Aerial view of the Orlík reservoir and power plant
The Orlík reservoir is a pivotal element of the Vltava cascade. Holding 720 million m3 of water, it is the largest capacity storage reservoir in the Czech Republic and together with the Lipno Lake it is crucial for multi-annual water-flow control on both the Vltava and the lower Elbe. The reservoir surface area is 26 km2 and it impounds the Vltava over a length of 70 km, the Otava over 22 km and the Lužnice over 7 km upstream of the confluence. In addition to Vltava and Elbe flow control, it allows extensive summer recreation, lake navigation and fish breeding.
Over the borderline between Central and Southern Bohemia
A flight over the Orlík reservoir is also an excursion to a region where the Vltava is definitely leaving Southern Bohemia’s landscape and enters Central Bohemia. The massive body of impounded Vltava waters, which has “fuelled” the Orlík power plant and other power plants downstream for more than 50 years, is the focal point of a relatively sparsely populated area between Krásná Hora nad Vltavou, Milín and the Žďákov Bridge.
Up to the north, where the tame waters gather strength again to produce environmentally friendly electricity at the Kamýk power plant, the first clearly visible point is the village of Solenice and the bridge between the two banks of the Kamýk Lake. Here the meandering Vltava is enclosed by a romantic rocky canyon with the dominating peaks of Na Altánku (516 m), Jezerná (446 m) and Homole (400 m). The landscape then opens up up-country and you can see the region of Dobříš and Nový Knín across the ruins of Kamýk Castle. Eastwards, beside the Krchov transmitter (489 m) and behind an undulation, lies the village of Přední Chlum, while one of the southeastern bays conceals the remnants of the former village of Orlické Zlakovice underwater. This is one of tens of small villages that had to give place to the Orlík reservoir built in the 1950s and 1960s. Spreading southwards are the endless waters of the Orlík Lake with clearly visible bays formed by the former narrow valleys of small influent streams. The nearest dominating peaks on the western bank are Čistá (507 m) and Taterův vrch (440 m), on the eastern bank then Bořím (421 m) and Ráj (445 m).
Orlík reservoir and hydro power plant
The Orlík hydro power plant plays a major role in the management of the nationwide power system and in the production of cheap, environmentally friendly, peak-load electricity. This is allowed by its total capacity of 364 MW, very quick and flexible start-up with full-load operation reached in 128 seconds and remote control from the Štěchovice control centre.
The reservoir was built in 1954-1962. The lake was created by constructing a concrete gravity dam 91.5 m high, with a crest 450 m long. The dam body has 3 spillways measuring 15 x 8 m and two bottom outlets with a diameter of 4,000 mm and a total capacity of 2,300 m3/s.
The hydro power plant with dimensions of 17 x 127.5 m and a turbine hall 20 m high is located on the left side of the river at the toe of the concrete dam. Water is fed to its turbo-generators by four steel pipes, 6,250 mm in diameter, embedded in the dam. The inlet is fitted with quick-closing valves and emergency gates. The power plant was put into operation in 1960-1962 and is fitted with 4 fully automated turbo-generators with Kaplan turbines for a 70.5m head. One ten-blade runner, unique in the world at the time of commissioning, was awarded a gold medal at EXPO 58 in Brussels. Today the turbo-generators have modern, eight-blade runners with higher efficiency. Electricity from 15kV generators is transformed in six single-phase transformer units to 220 kV for two output lines.
Power plant entrance hall
The area is dominated by an artistic mosaic by Emil Cimbura from 1959.
History of reservoir construction
The history of the construction of the Orlík Reservoir through the eyes of Mgr. Jan Kouba, a historian and curator of the Ethnographic Sub-collection, Prácheň Museum in Písek.
The turbine hall hosts 4 vertically installed turbo-generator sets, each consisting of an exciter, a 91MW synchronous generator and a Kaplan turbine. As the majority of the machinery is located under the floor, you can only see the top covers of the generators in the turbine hall floor and an architectural cover concealing a thrust bearing structure, exciter and oil head. Two gantry cranes installed under the turbine hall ceiling are used to move the machinery during repairs. A control room is in the middle of the turbine hall, in the direction of the dam.
Thrust bearing space
The first part of the turbo-generator set that protrudes above the turbine hall floor, the thrust bearing structure contains two bearings – an upper generator guide bearing and an actual thrust bearing that absorbs all axial stress vertically on the turbo-generator axis. The forces result from the weight of all rotating parts of the set, which is 650 tonnes in total, and the vertical hydraulic thrust of water transmitting its energy to the turbo-generator, which is 600 tonnes at full capacity. The thrust bearing structure transfers the load of over 1,200 tonnes to the generator stator structure and through it to the power plant’s concrete structures. The total length of the turbo-generator rotating parts is 27 metres.
