"HPPs ON DRINA RIVER" Višegrad

After investigation works, design documentation and other preparations having been finished; other preparations started in 1983 in order to provide preconditions to start capital works on 1.4.1985. Thanks to maximum usage of technology, engines, intellect and experience as by investors as by planners and contractors, the planned term for putting the first unit in operation was realized on 25.11.1989.

Construction and electromechanical work group managed to carry out the following actions within the planned term:
- to dig up 2.100.000 m3 mostly stone material
- to enbank 200.000 m3
- to build in 650.000 m3 of concrete
- to transport and build in 30.000 tones of armature
- to build in 12.500 tones of hydromechanical and electromechanical equipment
- to build in 25.000 m of anchor
- to drill and inject 65.000 m of holes
- to build 16 km of the main road and 25 km of the regional road with 51 tunnels, total length of 10,5 km, 22 bridges total length of 2.500 m and more than 60 km of local roads.
- to expropriate more than 1.000 hectares of land, dislocate 266 households, 3 schools, 2 sacral objects etc.

Beside common problems in construction such as getting in concrete on very cold weather (the water for concrete was warmed up); as well as on high temperatures (components for concrete were cooled with water), a particular problem was arising of huge waters, so that constructing pit was flooded 4 times.

A special problem was also regular supply with financial resources.

It is worth to mention the following facts from constructing phase:

- 2.500 m3 of concrete was built in per day
- active turbine runner was installed together with turbine cover and shaft total weight of 280 tones
- during 54 months more than 4.000-5.000 workers from more than 50 companies were engaged in constructing this object.

Due to permanent lack of financial resources, since 400 million dollars were needed for construction of the object, it was decided to build and finance only those objects, which would put plant in operation. So the aim was to provide production, i.e. the own financial sources, and building of the objects that are not used to put the plant in operation was postponed for some better times. That is why arranging of the riverbanks from the dam to the old bridge, reconstruction of the road on the right bank, spawning place, administration building and other works were postponed for later.

Experiences in mounting of equipment

Timely announced tender, at first for large, then for small equipment made possible realization within deadline. The equipment was delivered on time, although it is important to mention that one transformer 115 MVA broke at voltage test. Since transformer circuit was american mode, i.e. 3+1, its delivery did not endanger the term for finalization of the object. It also has to be mentioned that deadlines for delivery of the biggest part of the equipment were exactly and thoroughly defined. During the design work of generators, transformers, cranes, gas insulated switchgear (GIS); suppliers were timely warned about mistakes or bad decisions, so that they were solved in time.

Complete HME was delivered by "Metalna" Maribor. Some characteristics of HPP Višegrad are radial gates (and auxiliary gates) big dimensions of 18,9 x19 m two in total capacity of 3.000 m3/s each equipped with flapes of 300 m3/s each what was proved to be very good in exploitation.

Bottom outlets have capacity of 4x1.200 m3/s. Total evacuation is about 10.800 m3/s what was conditioned by quantity of 10.000 m3 of water that appeared in Višegrad in 1896. Intake structures are separated in halves as well as draft tube. This is done because of capacity of 270 m3/s per unit and in order to reduce dimensions of entrance and draft tube gates. At the entrance there is a quick flat gate which serves as turbine inlet valve since it is very short spiral. The spiral is made of concrete, and it was proved that has no important damages. It is operated by servomotors, and operating units "Prva petoletka" are proved to be very reliable.

The main problem was inappropriate choice of turbine installers as well as bad supervision of turbines. "Metalna" from Maribor that had never mounted such turbine before, was chosen here. As a result, basic tolerances such as: flatness of thrust bearing, inclination of the unit, clearance on the seal, clearance on the runner were not respected. It is important to mention a bad solution for thrust bearing because there is no appropriate acces to it what makes maintenance difficult.

The supplier shortened the shaft from conceptual design, what caused these problems. This also caused closing of the down part of generation ventilation, what also causes damages on isolation. Disobedience of tolerances at already narow designed clearance on the runner blades (3 mm), and inadmissable inclination make alignment of the unit difficult. At the unit 1, this caused a bad damage of oil regulating pipe in the oil head and insufficient number of hours until 1994, when the unit was repaired and this deviation reduced to alowed level.

