Large Grid Research Articles

A global super-grid: sociotechnical drivers and barriers

BackgroundOne way to design an electricity system wholly based on renewables is referred to as the global Super-grid, a vision of a transmission network of unprecedented geographical scope that uses advanced technology to balance spatially and temporally varying supply and demand across the globe. While proponents, since the 1960s, have argued that a global Super-grid is technologically possible and socially desirable, and significant technical progress has been made since the 1990s, development is slow with new transmission lines being built predominantly with established technology and within the boundaries of single countries. The aim of this study is to explore sociotechnical drivers and barriers of global Super-grid development.ResultsA main driver is the century old ideas that larger grids are more efficient and contribute to cooperation and peace. Over the last decades, the level of technical knowledge and networks of proponent have grown. The Super-grid also benefits from the potential opportunity of building on existing grids. Barriers stem from the scale of investments needed to experiment, path dependences in established industry and competition from novel smaller scale solutions based on local production, energy storage and smart grid technology. Other barriers originate in the organisational and institutional complexities of international electricity trade, and in the lack of trust at local and global levels, which hinder the development of necessary coordination.ConclusionsThe analysis suggests that if the Super-grid is to become part of a future electricity system, the discourse needs to open up, move beyond simplistic ideas of efficiency and ‘technocratic internationalism’, and take into account a broader set of social benefits, risks and trade-offs.

Robust Control and Optimization Method for Single-Phase Grid-Connected Inverters Based on All-Pass-Filter Phase-Locked Loop in Weak Grid

In a distributed generation system, the all-pass-filter phase-locked loop (APF-PLL) is a commonly used method for grid synchronization. However, the coupling effect between APF-PLL and current control loop increases the risk of oscillation instability for the inverter in the weak grid. At present, there are few effective methods to solve the adverse effect of APF-PLL on the inverter-grid interconnection system in the weak grid. Therefore, a small-signal impedance model of the inverter considering the dual d-q frame brought by APF-PLL is first established. Then the reason for the inverter instability caused by APF-PLL in the weak grid is analyzed. Subsequently, an impedance reshaping method based on a modified first-order filter PLL with a complex coefficient filter (CCF-MFOF-PLL) and its parameter optimization design method are proposed. Finally, the experimental results verify that the proposed method widens the stable range of the inverter and ensures the stable operation of the inverter even with the large grid impedance.

Experimental and mathematical model of a novel viscoelastic bio-inspired multi-dimensional vibration isolation device

The earthquake usually causes multi-dimensional vibration for the large span grid structure, how to effectively isolate and mitigate the vibration response of structure is an important problem in engineering. In this paper, a novel viscoelastic bio-inspired multi-dimensional vibration isolation (VBM-VI) device is proposed under the inspiration of bionic design of the bird legs, which possesses multi-dimensional vibration isolation performance by constituting of viscoelastic core pad and VE-LLS device together. The property experiment of the VBM-VI device is performed to explore the mechanical performance at vertical and horizonal directions, the hysteretic curves and characteristic parameters are discussed to reveal the influence of the excitation frequency and amplitude on the mechanical properties of the VBM-VI device, the experimental results show that the mechanical properties express significant frequency and amplitude dependence at vertical and horizontal directions. To further describe the mechanical proprieties of the VBM-VI device, the mathematical models are established at vertical and horizontal directions respectively, and the comparison analysis between experimental and theoretical results indicates that the established mathematical models can precisely describe mechanical performance of the VBM-VI device.

