Page 1 of 6
European Journal of Business &
Social Sciences
Available at https://ejbss.org/
ISSN: 2235-767X
Volume 07 Issue 04
April 2019
Available online: https://ejbss.org/ P a g e | 2081
Load Frequencycontrol of Interconnected Power
Systems Using Pi Based PSO
SRIRAM.S.R.
M.Tech – Power System Engineering,
PRIST Deemed University, Madurai, Tamilnadu
Abstract - Proportional-plus-integral controller is designed here
based on Particle Swarm Optimization(PSO) for controlling the
frequency deviation which is a major problem of a two area
interconnected power system. In order to improvise the
performance of supplying power of a power system, error
function is minimized. The objective function taken into
consideration over here is Integral Time multiplied with
Absolute Error (ITAE). To optimize the gain values of
controller, the PSO algorithm is used. The choice of this
algorithm over other recent well known algorithms such as
Bacteria Foraging Optimization Algorithm (BFOA) and Genetic
Algorithm (GA) is explained for the same interconnected
system. Tuning of controllers is done in order to get the gain
values or controller parameters such that the desired frequency
and power interchange with neighboring systems are maintained
within specific value. Controllers must possess the property of
being sensitive against changes in frequency and load. Tuning of
controllers based on PSO algorithm is justified by making a
comparison with Conventional method and LQR method.
Key Words: frequency and load, Integral Time multiplied with
Absolute Error (ITAE)
I.INTRODUCTION
A power plant got to monitor the load conditions and
serve consumers entire day. It is therefore irrelevant to consider
that uniform power is generated throughout. So depending on
load power generation varies. The objective of control strategy is
to deliver and generate power in an interconnected system as
reliably and economically as possible while maintaining the
frequency and voltage within the limits. The system frequency is
mainly affected due to change in load, while reactive power
depends on changes in voltage magnitude and is less sensitive to
frequency. To keep the frequency constant Proportional plus
Integral (P-I) controller is used which controls the turbines used
for tuning the generators and also the steady state error of
systems frequency is reduced by tuning the controller gains.
There are different algorithms to optimize the controller gains
for load frequency control of an interconnected power system
like Genetic Algorithm (GA) but this one is difficult to
implement because of its complexity in coding and low speed of
convergence. Another method Bacteria Forging Optimization
Algorithm (BFOA) deals with the problem of reproduction
process which gives rise to a population of N individuals. Here
in this work Particle Swarm Optimization (PSO) is used because
of its simplicity and is not affected size of problem and
effectively solve large-scale non-linear optimization problems.
Before these algorithms got attention there were methods like
Conventional method, Ziegler-Nicholas and LQR method were
used to tune the controller
II.LOAD FREQUENCY CONTROL
Power systems are used to produce electrical power
from natural or renewable energy. Load frequency control (LFC)
is really important in power systems to supply reliable and better
electric power at consumer end. However, the consumers of the
electric power vary the loads randomly and frequently. Change
in load leads to adjustment of generation so that there is no
power imbalance whereas controlling the power generation is a
problem. To nullify the effects of the haphazard load changes
and to keep the voltage as well as frequency within prespecified
values a control system is essential. The frequency is closely
related to the real power balance whereas voltage is related to
reactive power. The real power and frequency control is referred
to as load frequency control (LFC) [1]. If in a system there are
changes in load then those changes will affect both frequency
and bus voltages. LFC as the name signifies adjusts the power
Page 2 of 6
European Journal of Business &
Social Sciences
Available at https://ejbss.org/
ISSN: 2235-767X
Volume 07 Issue 04
April 2019
Available online: https://ejbss.org/ P a g e | 2082
flow between different areas while holding the frequency
constant. LFC is actually a loop that regulates output in the
range of megawatt and frequency of the generator [9]. This
consists of two loops i.e. primary loop and secondary loop. The
problems of frequency control of interconnected areas are more
important than those of single area systems.
Reasons to hold frequency constant are:
1. Most types of ac motors run at speeds which are related to the
frequency directly.
2. If normal frequency is 50 Hertz and the turbine run at speeds
corresponding to ±2.5 Hertz then the blades of the turbine are
likely to get damaged.
3. The electrically operated clocks are driven by the synchronous
motors. The accuracy of these clocks dependent on the
frequency as well as an integral of this frequency error.
Nowadays power systems are connected to neighbouring areas.
But interconnection of the power systems leads to high
increment in the order of the system. This connection is made
possible by tie-lines.
With many loads linked to a system in a power system,
speed and frequency vary with the characteristics of the
governor with variations in loads. No need to modify the setting
of the generator if maintaining of constant frequency is not
needed. When constant frequency is needed the turbine speed
can be adjusted by varying the governor characteristic.
Complications arise when 2 generating stations in parallel, are
handling the variation in load. Chances of distribution of load in
two systems are mentioned below:
Let both generating stations are interconnected through a tie line.
