Saturday, February 4, 2017

Upper Kotmale Hydro Power Station (UKHP)

Upper Kotmale Hydro Power Station (UKHP)

This is the latest hydro power plant which joins to the Mahaweli complex and located at Niyamgamdara. UKHP is the 3rd largest hydro power plant in Sri Lanka and it has the longest tunnel (with penstock). This is one of the underground power stations in Sri Lanka and this feeds 150 MW to national grid. There are some special features like electrical breaking, remote control and flood forecasting system in this run-off river type power plant. There are 2 Fransis type turbines which are covered by spiral casings were used to generate electricity. The dam and the reservoir is located in Thalawakale area and the catchment area of the power plant is “Agra Oya” and “Nanu Oya”. Generating voltage of UKHP is 13.8 kV. This is stepped up to 220 kV and connects to the Kotmale switch yard using double circuit transmission lines and then to Biyagama grid. And also UKHP has head of 473m. In upper Kotmale power plant use Francis type Turbines. Upper Kotmale power plant was Japanese funded project. This was JAICA Loan and Constructor and Consultant both are Japan.

Reservoir and Dam


This is a run-off river type hydro plant. Therefore this is actually not a reservoir, it is called regulating pond. This concrete gravity dam consists of 5 radial type spillway gates one with a flap gate. St.Clair waterfall is taking water from this pond. Therefore there is another gate for release water for the waterfall. Details of the dam and gates are as follows.

·         Height of the dam = 33.5 m
·         Length of the dam = 180 m
·         Width of the dam = 7 m
·         Catchment area = 310 km2
·         Max. Spillway capacity = 660 m3/s
·         Spill Level = 1194 m
·         Minimum Operating level = 1190 m
·         Effective Storage = 0.8 MCM
·        St. Clair discharge = 1.3 m3/s
5   spillway gates are used because the storage capacity is very small compared with others. Area of a gate is 12 x 10 m2 and weight of a gate is 90 tons. Each gate consists of 2 hydraulic cylinders. When the water level rises suddenly all these gates are operated according to a sequence. Flap gate is used to control the water level of the pond. There are 2 types of sensors used to indicate the water level to the control room. Either pressure sensor or floating sensor will give an indication to the control room and then the flap gate or spillway gates may be open.
Floater is connected to the gear mechanism and there is an Analogue to Digital converter. The digital output is then goes to the control as a PLC input.
Intake gate is used to take water in to the headrace tunnel from the pond and it is always kept open and holds on two steel bars to avoid the load coming to the hydraulic cylinder. This is a steel fixed wheel type gate with height of 8.8 m and span of 5 m. The gate can be operated both local and remote modes. Normal operating speed is 0.3 m/min, but in an emergency case it can be increased to 3 m/min
Spill gate & Flap gate

Headrace tunnel and Penstock

This is a horseshoe shaped type concrete tunnel with total length of 13.8 km with the penstock. The tunnel has uniform diameter of 5.8 m and 400 mm thickness. Circular steel lined penstock has length of 793 m and the pressure is 5 MPa.  This penstock inclines 480 from horizontal and has 4 different diameters from headrace tunnel to main inlet valve.

Surge Chamber

There are different types of surge tanks. They are,        

In UKPS used two surge tanks one is upstream and other is downstream. Upstream surge tank is restricted orifice type and has the diameter of 12 m.

§  Upstream surge tank
Sudden closing of guide vanes make reverse surge along the penstock. It didn’t damage to the penstock, because penstock made out of steel. But surge can damage to the headrace tunnel. So upstream surge tank used for protection of headrace tunnel. (Water hammer effect)
§  Downstream Surge tank
Used to protect draft tube and turbine from surge build because of the tailrace water flow behaviors.     

Main Inlet Vale (MIV)

Surge chamber is built to minimize the water pressure due to sudden closing of Main Inlet Valve (MIV). Upstream surge chamber is restricted orifice type and has the diameter of 12 m. MIV is used to control the water flow to guide vanes of the turbine. It is spherical type and it will take 90 s to open or close and operates using a servo motor (both open and close operations). There are two seals as service seal (downstream sealing ring) and maintenance seal (upstream sealing ring) to prevent water leakages after MIV is closed, they are operated by the high pressure of water path.

