Multilevel inverters (MLIs) with switched-capacitor (SC) units have been a widely rehearsed research topic in power electronics since the last decade. Inductorless/transformerless operation with voltage-boosting feature and inherent capacitor self-voltage balancing performance with a reduced electromagnetic interference make the SC-MLI an attractive converter over the other available counterparts for various applications. There have been many developed SC-MLI structures recently put forward, where different basic switching techniques are used to generate multiple (discrete) output voltage levels. In general, the priority of the topological development is motivated by the number of output voltage levels, overall voltage gain, and full dc-link voltage utilization, while reducing the component counts and stress on devices for better efficiency and power density. To facilitate the direction of future research in SC-MLIs, this article presents a comprehensive review, critical analysis, and categorization of the existing topologies. Common fundamental units are generalized and summarized with their merits and demerits. Ultimately, major challenges and research directions are outlined leading to the future technology roadmap for more practical applications.

Switched-Capacitor Multilevel Inverters: A Comprehensive Review

Tunnels and underground space have been evolved as an integrated part of sustainable transportation infrastructure of both the developed and developing countries. Tunnels also have strategic importance in the defence sector of the nation. In recent decades, tunnels have been subjected to several events of intentional and unintentional blast incidents. The present study investigates the response of shallow unlined rock tunnels subjected to an internal blast. The effect of different unconfined compressive strengths (UCS) of rock on the extent of damage produced due to blast loading has been investigated. UCS representing dolomite, shale, sandstone, granite, basalt and quartzite medium has been investigated through a three-dimensional finite element model having dimensions of 30 m × 30 m in cross section and 35 m in length generated in Abaqus. The diameter of the unlined tunnel has been taken as 5 m having overburden depth of 12.5 m. Coupled-Eulerian–Lagrangian modelling has been adopted to simulate the blast loading. Nonlinear Mohr–Coulomb failure criteria have been adopted for different rock media. Trinitrotoluene (TNT) explosive has been used to simulate the blast loading, and Jones–Wilkins–Lee material model has been adopted to model the characteristics of TNT. Subsequently, a relation has been proposed to understand the damage caused by TNT blast loading in a shallow unlined tunnel. The total induced energy due to internal blast loading has been found to be maximum and minimum in dolomite and quartzite, respectively.

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Effect of unconfined compressive strength of rock on dynamic response of shallow unlined tunnel

The tunnels are the lifeline of urban centers and play a significant role in the mass transportation of goods and services. They are the locales of accident and target for subversive activities, including terror attacks. Society needs to be well aware of such accidents and mitigation methods. We have carried out the analysis of internal blast loading using finite element analysis for two different rock tunnels constructed in Manhattan-Schist and Quartz-Schist rock employing Abaqus. The coupled-Eulerian–Lagrangian method has been considered for advanced modeling of the air inside the tunnel and 100 kg of Trinitrotoluene (TNT) explosive. TNT has been considered as material for better results by using Jones–Wilkins–Lee model and Eulerian volume fraction tool in the Abaqus have been used. This study concludes that rocks having a higher angle of friction are more resistant to blast loading. Further, rocks having lower cohesion are more blast resistant. However, if the design of tunnel support remains constant in rocks, then, damage in the tunnel lining remains the same, but deformations vary.

Blast analysis of tunnels in Manhattan-Schist and Quartz-Schist using coupled-Eulerian–Lagrangian method

The tunnels are being constructed for the faster movement of goods and services from different locations. Therefore, Underground Urban Space (UUS) has evolved as an integral part of metro cities. C...

Blast resistant behaviour of tunnels in sedimentary rocks

The need for faster and safer means of land transportation has increased due to scarcity of land. The number of terror attacks on the underground metro tunnels has increased in the past couple of decades. These events have presented the increased need for blast resistant design of metro tunnels. The stability of underground metro tunnels constructed in the different rocks has been analysed in the present paper. The analysis of impact loading has been carried out using nonlinear finite element technique. The coupled Eulerian–Lagrangian which is an advanced method of modelling for trinitrotoluene (TNT) and air inside the tunnel has been used. The explosive charge has been assumed at the centre of the tunnel having 100 kg mass, and the analysis is carried out for 30 ms of duration. The nonlinear behaviour of TNT has been simulated by using Jones–Wilkins–Lee material model of the equation of state. The nonlinear behaviour of the different rocks, i.e. Quartzite, Quartz-Schist, Sandstone, Shale and Dolomite, has been considered through the Mohr–Coulomb constitutive model. The rock model has 30 m × 30 m × 35 m dimension having an opening of 5 m at the centre of the model. The tunnel has 12.5 m of the depth of overburden and tunnel lining has 0.35 m of thickness. The tunnel lining has been reinforced with steel bars of Fe415 steel grade, and concrete has M30 grade. The rock tunnel constructed in Shale is susceptible to a higher magnitude of damage as compared to other rocks in this paper when exposed to internal blast loading. The Quartzite rock tunnel is the most suitable choice for constructing the blast-resistant tunnel. It has been concluded that Quartzite has higher safety against an internal blast loading caused by 100 kg TNT explosive charge. Heaving has been observed for all the cases on the ground above the location of TNT.

