If we have hydro power plant connected to grid, the generator needs to be synchronized to the grid for power evacuation.To synchronize generator with the grid, the frequency of the grid must match with the frequency of the current generated by the generator.For this, generator must be always rotated at same speed which is called synchronous speed. However, the speed of generator varies as the load varies.The speed of generator decreases as load increases and increase as the load decreases. The load fluctuation is inevitable and therefore a mechanism must be developed to run the generator at constant synchronous speed which is called the governing mechanism.
The governing of water turbine generator is similar to the diesel generator.In diesel generator we vary the mass flow rate of the fuel (diesel) going to the engine. However, in hydro generator we vary the flow rate of the water.Whenever the speed of generator decreases we increase the flow rate of water and thus higher flow of water helps to maintain its synchronous speed.The vice-versa will occurs when the speed of generator decreases.
The flow rate is varied by varying the area of the flow.Different type of mechanism has been developed for different types of turbine to vary the area of the flow.Following are the mechanism for the different turbines.
1) Francis Turbine and Kaplan Turbine : Guide Vane Mechanism
Figure: Guide Vane Mechanism
As shown in the left side of the figure, the guide vane (also called wicket gates) is in completely closed position which will block the flow of water to the the turbine.In the right side the guide vane is fully opened allowing all the water to flow to turbine. The guide vane can also be partially opened.
Since the energy of water reaching the turbine,is very high, and since the guide vanes are heavier in weight, large amount of power needs to be applied to move the wicket gates.This is made possible by the use of hydraulic servo-motor and linkage bar assembly.The servo motor is supplied with the pressurized hydraulic fluid provided by the Oil Pumping Unit (OPU) of the Power House.
Pelton Turbine: Spear Valve mechanism/ Deflector plate mechanism
Spear Valve Mechanism: Spear Valve is a conical type valve having it's pointed edge facing toward the runner. The linear motion of the spear valve varies the area of the water flow inside the nozzle.. Whenever the spear valve comes near toward the runner the area of the flow inside nozzle decrease and vice versa will occurs when the moves backside.This type of mechanism is used for slow response to the load fluctuation (5-10 sec)
Deflector Plate Mechanism:
This is the mechanism which is for very fast response to the load fluctuation. It brings the generator back to its original speed within 2-3 second. In this mechanism instead of varying the flow of water , the water is diverted to the tailrace instead of hitting the turbine .This will cause the loss of water
Control Mechanism: The function of control mechanism is to sense the rotational speed of the generator and provide signal to the mechanical device (Guide Vane/Spear Valve/Deflector Plate assembly) for opening or closing.Previously, the control mechanism was mechanical using the principle of centrifugal governor. However, regarding the issue of accuracy and the response to the fluctuation of load, it has become obsolete. Digital Governor is widely used recently which uses the principle of PID controller.
For detail understanding, I have also posted the video of working of governing of Francis turbine and Pelton turbine. The video is courtesy of Voith Hydro GmBH Germany.
Please feel free to comment and have further discussion
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