named recommendation. This is an important and limiting demand. The conductor and armoring may be deformed in terms of local length increase and local diameter decrease. This happens typically at the weakest point of the sample, often the joint. This deformation depends on design parameters as for example maximum allowable tension in conductor and armoring. As no exact definition is given in Electra's recommendation concerning "permanent detormation" the allowable conductor tension and armor tension are more or less free to choose. Manufacturers tend to stay on the safe side and choose values between 70 and 100 N/mm2 for jointed copper conductors. When one allows larger tension, greater laying depths can be reached. To get a rough feeling, some depths are calculated for two different cables and two different wave conditions (See Table 1). The test was performed with shorter temperature cycle duration to include more cycles within 20 days. However, it was ensured that steady state temperatures for conductor and sheath were reached in each cycle. 3.3 lncreasing the voltage Extruded HVDC cable systems are commercially available up to 150 kV DC. The next step is to develop and commercialize these cable systems at 300 kV DC using the knowledge and design base of the 80 and 150 kV levels. A prototype polymeric 300 kV cable was produced and tested together with prototype accessories. The test program was set up according to CIGRE publication "Recommendations tor testing DC extruded cable systems tor power transmission ata rated voltage up to 250 kV". The test steps are listed here below. Table 1. Laying depths for different wave heigts b h , movement peridost and cable desing. The tension in the armoning never exceeded 170 N/mm 2 Max. Laying Max. Laying Conductor depth [m] depth [m] Cable type tension Wave Wave conditions 1 conditions 1 [N/mm2] tbh= 3 meter bh= 3 meter = 6 seconds t = 12 seconds Very Single core, 300 mm2 conservative 1050 1 150 copper, 8 mm < 70 isntrseuelaltwioirne, 4armmmoridnoguble Conservation 1500 1650 < 70 Very Single core, 1300 mm2 conservative 950 1050 copper, 12 mm < 70 isntrseuel al twioirne, 5armmmoridnoguble Conservation 1350 1500 < 70 GÜNDEM / AGENDA O 12 cycles at a negative DC voltage of 1.85 x 300 =-555 kV. Every cycle consisted of 8 hours heating followed by 16 hours of natural cooling. □ 12 cycles at a positive DC voltage of 1.85 x 300 =+555 kV. O 3 long cycles at a positive DC voltage of 1 .85 x 300 =+555 kV. Every cycle consisted of 24 hours heating followed by 24 hours of natural cooling. O 1 O switching surges with U p2s =+630 kV and 1 O switching surges with U p2o =-350 kV superimposed on a DC voltage of U O =+300 kV DC. O 1O switching surges with U p2s =-630 kV and 1 O switching surges with U p2o =+350 kV superimposed on a DC voltage of U O =-300 kV DC. O After the superimposed switching surge withstand test,the test object was subjected to a negative DC voltage of -555 kV DC during 2 hours. The test objects passed the tests without any problems. After this type test extra 8/16 cycles were performed at voltages increasing 50 kV per cycle starting at -600 kV. At a negative DC voltage of-740 kV the voltage was not increased anymore, but kept at that level. in total five consecutive 8/1 6 cycles were performed at this level. After that the test was stopped. No breakdown occurred. 4. System design When designing a VSC HVDC system, a large range of parameters has to be included in the optimization. Two of the main components are investment cost and electrical losses. Converters are usually more cost-efficient in the high current end of the spectrum, in terms of money per MW. Losses in the converter are basically scalable with power rating. DC cable systems are generally more cost efficient in the high voltage end of the spectrum and the losses are a function of conductor area and current, independent of voltage, in contrast to AC-cables. Any actual project will in the end have to be justified in economical terms after an evaluation of the total cost of installation, operation ar:ıd losses compared to system benefits and payback. One important and challenging task far the grid owner will be to put a value on the increased functionality and stability of the grid that comes from introducing VSC HVDC. 4.1 System development The main purpose of the HVDC system development is to realize a robust and simple construction, while keeping the total operation losses low. The development process involved comprehensive evaluation of station topology, switching frequency of I GBT's, and pulse pattern applied. The first VSC HVDC system built mainly for demonstration purposes, connecting Hellsjön to Grangesberg in Sweden, was inaugurated in May 1997.HVDC VSC and STATCOM's with IGBT 's as switching elements have been in commercial operation since 1999. Since then, different topologies and system designs suited for different applications have been ENERJi D0NYASI EYLÜL 2006 29
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