UNIT 2

FREQUENCY CONTROL AND AUTOMATIC GENERATION CONTROL

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.

AUTOMATIC GENERATION CONTROL

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).

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!

3 COMPONENTS OF AGC

Primary control Immediate (automatic) action to sudden change of load.For example, reaction to frequency change.

Secondary controlTo bring tie-line flows to scheduled.Corrective actions are done by operators.

Economic dispatchMake sure that the units are scheduled in the most

economical way. This is function of primary and secondary control of AGC.

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

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

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

PP CM

1

Slope = -R

1MP 2MP1CP 2CP

MP

= =

1

2

0

AGC FOR SINGLE AREA

• System Modeling• Single Generator• Multi Generators, special case: two

generators

LFC FOR SINGLE AREA

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 toRestore system frequency, Restore tie-line capacities to the scheduled value, and,Make the areas absorb their own load.

This is called “secondary control”.

AGC FOR MULTIPLE AREA SYSTEM

AGC FOR 2-GENERATOR: BLOCK DIAGRAM

sGM 2

2

1

R

+-

sGP222MP

2LP

2CP212 PP

+

-

- 2s

-

sGM1

1

1

R

+-

sGP11

1MP1LP

1CP

121 PP

+

-

-

1s

+

-112T

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

AGC FOR 2-GENERATOR: STATIC SPEED-POWER CURVE

Load increases. Frequency drops. Steady state is reached

when frequency of both generators is the same.

1MP 2MP

MP

1

2

0

+ = Change in total load

1MP 2MP

STEADY STATE FREQUENCY CALCULATION:2 GENERATORS

• From• Consider the frequency at steady state,

• But, , ,and• Then,

iLiiiiLiiiiiMi PPDPPDMP ~~

linetieLM PPDP 1111

~

linetieLM PPDP 2222

~

21 1

1

1

RPM

22

1

RPM

2121

21

11~~RR

DD

PP LL

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

1121 BPACE

2212 BPACE

iB

iLii RDB

1

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

iP

n

kkiikkii BVVP

1

sin

n

kkiiki BP

1

n

kkiik

n

kkiiki TBP

11

ikT

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.

CiP

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.

1MP 2MP

MP

1

2

0

+ = Change in load 2

1MP 2MP

2’

1MP 2MP