BASICS OF LOAD FLOW ANALYSIS

1. LOAD FLOW STUDY

2. AGENDA
 What is a load flow study?
 Why have a load flow study completed?
 When to have a load flow study completed?
 What is the use of load flow study?
 Load flow objective
 What are the inputs & output of load flow analysis?
 Load flow assumptions, constraints and limitations
 Load flow calculation methods

3. WHAT IS LOAD FLOW STUDY?
 Load flow (power flow) analysis is a basic analysis for the study of power systems
and also trickiest of the critical four power system studies.
 The reason these four studies are so powerful are that they require much of the
same data, and when completed together they have the ability to save time and
money, versus completing them one at a time.
 it evaluates your power system’s capability to adequately supply the connected
load while staying within proper voltage and current ranges.
 The load flow study report will determine the voltages and power factor at all your
buses, as well as currents or power flow on all your feeders.

4. WHY HAVE A LOAD FLOW STUDY COMPLETED?
 Completing a load flow study on an existing system will provide
recommendations for system operation and optimize the system
operation to minimum operational costs.
 Understanding the power flows on various system feeders will allow
the operators to understand if there is spare capacity, if there are
areas of the plant that are overloaded, and if there are operational
configurations that will save energy and the associated costs.

5. WHEN TO HAVE A LOAD FLOW STUDY
COMPLETED?
 The information that is critical from a proper load flow is the voltages and power
factor at all your buses, and currents or power flow on all your feeders.
 With this information you will be able to make important decisions on where to
add or remove load, and where power factor correction can be added to increase
the efficiency of your system.

6. WHAT IS THE USE OF LOAD FLOW STUDY?
 It is used for normal, steady-state operation.
 It gives you the information what is happening in a system.
 The load flow helps in continuous monitoring of the current state of the power
system, so it is used on daily basis in load dispatch/power system control centers.
 It can also be a support during examining effectiveness of the alternative plans for
future system expansion, when adding new generators or transmission lines is
needed.

7. LOAD FLOW OBJECTIVE
 The objective of load flow calculations is to determine the steady-state operating
characteristics of the power system for a given load and generator real power and
voltage conditions.
 Once we have this information, we can calculate easily real and reactive power flow
in all branches together with power losses.

8. WHAT ARE THE INPUT & OUTPUT OF LOAD FLOW
ANALYSIS?
BUS DATA
LINE DATA
GENERATOR DATA
LOAD DATA
VOLTAGE MAGNITUDE
VOLTAGE ANGLE
REALREACTIVE POWER
CURRENT FLOW
POWER LOSSES
LOAD
FLOW

9. BUS DATA
For PV
buses:
• Real power (generation and demand),
• Reactive power (demand),
• Voltage magnitude.
For PQ
buses:
• Real power (generation and demand),
• Reactive power (generation and demand).
For slack
bus:
• Voltage magnitude (usually 1 per unit),
• Voltage angle(specified to be zero),
• Real power (demand),
• Reactive power (demand).

10. LINE DATA
Transmission
lines:
• Resistance,
• Reactance,
• Capacitance (can be negligible).
Transformers:
• Winding resistances on low and high voltage side,
• Leakage reactance on low and high voltage side,
• Magnetization reactance,
• Iron loss admittance

11. LOAD FLOW ASSUMPTIONS, CONSTRAINTS AND
LIMITATIONS
 Assumptions for load flow calculation:
 System is in steady state (no transient changes)
 Three phases system is assumed to have balanced loading
 Per-unit system is used for simplification
 There also some constraints in load flow problem for:
 Voltage magnitude, |Ui|min≤|Ui|≤|Ui|max|Ui|min≤|Ui|≤|Ui|max
 Voltage angle, |δi−δk|≤|δi−δkmax||δi−δk|≤|δi−δkmax|
 Physical limitations:
 Real generated power, PGimin≤PGi≤PGimax
 Reactive generated power, QGimin≤QGi≤QGimax

12. LOAD FLOW CALCULATION METHODS
Gauss-
Seidel
Newton-
Raphson
Fast-
decoupled
The number of nodes in real power systems is so high that the calculation
are to complex to make it by hand. That’s why, we use numerical methods.
They are,

13. GAUSS - SEIDEL NEWTON - RAPHSON FAST - DECOUPLED
Complexity Easy Complex Less complex (constant
Jacobian, you do not
have to inverse it)
Convergence Linear Quadratic - the fastest Geometric
Sensitivity Not available Available Available
System size Problematic with large
systems (the time of
calculation increases
linearly with system size)
Appropriate for any
system size (the time of
calculation does not
depend on system size,
3-4 iterations usually
needed)
Appropriate for any
system size (the time of
calculation does not
depend on system size,
5-6 iterations usually
needed)
Accuracy Good The best Average

14. GAUSS - SEIDEL NEWTON - RAPHSON FAST - DECOUPLED
Type of system May have a convergence
problem with ill-
condition system
No problem with ill-
condition system
No problem with ill-
condition system
Accuracy Good The best Average
Sensitivity
It is sometimes advisable to have one or two
Gauss-Seidel iterations before Newton- Raphson,
which may decrease the iteration to some extent
and it helps with the “flat start” which is sometimes
causing non-convergence for Newton - Raphson
method.