Generic Grid Research Articles

Dynamic Online Grid Impedance Estimation and Its Effects on DC Capacitor Lifetime in Variable Frequency Drive

Grid impedance plays a significant role in the performance and design considerations of nonlinear devices like Variable Frequency Drive (VFD). VFD operates across domestic, industrial, and residential applications under different grid configurations and capacities including renewables. IEEE 519 standard recommends grid short circuit ratio and thereby grid impedance as an index for the individual and overall current harmonic performance of the connected utilities. This research investigation details a comprehensive approach for an online dynamic grid impedance estimation and may also give an insight into the hardware implementation within a VFD to achieve optimum performance and reliability during installation. A VFD of 250 kW is used for real-time measurements and simulation with varying grid configurations to determine grid impedance and lifetime of the DC capacitors. A fuzzy model is developed based on the established knowledge base for the estimation of the grid impedance of a generic grid. The fuzzy model is also developed to estimate the lifetime of the DC capacitor.

Fast Frequency Support from Grid-forming Converters Under dc- and ac-side Current Limits

A decentralized nonlinear feedback controller is proposed for droop-based grid-forming converters (GFCs) with both dc-side current limitation and ac-side current limit to provide fast frequency support from renewable resources after a large generation trip or load increase. The controller is designed to guarantee asymptotic stability for a generic grid with multiple synchronous generators and GFCs represented by a reduced-order model. The controller is based on the integral sliding mode approach and requires frequency information of the neighboring buses if GFCs are connected to them. The proposed approach also leads to a quantifiable frequency support mechanism that aims to maximize the improvement in frequency nadir and minimize the steady-state frequency deviation. In addition, it allows a GFC to refrain from participating in frequency support if it chooses to do so. Simulations on the electromagnetic transient (EMT) model of a 4-machine 2-area system with 1 GFC and a detailed phasor model of the modified IEEE 16-machine system with 4 GFCs equipped with inner control loops demonstrate the effectiveness of the suggested control.

A quadrature-based scheme for numerical solutions to Kirchhoff transformed Richards' equation

<p style='text-indent:20px;'>In this work we propose a new numerical scheme for solving Richards' equation within Gardner's framework and accomplishing mass conservation. In order to do so, we resort to Kirchhoff transformation of Richards' equation in mixed form, so to exploit specific Gardner model features, obtaining a linear second order partial differential equation. Then, leveraging the mass balance condition, we integrate both sides of the equation over a generic grid cell and discretize integrals using trapezoidal rule. This approach provides a linear non-homogeneous initial value problem with respect to the Kirchhoff transform variable, whose solution yields the sought numerical scheme. Such a scheme is proven to be <inline-formula><tex-math id="M1">\begin{document}$ l^{2} $\end{document}</tex-math></inline-formula>-stable and convergent to the exact solution under suitably conditions on step-sizes, retaining the order of convergence from the underlying quadrature formula.</p>

A scalable life cycle assessment of alternating and direct current microgrids in office buildings

Microgrids integrating local renewable energy sources at low-voltage level show promising potentials in realizing a reliable, efficient, and clean supply of electricity. Further improvements are expected when such a microgrid is operated based on direct current (dc) instead of alternating current (ac) infrastructure for power distribution commonly in use today. Our study aims to systemically quantify the gap between environmental impacts of microgrids at building level using the case study of power distribution within office buildings. For this purpose, a scalable comparative life cycle assessment (LCA) is conducted based on a technical bottom-up analysis of differences between ac and dc microgrids. Particularly, our approach combines the micro-level assessment of required power electronic components with the macro-level requirements for daily operation derived from a generic grid model. The results indicate that the environmental impacts of employed power electronics are substantially reduced by operating a microgrid based on dc power distribution infrastructure. Our sensitivity analyses show that efficient dc microgrids particularly lead to savings in climate change impact emissions. In addition, our study shows that the state-of-the-art scaling rules of power electronics currently used in LCAs leads to inaccurate results. In contrast, the proposed methodology applies a more technical approach, which enables a detailed analysis of the environmental impacts of power electronic components at system level. Thus, it provides the foundation for an evaluation criterion for a comprehensive assessment of technological changes within the framework of energy policy objectives.

