Outer Grid Research Articles

Potential of Power Electronics to Improve the Power Quality of Network Grid

This study deals with voltage drop characterization of transmission systems by using power electronics circuits supplied from embedded generation. Many grid-connected power electronic systems, such as STATCOMS, UPFCs, and distributed generation system interfaces, use Voltage Source Inverters (VSI) connected to the supply network through a filter. This filter, typically a series inductance acts to reduce the switch harmonics entering the distribution network an alternative filter is an LCL network, which can achieve reduced levels of harmonic distortion at lower switching frequencies and with less inductance, and therefore has potential benefits for higher power application. However, systems incorporating LCL filters require more complex control strategies. This dissertation proposes a robust strategy for regulating the grid current connected via an LCL filter. The strategy integrates an outer loop grid current regulator with inner capacitor current regulation to stabilize the system. An asynchronous farm PI current regulation strategy is used for the outer grid current control loop. Linear analysis, simulation, and experimental results are used to verify the stability of the control algorithm across a range of operating conditions and finally, expressions for "harmonic impedance" of the system are derived to study the effects of supply voltage distortion on the harmonic performance of the system.

Numerical Analysis of Pressure Load on Hypersonic Wing

The flight vibration environment of hypersonic vehicles has always been a hot topic in the field of aerospace at home and abroad. In this paper, the simple wing model and the outer flow field grid of the wing are established by using fluent software. A series of environmental parameter conditions are set in the hypersonic flow field, and the steady and unsteady flow fields are analyzed. The distribution characteristics of the pressure load on the wing surface in the hypersonic flow field are obtained. The results show that the pressure load measured at the head position of the wing model is larger, and the pressure load at the fuselage position is smaller and changes more gently. With the increase of Mach number, the pressure load on the wing model will also increase. Then MatLab software is used to convert the time domain curve of wing surface pressure load into a power spectral density curve, which provides load input for subsequent random vibration response simulation on finite element software.

CABARET on rotating meshes

The Compact Accurately Boundary-Adjusting high-REsolution Technique (CABARET) method for 3D gas dynamics equations is generalised to rotating hexahedral mesh zones using the sliding interface approach, which extends applications of the original algorithm to a wide range of unsteady rotor flow problems. Important properties of the suggested extension of the CABARET scheme include the preservation of good linear dispersion and dissipation properties of the original CABARET method on fixed grids. To strictly preserve flux conservation at the sliding interface, the so-called supermesh approach is implemented that uses the projection of the contact faces on both sides of the sliding interface onto an intermediate virtual surface. The implementation is based on applying the sub-cell and sub-face solution reconstruction on the sliding interface of hexahedral meshes rather than deforming unstructured meshes. The sub-faces are defined by constructing projection overlap polygons using the Sutherland-Hodgman clipping algorithm. A local characteristic flux splitting is applied at each overlap polygon face, while updating the local stencil connectivity table at each time step. To further improve the dispersion properties of the CABARET grid at small Courant–Friedrichs–Lewy numbers, the dispersion-improved version of the CABARET method previously developed on rectangular Cartesian meshes is extended to non-uniform curvilinear meshes typical of open-rotor applications. Numerical examples are provided for the analytical problem of normal acoustic wave propagation through a rotating cylindrical grid zone embedded in an outer stationary grid. The error decay rates are calculated and compared with the theoretical order of CABARET convergence. In addition, the same algorithm accelerated on Graphics Processing Units (GPUs) is applied for simulations the flow around a generic two-bladed rotor in hover in the Wall Model Large Eddy Simulation framework. For the latter case, flow solutions are analysed, and their numerical grid sensitivity is discussed.

Tc-cylinder: An optimized algorithm for accurate topography effect from high-resolution digital elevation models

