SPEED REGULATION (GOVERNING) OF THE PELTON WHEEL
The governor uses either mechanical or electronic feedback to sense the speed of the turbine. Proportional or directional valves controlled by the governor operate cylinders that open and close wicket gates or needle valves to adjust the flow of water to the turbine in order to maintain a constant turbine speed. Hydroelectric turbines rotate at relatively low speeds compared to steam turbines, with larger hydroelectric turbines rotating at 35-75 rpm, and smaller ones as fast as 150 rpm. The large turbine diameter combined with the massive inertia of the water flowing through it makes precise control of rotational speed a critical concern. If governor proportional or directional valves do not respond instantly and accurately to fluctuating generator loads, there is a lagging behind of the wicket or needle valve position. This results in an oscillating condition where by the turbine is constantly speeding up and slowing down. This inefficient power production, although difficult to quantify, leads to loss in revenue for the utility. Furthermore, if this oscillation exceeds the maximum allowable frequency, then the turbine must be shut down, resulting in temporary loss of generating production. Also, in the event of a sudden loss of load, it is important that the governor act instantly to shut down the turbine to prevent a “runaway” speed condition. Runaway speed is the speed at which the turbine exceeds its designed maximum rotational speed. When this occurs it is possible for the turbine to disintegrate due to massive centrifugal forces. Usually, hydraulic turbines are coupled to electronic generators. These generators are required to run at constant speed irrespective of variations in the head and power output. When the load on the turbine changes, the speed may also change, (i.e., without load the speed increases and with over load, the speed decreases). Hence, the speed of the runner must be maintained constant to have a constant speed of generator. This is done by controlling the quantity of water flowing on the runner according to the load variations. This speed regulation is known as governing and it is usually done automatically by a governor. Main Functions of the Governing System • Control of the turbine start-up and shutdown sequences • Synchronization of the turbine with the grid • Control of the active power supplied by the generator to an interconnected network • Control of network frequency on an isolated electrical network • Protection of the unit against over speed in case of load rejection • Control of advanced sequences
Governing Mechanism for the Pelton wheel
A servomotor governor (also known as oil pressure governor) is shown in Figure. It consists of (l) a servomotor, (2) relay valve or control valve, (3) actuator (centrifugal governor), (4) oil sump, (5) oil pump and (6) oil supply pipes. Working
The centrifugal governor (actuator) is driven by the turbine shaft through a belt or gear. When the load on the generator reduces, the turbine speed increases. It causes the following actions to take place one after another. 1. Fly ball of the governor moves upward, 2. Sleeve moves upward, 3. Left hand end of main lever rises, 4. Bell crank lever moves down and simultaneously the piston of the control valve moves down in the cylinder.
The movement of bell crank lever brings the deflector in front of the jet. The deflector diverts a portion of the water jet away from the runner buckets. Thus, rapid closure of the nozzle opening is eliminated and at the same time the quantity of water striking the runner is reduced. The rapid closing of the nozzle increases the pressure of water which may result in water hammer problems. The downward movement of the piston in the control valve forces oil under pressure from the control valve to the left side of the piston in the servomotor. The piston of the servomotor moves to the right pushing the spear forward. The oil in the right side is returned to the oil sump. The forward motion of the spear reduces the opening of the nozzle. Consequently, the rate of flow is decreased and normal speed is restored. Once the normal turbine speed is restored, the main lever returns to its initial position. The deflector is brought to its original position by means of cam arrangement. When the load on the generator increases, the turbine speed decreases. This causes the following actions to take place one after another. 1. Fly balls move downward, 2. Sleeve moves downward, 3. Left hand end of the main lever lowers down, 4. The piston in the control valve moves upward in the cy binder, 5. Oil under pressure is forced from the control valve to the right side of the piston in the servomotor, 6. Servomotor piston moves to the left pushing the oil in the less side to the oil sump. Simultaneously, the spear moves backward. The backward movement of the spear increases the opening of the nozzle outlet. Thus, a large quantity of water strikes the runner and the normal speed of the turbine is restored. PERFORMANCE CURVES FOR TURBINES
Turbines are always designed and fabricated for a given set of specifications containing variables, like speed, power output, head and discharge. And the efficiency of the unit is maximum when it operates under designed conditions. But in practice, it operates under varying conditions because the level and quantity of water in a storage reservoir does not remain constant throughout the year, and also the load on the turbine is variable. Thus, it is essential that the exact behavior of the unit under varying working conditions is predetermined. These are obtained by manufacturers by conducting experiments on models in a laboratory and by doing field tests on the site. The physical parameters controlling the performance of a turbine are speed N, power output P, head H, discharge Q, the position of gate opening (nozzle opening or guide blade opening) G, and the efficiency of the turbine. The behaviour of the units are represented by curves called turbine characteristics. The characteristics of turbines are obtained under three different categories-' (a) Main characteristics (Head constant) (b) Operating characteristics (Speed constant) (c) Muschel curves (Efficiency constant)
A. Main characteristics: In this case, the head is kept constant and the speed is varied by varying the load on the turbine. The governing mechanism is disconnected from the system so that the experiments are performed at constant gate openings. Speed vs discharge curve: For a given area of flow, the discharge depends upon H 1/2 for a Pelton turbine. Since H is constant, the peripheral speed of the turbine is constant and therefore, the discharge is independent of speed. Speed vs power curve: Power is proportional to angular speed. When N is zero, the angular speed is zero and when N equals runaway speed, the output power is again zero. Thus, the speed-power curves for turbines are parabolic in nature. Speed vs efficiency curve: The speed-efficiency curves for turbines are similar to speedpower curves.
