- Frequency control is important to maintain required receiving end voltage and stable operation when systems are interconnected. Automatic generation control (AGC) is used to maintain power balance and constant system frequency as load changes. - AGC has three components - primary control provides immediate response to load changes, secondary control corrects tie-line flows, and economic dispatch schedules units economically. It acts based on changes in generator speed and frequency. - For multi-area systems, AGC must restore frequency and scheduled tie-line flows in each area while ensuring areas absorb their own load changes. Area control error (ACE) is used to adjust control settings to drive ACE to zero and balance the system.

1. UNIT 2
FREQUENCY CONTROL AND AUTOMATIC
GENERATION CONTROL

2. IMPORTANCE OF FREQUENCY
CONTROL
 If frequency changes there won’t be get required receiving end
voltage.
 When two systems working at diff. frequency are tied together
to make same frequency by frequency converting station.
 Change in real power affect mainly the system frequency,
while reactive power is less sensitive to changes in frequency
and is mainly dependent on changes in voltage magnitude.
 The load frequency control (LFC) loop controls the real power
and frequency and the automatic voltage regulator (AVR) loop
regulates the reactive power and voltage magnitude.

3. AUTOMATIC GENERATION CONTROL

4. PURPOSE OF AGC
 To maintain power balance in the system.
 Make sure that operating limits are not exceeded:-
 Generators limit
 Tie-lines limit
 Make sure that system frequency is constant (not
change by load).

5. OVERVIEW OF AGC
 Load is always changing.
 To maintain power balance, generators need to
produce more or less to keep up with the load.
 When Gen < Load (Gen > Load), generator speed and
frequency will drop (rise).
 => We use this generator speed and frequency as
control signals!

6. 3 COMPONENTS OF AGC
 Primary control
Immediate (automatic) action to sudden change of load.
For example, reaction to frequency change.
 Secondary control
To bring tie-line flows to scheduled.
Corrective actions are done by operators.
 Economic dispatch
Make sure that the units are scheduled in the most
economical way.
 This is function of primary and secondary control of AGC.

7.  If the load on the system is increased, the turbine speed drops
before the governor can adjust the input of the steam to the
new load.
 The speed of the rotor can be maintained constant by adding
an integrator. The integral unit monitors the average error over
a period of time and will overcome the offset.
 Thus, as the system load changes continuously, the generation
is adjusted automatically to restore the frequency to its
nominal value. This scheme is known as automatic generation
control (AGC).
CONCEPT OF AUTOMATIC GENERATION CONTROL

8.  As shown in the block diagram, the valve opening changes
either by changing the reference setting or by the change in
speed (or frequency). This is called primary regulation.
 The reference setting can also be changed remotely by power
system load frequency control. This is called secondary
regulation.
 Only some generating units in a power system may be used
for secondary regulation

9. STATIC SPEED-POWER CURVE
• From,
• Primary control: Immediate
change corresponding to sudden
change of load (frequency)
• Secondary control: Change in
setting control power to maintain
operating frequency.
• The higher R (regulation), the
better.






R
P
P C
M
1

Slope = -R
1
M
P 2
M
P
1
C
P 2
C
P
M
P
= =
1
2
0


10. AGC FOR SINGLE AREA
• System Modeling
• Single Generator
• Multi Generators, special case: two
generators

11. LFC FOR SINGLE AREA

12. AGC FOR MULTI AREAS
 During transient period, sudden change of load causes each
area generation to react according to its frequency-power
characteristics.This is “called primary control”.
 This change also effects steady state frequency and tie-line
flows between areas.
 We need to
Restore system frequency,
Restore tie-line capacities to the scheduled value, and,
Make the areas absorb their own load.
 This is called “secondary control”.

13. AGC FOR MULTIPLE AREA SYSTEM

14. AGC FOR 2-GENERATOR: BLOCK
DIAGRAM
 
s
GM 2
2
1
R
+
-
 
s
GP2
2


2
M
P

2
L
P

2
C
P

21
2 P
P 


+
-
- 2


s
-
 
s
GM1
1
1
R
+
-
 
s
GP1
1


1
M
P

1
L
P

1
C
P

12
1 P
P 


+
-
-
1


s
+
-1
12
T
Load ↑
Frequency 2 ↓
Different phase angle ↑
Transfer power 12 ↑
Frequency 1 ↓
Governor 2 sense
speed ↓, try to ↑
mechanical power
Governor 1 sense
speed ↓, try to ↑
mechanical power
Steady state: New
(lower) system
Frequency

15. AGC FOR 2-GENERATOR:
STATIC SPEED-POWER CURVE
 Load increases.
 Frequency drops.
 Steady state is reached
when frequency of both
generators is the same.

1
M
P 2
M
P


M
P
1
2
0

+ = Change in
total load
1
M
P 2
M
P

16. STEADY STATE FREQUENCY CALCULATION:
2 GENERATORS
• From
• Consider the frequency at steady state,
• But, , ,and
• Then,
i
Li
i
i
i
Li
i
i
i
i
Mi P
P
D
P
P
D
M
P 













 


~
~

line
tie
L
M P
P
D
P 






 1
1
1
1
~

line
tie
L
M P
P
D
P 






 2
2
2
2
~

2
1 

 



 




1
1
1
R
PM





2
2
1
R
PM

















2
1
2
1
2
1
1
1
~
~
R
R
D
D
P
P L
L


17. Area Control Error (ACE)
Control setting power of each area needs to
be adjusted corresponding to the change of
scheduled tie-line capacity and change of
system frequency.
ACE measures this balance, and is given by,
for two area case.
 Where = Frequency bias setting of area i (>0) and




 1
12
1 B
P
ACE




 2
21
2 B
P
ACE
i
B 









i
Li
i
R
D
B
1

18. TIE-LINE MODEL ( )
From power flow equation,
Approximate at normal operating condition,
we have
Then, for small change,
 Where is called stiffness or synchronizing power coefficient
i
P

 




n
k
k
i
ik
k
i
i B
V
V
P
1
sin 

 




n
k
k
i
ik
i B
P
1


   

 










n
k
k
i
ik
n
k
k
i
ik
i T
B
P
1
1




ik
T

19. 19
ACE: TIE-LINE BIAS CONTROL
Use ACE to adjust setting control power, ,
of each area.
Goal:
To drive ACE in all area to zero.
To send appropriate signal to setting control
power,
Use integrator controller so that ACE goes to
zero at steady state.
Ci
P


20. 20
AGC FOR 2-AREA WITH TIE-LINE BIAS CONTROL :
STATIC SPEED-POWER CURVE
 Load in area 2 increases.
 Frequency of both area
drops.
 ACE makes Control power
of area 2 increases.
 Steady state is reached when
frequency is back at the
operating point and
generator in area 2 take its
own load.

1
M
P 2
M
P


M
P
1
2
0

+ = Change in load 2
1
M
P 2
M
P
2’
1
M
P 

2
M
P 
