Salient pole Synchronous generators and induction generators are used in hydroelectric power plant to convert the mechanical energy output of the turbine to electrical energy. Normally in hydro plants synchronous generators are employed for generating electricity, however for small power plant generator applications (plants less than 5 MVA generation capacity) Induction generators are used because of their lower cost due to the absence of the exciter, voltage regulator, and synchronizer which should be associated with synchronous generators.
Salient pole Synchronous generators are used in hydoelectric plant because:
The majority of hydroelectric installations utilize salient pole synchronous generators. Salient pole machines are employed because the hydraulic turbine in the hydroelectric plants operates at low speeds compared to steam plants, therefore generators in hydro plant requires large number of field poles to produce the rated frequency. Rotor of synchronous generator requires salient poles is mechanically best suited for low-speed applications, compared to round rotor machines which are employed in horizontal axis high-speed applications in turbo-generators. Round rotor or non salient pole synchronous generators are used in Thermal power plant because of high speed turbine.
Questions : Hydroelectric Power Plant
What is Forebay and its importance?
Answer: Forebay serves as regulating reservoir storing water temporarily during the light load period and providing the water for initial increase of the load during which water in the canal is being accelerated. In simple words Forebay may be considered as enlarged body of water just above the intake to store water temporarily to meet the hourly load fluctuation. When the hrdroelectric plants are located just at the base of the dam, no forebay is required because the reservoir itself serves the purpose of forebay. However, where the plants are situated away from the storage reservoir forebay is provided.
What is Penstock?
Answer: It is the closed conduit or pipe which connects the forebay or surge tank to the scroll case of the turbine. Penstocks are built of steel or reinforced concrete. The thickness of the penstock must be adequate to withstand both the normal hydrostatic pressure and also sudden surges both above and below normal caused by fluctuations in the load and in emergency conditions.
What is water hammering phenomenon?
Answer: When the load on the generator reduces suddenly, governor mechanism will close the turbine gates and thus create an increased pressure in the penstock. This results in ' water hammering phenomenon' and may need pipe (penstock) of extra ordinary strength to withstand the pressure otherwise the penstock will burst. To avoid this positive water hammering pressure some means are provided to take the rejected flow. This job will be carried out by small storage tank or reservoir for receiving the rejected flow. This storage reservoir or tank is called as 'surge tank'.
What is Trash Racks?
Answer: Trash Racks are long, flat vertical bars spaced accordingly such that it provides the minimum passage width to the water through the turbine. They are employed to prevent the ingress of floating and other material to the turbine
What is Tailrace?
Answer: Water after doing the useful work in the turbine is discharged to the tail race. It is the path through which water will flow down stream after running the turbine.Tailrace storage will acts as reservoir for the down stream projects for generating electricity or for irrigation purposes
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