An efficient algorithm, tc-cylinder, has been developed to obtain terrain corrections by considering computational efficiency, data storage and use, spherical curvature effects, water bodies, etc. High resolution Digital Elevation Models (DEMs) are fundamental data sources for accurate terrain corrections. But a high-resolution DEM like SRTM1 poses some difficulties during processing, such as data storage, handling, efficiency and computation speed over large areas. In order to overcome these issues, a shortened computation area radius (e.g., 50 km) is preferred for practical studies. This kind of limitation may cause an accuracy loss of over 20 mGal in steep topography. Similarly, spectral methods used to speed up the computations have drawbacks in areas with high slopes. The presented algorithm for segmenting cylinder bodies proves more realistic and effective terrain correction results in all physical conditions. An optimization approach that belongs to the Intel’s processor and compiler architecture improved processing efficiency by a factor of 2.5 times with non-temporal stores, which copies large data directly to memory. Synthetic masses based on known geometric bodies were considered to test program efficiency and numerical results. The contribution was examined at gravity stations in the Konya Closed Basin, which has a sophisticated topography. Additionally, an investigation was conducted in extreme conditions, such as Everest Mountain and the Ağrı Mountain in Turkey, with abrupt elevation profile changes. The numerical analyses of the accuracy of terrain correction values is performed on different combinations of DEM resolutions. Consequently, it is verified that 1′′ inner and 15′′ outer grid resolutions lead to sub-mGal accuracy with less processing time, even in rough topography.

To E or Not to E : Numerical Nuances of Global Coronal Models

In recent years, global coronal models have experienced an ongoing increase in popularity as tools for forecasting solar weather. Within the domain of up to 21.5 R ⊙, magnetohydrodynamics (MHD) is used to resolve the coronal structure using magnetograms as inputs at the solar surface. Ideally, these computations would be repeated with every update of the solar magnetogram so that they could be used in the ESA Modeling and Data Analysis Working Group magnetic connectivity tool (http://connect-tool.irap.omp.eu/). Thus, it is crucial that these results are both accurate and efficient. While much work has been published showing the results of these models in comparison with observations, not much of it discusses the intricate numerical adjustments required to achieve these results. These range from the details of boundary condition formulations to adjustments as large as enforcing parallelism between the magnetic field and velocity. By omitting the electric field in ideal MHD, the description of the physics can be insufficient and may lead to excessive diffusion and incorrect profiles. We formulate inner boundary conditions that, along with other techniques, reduce artificial electric field generation. Moreover, we investigate how different outer boundary condition formulations and grid design affect the results and convergence, with special focus on the density and radial component of the B -field. The significant improvement in accuracy of real magnetic map–driven simulations is illustrated for an example of the 2008 eclipse.

A robust controller design procedure for LCL‐type grid‐tied proton exchange membrane fuel cell system in harmonics‐polluted network

AbstractProton exchange membrane fuel cell (PEMFC), due to its high efficiency and good stability, is one of the most efficient and effective sources of alternative renewable energy. The LCL filters are utilized to attenuate the harmonics caused by inverter‐based distributed power generation systems (DPGS). In this paper, a systematic robust control design is presented for an LCL‐type grid‐tied PEMFC inverter. The proposed nested control includes the capacitor current feedback and the outer grid current loops, considering the computational delays. The proposed control results in more accurate and realistic modeling to transfer the power of PEMFC and inject a high‐quality current into a weak and harmonics‐polluted network. Also, by utilizing the proposed method, the computational delays that affect the damping capability are eliminated, and the performance of the control system is improved. In addition, designing a virtual series and parallel impedances strengthens the phase and amplitude of the inverter output impedance. As a result, the robust operation of the inverter for significant changes in network impedance is improved, and the system well attenuates the mains voltage harmonics. The simulation results in Matlab/Simulink environment confirm the performance of the control system and the suitable quality of the injected current of the PEMFC system into the weak and harmonic‐polluted network.

Decomposition and coordination calculation of economic dispatch for active distribution network with multi-microgrids

With the development of the active distribution system with multi-microgrids, increasingly numbers of distributed energy resources are integrated into the distribution network through multi-microgrids. As a result, the economic dispatch problem presents high dimensional complexity. Hence, this paper proposes a decomposition and coordination calculation method to reduce the computational dimension and improve the calculation accuracy. In this paper, the active distribution system with multi-microgrids is divided into two parts: one is the outer power grid and the other is the inner power grid, and the optimal value is obtained by applying the differential evolution algorithm. To improve the efficiency of the traditional network loss calculation, the network loss sensitivity is adopted to calculate the network loss. In order to verify the effectiveness of the proposed method, the test model is constructed based on the IEEE 33 node and 69 node distribution systems. The simulation results show that the proposed method can obtain the global optimal solution, and its economy is better than the overall centralized calculation method. This method is conducive to the subsequent application in the actual power grid system.

