Analysis of the electrical characteristics and surge protection of EHV transmission lines supported by tall towers

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Numerical Electromagnetic Analysis of Current and Voltage Distribution along Down Conductors and Towers Hit by Direct Lightning

In order to answer the practically important question of whether the down conductors of lightning protection systems to tall towers and buildings can be electrically isolated from the structure itself, this work is conducted. As a first step in this regard, it is presumed that the down conductor placed on metallic tower will be a pessimistic representation of the actual problem. This opinion was based on the fact that the proximity of heavy metallic structure will have a large damping effect. The post-stroke current distributions along the down conductors and towers, which can be quite different from that in the lightning channel, govern the post-stroke near field and the resulting gradient in the soil. Also, for a reliable estimation of the actual stroke current from the measured down conductor currents, it is essential to know the current distribution characteristics along the down conductors. In view of these, the present work attempts to deduce the post-stroke current and voltage distribution along typical down conductors and towers. A solution of the governing field equations on an electromagnetic model of the system is sought for the investigation. Simulation results providing the spatio-temporal distribution of the post-stroke current and voltage has provided very interesting results. It is concluded that it is almost impossible to achieve electrical isolation between the structure and the down conductor. Furthermore, there will be significant induction into the steel matrix of the supporting structure.

Lightning induced currents in isolated towers

During a lightning strike to ground or structure nearby, currents are induced in all conducting structures including tall towers. As compared to the case of a direct strike, these induced currents will be of much lower amplitude, however, appear more frequently. A quantitative knowledge on these induced currents will be of interest to instrumented and communication towers. A preliminary analysis on the characteristics of the induced currents was reported in an earlier work 1], which employed simplifications by neglecting the induced charge on the tower and also the contribution from the upward connecting leader. This work aims to make further progress by considering all the essential aspects in ascertaining the induced currents. For determining the field produced by the developing return stroke, a macro-physical model for the return stroke is employed and for the evaluation of the induced currents, an in-house time domain numerical electromagnetic code along with suitable modifications for incorporating the dynamics of upward leader is employed.

Role of upward leaders in modifying the induced currents in solitary down-conductors during a nearby lightning strike to ground

Electromagnetic field produced by a lightning strike to ground causes significant induction to tall objects in the vicinity. The frequency of occurrence of such nearby ground strikes can be higher than the number of direct strikes. Therefore, a complete knowledge on these induced currents is of practical relevance. However, limited efforts towards the characterisation of such induced currents in tall down-conductors could be seen in the literature. Due to the intensification of the background field caused by the descending stepped leader, tall towers/down-conductors can launch upward leaders of significant length. The nonlinearity in the conductance of upward leader and the surrounding corona sheath can alter the characteristics of the induced currents. Preliminary aspects of this phenomenon have been studied by the author previously and the present work aims to perform a detailed investigation on the role of upward leaders in modifying the characteristics of the induced currents. A consistent model for the upward leader, which covers all the essential electrical aspects of the phenomena, is employed. A first order arc model for representing the conductance of upward leader and a field dependant quadratic conductivity model for the corona sheath is employed. The initial gradient in the upward leader and the field produced by the return stroke forms the excitation. The dynamic electromagnetic response is determined by solving the wave equation using thin-wire time-domain formulation. Simulations are carried out initially to ascertain the role of individual parameters, including the length of the upward leader. Based on the simulation results, it is shown that the upward leader enhances the induced current, and when significant in length, can alter the waveshape of induced current from bipolar oscillatory to unipolar. The duration of the induced current is governed by the length of upward leader, which in turn is dependant on the return stroke current and the effective length of the down-conductor. If the current during the upward leader developmental phase is considered along with that after the stroke termination to ground, it would present a bipolar current pulse. (C) 2015 Elsevier Ltd. All rights reserved.

A First approach on the new transmission system in northern Brazil

This article shows an analysis of the electrical parameters of a three-phase transmission line section using a relatively new design for steel towers characterized by 280 meters high. These main features, the height of the line conductors and distance between them, are intrinsically related to the longitudinal and transversal parameters of the line. By this means, an analysis was carried out in order to show the electrical variations between a transmission line using the new technology and a conventional 440-kV line through a wide range of frequencies and a variable soil resistivity. © 2012 The Institution of Engineering and Technology.