Application of Enhanced CPC for Load Identification, Preventive Maintenance and Grid Interpretation

Currents’ Physical Components (CPC) theory with spectral component representation is proposed as a generic grid interpretation method for detecting variations and structures. It is shown theoretically and validated experimentally that scattered and reactive CPC currents are highly suited for anomaly detection. CPC are enhanced by recursively disassembling the currents into 6 scattered subcomponents and 22 subcomponents overall, where additional anomalies dominate the subcurrents. Further disassembly is useful for anomaly detection and for grid deciphering. It is shown that the newly introduced syntax is highly effective for identifying variations even when the detected signals are in the order of 10−3 compared to conventional methods. The admittance physical components’ transfer functions, Yi(ω), have been shown to improve the physical sensory function. The approach is exemplified in two scenarios demonstrating much higher sensitivity than classical electrical measurements. The proposed module may be located at a data center remote from the sensor. The CPC preprocessor, by means of a deep learning CNN, is compared to the current FFT and the current input raw data, which demonstrates 18% improved accuracy over FFT and 45% improved accuracy over raw current i(t). It is shown that the new preprocessor/detector enables highly accurate anomaly detection with the CNN classification core.

A Gradient Discretisation Method for Anisotropic Reaction–Diffusion Models with Applications to the Dynamics of Brain Tumors

Abstract A gradient discretisation method (GDM) is an abstract setting that designs the unified convergence analysis of several numerical methods for partial differential equations and their corresponding models. In this paper, we study the GDM for anisotropic reaction–diffusion problems, based on a general reaction term, with Neumann boundary condition. With natural regularity assumptions on the exact solution, the framework enables us to provide proof of the existence of weak solutions for the problem, and to obtain a uniform-in-time convergence for the discrete solution and a strong convergence for its discrete gradient. It also allows us to apply non-conforming numerical schemes to the model on a generic grid (the non-conforming ℙ ⁢ 1 {\mathbb{P}1} finite element scheme and the hybrid mixed mimetic (HMM) methods). Numerical experiments using the HMM method are performed to assess the accuracy of the proposed scheme and to study the growth of glioma tumors in heterogeneous brain environment. The dynamics of their highly diffusive nature is also measured using the fraction anisotropic measure. The validity of the HMM is examined further using four different mesh types. The results indicate that the dynamics of the brain tumor is still captured by the HMM scheme, even in the event of a highly heterogeneous anisotropic case performed on the mesh with extreme distortions.

Progressive Surface Reconstruction of Cardiac Anatomy from Sparse and Scattered Points for Heart Mapping Procedure

In computer aided radiofrequency catheter ablation of the heart, intraoperative three-dimensional reconstruction of the intracardiac cavity is a very important step. In this paper, we propose a point-by-point surface reconstruction algorithm based on scattered points and guarantee the properties of local reconstruction and surface smoothing. The algorithm firstly builds a generic self-adapted grid model to enclose and approximate to the initial sampled points (at least four points) captured from endocardial surface. Then, the grid model is deformed when adding a newly sampled point. To avoid the linear increase of computation with the increasing of the number of points and unbalanced neighborhood, we propose a local region search criterion based on orthogonal least squares and a neighborhood partition method. Such criterion can guarantee selecting the minimum local balanced points to complete each step of grid deformation. Four normal and six abnormal cardiac computed tomography angiography models specific to the left atrium are used to verify our method. The results show that the surface reconstruction errors as well as the reconstruction time per update are smaller than the state-of-theart methods applied to progressive reconstruction using sparse and scattered points. Furthermore, a cardiac phantom of right ventricle is used to verify the 3D surface reconstruction by our method. The experimental results confirm the feasibility of the proposed algorithm in practical operation.

