Publications

The increasing demand to utilize renewable energy necessitates the transmission of power over long distances. HVDC technology has emerged as the optimal solution for this purpose due to fewer losses and good economic factors. Multi-terminal DC (MTDC) systems are being more focused nowadays, as they offer more advantages over Point to Point (P2P) HVDC scheme because the MTDC network adds more reliability and flexibility to the system. DC/DC converters are emerging as an important device for future MTDC transmission systems. They are required to interconnect HVDC links with different system characteristics such as different DC voltage levels, grounding schemes, and technologies. In addition to this, DC/DC converters are capable of providing additional features in the system like grid protection, DC voltage control, and power flow control. The majority of studies in the literature on DC/DC converters are predominantly focused on the context of either exploring different DC/DC converter topologies or their control and operation in constant power mode for interconnecting HVDC links, suitable for future MTDC grids. This paper presents an MTDC test case integrating a DC/DC converter where the converter is working with a DC voltage controller and participating in the DC voltage management system. The influence of voltage-controlled DC/DC converter is studied by introducing power disturbances in the MTDC system. The system is modeled and simulated in EMTP software. The droop control technique known for the VSC converter for DC voltage control is extended to obtain a dual droop controller which is used with a DC/DC converter for controlling both DC grid voltages simultaneously. However, this control approach involves designing two droop coefficients for their respective DC grids, complicating the examination of their interaction. Another possibility is to use a new technique called “virtual resistance DC voltage control” which requires tuning only one parameter. The objective is to control DC grid voltages and establish a link between the interconnected networks. The control approach is validated through electromagnetic transient (EMT) simulations. Through the virtual resistance DC voltage control, the interconnected DC grids can share the power disturbance in the system and maintain the DC voltages under their specified limits. This makes the MTDC system more reliable and reduces the stress on the DC voltage management system. Modular multilevel converter (MMC) based topologies are used for DC/DC converters, namely F2F-MMC (front-to-front MMC) which can provide galvanic isolation between the two links and MMC-DC (M2DC) which does not provide galvanic isolation. A comparison analysis has also been made to compare their behavior with a virtual resistance controller. All converters are modeled using reduced order modeling methodology and the DC cables are modeled with wideband models. The observations in this paper indicate that by employing the virtual resistance DC voltage controller, a connection has been established between interconnected networks, enabling HVDC links to actively participate in and share the power disturbances within the MTDC system. Apart from this, the virtual resistance control behavior remains consistent regardless of the topology of the DC/DC converter, thus demonstrating its robustness as a DC voltage controller.

AbstractThe future multi‐terminal direct‐current (MTDC) grid will require the interconnection of point‐to‐point high‐voltage (HV) DC links with different specifications such as DC voltage level, system grounding configuration and HVDC technology. To adapt these differences, it is obligatory for DC/DC converters to interconnect HVDC links. Additionally, they are capable of providing supplementary functionalities as they are highly controllable devices. In this article, a primary virtual resistance DC voltage controller associated with DC/DC converter is proposed for managing DC grid voltages of the interconnected HVDC grids, increasing the reliability of the system. The commonly known topology, Front‐to‐Front Modular Multilevel Converter (F2F‐MMC) is adopted for DC/DC converter. Time‐domain simulations are performed using EMTP software for validating the controller behaviour under power disturbances and large events of loss of one converter in a MMC‐based MTDC system. The converters are modelled using reduced order modelling (ROM) methodology. Apart from this, dynamic studies have been carried out using a linear state space model for small‐signal stability analysis of a HVDC system integrating DC/DC converter with a virtual resistance DC voltage controller. The results are examined through parametric sensitivity analysis.

The Modular Multi-Level DC-DC Converter (M2DC) is an attractive non-isolated DC-DC converter topology for HVDC grid. In order to carry out MTDC grid stability studies, the development of reduce order models of converters is necessary. This article first presents the M2DC converter. Then, the reduce order model will be developed in the second part. The development of the control of this model will be carried out in the third part. Atlast, the comparison of the reduce order model and its control with the average arm model will be performed in the later section of the paper.

The development of multi-terminal DC (MTDC) networks has various challenges as interconnecting grids of different voltages and grounding schemes, DC grid protection and power flow. DC/DC converter has emerged as the solution for interconnecting HVDC links with different specifications. In this paper, the Front-to-Front Modular Multilevel Converter (F2F-MMC) topology is adopted for DC/DC converter, which can act as a firewall between the healthy and faulty grid during DC faults. Along with this, DC/DC converters when operated in DC voltage control mode can provide supplementary functionalities such as participating in DC grid voltage management and increasing the reliability of the system. In this study, the F2F-MMC converter is operated with a virtual resistance DC voltage controller integrated into an MTDC system. A pole-to-pole DC fault is applied on the MTDC grid and the influence of the virtual resistance controller associated with the DC/DC converter is studied for re-establishing the power flow after DC faults.

The mechanical properties of structural timber largely depend on the occurrence of knots and on fibre deviation in their vicinities. In recent strength grading machines, lasers and cameras are used to detect surface characteristics such as the size and position of knots and local fibre orientation. Since laser dot scanning only gives reliable information about the fibre orientation in the plane of board surfaces, simple assumptions are usually made to define the inner fibre orientation to model timber boards. Those models would be improved by better insight into real fibre deviation around knots. In the present work, a laboratory method is developed to evaluate growth layers geometries and fibre orientation, solely based on the fact that the fibers are parallel to the tree rings and without any further assumptions. The method simply relies on color scans and laser dot scans of Douglas fir (Pseudotsuga menziesii) timber specimen sections revealed by successive planing. The proposed method provides data on fibre orientation in 3D with an accuracy that is relevant for the calibration of detailed models.

