governor modes of operation

8/3/2019 Governor Modes of Operation

1/21

Governor Modes of Operation

The Governor is the primary speed/load controller of the turbine. Turbine

governor is the one which controls the fuel/steam input to the turbine thus controlling

the speed and power output from the turbine. The turbine governor usually has

different modes of operation which can be selected depending on the operating

conditions of the generator. The response of the governor during system disturbance,

both small signal which is common during normal operating conditions and large

signal which is during a rare fault condition; depends on which mode of turbine

control is selected in the governor.

There are mainly three Governor Modes

a. Turbine Droop Control

b. Turbine Load Control

c. Turbine Isochronous Control

The turbine droop control is the basic turbine governor mode. In this mode the

speed reference is reduced with the increase in load of the turbine. The droop refers to

the change in speed expressed as a percentage of rated speed when the machine is

loaded from no load to full load. In this mode the speed is the reference to which the

governor takes action. The droop controlled is a proportional controller. The droop

control is used for both Islanding operation and also for operation with the gird.

The turbine load control is a modified droop control. In this mode the Load of the

turbine is the reference and the governor tries to maintain a constant load under all

operating conditions irrespective of the frequency changes. The turbine load control is

a either a PI or a PID controller depending on how it is configured in the governorcontrol system. The Turbine load control is used only for operation with the grid and

not for operation in independent island condition.

The Turbine Isochronous control is the frequency control. In this mode the

frequency of the turbine is the set point and the governor tries to maintain a constant

frequency under all operating conditions irrespective of load changes. The Turbine

isochronous control is either a PI or a PID controller depending on how it is

configured in the governor control system. The turbine isochronous control is used

only for operation in island condition and not for operation with the grid.


2/21

Turbine Droop control Mode

When the Turbine is put in droop control mode, the speed reference is

decreased with increase in the load of the machine. The droop in effect refers to the

change (reduction) in frequency when the machine is loaded from no load to full load.

The droop is expressed as a percentage of the change in frequency from no load to fullload to the turbine rated frequency.

For example, if the turbine is rated at 3000 rpm, and the machine speed

reduces from 3000 rpm to 2880 rpm when it is loaded from full load to no load, then

the droop % is given by

Droop % (Turbine) = Rated No load speed Full load speed / Rated No load speed

= 3000 2880 / 3000

= 4%

Thus the above machine has 4% droop. In frequency terms, if the Generator

connected to the machine is rated at 50 Hz, the generator output frequency would

have reduced to 48 Hz when it was loaded to full load. Thus the generator droop is

given by

Droop % (Generator) = Rated No load speed Full load speed / Rated No load speed

= 50 48 / 50

= 4%

The Figure shows the Droop characteristics of the Turbine in the given example.


3/21

The Turbine Droop Mode can be used during both parallel operation with the

grid and also during independent island operation. It is a preferred operational tool in

gird level as it yields well to multi machine operation. In small islands which do not

go for isochronous control, droop gives stable operation.

Turbine Droop Control in Independent OperationWhen a Turbine is put in droop control in a independent island the turbine the

frequency of the turbine is now dependent on the section load.

a. Operation when the system load changes -

In the figure, it can be seen that the initial condition, the turbine was feeding a

load L with frequency F. In case the section load increases to L2 the frequency gets

reduced to f2. The change in frequency is in accordance with the droop of the

machine. It can be calculated by the formula

Change in Frequency = (L2-L) * Droop % / Full load of the turbine.

New frequency f2 = f Change in frequency

The reverse happens when the section load reduces. In this case as the load

gets reduced to L2 the frequency gets increased from f to f1. The new frequency will

be in accordance with the droop characteristics of the machine. It can be calculated by

the formula

Change in Frequency = (L1-L) * Droop % / Full load of the turbine.

New frequency f1 = f + Change in frequency


4/21

b. Operation when the manual raise/lower command is given -

In an independent operation, the turbine frequency is the output frequency,

and in case we give a manual raise lower command the frequency of the turbine raises

and lowers.