Reducing Environmental Impacts through a Smart Design of Microgrids: Life Cycle Assessment for AC and DC

Microgrids with integrated renewable energies show promising potentials in terms of reliable, efficient, and clean smart grid concepts. Further improvements are expected when such a microgrid is operated on a direct current instead of an alternating one. Our study aims to systemically analyze the environmental impacts of microgrids operating on ac and on dc using power distribution within office buildings as an example. For this purpose, a comparative and scalable life cycle assessment is conducted based on a technical bottom up analysis of differences between ac and dc microgrids. Particularly, our approach combines the assessment of required power electronic components on a micro-level using a generic grid model on a macro-level. The results indicate that the environmental impacts of employed power electronics can be substantially reduced when utilizing a microgrid operating on dc instead of ac.

Future-oriented generic grid code regarding wind power plants in Europe

The assessment of compliance with grid codes requirements can be challenging for researchers and (early stage) technology developers. To tackle this problem, a generic grid code is proposed in this article that defines certain requirements aimed for future scenarios with high shares of renewables. The proposed code is not linked to a particular grid of a certain region or country. The requirements are expressed in a mathematical way, in contrast to the legal format of real grid codes, to enable for easier implementation. The parameterised expressions enable for easy customisation for specific use cases. The compliance is assessed through reference under/over voltage test events and a test frequency disturbance, to which desired responses are specified. Detailed dynamic tolerance bands are defined around the desired responses, using linear parameterised functions providing a clear and continuous definition that leaves little room for interpretation. This generic grid code is inspired by the present regulations, especially the European grid code framework (ENTSO-E RfG) and the Irish grid code, which is seen by many as a benchmark regarding wind power integration. However, the generic grid code includes additional requirements that are not commonly part of grid codes today, including requirements covering asymmetric faults and inertia-like response to frequency deviations.

Transient nonlinear heat transfer analysis using a generic grid refinement for structure parameter variations

Traditional finite element models are usually established based on specific structures to address the transient nonlinear heat transfer analysis; however, if the structure parameters change one has to reanalyze/rebuild the finite element models, resulting in a lot of additional computational cost. To address this problem, this work develops a generic grid refinement approach to improve the computational efficiency of nonlinear heat transfer analysis. The advantage of the proposed method lies that the grids can be used to establish connections between original and modified structures through transfer operators, which is calculated only once for the original structures and can be reused in following grid iterations. Then, the transient nonlinear heat transfer equations of modified structures can be efficiently solved based on the grid iterations in each time step. As a result, the computational cost can be magnificently reduced. Several numerical examples are investigated to comprehensively evaluate the performance of the present algorithm when dealing with the transient nonlinear heat transfer analysis of structures with modifications.

Application of Flow Diagnostics to Rapid Production Data Integration in Complex Grids and Dual-Permeability Models