Supplier of turbines "TOSHIBA" demanded maximum delevelling of 42,7- 47,0 m net head. We are not informed about reason. It is evident that construction of the structure is adopted to it. Guaranteed η is within these limits. High axial power on the thrust bearing is about 1.550 t max what gives 46 kp/cm2 specific pressure on thrust bearing pads, what is very high.

It is worth to mention problem of choice for oil pumps in regulation of turbine. Two pumps power of 200 kW each were chosen and this choice caused a number of problems:

- increased self-consumption
- oil oxidation
- replacement of cables with bar connection in circles of self-consumption
- problems with GIS (420 kV) are basically in the choice of supplier. Energoinvest has delivered prototype for this voltage level.

Several bad damages have been noticed. They are overhauled within 3-30 days each. Favourable conditions were that it was suggested to put in bypass from TL (transmission line) Pljevlja to overcome problems at TL Višegrad. Installations 15,75 kV were delivered by "Simelectro", France.

On planners demands, switches were chosen for short circuit on common point of generators in 2010 for former Yugoslavia conditions.

Producer of equipment is BBC and switches are pneumatic with In= 7.000 A and Ik=150 kA. Collecting points are armoured with overpressure so that do not allow coming of dust. They are grounded only in one point in order to reduce electro-dynamic forces at short circuits. At mounting, contact bars were not aligned properly. Experts in HPP Višegrad solved this problem by themselves, later.

It is a rare example that three units of this power are connected into joint busbars of 15,75 kV. According to the "Siemens" experts, they do not know such example with power of 3x105 MW. Such conception demanded a special 100% protection for generator coil. It is also the first time in former Yugoslavia that transformers are directly connected with GIS. This demanded special bushings for the same connection on the voltage of 400 kV. Generators were made by "Rade Končar", a Zagreb factory. There were no problems during mounting because parts in concrete were correctly positioned and delivery deadlines were exactly defined.

But, supplier did not pay attention to arrangement of the phases, what is very important since it had to be finished off on the spot. The planner, also, overlooked clearance on feroda brake pads, so the brake ring was finished off in the factory. Supervision asked for that in time so that there was no loosing of time. It is important that geometer at supervision was able to solve the problem.

Cooling of the generator is designed properly, so the temperature of the stator coil and of package is kept at 55 °C.

Transformers are produced by "Nie di Legnano", Milan-Minel-Belgrade. Complete design calculation is worked out by "Nie di Legnano", as well as type and routine tests for the first unit. Heat exchangers are water-oil. Note that supplier failed in the first construction, so the heat test was not successfull.

An important problem at assembling is alignment of transformer, since it has to be fixed in all three dimensions because of rigid connection on GIS.

Cranes are produced by "Litostroj" capacity of 3.500 kN and 500 kN and difusors are produced by "Metalna".

The main problem was to put draft tube crane in operation, we solved it later by ourselves.

Drainage is solved with pumps "Jastrebac". These are vertical pumps 30 kWx2, 90 kWx2 and 25 kWx2. They proved to be reliable. At first UPA emerged pumps were installed. They had to be replaced immediatelly as completely inadequate (For construction drainage well).

Experiences in assembling small equipment

As above mentioned it was delivered in time. Suppliers are:
- 10/20 kV „Minel”, Belgrade, SCG
- 0,4 kV „TSN”, Maribor, Slovenija
- automatics and control, protection, signalization and measurement, telecommunications „Energoinvest”, BiH
- hydraulic measurement - „Rittmayer”, Switzerland

Note that LV switches of great power are delivered instead from Italy, from Romania, and they are rather reliable.

It is important that constructors were suggested to build control hall at the same time when the dam (although there was resistance). As a result, when the first unit was put in operation, equipment in the control hall was assembled completely: panel board, signal system, measurements, switch 220V=220V, invertors, rectifiers, Rittmayer, AKU and telecomunications. On this level we had a big progress in velocity of finalization.

Connecting TL 400 kV HPP-TS Višegrad had been built two years before the first unit was put in operation (note problem with phase).

Mounting of small equipment was done by „Energomontaža” and „Unioninvest” from Sarajevo.