Initially the machine was feeding to a section load L with frequency f, now let

the sped reference in this case be R. If the section load is a constant and a manual

speed increase command is given in the governor, it shifts the speed reference to R2.This increased speed reference means the turbine frequency goes up from f to f2 as

shown in the figure.

In case a manual speed lower command is given to the governor, it shifts the

reference to R1. This reduced speed reference causes the turbine frequency to go

down from f to f1 as shown in the figure.

c. Operation when More than one machine is put in droop in the same island -

In case more than one machine is present in the island and all the machines are

put in droop, the turbine governor behaviour is different. Let us assume a small island

with two machines put in droop mode. In case there are more than two machines putin droop then the load is shared by the turbines depending on the droop percentage

and also on the droop reference on which the turbines are present.

A turbine with a greater droop reference will take up a greater load and vice

versa. In the same manner the machine with the lowest droop will take up more load

than the machine with a greater droop. In practice all machines put in droop mode will

have a same droop so that in case of a system disturbance they can take up load or

reject load in the same manner.


5/21

The figure shows how two machines with same droop and same speedreference react to the load changes. The initial load of the machines is L1 and L2,

were L1+L2 forms the total system load. In case of an increase in the section load,

both machines take up the load as shown in the figure. Now the system frequency

reduces to f and the load by each machine is L1 and L2. The reverse happens in

case there is a reduction in the section load, the two machines uniformly shed the load

and the system frequency rises to a new steady state point. The basic concept is both

the machine when paralled to one another operate on the same frequency.

The figure shows how the machine operates in case when a manual speed raise

is given to only one of the machine. A manual speed rise in generator 2 increases the

reference to a new point as shown in the figure. When the speed reference of

generator 2 raises from R1 to R2, the system frequency also rises from f to f. In the

initial condition, generator 1 was supplying load L1 and generator 2, load L2 with

system frequency f. When the reference of generator 2 is raised, it takes up more load,

and its load changes to L2. As the section load remains the same the, generator

reduces its load to L1. The reverse happens in case a manual speed lower command

is given. In that case the net system frequency drops, generator on which the speedlower command is given reduces its load and the other generator increases the load.


6/21

d. Operation of Two or more machine with unequal droop in the same island -

The figure shows how two machines with different droops behave when put in

parallel to one another in an island condition. From the figure it can be seen that

generator 1 has a much higher droop than generator, but both have the same reference.

In the initial condition, at frequency F, we see that the generator with the smaller

droop is taking up more load. If generator 1 load is L1 and generator 2 load is L2, wehave L2 > L1. Now in case of a section load increase it is seen that the machine with

the smaller droop is taking more load. At a increased load and reduced system

frequency f, the generator 1 is supplying L1 and generator 2 is supplying L2, but it

is seen that L2- L2 > L1-L1. Thus the machine with a smaller droop will take up

more load in case of a section load rise and will shed more load in case of a section

load reduction.

This is however an unwanted condition, it is desirable that all machine which

are put in droop and operating in an island have the same droop. This will help in

uniform rise and fall in the loading pattern and help in maintaining a stable operation.


7/21

Turbine Droop Control in operation with the Grid

A generator when paralled to the grid has no control over its frequency, the

frequency of the gird in the machine frequency for all practical steady state

considerations.

The figure shows how the machine when parallel to the grid behaves to grid

frequency changes. In the initial condition the generator is supplying a load L at

frequency f. when the grid frequency increases to f1, the generator loading reduces to

L1 as shown in figure. The reverse happens in case of a lowering of grid frequency,when the grid frequency reduces to f2, the generator loading rises to L2 and shown in

the figure. Thus the loading and unloading of the machine depends on the grid

frequency and the droop % of the controller.


