A general formulation for computational design of grounding systems in underground electrical substations
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- Teses de doutoramento [2147]
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A general formulation for computational design of grounding systems in underground electrical substationsAuthor(s)
Directors
París, JoséColominas Ezponda, Ignasi
Date
2018Abstract
[Abstract]
Electricity has become an essential element in our lives, which is necessary in our daily
routine to do basic tasks as work or communicate. This makes the correct supply
of electricity a fundamental issue. One of the elements in charge of the electricity
supply is the electrical substations. As a consequence of the considerable population
growth in cities and the increasing demand for power and power supply have caused the
construction of new electrical substations in urban areas. However, this construction
has become in a challenge because of factors as the limited availability of space or
the cost of urban land. Consequently, engineers developed the underground electrical
substations, which consist of compact solutions buried in the ground.
A key element to keep in mind in electrical substations is safety, especially during
a fault condition. Grounding systems are the devices in charge of guaranteeing the
proper functioning of electrical substations and the safety conditions for people in
these situations. The correct design and analysis of grounding grids have become an
essential procedure in the underground electrical substations, since they are located in
urban areas. A grounding grid is mainly characterised by the ground resistance, the
ground potential rise, and the step, touch and mesh voltages.
In this thesis a general formulation to design and analyse properly grounding systems
of underground electrical substations has been developed, which allows to represent
a realistic soil structure in order to calculate with a good accuracy their main
parameters. The function of a grounding system is to carry and dissipate the fault electric
current into the ground, causing the appearance of potential gradients within and
around substations. To achieve the formulation, the physical phenomenon of a fault
current derivation into the ground through a grounding system was study by means of
the Maxwell's Equations. Thus, a mathematical approach based on the steady-state
behaviour of the general equations of electromagnetism has been carried out in order
to model this derivation of electric current. The procedure to obtain this mathematical
model was to study each conductive domain that formed the soil structure, a uniform
soil and a non-homogeneous finite volume, as if they were separated from each other,
and then coupled for their resolution. After that, two systems of equations defined by
potential functions are obtained, which are related between them by the compatibility
conditions applied on the surface of the enclosure. To achieve a problem which can
be solved by numerical techniques, the mathematical approach was recast into three
boundary integral equations by means of the concepts of potential theory. The numerical technique chosen to solve these boundary integral equations is the Boundary
Element Method (BEM), since it is the appropriate method to solve the governing
equations proposed to model the electric current derivation to an infinity domain like
the ground. Before applying this technique, the strong form of the boundary integral
equations are reformulated in their weak form from which accurate approximate
solutions can be calculated by means of the weighted residual methods. Thus, two numerical
approaches are carried out to solve the boundary integral equations, one based
on the Point Collocation and other on the Bubnov-Galerkin Method. The results obtained
from the resolution of both numerical approaches allows to calculate the values
of the leakage current densities emanating from the surface of the grounding grid to
the ground, as well as the current densities and electric potential distributions on the
substation enclosure. Therefore, the electrical potential at any point in the ground
can be calculated, and so the earth surface potential distribution. Then, from this
surface distribution, the main parameters that characterised a grounding system can
be calculated.
Finally, the general formulation developed is applied to analyse grounding systems
for real underground electrical substations. The model shows excellent results for these
analyses and proves that the formulation allows to calculate the main parameters of
these facilities with good accuracy.
Keywords
Subestaciones eléctricas-Diseño y construcción
Corrientes eléctricas-Puesta a tierra
Corrientes eléctricas-Puesta a tierra
Description
Programa Oficial de Doutoramento en Enxeñaría Civil . 5011V01
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