Subtropical towers typology design : RNA showground case study

The QUT Team developed an idea for a new residential housing typology that is appropriate for sites where the best views are in the opposing direction to the preferable climatic orientation. The interlocking configuration creates a double height external living space in every apartment, creating further opportunities for cross ventilation and natural daylight. Unlike conventional double loaded housing typologies, the interlocking configuration only requires a continuous public circulation corridor every second level. The cores that service this corridor are separated to either end of the tower and open areas. The configuration of the interlocking apartments creates an interesting composition of solid and void when viewed externally. This undulating facade petternation assists in articulating the large building mass. The project was evaluated by independent consultants and found to be cost effective, and at the same time delivering energy efficient high density liveability. The project was presented to a meeting of the Australian Council on Tall Buildings seminar on 15 September 2010.

The Shape of Global Wealth: Slender Towers from New York to Seattle

Thesis (Master's)--University of Washington, 2016-06

Response of tall buildings with symmetric setbacks under blast loading

This study explores three-dimensional nonlineardynamic responses of typical tall buildings with and without setbacks under blast loading. These 20 storey reinforced concrete buildings have been designed for normal (dead, live and wind)loads. The influence of the setbacks on the lateral load response due to blasts in terms of peak deflections, accelerations, inter-storey drift and bending moments at critical locations (including hinge formation) were investigated. Structural response predictions were performed with a commercially available three-dimensional finite element analysis programme using non-linear direct integration time history analyses. Results obtained for buildings with different setbacks were compared and conclusions made. The comparisons revealed that buildings have setbacks that protect the tower part above the setback level from blast loading show considerably better response in terms of peak displacement and interstorey drift, when compared to buildings without setbacks. Rotational accelerations were found to depend on the periods of the rotational modes. Abrupt changes in moments and shears are experienced near the levels of the setbacks. Typical twenty storey tall buildings with shear walls and frames that are designed for only normaln loads perform reasonably well, without catastrophic collapse, when subjected to a blast that is equivalent to 500 kg TNT at a standoff distance of 10 m.

Of ivory towers and boundary spanners

We all live in a yellow submarine… When I go to work in the morning, in the office building that hosts our BPM research group, on the way up to our level I come by this big breakout room that hosts a number of computer scientists, working away at the next generation software algorithms and iPad applications (I assume). I have never actually been in that room, but every now and then the door is left ajar for a while and I can spot couches, lots (I mean, lots!) of monitors, the odd scientist, a number of Lara Croft posters, and the usual room equipment you’d probably expect from computer scientists (and, no, it’s not like that evil Dennis guy from the Jurassic Park movie, buried in chips, coke, and flickering code screens… It’s also not like the command room from the Nebuchadnezzar, Neo’s hovercraft in the Matrix movies, although I still strongly believe these green lines of code make a good screensaver).

The subtropical residential tower: An investigation by Design Charrette

QUT's Centre for Subtropical Design (CSD) partnered with a major developer to bring together some of Brisbane’s most experienced and creative architects and designers in a two-day intensive design charrette to propose innovative design strategies for naturally-ventilated high rise residential buildings. An inner-urban renewal site in Queensland’s capital city Brisbane gave four multi-disciplinary teams the opportunity to address a raft of issues that developers and consultants will confront more and more in the future in warm humid climates. The quest to release apartment dwellers from dependence on energy-hungry air-conditioning and artificial lighting was central to the design brief for the towers. Mentored by Richard Hassell of WOHA, the creative teams focussed on climate-responsive design principles for passive climate control including orientation, cross-ventilation and outdoor living in order to reduce greenhouse gas emissions and offset occupants’ rising energy costs. This article discusses how outcomes of the charrette take their cue from the city’s subtropical climate and demonstrate how high-density high-rise living can be attractive, affordable and sustainable through positive engagement with the subtropical climate’s natural attributes.