8/21

The figure shows how a machine will react when a manual speed raise or

lower is given when the machine is paralled to the grid. In the initial condition, the

machine is supplying a load L with frequency f and a speed reference R. when a

manual speed raise is given; the speed reference is raised to R2, and for a unchanged

system frequency, the load supplied by the generator increases to L2. The reverse

happens for a manual speed lower command. Here the droop reference is shifted toR1 and for an unchanged system frequency the load reduces to L1 as shown in figure.


9/21

Implementation of Droop in GE Gas turbine systems

Droop is a proportional controller. In GE gasturbine systems, the FSRN block

is used to configure the droop.

Speed Control Fuel Stroke Reference

190:FSRNV4

100 %G3\FSRMAX17.3006 %G3\FSRMIN

22.2 %G3\FSKRN10 %G3\TLC_COMP

14.1 %/%G3\FSKRN2100.3 %G3\TNR

0 %G3\TNH0G3\L83ISOK

100 %G3\TNRI

1 %/%G3\FSKRN30 %G3\FSR

0.06 %/secG3\FSKRN5

0G3\L83SCI_CMD

0.5 %G3\FSKRN6

100 % G3\FSRN

0 % G3\FSRNI

0 G3\L60IR

0 G3\L60IL

x

+

+

-+

0

+ -

++

+

+

+-

x

+- +

FSRN

MIN

-1

Z

x

FSRMAX

A > B

A

B

A < -BA

B

CLAMP

max

min

FSRMAX

FSRMIN

FSKRN1

TLC_CMP

FSKRN2

TNR

TNH

LISOK

TNRI

FSKRN3

FSR

FSKRN5

LI_CMD

FSKRN6

L60IL

L60IR

FSRNI

FSRN

The Block shown above is taken from a GE gas turbine application code. In the droop

mode of operation, the FSRN , which is the fuel flow reference to the turbine is given

by the formula

FSRN = FSKRN1 + (TNR-TNH)*FSKRN2

Where

FSRN furl stroke refernce

TNR Turbine speed/droop reference

TNH turbine speed

FSKRN1 no load fuel reference

FSKRN2 Droop gain for fuel

The control equation can be redrawn as the control system block as shown in the

figure , as seen the droop mode is a P (proportional) controller. In the figure , TNR is

the setpoint ,or it is the droop reference for turbine , The variation of TNR is doneusing the TNRV1 block.


10/21

Speed/ Load Setpoint

770:_COMMENT

780:_MOVE_B

SRC1TRUE

ENABLE1TRUE

DEST 1 G3\L83TNROP

800:TNRV1

0G3\L83HOST

0FALSE113.5 %G3\TNKHOST

00

107 %G3\TNKR3

95 %

G3\TNKR4 1G3\L83TNROP

100 %G3\TNKR5

0G3\L70L

0G3\L70R

0FALSE

0FALSE

0G3\L83JD2

0G3\L83JD3

0G3\L83JD4

0G3\L83JD50FALSE

0FALSE

0G3\L83JD8

0FALSE

0G3\L83JD10

0FALSE

0 %/secG3\TNKR1

100.3 %G3\TNKR2

100.3 %G3\TNKR7

0G3\L83PRES1

0G3\L83PRES2

0 G3\L33CDMN

0 G3\L33CDMX

100.3 % G3\TNR

A=BA

B

A=BA

B

CLAMP

max

min

+

+

-1

Z

-+

L83PRS2

L83PRS1

TNKR7

TNKR2

TNKR1_0

L83JD11

L83JD10

L83JD9

L83JD8

L83JD7

L83JD6

L83JD5

L83JD4

L83JD3

L83JD2

L83JD1

L83JD0

L70R

L70L

TNKR5

LTNROP

TNKR4

TNKR3

TNKLOST

TNKHOST

L83LOST

L83HOST

TNR

L33CDMX

L33CDMN


11/21

The control equation for TNR can be writtern as

TNR = TNR * Z^-1 + (L83JD#) ( L70R L70L)

In the time domain it can be writtern as

TNR(t) = TNR(t-1) + (L83JD#) ( L70R L70L)Where

TNR Speed/Droop reference

L83JD# - Load rate of the turbine

L70R Manual Raise command

L70L Manual Lower command

This is pure integration using backward integration. As the auto load rate is

already specified, it cannot be taken to be a controller as such as the response is fixed

and not dependent on the error value. the L70R and L70l can be triggered by either

the manual raise command , the auto synch command , the temperature limit

command or more importantly the MW control block which is what is used by thepreselect load controller.