Out-of-step Splitting Control based on Network Wide Area Response Information

Influenced by the continuous growth of renewable energy installation scale and the increasingly prominent characteristics of AC/DC hybrid structure, the disturbance behavior of large power grid is becoming more and more complex. To ensure the safety of power grid, higher requirements are put forward for the reliability, accuracy and orderliness of the out-of-step splitting control which is belonged to the third line of defense. Firstly, this paper analyzes the principle and application limitation of out-of-step splitting control of ucosφ based on local measurement information, studies the relationship and error between ucosφ and oscillation center, and the location coefficient ζ of oscillation center is defined. On this basis, the criteria of branch splitting and the control strategy of multi-branch wide area cooperative out-of-step splitting are proposed. The simulation results of AC and HVDC hybrid power grid show that the proposed strategy can adapt to the complex dynamic behavior of large power grid in which multiple generator clusters lose synchronization one after another, accurately identify the cutset corresponding to the out-of-step generator cluster, and realize the unified and orderly splitting of each branch in the cutset.

Atomic diffusion and turbulent mixing in solar-like stars: Impact on the fundamental properties of FG-type stars

Context. Chemical composition is an important factor that affects stellar evolution. The element abundance on the stellar surface evolves along the lifetime of the star because of transport processes, including atomic diffusion. However, models of stars with masses higher than about 1.2 M⊙ predict unrealistic variations at the stellar surface. This indicates the need for competing transport processes that are mostly computationally expensive for large grids of stellar models. Aims. The purpose of this study is to implement turbulent mixing in stellar models and assess the possibility of reproducing the effect of radiative accelerations with turbulent mixing for elements like iron in order to make the computation of large grids possible. Methods. We computed stellar models with the Module for Experiments in Stellar Astrophysics code and assessed the effects of atomic diffusion (with radiative acceleration) in the presence of turbulent mixing. Starting from a turbulent mixing prescription already calibrated on helium surface abundances of F-type stars as a reference, we parametrised the effect of radiative accelerations on iron with a turbulent diffusion coefficient. Finally, we tested this parametrisation by modelling two F-type stars of the Kepler Legacy sample. Results. We found that, for iron, a parametrisation of turbulent mixing that simulates the effect of radiative acceleration is possible. This leads to an increase in the efficiency of the turbulent mixing to counteract the effect of gravitational settling. This approximation does not affect significantly the surface abundances of the other elements we studied, except for oxygen and calcium. We demonstrate that this parametrisation has a negligible impact on the accuracy of the seismic properties inferred with these models. Moreover, turbulent mixing makes the computation of realistic F-type star models including the effect atomic diffusion possible. This leads to differences of about 10% in the inferred ages compared to results obtained with models that neglect these processes. Conclusions. The inclusion of turbulent mixing and atomic diffusion with radiative accelerations allows a more realistic characterisation of F-type stars. The parametrisation of the effect of radiative acceleration on iron opens the possibility to compute larger grids of stellar models in a reasonable amount of time, which is currently difficult when the different chemical transport mechanisms, especially radiative accelerations, are considered, although this parametrisation cannot simulate the evolution of abundances of all elements (e.g. calcium).

Research on Power Converter System (PCS) Control for Energy Storage in Local Area Network (LAN) Projects Based on Virtual Synchronous Generator (VSG)

Since the LAN is not supported by large power grids, energy storage systems usually serve as voltage sources during their normal operation to maintain stable frequency and voltage. In this paper, we first studied PCS control technologies for energy storage systems applicable to the LAN project and determined a VSG technical route through technical comparison. Then, we analyzed the influence of key parameters on the operating characteristics of VSG. Finally, we focused our research on VSG-based control strategy with virtual impedance, power allocation strategy based on multi-parallel VSGs, and VSG-based control strategy for unbalanced voltage. The results show that these strategies are easy to implement, with stable and reliable system operation, and can be widely promoted, offering insights for the future development of LAN projects.