Rotating exciter space
The Orlík power plant’s turbo-generators have an exciter on the rotating assembly shaft. The lower portion shows the commutator assembly of this DC machine and the upper portion shows the rings that feed power to the generator’s rotor.
The exciter is the primary source of energy for electricity generation in the generator’s stator. Electricity generation in the stator requires a rotating magnetic field. This is created by using a DC source to create a DC magnetic field and turning the assembly to rotate the field.
Oil head space
The oil head space is located under the pyramidal dome at the top of the turbo-generator set. It serves to feed pressure oil for runner blade adjustment through two ducts to the runner actuator – but through the generator shaft. Pressure oil is delivered to the shaft in the oil head and the transfer area must be movable but also tight. The entire mechanism is highly complex because there is a “floating” seal that needs to be properly lubricated during operation and ensures tightness in the transfer of oil from stationary parts to rotating parts.
The oil head space also hosts a centrifugal switch – a safety overspeed sensor. It evaluates whether speed exceeds the permitted limit. This switch is also used for emergency shutdown of the turbo-generator set.
Current limiting reactor
The current limiting reactor has no magnetic circuit, is three-phase and serves to reduce short-circuit currents in the station service branch of the generator power output.
Generator switch cubicle
The Orlík hydro power plant has a generator switch with a unique design – an air-actuated, pressure air system. Pressure air is used to make and break contact in the generator power output to unit transformers and the power output path is interrupted at multiple locations when switching off, using pipes installed at both sides of the switch. In addition to path interruption, the pressure air also quenches the arc that results from disconnecting generator currents of up to 4000 A. The connectors move back with the aid of strong springs after a while; by that time, the pressure air has already disconnected the cut-off blades in the centre of the switch.
The generator switch still has excellent switching parameters. Its only disadvantage is noisy disconnection, which is similar to a loud gunshot. The switches were installed in the 1980s and this is the last place they are used in the Czech Republic.
Station service consumption
Electricity for station service consumption flows from the current limiting reactor to a station service transformer that transforms 15 kV to 400 V. There is the high-voltage input on the left and one output with a pair of disconnectors on the right, with branched output lines for two turbo-generator switchboards. These are just disconnectors; a power on/off switch is located in the station service switchgear area.
One half of station service consumption at each turbo-generator set is usually covered by this power source and the other half by the station service substation.
The diesel generator provides power supply in case of a power cut. It must protect the power plant against damage even if one of the turbo-generators is blocked out. Its power output is 440 kVA. The power output is sufficient when one of the turbo-generator sets is dismantled and power needs to be supplied to the draft tube pump that has a power consumption of 185 kW. The diesel generator is located in its own space at the turbine hall level, on an elevated platform that protects it from extreme floods.
The area in the second basement contains filters for cooling water used to cool air in the generators and to cool turbo-generator bearings. Each turbo-generator has a pair of filters, with one in operation and the other in reserve; when clogged, they can be cleaned by an operation run from the local control box.
Turbine hall – first basement
The first basement, or the generator floor, contains generators as well as adjuster pumping sets, i.e. sources of pressure oil for the adjustment of Kaplan turbine runners and guide vanes. The volume of adjusting oil filling for each turbo-generator set is 32 m3. These are original low-pressure oil sources for the adjustment of runner blades and guide vanes at the Kaplan turbine.
Alongside the adjuster pumping units, there are also three thrust bearing coolers at each turbo-generator set. A braking unit is installed at each generator, which is a unit that generates power needed for electric braking of the turbo-generator set. It is used to brake the turbo-generator set when electricity generation is suspended and the rotating parts of the turbo-generator set need to be stopped. The set is braked to ensure good bearing lubrication, which deteriorates at lower speeds. The braked rotating parts of the turbo-generator set weigh 650 tonnes in total.
The generator has a power output of 90 MW at 15 kV and is cooled with air that is cooled down by water coolers; forced air circulation is ensured by fan blades attached to a 400-tonne rotor.
Mechanical brakes serve as a generator parking brake and for emergency stopping, e.g. when there is an electrical fault that makes electric braking impossible.
The braking unit is a set of three rotating machines, an induction motor and two dynamos. The auxiliary dynamo generates excitation energy for the main dynamo; the main dynamo then generates power for electric braking. The power is then fed to the generator’s rotor, while the stator is short-circuited during electric braking. The resulting interaction of two magnetic fields generates very high braking torque.