Daylighting performance of subtropical multi-residential towers : simulations tools for design decisions

During an intensive design-led workshop multidisciplinary design teams examined options for a sustainable multi-residential tower on an inner urban site in Brisbane (Australia). The main aim was to demonstrate the key principles of daylight to every habitable room and cross-ventilation to every apartment in the subtropical climate while responding to acceptable yield and price points. The four conceptual design proposals demonstrated a wide range of outcomes, with buildings ranging from 15 to 30 storeys. Daylight Factor (DF), view to the outside, and the avoidance of direct sunlight were the only quantitative and qualitative performance metrics used to implement daylighting to the proposed buildings during the charrette. This paper further assesses the daylighting performance of the four conceptual designs by utilizing Climate-based daylight modeling (CBDM), specifically Daylight Autonomy (DA) and Useful Daylight Illuminance (UDI). Results show that UDI 100-2000lux calculations provide more useful information on the daylighting design than DF. The percentage of the space with a UDI <100-2000lux larger than 50% ranged from 77% to 86% of the time for active occupant behaviour (occupancy from 6am to 6pm). The paper also highlights the architectural features that mostly affect daylighting design in subtropical climates.

Loading of a very tall building in a simulated downburst wind field

Thunderstorm downbursts are important for wind engineers as they have been shown to produce the design wind speeds for mid to high return periods in many regions of Australia [1]. In structural design codes (e.g. AS/NZS1170.02-02) an atmospheric boundary layer (ABL) is assumed, and a vertical profile is interpolated from recorded 10 m wind speeds. The ABL assumption is however inaccurate when considering the complex structure of a thunderstorm outflow, and its effects on engineered structures. Several researchers have shown that the downburst, close to its point of divergence is better represented by an impinging wall jet profile than the traditional ABL. Physical modelling is the generally accepted approach to estimate wind loads on structures and it is therefore important to physically model the thunderstorm downburst so that its effects on engineered structures may be studied. An advancement on the simple impinging jet theory, addressed here is the addition of a pulsing mechanism to the jet which allows not only the divergent characteristics of a downburst to be produced, but also it allows the associated leading ring vortex to be developed. The ring vortex modelling is considered very important for structural design as it is within the horizontal vortex that the largest velocities occur [2]. This paper discusses the flow field produced by a pulsed wall jet, and also discusses the induced pressures that this type of flow has on a scaled tall building.

The impact of private and shared open space on liveability in subtropical apartment buildings

The study explores the relationship between open space design, factors impacting open space provision, and resident satisfaction with open space in multistorey apartment buildings in the context of the subtropical lifestyle and climate of Brisbane Australia. The purpose of the paper is to identify the specific physical and spatial design characteristics residents perceive to be important in open spaces associated with their private dwellings and with shared open spaces. Firsthand resident evaluations of everyday experiences of residing in inner urban high density environments are explored through a survey of 636 residents and interviews with 24 residents. Private balconies are highly valued, but residents’ satisfaction would be enhanced by spaciousness for diverse activities, privacy and climate responsive design. Communal spaces and facilities are used infrequently by many residents who prefer interactions with community outside of the building. This is related to preferences for a level of anonymity in a setting where privacy is difficult to achieve due to physical proximity of neighbours.

The performance of irrigated mixtures of tall fescue, ryegrass and white clover in subtropical Australia. 2. The effects on yield and botanical composition of managing for quality.

