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.