SummaryStreamline-based methods, as repeatedly demonstrated in multiple applications, offer a robust and elegant framework for reconciling high-resolution geologic models with observed field responses. However, significant challenges persist with the application of streamline-based methods in complex grids and dual-permeability media due to the difficulty with streamline tracing in these systems. In this work, we propose a novel and efficient framework that circumvents these challenges by avoiding explicit tracing of streamlines but exploits the inherent desirable features of streamline-based production data integration in high-resolution geologic models.Our approach features the application of flow diagnostics to inverse problems involving the integration of multiphase production data in reservoir models. Here, time-of-flight as well as numerical tracer concentrations for each well, on the basis of a defined flux field, are computed on the native finite-volume grid. The information embedded in these metrics are used in the dynamic definition of stream-bundles and, eventually, in the computation of analytical water arrival-time sensitivities with respect to model properties. This calculation mimics the streamline-derived analytical sensitivity computation used in the well-established generalized travel-time inversion (GTTI) technique but precludes explicit streamline tracing. The reservoir model property field is updated iteratively by solving the LSQR (sparse least-squares with QR factorization) system composed of the computed analytical sensitivity and the optimal water travel-time shift, augmented with regularization and smoothness constraints.The power and efficacy of our approach are demonstrated using synthetic model and field applications. We first validate our approach by benchmarking with the streamline-based GTTI algorithm involving a single-permeability medium. The flow-diagnostics-derived analytical sensitivities were observed to show good agreement with the streamline-derived sensitivities in terms of correctly capturing relevant spatiotemporal trends. Furthermore, the desirable quasilinear behavior characteristic of the traditional streamline-based GTTI technique was preserved. The flow-diagnostics-based inversion technique is then applied to a field-scale problem involving the integration of multiphase production data into a dual-permeability model of a large naturally fractured reservoir. The results clearly demonstrate the effectiveness of the proposed approach in overcoming the limitations of classical streamline-based methods with dual-permeability systems. By construction, this approach finds direct application in single/multicontinuum models with generic grid designs, both in structured and fully unstructured formats, thereby aiding well-level history matching and high-resolution updates of modern geologic models.This work presents, for the first time, an application of the GTTI to dual-permeability models of naturally fractured reservoirs. This is facilitated by a simplified, yet effective approach to travel-time sensitivity computations directly on finite-volume grids. The proposed approach can be easily applied to subsurface models at levels of complexity identified as challenging for classical streamline-based methods.

Fully scalable forward model grid of exoplanet transmission spectra

Simulated exoplanet transmission spectra are critical for planning and interpretation of observations and to explore the sensitivity of spectral features to atmospheric thermochemical processes. We present a publicly available generic model grid of planetary transmission spectra, scalable to a wide range of H$_2$/He dominated atmospheres. The grid is computed using the 1D/2D atmosphere model ATMO for two different chemical scenarios, first considering local condensation only, secondly considering global condensation and removal of species from the atmospheric column (rainout). The entire grid consists of 56,320 model simulations across 22 equilibrium temperatures (400 - 2600 K), four planetary gravities (5 - 50 ms$^{-2}$), five atmospheric metallicities (1x - 200x), four C/O ratios (0.35 - 1.0), four scattering haze parameters, four uniform cloud parameters, and two chemical scenarios. We derive scaling equations which can be used with this grid, for a wide range of planet-star combinations. We validate this grid by comparing it with other model transmission spectra available in the literature. We highlight some of the important findings, such as the rise of SO$_2$ features at 100x solar metallicity, differences in spectral features at high C/O ratios between two condensation approaches, the importance of VO features without TiO to constrain the limb temperature and features of TiO/VO both, to constrain the condensation processes. Finally, this generic grid can be used to plan future observations using the HST, VLT, JWST and various other telescopes. The fine variation of parameters in the grid also allows it to be incorporated in a retrieval framework, with various machine learning techniques.

The on-line coupled atmospheric chemistry model system MECO(n) – Part 5: Expanding the Multi-Model-Driver (MMD v2.0) for 2-way data exchange including data interpolation via GRID (v1.0)

Abstract. As part of the Modular Earth Submodel System (MESSy), the Multi-Model-Driver (MMD v1.0) was developed to couple online the regional Consortium for Small-scale Modeling (COSMO) model into a driving model, which can be either the regional COSMO model or the global European Centre Hamburg general circulation model (ECHAM) (see Part 2 of the model documentation). The coupled system is called MECO(n), i.e., MESSy-fied ECHAM and COSMO models nested n times. In this article, which is part of the model documentation of the MECO(n) system, the second generation of MMD is introduced. MMD comprises the message-passing infrastructure required for the parallel execution (multiple programme multiple data, MPMD) of different models and the communication of the individual model instances, i.e. between the driving and the driven models. Initially, the MMD library was developed for a one-way coupling between the global chemistry–climate ECHAM/MESSy atmospheric chemistry (EMAC) model and an arbitrary number of (optionally cascaded) instances of the regional chemistry–climate model COSMO/MESSy. Thus, MMD (v1.0) provided only functions for unidirectional data transfer, i.e. from the larger-scale to the smaller-scale models.Soon, extended applications requiring data transfer from the small-scale model back to the larger-scale model became of interest. For instance, the original fields of the larger-scale model can directly be compared to the upscaled small-scale fields to analyse the improvements gained through the small-scale calculations, after the results are upscaled. Moreover, the fields originating from the two different models might be fed into the same diagnostic tool, e.g. the online calculation of the radiative forcing calculated consistently with the same radiation scheme. Last but not least, enabling the two-way data transfer between two models is the first important step on the way to a fully dynamical and chemical two-way coupling of the various model instances.In MMD (v1.0), interpolation between the base model grids is performed via the COSMO preprocessing tool INT2LM, which was implemented into the MMD submodel for online interpolation, specifically for mapping onto the rotated COSMO grid. A more flexible algorithm is required for the backward mapping. Thus, MMD (v2.0) uses the new MESSy submodel GRID for the generalised definition of arbitrary grids and for the transformation of data between them.In this article, we explain the basics of the MMD expansion and the newly developed generic MESSy submodel GRID (v1.0) and show some examples of the abovementioned applications.

