GK Dürnrohr
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The GK Dürnrohr ( German abbreviation for Gleichstromkurzkupplung Dürnrohr, that means Dürnrohr HVDC-back-to-back station) was a HVDC back-to-back scheme west of Dürnrohr substation, which was used for the energy exchange between Austria and Czechoslovakia between 1983 and 1996. The GK Dürnrohr had a nominal transmission rating of 550 MW. The nominal value of the DC voltage in the intermediate circuit was 145 kV. The transmission losses of the facility were 1.4%.
Planning and construction
With that planning of GK Dürnrohr was started in 1975, after a contract between Austria and Poland was agreed over electric power exchange by a power line running over the territory of Czechoslovakia. Construction work at the facility started at the end of 1980. At beginning of 1982 the facility was so far that with the assembly of the electrical components could be started. In the middle of 1983 the process of taking the station in service took place. After the middle of June 1983 first experimental energy exchange with Czechoslovakia took place. The official inauguration took place on 1 September 1983
Valve Hall
The static inverter of the plant is situated in a 29.8 meters long, 15.4 meters large and 13.8 meters high reinforced concrete building, which has at both long sides two bays for the static inverter transformers. For fire protection reasons the walls of the static inverter hall, which have a strength of 20 centimeters, were equipped with stone wool insulation. The entire building is covered with zinc coated sheet, which serves as electrical shielding and enclosure.
The static inverter hall has also a cellar, in which the air conditioning system, the water cooling system for the static inverters and the water treatment plant are installed. At the western end to the narrow side of the static inverter hall there is only separated by a fire protection wall the equipment building, in which there is an auxiliary control room, battery and electric rectifier rooms, storage and test equipments rooms and the ventilation devices for the air conditioning system. A window consisting of 3 glasses, which cannot be opened, allows a view from the equipment building to the valves of the static inverter. At the eastern end to the narrow side there is the smoothing coil.
Transformer
On both sides of the static inverter there are two three-phase transformers each dimensioned for a power of 335 MVA with a nominal winding ratio of 400: 63. At all used static inverter transformers the primary coils are switched in the star, while on each side of the static inverter one transformer has a secondary winding switched in star and the other in triangle. The transformers, at which primary and secondary windings are switched in the star, are of same type as the transformers at which primary windings are switched in the star and secondary windings in triangle. The transformers with secondary winding switced in triangle are however not identically with those with secondary winding switced in triangle.
Static inverter
The static inverter, which is implemented as twelve-pulse inverter, uses for each valve function a serial switch of 44 thyristors with a maximum blockade voltage of 4,2 kV and a maximum nominal DC current rating of 3790 A. The total number of thyristors used in the facility is 1056. They were with a wafer diameter of 100 mm at construction time the largest thyristors of the world.
Each static inverter consists of 3 thyristor towers, which are housed in the static inverter hall. Each thyristor tower contains a complete twelve-pulse branch of the static inverter. In these thyristor towers for each valve function 4 thyristor modules each switched into row are used, which are arranged in two floors. Between the thyristor modules of a floor e.g. the 1st and 2nd and/or 3rd and 4th is in each floor, there is a coil with an iron core. Parallel to the thyristor modules of a floor there is a capacitator. Parallel to each valve function there is a surge arrester in form of a varistor.
Each thyristor module consists of a series connection of 11 thyristors, to which all a series connection from a capacitaor and a resistor is parallelized. From the series connection of capacitator and resistor also the energy for the steering circuits of the thyristors is taken. As the thyristors and their steering electronics are on high voltage potential, the transmission of the ignition impulses from the control electronic on ground potential takes place via fibreoptical cables. A second optical waveguide cable allows the transmission of data from the thyristor module to the main control electronics on ground potential. As control a programmable controller of the system SIMATIC S5 is used.
The thyristors and the coils switched with them in series are cooled with deionozed water, which circulates in a closed cycle. The developed heat is delivered to a second cycle, in which there is a mixture of glycol and water. Over evaporation radiators the heat of this cycle is transferred to the environment. For maintenance purpose dedicated modules are exchanged against intact modules and carried in the repair and inspection room. For this a telescope lifting platform and a crane are installed in the static inverter hall.
Smoothing coil
At the eastern side of the static inverter hall there is a smoothing coil with iron core of 85 mH. It was built by the company ELIN and is as high-voltage transformers oil-cooled.
AC filters
As AC-fiters 4 series resonance circuits are installed on both sides of the plant. Each of the filter consists of a series-connection of a 2 microfarad capacitator with a coil to which a 615 ohm resistor is parallelized. One filter of each side uses a 41 mH air-core coil, while the other has a 29 mH-air core coil. For the purpose of reactive power compensation there is also on each power exit a bank of capacitors available. Their value is 2 microfarads for the exit of the line toward Czech Republic and 1 microfarad for the exit toward Austria.
Power line to Czech
The 102 kilometers long powerline to Slavetice substation in Czech Republic is a single-circuit 380 kV-line, which is installed on pylons for 2 electric circuits. In Czech two-level arrangement of conductors is used, while in Austria three-level arrangement is used. The line crosses the border near Kleinhaugsdorf.
Decommissioning
After the synchronisation of electricity grids of West and Eastern Europe on 17 October 1995 the plant remained in operatation until 31 October 1996, as Austria has in contrast to Germany no large 380 kV-grid. However after in Poland some power stations were equipped with an efficient frequency regulation device the power grids of Czech and Austria could be done interconnected directly without using a HVDC-interconnection. The high voltage switchgears were used in Vienna Southeast Substation and in Southern Burgenland, like the transformers, which had to be however modified for a secondary voltage of 110 kV. As the originally planned sell of the facility to Eastern Europe for an HVDC-back-to-back station, which would serve as an interconnection between the power grids of Eastern Europe and former Soviet Union, never took place, in 2007 the remaining parts of the facility were dismantled in 2007. The valve hall is used today by the former operator Verbund AG for operational uses.