Enhancement of fault current contribution from inverter-based resources

The reduction in levels of fault current infeed as inverter-based resources (IBR) displace synchronous machines undermines the ability of a conventional protection system to identify and isolate faults in an effective manner and is therefore a concern for system operators (SOs). This observation provided the motivation to investigate the limitations of IBRs when injecting fault current and to explore how these limitations might be overcome. This thesis investigates techniques aimed at significantly increasing Fault Current Contribution (FCC) from an IBR system so that renewable energy resources can continue to be deployed without compromising the protection system. The techniques for enhancing FCC are at three different levels of an IBR system: at semiconductor or device level, circuit level and system level. The first study uses phase change materials (PCM) to provide a short-term overload rating to insulated-gate bipolar transistors (IGBTs) and found them to have very limited potential to provide FCC. A Finite Element Analysis (FEA) of heat-flow concluded that, although the PCM was useful for dealing with short over-load currents, it was unsuitable for facilitating large fault currents of several times normal load current. The view was that if the fault current cannot be created at device level through better thermal management, then a circuit level innovation would be required. The second study investigates series/parallel switching of submodules in modular converters. This takes advantage of the fact that during a fault, the line voltage is reduced, and if it falls below 0.5 pu then half of the sub-modules (SMs) can be put into parallel with the other half to double the FCC (2 pu) at half the voltage (0.5 pu). Similarly, if the voltage drops below 0.25 pu, parallel connection of four groups of SMs would enable 4 pu current capability. A model of a static synchronous compensator (STATCOM) was developed, inspired by the alternate arm converter (AAC), with the director switch of the AAC used as part of the reconfiguration circuit. The conclusion of this study was that the penalty paid in power losses in the additional semiconductor devices used for reconfiguration is reasonable for the 2 pu FCC case but not at the 4 pu FCC case. The third study was based on circuit reconfiguration but beyond the converter itself and in this case the windings of the coupling transformer of a STATCOM. Sections of winding were switched using thyristors to tap-change the transformer by a large factor. Using the proposed thyristor-based electronic tap-changer (eTC), the number of turns of the grid-side winding was reduced during a voltage dip, so that larger current can be delivered to the network for the same converter current. The STATCOM was controlled in the natural frame (abc frame) and this control is used to actively drive the currents in the tap-changer thyristors to zero when needed so that they can be commuted rapidly. The transformer was configured to give a normal ratio of 1:4 and be able to tap-down to 1:2 and 1:1 to increase FCC to 2 pu or 4 pu. Theoretical analysis of, and operating principles for, the proposed eTC, together with their associated control schemes, are verified by time-domain simulation at full-scale. The case-study circuit demonstrates delivery of substantial fault current contribution (FCC) of up to 4 pu at the point of common coupling (PCC) in less than half a cycle (10 ms) after detection of three- and single-phase faults. The results demonstrate that the proposed eTC is a good candidate for the enhancement of fault current from IBR systems that employ coupling transformers, allowing them thereby to make a contribution to future electricity networks dominated by IBR.Open Acces

Doctor of Philosophy

dissertationAn important aspect of medical research is the understanding of anatomy and its relation to function in the human body. For instance, identifying changes in the brain associated with cognitive decline helps in understanding the process of aging and age-related neurological disorders. The field of computational anatomy provides a rich mathematical setting for statistical analysis of complex geometrical structures seen in 3D medical images. At its core, computational anatomy is based on the representation of anatomical shape and its variability as elements of nonflat manifold of diffeomorphisms with an associated Riemannian structure. Although such manifolds effectively represent natural biological variability, intrinsic methods of statistical analysis within these spaces remain deficient at large. This dissertation contributes two critical missing pieces for statistics in diffeomorphisms: (1) multivariate regression models for cross-sectional study of shapes, and (2) generalization of classical Euclidean, mixed-effects models to manifolds for longitudinal studies. These models are based on the principle that statistics on manifold-valued information must respect the intrinsic geometry of that space. The multivariate regression methods provide statistical descriptors of the relationships of anatomy with clinical indicators. The novel theory of hierarchical geodesic models (HGMs) is developed as a natural generalization of hierarchical linear models (HLMs) to describe longitudinal data on curved manifolds. Using a hierarchy of geodesics, the HGMs address the challenge of modeling the shape-data with unbalanced designs typically arising as a result of follow-up medical studies. More generally, this research establishes a mathematical foundation to study dynamics of changes in anatomy and the associated clinical progression with time. This dissertation also provides efficient algorithms that utilize state-of-the-art high performance computing architectures to solve models on large-scale, longitudinal imaging data. These manifold-based methods are applied to predictive modeling of neurological disorders such as Alzheimer's disease. Overall, this dissertation enables clinicians and researchers to better utilize the structural information available in medical images

