In this article we will discuss about:- 1. Characteristics of Water Turbines 2. Governing of Water Turbines 3. Selection.
Characteristics of Water Turbines:
Reaction turbines of various types can be used for operating heads up to 500 m and Pelton turbines are used for operating heads above 500 m.
2. Specific Speed.
3. Turbine Setting:
A Pelton wheel is always set at a higher level than the highest tailrace level (usually 2 m above) while a Francis turbine runner is placed at a level very near or below the lowest tailrace level.
4. Runaway Speed:
This is the maximum speed at which a turbine wheel would run under the worst operating conditions with all gates open so as to allow all possible water inflow under maximum head. The generator coupled to the turbine must be capable of withstanding the full runaway speed of turbine under permissible head.
5. Constant Speed Curves:
In hydroelectric power plants, the turbines operate at constant speed and, therefore, variables are operating head H and discharge Q. As the discharge and head vary so as to keep the speed constant, the turbine output Pt is measured by brake arrangement. The turbine efficiency η is then calculated for various values of Q and H. Now the output discharge (Pt– Q), efficiency- discharge (η – Q) curves, as shown in Fig. 2.19 and efficiency-percentage full load curves are drawn as shown in Figs. 2 20 and 2.21. From the curves drawn, it can be concluded that the Kaplan and Pelton turbines perform well at part loads but Francis and Propeller turbines do not.
Governing of Water Turbines:
In order to have electrical output of constant frequency it is necessary to maintain speed of the alternator driven by the turbine constant. This is achieved by controlling the flow of water entering the turbine by the automatic adjustment of guide vanes in case of reaction turbines and of the nozzle needle in the case of impulse turbines. Such an operation of speed regulation is called the governing, and it is attained automatically by means of a governor. In case of impulse turbine the governor also operates the auxiliary relief valves or jet deflectors.
For the regulation of water below the penstock connection, at the time of decrease in load on the impulse turbines, the governor reduces the water flow from the power nozzle and the surplus water is diverted with the help of auxiliary relief nozzles. In the case of multi-nozzle turbines, a deflector plate deflects some water from the runner buckets by swinging into the water jet from each nozzle. With the movement of deflector plate out of the path of water jets, the needles slowly reduces the flow of water so as to keep the output of the turbine constant at the level of new load.
In the case of Francis turbine, there are pressure regulators for discharging the water from the casing to the tailrace at the time of drop in load. The regulators close as fast as the guide vanes open and vice versa.
The governor should be quite sensitive to variations in the shaft speed and should be rapid in action but not so rapid as to cause water hammer in the penstock. The governing systems for the modern hydraulic turbines have a regulating time of 3-5 seconds.
Simplified arrangement of a water turbine governor is illustrated in Fig. 2.22. The principal elements of the governor are:
1. The speed-responsive element—usually flyball mechanism or speed (centrifugal) governor.
2. Control valve or relay valve to supply fluid under pressure to the power cylinder (servomotor) in order to actuate the turbine control mechanism. The use of control valve and servomotor is to amplify the small force created by the flyballs.
3. The restoring mechanism or follow-up linkage to hold the servomotor in required fixed position when the turbine output and load demand are equalised.
4. The fluid pressure supply required for the action of servomotor.
The flyballs may be belt driven, as shown in Fig. 2.22 or driven by a small electric motor fed from a separate generator operated in synchronism with the turbine. When the load on the turbine decreases, the speed of the turbine increases, consequently, the flyballs also rotate at high speed and move outwards. The floating lever gets lifted up, control valve is displaced upwards from its central or dead beat position, the upper port is uncovered and the oil flows from a pressure tank through the port into the right hand end of the servomotor cylinder.
The piston moves to the left and closes the nozzle with the help of a spear in the case of Pelton wheels and adjusts the guide vanes in the case of reaction turbines. In case of increase of load on the generator, the speed of the turbine will decrease and the reverse action would take place. The restoring or follow up linkage resets the relay; pilot or control valve after the servomotor piston has adjusted the water control mechanism.
In case of Pelton wheel a combined spear and deflector regulation is employed in order to avoid water hammer in the penstock. In case of decrease of load on turbine, the deflector, which is usually a plate connected to the servomotor by means of levers, is brought in between the nozzle and buckets, thereby, diverting water away from the runner and directing into the tailrace. In the mean time, the spear has been adjusted to the new position of equilibrium and the deflector plate is moved out of the path of water nozzle.
Selection of Water Turbines:
The selection of a water turbines depends upon various factors such as working head, available discharge, speed, output and nature of load. The effective head under which the turbine is to operate gives the first guide to the selection of the type of turbine. For very high head i.e., 500 metres and above, Pelton turbine is usually employed.
Reaction turbines are not suitable for such high heads as due to high velocity of water, there will be rapid wear and friction losses. For medium heads i.e., above 30 metres and below 500 metres, Francis turbine is usually adopted but impulse turbine may also be adopted even at the expense of efficiency. For low heads i.e., below 70 metres, propeller type of turbine is used.
In case of considerable variations in head and load, Kaplan turbine gives improved efficiency as compared to the fixed blade propeller turbine. Even for the same head, two different types of turbines may be employed. For example, for a head of 200 m either of the Pelton or Francis turbines can be used. Similarly for a head of 30 m, either of the Kaplan or Francis turbine can be employed. It is a general practice to select a runner of higher specific speed.
A runner of high specific speed will generate more power for the same head resulting in small size of the turbo-alternator and the power house. For larger output but at low head, a runner of high specific speed is used to keep the size of the turbo-alternator and power house smaller and economical. For high head and even with medium output plants, even a low specific speed runner will attain high rotational speed and a high specific speed runner need not be selected.