Operation and control of power systems

1. Dr Md. Sohel Rana
PhD (UNSW, Australia), MIFAC, MACA, MIEEE, MIEB
Professor
Dept. of Electrical & Electronic Engineering
Rajshahi University of Engineering & Technology
E-mail: sohel.unsw@gmail.com; sohel@eee.ruet.ac.bd
Cell: +8801725431631 (BD); +61424029040 (AU)
Web: http://www.ruet.ac.bd/teacher/EEE/sohel
Power System Operation and Control
EEE 4243
Imagination is more important than knowledge – Albert Einstein

2. Course Material RUET
Bangladesh
 Power Generation Operation and Control
----- by Allen J. Wood and Bruce F. Wollenberg
 Operation and Control in Power Systems
----- by P S R Murty
 Power System Analysis
----- by Hadi Saadat
2

3. 3
Course Contain
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Course Contain:
1. Principle of power system operation: SCADA,
convention and competitive environment.
2. Unit commitment, static security analysis, state
estimation, optimal power flow
3. Automatic generation control
4. Dynamic security analysis

4. Introduction
4
► The frequency of a power system should maintain its nominal value
 Certain frequencies can harm important equipment in the power system
• Harmonic vibrations in turbine blades and shafts.
• Heating of generators and transformers.
 Some sensitive loads may be disturbed.
► Any changes in real power affects mainly the system frequency.
► Load frequency control (LFC) loop is the traditional mean of
controlling the frequency.
► LFC loop controls the real power and frequency.
Introduction
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5. Introduction (cont’d.)
5
► Case 1:
 Load increases, generation unchanged
 Kinetic energy of synchronous machine is used to tackle increased demand
 Machine becomes slower, frequency decreases
► Case 2:
 Load decreases, generation unchanged
 Kinetic energy of synchronous machine increases
 Machine becomes faster, frequency increases
Effect of generation-demand mismatch on frequency
Introduction (cont’d)
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6. 6
AVR with LFC
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7.  The two basic ways of controlling the frequency and voltage are by
using Load frequency Control loop (LFC) and Automatic Voltage
regulator loop (AVR).
 Changes in real power affect mainly the system frequency. The LFC
loop controls the real power and frequency.
 Reactive power is less sensitive to changes in frequency and mainly
dependent on voltage magnitude. The AVR loop controls the
reactive power and voltage magnitude.
 Cross-coupling between LFC loop and AVR loop is negligible so
the frequency and voltage controls can be analyzed independently.
AVR with LFC
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8. 8
AVR with LFC
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Fig: Schematic diagram of LFC and AVR of a synchronous generator
∆f
Governor

9. 9
LFC/ALFC
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Valve Control
Mechanism
G
Valve
Frequency
deviation Frequency
Sensor
Change in system
power
LFC
Real Power
Command Signal
Load
Steam
Turbine
Governor

10. From the swing equation we know that
For small disturbances
If we express speed in per unit with respect to synchronous speed
then finally after calculation we get
Generator Model
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e
m
s
P
P
dt
d
H


2
2
2 

e
m
s
P
P
dt
d
H





2
2
2 

]
[
)
(
2
1









dt
d
P
P
H
dt
d
e
m 

11. By laplace transformation
Generator Model
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))
(
)
(
(
2
1
)
( s
P
s
P
Hs
s e
m 




So the generator block diagram is

12. The speed load characteristic of a load is
where ΔPL = non frequency sensitive load change
DΔω = frequency sensitive load change
Load Model
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




 D
P
P L
e
Now the generator and load combined block diagram is

13. Load Model
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14. 14
 When the generator electrical load is suddenly increased, the
electrical power exceeds the mechanical power input.
 The power lack is supplied by the kinetic energy stored in the
rotating system.
 The reduction in the kinetic energy causes the turbine speed and
so the generator frequency to fall.
 The change in speed is sensed by the turbine governor which
acts to adjust the turbine input valve to change the mechanical
power output to bring the speed to a new steady state.
Governor Model
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15. The governor output is ΔPg = ΔPref– 1/R Δω;
where ΔPref = Reference power
1/R Δω = Power from governor speed characteristics
In the S domain it can be written as
ΔPg(s) = ΔPref(s) – 1/R ΔΩ(s)
Governor Model
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16. The command ΔPg is transformed through hydraulic amplifier to the
turbine input valve command ΔPv. Relation between them in the S
domain is
Governor Model
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So the block diagram of governor is
)
(
1
1
)
( s
P
s
s
P g
g
V 





17. Prime mover is the source of mechanical power. The model for the turbine
relates changes in mechanical power output ΔPm to the changes in the
turbine input ΔPv.
The simplest prime mover model can be approximated as
ΔPm(s) =
1
1+τ𝑇𝑆
ΔPv(s)
So the block diagram can be drawn as
The time constant τ𝑇 is in the range of 0.2 to 2 seconds.
Prime Mover (Turbine)
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18. We can get the complete block diagram of LFC for an
isolated power system by combining the governor, prime
mover, generator and load models.
Governor Turbine Rotating mass
and load
LFC/ALFC
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19. Closed-loop transfer function:
LFC/ALFC
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R
s
s
D
Hs
s
s
P T
g
T
g
L /
1
)
1
)(
1
)(
2
(
)
1
)(
1
(















20. LFC/ALFC
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0 5 10 15
-0.015
-0.01
-0.005
0
Frequency
deviation
(Hz)
Time (seconds)
Fig. : Frequency deviation step response of LFC.