This paper describes an innovative method developed by the authors to support basic decisions concerning the structural restoration of a large historical panel painting which had been damaged by inappropriate attachment to a wall and ongoing exposure to severe changes in environmental humidity. The Lapidazione di Santo Stefano is a large panel (2.78 × 3.92 m2) painted by Giorgio Vasari in 1571 and has been housed since then in the Church of Santo Stefano dei Cavalieri in Pisa (Italy). Its wooden support is made of large horizontal planks glued together along their edges and stiffened by vertical, dovetailed crossbeams. The panel was tightly fastened to a church wall with several rigid bolts; due to the moisture cycling produced by rainwater leakage and a subsequent “compression set”, it had developed severe tension stresses perpendicular to the grain, resulting in cracks affecting both the wood and the paint layers. To decide how to carry out the structural restoration of the panel, it was necessary to know whether slippage could occur between the panel and crossbeams during seasonal variations in environmental humidity. Without slippage, tensile stresses would be generated in the wood and could produce further cracks and damage the paint layers. An in situ monitoring method for assessing the possibility of slippage was developed and implemented. An analysis of data collected over a period of 6 months before the structural restoration confirmed that adequate slippage was possible; hence, the decision to fully repair the cracks was taken. Monitoring continued for a year after restoration and confirmed the previous findings. This paper describes the monitoring method, the equipment used, the results of its implementation and its value as a preventive conservation tool.

Automatic control of a workpiece being manufactured is a requirement to ensure in-line correction and thus move towards a more intelligent manufacturing system. There is therefore a need to develop control strategies which are capable of taking precise account of real working conditions and enabling first-time-right control. As part of such a smart-control strategy, this paper introduces a machine learning-based approach capable of accurately predicting a priori the 3D coverage of a part according to a scan configuration given as input, i.e. predicting before scanning it which areas of the part will be acquired for real. This corresponds to a paradigm shift, where coverage estimation no longer relies on theoretical visibility criteria, but on rules learned from a large amount of data acquired in real-life conditions. The proposed 3D Scan Coverage Prediction Network (3DSCP-Net) is based on a 3D feature encoding and decoding module, which is capable of taking into account the specifics of the scan configuration whose impact on the 3D coverage is to be predicted. To take account of real working conditions, features are extracted at various levels, including geometric ones, but also features characterising the way structured-light projection behaves. The method is thus able to incorporate inter-reflection and overexposure issues into the prediction process. The database used for the training was built using an ad-hoc platform specially designed to enable the automatic acquisition and labelling of numerous point clouds from a wide variety of scan configurations. Experiments on several parts show that the method can efficiently predict the scan coverage, and that it outperforms conventional approaches based on purely theoretical visibility criteria.

Complex mechanical parts with high geometry and dimensional accuracy require multi-axis machining with low volumetric errors. To achieve such precision in the micrometer range, the geometric errors of the machine tool must be correctly identified and compensated. This article proposes a new methodology to measure directly the geometric errors of the rotary axes on machine tool. The methodology consists of a system of several non-contact sensors that are strategically placed around a datum cylinder. This system is held on the machine spindle and remains static over the entire measurement. The relative motion between the sensors and the cylinder is obtained by rotating the table of the multi-axis machine tool. The cylinder could be fast centered and leveled by using micro linear stages. A rotation stage enables to decouple the rotary axis motion from the datum cylinder rotation, consequently carry out methodologies of errors separation associated with multiprobe and multistep methods.

The dimensional accuracy of machined parts can be influenced by numerous factors, among which inaccuracies in the machine’s structural loop and thermal expansion of components have the biggest impact. Hence, highly accurate machining requires effective error compensation. This motivates the development of a real-time compensation system implemented on a five-axis machine tool. In this study, a physical monitoring device is installed in the feedback loops of the machine’s axial position control circuit, to intercept and modify linear encoder signals. It communicates with a custom software application that processes the data and generates corrected signals according to geometric model based on the rigid body assumption. The numerical controller (NC) is then induced to perform volumetric error correction based on its default programming. The key advantage of this software-based compensation strategy over the use of look-up tables or NC program modification is the total independence from the NC. The same real-time program is also used for the characterization of linear axis controls. This article outlines the behaviour of an NC machine when an axis displacement is generated via the modification of measuring systems feedback. The rate of change of the virtually added movement appears to be more of a limiting factor to the controller than the magnitude. Moreover,

Liquid nanofilms are ubiquitous in nature and technology, and their equilibrium and out-of-equilibrium dynamics are key to a multitude of phenomena and processes. We numerically study the evolution and rupture of viscous nanometric films, incorporating the effects of surface tension, van der Waals forces, thermal fluctuations, and viscous shear. We show that thermal fluctuations create perturbations that can trigger film rupture, but they do not significantly affect the growth rate of the perturbations. The film rupture time can be predicted from a linear stability analysis of the governing thin film equation, by considering the most unstable wavelength and the thermal roughness. Furthermore, applying a sufficiently large unidirectional shear can stabilize large perturbations, creating a finite-amplitude traveling wave instead of film rupture. In three dimensions, unidirectional shear does not inhibit rupture, as perturbations are not suppressed in the direction perpendicular to the applied shear. However, if the direction of shear varies in time, then the growth of large perturbations is prevented in all directions, and rupture can be impeded. © 2024 American Physical Society.