Review of Load Frequency Control Methods
Part-I: Introduction and Pre-deregulation Scenario
Sachin K. Jain
S. Chakrabarti and S. N. Singh
Department of Electronics & Communication Engineering
PDPM Indian Institute of Information Technology, Design
& Manufacturing, Jabalpur, India
skjain@ieee.org
Department of Electrical Engineering
Indian Institute of Technology Kanpur
Kanpur, India
saikatc@iitk.ac.in, snsingh@iitk.ac.in
Abstract— In an electric power system, which is one of the
most complicated systems among any non-natural systems, Load
Frequency Control (LFC) has always been an important and
critical issue. Therefore, many configurations of power system
model, variety of control strategies and a number of controllers
have been proposed in the literature for making LFC more
effective, robust, adaptive and efficient. In this work, which is
divided into two parts, namely, ‘Part-I: Introduction and Prederegulation Scenario’ and ‘Part-II: Post-deregulation Scenario
and Case Studies’ attempt has been made to provide literature
review on LFC. This paper discusses fundamental aspects of LFC
followed by some LFC methods that were proposed for prederegulation scenario. Significance of these methods in the
present scenario has been highlighted.
Keywords—Automatic Generation Control (AGC); Deregulated
Power Market; Load Frequency Control (LFC); Power System;
Real Power Control.
I.
INTRODUCTION
A power system is a highly non-linear and large-scale
multi-input multi-output (MIMO) dynamical system with
numerous variables, protection devices and control loops, with
different dynamic responses and characteristics. Hundreds of
small and large interconnected generating units supply a
variety of loads across the huge geographical area through
highly meshed transmission lines, and distribution network. It
is highly desirable to improve the performance and functions of
power systems during normal and abnormal operations, but it is
not an easy task, due to large interconnected network with
constantly changing load, operational limits, rotor angle
instability, voltage instability, frequency instability, economy
in operation and physical disturbances. In real power systems,
there is some overlap between the different forms of instability;
one phenomenon may lead to others and as systems fail, more
than one form of instability may ultimately emerge [1]. For
example, during frequency excursions following a major
disturbance, voltage magnitudes and power flow may change
significantly.
Various power system controls have evolved over the past
few decades to keep the power system in a secure state and
protect it from dangerous phenomena [2]-[5]. Some of these
are generation excitation controls, prime mover controls,
generator/load tripping, fast fault clearing, high speed re-
closing, voltage control/reactive power compensation, load–
frequency control and current injection etc. In the recent times,
static VAR compensator (SVC), high voltage direct current
(HVDC) converters, FACTS controllers and wide-area power
system stabilization and control are increasingly being used to
solve power system oscillation problems [6] but loadfrequency control still retains its importance because it forms
an important, integral part of automatic generation control.
Automatic generation control, a centralized integrated control,
balances the total generation and load (plus losses) to reach the
nominal system frequency (usually 50 or 60 Hz) and scheduled
power interchange with neighboring systems.
This work aims at emphasizing the challenges presented by
deregulation (restructuring) of the power system to the LoadFrequency Control and the state of the art in exertion of these
challenges. The entire work has been divided into two parts,
namely, ‘Part-I: Introduction and Pre-deregulation Scenario’
and ‘Part-II: Post-deregulation Scenario and Case Studies’.
Part-I discuss about the motivation and fundamentals of LoadFrequency Control (LFC), and comparison of LFC techniques
proposed for pre-deregulation scenario. Part-II presents, impact
of deregulation on power system, and review of LFC
techniques for post-deregulation scenario. Finally, case studies
of small power system are presented to explain the LFC
approaches in deregulated environment.
II.
OVERVIEW OF LFC
It’s amazing but true that all the generators (can be
hundreds, thousands or even more) interconnected to a network
run at the same speed. To understand how this speed can be
controlled, let us consider an isolated generator, supplying a
local load, as shown in Fig. 1. The generator experience two
opposing torque; the mechanical torque (input), Tmech, in the
direction of rotation that acts to increase rotational speed, and
the electrical torque (load/output), Telect, in the opposite
direction of rotation that acts to slow down the speed. In the
steady state condition, both torques will be equal in magnitude
and speed will remain constant. When there is a change in
electric load on the generator (reflected instantaneously as a
change in the electrical output torque (Telect)) and/or
mechanical input to the turbine, equilibrium between the
mechanical torque (Tmech) and the electrical torque (Telect) is
disturbed, which in turn results in speed variation. This
Telect
Mechanical
Energy Input
Turbine
Electrical Energy
Output
Generator
Tmech
Δf
Speed
Governor
factual
Controller
Frequency
Measurement
Reference
Frequency fref
Fig. 1. Mechanical and Electrical Torque Balance in a Generating Unit.
continues until system attains a new equilibrium point.
