ISBN:378-26-138420-0245
LOAD FREQUENCY CONTROL A DISTRIBUTED INTERNATIONAL CONFERENCE ON CIVIL AND MECHANICALFOR ENGINEERING, ICCME-2014
GRID
SYSTEM INVOLVING WIND, HYDRO AND THERMAL POWER PLANTS P Suresh Kumar Dr.K.Rama Sudha PG scholar Professor Department Electrical & Electronics Engineering, Andhra University, Visakhapatnam, Andhra pradesh
Abstract- In an interconnected power system, as a
Rt
Temporary droop
power load demand varies randomly both area frequency and tie-line power interchange also vary. The objectives of load frequency control (LFC) are to minimize the deviations in these variables (area frequency and tie-line power interchange) and to ensure their steady state errors to be zero. In this area of energy crisis, renewable energy is the most promising solution to man’s ever increasing energy needs. But the power production by these resources cannot be controlled unlike in thermal plants. As a result, standalone operation of renewable energy is not reliable. Hence grid-connection of these along with conventional plants is preferred due to the improved performance in response to dynamic load. It is observed that fluctuations in frequency caused due to load variations are low with increase in penetration of renewable resources. Load frequency control (LFC) including PI controller is proposed in order to suppress frequency deviations for a power system involving wind, hydro and thermal plants owing to load and generating power fluctuations caused by penetration of renewable resources. A system involving four thermal plants, a wind farm and a hydro plant will be modeled using MATLAB.
Tg
Main servo time constant
Index Terms—Continuous power generation, load frequency, Control (LFC), wind power, hydro power, and thermal power plants LFC of multi area system, Frequency deviation in the multi area system.
operation of renewable resources is not reliable as they are
D
Change in load with respect to frequency
Tw
Water starting time
Tr
Reset time
Kt
Gain of turbine
Tt
Time constant of turbine
II. INTRODUCTION
The high Indian population coupled with increase in industrial growth has resulted in an urgent need to increase the installed power capacity. In India, majority of power production, around 65 percent is from thermal power stations. Due to problems related to uncertainty in pricing and supply of fossil fuels, renewable resources have been identified as a suitable alternative [7]. However, standalone
intermittent in nature. The intermittent nature of resource increases the frequency deviations which further add to the deviation caused by load variation. This necessitates the
I. NOMENCLATURE
grid connection of renewable resources [4] [2]. Frequency deviation is undesirable because most of the AC motors run
∆PC
Command signal
at speeds that are directly related to frequency. Also the
∆F
Frequency change
generator turbines are designed to operate at a very precise
∆YE
Changes in steam valve opening
speed. Microcontrollers are dependent on frequency for
R
Speed regulation of the governor
their timely operation. Thus it is imperative to maintain
Ksg
Gain of speed governor
system frequency constant. This is done by implementing
Tsg
Time constant of speed governor
Load Frequency Control (LFC). There are many LFC
Rp
Permanent droop
methods developed for controlling frequency. They include
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flat frequency control (FFC), tie-line bias control (TBC) and flat tie-line control (FTC) [1]. In FFC, Some areas act as load change absorbers and others as base load. The
∆ = ∆ − ∗ ∆ ∗
∗
(1)
advantage is the higher operating efficiencies of the base load as they run at their maximum rated value at all times. But the drawback here is the reduced number of areas absorbing load changes which makes the system more transient prone. In FTC load changes in each area are controlled within the area, thereby maintaining tie line frequency constant. The most commonly used method is the tie-line load bias control in which all power systems in
Fig 1.Block diagram representation of speed governing
the interconnection aid in regulating frequency regardless
system
of where the frequency change originates. In this paper, the power system considered has a Thermal system with four thermal areas, a Hydro plant and a wind farm.
B. Mathematical modeling Turbine model
III. MODELING OF THERMAL AREAS
The dynamic response of steam turbine is related to changes in steam valve opening ∆YE in terms of changes in power output. Typically the time constant Tt lies in the
The thermal areas have been modeled using transfer function. Speed governor, turbine and generator constitute the various parts namely the speed governing system,
range 0.2 to 2.5 sec. The dynamic response is largely influenced by two factors (i) entrained steam between the inlet steam valve and first
turbine model, generator load model .A complete block diagram representation of an isolated power system
Stage of the turbine, (ii) The storage action in the reheater which causes the
comprising Speed governor, turbine and generator and load is easily obtained by combining the block diagrams of individual components. [7].
