One of the most common computational procedures used in
power system analysis is the load flow calculation. The planning, design, and
operation of power systems require such calculations to analyze the
steady-state (quiescent) performance of the power system under various
operating conditions and to study the effects of changes in equipment
configuration.
These load flow solutions are performed using computer
programs designed specifically for this purpose. The basic load flow question
is this: Given the load power consumption at all buses of a known electric
power system configuration and the power production at each generator, find the
power flow in each line and transformer of the interconnecting network and the
voltage magnitude and phase angle at each bus.
Analyzing the solution of this problem for numerous
conditions helps ensure that the power system is designed to satisfy its
performance criteria while incurring the most favorable investment and
operation costs. Some examples of the uses of load flow studies are to
determine the following:
— Component or circuit loadings
— Steady-state bus voltages
— Reactive power flows
— Transformer tap settings
— System losses
— Generator exciter/regulator voltage set points
— Performance under emergency conditions
Modern systems are complex and have many paths or branches
over which power can flow. Such systems form networks of series and parallel
paths. Electric power flow in these networks divides among the branches until a
balance is reached in accordance with Kirchoff’s laws.
Computer programs to solve load flows are divided into two
types—static (offline) and dynamic (real time). Most load flow studies for
system analysis are based on static network models.
Real time load flows (online) that incorporate data input
from the actual networks are typically used by utilities in automatic
Supervisory Control And Data Acquisition (SCADA) systems.
Such systems are used primarily as operating tools for
optimization of generation, var control, dispatch, losses, and tie line
control. This discussion is concerned with only static network models and their
analysis.
Because the load flow problem pertains to balanced,
steady-state operation of power systems, a single-phase, positive sequence
model of the power system is used. Three-phase load flow analysis software is
available; but it is not normally needed for routine industrial power system
studies.
A load flow calculation determines the state of the power
system for a given load and generation distribution. It represents a
steady-state condition as if that condition had been held fixed for some time.
In actuality, line flows and bus voltages fluctuate
constantly by small amounts because loads change constantly as lights, motors,
and other loads are turned on and off. However, these small fluctuations can be
ignored in calculating the steady-state effects on system equipment.
As the load distribution, and possibly the network, will
vary considerably during different time periods, it may be necessary to obtain
load flow solutions representing different system conditions such as peak load,
average load, or light load.
These solutions will be used to determine either optimum
operating modes for normal conditions, such as the proper setting of voltage
control devices, or how the system will respond to abnormal conditions, such as
outages of lines or transformers. Load flows form the basis for determining
both when new equipment additions are needed and the effectiveness of new
alternatives to solve present deficiencies and meet future system requirements.
The load flow model is also the basis for several other
types of studies such as short-circuit,
stability, motor starting, and harmonic studies. The load
flow model supplies the network data
and an initial steady-state condition for these studies.
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