Frequency, being a direct function of speed of the prime
mover, also varies along with any imbalance between electrical
and mechanical torque. For a lossless system, the relationship
between imbalance and speed can be expressed as,
Pmech  Pelect  M
ds
dt
(1)
where, Pmech is mechanical power input to the turbine, Pelect is
electrical power output of the generator, s is rotational speed
of the rotor and M is inertia coefficient.
A. Need for LFC [1, 4, 7]
For satisfactory operation of a power system, frequency
should remain nearly constant. Frequency deviations can
directly impact on a power system operation, system reliability
and efficiency. Large frequency deviations can damage
equipments, degrade load performance, overload transmission
lines and interfere with system protection schemes. These
large-frequency deviation events can ultimately lead to a
system collapse [4]. Variation in frequency adversely affects
the operation and speed control of induction and synchronous
motors. The reduced speed of motor-driven generating station
auxiliaries, associated with the fuel, the feed-water and the
combustion air supply systems, such as fans, pumps, and mills,
will bring down plant output. Considerable drop in frequency
could result in high magnetizing currents in induction motors
and transformers thereby increasing reactive power
consumption. The extensive use of electric clocks and the use
of frequency for other timing purposes require accurate
maintenance of synchronous time which is proportional to
integral of frequency. In domestic appliances, where
refrigerators’ efficiency goes down, Television and Air
Conditioners reactive power consumption increases
considerably with reduction in power supply frequency.
Therefore it is very important to maintain the frequency within
allowable range. Due to the statistical nature of the load
fluctuations, we cannot avoid continuous load changes but we
can hope to keep the system frequency within sufficiently
small tolerance levels by adjusting the generation continuously.
This is achieved using automatic generation control, which
requires LFC mechanism for its implementation. Shayeghi et
al. [5] have mentioned main objectives of LFC as:
1. Ensuring zero steady-state error for frequency deviations.
2. Minimizing unscheduled tie line power flows between
neighboring control areas.
3. Getting good tracking for load demands and disturbances.
4. Maintaining acceptable overshoot and settling time on the
frequency and tie line power deviations.
B. Defining LFC
Load Frequency Control (LFC) is an ancillary service that
is related to the short-term balance of energy and frequency of
the power systems and acquires a principal role to enable
power exchanges and to provide better conditions for
electricity trading. The main goal of the LFC is to maintain
zero steady state errors for frequency deviation and good
tracking of load demands in a multi-area power system. At the
generation unit level, speed governor controls the speed of the
prime mover as the load on the generator change. This is
referred as primary speed control function. The setting of the
governor can be modified as per the load to keep prime mover
speed and hence frequency constant. In a single machine
system, it is not a big deal to control the frequency. The basic
scheme of Load Frequency Control of single generating unit is
shown in Fig. 1. In this scheme, the speed governor senses the
change in speed (frequency) via the primary and supplementary
control loops. A hydraulic amplifier provides the necessary
mechanical forces to position the main valve against the highsteam (or hydro-) pressure, and the controller provides a
steady-state power output setting for the turbine. For real
interconnected system the control is not as simple as for single
unit.
III.
LFC METHODS FOR PRE-DEREGULATION SCENARIO
For almost a century, electricity business (Generation,
Transmission and Distribution) was considered as a natural
monopoly. The electricity business was operated by vertically
integrated utilities, under regulatory control of government/
public bodies. It means generation, transmission and
distribution of electricity were under control of a single entity.
Hence, in an isolated power system, owned by single entity or
utility, consisting of a group of generators and loads, where all
the generators respond to changes in load or speed-changer
settings, in unison, regulation of power imbalance in generation
and supply is not a serious issue. The LFC task, in such
systems, is limited to restore the system frequency to the
specified nominal value. There are following possible ways to
share the change in the load to maintain frequency [8]:
1. Either of generating units cater the change in load (Flat
Frequency Regulation)
known as control areas. A centralized system then uses
automatic equipments to control the frequency of complete
system. This concept is known as Multi Area Control. Fig. 2
shows the vertically integrated utility structure of power system
with three control areas.
