydrogen production.