1. Automatic Generation Control (AGC)
by Dr. Deependra Kumar Jha ME (Power Systems), PhD (Electric Power System Engineering) Professor, Department of Electrical Engineering School of Engineering & Technology, Galgotias University

2. Purpose and Overview of AGC Automatic Generation Control (AGC)
Outline
Purpose and Overview of AGC
Automatic Generation Control (AGC)
System modeling: control block diagram
AGC for single generator
AGC for 2 generators
AGC for multi generators
Area Control Error (ACE)

3. 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).

4. 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!

5. 3 Components of AGC Primary control Secondary 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 presentation covers only primary and secondary control of AGC.

6. AGC for Single Area System Modeling Single Generator
Multi Generators, special case: two generators

7. System Modeling: Turbine-Governor Model
Small signal analysis model, relating mechanical power to the control power and the generator speed.
Where = Small change in control setting power
= Small change in governor synchronous speed
= Small change in mechanical output power
= Regulation constant
= Transfer function relating mechanical power to control signals + -

8. Speed-Power Relationship
From synchronous turbine-governor: small signal analysis model,
At steady state (s → 0, → 1), we have

9. Static Speed-Power Curve
Slope = -R = 1 2
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.

10. Turbine and Generator Load Model
Turbine Model
Generator load model - +

11. AGC for Single Generator
closed loop power control system as below. + -

12. AGC for Multi Generators
Consider effect of
power flows in transmission lines, and
loads at each bus
to mechanical power of each generator.
This analysis assumes that every bus is a generator bus.

13. Power Balance Equation at Each Bus
Where
= Generator i power
= Load power at bus I
= Power flow from bus i
Consider small changes, G1 G2 G3

14. Load Power Equation ( ) Assume that
Where
= Small change of load input
= Small change of load power
= Small change of voltage angle
Substitute in power balance equation,
We have

15. Mechanical Power of Each Generator ( )
Linearized equation relating mechanical power to generator power and generator speed.
Where
= Small change in mechanical power of generator i
= Small change in electric power of generator i
= small change in internal voltage angle of generator i
From,
We have

16. Generator Block Diagram
From,
We can write
where + -

17. AGC for Multi Generators: Block Diagram
Change in tie-line power flow + -

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

19. Tie-Line Block Diagram
From and
We have, + -

20. AGC for 2-Generator: Block Diagram+ - -1

21. 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

22. Steady State Frequency Calculation: 2 generators
From
Consider the frequency at steady state,

23. Simplified Control Model Area Control Error (ACE)
AGC for Multi Areas
Simplified Control Model
Area Control Error (ACE)

24. Simplified Control Model
Generators are grouped into control areas.
Consider
An area as one generator in single area, and,
Tie-lines between areas as transmission lines connecting buses in single area.
We can apply the same analysis to multi-area!!
However, we have to come up with frequency- power characteristics of each area.
Actual application of this model is for power pool operation.

25. Power Pool Operation
Power pool is an interconnection of the power systems of individual utilities.
Each company operates independently, BUT,
They have to maintain
contractual agreement about power exchange of different utilities, and,
same system frequency.
Basic rules
Maintain scheduled tie-line capacities.
Each area absorbs its own load changes.

26. 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”.

27. 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

28. 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.

29. AGC for 2-Area with Tie-line Bias Control: Block Diagram+ - -1