How to set the droop% in the controller

Note - this is a sample calculation only, some typical values here, and it may not be

what is present at your site.

Let us assume that the droop percentage needed to be set is 4 %. This means

that at full load of the machine the difference between the TNR and the TNH needs to

be 4%. so in the above equation we can fix

TNR - TNH = 4

The FSNL FSR is already known from chara graphs of the machine or by a

field test at site. Let us say that it is 20%.

The maximum base load FSR value is also calculated from graphs in

accordance to the site conditions and in later stages from the base load test of the

machine. Let us say that this is 75%. There is usually some amount of changes to this

constant during the commissioning.

So from the above it can be seen that at base load conditions

75 = 4*FSRKN2 + 20

FSRKN2 = 13.75%

There is usually some amount of changes to this constant during the

commissioning to reflect the field conditions.


12/21

Turbine Load control Mode (Preselect Mode in GE Terminology)

When the turbine is put in the load control mode, the reference is the generator

power output and the governor tries to maintain a constant power output from the

generator irrespective of the frequency changes in the gird. Turbine load control mode

is used only when parallel to the grid and a theoretical Load controller would have agraph like figure

In the load control mode the governor tries to maintain a constant power

according to the setpoint in the whole range of frequency operation. Fl and Fh

indicates the lowest and highest operational frequency, thus making Fh-Fl the

operational frequency range of the generator. In the initial condition, the machine is

supplying a power L at frequency f. In case the gird frequency changes in the

operational frequency range the power output remains the same. When the set point is

raised to L1, the machine loads to load L1 and when the set point is reduced to load

L2, the machine unloads to load L2 as shown in the figure.

The Load control mode is an extension of the droop control. When you put the

machine in droop control and parallel it to the grid the machine load is dependent on

the frequency of the gird as shown in figure


13/21

The figure shows how the droop reference is varied by the turbine governor to

maintain a steady power output in the generator.

In the initial condition let the generator was supplying a load L with frequency

f. The set point of the machine is L. Now if there is a reduction in frequency to f1 , the

load point shifts from point a to point b such that the power output is now b. Now thecontroller reduces the droop reference from R to R1 so that the new frequency f1 the

power output remains the same at L. thus with the lowering of the droop speed

reference to R1 the operating point is shifted to c as shown in the figure.

If the frequency had risen to f2 , the new operating point of the generator

would have been point d such that the power output from the generator would have

reduced to L2. Now the governor would raise the droop reference to R2 such that for

the frequency f2, the power output remains the same at L, thus shifting the point of

operation to point e.

Why Load control should not be used for a independent generator in an island

The load control tries to maintain a fixed power output from the generator. In

case of independent operation the section load varies a lot depending on the nature of

the process load. If a fixed setpoint is given to the generator and the section load

rises , the governor tries to reduce the load and decreases the droop reference.

However as the section load remains the same the governor goes on reducing the

droop reference such that the machine frequency goes on reducing till the machine

trips in under frequency. If the section load had reduced, the reverse would have

happened, in order to increase the load the governor would go on increasing the droop

reference and thus the speed till the machine trips on overspeed. Thus from the above

it is clear that load control is not suitable for independent operation.


14/21

When can a Machine be put in load control in an independent island

A generator can be put in load control mode in an independent island if there

is at least one machine in the same section which can crater to the changes in the

section load. Let us consider the following example.

Let us suppose that there are two generators, gen-1 and Gen-2 with equal

droop and equal droop reference feeding to a common section load as shown in the

figure.

