Onaizah Colleges – College of Engineering and IT, Saudi Arabia,
Electrical Power System Laboratory (EE344)
EXPERIMENT # 7
Power Flow Solutions and Control
OBJECTIVES

To investigate the power flow solution under different operating conditions.

To study system performance under different control strategies.
BACKGROUND
General
The power flow problem is a very well known problem in the field of power systems, which
means the flow of power in all system components. In other words, the voltage magnitudes and
angles for one set of buses are desired, given that voltage magnitudes and power levels for
another set of buses are given.
A power flow solution procedure is a numerical method that is employed to solve the power flow
problem. A power flow program is a computer code that implements a power flow solution
procedure. The power flow solution contains the voltages and angles at all buses, from which the
real and reactive generation and load levels at all buses and the real and reactive flows across all
lines, are computed.
Power flow analysis is a very important task in power engineering. It concerns the system
performance in its normal operating conditions. It is performed to obtain the magnitude and phase
angle of the voltage at each bus and the real and reactive power flows in the system components.
Power flow analysis has a great importance in:
1- the future expansion planning;
2- the system economic operation;
3- the maintenance schedules;
4- the exchange of power between utilities;
Onaizah Colleges – College of Engineering and IT, Saudi Arabia,
Electrical Power System Laboratory (EE344)
5- the stability studies;
6- the setting of proper protection devices.
In order to perform a power flow study, full data must be provided about the studied system, such as
connection diagram, parameters of transformers and lines, rated values of each equipment, and the
assumed values of real and reactive power for each load.
The power flow problem was originally motivated within planning environments where
engineers considered different network configurations necessary to serve an expected future
load. Later, it became an operational problem as operators and operating engineers were required
to monitor the real-time status of the network in terms of voltage magnitudes and power flows.
Today, the power flow problem is widely recognized as a fundamental problem for power
system analysis, and there are many advanced, commercial power flow programs to address it.
Most of these programs are capable of solving the power flow program for tens of thousands of
interconnected buses. Engineers that understand the power flow problem, its formulation, and
corresponding solution procedures are in high demand, particularly if they also have experience
with commercial grade power flow programs.
It is well known that generators have maximum and minimum real power capabilities. In
addition, they also have maximum and minimum reactive power capabilities. The maximum
reactive power capability corresponds to the maximum reactive power that the generator may
produce when operating with a lagging power factor. The minimum reactive power capability
corresponds to the maximum reactive power the generator may absorb when operating with a
leading power factor. These limitations are function of the real power output of the generator,
that is, as the real power increases, the reactive power limitations reduces.
The typical generator capability curve shows the lagging and leading reactive limitations as the
real power is varied. Most power flow programs model the generator reactive capabilities by
assuming a somewhat conservative value for Pmax (perhaps 95% of the actual value), and then
fixing the reactive limits Qmax (for the lagging limit) and Qmin (for the leading limit).
Bus Types
Onaizah Colleges – College of Engineering and IT, Saudi Arabia,
Electrical Power System Laboratory (EE344)
Each bus in the system has four variables: voltage magnitude and angle, real and reactive powers.
During the operation of the power system, each bus has two known variables and two unknowns.
Generally, each bus must be classified as one of the following types:
(a) Slack or Swing Bus
This bus is considered as the reference bus. It must be connected to a generator of relatively the
highest rating. During the operation, the voltage of this bus is always specified and remains constant
in magnitude and angle. In addition, this bus is responsible for supplying the losses of the system.
(b) Generator or Voltage Controlled Bus
During the operation, the voltage magnitude at this bus is kept constant. Also, the active power
supplied is kept constant according to the dispatch center requirement. Most probably, this bus is
connected to a generator where the voltage is controlled using the excitation and the power is
controlled using the prime mover control.
(c) Load Bus
This bus is not connected to a generator so that neither its voltage nor its real power can be
controlled. On the other hand, the load connected to this bus has a given active and reactive power
demands.
In summary, the known and unknown variables of each bus type are given in Table-A.
Table A : Known and unknown variables for each Bus Type
Type of Buses
Known
Unknowns
Swing Bus (Slack)
V 

