Modelling of energy service networks - Modelling Distributed Energy Resources in Energy Service Networks - page 37

The natural gas delivery system comprises different sets of equipment, and they
can be classified as:

Gas wells : Are the reservoirs where the fossil fuel is located and just like oil
fields they can be classified as onshore or offshore sites.

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Transmission pipelines : Have the responsibility of transporting natural gas at
high pressure levels from wellheads either to local distribution companies or
directly to gas-fired power plants and/or large industrial users.

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Compressors : Are devices that increase and regulate the gas pressure level in the
networks, this is because pressure values during gas transportation drop due to
frictional resistance. These units are abundant in the system and can be installed
at the transmission level, near a storage facility, or at distribution levels.

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Storage tanks : Are units filled with gas during off-peak periods in order to mitigate
the high demand during peak periods. Additionally, these facilities keep a steady
flow of supply when contingencies occur.

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City gate stations : Meter the gas volumes delivered, control pressure values
through regulation and introduce odorant into the gas stream. Naturally, these
facilities are considered to be the link between transmission and distribution
systems.

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Distribution pipelines : Provide the final stage in the natural gas delivery chain
by transferring gas at low pressure levels from city gate stations to end-use
customers, they are also popularly referred to as local distribution zones (LDZ).

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Pressure regulators : Are located throughout the distribution network to alter the
pressure levels locally by varying the gas flow rate. Therefore, they complement
compressor stations and help meet the different pressure level requirements that
customers have and which distribution network operators (DNOs) must satisfy.

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Valves : Are devices that allow selected sections of the gas network to be cut off.
In addition, they also provide control of the gas flow rate and may prevent the
flow of gas in an undesired direction.

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For modelling purposes, the natural gas networks are represented by branches
and nodes which serve to illustrate, respectively, pipelines and consumption points.

3.2.2 Defining the natural gas load flow problem
As natural gas flows through the many pipes of the system, energy and pressure are
lost due to friction and heat transfer. The steady-state gas load flow problem consists
of calculating the values of nodal pressures and flow rates in the individual pipes by
knowing the source pressure values and gas injections at load nodes .
The input data known for this particular problem are the load demands at nodes,
the incidence matrix , and the pressure value at the slack node. Furthermore, it is
implied one of two variables is known for each node in the system; being either
pressure p or flow injection F . According to the data available, the gas nodes can be
classified into the following groups:

Slack node or pressure node : Is the reference or generation node in which the
pressure p is fixed and used as a reference for the rest of the network, thus the
flow injection F into the node needs to be calculated.

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