Environmental Engineering Reference
In-Depth Information
For modelling purposes, when PHEVs are operational, this particular type of DER
will be represented in the electrical network as a positive load while charging and as
a negative load whenever they are discharging back to the grid. As a consequence,
these power fluctuations modify the net electric power demands seen by DNOs from
their grid supply points. To portray this type of behaviour, electrical systems need
to include several terms into each nodal balance equation wherever these units are
under consideration. Therefore, this section details the set of nodal terms required
to describe the presence of plug-in hybrid electric vehicle technologies with V2G
features in energy service networks.
From the perspective of electric networks, all load nodes must sum the electric
power supplied from the grid with the power available from distributed resources
to satisfy the required demands from end-users. For this to happen, it is assumed
that at any node the incoming electricity will either satisfy the conventional load or
help charge the PHEV units, while not forgetting the influence cogeneration injec-
tions can have within the node. In particular, regarding PHEV operation, the energy
stored in the battery systems will primarily be employed to power the electric motors
for transportation purposes, while also being reserve capacity to satisfy electric load
when deemed necessary. Naturally, the capacity to store energy in an electrochem-
ical battery will rely on both the specifications of the unit and efficiency equations
(4.37)-(4.39) described in the previous subsection, while the proportion of the PHEV
employed within a node will depend on its degree of penetration. The penetration
level is defined as the percentage of dwellings with a PHEV unit connected, as
expressed by term (4.40). Furthermore, for simplicity all G2V and V2G injections are
assumed to be operating at a unity power factor or have their power factor corrected
to unity [96].
The applied approach implies the nodal capacities of PHEV and cogenera-
tion devices are an aggregated quantity, which in turn are equivalent to the sum
of all individual units present within a particular node for a specific time inter-
val. Figure 4.21 shows a schematic of the possible power injections involved in
an electrical node with the final objective of satisfying the electric power demand
(while at the same time considering reverse power flows). This figure is analogous to
Figure 4.14 which represented the injections occurring in natural gas nodes, and
together they represent the main aspects the TCOPF modelling framework covers for
DER technologies.
As seen from Figure 4.21, the electrical demand required by end-users can
be satisfied through a combination of flows coming from either the grid, micro-
CHP units or PHEV storage units. To successfully portray these power injections,
the modelling framework for electric power conversion must establish the basic
principles and variables that will create coordinated interactions within the electric
network nodes. As can be expected, these nodal conversion equations require being
coherent and easily adaptable to the basic electric load flow formulation explained in
Chapter 3.
Taking Figure 4.21 as reference, details are given on the set of nodal equations
needed to portray the presence of PHEV and cogeneration technologies in energy
service networks.
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