P
Q
Generator Bus (PV)
V 
P

Q
P
Q
V 

Load Bus (PQ)
Load Flow Equations
For a system of n buses, the injected complex power at bus k ( k = 1,2, ..,n) can be expressed as:
*
*
Sk = Vk Ik = Pk - j Qk
N
*
Pk - j Qk = Vk  Ykn Vn
n =1
N
Pk = | Vk |  | Ykn | | Vn | cos( k -  n   kn )
n =1
N
Qk = | Vk |  | Ykn | | Vn | sin ( k -  n   kn )
Onaizah Colleges – College of Engineering and IT, Saudi Arabia,
Electrical Power System Laboratory (EE344)
where Ykn is the proper element in the bus admittance matrix Ybus and, Ykn =Yknkn , Vk =
Vkk and Pk and Qk are the active and reactive power injection at bus k, respectively.
Solution Methods
The nonlinear algebraic power flow equations can be solved using the following iterative
methods: 1. Gauss-Seidel method (GS). 2. Newton-Raphson method (NR).
The first method (GS) is simpler but the second method (NR) is reported to have better
convergence characteristics and is faster than (GS) method.
EXPERIMENTAL PROCEDURE
1- Base case:
1- Consider the power system given in Fig. 1. All system data are shown on actual units.
2- Run the power flow program on the given system, using the Power-World software.
3- Observe the relation of active and reactive power flow with the angle and voltage of each bus.
4- Record your output results in Table 1.
2- Generator Voltage Control
1- Referring to the base case, adjust the voltage of the generator at bus #2 to 1.05 and 0.95 p.u (1
p.u = 345 kV), one at a time.
2- Run the power flow program in each case.
3- Record your output results in Table 1.
4- Observe the direction of reactive power flow in each case.
3- MW Generation Control
Onaizah Colleges – College of Engineering and IT, Saudi Arabia,
Electrical Power System Laboratory (EE344)
1- Referring to the base case, adjust the output power of the generator at bus #2 to 100 and 200
MW, one at a time.
2- Run the power flow program in each case.
3- Record your output results in Table 2.
4- Observe the direction of active power flow in each case.
4- Outage Analysis
1- Referring to the power system in the base case.
2- Take each line, each load & each generator one at a time, out of service.
3- Record and print only the abnormal condition results in Table 3.
Fig. 1 Power System Under Study (Base Case)
SIMULATION RESULTS/OBSERVATIONS
Table 1: Generator Voltage Control
Bus Voltages
V2
V3
2
Line flows
3
P1-2
P1-3
P2-3
Q1-2
Comments
Q1-3
Q2-3
Onaizah Colleges – College of Engineering and IT, Saudi Arabia,
Electrical Power System Laboratory (EE344)
1.05
1.01
0.95
Table 2: MW Generation Control
MW
Line flows
Comments
Generation
G-2
P1-2
P1-3
P2-3
Q1-2
Q1-3
Q2-3
100
150
200
Table 3: Outage Analysis
Outage
Bus Voltages
V2
L1-2
L1-3
L2-3
Ld-1
Ld-2
Ld-3
G-2
G-3
V3
MVA Line Flows
S1-2
S1-3
Normal/
S2-3
abnormal
Onaizah Colleges – College of Engineering and IT, Saudi Arabia,
Electrical Power System Laboratory (EE344)
CHECKLIST
The report should contain

A printed cover page

Objectives

Introduction/Background

Equipment list – Computer and PowerWorld software

Procedure,

Simulation Results/Observations,

Comments and Conclusion:
 One picture for each control i.e.; Generator voltage control, MW generation control and
outage analysis
 Discuss the generator voltage control results and its effect on the MVAR flow. Determine
the % change in MW & MVAR flow w.r.t. the voltage change.
 Comment on the MW generation control and its effect on the MW flow. Find the %
change in MW & MVAR flow w.r.t the generation change in MW.
 Identify the serious outages on the power system under study.

Observation sheet used during lab session with signature of the instructor