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This report is based on the 2 day field visit on 26 th and 27 th of June 2014 . We were about to visit Victoria power station and victoria dam on the firs t day. And on the following day we were about to visit Rantambe power station and dam. First day we covered both victoria power plant and dam, and on the second day we visited Rantambe power station and Randenigala reservoir, dam.
VICTORIA POWER PLANT
INTRODUCTION AND LOCATION The biggest hydro power plant project in Sri Lanka is Victoria power plant project which is situated at Theldeniya in central province. province. The project completed under under the Mahaweli development program program in 1985 with producing 210MW power to the Sri Lankan electrical power supply grid. Victoria dam is built across the Mahaweli River and the water from this huge dam and 5.8km 5.8km long tunnel for penstock, produces a 190m head to the three turbines turbines to generate power. Victoria power plant is one of the power plants in Sri Lanka that uses to control the frequency of main grid.
DAM AND PENSTOCK The height of the dam is about 120m and crest length is 520m, maximum thickness of dam is 25m. It makes the victoria reservoir which is for irrigational purposes and as well as power generation. It is an arch dam. The dam is constructed in 32 vertical concrete pieces and this special arrangement allows the dam to move horizontally maximum of 6cm when water is fully filled in the reservoir. This can be considered consider ed as a brilliant piece of engineering work. There are 8 radial spill way gates wide of 12.5m each in the middle of the dam which operates automatically for spilling purposes when the dam cannot bare the pressure of stored water. Dam has another system of closing cl osing the spillway gates. And it used when there is a repair to be done in the gates. It consists of ten large metal pieces which can be stocked one over another to stop the water flow. Water from the dam runs through 3 penstocks which has 6m diameter each. The 3 penstocks are housed by 6km long tunnel which provides the required 190m head for the turbines. These 3 penstocks directl y go to the 3 turbines.
TURBINE Matching to the head produced, the water flow rate and the rotating speed the type of the turbine selected is vertical Francis type. The rated speed of turbine is 333rev/min. Turbine is made from stainless steel in one piece. There are 3 turbines for 3 penstocks and each tu rbine is connected to 70MW generator. There are 24 guide veins around the turbine which are controlled by oil. Cooled water is supplied to cool the bearings.
GENERATOR The rated speed of a generator is 333rev/min. these generators produce power in 12.5kV rated and rated current output is 4388A. With full storage the 3 generators all together can produce 285GWh energy. When the generators give the rated power output they work with 98.2% efficiency. Excitation voltage for the generators is 220V DC and it is supplied by separate transformer. Battery backup is also produced. Thyrister bank is used for the AVR unit to control the voltage level.
TRANSFORMERS AND GRID CONNECTION
The 12.5kV output of 3 generators is then step up to 132kV for transmission purpose. There are 9 single phase transformers for this. Three transformers for three phases in each generator. And there is an additional one to be connected if one transformer fails. Victoria power plant is connected to Kothmale and Randenigala through transmission lines.
RANDENIGALA RESOVOIR AND DAM
Randenigala dam is located about 20km downstream to the victoria dam. It is constructed under the mahaweli development project in 1986. It is a rock fill type dam of 94m of height, 485m of length and 303m of base width. Rock fill type means it is constructed based on rock and on a bed rock for the strength and filled with clay core in order to stop leakage of water. There is a “grooting gallery” which is located 20m below the centerline of the dam in the bedrock layer. Bed rock is the foundation of the dam and clay seals the water leakages through the dam, but if water leaks it would be a threat to the stability of the dam.
So that this gallery is built to look after leakages in bed rock. If there are cracks on the rock, high pressurized clay or a compound is send through them to seal and stop being spread. Pressure meters are placed to take readings of the water pressure. There are 3 large spillway gates made on the dam for spilling purposes. These gates not like in Victoria, should be controlled manually. When the water level reaches 132m the gates should be opened. There is no long penstock tunnel in Randenigala dam to power station as the power station located immediatel y downstream of the dam.
RANTAMBE POWER STATION
Rantambe dam is constructed in 1990 mainly for irrigation purposes. It uses to generation electrical energy using the water releases from Randenigala reservoir and the natural flow of the Uma Oya. The total installed capacity of power station is 51MW. Through the design the randenigala power plant can be controlled from here.
TURBINES Rantambe dam and tunnel gives the turbine only a 33m head. But the turbines should have 166.7 rated rpm for health work. Because of the low head the flow rate of water has to be arised. So that here in Rantambe power station the turbines are withstanding 90m3/s/per turbine. This is larger than the value at victoria. To match these design parameters, the turbine type is chosen as Fransis. It has 13 blades and weights 18.7 tonns. It is made out of stainless s teel and made as one piece. That means there are no welded joints on it. The governor for the turbine is controlled by oil which are pressurized by air at
about 100kPa. The low head of 33m to the turbine results in cavitation on the turbine blades. In order to stop this happening, high pressurized air is in injected to turbine blades from an external motor.
GENERATOR To match the total capacity of power plant two generators rated at 26.5MW are installed for the 2 turbines in the power plant. The two generators are manufactured by Asia Brown Boveri (ABB). Each generator’s rated voltage is 12.5kV. The rated speed of the generators is 166rpm, that is when the guide veins opened fully. But when we were there at that moment the speed was about 100rpm, and the guide veins were opened only 60%. There were separate control panels for each generator, turbine set which allows controlling and displaying the speed, percentage open of guide veins, the pressure levels of different places. For safety purposes the entire building is air conditioned by chilled air system. Rantambe power plant is connected to victoria by stepping up the voltage level and 220kV line and, through 132kV lines this power plant is connected to mahiyangana and ampara.
