Smart Transformer-based Medium Voltage Grid Support by Means of Active Power Control

In the last decades the voltage regulation has been challenged by the increase of power variability in the electric grid,due to the spread of non-dispatchable generation sources.This paper introduces a Smart Transformer(ST)-based Medium Voltage(MV)grid support by means of active power control in the ST-fed Low Voltage(LV)grid.The aim of the proposed strategy is to improve the voltage profile in MV grids before the operation of On-Load Tap Changer in the primary substation transformer,which needs tens of seconds.This is realized through reactive power injection by the AC/DC MV converter and simultaneous decrease of the active power consumption of voltage-dependent loads in ST-fed LV grid,controlling the ST output voltage.The last feature has two main effects:the first is to reduce the active power withdrawn from MV grid,and consequently the MV voltage drop caused by the active current component.At the same time,higher reactive power injection capability in the MV converter is unlocked,due to the lower active power demand.As result,the ST increases the voltage support in MV grid.The analysis and simulation results carried out in this paper show improvements compared to similar solutions,i.e.the only reactive power compensation.The impact of the proposed solution has been finally evaluated under different voltage-dependence of the loads in the LV grid.

Electronic band transformation from indirect gap to direct gap in Si-H compound

The electronic band structures of periodic models for S^H compounds are investigated by the density functional theory. Our results show that the Si H compound changes from indirect-gap semiconductor to direct-gap semiconductor with the increase of H content. The density of states, the partial density of states and the atomic charge population are examined in detail to explore the origin of this phenomenon. It is found that the Si-Si bonds are affected by H atoms, which results in the electronic band transformation from indirect gap to direct gap. This is confirmed by the nearest neighbour semi-empirical tight-binding （TB） theory.

Localized Vibrational Modes of Excitation in Electroluminescent Polymers

The localized vibrational modes of exciton and polaron are investigated, and the results show that the frequencies of the three even parity modes of exciton are very different from those of polaron. For the exciton, three modes are distinctly separated;but, for the polaron, two modes with higher frequencies are close to each other, Then, it is possible to distinguish exciton from polaron by watching their Roman spectra. Therefore, the localized mode may be used to specify the exciton and polaron in the electroluminesent polymers.

First Principles Study on the Magnetism of Rectangular Nanosilicenes

We present first-principle calculations on the magnetism in finite rectangular nanosilicenes（RNSs）. An antiferromagnetic（AFM） state at two zigzag edges is found when the RNSs approach a critical size. This AFM state originates from the localized p_z orbits of Si atoms at the edges, similar to those in the infinitely long zigzag-edged silicon nanoribbons. The smallest RNS that can maintain the AFM phase as the ground state is identified. It is also found that aluminum dopants can regulate the distribution of the spin density and the energy difference between AFM and FM states.

The inelastic electron tunneling spectroscopy of edge-modified graphene nanoribbon-based molecular devices

The inelastic electron tunneling spectroscopy（IETS） of four edge-modified finite-size grapheme nanoribbon（GNR）-based molecular devices has been studied by using the density functional theory and Green＇s function method. The effects of atomic structures and connection types on inelastic transport properties of the junctions have been studied. The IETS is sensitive to the electrode connection types and modification types. Comparing with the pure hydrogen edge passivation systems, we conclude that the IETS for the lower energy region increases obviously when using donor–acceptor functional groups as the edge modification types of the central scattering area. When using donor–acceptor as the electrode connection groups, the intensity of IETS increases several orders of magnitude than that of the pure ones. The effects of temperature on the inelastic electron tunneling spectroscopy also have been discussed. The IETS curves show significant fine structures at lower temperatures. With the increasing of temperature, peak broadening covers many fine structures of the IETS curves.The changes of IETS in the low-frequency region are caused by the introduction of the donor–acceptor groups and the population distribution of thermal particles. The effect of Fermi distribution on the tunneling current is persistent.

First-Principles Study of Electronic Properties in PbS（1^-00） with Vacancy Defect

Electronic properties of both Pb and S vacancy defects in PbS（1^-00） have been studied using the first-principles density functional theory （DFT） calculations with the plane-wave pseudopotentials. It is found that the density of states （DOS） near the top of the valence band and the bottom of the conduction band is significantly modified by these defects. Our calculation indicates that in the case of S vacancy defects the Fermi energy shifts to the conduction band making it as an n-type PbS （donor）. However, in the case of Pb vacancy, because of the appreciable change of the DOS, the system acts as a p-type PbS （accepter）. In addition, the structural relaxation shows that the defect leads to outward relaxation of the nearest-neighbouring atoms and inward relaxation of the next-nearest neighbouring atoms.