Hammerstein Models and Real-Time System Identification of Load Dynamics for Voltage Management

Distributed generators, controllable appliances, electric vehicle charging infrastructures, and energy storage systems introduce new technical challenges to the management of distribution networks, especially when there are large power fluctuations. Interactive dynamics between load and distributed generators in a distribution network carry significant impact on voltage variation and transient during load power disturbances. It is shown in this paper that the traditional static power flow analysis, in which load dynamic behavior is not counted, is not sufficient to model and predict voltage excursion after a power disturbance. To capture the behavior of load types and dynamics, this paper employs Hammerstein model structures to represent such behavior and explore their real-time identification. This is especially important for voltage quality management since the load dynamics depend on active and reactive load power, and hence change substantially due to load/generator power perturbations, electric vehicle charging activities, and subsystem load type varieties. Identification algorithms are introduced and their convergence properties are established. The algorithms are applied to a generic grid structure first, then evaluated on a 33-Bus system with multiple dynamic loads.

Secure data hiding scheme using shape generation algorithm: a key based approach

Data hiding in images has evolved as one of the trusted methods of secure data communication and numerous approaches have been introduced over the years using gray scale images as the cover media. Most of the methods are based on data hiding in least significant bit planes of cover images. Many such methods purely depend on data substitution algorithms by defining a pattern in which data is embedded. One can gain access to the secret data in a few attempts, if the algorithm is known. Keeping this in view several approaches based on secret keys have also been proposed by researchers. This paper proposes an efficient data embedding scheme using a key and an embedding pattern generated through midpoint circle generation algorithm. The pattern can be applied to a carrier that is mapped onto a grid/image. The cryptosystem uses the concept of steganography and is computationally light and secure. The secret-key is generated in such a way that Avalanche effect is ensured except in very rare cases. The proposed data embedding method is shown to be robust and highly secure while maintaining good hiding capacity and imperceptibility. It is applicable for data hiding in a generic grid that could be of pixels or bits.

Resiliency optimization of biomass to biofuel supply chain incorporating regional biomass pre-processing depots

Biomass to biofuel supply chain is subject to several potential disruptions such as flood, drought, pest attack, and equipment failure. These disruptions must be considered while designing the supply chain; especially if capital cost intensive components such as regional biomass pre-processing depots (RBPD) are to be implemented. This work develops a supply chain design optimization model that incorporates the possibility of such disruptions at the design stage. The objective function is the sum of the total cost incurred during the non-disruption and disruption scenarios weighted by their respective probability of occurrence. This also quantifies the expected disruption cost (EDC) on the operation of the supply chain. The decision variables are the locations and capacities of RBPDs and biorefinery, as well as the biomass flow across the supply chain. The model was applied to two separate case studies, namely, procurement of corn stover from farms arranged in a generic grid pattern, and procurement of corn stover, switchgrass, and Miscanthus for a region of thirteen counties in Southern Illinois. The simulation results showed that the consideration of resiliency in design reduced the EDC of supply chain by up to 38% by optimizing the RBPD locations. The results were shown to depend on the intensity and nature of disruptions. This was especially true for feedstock with higher yield such as Miscanthus. Local parameters such as yield and biomass price also affected the optimal results. Moreover, the presence of RBPDs was shown to increase supply chain resiliency.