21. With the LFC loop there will be a steady state error. To
overcome this frequency deviation we use AGC. The
integral controller gain Ki must be adjusted for satisfactory
response.
AGC
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22. Fig.: Block diagram of AGC of an isolated power station
AGC
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23. Just like the LFC loop is we consider the same values of 𝜏𝑇, 𝜏𝑔, H,
R, D and ΔPl and if we set the additional integral controller gain Ki
to 7, then by using SIMULINK we get the following output.
0 5 10 15 20
-15
-10
-5
0
5
10
-3
Frequency
deviation
(Hz)
Time (seconds)
Fig.: Frequency deviation step response of AGC
AGC
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24. Closed-loop transfer function:
AGC
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R
K
s
s
D
Hs
s
s
s
s
P I
T
g
T
g
L /
1
)
1
)(
1
)(
2
(
)
1
)(
1
(















 Stability analysis using Routh-Hurwitz Criterion

25. Introduction (cont’d.)
2
5
► Variation of voltage is harmful for maintaining supply quality
 At low voltage
• Devices may be unable to start
• Maloperation of sensitive load
 At high voltage
• may damage electrical equipment
• shortening lifetime
► Voltage magnitude is mainly dependent on reactive power.
► Automatic voltage regulator (AVR) loop is the conventional way
of controlling terminal voltage.
► The AVR loop controls the reactive power and hence voltage
magnitude.
AVR
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26. AVR
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Fig.: A typical arrangement of a simple AVR.
𝑉𝑅 𝑉𝐹
𝑉
𝑒

27. The excitation system amplifier may be magnetic, rotating
amplifier or modern electronic amplifier.
𝑉𝑅(𝑠)
𝑉𝑒(𝑠)
=
𝐾𝐴
1+τ𝐴𝑆
Typical values of 𝐾𝐴 are in the range of 10 to 400.
The amplifier time constant is very small in the range of
0.02 to 0.1 sec.
Amplifier Model
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28. Modern exciters use AC power sources through solid state rectifiers
like SCRs. The output voltage of the exciter is a nonlinear function of
the field voltage. A reasonable model of a modern exciter is linearized
which takes into account the major time constant and ignores
nonlinearities.
𝑉𝐹(𝑠)
𝑉𝑅(𝑠)
=
𝐾𝐸
1+τ𝐸𝑆
Exciter Model
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The time constant of modern exciter are very small.

29. The synchronous machine generated EMF is a function of
machine magnetization curve and its terminal voltage is
dependent on the generator load.
𝑉𝑡(𝑠)
𝑉𝐹(𝑠)
=
𝐾𝐺
1+τ𝐺𝑆
The constants are load dependent. 𝐾𝐺may vary between 0.7
to 1.
The time constant varies between 1 and 2 seconds.
Generator Model
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30. The voltage is sensed through a potential transformer and is
rectified through a bridge rectifier.
𝑉𝑆(𝑠)
𝑉𝑡(𝑠)
=
𝐾𝑅
1+τ𝑅𝑆
The time constant varies from 0.01 to 0.06 seconds.
Sensor Model
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31. The complete block diagram of AVR is as following
AVR
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𝐾𝐸
1 + 𝜏𝐸𝑠
Amplifier Exciter Generator
Vref (s)
Vs (s)
Ve (s) Vr (s) Vf (s)
Sensor
Vt (s)
𝐾𝐺
1 + 𝜏𝐺𝑠
𝐾𝐴
1 + 𝜏𝐴𝑠
𝐾𝑅
1 + 𝜏𝑅𝑠

32. Closed-loop transfer function:
AVR
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R
G
E
A
R
G
E
A
R
R
G
E
A
ref
t
K
K
K
K
s
s
s
s
s
K
K
K
K
s
V
s
V







)
1
)(
1
)(
1
)(
1
(
))
1
((
)
(
)
(






33. Voltage
(v)
0 5 10 15 20 25
0
0.5
1
1.5
2
Time (seconds)
Fig.: The terminal voltage step response of AVR
Terminal Voltage Step Response
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34. Block diagram of the AVR system using rate feedback stabilizer is
AVR
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35. After using the rate feedback stabilizer the terminal voltage
step response of AVR
Rate Feedback Stabilizer
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Fig.: Terminal voltage step response using rate feedback stabilizer
0 5 10 15
0
0.5
1
Voltage
(v)
Time (seconds)

36. The block diagram of AVR compensated with a PID controller is
AVR
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37. Fig.: Terminal voltage step response of AVR after using PID controller
AVR
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0 1 2 3 4 5
0
0.4
0.8
1.2
Voltage
(v)
Time (seconds)