The turbine cover area hosts all commonly accessible parts belonging to the turbine:
- Runner actuator – with its position on the turbine shaft, it dominates the area.
- Guide wheel actuators – located at the wall, they turn the orange ring that actuates guide vane mechanisms; see the close-up.
- Leaked oil and leaked water pumping systems.
- Carbon seal cooling system. The seal is inaccessible and serves as a barrier preventing water from leaking from the turbine along the shaft to the turbine cover – it is lubricated and cooled by water.
In addition, the turbine cover includes a manhole that provides access to the guide vane area in the waterway when the turbo-generator is blocked.
The rotating parts weigh 650 tonnes, with the generator’s rotor alone weighing 400 tonnes. If the generator rotor is to be moved during repairs, it is necessary to use all 4 hoists of both cranes, which are interconnected using a special carrier beam kept in the turbine hall for that purpose.
The machinery has undergone two overhauls in its lifetime so far. During the second overhaul, the worn-down ten-blade runners and their hubs were replaced with new, eight-blade runners with higher efficiency at all turbo-generator sets. Another major repair was done after the 2002 flood, when the turbo-generators were flooded and the insulation of all generators had to be replaced.
Guide vane area
The man is standing in the guide vane area on the blades of a runner with a diameter of 4.6 m. Around him you can see 24 guide vanes, which serve as a working water stop when closed and regulate generator power output in the open position. The diameter of the ring of guide vanes is 5.52 m and the guide vanes are 1.5 m high. The Kaplan turbine’s runner blades are currently in their 30% open position, the usual start-up position. The runners of the Kaplan turbines at the Orlík power plant have eight blades each.
The pink colour at blade roots is a residue from a penetration test. The penetrating paint is able to seep deep into cracks and if a fracture occurs at the tested stressed point, it is identified by white colour. Penetrations tests are conducted on blade roots in order to check their strength and integrity – remember that the blades are stressed by hydraulic thrust of up to 600 tonnes from the above during full-capacity operation.
The spiral casing is attached to the penstock and its area gradually decreases to ensure uniform water flow at the guide vanes. Stay vanes, which are part of the spiral casing, can be seen before them.
Under the runner
The man is standing on a scaffold built here for inspections and repairs during a shutdown. Runner blades can be seen above his head. The object in the middle is the runner hub with a blade adjusting mechanism. Around the perimeter, you can see the transition between a round area with stainless steel welded on and the cylindrical part of the draft tube with painted surface – a caulked transition belt must be maintained between those parts to prevent corrosion where the two surfaces get into contact.
Blade bottoms, subjected to negative pressure, are the places where cavitation is most likely to occur. Cavitation is the formation of cavities when pressure in a liquid drops, followed by implosion. When the negative pressure that formed a cavity disappears, its bubble collapses, generating a shockwave with destructive effect on the material of turbine blades.
Expansion joint area – lower level
This is the area where the concrete blocks of the dam and those of the power plant meet; therefore, penstocks embedded in the dam must be connected to the spiral casings using expansion couplings that allow mutual movement without damaging the waterways. The space under the runners is accessed from this area during repairs of turbo-generators.
Expansion joint area – passageway
The peek into the passageway to dam galleries shows, by the length of the passageway, that the Orlík reservoir’s concrete dam is a gravity dam, impounding water by its own weight, and its width at the foundation is 60 metres. The lower level of this area is only accessible by ladders. The passageway door is pressure-resistant from both sides and was newly installed after the 2002 flood. It serves to isolate the power plant from the dam in case of a penstock failure or during a flood.
Dam – grouting gallery
The grouting gallery is the lowest-level gallery in the dam; it allows dam inspections as well as dam safety monitoring using pressure-measuring grout holes. This level is also used to evaluate the movements of dam blocks, monitored with “pendulums” – strings suspended from the top of the dam with a weight at the bottom. The dam “sways” periodically due to uneven temperature changes on its upstream and downstream faces.
The grouting gallery passes through a number of isolated dam blocks (see block numbers on the upstream side of the gallery).
Grouting gallery – stairs
The shape of the grouting gallery corresponds to the shape of the valley under the dam, so it turns into stairs in sloping areas. On the right-bank side, the stairs are interrupted by a horizontal part with entrance to the space under dam spillways.