Output of the low pressure stage to lag behind that of the High pressure stage
A. Mathematical modeling of speed Governing System
The command signal ∆PC initiates a sequence of events-the pilot valve moves upwards, high pressure oil flows on to the top of the main piston moving it downwards; the steam
Fig 2.Turbine transfer function model
valve opening consequently increases, the turbine generator speed increases, i.e. the frequency goes up which is modeled mathematically.
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C. Mathematical modeling Generator Load Model
The increment in power input to the generator-load system is related to frequency change as ∆ = ∆ − ∆ ∗
(2)
Fig 3. Block diagram representation of generator-load
IV. MODELING OF HYDRO AND WIND
Model
AREA
D. Entire thermal area
A. Modeling of hydro area The representation of the hydraulic turbine and water column in stability studies is usually based on certain
Typical values of time constants of load frequency control
assumptions. The hydraulic resistance is considered
system are related as Tsg< Tt << Tps. Fig. 4 shows the
negligible. The penstock pipe is assumed inelastic and
required block diagram and Table 1 shows the different
water incompressible. Also the velocity of the water is
parameters of the four thermal areas.
considered to vary directly with the gate opening and with the square root of the net head and the turbine output power is nearly proportional to the product of head and volume flow [3]. Hydro plants are modeled the same way as thermal plants. The input to the hydro turbine is water instead of steam. Initial droop characteristics owing to reduced pressure on turbine on opening the gate valve has to be compensated. Hydro turbines have peculiar response
Fig 4. Block diagram of entire thermal area
due to water inertia; a change in gate position produces an initial turbine power change which is opposite to that
Table 1 Parameters of all four thermal areas
sought. For stable control performance, a large transient (temporary) droop with a long resettling time is therefore required in the forms of transient droop compensation as shown in Fig. 5. The compensation limits gate movement until water flow power output has time to catch up. The
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result is governor exhibits a high droop for fast speed
uses a torque controller in order to maintain the speed at 1.2
deviations and low droop in steady state.
pu [6] [9] [10].
Fig 6. Block diagram of simple wind turbine Fig 5.Block diagram of hydro area.
V. LFC FOR A MULTI-AREA SYSTEM B. Modeling of wind farm An extended power system can be divided into a number of Wind passes over the blades, generating lift and exerting a
load frequency control areas interconnected by means of tie
turning force. The rotating blades turn a shaft inside the
lines. The control objective now is to regulate the frequency
nacelle, which goes into a gearbox. The gearbox increases
of each area and to simultaneously regulate the tie line
the rotational speed to that which is appropriate for the
power as per inter-area contacts. As in case of frequency,
generator, which uses magnetic fields to convert the
proportional plus integral controller will be installed so as
rotational energy into electrical energy. The power in the
to give zero steady state error in the tie line power flow as
wind that can be extracted by a wind turbine is proportional
compared to the contracted power. It is conveniently
to the cube of the wind speed and is given in watts by
assumed that each control area can be represented by an
P= ( Aν3 Cp)/2 where ρ is the air density, A is the rotor
equivalent turbine,
swept area, ν is the wind speed and Cp is the power
Symbols used with suffix 1 refer to area 1 & those with
coefficient. A maximum value of Cp is defined by the Betz
suffix 2 refer to area 2 and so on. Incremental tie line power
limit, which states that a turbine can never extract more
out of area 1 given by [5].
than 59.3% of the power from an air stream. In reality, wind turbine rotors have maximum Cp values in the range
generator
and
governor
∆ !"#, = 2&' ( ) ∆* +, − ) ∆*( +,
system.
(3)
25–45%. A wind farm consisting of Doubly-fed induction generator
Similarly, the incremental tie line power output of area 2
(DFIG) wind turbine is considered. DFIG consists of a
is given by
wound rotor induction generator and an AC/DC/AC IGBTbased PWM converter. The stator winding is connected
∆ !"#,( = 2&' ( ) ∆*( +, − ) ∆* +,
(4)
directly to the 50 Hz grid while the rotor is fed at variable frequency through the AC/DC/AC converter. The wind
Where T12 = synchronizing coefficient
speed is maintained constant at 11 m/s. The control system
f1 and f2 represent frequency of the respective area.