In the Initial Condition Generator 1 is at point a feeding a load L1 and

generator L2 at point a feeding a load L2 such that L1+L2 is equal to the total section

load in the independent island. Now let there be a increase in the section load such

that both the generators take up the load with the system frequency reducing to f.Now the new point of operation is b for generator 1 with load L1 and b for generator

2 with load L2. Now if the generator 1 is put in load control mode and given a

setpoint L1 , the governor tries to reduce the load by reducing the droop reference ,

now the overall system frequency reduces as the generator 1 sheds the load on its way

to the new reference R. The load shed by generator 1 is taken up by generator 2

which is in droop mode , finally the generators settle at a new frequency f . Here

generator 1 is in a new operating point with Load L1 and generator 2 with load L2

such that L2 L2 is equal to (L2-L2)+(L1-L1). Thus the generator 2 takes up the

entire increase in the section load. Thus as long as there is a machine put in droop

mode which can take up the load changes Load control will offer a stable operation in

independent island operation.


15/21

Implementation of Load control in GE Gas turbine Systems

The Turbine Load control is implemented in mainly two blocks

Preselcted Load Setpoint

290:_COMMENT

300:MANSET3

29.4 MWG3\K90PSMX

2 MWG3\K90PSMN0FALSE

0.1 Meg/sG3\K90PSR0.1 Meg/sG3\K90PSR

2 MWG3\90PSEL_CMD

0FALSE0

0FALSE0FALSE0FALSE

0 G3\L33PSMX

1 G3\L33PSMN

2 MW G3\90PSEL

A=BA

B

A=BA

B

MED

-1

Z

x

-

+

-1

RATE

CLAMP

+

+

rate

MEDMAX_LIMIT

MIN_LIMIT

dt

+

+

+

-rate

CDM

-1

Z

MAXLMT

MINLMT

RT2SEL

RATE2

RATE1

CMD

LPRESET

VPRESET

RAISE

LOWER

RL_ENA

AT_MAX

AT_MIN

REF_OUT

Megawatt Load Control

310:_COMMENT

320:L90LV2

100 %G3\FSRN

100 %G3\FSRT

3 %G3\LK90DB3

6 %G3\LK90DB4

0G3\L83RMAX

0G3\L83PS29.4 MWG3\LK90MAX

2 MWG3\90PSEL

2.6 MWG3\LK90SPIN

0.0178608 MWG3\DWATT

0.13 MWG3\LK90DB1

0.33 MWG3\LK90DB2

1 G3\L90LR

0 G3\L90LL

-+

A > BA

B

A

B

inhibit raise

lower

A

B

|A -B|A

B

raise

A

B

A

B

db1

db2

db2 db1 raiseInhibit

raise

L90LR

L90LL

lower

"1

"

A > B

A > B

A > B

A > B

L90LR

L90LL

K90DB2

K90DB1

LOAD

K90SPIN

PRESEL

K90MAX

L83PS

L83RMAX

K90DB4

K90DB3

FSRT

FSRN


16/21

Physical implementation of the Load controller

In the above diagram let point a be the initial machine position. In this condition the

grid frequency is f and load is L . Now let us suppose that the grid frequency has

reduced to f1 , now the machine for a instant will shift from load L to load L1 , ie tooperating point b. The load controller now comes into action and reduces the

reference from R to R1 , such that the initial load condition is attained , with the dead

band gap of L+ub & L-lb. Now the new machine operating point is c , with frequency

f1 and load L with a new reduced reference R1.

The reverse happens in case there is a increase in the grid frequency , now the

operating point shifts to point d from the initial position a. Now the load controller

comes into action and shifts the reference to R2 such that the initial load condition is

obtained. Now the new machine operating point is e , with frequency f2 and load L

with a increased reference R2.


