governor and droop basics_final

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11/10/20

Awareness Program for Operation Staff

Understanding of

Generator Governor and Droop

Kotmale Power Station

January , 2010

By:

Eng., Nizam Salih

Electrical Engineer (Operations)

Session 2

Contents

1. What is a Governor

2. Purposes of a Speed Governor

3. Isochronous Operation

4. What is Droop

5. Percentage Speed Droop

6. Parallel Operation on a Large (Infinite) Bus

7. During Start and Normal Operation

1. What is a Governor

A governor senses the speed (or load) of a prime mover and controls the fuel to

the prime mover to maintain its speed (or load) at a desired level.

Prime mover - engine, turbine, or other source of power

Example:

If you’ve ever driven a car, you’ve functioned as a governor when you control

the car’s speed under varying driving conditions.The driver (governor) adjusts the fuel to maintain a desired speed. If the speed limit is 100

(this is the desired speed), you check the speedometer (the car’s actual speed). If actual

speed and desired speed are the same, you hold the throttle steady. If not equal, you

increase or decrease throttle position to make the desired speed and the actual speed thesame.

As the car starts uphill, the load increases and actual speed decreases. The driver notes

that actual speed is less than desired speed and moves the throttle to increase speed back

to the desired speed at the increased load.As the car goes downhill, the load decreases and actual speed increases. The driver notes

that actual speed is greater than desired speed and decrease the throttle to return to thedesired speed with the decreased load.

The Driver is the Governor

2. Purposes of a Speed Governor

The speed governor has five purposes:

1. To allow the synchronization the generator.

2. To act so that the turbine-generator set supplies the customerMW loadon demand.

3. To change the load on the generator.

4. To carry outfrequencycorrection (if it is necessary).

5. To control the turbine-generator duringdisturbance conditions.

Diagram of the controlled and the governing systems



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Structure of the Governor Kotmale PS

3. IsochronousLet us look at the simple case of a governor with zero droop. This is

termed an Isochronous governor because the speed is maintainedconstant regardless of generator loading.

4. What is Droop ?

Droop is expressed as a percentage of the original speed setting from

no load to full load. In other words the term droop, is the decrease of

generator shaft speed as the load (required torque output) increases

If, instead of a decrease in speed setting an increase takes place, the

governor is showing negative droop. Negative droop will cause instability

in a governor.

Without some form of droop, engine-speed

control would be unstable in most cases

Why is Droop Necessary

In a system without droop, a load increase will cause the engine to slow down. The governor

will respond by increasing the fuel until the engine speed has returned to the original speed.

Due to the combined properties of inertia and power lag, the engine speed will continue to

increase beyond the original speed setting, causing an overshoot in speed. The governor

again will respond to decrease speed to correct for the overshoot. It will over-correct the

speed in the other direction causing an undershoot. This overcorrection of speed in both

directions (instability) will amplify until the engine trips out on over speed.

5. Speed Regulation

The purpose of the speed regulation block is to provide adjustable

gain factor in permanent feedback loop.

According to IEC Test Code (Publication 308) this factor is referred

to as:

bg – permanent speed droop - if the feedback signal is taken

from the gate position.

ep – permanent speed regulation - if the feedback signal is taken

from the power transducer.



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Speed Droop

In operation, the governor opens/closes the water valves in proportion

of the speed dec rease/increase detected as compared to the total speed

change allowed from no-load to full-load. The governor feedback

control will continue to adjust the valves until the speed error is zero.

The allowable speed change is determined from the droop setting.

Proportional Band

Tmech

Telec.

n0 = shaft rated

speed (rpm)

When Telec. = Tmech , n0 = constant.

When Telec. > Tmech , n < n0

When Telec. < Tmech , n > n0

freq.

freq.

Analogy for understanding of Speed/Load Variation

T = torque

Case 1

Case 2

The Droop Curve

Droop is a straight line function, with a

certain speed reference for every fuel

position.

Normally a droop governor lowers the

speed reference , from no load to full

load.

Thus, a 2.4% droop governor with a

reference speed of 375 rpm at no load

would have a reference speed of 366

rpm at full load. ie frequency will

decrease from 50Hz to 48.8 Hz.

Example

Percentage Speed Droop

When a slope is added we obtain a characteristic similar to figure below.Five droop settings are shown all with the same slope. The slope is 4%

droop per Total Rated Load.

6. Parallel Operation on a Large (Infinite) Grid

On a larger system, the voltage and frequency will remain fairly constant

regardless of the loading of the unit. Hence the generator will pick up loadaccording to the arrows in the figure.

G1 G2 Freq.

Initial Load 350 350 50Hz

G1 G2 Freq.

Final Load 600 450 49.64

G1 has a less of a droop than G2

System load increases by 350MW, How the

Generators are loaded and the final frequency?



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50 MW50 MW

Initial state, G1 Initial state, G2

In this case the NL set point of G2 is raised and G1 is lowered to shift 50 MWof load from G1 onto G2. This action of changing the NL sett-point to change the

generator loading Occurs many times a day on hydraulic turbines supplying the grid.

AUTO mode in Freq and Power control

Ex. Draw the same for ep1 = 2.4% and ep2 = 8.9%

Discussion

Discuss Power / frequency variation when operating in

1. AUTO mode with speed regulation ep1

2. AUTO mode with speed regulation ep2

3. MAN mode with speed regulation ep1

4. MAN mode with speed regulation ep2

with load increase and decrease?

7. Start Sequence diagram

During Runup

The speed governor has to bring the turbine from turning gear (a few rpm) to

synchronous speed (ex. 375 rpm). B ecause of the small amount of water

(power) required, a separate high droop setting termed wide range speed

control is used. This allows a very accurate control of the valves as the machine

runs up from 0% to 90% rated speed with very little water input to the turbine.

Normal Operation

Once the turbine reaches about 90% rated speed the error signal produced

by the previous controller decreases and the running or narrow range speed

control takes over with a low droop setting. The droop can then handle the

large variations in steam valve operations from zero to full load.



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START

The actuator (gate) opens from 0 to pre-adjustedSTART 1 position. The Governor

is in manual control mode.

The Governor will switch over to AUTO mode at 90% rated speed of the turbine.Simultaneously the actuator (gate) is forced by the electronic gate limiter to

START 2 position (selected by a set point in the c ontrol cubicle).

Proper pre-adjustment of START 1 and START 2 gate position ensures that the

speed of the turbine quickly reaches 90% speed, but passes the last 10% slowly to

settle at the rated value.

ie. rapid start and synchronizing can be achieved.

Start Sequence – Kotmale PS

Kotmale Governor Parameter Settings

Parameter Units U1 U2 U3 Remarks

bgp1 9 9 9 No-Load

operationBgi1 1/Sec 6 6 6

Bgd1 (Sec) Sec 2 2 2

bgp2 9 9 9 System

OperationBgi2 (1/Sec) 1/Sec 8 8 8

Bgd2 (Sec) Sec 2 2 2

ep1 % 2.4 2.4 2.4 Freq. control

ep2 % 8.9 8.9 8.9 Power control

Gate Limiter % >100 >100 >100

Balance +4 +0.5 +3

Gate Start Opening % 15 11 13

Power Set Rate % / Sec 1 1.4 1.6

END ?

No, this is the beginning !!

governor and droop basics_final

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