Modelling of energy service networks - Modelling Distributed Energy Resources in Energy Service Networks - page 42

3.3 Analogies in energy service networks

Comprehending the basic characteristics of electrical and natural gas systems allows
us to progress by building analogies for the modelling framework of an inte-
grated optimal power flow tool. These analogies are grouped and described into the
following categories:

Modelling components and variables;

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Newton-Raphson's iterative solution algorithms.

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3.3.1 Modelling components and variables
The basic types of components needed to model energy service networks are nodes
and branches; and as this chapter has detailed, nodal classifications share similar
traits. For instance, in gas networks load nodes are known and the pressure values
need to be determined; hence, they are similar to PQ nodes in electrical systems. On
the other hand, pressure nodes are commonly employed as the slack since their fixed
pressure will serve as a reference for the gas system; electrically, this is analogous
to the characteristics of PV nodes . Furthermore, aside from nodal analogies, the
variables considered when conducting electric and natural gas load flow calculations
also share similarities. Table 3.1 classifies the analogous variables for the energy
service networks discussed in this topic.
The heat energy of a fluid is measured in units of calorific value, which in turn is
defined as the number of heat units released when a unit volume of gas burns. This
relationship in fluids between volume flow and power is proportional to the gross
heating value (GHV), natural gas has a value around 96 and can be measured in
megawatt-hour per cubic metre [174]; hence, the relationship can be expressed for
the flow between nodes k and m as:

G km = GHV · F km (3.53)

Multiplying potential times flux gives power in both gas and electrical systems.
Thus, both thermal and electrical power share the same SI base units (m 2 kg/s 3 ),
which is equivalent to (J/s) and popularly known as watts ; this fact is depicted in
Table 3.2.

Table 3.1 Variable analogies for energy networks

Variable Natural gas system Electrical system

Pressure (N/m 2 )

Potential Voltage (V)

Flow (m 3 /s)

Flux Current (A)

Power Pressure × Flow (W) Voltage × Current (W)

Power losses Pressure × Flow (W) Voltage × Current (W)

Resistance Friction factor Impedance ( )

Topology Incidence matrix Admittance matrix