28 11�!! AGENDA 2.1 Valve Voltage The largest VSC HVDC converter valves so far delivered were for the Cross Sound Cable project in the North Eastern USA. Those valves are rated ±150 kV de and handle 330 MW active power at 1 050 Arms AC-current. Valves with the same voltage rating are also installed in the Murray Link project. The Estlink project increases the active power to 350 MW, but keeps the dc-voltage at the same level. These projects have prefabricated converter valves mounted in modular metal enclosures. Most of the assembly and testing of the valves is performed at the factory in a controlled environment, so the time at site is reduced. The enclosures also separate the valves mechanically and shield the electromagnetic disturbance caused by switching. When increasing the voltage to 300 kV, the enclosure concept will have to be abandoned due to insulation distances. lncreasing the valve switching voltage leads to higher influence of stray capacitances. The effect of !his, combined with the commutation inductance in the valve, has to be accounted for in the design. The challenge of increasing the DC voltage to ± 300 kV is to achieve sufficient voltage sharing between the series-connected IGBT components in both switching and blocking conditions. 2.2 Valve current Lowest valve cost per MW is achieved at maximum AC-current. The main limiting factors are maximum semiconductor temperature and switching capability of the IGBT module. The temperature of the IGBT is governed by the losses generated and the thermal resistance of the cooling arrangement. Losses come from two main contributors: conduction and switching. Conduction losses may be reduced by using larger semiconductor area. Switching losses depend on the switching time and the voltage and current at the switching instant. The switching frequency then determines the average switching losses. Fas! switching and a low switching frequency reduce the power dissipation. However, the switching frequency affects controllability of the converter so there is a trade-off between them. High switching speed, i.e. fası transition between conducting and blocking state, is realized through specially adapted gate driving. The Estlink project, to be commissioned by the end of 2006, has driven the AC-current requirement from 1050 A to 1130 A at rectifier. This increase of current handling on the same IGBT has been reached by improving the heat sink and by optimizing the switching pattern. For higher current ratings, !here is a need to increase the semiconductor afea. Such bigger components have been developed.Using extensive test results from single-pulse tests, fre uenc tesis ower c c ıng es s, sımu a ıons an measurements it is predicted !hat up to 1 740 A converter AC current may be reached. 3. Cable system The installed amount of polymeric insulated HVDC cable has become impressive. Circa 1200 km cable has been installed ENERJi DÜNYASI EYLÜL 2006 since the start at Gotland, Sweden in 1999. The number of inservice years multiplied with the length of cable gives 4200 km x years at the enci of 2005. The first three installations were land cables, but the latest installed cables, the Cross Sound Cable projeci and the Troll A projeci, are submarine. in the year 2006 another submarine cable projeci will be realized, the Estlink project. 3.1 Joints Terminations, stiff prefabricated joints as shown in Figure 2 for land installation and submarine repair as well as flexible Figııre 2. Prefabricated sıiff joints. Leji: ıhe electrical design, right: the meclıanical design. joints were used in these projects. The electrical design of the stiff submarine prefabricated joint is the same as for the land design, but the mechanical design demands a combination of mechanical strength and water tightness in a more harsh environment. The water tightness was achieved by swaging down a lead sheath over the joint after which it is soldered against the lead sheath of the cable. To maintain and ensure the continuity of the PE-sheath, a heat shrinkable tube was employed over the lead tube. The mechanical protection consists of a galvanized steel pipe and stiffeners giving the joint good crush resistance against a rocky seabed. The mechanical, thermal and electrical properties of the flexible joint have to match the cable. 3.2 lncreasing the laying depths The laying depths of the above mentioned projects are moderate; 50 m for Cross Sound Cable and 340 m for Troll A. it is of interest how deep cables can be laid using conventional design technique. The maximum laying depth of a cable is dictated by its design and by the test force during a mechanical tensile bending test. The test force is calculated using CIGRE "Recommendations for mechanical tests on submarine cables" published in Electra No.171, April 1997.The test force depends on the cable weight, the allowable bottom ns on. e ynamıc ensıon depends on the mass of the cable and the vertical movement and the circular frequency of the laying sheave. One could say that the dynamic tension is defineci by the weather and wave conditions. After test the cable sample undergoes a visual inspection. The test shall not give "permanent deformation of the conductor or armoring" according to the above
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