Area-2
G
Area-1
G1
G2
G3
The real system is much more complex and has large
number of generating units and diverse load centers
interconnected in highly meshed way via transmission lines.
There may also be some Independent Power Producers (IPPs).
Therefore, in real power system, the controlling of generationsupply imbalance is not as simple as it appears above. To make
the control task simpler, the most widely used technique is to
divide whole system into many relatively smaller systems
D
Area-3
D1
D2
+
D
A. Tie-Line Bias Control
The LFC uses an indicator called Area Control Error (ACE)
to send control signals to appropriate units under its control. A
Turbine
ΔPmech1
1
M 1s  D1
ΔPtie
Δ1
+
T/s
-
+
+
Governor
+
Turbine
ΔPmech2
-
1/R2
B2
Fig. 3. Tie Line Bias Supplementary Control Loops for Two Area Control.
D
A multi-area power system comprises areas that are
interconnected by high voltage transmission lines or tie-lines.
The trend of frequency measured in each control area is an
indicator of the trend of the mismatch power in the
interconnection and not in the control area alone. Therefore, in
addition to regulating area frequency, the supplementary
control should maintain the net interchange power with
neighboring areas at scheduled values. This is generally
accomplished by adding a tie-line flow deviation to the
frequency deviation in the supplementary feedback loop. Fig. 3
shows the block diagram of tie line bias supplementary control
loops for two area control.
-
K/s
G
Fig. 2. Vertically Integrated Utility Multi Area Structure of Power System.
-
ACE2
G
TRANSMISSION
IPP
-
Governor
G
Tie Line
1/R1
K/s
D
TRANSMISSION
B1
ACE1
G
TRANSMISSION
2. All units share the change in load (Parallel Frequency
Regulation)
Control signals based on any of the above two methods can
be issued to the speed governor of generating units to balance
the load change. Primary control action is not usually sufficient
to restore the system frequency, especially in an interconnected
power system and the supplementary control loop is required to
adjust the load reference set point through the speed-changer
motor. The supplementary loop performs a feedback via the
frequency deviation and adds it to the primary control loop
through a dynamic controller.
G
Tie Line
1
M 2 s  D2
Δ2
unit will react to a raise/lower control signal by
raising/lowering its generation output. The ACE can be
represented as,
ACE  Ta  Ts   B *  f a  f s 
(2)
where, Ta and Ts are actual and scheduled interchange (in MW),
respectively, fa and fs are actual and scheduled frequency, and B
is ACE frequency bias (in MW/Hz).
A positive value of ACE indicates an excess area generation
and a negative value of ACE indicates generation deficiency in
the area. The above equation (2) is referred to as tie-line bias
control. The effects of local load changes and interface with
other areas are properly considered as two input signals. Each
control area monitors its own tie-line power flow and
frequency at the area control centre. The ACE signal is
computed and allocated to the controller. Finally, the resulting
control action signal is applied to the turbine–governor unit.
Power to compensate the tie-line power change initially comes
from all areas to respond to the step load increase in any area,
and results in a frequency drop sensed by the speed governors
of all generators. However, after a few seconds (at steady
state), the additional powers against the local load changes
come only from respective area. Glover and Schweppe [9] has
proposed an advanced LFC based on discrete time, linear-plusdeadband feedback control law.
B. Optimal Control
Elgerd and Fosha [10], [11] first time addressed optimal
control concept for frequency control design of interconnected
power systems using a state variable model to develop new
feedback control laws. They developed an optimal feedback
controller and suggested feasible ways of improving dynamic
response and stability margins of the load-frequency control.
However, it was found that above approach requires
knowledge of the new steady-state operating point, that is not
available for implementation, and hence, the control is not a
feasible optimum control. A modified Kalman filter was
introduced in [12] for the identification of the incremental
power demand in order to optimally compensate for loadfrequency deviations. Miniesy and Bohn [13] presented
optimum load-frequency continuous control using differential
approximation and a Luenberger observer based methods.
