Governor Modes Of Operation w5215

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 governor control 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.

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 full load 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 , 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.

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 Operation When 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

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 put in 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.

The figure shows how two machines with same droop and same speed reference 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 speed lower command is given reduces its load and the other generator increases the load.

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, we have 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.

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.

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 to R1 and for an unchanged system frequency the load reduces to L1 as shown in figure.

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 G3\FSRMAX

100 %f

G3\FSRMIN

17.3006 %

G3\FSKRN1 G3\TLC_COMP G3\FSKRN2 G3\TNR G3\TNH G3\L83ISOK

f

22.2 %f 0 %f 14.1 %/%f 100.3 %f 0 %f 0b

FSRMIN

+

FSKRN1 TLC_CMP FSKRN2 TNR TNH

G3\FSKRN3

1 %/%

+

100 %

G3\FSRN

0%

G3\FSRNI

+

+

x

-

+

0

LISOK

f

0 %f 0.06 %/secf

TNRI FSKRN3

-

x

+ +

-

FSR FSKRN5

x

+

-

MIN

+ Z

b

0

LI_CMD

A A>B B A A < -B

G3\FSKRN6

f

min

-1

G3\L83SCI_CMD

FSRN

+

+

100 %

G3\FSKRN5

max

FSRN f

G3\TNRI G3\FSR

CLAMP

FSRMAX

f

0.5 %

FSKRN6

FSRNI

f

FSRMAX

L60IR L60IL

b

0

b

0

G3\L60IR G3\L60IL

B

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 done using the TNRV1 block.

Speed/ Load Setpoint 770:_COMMENT 780:_MOVE_B TRUE TRUE

1 SRC 1 ENABLE

DEST 1

800:TNRV1 G3\L83TNROP

b

0 L83HOST b 0 L83LOST

G3\L83HOST FALSE G3\TNKHOST 0

f

f

107 %

TNKR3

G3\TNKR4

95 %

TNKR4

G3\L83TNROP

1

G3\L70L

f

b f

G3\L83JD4 FALSE

min

0b L83JD7 0b L83JD8

FALSE G3\L83JD8

0b L83JD9 0b L83JD10

FALSE G3\L83JD10 G3\TNKR1 G3\TNKR2

%f

100.3

TNKR1_0

-

+

0b L83JD11

0 %/secf

L33CDMX

b0

TNR

0b L83JD5 0b L83JD6

G3\L83JD5

b0

max

LTNROP

0b L83JD3 0b L83JD4

G3\L83JD3

L33CDMN

G3\L33CDMN G3\L33CDMX

CLAMP

0b L83JD1 0b L83JD2

G3\L83JD2

G3\L83PRES2

A=B

b

FALSE

G3\L83PRES1

B

A=B

0 L70R 0b L83JD0

FALSE

G3\TNKR7

A

100 % TNKR5 0b L70L

G3\L70R

FALSE

B

113.5 % TNKHOST f 0 TNKLOST

G3\TNKR3

G3\TNKR5

A

+ +

TNKR2

100.3 %f TNKR7 0b L83PRS1 0b L83PRS2

-1 Z

f 100.3

%

G3\TNR

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 the preselect 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.

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 a graph 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

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 the controller 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.

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.

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 G3\K90PSMX

29.4 MWf

G3\K90PSMN

2 MWf

A

MAXLMT

B

FALSE G3\K90PSR G3\K90PSR

A

MINLMT

0b RT2SEL 0.1 Meg/sf 0.1 Meg/sf

B rate

RATE2

2 MWf

A=B

+ -

CMD

REF_OUT

+ +

0

G3\L33PSMX

b

1

G3\L33PSMN

f

2 MW

G3\90PSEL

-1

MED

MIN_LIMIT

-

rate

dt

+

CDM

+ + -1

0b RAISE 0b LOWER

FALSE

b

Z

MAX_LIMIT

FALSE

AT_MIN

MED x

0b LPRESET 0f VPRESET

FALSE

AT_MAX

RATE CLAMP

RATE1 -1

G3\90PSEL_CMD

A=B

Z

0b RL_ENA

FALSE

Megawatt Load Control 310:_COMMENT 320:L90LV2

G3\FSRT

100 %f 100 %f %f

G3\LK90DB3

3

G3\LK90DB4

6 %f

G3\L83RMAX

0b

FSRN

-

+

G3\FSRN

inhibit raise

K90DB3

A A>B B

lower

K90DB4

A A>B B

raise

FSRT

L83RMAX

G3\LK90MAX

0b L83PS 29.4 MWf K90MAX

G3\90PSEL

2 MWf

PRESEL

A A>B B

G3\LK90SPIN

MWf

K90SPIN

A |A -B| B

G3\L83PS

2.6

f G3\DWATT 0.0178608 MW 0.13 MWf G3\LK90DB1

G3\LK90DB2

0.33 MWf

K90DB1

A A>B B

db1

A A>B B

db2

K90DB2

LOAD

"1 "

db2

db1

raise

Inhibit raise

L90LR

L90LL lower

L90LR

b1

G3\L90LR

L90LL

b0

G3\L90LL

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 to operating 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.

Turbine Isochronous control Mode When 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 is supplying 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

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 L2 the 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 grid When 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.

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 G3\FSRMAX

100 %f

G3\FSRMIN

17.3006 %

G3\FSKRN1 G3\TLC_COMP G3\FSKRN2

f

22.2 %f f

0% 14.1 %/%f

G3\TNR

100.3 %f

G3\TNH

0%

f

G3\L83ISOK

0b

G3\TNRI

100 %f

G3\FSKRN3

1 %/%

G3\FSKRN5

max

FSRMIN

+

FSKRN1 TLC_CMP FSKRN2 TNR TNH

+

100 %

G3\FSRN

0%

G3\FSRNI

+

+

x

-

+

0

LISOK FSRN

f

0 %f 0.06 %/secf

TNRI FSKRN3

-

x

+ +

FSR FSKRN5

x

-

+

-

MIN

+ Z

b

0

LI_CMD

A A>B B A A < -B

G3\FSKRN6

f

min

-1

G3\L83SCI_CMD

FSRN

+

+

G3\FSR

CLAMP

FSRMAX

f

0.5 %

FSKRN6

FSRNI

f

FSRMAX

L60IR L60IL

b

0

b

0

G3\L60IR G3\L60IL

B

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.

Else The 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.

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 can be 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.

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 go down 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.