17/21

Turbine Isochronous control ModeWhen a Turbine is put in isochronous control Mode, the reference to the

controller is the frequency of the generator and the controller tries to maintain a

constant frequency irrespective of load conditions. The Turbine isochronous mode is

used only on independent island conditions and should not be used when in parallel to

the grid. The theoretical isochronous controller would be

In the isochronous control mode the governor tries to maintain a constant

frequency according to the setpoint in the whole range of frequency operation. Fl and

Fh indicates the lowest and highest operational frequency, thus making Fh-Fl the

operational frequency range of the generator. In the initial condition, the machine issupplying a power L at frequency f. In case the section load changes the frequency of

the machine remains the same. When the set point is raised to f , the machine

accelerates to frequency f and when the set point is reduced f, the machine

decelerates to frequency f as shown in the figure.

The Isochronous control mode can also be taken as a extension of the droop

control. When you put the machine in droop control in an independent island the

frequency of the machine is dependent on the section load as shown in fig


18/21

The figure shows an implementation of the isochronous control. The figure

shows how the droop reference is varied to keep the output frequency constant. Let

the machine have a frequency setpoint of f.

In the initial condition the generator is at operation point a supplying as load L

with frequency f. Now if the section load increases to L2 the frequency drops down to

f2 with the new operating point b. Now the isochronous controller sensing a reduction

in speed increases the speed reference to R2 such that for the same operating load L2the frequency is f. Thus the new operating point of the machine is c, where the

machine is supplying to a increased load L2 with the same frequency f.

In case there is a reduction in the load to L1, the frequency rises to f1 and the

machine moves to the operating point d. Now the isochronous controller reduces

droop reference to R1 such that the frequency recues to the initial operating frequency

f. Now the new machine operating point is e as shown in the figure. Thus by

increasing and reducing the droop reference a constant frequency of f is maintained

even with changing section loads.

Why isochronous controller should not be turned on when the machine is with

the gridWhen a machine which is in parallel to the grid, it has no control over the

frequency, the grid frequency is the machine frequency. Let us suppose that the

machine is put in iso mode and given a particular frequency setpoint. If this setpoint is

higher than the grid frequency, the controller will go on increasing the droop

reference, but the machine speed will not rise, instead the machine power output rises

and goes on rising till it reaches it maximum load. In case the given frequency

setpoint is lower than the grid frequency , the controller will go on reducing the droop

reference thus unloading the machine , it will go on unloading till the machine trips on

reverse power. Thus depending on the frequency setpoint and the actual gird

frequency the machine will either load itself to base load or unload itself to reverse

power trip. Thus a machine in parallel to the grid must never be put in isochronous

controller mode.


19/21

Implementation of Isochronous controller in GE Gas turbines

The Isochronous controller is a PI controller and it is predominantly implemented in

the FSRN block.

Speed Control Fuel Stroke Reference

190:FSRNV4

100 %G3\FSRMAX17.3006 %G3\FSRMIN

22.2 %G3\FSKRN10 %G3\TLC_COMP

14.1 %/%G3\FSKRN2100.3 %G3\TNR

0 %G3\TNH0G3\L83ISOK

100 %G3\TNRI1 %/%G3\FSKRN3

0 %G3\FSR0.06 %/secG3\FSKRN5

0G3\L83SCI_CMD

0.5 %G3\FSKRN6

100 % G3\FSRN

0 % G3\FSRNI

0 G3\L60IR

0 G3\L60IL

x

+

+

-+

0

+ -

++

+

++

-x

+- +

FSRN

MIN

-1

Z

x

FSRMAX

A > B

A

B

A < -BA

B

CLAMP

max

min

FSRMAX

FSRMIN

FSKRN1

TLC_CMP

FSKRN2

TNR

TNH

LISOK

TNRI

FSKRN3

FSR

FSKRN5

LI_CMD

FSKRN6

L60IL

L60IR

FSRNI

FSRN

The isochronous mode algorithm is explained below

a. once you put the iso mode on in a independent machine , the LISOK signal goes

high.

the following calculation takes place

FSRNI(t) = FSRNI(t-1) + (TNRI -TNH(t))*FSRKN3

(as you can see this is a form of the velocity PI algorithm , and this were the PI is

actually implemented in the controller)

b. After the calculation the following check is done.