Magnetic bearings with double crossed loops modelled with T-A formulation and electric circuits

The application of High‐Temperature Superconductor(HTS)coils made of coated conductors has been investigated for many years.A possible configuration for such coils is the jointless loop,also known as the ring coil.The double crossed loop coil(DCLC)has been successfully applied in superconducting magnetic bearings(SMBs).The design of SMBs with DCLCs requires flexible modelling to allow all parts of the device to be represented.This work proposes the T‐A formulation with a thin‐film approximation for modelling SMB with DCLCs in the finite element analysis framework.A 2D representation of the system is coupled with an external electric circuit to model the continuity of the lines that represent the parts of each jointless loop.To couple the T‐A formulation and the circuit,an average of the total electric field,with both resistive and inductive components,is applied to the circuit.The total current computed by the circuit is applied to the T‐A formulation.The proposed methodology was validated by comparison with levitation force experimental data.Two types of tests were simulated:five levitation force tests and three guidance force tests.It is shown that there is a limit to the behaviour of the levitation force related to the high‐loss state.Below this limit,the stack of DCLCs behaves as an equivalent bulk.Beyond this limit,a high‐loss state appears as a linear growth of the levitation force.It is also shown that this high‐loss state in vertical displacement influences the lateral force.

A Simple Kinetic Monte Carlo Approach for Epitaxial Submonolayer Growth

Two-component submonolayer growth on triangular lattice is qualitatively studied by kinetic Monte Carlo techniques.The hopping barrier governing surface diffusion of the atoms is estimated using pair interaction potentials.Several degrees of freedoms enhancing the surface diffusion of atoms are also introduced.The main advantages of the presented technique are the reduced number of free parameters and the clear diffusion activated mechanism for the segregation of different types of atoms.The potential of this method is exemplified by reproducing(i)phase-boundary creation and dynamics related to vacancies and stacking faults;(ii)a special co-deposition and segregation process where the segregated atoms of the second component surround the islands formed by the first type of atoms.

A Kinetic Monte Carlo Approach for Self-Diffusion of Pt Atom Clusters on a Pt(111)Surface

A lattice Kinetic Monte Carlo(KMC)approach is considered to study the statistical properties of the diffusion of Pt atom clusters on a Pt(111)surface.The interatomic potential experienced by the diffusing atoms is calculated by the embedded atom method and the hopping barrier for the allowed atomic movements are calculated using the Nudged Elastic Band method.The diffusion coefficient is computed for various cluster sizes and system temperatures.The obtained results are in agreement with the ones obtained in previous experimental and theoretical works.A simple scaling argument is proposed for the size dependence of the diffusion coefficient’s prefactor.A detailed statistical analysis of the event by event KMC dynamics reveals two important and co-existing mechanisms for the diffusion of the cluster’s center of mass.At low temperatures(below T=400K)the dominating mechanism responsible for the displacement of the cluster’s center of mass is the periphery(or edge)diffusion of the atoms.At high temperatures(above T=800K)the dissociation and recombination of the clusters becomes more and more important.

A statistical model for the design of rotary HTS flux pumps based on deep-learning neuron network

Rotory high temperature superconducting(HTS)flux pumps can consistently generate a DC voltage by rotating magnets over superconducting tapes,and thus energize the circuit if a closed loop is formed.The voltage output is a crucial factor to reflect the performance of such an HTS flux pump,which is determined by a set of design specifications,and some of them have been investigated extensively in the current literature.However,no work has been done yet to study the HTS dynamo output voltage by efficiently integrating all the design parameters together.In this paper,a well‐trained deep‐learning neuron network(DNN)with back‐propagation algorithms has been put forward and validated.The proposed DNN is capable of quantifying the output voltage of an HTS dynamo instantly with an overall accuracy of approximately 98%with respect to the simulated values with all design parameters explicitly specified.The model possesses a powerful ability to characterize the output behavior of HTS dynamos by considering multiple design parameters,e.g.,airgap,superconductor tape width,operating frequency,remanent flux density,rotor radius,and permanent magnet width,which have covered all the typical design considerations.The output characteristics of an HTS dynamo against each of the design parameters have been successfully demonstrated using this model.Compared to conventional time‐consuming finite element method(FEM)based numerical models,the proposed DNN model has the advantages of automatic learning,fast computation,as well as strong programmability.Therefore the DNN model can greatly facilitate the design and optimization process for HTS dynamos.An executable application has been developed accordingly based on the DNN model,which is believed to provide a useful tool for learners and designers of HTS dynamos.

