Distributed Power System Virtual Inertia Implemented by Grid-Connected Power Converters

Renewable energy sources (RESs), e.g., wind and solar photovoltaics, have been increasingly used to meet worldwide growing energy demands and reduce greenhouse gas emissions. However, RESs are normally coupled to the power grid through fast-response power converters without any inertia, leading to decreased power system inertia. As a result, the grid frequency may easily go beyond the acceptable range under severe frequency events, resulting in undesirable load-shedding, cascading failures, or even large-scale blackouts. To address the ever-decreasing inertia issue, this paper proposes the concept of distributed power system virtual inertia, which can be implemented by grid-connected power converters. Without modifications of system hardware, power system inertia can be emulated by the energy stored in the dc-link capacitors of grid-connected power converters. By regulating the dc-link voltages in proportional to the grid frequency, the dc-link capacitors are aggregated into an extremely large equivalent capacitor serving as an energy buffer for frequency support. Furthermore, the limitation of virtual inertia, together with its design parameters, is identified. Finally, the feasibility of the proposed concept is validated through simulation and experimental results, which indicate that 12.5% and 50% improvements of the frequency nadir and rate of change of frequency can be achieved.NRF (Natl Research Foundation, S’pore)Accepted versio

Layout-Aware Information Extraction for Document-Grounded Dialogue: Dataset, Method and Demonstration

Building document-grounded dialogue systems have received growing interest as documents convey a wealth of human knowledge and commonly exist in enterprises. Wherein, how to comprehend and retrieve information from documents is a challenging research problem. Previous work ignores the visual property of documents and treats them as plain text, resulting in incomplete modality. In this paper, we propose a Layout-aware document-level Information Extraction dataset, LIE, to facilitate the study of extracting both structural and semantic knowledge from visually rich documents (VRDs), so as to generate accurate responses in dialogue systems. LIE contains 62k annotations of three extraction tasks from 4,061 pages in product and official documents, becoming the largest VRD-based information extraction dataset to the best of our knowledge. We also develop benchmark methods that extend the token-based language model to consider layout features like humans. Empirical results show that layout is critical for VRD-based extraction, and system demonstration also verifies that the extracted knowledge can help locate the answers that users care about.Comment: Accepted to ACM Multimedia (MM) Industry Track 202

Identification and characterization of miRNA169 family members in banana (Musa acuminata L.) that respond to fusarium oxysporum f. sp. cubense infection in banana cultivars

MicroRNAs (miRNAs) play an important role in plant resistance to pathogen infections. However, little is known about the role of miRNAs in banana Fusarium wilt, which is the most economically devastating disease in banana production. In the present study, we identified and characterized a total of 18 miR169 family members in banana (Musa acuminata L.) based on small RNA sequencing. The banana miR169 family clustered into two groups based on miRNA evolutionary analysis. Multiple sequence alignment indicated a high degree of sequence conservation in miRNA169 family members across 28 plant species. Computational target prediction algorithms were used to identify 25 targets of miR169 family members in banana. These targets were enriched in various metabolic pathways that include the following molecules: glycine, serine, threonine, pentose, glycerolipids, nucleotide sugars, starch, and sucrose. Through miRNA transcriptomic analysis, we found that ma-miR169a and ma-miR169b displayed high expression levels, whereas the other 16 ma-miR169 members exhibited low expression in the HG and Baxi banana cultivars. Further experiments indicate that there were negative relationships between ma-miR169a, ma-miR169b and their targets basing on their expression levels to Foc4 (Fusarium oxysporum f. sp. cubense tropical race 4) infection in resistant cultivars. But they were low expressed in susceptive cultivars. These results suggested that the expression levels of ma-miR169a and ma-miR169b were consistent with the resistance degree of the banana cultivars to Foc4. The analysis presented here constitutes a starting point to understand ma-miR169-mediated Fusarium wilt resistance at the transcriptional level in banana and predicts possible candidate targets for the genetic improvement of banana resistance to Foc4

Relay Selection for Over-the-Air Computation Achieving Both Long Lifetime and High Reliability