Compact inertial electrostatic confinement D-D fusion neutron generator

A compact Inertial Electrostatic Confinement (IEC) system is designed and fabricated for D-D fusion neutron generation. The IEC system consists of two concentric spherical grids connected to high voltage power supply inside a vacuum chamber filled with deuterium gas. The diameter of inner grid cathode is 40 mm and the diameter of outer grid anode is 120 mm. These grids are placed inside a SS304L cylindrical vacuum chamber of 300 mm diameter and 450 mm length. The IEC system has been operated at 24 kV in deuterium gas medium at 0.01–0.02 mbar, and the neutron yield of ~ 105n/s is measured with neutron monitor. The temperature inside the IEC system is also measured using Fiber Bragg Grating (FBG) during D-D gas discharges. Degradation in vacuum inside the chamber causes the instability in deuterium plasma which reduces the neutron yield and increases the cathode temperature.

ОСОБЕННОСТИ ПРИМЕНЕНИЯ МЕТОДА ПОСЛЕДОВАТЕЛЬНОГО УТЯЖЕЛЕНИЯ ПРИ АНАЛИЗЕ СТАТИЧЕСКОЙ УСТОЙЧИВОСТИ ИСТОЧНИКОВ РАСПРЕДЕЛЕННОЙ ГЕНЕРАЦИИ

Design and operation of distributed generation facilities needs to provide for sufficient margin of static sta-bility in a variety of operating situations, including normal and post-emergency operation in parallel with the outer grid (the power system). Power system protections are often placed where the on-site power plants connect to the power system; these protections enable the loaded generators to become islanded in case of emergency; rated margin of static stability must be available in such post-emergency operation. To analyze static stability of synchronous generators and motor loads, we propose combining gradual equalization and gradual tran-sition to post-emergency parameters. However, the latter method has different quirks when applied to static stabi-lity testing of generators, depending on whether they are running in parallel or islanded. Besides, application of this method requires multiple constraints pertaining to the functioning of on-site power plants, among other things. The authors hereof have developed a single algorithm for static stability testing of generators and loads that applies well to on-site power plants whether they are islanded on running in parallel to the grid. The algorithm adjusts for constraints on excitation systems and is implemented in KATRAN software. A computational experiment is described herein that concerns an electricity delivery system that is powered by distributed generation facilities and carries heterogeneous motor loads. Specific features of such studies are additionally described.

Distributed model predictive control for energy management in a network of microgrids using the dual decomposition method

SummaryThis paper deals with the application of model predictive control (MPC) to optimize power flows in a network of interconnected microgrids (MGs). More specifically, a distributed MPC (DMPC) approach is used to compute for each MG how much active power should be exchanged with other MGs and with the outer power grid. Due to the presence of coupled variables, the DMPC approach must be used in a suitable way to guarantee the feasibility of the consensus procedure among the MGs. For this purpose, we adopt a tailored dual decomposition method that allows us to reach a feasible solution while guaranteeing the privacy of single MGs (ie, without having to share private information like the amount of generated energy or locally consumed energy). Simulation results demonstrate the features of the proposed cooperative control strategy and the obtained benefits with respect to other classical centralized control methods.

Characterization of Electrical Breakdown for Low and High Pressure Hydrogen Gas in Coaxial Virtual Cathode Device

THE DC gas discharge is established between a coaxial cylindrical stainless steel electrodes. The coaxial cylindrical DC discharge device consists of an outer grid cathode and inner rods anode with 4 mm gap between them. This experimental study is focused on the effect of gas pressure and electric field strength on breakdown voltage. There is a balance between the electron attachment and first ionization collision, so the Second Townsend emission is the responsible for maintain the discharge.The I-V characteristic curvesfor different pressures ofhydrogen gas indicate that the highest current appears for the highest gas pressure. The gas breakdown voltage varies as a product of Pd and Townsend’s coefficients depends on gas pressure and E/P, where E is the electric field and P is the gas pressure.

An ODE-Based Wall Model for Turbulent Flow Simulations

This paper presents a new wall model to compute turbulent boundary layers using the Reynolds-averaged Navier–Stokes equations in high Reynolds number flows. The model solves a two-point boundary value problem for a coupled set of equations for the streamwise velocity and the turbulent viscosity. Since it includes both the pressure gradient and the momentum balance of the full Navier–Stokes system, the ordinary differential equation is valid farther from the wall. We implement the model within a Cartesian cut-cell method and use one-dimensional linelets in each cut cell to avoid the excessive mesh refinement that would otherwise be needed. The linelets are coupled to the outer Cartesian grid in a fully conservative manner, with two-way interaction between the linelets and the background grid. Detailed comparisons of velocity and eddy viscosity with three well-studied examples from the Turbulence Modeling Resource website are presented to demonstrate the model’s performance in two space dimensions, including an example with smooth-body separation. The results show the new model gives excellent results even when the value of the first point is in the wake layer.