Space under dam spillways
This diversion pit area served to divert the Vltava River around the construction site during dam construction. The Vltava was first diverted to its left bank at which the pit was made from concrete. Then the river channel was dammed, the river was diverted to the newly built, concrete-lined diversion pit and the dam was constructed. At a certain stage, the pit was divided into two parts by a central section with pillars, and the two blocks above the two newly created halves were made. The division gave rise to two tunnels, which were fitted with steel gates at the inlet. The dam was ceremonially closed on 30 September 1961. This was also the day on which filling started. The steel plates were closed and the space behind them was filled with concrete and became an inner area in the dam. These premises are under the spillways.
12kV switchgear system
The station service switchgear for Orlík and Kamýk is supplied from the Milín substation by two high-voltage cables. One cable runs from each section to the Kamýk power plant’s substation. The process can also be reversed.
Access to the high-voltage part of each cubicle is duly secured. A partition must be inserted into a slot in the cubicle, then the contacts of the switching device are pulled out from the live parts and only then is it possible to open the panel and remove the switching device.
Short-circuiting sets safely interlock a bay when work is performed on the line or in a substation.
The four generators output power at 15 kV to two trios of single-phase transformer units, where it is transformed to 220 kV and is outputted, through gear in a 220kV substation, to the Milín substation by two lines. This is a “two-unit” generator arrangement.
The view in this area also shows the outlet slab above the stilling basin, which can be travelled by a 25-tonne gate handling crane. It is used to lower blocking slabs, which are usually suspended in slots under cover plates, to block the outlets of turbo-generator sets during repairs.
View from the crest
The crest of the dam consists of a roadway connecting both banks. There is the village of Solenice downstream of the dam, the “Kostínek” and “Solenice” hotels on the left bank and a pair of navigation structures on the right bank – the smaller one is in operation and transports ships up to 8 metres long, 3 metres wide and weighing no more than 3 tonnes; the larger navigation structure has not been completed yet.
The intake work platform provides access to the radial gate machinery rooms and to pumping units for quick-closing valves, which are emergency gates at the intakes of individual turbo-generators that are able to close the intake even at full operating flow rate. The actual emergency gates are in shafts under the intake platform, and those are preceded by slots for working intake gates hoisted by a gate handling crane working there. On the upstream side, the intakes begin with panels of bars that serve to prevent floating objects from entering the generator waterways.
Under flood conditions and when the upper level is expected to exceed the upper retention limit, the entrances to the dam interior at the intake platform can be protected by flood gates at a higher level.
Under standard operating conditions, the emergency gate shaft is flooded with water up to the upper water level. Now it is accessible for repair. In the middle you can see the piston rod of the emergency gate actuator, with the actuator body on it at the bottom (it travels up and down on the piston rod when in operation). At the very bottom you can see the emergency gate slab on a system of rods. The platforms and approach ladders serve for inspections and repairs of the emergency gate technology.
The slab, weighing 65 tonnes, is used as an emergency gate and is suspended from a system of rods attached to a piston. When falling down, the piston pushes oil out of the actuator to the tank of the pumping unit located under the roadway. To raise the slab, pumps are switched on to pump oil into the piston rod, the piston rod lifts the piston and the slab travels up. Raising it takes 6 minutes, while its fall takes just 36-40 seconds. The fall is controlled through flow reduction to make the landing very gentle.
The whole slab moves on large undercarriages that travel in a steel pathway where water pressure is present. This makes the slab able to move into full flow. The penstock space behind the emergency gate is fitted with a vent pipe embedded in the dam. This pipe is used to suck in air to fill waterways that are being emptied.
Filling an empty, air-filled penstock requires the presence of an operator in the area under the roadway. The air pushed out during penstock filling might have high energy if the filling process were too quick, so filling is performed in steps. The slab is raised by about 4 cm first and a narrow stream of water fills the entire water system until it is filled up to the slab. As soon as the level is within 1 m of the lake’s level, the slab can travel up at full speed.
Emergency gate at TG4 intake
The emergency gate consists of a 65-tonne steel slab. It is moved by a linear actuator; the actuator’s piston is firmly suspended and the piston rod is moved by a system of connecting rods so that it controls the emergency gate that is capable of stopping water at full flow rate, i.e. 150 m3 of water per second.
Lifting the slab to 9 m takes about 6 minutes, lowering it takes 36-40 seconds. When the slab is down, the piston is at the top with minimum oil; when oil is pumped in through a borehole in the piston rod, the piston rod moves up along the piston. If the emergency gate needs to be lowered, the valve through which oil is transferred from the piston rod back to the pumping unit tank is opened.
The whole emergency gate assembly is more than fifty metres high.
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