If load varies at X or Y & A generation has to maintain the
constant frequency, at that time it’s known as Flat Frequency
Regulation.
Secondly where both X & Y have to maintain the
constant frequency. It’s known as parallel frequency
regulation.
Thirdly where frequency maintenance is done of a
certain Area by its own generator & keeping constant
the tie-line loading. It’s called flat tie-line loading
control.
In Selective Frequency control individually system
handles the variation in load itself & without
interfering, beyond its limits, the maintenance of the
other one in that group.
In Tie-line Load-bias control all systems in the
interconnection help in maintaining frequency no
matter where the variation is created. It has a principal
load frequency controller & a tie line plotter
determining input power on the tie for proper control of
frequency.
Multi Area System
Power systems have variable and complicated
characteristics and comprise different control parts and also
many of the parts are nonlinear . These parts are connected to
each other by tie lines and need controllability of frequency and
power flow. Interconnected multiple area power systems can be
depicted by using circles. A simplified four area interconnected
power system used in this study, each area can be represented as
equivalent generating unit and interconnected through lossless
tie-lines with some reactance. As simplified four-area
interconnected power systems a four-area interconnected system
block diagram is depicted. The system frequency deviation Δfi,
the deviation in the tie-line power flow ΔPtie,i, load disturbance
ΔPDi. The system parameter values are given in Appendix.
Frequency Control
The frequency of a system is dependent on the generated and
consumed active power balance. In order to control the
image171
Page 3 of 6
European Journal of Business &
Social Sciences
Available at https://ejbss.org/
ISSN: 2235-767X
Volume 07 Issue 04
April 2019
Available online: https://ejbss.org/ P a g e | 2083
frequency at its target value, a certain amount of active power
reserves is kept available to maintain the balance between the
active power generation and consumption in case of
disturbance. There are many different terms, definitions and
rules concerning what frequency control reserves entail [10].
The general hierarchical classifications of frequency reserve
services are: (i) frequency response, regulation, contingency
reserve and load following in the NERC and, (ii) primary
frequency control, secondary frequency control, tertiary
frequency control and time control in the ENTSOE [11]. The
ENTSOE has recently classified the frequency reserve services
as frequency containment reserve, frequency restoration reserve
and replacement reserve [12]. These frequency control reserves
can be activated automatically and/or manually. Following a
disturbance, the automatic fast controllers aim to stabilize the
frequency and then to bring the frequency back to its target
value. The manual controls are used to manage the contingencies
and to restore the automatic control reserves, frequency and
interchanges to their target values. These classifications are
commonly taking into account the time scale decomposition of
different control schemes. Likewise to the voltage control, the
frequency control reserves can be
provided from the preventive and/or corrective control actions
Basic Controller Types
PID controllers use a 3 basic behavior types or
modes: P - proportional, I - integrative and D - derivative. While
proportional and integrative modes are also used as single
control modes, a derivative mode is rarely used on it’ s own in
control systems. Combinations such as PI and PD control are
very often in practical systems.
PI Controller:
PI controller will eliminate forced oscillations and
steady state error resulting in operation of on-off controller and
P controller respectively. However, introducing integral mode
has a negative effect on speed of the response and overall
stability of the system. Thus, PI controller will not increase the
speed of response. It can be expected since PI controller does not
have means to predict what will happen with the error in near
future. This problem can be solved by introducing derivative
mode which has ability to predict what will happen with the
error in near future and thus to decrease a reaction time of the
controller.
PI controllers are very often used in industry, especially when
speed of the response is not an issue. A control without D mode
is used when:
a) fast response of the system is not required
b) large disturbances and noise are present during operation of
the process
c) there is only one energy storage in process (capacitive or
inductive)
d) there are large transport delays in the system
Interconnected areas in a power system.
We can therefore state that the load frequency control (LFC) has
the following two objectives:
Hold the frequency constant ( Δf = 0) against any load
change. Each area must contribute to absorb any load
change such that frequency does not deviate.
Each area must maintain the tie-line power flow to its pre- specified value.
The first step in the LFC is to form the area control error
(ACE) that is defined as
where Ptie and Psch are tie-line power and scheduled
power through tie-line respectively and the constant Bf is called
the frequency bias constant .
The change in the reference of the power setting ΔPref, i , of the
area- i is then
obtained by the feedback of the ACE through an integral
controller of the form
where Ki is the integral gain. The ACE is negative if the net
power flow out of an area is low or if the frequency has dropped
or both. In this case the generation must be increased. This can
be achieved by increasing ΔPref, i . This negative sign accounts
for this inverse relation between ΔPref, i and ACE. The tie-line
power flow and frequency of each area are monitored in its
control center. Once the ACE is computed and ΔPref, i is obtained
commands are given to various turbine-generator controls to
adjust their reference power settings.
IV.TUNING OF CONTROLLERS BASED ON PSO
Tuning Of Controller
image173