General Power Flow Calculation for Multi-Terminal HVDC System Based on Sensitivity Analysis and Extended AC Grid

The power flow calculation of the multiterminal high voltage direct current (MTDC) system is essential for planning sustainable energy sources and power flow analysis for the MTDC system. However, the traditional unified methods require a large system of non-linear equations leading to low calculational efficiency. Also, for a large DC grid, there is a concern about the convergence of sequential methods. This paper proposes a general power flow calculation method for voltage source converter (VSC) based MTDC systems. Based on an extended topology of an AC grid, a generalized calculation model of the MTDC power flow is proposed. Then, a novel sensitivity analysis-based power flow (SAPF) method is proposed, in which the state variables of the extended AC grid are calculated via sensitivity analysis. With a smaller system of equations, the proposed SAPF method has less computational burden than the traditional unified methods and causes no convergence problem compared to sequential methods. To further improve the calculational efficiency, the sensitivity-based variable updating is adopted to accelerate the iterative process. By comparing with the existing methods in calculating power flow for different MTDC systems, including large systems with multiple AC/DC grids, the effectiveness and scalability of the proposed methods are verified.

Research on Semi-physical Simulation of Direct Drive Fan Based on ADPSS

A high proportion of new energy equipment brings problems such as increased power source uncertainty and weaker moment of inertia to the power grid. Through large-scale grid simulation with refined models of new energy equipment, potential grid risks can be discovered and grid security can be effectively improved. The modeling accuracy of the new energy equipment model affects the simulation accuracy of the large power grid. In order to solve the accuracy problem of the new energy equipment model, a semi-physical simulation platform close to the actual equipment is used to carry out simulations of different working conditions. Based on ADPSS, a semi-physical simulation electromagnetic model of a dual-motor dual-power unit two-level direct-drive fan was built, and simulation data of different working conditions were obtained. The direct frequency control method is adopted to optimize the motor model, which reduces the steady-state fluctuation of the semi-physical simulation and improves the simulation accuracy.

Deep learning for fast photoacoustic wave simulations

Photoacoustic tomography involves absorption of pulsed light and subsequent generation of ultrasound, which when detected using an array of sensors can produce clinically useful images. Simulation tools for photoacoustic wave propagation have played a key role in advancing photoacoustic imaging by providing quantitative and qualitative insights into parameters affecting image quality. Classical methods for numerically solving the photoacoustic wave equation rely on a fine discretization of space and can become computationally expensive for large computational grids. In this work, we apply Fourier Neural Operator (FNO) networks as a fast data-driven deep learning method for solving the 2D photoacoustic wave equation in a homogeneous medium. Comparisons between the FNO network and pseudo-spectral time domain approach demonstrated that the FNO network generated comparable simulations with small errors and was an order of magnitude faster. Moreover, the FNO network was generalizable and could generate simulations not observed in the training data.

Research on Comprehensive Technology of Steel Grid Installation and Integral Jacking

In this paper, through the construction simulation of the installation and jacking of a large steel grid project, ANSYS software is used to select the corresponding finite element module, and by analyzing the construction models of different modes and different load conditions, the analysis results and structural models are displayed. The 12 supporting hydraulic jacks can meet the overall steel structure hoisting requirements on site. At the same time, the calculation model takes into account the deformation of the overall grid during the unloading process. After testing, the maximum displacement is 7mm, which meets the requirements of national regulations. This method better supports the construction guidance, and also achieved good benefits during the on-site implementation process. The installation plan and jacking technology are worthy of reference.

A voltage coordination control strategy based on the reactive power-active network loss partitioned aggregation domain

As the scale of the power system continues to expand, it becomes more difficult to regulate the voltage of large grids in a unified manner by traditional control methods. In this paper, based on the theory of modularity function model in complex networks, a voltage coordination control method of partitioning the aggregated domain of reactive-voltage sensitivity weights and active network loss-voltage sensitivity weights of power systems is proposed. Firstly, the system is partitioned according to the modularity function model and the concept of aggregation domain is proposed. Secondly, the reasonableness of the aggregation domain is judged by using the proposed reactive power support capability index, and the partitioned voltage control strategy is proposed to realize the optimal and reasonable control of the voltage crossing point. Finally, the simulation experiment compares the voltage regulation results of the centralized control method and the zonal control method. The zoned control method provides a relative voltage improvement of 3.27% over the centralized control method, the active network loss of the zoned control method is 3.30% less than that of the centralized control method, and a relative reduction in time of use of 90.70%. The simulation results show that the proposed method can effectively reduce the search range of reactive power compensation devices, reduce the number of nodes for coordinated control, reduce the active network loss of the system due to voltage regulation, and ensure the control capability of reactive power compensation devices within the partition.