C. Multi-level and Decentralized Control
The concept of multi-level and decentralized control was
originated because of structure of the power system as a large
interconnected system. In these controls, a power system is
viewed as an interconnection of several lower-order
subsystems. The local controller controls each subsystem
according to the interaction variables provided by a second
level controller (coordinator). The coordinator improves the
interaction variables to satisfy interaction feasibility by means
of an iteration technique. Many multi-level control schemes
have been suggested [14]-[17] for such interconnected systems
as a means of providing a more reliable control strategy. The
controller proposed in [14] minimizes the deviations in
frequency and scheduled tie-line power resulting from sudden
disturbances. The coordinator, a second level controller,
improves the interaction variables to satisfy interaction
feasibility by means of an iteration technique. The work
reported by Premakumaran et al. [16] utilizes certain possible
beneficial aspects of interconnection to permit more desirable
system performances in the LFC of a two-area power system.
This work also includes the effects of excitation system and
governor controls of the power-system model studied in this
work.
Shirai [18] has proposed a decentralized LFC scheme,
where, both governor and voltage controls are used in each
area, the disturbances occurred in each area can be absorbed by
only the controller installed in own area. There have been
many works since than on decentralized LFC. Davison and
Tripathi [19] has proposed two minimum order controllers for
the tie-line/inter-area power flow and frequency control
problems as an optimal decentralized control. Feliachi [20] has
also proposed an optimal decentralized load frequency control
scheme, however, this uses a fixed mode evaluation algorithm
based on eigenvalue dynamics to determine its feasibility.
Geromel et al. [21] proposed numerical procedure that solves a
Riccati equation iteratively and the proposed method allowed
to design the LFC with pre-specified structures.
D. Adaptive Control
Since, power system parameters, are a function of the
operating point, many adaptive control schemes were reported
to keep the system performance near its optimum, by tracking
the operating conditions and using the updated parameters to
compute the control [22]-[27]. Kanniah et al. [22] used
microprocessor for application of self-tuning regulators in
power system LFC. This adaptive control strategy combined a
control algorithm with a parameter estimation algorithm for
online update of the parameters of a model of the system. Pan
and Liaw [23], [24] proposed an adaptive controller using a PI
adaptation that only required parameters for the control are the
available information of the states and output of the model as
well as the plant output. Rubaai and Udo [25] proposed a
multilevel adaptive technique based on self-tuning regulator.
As claimed by the authors, the proposed technique permits
optimality at all levels while providing a decomposed solution.
Wang et al. [26] proposed a robust, adaptive control based on a
combination of the robust control approach and an adaptive
control technique that considered power system LFC with
system parametric uncertainties. In [27], a neural network (NN)
was used to act as the control intelligence in conjunction with a
standard adaptive LFC scheme. In this approach a NN is
operated in parallel with a full load frequency adaptive control
scheme to monitor the system frequency, as the controller
issues its control commands. If there appears a condition where
the frequency values are corrupted or the system is not
sufficiently excited, then the NNs will be able to provide the
power set points that may be directly communicated for
necessary system frequency control.
IV.
SIGNIFICANCE OF THESE METHODS IN PRESENT
SCENARIO
Introduction of deregulation of power industry worldwide
has changed the way the power system is operated. Elements of
power system are now owned by different utilities, which
compete with each other in the market for selling their product
(energy) and other services. LFC is treated like an ancillary
service. Therefore, it appears that earlier methods of load
frequency control are absolutely obsolete. However, this is not
completely true. Although, power system has been deregulated,
it has only changed the management of the system, basic
control requirements remain same even in present scenario. As
evident from companion paper [28] and a review article by
Shayeghi et al. [5], these methods provide firm platform for
development of advanced techniques that are suitable for
deregulated scenario. Many recent works on LFC in
deregulated power system are reported based on the concepts,
such as, adaptive control, multi-level and decentralized
Control, etc. Therefore, significance of these methods in
present scenario cannot be overlooked.
V.
CONCLUSION
Load frequency control has been an important issue since
the establishment of first power system. Major objectives of
LFC includes following the changes in load demands and
regulating frequency. Specific LFC methods also help in
maintaining tie line power inter-changes to specified values.
This paper has reviewed key literature of pre-deregulation
scenario in the area of LFC and has highlighted some of the
significant developments that are worthy enough even in
present deregulated scenario. Salient features of various LFC
schemes reported in the literature have been discussed under
four categorizing, namely, Tie-Line Bias Control, Optimal
Control, Multi-level and Decentralized Control, and Adaptive
Control. Significance of these methods has been established in
post-deregulation using recent research outcomes. It is believed
that this work, presented in two parts, will serve as a valuable
resource to practicing engineers and researchers are working in
this important area.
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