Investigation of gas breakdown in cylindrical inertial electrostatic confinement device with inner cage anode

The gas breakdown in the Inertial Electrostatic Confinement (IEC) device has been studied by DC discharge of Nitrogen gas in the pressure range between 0.03 Torr and 0.7 Torr. Paschen curves and Townsend coefficients are obtained in a cylindrical system with inner anode in shape of rods and the outer grid cathode. The breakdown occurs and the plasma is formed at the center of the electrodes. Townsend coefficients are calculated for anode transparencies of 84%, 92%, and 96% (24, 12, and 6 rods respectively). On the left hand side of Paschen curves, the high transparency has a higher breakdown voltage. More electrons are allowed to pass the anode. So, the collisions increase, leading to a slight increment of the first Townsend coefficient (α) decreasing of the second Townsend coefficient (γ) while the ionization efficiency (η) is not affected. For high reduced electric field E/P, i.e. a high electric field with low pressure, the probability of ionizing collision events decreases. This causes a decrement of α while γ is raised because the ions' energy is not exhausted in collisions at the center. The coefficients α and γ are working in harmony to balance the rates of electrons generated in gas ionization so that the relation between the two coefficients is satisfied.

Reducing the dimensionality of grid based methods for electron-atom scattering calculations below ionization threshold

For total energies below the ionization threshold it is possible to dramatically reduce the computational burden of the solution of the electron-atom scattering problem based on grid methods combined with the exterior complex scaling. As in the R-matrix method, the problem can be split into the inner and outer problem, where the outer problem considers only the energetically accessible asymptotic channels. The (N+1)-electron inner problem is coupled to the one-electron outer problems for every channel, resulting in a matrix that scales only linearly with size of the outer grid.

Numerical analyses of flow distributions in nuclear fuel assemblies affected by grid deformations

In the event of a safety shutdown earthquake (SSE) in a nuclear power plant, the spacer grid of the fuel assembly will be deformed as a result of the vibrations. If the flow area in a subchannel is reduced due to the grid deformation, the coolant flow will be restricted and consequently a loss of flow occurs in the affected fuel assembly during the accident. In this study, computational fluid dynamics (CFD) analyses are conducted in order to assess the flow redistribution and flow recovery in fuel assemblies. The real geometries of an outer grid and mixing vane are used in the simulation, and the region including the inner grid is modeled as a porous media zone. The resistance coefficients of the porous media model are determined using CFD analyses. The Reynolds-averaged Navier–Stokes equation with a non-linear turbulence model was used to solve the three-dimensional anisotropic turbulence flow in the rod bundles during normal operation, blowdown, and reflood phases following a loss-of-coolant accident (LOCA). In these analyses, it is assumed that forty percent of the flow area is blocked by grid deformations. The results demonstrate that a downstream distance of 45 times the hydraulic diameter is required for the coolant flow to recover to 95% of the original flow rate in the affected fuel assembly.

Design of a linear neutron source

In this paper, we present the design of a linear neutron source based on the concept of inertial electrostatic confinement fusion. The source mainly comprises of a concentric coaxial cylindrical grid assembly housed inside a double walled cylindrical vacuum chamber, a gas injection system, a high voltage feedthrough and a high voltage negative polarity power supply. The inner grid will be kept at a high negative potential with respect to the outer grid that will be grounded. The effect of grid transparency on electric potential distribution and ion trajectories has been studied using SIMION. A diffuse deuterium plasma will be initially created by making filament discharge and subsequently, on application of high negative voltage to the inner grid, deuterons will be accelerated towards the axis of the device. These deuterons will oscillate in the negative potential and consequently fuse in between the grids to produce neutrons. This source is expected to produce 107–108 neutrons/s. The proposed linear neutron source will be operated both in the continuous and pulse modes and it will be utilized for a few near term applications namely fusion reactor material studies and explosive detection.

