Governing of Turbines

Governing of Turbines

The load on the generator keeps on fluctuating while the input power to the turbine is constant. Therefore, the speed of turbine will either increase or decrease with the decrease or increase of load respectively. This leads to fluctuation of speed of the turbine. This in turn will fluctuate the speed of generator which is not desirable because it would vary its frequency and voltage.

In order to run the generator at constant speed, the turbine speed is required to be maintained under variable load conditions. It is achieved by varying the discharge rate to the runner of the turbine according to the load on the turbine.

The regulation of the speed of the turbine within prescribed limits according to the load on the turbine is called the governing of turbines.

The functions of a turbine governor are :

1. It should control the speed of turbine with fluctuating load at synchronous speed of the generator.
2. It helps in starting and shutting down the turbine unit by opening and closing the nozzles of a pelton wheel and wicket gates in case of reaction turbines.

1. Governing System for Turbines of a Hydro-Electric Power Plant :

• The governing system of a turbine is based on a closed loop feed back control system. The block diagram for such a system is shown in Figure A.
• The basic elements and their functions are as follows :

Figure A

• The variable which is required to be controlled is called controlled variable. In this case turbine speed is the controlled variable which fluctuates according to the load on the Turbine. The first step in this closed loop control system is done by measuring element called feed back element, which is speed in this case. This information is fed back to the controller through a summing point or comparator.
• This comparator compares the present value of controlled value (i.e. speed) with the reference point. (it represents the speed at which turbine is required to run).
• On comparison between the present value of speed and reference point speed, if any difference between the two, an error signal is produced. The error signal is transmitted to the controller.
• The function of controller is to produce an output corresponding to the error and initiates an action so as to bring down the error to zero.
• The output of the controller is given to a final control element which is generally a heavy actuator. This actuator will actuate the mechanism to control the rate of discharge into the turbine.
• Thus it could be seen from the above discussion that the closed loop control system would function automatically in a loop till such time it reduces the error to zero and bring back the speed of the turbine to desired constant speed under all varying load conditions.

2. Governing Mechanism :

• Input to a turbine is proportional to the product of head, H and discharge rate, Q. Consider the case when load on turbine increases, the speed of turbine reduces since the input to the turbine, hence its output remains the same.
• In order to increase the speed of turbine to its constant value, it is necessary to increase its input according to the increase load which can be achieved either by increasing the head or discharge rate.
• Since the head on turbine cannot be changed, therefore the input to turbine can only be increased by increasing the discharge rate into the turbine.
• Again the discharge rate can be increased by either increasing the velocity of flow or the discharge area. The change in velocity will affect the velocity triangles.
• Thus the angle of relative velocity at inlet will differ from the inlet tip angle of blades causing shock and hydraulic losses.
• Hence, the rate of discharge in turbines is adjusted by changing the flow area in order to meet the varying load conditions with the help of governor mechanism. The governor mechanisms are now being discussed for different turbines in succeeding paragraphs.

3. Governing of Pelton Turbine :

The governing of Peiton turbine is done by oil pressure governors as shown in Figure B.

Figure B

The major components of the governor mechanism are :

1. Oil pump and oil sump : System uses oil in servomotor or relay cylinder since the force required to actuate the spear valve would be enormous. For this reason, the system requires an oil sump to store the oil and an oil pump to regulate the oil supply in the mechanism. Oil pump is a positive displacement type of pump like gear pump or axial piston pump. It’s function is to pressurise the oil.
2. Relay or control valve : Relay valve is a spool valve having 5 ports. It is also called as control valve or distributor valve. It receives the pressurised oil from the oil pump which is diverted towards the ports connected to pipe A or pipe B. Through these pipes the oil is transferred to corresponding sides of double acting servomotor cylinder. Simultaneously, the oil will be returned from the servomotor from the opposite pipe to the sump.
3. Servomotor or relay cylinder : It is a double acting cylinder which acts as hydraulic actuator. It receives oil from relay valve say through pipe A. The piston of the cylinder will be displaced towards left, thus forcing the oil through the pipe B into the relay valve and finally to oil sump. It will simultaneously more the spear valve to the left and increase the area of flow through the nozzle.
4. Spear valve : Spear valve controls the flow area of the nozzle. It is directly connected to the piston of relay cylinder.
5. Governor and linkages : A centrifugal governor is used as the measuring element of the closed loop control system. It is driven by the turbine shaft through bevel gears.

