Civil Engineering Reference
In-Depth Information
It can be observed that in the above set of equations the end-user
demands
Q
(
h
N
) are written as functions of unknown heads in the internal
nodes. The solution of the system in this form is called 'head-driven', and is
preferred for the perturbed seismic conditions where satisfaction of pre-
scribed demands is not guaranteed. It is customary in the analysis of WSS
to treat the end-user demands
Q
as fi xed boundary conditions (the system
must be proportioned in order to satisfy them). The solution of the system
with
Q
independent of
h
N
is called 'demand-driven'.
The set of nonlinear equations holds in so-called stationary conditions,
i.e. it assumes end-user demands constant over time. This is a simplifi cation,
and is valid as long as the boundary conditions vary smoothly with time (i.e.
quasi-stationary conditions). In seismic conditions, this is not the case where
abrupt variation of water demands due to pipe ruptures and leakages is
soon replaced by a new stationary state.
Model of an EPN
In an EPN, edges are transmission or distribution lines, while nodes are
either source buses, transformation buses or demand/load buses (a bus is
the EPN-specifi c term denoting a node). The EPN function is to provide
end-users with electric power having a voltage level within a narrow range
for the functioning of electrical devices. Source buses, enclosed in power
plants, can be of two different types, slack or generation. Each EPN has
only one slack bus that has the function of compensating the unbalance
between the power required by end-users and the power provided by gen-
erator buses.
The EPN lines compose a Transmission and Distribution (T&D) system,
divided into two general tiers: a transmission system with a grid-like topol-
ogy, which spans long distances at high voltages, usually between 60 and
750 kV, and a more local distribution system at intermediate voltages, char-
acterised by a tree-like topology. The latter is further divided into a
medium
voltage distribution system at voltages in the low tens of kV, and a
low
voltage distribution system, which consists of the wires that directly connect
most domestic and small commercial customers, at voltages that vary world-
wide between 100 and 240 V. The distribution system can be both overhead
and underground.
Lines at different voltages are terminated in substations. In general, an
electric substation is a facility that serves as a source of energy supply for
the local distribution area. Depending on their functions, substations can
be of three different typologies, i.e. transformation, distribution and trans-
formation/distribution. When the transformation function is required, two
buses at two different voltages are present, while in distribution substations
only one bus is present (load bus).
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