In a general wireless sensor network, a sink node collects data from each node successively and then post-processes the data to obtain useful information. However, conventional methods have a scalability problem: the data collection/processing time increases with the number of nodes, and frequent transmission collisions degrade spectrum efficiency. If only statistical values of the data are needed, using over-the-air computation (AirComp) can efficiently perform data collection and computation. However, AirComp also has its problems: when the channel gain of a node is too low, (i) the transmission power of that node will be high, decreasing the lifetime of that node and the entire network, and (ii) sometimes, the computation error still occurs even though the maximal transmission power is used. To jointly solve these two problems, in this paper we investigate the relay communication for AirComp and study a relay selection protocol. The basic method selects an ordinary node with a good channel condition as a relay node, considering both computation error and power consumption. This method is further enhanced to explicitly consider network lifetime in relay selection. Extensive simulation evaluations confirm that the proposed method helps to prolong the lifetime of the entire network and reduce computation errors as well

Utilizing the dead-time effect to achieve decentralized reactive power sharing in islanded AC microgrids

In islanded ac microgrids, distributed energy storage systems (DESSs) are normally coupled to the grid through voltage source inverters (VSIs). To improve the operation efficiency and to avoid overloading, it is desirable that VSIs can share active and reactive powers in proportion to their respective power ratings. Although accurate active power sharing can be easily guaranteed by frequency droop control, it is difficult to achieve reactive power sharing as desired due to mismatched grid impedances and voltage sensor scaling errors. To overcome this challenge, a decentralized reactive power control scheme is proposed and used in conjunction with the conventional voltage droop control to enhance the reactive power sharing performance. Specifically, the proposed control scheme utilizes the dead-time effect to equalize the power factors of all VSIs. As a result, the reactive power sharing performance is improved in a fully decentralized manner. Finally, simulation and experimental results are provided to validate the effectiveness of the proposed control scheme.Ministry of Education (MOE)This work was supported by the Singapore Ministry of Education Academic Research Fund Tier 1 under Grant 2018-T1-001-150 (RG 90/18)

Exploration of the relationship between inertia enhancement and DC-link capacitance for grid-connected converters

Grid-connected converters (GCCs) are showing potential in providing virtual inertia and has attracted wide attention recently. For the virtual inertia emulated by GCCs, it is proportional with the DC-link capacitance, and thus, the DC-link capacitance can directly affect the dynamic performance for the GCC emulating inertia through modifying the inertia constant. However, the impacts of the DC-link capacitance have never been discussed in any literature before. Considering this issue, the influence of the DC-link capacitance on the dynamic performance for the GCC providing virtual inertia is analyzed in this paper. In addition to that, the selection approach of the DC-link capacitance is presented by tuning the system damping ratio within its optimal range. Simulations verify the correctness.Accepted versio

Autonomous DC-link voltage restoration for grid-connected power converters providing virtual inertia

Renewable energy sources have been increasingly adopted to reduce greenhouse emissions. However, they are normally interfaced with power grids through grid-connected power converters without any inertia contribution. This will lead to the decreased power system inertia. As a solution, the method of inertia emulation by power converters has been reported to handle this problem. However, the DC-link capacitors of power converters cannot restore their voltages after injecting the power required by inertia emulation. Thus, if the load change causes another DC-link voltage drop, the undesirable overmodulation may appear. Moreover, power converters cannot provide multiple inertia support during cascading frequency events without the DC-link voltage recovery. To address the above concerns, this paper proposes an autonomous DC-link voltage restoration method that allows the restoration of DC-link voltages after individual frequency events. Simulation and experimental results verify the feasibility of the proposed method.National Research Foundation (NRF)Accepted versionThis research is supported by the National Research Foundation, Prime Minister's Office, Singapore under the Energy Innovation Research Programme (EIRP) Energy Storage Grant Call and administrated by the Energy Market Authority (NRF2015EWT-EIRP002-007)

Developing a model of chronic subdural hematoma

Chronic subdural hematoma (CSDH) is a common neurosurgical condition that has a high incidence in the increasing elderly population of many countries. Pathologically, it is defined as a persistent liquefied hematoma in the subdural space more than 3 weeks old that is generally encased by a membraneous capsule. CSDHs likely originate after minor head trauma, with a key factor in its development being the potential for a subdural cavity to permit its expansion within, which is usually due to craniocerebral disproportion. The pathogenesis of CSDH has been attributed to osmotic or oncotic pressure differences, although measurements of these factors in the CSDH fluid do not support this theory. Current belief is that CSDH arises from recurrent bleeding in the subdural space, caused by a cycle of local angiogenesis, inflammation, coagulation and ongoing fibrinolysis. However, because of a lack of detailed knowledge about the precise mechanisms, treatment is often limited to surgical interventions that are invasive and often prone to recurrence. Thus, it is possible that an easily reproducible and representative animal model of CSDH would facilitate research in the pathogenesis of CSDH and aid with development of treatment options. © 2011 Springer-Verlag/Wien