Effect of rainfall transients on thermal and moisture exposure of underground electric cables

Cable ampacity analysis is generally performed assuming constant worst-state environmental conditions, which often correspond to a dry soil condition or to a condition with uniform ambient soil moisture content. The characteristic time scale of thermal variation in the soil is large, on the order of several weeks, and is similar to the time scale between rainfall events in many geographic locations. Intermittent rainfall events introduce significant transient fluctuations that influence the thermal conditions and moisture content around a buried cable both by increasing thermal conductivity of the soil and by increasing the moisture exposure of the cable insulation. This paper reports on a computational study of the effect of rainfall events on the thermal and moisture transients surrounding a buried cable. The computations were performed with a finite-difference method using an overset grid approach, with an inner polar grid surrounding the cable and an outer Cartesian grid. The thermal and moisture transients observed in computations with periodic rainfall events were compared to control computations with a steady uniform rainfall. Under periodic rainfall conditions, the temperature and moisture fields are observed to approach a limit-cycle condition in which the cable surface temperature and moisture content oscillate in time, but with mean values that are significantly different than the steady-state values.

Simulations of solid–liquid mass transfer in fixed and fluidized beds

Abstract Solid-to-liquid mass transfer as a result of liquid flow through static and dynamic assemblies of mono-sized solid spheres has been simulated. The three-dimensional, transient simulations fully resolve the flow of liquid in the interstitial space between the spheres by means of a lattice-Boltzmann scheme. For the mass transfer process, a finite volume method on Coupled Overlapping Domains (CODs) is used. Spherical grids around the particles allow for resolving the thin scalar concentration boundary layers that are the result of high Schmidt numbers (Sc up to 1000). The spherical grids – one for each sphere – are coupled to an outer cubic grid. Particle-based Reynolds numbers are in the range 1–10; solids volume fractions are 0.15–0.40. The results demonstrate the validity of the COD approach for multi-particle systems, and provide insight in the dependencies of the Sherwood number on the solid–liquid system characteristics. It is shown that for moderate solid-over-liquid density ratios, fluidized particle assemblies have lower Sherwood numbers than fixed beds.

The Research of Two Staged Non-Isolated Photovoltaic Grid Connected Inverter Control Method

Grid-connected inverter topologies and control methods are analyzed and compared on the basis of two non-isolated PV grid-connected inverter circuit topology as 3kVA grid connected PV systems. In this paper, the current control method, the DC bus voltage outer grid current inner loop of the dual closed-loop PI control method. Sampled signal through the system circuit for processing, resulting in a SPWM drive signal to control the conduction of the power devices off the grid current control.

Structural and Functional Changes of the Human Macula during Acute Exposure to High Altitude

BackgroundThis study aimed to quantify structural and functional changes at the macula during acute exposure to high altitude and to assess their structure/function relationship. This work is related to the Tuebingen High Altitude Ophthalmology (THAO) study.Methodology/Principal FindingsSpectral domain optical coherence tomography and microperimetry were used to quantify changes of central retinal structure and function in 14 healthy subjects during acute exposure to high altitude (4559 m). High-resolution volume scans and fundus-controlled microperimetry of the posterior pole were performed in addition to best-corrected visual acuity (BCVA) measurements and assessment of acute mountain sickness. Analysis of measurements at altitude vs. baseline revealed increased total retinal thickness (TRT) in all four outer ETDRS grid subfields during acute altitude exposure (TRTouter = 2.80±1.00 μm; mean change±95%CI). This change was inverted towards the inner four subfields (TRTinner = −1.89±0.97 μm) with significant reduction of TRT in the fovea (TRTfoveal = −6.62±0.90 μm) at altitude. BCVA revealed no significant difference compared to baseline (0.06±0.08 logMAR). Microperimetry showed stable mean sensitivity in all but the foveal subfield (MSfoveal = −1.12±0.68 dB). At baseline recordings before and >2 weeks after high altitude exposure, all subjects showed equal levels with no sign of persisting structural or functional sequels.Conclusions/SignificanceDuring acute exposure to high altitude central retinal thickness is subject to minor, yet statistically significant changes. These alterations describe a function of eccentricity with an increase in regions with relatively higher retinal nerve fiber content and vascular arcades. However, these changes did not correlate with measures of central retinal function or acute mountain sickness. For the first time a quantitative approach has been used to assess these changes during acute, non-acclimatized high altitude exposure.