16 research outputs found
Mechanism of degradation and breakdown in PET films under high intensity AC fields
No full textThe mechanism of electrical degradation and breakdown of polymeric
insulating films is examined by this paper as a two-stage process.
Electron injection by Fowler-Nordheim tunnelling, provides the
energy for the creation of the first cavity near to the injection
contact (the beginning of the formation of the so-called
low-density region) where impact ionisation of molecules and hence
electron avalanche, can occur. This stage is assisted by intrinsic
tunnelling of electrons through local potential barriers. The
second stage, the macroion bond scission and the creation of
another macroion and a free radical is a thermofluctuational
process which involves the action of the stretching force by the
local electric field. From the other hand breakdown initiation
starts when an electron following a Poole-Frenkel hopping
mechanism, is accelerated in a hole with sufficiently large
dimensions. The role of the applied electric field Fa and its
relation to the local electric field Fl in both stages is
examined. Experiments were executed for measuring the breakdown AC
voltage Vb and dielectric strength Fb, with voltage
rising rate 3 KV/s, in order to examine their relation with
specimen thickness d and hence to derive a steady state
breakdown criterion. Measurements of PET films life-time were also
taken in order to examine the degradation mechanism and the
polymer's lifetime
Condition assessment of power cable accessories using advanced VHF/UHF PD detection
No full textElectrical Engineering, Mathematics and Computer Scienc
The role of interfaces in the dielectric strength of polymeric films under high intensity fields
No full textThis paper examines the role of interfaces in the polymeric
insulating films under the scope of their dielectric strength. PET
(Polyethylene terephthalate) films of different thickness are put
together forming a planar sample of a typical thickness of m
and subjected to AC ramp voltage, until the dielectric
breakdown. The role of interface as a potential barrier for
electrons was examined as well as the influence of interface
number for a standard thickness. Experiments were also executed
for investigating the role of the position of the first, the
second and the last interface in the sample. All the experiments
were executed with the same voltage rising rate, the breakdown AC
voltage Vb was measured and the dielectric strength Fb
was calculated. It was shown that interfaces act as deep trapping
centers for electrons giving rise to the sample dielectric
strength for sort time experiments. Moreover the thinner the first
film, the higher the dielectric strength for the sample. It was
also found that the position of the second interface does not play
an important role in the process
Modeling of the Partial Discharge Process in a Liquid Dielectric: Effect of Applied Voltage, Gap Distance, and Electrode Type
No full textThe partial discharge (PD) process in liquid dielectrics is influenced by several factors. Although the PD current contains the information representing the discharge process during the PD event, it is difficult to determine the detailed dynamics of what is happening in the bulk of the liquid. In this paper, a microscopic model describing the dynamics of the charge carriers is implemented. The model consists of drift-diffusion equations of electrons, positive and negative ions coupled with Poisson’s equation. The stochastic feature of PD events is included in the equation. First the model is validated through comparison between the calculated PD current and experimental data. Then experiments are conducted to study the effects of the amplitude of the applied voltage, gap distance and electrode type on the PD process. The PD currents under each condition are recorded. Simulations based on the model have been conducted to analyze the dynamics of the PD events under each condition, and thus explain the mechanism of how these factors influence the PD events. The space charge generated in the PD process is revealed as the main reason affecting the microscopic process of the PD events
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A conceptual design of a wireless sensor actuator system for optimising energy and well-being in a building
No full textThis article presents a prototype model based on a wireless sensor actuator network (WSAN) aimed at optimizing both energy consumption of environmental systems and well-being of occupants in buildings. The model is a system consisting of the following components: a wireless sensor network, `sense diaries', environmental systems such as heating, ventilation and air-conditioning systems, and a central computer. A multi-agent system (MAS) is used to derive and act on the preferences of the occupants. Each occupant is represented by a personal agent in the MAS. The sense diary is a new device designed to elicit feedback from occupants about their satisfaction with the environment. The roles of the components are: the WSAN collects data about physical parameters such as temperature and humidity from an indoor environment; the central computer processes the collected data; the sense diaries leverage trade-offs between energy consumption and well-being, in conjunction with the agent system; and the environmental systems control the indoor environment
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Measurement of solute fluxes in isolated rat lungs
No full textMost previous studies in isolated perfused lungs have utilized measurements of solute flow from alveolar to vascular space to characterize the barrier and transport properties of the alveolar epithelium. In this study, we measured flux of a series of nonionic hydrophilic solutes and sodium across the alveolar epithelium of the isolated rat lung from perfusate to airspace(P→A), as well as from airspace to perfusate(A→P). Apparent permeability-surface area products (PS) were calculated from the rates of isotope appearance downstream in either the airspace or the perfusate. Equivalent pore analysis of data forP→A solute flow demonstrated a small pore population with radius 0.6 nm occupying 85% of the total pore area and a large pore population with radius 3.8 nm occupying 15% of the total area. Similar analysis ofA→P solute flux demonstrated a small pore population of 0.6 nm occupying 86% of the total pore area and a large pore population with radius 2.9 nm occupying 14% of total pore area. The ratio (R) ofPS
P → A divided byPS
A → P was 0.8 for the nonionic hydrophilic solutes, while R for sodium was 0.5. In the presence of amiloride and ouabain, R for sucrose was unchanged while R for sodium increased to 0.8 due to a fall inPS
A → P. The difference between R for sodium and R for the passively transported solutes, and the reduction in this difference in the presence of sodium transport inhibitors, are consistent with active sodium reabsorption by the intact alveolar epithelium. Differences in measured unidirectional passive solute fluxes probably result from unequal effective surface areas for diffusion from vascular space and
vice versa in the anatomically complex mammalian lung
An analysis of the feasibility of using a solar water heating system in a hotel : a case study
No full textIncludes bibliographical references (p. 53-57).The use of solar energy to heat water will not only contribute to the mitigation of global climate change due to the reduced combustion of fossil fuels, but it will also reduce the demand on South Africa's strained electricity supply infrastructure. The hotel industry is particularly energy intensive and the production of hot water makes up a significant portion of its electricity consumption. Furthermore, South Africa boasts one of the world's most abundant solar radiation resources. Thus far, however, the use of large-scale solar water heating (SWH) systems in South African hotels is limited, with the lack of economic feasibility likely to be the core reason