A Simple Approximate Simulation Using Coupled Eulerian–Lagrangian (CEL) Simulation in Investigating Effects of Internal Blast in Rock Tunnel

High gain dc-dc converters are used in several applications which include solar photovoltaic system, switch-mode power supplies and fuel cells. In this paper, an ultra-high gain dc-dc boost converter is proposed and analyzed in detail. The converter has a gain of six times as compared with the boost converter. The high gain is achieved by utilizing switched inductors and switched capacitors. A modified voltage multiplier cell (VMC) with switched inductors is proposed. The converter has a single switch which makes its operation easy. Moreover, the voltage across the switch, diodes and capacitors are less than the output voltage which increases the overall efficiency of the converter. The converter performance in steady-state is analyzed in detail and it is compared with other latest high gain converters. The working of the converter in non-ideal conditions is also discussed in detail. The loss analysis is done using PLECS software by incorporating the real models of switches and diodes from the datasheet. To confirm and validate the working of the proposed converter a hardware prototype of 200 W is developed in the laboratory. The converter achieves high gain at low duty ratios and its performance is found to be good in open and closed loop conditions.

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A New Transformerless Ultra High Gain DC–DC Converter for DC Microgrid Application

Tunnels have been an integral part of human civilization. Due to complexity in its design and structure, the stability of underground structures under extreme loading conditions has utmost importance. Increased terrorism and geo-political conflicts have forced the engineers and researchers to study the response of underground structures, especially tunnels under blast loading. The present study has been carried out to seek the response of tunnel structures under blast load using the finite element technique. The tunnel has been considered in quartzite rock of northern India. The Mohr-Coulomb constitutive model has been adopted for the elastoplastic behaviour of rock. The rock model surrounding the tunnel has dimensions of 30 m x 30 m x 35 m. Both unlined and lined (concrete) tunnel has been studied. Concrete Damage Plasticity model has been considered for the concrete lining. Four different parameters (i.e., tunnel diameter, liners thickness, overburden depth and mass of explosive) have been varied to observe the behaviour under different condition. To carry out blast analysis, Coupled-Eulerian-Lagrangian (CEL) modelling has been adopted for modelling of TNT (Trinitrotoluene) and enclosed air. JWL (Jones-Wilkins-Lee) model has been considered for TNT explosive modelling. The paper concludes that deformations in lined tunnels follow a logarithmic pattern while in unlined tunnels an exponential pattern has been observed. The stability of the tunnel has increased with an increase in overburden depth in both lined and unlined tunnels. Furthermore, the tunnel lining thickness also has a significant effect on the stability of the tunnel, but in smaller diameter tunnel, the increase in tunnel lining thickness has not much significance. The deformations in the rock tunnel have been decreased with an increase in the diameter of the tunnel.

Numerical modelling of internal blast loading on a rock tunnel

Tunnels are proven to be the most important mode of transportation, these days. Due to rapid development of underground space in big cities, it is important to study and analyze huge stresses that are coming to the u/g tunnels due to different loading conditions. Mostly, urban tunnels are highly susceptible to deformation even with the small change in stress distribution of surrounding rock mass. Due to moving traffic and construction and demolition of new and existing structure, u/g structures are always subjected to huge stresses which need to be distributed uniformly for the safety of lining and lives of people. In this paper, finite element method is used to study the behavior of lined and unlined tunnels under the weight of overlying strata or litho-static pressure. In the analysis, vertical stresses on tunnel and lining are being applied in terms of unconfined compressive strength of rocks and geo-material (sm1) for better understanding. Authors have also simulated the effect of weathering on the deformation and stresses of different rock masses. The finite element software ABAQUS is used for simulation work. Mohr–Coulomb plasticity model is used for the different rocks. 3D finite element model is considered of size 0.3 × 0.3 × 0.35 m for 0.05 m overburden. UCS of different weathered basalt and a geo-material prepared in the laboratory applied as a load for both lined and unlined tunnel element. The results conclude that with the increase in the weathering indices of surrounding rock mass, massive deformation was encountered even with very less magnitude of applied vertical and horizontal stress.

Mohammad Zaid

Papers

A New Transformerless Ultra High Gain DC–DC Converter for DC Microgrid Application

Effect of unconfined compressive strength of rock on dynamic response of shallow unlined tunnel

Blast analysis of tunnels in Manhattan-Schist and Quartz-Schist using coupled-Eulerian–Lagrangian method

Numerical modelling of internal blast loading on a rock tunnel

Finite Element Analysis of Static Loading on Urban Tunnels