Target location service system in microgrid operation based on internet of things

Abstract With the gradual reduction of conventional energy sources, the problem of environmental pollution is becoming more and more serious. Environmentally friendly, efficient and flexible power generation models are widely favored, and microgrids emerge as the times require. Microgrid is a medium and low voltage network including power distribution, storage devices, loads and other equipment. It can operate in grid-connected mode or as a stand-alone system in an island environment, continuing to supply power to users even when the large grid is disrupted. Compared with traditional power grids, microgrids have small capacity and changeable operation modes, making it difficult to locate distribution areas and short-circuit currents. At the same time, it is difficult to trace the faults during operation. So far, microgrids have had a lot of technical issues in operation that need to be resolved quickly. In recent years, IoT technology has developed rapidly. The Internet of Things is a comprehensive network that integrates location services, monitoring services, information identification and monitoring services through information exchange and communication technologies, such as video recognition technology, positioning technology, and wireless sensor technology. In order to monitor the operation mode of the microgrid in real time, find the source of the fault and locate the operation target, based on the Internet of Things technology, this paper installs wireless sensors in the line segment to monitor the line and other applications. This paper uses wireless sensor networks to record positioning variables, processes and data analysis to achieve the accuracy of microgrid operation target positioning. In addition, in order to improve the positioning accuracy of wireless sensor monitoring network and solve the problem of energy limitation, DV-Hop algorithm EB-IDMF algorithm and R-A algorithm are also introduced. Finally, the feasibility of the system is verified through experiments and tests. With the blessing of the positioning algorithm, the positioning accuracy is improved by 4.9%, which has great practical value for the target positioning of microgrid operation.

The Method of Local Approximations in the Construction of High-Precision Small-dimensional MgFE for the Calculation of Composite Bodies

The method of multigrid finite elements is effectively used to analyze the stress state in composite bodies (CB). When constructing a multigrid finite element (MgFE), briefly a standard MgFE, using known procedures, a fine grid, and large ones nested in a fine one are used. The fine grid is generated by partitioning, which takes into account the heterogeneous structure of the MgFE, large grids are used to reduce its dimension. For a standard MgFE, it is characteristic that every large grid and the corresponding approximations of displacements are determined throughout its entire area. This leads to an increase in the dimension of the standard MgFE when constructing high-order approximations on large grids, which are used to increase its order of accuracy. Standard high-precision MgFE, i.e. of high order of accuracy, have a large dimension, which makes their application difficult.
 In this paper, a method of local approximations (MLA) for constructing high-precision small-dimensional MgFE (short — small-sized MgFE) is proposed. Such MgFE are used to calculate elastic CB and are designed on the basis of standard. The main idea of the MLA is that local approximations of high-order displacements are determined on large grids in the central part of the region of a small-sized MgFE, and in the vicinity of the boundary of the region — of a small order, which allows using various local approximations to vary the dimension and order of accuracy of a small-sized MgFE. Two approaches to the construction of small-sized MgFE are shown, in the case of their complex shape, forming finite elements are used. Calculations show that small-sized MgFE generate stresses in the CB, the errors of which are 15÷50 smaller than the errors of similar stresses corresponding to standard MgFE, i.e. small-sized MgFE are more effective than standard ones. The use of smallsized MgFE in calculations makes it possible to determine stresses with a small error for large CB partitions.

