1. FREQUENCY CONTROL AND AUTOMATIC GENERATION CONTROL
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. CONCEPT OF AUTOMATIC GENERATION CONTROL
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).

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.
Slope = -R = 1 2

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
Governor 1 sense speed ↓, try to ↑ mechanical power + - -1
Frequency 1 ↓
Different phase angle ↑
Transfer power 12 ↑
Steady state: New (lower) system Frequency
Frequency 2 ↓
Governor 2 sense speed ↓, try to ↑ mechanical power
Load ↑

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. 2 1 +
= Change in total load

16. STEADY STATE FREQUENCY CALCULATION: 2 GENERATORS
From
Consider the frequency at steady state,
But, , ,and
Then,

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

18. From power flow equation,
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

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.

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. 2’ 2 1 +
= Change in load 2