Charging process simulation of a coil by a self-regulating high-T_(c)superconducting flux pump

Self‐regulating high‐temperature superconducting(HTS)flux pumps enable direct current injection into a closed‐loop superconducting coil without any electrical contact.In this work,the process of charging a coil by a self‐regulating HTS flux pump is examined in detail by numerical modeling.The proposed model combines an H‐formulation finite element method(FEM)model with an electrical circuit,enabling a comprehensive evaluation of the overall performance of self‐regulating HTS flux pumps while accurately capturing local effects.The results indicate that the proposed model can capture all the critical features of a self‐regulating HTS flux pump,including superconducting properties and the impact of the secondary resistance.When the numerical results are compared to the experimental data,the presented model is found to be acceptable both qualitatively and quantitatively.Based on this model,we have demonstrated how the addition of a milliohm range,normal‐conducting secondary resistance in series with the charging loop can improve the charging process.In addition,its impact on the charging performance is revealed,including the maximum achievable current,charging speed,and the generated losses.The modeling approach employed in this study can be generalized to the optimization and design of various types of flux pumps,potentially expediting their practical application.

Time-dependent development of dynamic resistance voltage of superconducting tape considering heat accumulation

In flux pumps,motors and superconducting magnets,the high temperature superconductor(HTS)coated conductor frequently carries a DC transport current when an oscillating magnetic field is present in the background.Under this circumstance,the interesting effect of dynamic resistance takes place,which can affect the operating performance of superconducting devices:heat accumulation can contribute to the rising temperature of the HTS tape and the dynamic resistance voltage can change accordingly.This article explores the time‐dependent development of the dynamic resistance voltage using a numerical modeling considering the thermal effects.After a validation against experimental results,this work investigates the effects of several factors on the structure of the HTS tape on the time‐dependent development of the dynamic resistance,thus providing insights toward a better understanding of the time‐dependent behavior of HTS tapes under external magnetic fields.

Crystallographically Selective Nanopatterning of Graphene on SiO_(2)

Graphene has many advantageous properties,but its lack of an electronic band gap makes this two-dimensional material impractical for many nanoelectronic applications,for example,field-effect transistors.This problem can be circumvented by opening up a confinement-induced gap,through the patterning of graphene into ribbons having widths of a few nanometres.The electronic properties of such ribbons depend on both their size and the crystallographic orientation of the ribbon edges.Therefore,etching processes that are able to differentiate between the zigzag and armchair type edge terminations of graphene are highly sought after.In this contribution we show that such an anisotropic,dry etching reaction is possible and we use it to obtain graphene ribbons with zigzag edges.We demonstrate that the starting positions for the carbon removal reaction can be tailored at will with precision.

A superconducting wireless energiser based on electromechanical energy conversion

A superconducting magnet(SM)can produce high magnetic fields up to a dozen times stronger than those generated by an electromagnet made of normal conductors or a permanent magnet(PM),and thus has attracted increasing research efforts in many domains including medical devices,large scientific equipment,transport,energy storage,power systems,and electric machines.Wireless energisers,e.g.,high temperature superconducting(HTS)flux pumps,can eliminate the thermal load from current leads and arc erosion of slip rings,and are thus considered a promising energisation tool for SMs.However,the time‐averaged DC output voltage in existing HTS flux pumps is generated by dynamic resistance:the dynamic loss is unavoidable,and the total AC loss will become significant at high frequencies.This study introduces a highly efficient superconducting wireless energizer(SWE)designed specifically for SMs.The SWE takes advantage of the inherent properties of a superconducting loop,including flux conservation and zero DC resistivity.Extensive theoretical analysis,numerical modelling exploiting the H‐ϕformulation,and experimental measurements were conducted to demonstrate the efficiency and efficacy of the novel SWE design.The electromechanical performance and loss characteristics of the SWE system have also been investigated.Compared to conventional HTS flux pumps,the proposed SWE has lower excitation loss,in the order of 10−1 mW,and thus can achieve a high system efficiency of no less than 95%.Furthermore,it has a simpler structure with higher reliability,considered ready for further industrial development.In addition to deepening the understating of the intricate electromechanical dynamics between magnetic dipoles and superconducting circuits,this article provides a novel wireless energisation technique for SMs and opens the way to step changes in future electric transport and energy sectors.