Reactive Current Assignment and Control for DFIG Based Wind Turbines during Grid Voltage Sag and Swell Conditions

This paper proposes a reactive current assignment and control strategy for a doubly-fed induction generator (DFIG) based wind-turbine generation system under generic grid voltage sag or swell conditions. The system’s active and reactive power constrains during grid faults are investigated with both the grid- and rotor-side convertors (GSC and RSC) maximum ampere limits considered. To meet the latest grid codes, especially the low- and high-voltage ride-through (LVRT and HVRT) requirements, an adaptive reactive current control scheme is investigated. In addition, a torque-oscillation suppression technique is designed to reduce the mechanism stress on turbine systems caused by intensive voltage variations. Simulation and experiment studies demonstrate the feasibility and effectiveness of the proposed control scheme to enhance the fault ride-through (FRT) capability of DFIG-based wind turbines during violent changes in grid voltage.

New public management and the professions within cultural organizations: one hybridization may hide another

With the introduction into public organizations of management methods borrowed from the private sector, ‘new public management’ often comes across as a factor that weakens professional autonomy. Cultural industry is no exception to this phenomenon, as demonstrated by the case of the facilitators or interpreters in France ( médiateurs culturels). However, a careful study of the development of this occupation does reveal that, rather than a strict opposition between a professional logic and managerial logic, hybridization is possible. Indeed, professional groups can benefit from the establishment of methods that structure their activity and that take on board the dynamics of knowledge and career pathways. Points for practitioners The research presented here raises the question of how to reconcile the streamlining of public activities (new public management approaches) with the challenges of preserving or developing professions. Practitioners can thus see, through a case study and the presentation of a generic grid analysis, the different ways of structuring and managing the professions, with the emphasis on adopting an approach that ensures coherence between the skills necessary for each activity and the skills defined and career paths offered to the players. Far from being at odds, this kind of articulation makes it possible to hybridize the organizational logic of the new public management with the professional logic of the professions .

Connection and Disconnection Transients for Micro-Grids under Unbalance Load Condition

In this paper the design and the implementation of a method oriented to carry out a safe and a stable intentional disconnection, and re-connection, of local micro-grids from the main distribution network under generic grid conditions, avoiding the appearance of undesired transients, will be presented. In this work a seamless transfer between grid-connected and isolated mode is achieved by means of using an adapted frequency based synchronization control loop, which works in the a/3 stationary reference frame, whose response can be dynamically adapted in function of the micro-grid operating conditions. In addition to this feature, the inner loops of the control algorithm proposed in this application have been implemented in order to provide a proper operation of the system under generic conditions, even in the presence of unbalanced loads. In order to prove the reliability and good performance of the proposed method simulation and experimental results will be provided and discussed in the last sections of the paper.

A Stationary Reference Frame Grid Synchronization System for Three-Phase Grid-Connected Power Converters Under Adverse Grid Conditions

Grid synchronization algorithms are of great importance in the control of grid-connected power converters, as fast and accurate detection of the grid voltage parameters is crucial in order to implement stable control strategies under generic grid conditions. This paper presents a new grid synchronization method for three-phase three-wire networks, namely dual second-order generalized integrator (SOGI) frequency-locked loop. The method is based on two adaptive filters, implemented by using a SOGI on the stationary αβ reference frame, and it is able to perform an excellent estimation of the instantaneous symmetrical components of the grid voltage under unbalanced and distorted grid conditions. This paper analyzes the performance of the proposed synchronization method including different design issues. Moreover, the behavior of the method for synchronizing with highly unbalanced grid is proven by means of simulation and experimental results, demonstrating its excellent performance.