Solving Hydrodynamic Problems Based on a Modified Upwind Leapfrog Scheme in Areas with Complex Geometry

In recent years, the number of adverse and dangerous natural and anthropogenic phenomena has increased in coastal zones around the world. The development of mathematical modeling methods allows us to increase the accuracy of the study of hydrodynamic processes and the prediction of extreme events. This article discusses the application of the modified Upwind Leapfrog scheme to the numerical solution of hydrodynamics and convection–diffusion problems. To improve the accuracy of solving the tasks in the field of complex shapes, the method of filling cells is used. Numerical experiments have been carried out to simulate the flow of a viscous liquid and the transfer of substances using a linear combination of Upwind and Standard Leapfrog difference schemes. It is shown that the application of the methods proposed in the article allows us to reduce the approximation error in comparison with standard schemes in the case of large grid numbers of Péclet and to increase the smoothness of the solution accuracy at the boundary. The soil dumping and suspended matter propagation problems are solved using the developed schemes.

Elucidating the performance variations and critical issues of Zn electrodes in different types of aqueous electrolytes for Zn-based rechargeable batteries

Zn-based batteries attract extensive attention as the next-generation energy storage devices for both electronics and large power grids due to the high capacity, energy density, low cost, and safety derived from the inherent properties of metallic Zn and the safety of aqueous electrolytes. Herein, the performance variations and critical issues of the Zn electrodes in the alkaline and neutral/slightly acid electrolytes are systematically investigated by evaluating the cycle stability, electrode reaction kinetics, and stripping/plating reversibility in three kinds of electrolytes, including ZnSO4, Zn(CF3SO3)2, and KOH+Zn(CH3COO)2. The lifespan of the Zn electrode in the neutral/slightly acid electrolytes is >1400% higher than the one in the alkaline electrolyte at a current density of 1 mA cm−2. However, the Zn electrode in the alkaline solution exhibits better reaction kinetics and low polarization. Besides, in the neutral/slightly acid electrolytes, higher coulombic efficiency is obtained, corresponding to better stripping/plating reversibility. Therefore, research in neutral/slightly acid electrolytes should focus on elevating the electrode kinetics and reducing polarization, while in alkaline electrolytes, the dendrite, hydrogen evolution, and self-discharge should be treated to elevate the calendar life and reversibility. This work provides a guideline for further construction of high-performance Zn electrodes in different electrolyte systems.

Efficient Phasor-Based Dynamic Volt/VAr and Volt/Watt Analysis of Large Distribution Grid With High Penetration of Smart Inverters

As the penetration of power-electronics based smart inverters (SIs) is increasing in distribution grids, it adds computational challenges in solving dynamic models of large-scale distribution feeders. Voltage and reactive power (Volt/VAr), and voltage and active power (Volt/Watt) dynamics have been analyzed at slower time scales akin to the control of legacy grid devices. However, smart inverters, being power-electronics based devices, can provide dynamic active/reactive power support at a faster time scale, which necessitates Volt/VAr and Volt/Watt dynamics to be analyzed at a faster time scale. The existing dynamic models are overly detailed and computationally intractable for distribution feeders with a large number of inverters. In this context, this proposed work aims towards developing a computationally tractable, scalable, and accurate phasor-based model for dynamic Volt/VAr and Volt/Watt analyses of large distribution systems with high penetration of smart inverters. Case studies demonstrate that the proposed phasor-based model sufficiently captures the Volt/VAr and Volt/Watt dynamics, and is computationally faster by one order of magnitude compared to the average model and by two orders of magnitude compared to the detailed switching model. Case studies also demonstrate the efficacy and scalability of the proposed model in analyzing Volt/VAr and Volt/Watt dynamics of large-scale power networks with hundreds of SIs.

Fault Detection Utilizing Convolution Neural Network on Timeseries Synchrophasor Data From Phasor Measurement Units

An end-to-end supervised learning method is proposed for fault detection in the electric grid using Big Data from multiple Phasor Measurement Units (PMUs). The approach consists of preprocessing steps aimed at reducing data noise and dimensionality, followed by utilization of six classification models considered for detecting faults. Three of the models were variants of Convolutional Neural Network (CNN) architectures that consider a single type of measurement (voltage, current or frequency) at all PMUs or all types together also at all PMUs. CNN based models were compared to traditional methods of Logistic Regression (LR), Multi-layer Perceptron (MLP) and Support Vector Machine (SVM). Evaluation was conducted on two-year data measured by PMUs at 37 locations in a large electric grid. The response variable for classification were extracted from the grid-wide outage event log. Experiments show that CNN-based models outperformed traditional methods on one year out-of-sample outage detection over the entire grid.