For the net by the net: initial thoughts on India’s save the internet campaign

As the debate around net neutrality in India shows no sign of abating, Amandeep Singh and Nikhil George take a closer look at the Save the Internet Campaign and discuss its role in India’s regulatory governance space

Parametric Regression on the Grassmannian

We address the problem of fitting parametric curves on the Grassmann manifold for the purpose of intrinsic parametric regression. As customary in the literature, we start from the energy minimization formulation of linear least-squares in Euclidean spaces and generalize this concept to general nonflat Riemannian manifolds, following an optimal-control point of view. We then specialize this idea to the Grassmann manifold and demonstrate that it yields a simple, extensible and easy-to-implement solution to the parametric regression problem. In fact, it allows us to extend the basic geodesic model to (1) a time-warped variant and (2) cubic splines. We demonstrate the utility of the proposed solution on different vision problems, such as shape regression as a function of age, traffic-speed estimation and crowd-counting from surveillance video clips. Most notably, these problems can be conveniently solved within the same framework without any specifically-tailored steps along the processing pipeline.Comment: 14 pages, 11 figure

The Impact of Webpage Visual Characteristics on Consumer\u27s Initial Trust In E-Vendors

In making decisions regarding transacting with web-based vendors, consumers typically consider the uncertainty about vendor behavior or the perceived risk of having personal information misused by vendors. To mitigate these concerns vendors convey cues to improve consumer trust at a very early stage. Initial trust plays a central role in helping consumers overcome perceptions of risk and insecurity and makes them feel comfortable about interacting and transacting with an e-vendor. Therefore, initial trust is critical to both researchers and practitioners. The present paper describes a research-in-progress study that concentrates on webpage visual complexity and order as central factors in the design of web pages to enhance consumers’ initial trust in an online e-vendor

Weed flora dynamics and growth response of green gram (Vigna radiata (L.) R. Wilczek) under varied agri-horti system and weed management practices

Horizontal expansion of pulse production can be achieved by introduction of short duration pulse crop like, green gram (Vigna radiata (L.) R. Wilczek) under agri-horti system. Response of green gram under different agri-horti system and weed management practices is lacking. Therefore, an agronomic trial was conducted during monsoon season of 2011 at Agricultural Research Farm, Rajeev Gandhi South Campus, Banaras Hindu University, Barkachha, Mirzapur, Uttar Pradesh, in split plot design, consisting of three agri-horti systems [guava, custard apple and open field] in main plots and six weed management practices [pendimethalin 1000 g/ha (PE), imazethapyr 125 and 200 g/ha (PoE), 1-HW (20 DAS), 2-HW (15 and 30 DAS) and weedy check] were assigned to sub plots and replicated thrice. Green gram variety ‘Samrat’ was sown as per standard agronomic package of practices on August 5, 2011 in open field as well as within the alleys of, 5-year old, guava and custard apple agri-horti system. Agri-horti systems, did not significantly (P<0.05) affect the growth, yield attributes, yield and nutrient content in green gram. The weed management practices significantly affected the CGR, RGR and yield of green gram. Application of imazethapyr 200 g/ha recorded 79.08% reduction in weed biomass and 11.38% lower seed yield as compared to weedy check and 2-HW (15 and 30 DAS), respectively. 2-hand weeding effectively reduced weed biomass (88.07%) and showed highest yield (888.79 kg/ha), and CGR (13.61 g/day) followed by imazethapyr 200g/ha yield (787.66 kg/ha) and CGR (13.14 g/day)