Main Inlet Vale (MIV)


UKHP has 2 Fransis type vertical shaft hydraulic turbines. This turbine consists of runner, spiral case and guide vanes (also called Wicket gates) and draft tube. The tail race is open to the Kotmale reservoir. Rated speed of the turbine is 600 rpm. There are 20 guide vanes used to control the water flow and 20 stay vanes to control water flow equally to turbine blades. So all vanes should be operated to the same angle. For this operation all vanes are connected each other by a ring. If the only one vane will be stucked all vanes will be stucked due to the mechanical ring mechanism. To prevent this issue there is an overload pin for each and every vanes. Then stucked guide vane will be isolated from the 

operation.  Net head is 473m and head loss is 19m for both turbines. Discharge rate is 36.9 m3/min for both machines and output is 77 MW. Water comes perpendicular to the shaft and leaves parallel to the shaft. Turbine materials are Cr and Ni.


As Engineers said both Pelton and Fransis type turbines are suitable for UKHP. Pelton type turbines can be kept at atmospheric pressure but Fransis turbines should be submerged.  But there is a future plan to increase the height of Kotmale dam. Due to the height increment of Kotmale dam the water level at the tailrace can be increased. Therefore a Fransis turbine has selected for UKHP.
All to gather there are 3 bearings. They are two guide bearings and one thrust bearing.  One guide bearing and thrust bearing situated in upper part of the rotor. One guide bearing situated in lover part of the rotor. In upper Kotmale there is special type of thrust bearing. The material is PEEK (Poly Ether ketone). In PEEK there are some advantages. They are hard, low friction Co-efficient and insulation resistance is high. There are 3 types of bearing arrangement for Francis type turbines.


Continue Rated Output                      : 88 MVA
Rated Voltage at stator terminals       : 13.8 kV
Rated Current                                     : 3682A
Rated frequency                                 : 50 Hz
Rated Power Factor                            : 0.85
Rated Rotation Speed                        : 600rpm
Max. Runaway speed                         : 1050rpm
Excitation voltage of 150 VDC is provided through carbon brushes. The output of the generator is taken for the station service transformer and the for the excitation transformer before the GCB (Generator Circuit Breaker). The generator output terminals from generator to main transformer are made through Isolated Phase Bus (IPBS- this is an air insulated Cu bar).


There are 28 carbon brushes in rotor. Due to friction brushes are ablated. When the carbon brush length is become 30 mm it should be replaced by 70 mm brand new brush.

Generator Braking

There are two type of breaking use in Upper Kotmale power station. Electrical breaking and Mechanical Breaking.
1)                              Electrical Braking
To stop the generator rapidly they used electrical braking system. When the generator slows down to 50% of its rated speed (at 300 rpm) electrical braking is applied. The thing happens in that case is three stator phases are short circuited and earthed.
2)                              Mechanical Breaking
Electrical braking reduce speed slowly. To stop the machine rapidly we use mechanical braking. When the generator slows down to 5 % of its rated speed (at 30 rpm) mechanical brake is applied by a pressurized air system.

Governor control

There are mainly 2 parameters in governor; power setter (65P) and load limiter (77). Power setter controls the extent of opening of guide vanes. In UKHP when it opens 80%, we can get the 75 MW output. Load limiter gives the upper level of guide vane opening.


There are two numbers of special type main transformers in the UKHP for two generator units. A transformer unit consists of three single phase transformers with common conservator tank. All three phases are immersed in same oil system. Therefore the transformer is more compacted. Ratings of transformers are as below.
            Capacity          :   88 MVA                              Voltage ratio   :   220 kV / 13.8 kV
            LV current      :   3682 A                                HV current      :   231 A
            Vector Group  :   YNd11       

These are core type transformers and each one has a seven number of HV tap positions. A Transformer only has an off load tap changer. Transformer oil is water cooled. Transformer conservator tank has a rubber cell to separate the inside oil from the outside air. Both transformers equipped with online gas analyzers to analyze the faults occur inside the transformers. The sensor of this system is sensitive to Hydrogen (H2), Carbon Monoxide (CO), Ethylene (C2H4) and acetylene (C2H2) which are the primary indicators of inside faults of oil filled transformers. LV side of the transformer is connected to generator through Isolated Phase Bus and HV side is connected to GIS system through Gas Insulated Bus. Isolated Phase bus is filled with atmospheric air due to low voltage and Gas Insulated Bus is filled with SF6.

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