The sleeve of the governor is connected through linkages to relay valve. The movement of sleeve is transferred through the lever to more the piston rod of relay valve.

Working :

1. Figure B shows the position when the turbine is running at normal speed. Consider the case when the load on the generator increases, the speed of the generator and that turbine will decreases. Since the governor is driven by the turbine shaft, its speed will also decrease.
2. As a consequence, the fly-balls of the governor will move inwards due to reduced centrifugal force on the balls. As a result the sleeve of the governor will move downwards.
3. The downward motion of the sleeve will be transferred to the main lever through its fulcrum. It will cause the piston rod of the relay valve to move upwards and simultaneously the bell crank lever also moves upwards.
4. The upward motion of piston rod of control valve causes pressurised oil to flow through the pipe A to the relay cylinder and exerts a force on face A on the piston of servomotor. It moves the piston to the left, thus the spear rod with its valve will also move towards the left.
5. It will increase the nozzle area and the rate of flow of water to the turbine increases. Therefore, the input to turbine and consequently its speed increases, During this piston movement of servomotor to the left, oil held in the cylinder towards the face B is transferred through pipe B to the oil sump via the relay valve.
6. When the speed of turbine is adjusted to normal speed, the system would return to its original position Opposite will be action of the whole mechanism when the load on the generator decreases.
7. The increase in speed of turbine and generator will increase the speed of the governor. The balls flyout and the sleeve moves upwards.
8. It causes the piston rod of relay valve to move downwards, thus opening the valve towards pipe B. Pressurised oil flows into servomotor cylinder towards the face B and causes the spear to move towards right thus closing the nozzle area.
9. The reduced discharge to turbine runner reduces the input, hence its speed. When the speed attained by the turbine is its normal speed, the governing mechanism will return to its original position.

4. Governing Mechanism of a Francis Turbine :

• The basic mechanism for governing of various turbines remains the same as discussed above, the only difference being the method the control of discharge which varies from turbine to turbine.
• In case of pelton turbine, the discharge was controlled using the speed and spear nozzle in Francis turbine a different mechanism is used with modification in relay cylinder to control the discharge as shown Figure C.

Figure C

Mechanism has the following components to control the discharge.

1. Regulating ring :

• It is a circular ring having guide vanes pivoted at a point through the levers and links. 
• Therefore, when the regulator ring is rotated about its axis, all the guide vane would turn about their pivots.
• Due to turning of guide vanes the space between two consecutive guide vanes would chain. Space between guide vanes will increase in one direction of rotation of regulating ring and space will decrease in its opposite direction of rotation.

2. Regulating rod :

• It connects the regulating ring to relay cylinder (servomotor) through the various linkages as shown in Figure C. It can be seen by this mechanism, the linear motion of servomotor piston is converted into rotary motion of regulating ring.
• An alternate arrangement providing the rotary motion to regulating ring by using two servomotors is shown in Figure D.
• This mechanism gives better performance since the force required for operation of Francis turbine guide vanes is much more compared to Pelton turbine.

Figure D

5. Governing Mechanism for Kaplan Turbine :

• Speed control in Kaplan turbine is also achieved by varying the discharge by changing the guide vane angle as in case of Francis turbine with an additional feature.
• However, the governing achieved by changing the guide vane angles has a serious drawback. With the change in guide vane angle, the inlet velocity triangle is changes. It results into the change in direction of absolute velocity at inlet. Hence, the direction of relative velocity at inlet is also changed, thus the water will not enter the moving blades tangential to it.
• In other words, the tangential entry to runner blades can be achieved only at a particular speed of the turbine.
• Therefore, a separate servomotor is provided to operate the runner blades which are few in numbers. The arrangement is shown in Figure E.

Figure E

• In order to overcome this problem, the governing of Kaplan turbine is achieved by double regulation system which controls the movement of guide vanes as well as the moving vane.
• In this methods, the moving blades are pivoted on the hub. It carries a pair servomotors for controlling the movement of each moving blades as shown in Figure E.
• During the governing, the discharge is changed by changing the guide vane angles as discussed in case of Francis turbine, simultaneously the moving blade angles are also adjusted in such a way that for new position of guide vanes, the entry of water remains tangential to moving vanes over wide range of guide vane positions.
• Due to control of both guide and moving vanes, the Kaplan turbine gives better efficiency over wide range of loads as compared to Francis turbine.