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For purpose, a single from ∆f is fed through an integrator to ∆ !"#,( . = −
(/012 12
∗ ∆ − ∆ (
(5)
This has been represented by fig.7
the speed changer resulting in block diagram configuration shown .the system now modifies to a proportional plus integral controller, which is well known from control theory, gives zero steady state error.
Therefore In the case of an isolated control area, ACE (area control error) is the change in area frequency which when used in integral control loop forced the steady state frequency error to zero. In order that the steady state tie line power error in a two area control be made zero another Fig 7. Block diagram of Tie-line power flow
integral control loop (one for each area) must be introduced to integrate the incremental tie line power signal and feed it back to the speed changer as shown in Fig 8
With the primary LFC loop a change in the system load will result in a steady state frequency deviation, depending on the governor speed regulation. In order to reduce the frequency deviation to zero we must provide a reset action by introducing an integral controller to act on the load reference setting to change the speed set point. The integral controller increases the system type by 1 which forces the final frequency deviation to zero. The integral controller gain must be adjusted for a satisfactory transient response. It is seen from the above discussion that with the speed
Fig.8 Diagram for Proportional plus Integral Load
governing system installed on each machine, the steady
frequency Control
load frequency characteristic for a given speed changer setting has considerable droop, from no load to full load .system frequency system specifications are rather stringent and, therefore so much change in frequency cannot be tolerated. In fact, it is expected that the steady change in
For free governor operation the steady change in system frequency for a sudden change in load demand (Pd) is given as
frequency will be zero. While steady state frequency can be brought back to the scheduled value by adjusting speed changer setting, the system could undergo intolerable dynamic frequency changes with changes in load. It leads
∆ =
3 ∆45/7 8 9
: :;: < 81=> 9∗ 1=>;
(6)
to the natural suggestion that the speed changer setting be adjusted automatically by monitoring the frequency
This is accomplished by a single integrating block by
changes.
redefining ACE as a linear combination of incremental frequency and tie line power. Thus for control area 1
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ACE1 = ∆Ptie,1 +b1∆f1 Taking Laplace transform ACE1(s) = ∆Ptie,1(s) +b1∆f1(s) Similarly, for control area n, ACEn(s) = ∆Ptie,n(s) +bn∆fn(s) Combining the basic block diagrams of multiple control areas with ∆Pc1(s) to ∆Pcn(s) generated by integrals of respective ACEs (obtained through signals representing changes in tie line power and local frequency bias) and employing the block diagram of Fig. 7, we easily obtain the composite block diagrams.
VI. SIMULATION AND RESULTS
A. LFC for thermal system four area system
The four thermal systems have been combined and the composite block diagram is simulated in Simulink/Matlab R2010a as shown in Fig. 9.
Fig.9 Thermal system Let the loads ∆PD1 to ∆PD4 be simultaneously applied in control areas 1 to 4 respectively. The system parameters of 4 area system are given in Table I. The frequency deviation versus time scale of 4 thermal areas for step load change is shown in Fig. 10
Frequency deviation Vs time for thermal system only
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Frequency Deviation Vs Time(s) 0.025 0.02 0.015 0.01
f (Hz)
0.005 0 -0.005 -0.01 -0.015 -0.02 -0.025
0
50
100
150
200
250 300 Time (sec)
350
400
450
500
Fig. 10. Response for Fixed load (Thermal system only)
B. LFC for Thermal and Hydro System (multi area)
Fig.11 Hydro and thermal systems (multi area)
The four thermal systems along with hydro unit are combined and composite block diagram is simulated as
Frequency deviation versus time for integrated thermal and
shown in figure.11
hydro system for step load change is shown in Fig. 12 from the curves it can be concluded that penetration of hydro energy (renewable) does not affect the system frequency adversely as the frequency deviation is well within limits.
