An accurate model for the DC-side voltage control of the neutral point diode clamped converter

This paper presents an in depth analysis to show that the conventional power balance model for the dc-side control of the Neutral Point Diode Clamped (NPC) converter is not adequately accurate for all operating conditions. The inaccuracy is due to the fact that the instantaneous power of the ac-side inductors is assumed to be negligible in the power balance equation. This paper shows that this assumption obscures (i) the impact of operating point which is a direct consequence of the inherent nonlinear nature of the problem and (ii) the nonminimum-phase property of the plant in the rectifying mode of operation. Thus, the conventional dc-side control model may lead to unsatisfactory performance or even instability. To overcome the shortcoming, this paper also develops accurate nonlinear and linearized models for the dc-side controller design, including the impact of the ac-side interface reactors. The analytical results and the conclusions are validated by digital time-domain simulation of a study system in the PSCAD/EMTDC environment. The mathematical developments and conclusions of the paper are equally applicable to the conventional two-level VSC.     

Fast ferroresonance suppression of coupling capacitor voltage transformers

This paper describes a procedure for fast suppression of the phenomenon of ferroresonance in coupling capacitor voltage transformers (CCVT) without major change in the CCVT design. It is shown that it is possible to adjust parameters of the secondary overvoltage protection and the filter circuit so that the ferroresonance can be cleared in a very short time interval. The study cases reported in this paper show that ferroresonance is effectively cleared within two cycles. An implementation of metal oxide varistors (MOV) as part of passive ferroresonance protection is also addressed. The Electromagnetic Transients Program (EMTP) is used for modeling transients and fine-tuning the ferroresonance suppressing circuit. The studies are conducted on the Trench TEHMP161A CCVT.     

Order reduction of the dynamic model of a linear weakly periodic system-part II: frequency-dependent lines

The methodology for the order reduction of the dynamic model of a linear weakly periodic system presented in the first part of this paper is applied here to the case when the power network includes frequency dependent transmission lines. The technique for obtaining the external equivalent of the network is described. A comparison is given between systems having lines with constant or frequency-dependent parameters. Time-domain simulations have shown that the reduced-order equivalent leads to fast and accurate results.     

On the application of the complex torque coefficients method to the analysis of torsional dynamics

The complex torque coefficients method has been widely accepted for the analysis of the phenomenon of torsional interaction of turbine-generator units in power systems. This paper shows that, depending on the system parameters and the operating point, the complex torque coefficients method may exhibit limitations and not accurately and fully predict the system behavior in the frequency range of interest. These shortcomings consist of inability to i) predict monotonic instability due to real poles, ii) identify all electromechanical oscillatory modes, and iii)accurately predict damping (and consequently stability) of the oscillatory modes. This paper develops mathematical expressions to highlight the limitations of the complex torque coefficients method. Quantitative results based on three case studies, including a study on the first IEEE Benchmark System, are reported and results from eigenvalue analysis method, complex torque coefficients method, and time-domain simulation are presented and compared. This paper concludes that the complex torque coefficients method can be used only as a preliminary method for the investigation of torsional interactions and the results must be verified by other methods.     

Methodology and practice of in situ differential scanning electron microscopy

Contrast plays a crucial role both in qualitative and quantitative imaging in scanning microscopy. Usual methods of obtaining high contrast images in the scanning electron microscope (SEM) involve performing specific operations on the video signal already produced by the SEM. In this article, the concept of in situ differential imaging in the SEM is discussed. In this imaging modality, a true differential image of the sample is generated simultaneously with the normal video. The signal can be obtained at low and high video band-widths, thus allowing low contrast objects to be readily imaged. Various methodologies developed to perform in situ differential imaging are reviewed. A characteristic of all these techniques is their sensitivity to edges, a feature which is extensively used in a number of applications. The ability to obtain feature enhancement in any desired direction is another important attribute of this approach. Examples are given on the use of the method in general imaging as well as in the metrology of critical dimensions.     