Improvement of the Resilience of a Microgrid Using Fragility Modeling and Simulation

The modern microgrid is designed to withstand various disruptive events that have a high probability of occurrence but have a low impact on the system. This improves the reliability of the system but does not take into consideration the disruptive events that have a low probability of occurrence but have a large impact on the system, such as extreme weather or natural disasters. Redesigning a microgrid to withstand low probability high impact events is very costly and is not a feasible solution to existing microgrids. This paper proposes a method to improve the resilience of an existing microgrid to quickly recover from low probability high impact events. The method used for this purpose is a combination of Monte Carlo simulation and prioritization of load of the microgrid. The efficacy of the method is examined by modeling microgrids using a fragility model. Using the proposed novel resilience index, the resilience of the IEEE 5-Bus system and IEEE 14-Bus system and the effect of load shedding on the resilience of the microgrid are analyzed and presented. The effect on smaller and larger grids and their resilience is examined. A novel resilience index is used to quantify the improvement of resilience of the proposed method when compared to other methods available in the literature.

Selective laser ablation preparation of Ag grid transparent electrodes with fractal-combined structures and optimization of fractal grid pitch

New fractal-structured patterns with different grid period combinations were designed and applied to Ag grid transparent electrodes (TEs). Based on the principle of decreasing or increasing Ag grid line densities in local areas, the optical transmittance or electrical conductivity of the resulting Ag grid TEs could be selectively enhanced, thus effectively improving its comprehensive performance. Fractal-combined Ag grid TEs with different combined periods were firstly prepared by selective laser ablation of sputtered Ag films, and compared with the corresponding regular periodic Ag grid TEs to determine the optimal combined periods. The results indicated that the combined periods of 4 and 9 enabled the Ag grid TE to have a relatively high figure of merit of 4.92 × 10–2 Ω−1. This optimal fractal-combined Ag grid TE was chosen to optimize grid pitch for further comprehensive performance improvement. Using a large grid pitch of 1.1 mm could achieve the best trade-off between optical transmittance and electrical conductivity, and produced the best comprehensive performance with the maximum figure of merit of 5.17 × 10–2 Ω−1. The fractal-combined structure provides a unique scheme for flexibly designing and preparing grid patterns in metal TEs.

Adapting the Directed Grid Theorem into an FPT Algorithm

The Grid Theorem of Robertson and Seymour [JCTB, 1986], is one of the most important tools in the field of structural graph theory, finding numerous applications in the design of algorithms for undirected graphs. An analogous version of the Grid Theorem in digraphs was conjectured by Johnson et al. [JCTB, 2001], and proved by Kawarabayashi and Kreutzer [STOC, 2015]. Namely, they showed that there is a function $f(k)$ such that every digraph of directed tree-width at least $f(k)$ contains a cylindrical grid of size $k$ as a butterfly minor and stated that their proof can be turned into an XP algorithm, with parameter $k$, that either constructs a decomposition of the appropriate width, or finds the claimed large cylindrical grid as a butterfly minor. In this paper, we adapt some of the steps of the proof of Kawarabayashi and Kreutzer to improve this XP algorithm into an FPT algorithm. Towards this, our main technical contributions are two FPT algorithms with parameter $k$. The first one either produces an arboreal decomposition of width $3k-2$ or finds a haven of order $k$ in a digraph $D$, improving on the original result for arboreal decompositions by Johnson et al. The second algorithm finds a well-linked set of order $k$ in a digraph $D$ of large directed tree-width. As tools to prove these results, we show how to solve a generalized version of the problem of finding balanced separators for a given set of vertices $T$ in FPT time with parameter $|T|$, a result that we consider to be of its own interest.