Frequency deviation Vs time for (thermal hydro) system
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governor, turbo generator and load system consisting of four thermal areas, a hydro area and wind farm is controlled by a controller. By a batch control, the load is divided amongst various power plants in the ratio of their capacities by control system. This entire power system is modeled as shown in fig.13
Fig. 12. Response for Fixed load (thermal +hydro system)
C. LFC for Thermal, Hydro and Wind system
To compensate the intermittent nature of renewable, grid connection of the same is imperative for reliable power
Fig.13 LFC for Thermal, Hydro and Wind system
generation. The four thermal areas and the hydro unit are combined together in the ‘Thermal & Hydro’ subsystem which is same as the model shown in Fig. 8. The output ∆F of this subsystem gets reflected in the grid voltage. DFIG wind farm draws supply for stator from grid and the changing wind speeds has an impact on its output. Power from the DFIG is fed to the grid via stator and rotor depending on the wind speed. Higher the wind speed, higher is the power output, rotor feeds power; lower the wind speed, power It is possible to divide an extended power system into sub areas in which the generators are tightly coupled together so
output is low, hence rotor draws power from grid to have constant power flow through stator.
as to form a coherent group, i.e. all the generators respond in unison to changes in load or speed changer settings. Such a coherent area is called control area in which frequency is
Frequency Deviation (Hz) vs. Time(s) for the Integrated System
assumed to be same throughout in static and dynamic conditions. For the purpose of developing a suitable control strategy, a control area can be reduced to a single speed
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different units using Tie Line Bias Control method of LFC as it gives minimal frequency deviation.
VIII. REFERENCES
[1] R. Oba, G. Shirai, R. Yokoyama, T. Niimura, and G. Fujita, “Suppression of Short Term Disturbances from Fig. 14. Response for Fixed Load (Thermal + Hydro +
Renewable Resources by Load Frequency Control
Wind Systems)
Considering Different Characteristics of Power Plants”, IEEE Power & Energy Society General Meeting, pp.1 –
The output of wind farm is sent to the central control system to calculate the load distribution over thermal
7, Jul.2009. [2] N. R. Ullah, T. Thiringer, and Daniel
station. Random load of 1.7pu with maximum variation of
Karlsson,“Temporary Primary Frequency Control
0.8pu is considered here. Frequency deviation versus Time
Support by Variable Speed Wind Turbines— Potential
for Integrated Thermal, Hydro and Wind system for step
And Applications”, IEEE Transactions on Power
load change is shown in Fig. 14 Real time systems are best
Systems, vol.23, No.2, May 2008.
described by introducing random load variation. From the
[3]
curves, it can be concluded that in an integrated system with high penetration of renewable, frequency deviation has
ed., New York: McGraw-Hill, 1993. [4] L. Freris and D. Infield, Renewable Energy in Power
increased. Nevertheless, it is within limits thereby making renewable energy sources desirable.
P. Kundur, Power System Stability and Control, 1st
Systems, 1st ed., J.Wiley Sons Ltd., 2008. [5]
H. Saadat, Power System Analysis, 1st ed., Tata McGraw- Hill, 2002.
[6] O. Anaya-Lara, N. Jenkins, J. Ekanayake, P. Cartwright, M. Hughes, Wind Energy generation Modeling and Control, 1st ed., J. Wiley Sons Ltd.,
VII. CONCLUSION Load frequency control becomes more important, when a
2009. [7]
large amount of renewable power supplies like wind power generation are introduced. In this paper Load Frequency
Introduction, 2nd ed., Tata McGraw-Hill, 1983 [8] L.R. Chang-Chien, W.T. Lin and Y.C. Yin,
Control with considerable penetration of renewable has
“Enhancing frequency response control by DFIGs in
been analyzed in the presence of Thermal, Hydro and Wind
The high wind penetrated power systems,” IEEE
Systems with pi controller. It is observed that frequency deviation is low when wind system is introduced into the actual thermal systems, and it is within the tolerable limits for fixed load variations. The loads are distributed among
O. Elgerd, Electric Energy Systems Theory An
Transactions on power systems, 2010 [9]
G. Lalor, A. Mullane, and M. O’Malley, “Frequency Control and wind turbine technologies,” IEEE Trans. Power Syst., vol. 20, no. 4, pp. 1905–1913, Nov.
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2005. [10]
J. de Almeida and R. G. Lopes, “Participation of Doubly fed induction wind generators in system Frequency regulation,” IEEE Trans. Power Syst., vol. 22, no. 3, pp. 944–950, Aug. 2007.
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