Numerical Assessment of the Film Cooling Through Novel Sister-Shaped Single-Hole Schemes

In the present paper, a numerical investigation is conducted on film cooling performance from novel sister-shaped single-hole schemes. Based on the sister hole film cooling technique, shaped holes are formed by merging discrete sister holes to a primary hole. Simulations are performed at four blowing ratios of 0.25, 0.5, 1, and 1.5. The novel-shaped holes resulted in a significant reduction in the jet liftoff effect in comparison with a cylindrical and a forward-diffused shaped hole. Moreover, film cooling effectiveness is notably increased at the high blowing ratios of 1 and 1.5.     

Effective stress law for the permeability of a limestone

The effective stress law for the permeability of a limestone is studied experimentally by performing constant-head permeability tests in a triaxial cell with different conditions of confining pressure σ and pore pressure p_f. Test results show that a pore pressure increase and a confining pressure decrease both result in an increase of the permeability, and that the effect of the pore pressure change on the variation of the permeability is more important than the effect of a change of the confining pressure. A power law is proposed for the variation of the permeability with the effective stress (σ′=σ–n_kp_f). The permeability-effective stress coefficient n_k increases linearly with the differential pressure and is greater than unity as soon as the differential pressure exceeds few bars. The test results are well reproduced using the proposed permeability-effective stress law. A conceptual pore-shell model based on a detailed observation of the microstructure of the studied limestone is proposed. This model is able to explain the experimental observations on the effect of the total stress and of the pore pressure on the permeability of the limestone. Effective stress coefficients for the stress-dependent permeability which are greater than one are obtained. It is shown that the controlling factor is the ratio of the different bulk moduli of the various constituents of the rock. This ratio is studied experimentally by performing microhardness tests.     

Evaluation of a permeability–porosity relationship in a low-permeability creeping material using a single transient test

A method is presented for the evaluation of the permeability–porosity relationship in a low-permeability porous material using the results of a single transient test. This method accounts for both elastic and non-elastic deformations of the sample during the test, and is applied to a hardened class G oil well cement paste. An initial hydrostatic undrained loading is applied to the sample. The generated excess pore pressure is then released at one end of the sample while monitoring the pore pressure at the other end and the radial strain in the middle of the sample during the dissipation of the pore pressure. These measurements are back-analysed to evaluate the permeability and its evolution with porosity change. The effect of creep of the sample during the test on the measured pore pressure and volume change is taken into account in the analysis. This approach permits to calibrate a power law permeability–porosity relationship for the tested hardened cement paste. The porosity sensitivity exponent of the power–law is evaluated equal to 11, and is shown to be mostly independent of the stress level and of the creep strains.     

Seismic motion response and fragility analyses of cantilever retaining walls with cohesive backfill

The seismic motion response of a cantilever retaining wall with cohesive and cohesionless backfill materials was evaluated using fully dynamic analysis based on finite difference method. The dynamic analysis was validated based on experimental test results and then compared to analytical and empirical correlations based on Newmark sliding block method. Seven different earthquake events and the backfills with low to high levels of cohesion were considered. Nonlinear regression analyses were carried out to provide correlations between free-field peak ground acceleration (PGA) and maximum relative displacement of the retaining wall. These results were compared to results from empirical and analytical methods. Furthermore, fragility analyses were conducted to determine the probability of damage to the retaining wall for different free-field PGAs and backfill cohesions. It is demonstrated to what extent a small amount of cohesion in backfill material can influence displacement of the retaining wall and probability of damage in seismic conditions.     

Selective laser melting raw material commoditization: impact on comparative competitiveness of additive manufacturing

The paper analyses the impact of cheaper metal powder supplies on the comparative competitiveness of additive manufacturing (AM). By utilising two case studies, we compare the economic impact of an innovative titanium extraction method on Selective Laser Melting (SLM) and conventional methods of machining and casting. A switch-over analysis identifies the production quantities above which conventional manufacturing is more cost competitive than additive manufacturing. This analysis is performed for current raw material as well as cheaper raw material supply. The results illustrate the improved comparative competitiveness of SLM as the titanium supply is commoditised and more readily available in powder form. The responsiveness of the supply chain is improved as the switch-over point between SLM and conventional methods increases. Moreover, as the raw material supply chain for titanium is transformed through the use of this novel extraction method, the manufacturing supply chain is simplified.