The effects on yield, botanical composition and persistence, of using a variable defoliation schedule as a means of optimising the quality of the tall fescue component of simple and complex temperate pasture mixtures in a subtropical environment was studied in a small plot cutting experiment at Gatton Research Station in south-east Queensland. A management schedule of 2-, 3- and 4-weekly defoliations in summer, autumn and spring and winter, respectively, was imposed on 5 temperate pasture mixtures: 2 simple mixtures including tall fescue (Festuca arundinacea) and white clover (Trifolium repens); 2 mixtures including perennial ryegrass (Lolium perenne), tall fescue and white clover; and a complex mixture, which included perennial ryegrass, tall fescue, white, red (T. pratense) and Persian (T. resupinatum) clovers and chicory (Cichorium intybus). Yield from the variable cutting schedule was 9% less than with a standard 4-weekly defoliation. This loss resulted from reductions in both the clover component (13%) and cumulative grass yield (6%). There was no interaction between cutting schedule and sowing mixture, with simple and complex sowing mixtures reacting in a similar manner to both cutting schedules. The experiment also demonstrated that, in complex mixtures, the cutting schedules used failed to give balanced production from all sown components. This was especially true of the grass and white clover components of the complex mixture, as chicory and Persian clover components dominated the mixtures, particularly in the first year. Quality measurements (made only in the final summer) suggested that variable management had achieved a quality improvement with increases in yields of digestible crude protein (19%) and digestible dry matter (9%) of the total forage produced in early summer. The improvements in the yields of digestible crude protein and digestible dry matter of the tall fescue component in late summer were even greater (28 and 19%, respectively). While advantages at other times of the year were expected to be smaller, the data suggested that the small loss in total yield was likely to be offset by increases in digestibility of available forage for grazing stock, especially in the critical summer period.

The performance of irrigated mixtures of tall fescue, ryegrass and white clover in subtropical Australia. 1. The effects of sowing mixture combinations, nitrogen and oversowing on establishment, productivity, botanical composition and persistence.

In the subtropics of Australia, the ryegrass component of irrigated perennial ryegrass (Lolium perenne) - white clover (Trifolium repens) pastures declines by approximately 40% in the summer following establishment, being replaced by summer-active C4 grasses. Tall fescue (Festuca arundinacea) is more persistent than perennial ryegrass and might resist this invasion, although tall fescue does not compete vigorously as a seedling. This series of experiments investigated the influence of ryegrass and tall fescue genotype, sowing time and sowing mixture as a means of improving tall fescue establishment and the productivity and persistence of tall fescue, ryegrass and white clover-based mixtures in a subtropical environment. Tall fescue frequency at the end of the establishment year decreased as the number of companion species sown in the mixture increased. Neither sowing mixture combinations nor sowing rates influenced overall pasture yield (of around 14 t/ha) in the establishment year but had a significant effect on botanical composition and component yields. Perennial ryegrass was less competitive than short-rotation ryegrass, increasing first-year yields of tall fescue by 40% in one experiment and by 10% in another but total yield was unaffected. The higher establishment-year yield (3.5 t/ha) allowed Dovey tall fescue to compete more successfully with the remaining pasture components than Vulcan (1.4 t/ha). Sowing 2 ryegrass cultivars in the mixture reduced tall fescue yields by 30% compared with a single ryegrass (1.6 t/ha), although tall fescue alone achieved higher yields (7.1 t/ha). Component sowing rate had little influence on composition or yield. Oversowing the ryegrass component into a 6-week-old sward of tall fescue and white clover improved tall fescue, white clover and overall yields in the establishment year by 83, 17 and 11%, respectively, but reduced ryegrass yields by 40%. The inclusion of red (T. pratense) and Persian (T. resupinatum) clovers and chicory (Cichorium intybus) increased first-year yields by 25% but suppressed perennial grass and clover components. Yields were generally maintained at around 12 t/ha/yr in the second and third years, with tall fescue becoming dominant in all 3 experiments. The lower tall fescue seeding rate used in the first experiment resulted in tall fescue dominance in the second year following establishment, whereas in Experiments 2 and 3 dominance occurred by the end of the first year. Invasion by the C4 grasses was relatively minor (<10%) even in the third year. As ryegrass plants died, tall fescue and, to a lesser extent, white clover increased as a proportion of the total sward. Treatment effects continued into the second, but rarely the third, year and mostly affected the yield of one of the components rather than total cumulative yield. Once tall fescue became dominant, it was difficult to re-introduce other pasture components, even following removal of foliage and moderate renovation. Severe renovation (reducing the tall fescue population by at least 30%) seems a possible option for redressing this situation.