Environmental Engineering Reference
In-Depth Information
levels to the lower primary voltage for distribution. Also, voltage regulation by
on-load tap-changer (OLTCs) can also be applied at this stage to control and meet
adequate voltage values.
Primary system
: Consists of distribution feeders, including all power lines and
laterals, between the distribution substations and service transformers; usually
small industrial or big commercial clients can be connected at this juncture.
●
Service transformers
: Perform voltage reduction from the primary level and
route the power into the secondary system at a utilisation level.
●
Secondary system
: Takes the power from service transformers at close proximity
to residential and commercial customers to meet their respective loads.
●
For modelling purposes, the electrical networks are represented by branches and
nodes which serve to illustrate respectively, power lines and buses.
3.1.2 Defining the electrical load flow problem
Power systems have the ultimate goal of transporting electricity from the generators
to final consumers in the most efficient way possible. Because of the comprehensive
analysis they provide, steady-state power flow studies are the cornerstone for further
research into power system planning, expansion and operation studies.
The electric
load flow problem consists of determining complex voltages at all nodes in the net-
work by knowing the source voltage values and loads in the system; from which the
active and reactive power flows in all lines are then calculated
.
Each node in an electrical network has four variables that will aid in solving any
analysis that is needed. These variables are the voltage magnitude
V
, phase angle
θ
,
net real power
P
and net reactive power
Q
. According to the initial data available the
electrical nodes can be classified into the following groups:
Slack node
or
swing bus
: Is the reference node in which voltage magnitude
V
and
phase angle
θ
are specified,
P
and
Q
net injections are unknown.
●
Load node
or
PQ node
: Active
P
and reactive
Q
net power injections are known;
therefore, voltage magnitude
V
and angle
θ
need to be determined.
●
Source node
or
PV node
: These are the buses attached to a generator where active
power injection
P
and voltage magnitude
V
are given, but the voltage angle
θ
and the reactive net injection
Q
need to be specified.
●
Power systems are represented by a set of non-linear equations which can be
solved in a variety of ways. However, the nodal method approach is the most popu-
lar and is applied in this work as well. This approach in circuit analysis is based on
Kirchhoff 's Current Law (KCL), which states that at every node the sum of all cur-
rents injections will equal zero. In addition, the following annotations are considered
when deriving power system equations and are employed consistently throughout this
work when performing electric load flow studies:
A perfect symmetry exists between the phases of the three-phase power system;
●
The electrical system is assumed to be operating under balanced conditions and
is represented by its positive sequence network;
●
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