If (mod (FSRNI (t)) > FSRKN6)The respective l60ir ( iso setpoint raise) or the l60il (iso setpoint lower)

goes high. The LISOK signal goes low thus cutting off the TNRI signal from the FSR

block. The L60IR OR the L60IL increases/decreases the TNR, with the help of the

L70R/L70L logic, thus raising or lowering the droop reference.

ElseThe FSRNI (t) is added to the FSR to get the new FSR value.

FSR(t) = (TNR - TNH(t))*FSRKN2 + FSRKN1 + FSRNI(t-1) +

(TNRI -TNH(t))*FSRKN3

In the above equation, the (TNR - TNH (t)) is no longer a error signal because,

"TNR is not a setpoint any more". When the iso mode is selected, the TNR raise and

lower is inhibited from any other source.


20/21

Only the L60IR and the L60IL have the permissive to change the TNR. as this

particular code is executed only when L60IR AND L60IL is both zero , TNR is

nothing but a constant in the equation. Now rewriting the equation

FSR(t) = FSRNI (t-1 ) + ( (fsrkn3 * TNRI) - ( TNH(t) * (FSRKN3+FSRKN2)) ) +

FSRKN1 + (TNR*FSRKN2)

Here now the FSR and the FSRNI now represent the same common output canbe renamed as FSRI

FSRI(t) = FSRI (t-1 ) + ( (fsrkn3 * TNRI) - ( TNH(t) * (FSRKN3+FSRKN2)) ) +

FSRKN1 + (TNR*FSRKN2)

Here now the equation ((fsrkn3 * TNRI) - (TNH (t) * (FSRKN3+FSRKN2)) )

is the error signal E(t)

FSRI(t) = FSRI (t-1 ) + E(t) + + FSRKN1 + (TNR*FSRKN2)

The last equation is the final iso mode calculation equation. if you will see , it

is analogous to the velocity form of the pi algorithm.

Physical implementation of the isochronous controller

Now that we have looked into how the FSR is calculated in the iso algorithm ,

let us see the controller inaction during actual conditions.


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a. For small load variations

The small load variations are taken care by the FSRNI block. Let us suppose

that the machine is running in point a where the load (MW) is l1 and the frequency f1.

In this condition it is assumed that the machine is already in iso mode and reached a

steady state such that the TNRI and the TNH are now equal(near equal) , the now let

us suppose that there is a small load increase. The TNH value of the machine will godown as per the droop reference R1. Now FSRNI is calculated and it will be found

that FSRNI is less than the deadband limit. The value of FSRNI will be positive and it

will be added to the FSR block. This additional FSR will help in increasing the speed

of the machine to its previous value. The reverse happens if the load goes down by a

small amount. The speed of the machine will rise; the FSRNI will be negative and

reduce the actual FSR thus reducing the machine speed back to its previous value.

b. for large load variations

For large load variations the droop characteristics is changed before the TNRI

comes into play. Let us suppose that the machine is in the initial condition Point a.

now for a sudden load increase from l1 to l2, the frequency changes from f1 to f2.Now the FSRNI is calculated and it will be found that it is more than the deadband

FSRKN6. Now the L60IR command is issued which inturn increases the TNR value.

The rise in the TNR value will raise the droop reference and the speed of the machine

as already discussed in the droop mode study. Thus when the machine reaches the

point b, the droop characteristics is raised from Reference R1 to R2. This goes on till

the TNH reaches the TNRI and the calculated FSRNI is below the deadband limit.

From that point onwards the isochronous changes to the PI mode and FSRNI takes

care of the rest.

Thus from the above it can be seen that, the for small load variations the

FSRNI is responsible for maintaining constant frequency. In this mode, there is no

droop reference change. For large variations, the FSR reference itself is changed till

the speed becomes more or less equal to the setpoint value. Only then does the FSRNI

come into play.

governor modes of operation

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