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Ispitivanje specifične toplotne otpornosti i vlažnosti kablovske posteljice u laboratorijskim uslovima
The examination of the specific thermal resistivity and moisture of cable bed in laboratory conditions
Keywords: power cable; specific thermal resistivity of the soil; soil drying; temperature; cable bed
Sažetak
U ovom radu su najpre ukratko predstavljeni primeri promene maksimalne dozvoljene struje za jedan kabl u zavisnosti od vrednosti specifične toplotne otpornosti zemljišta rz. Istaknuta je projektantska praksa da se za proračun trajno dozvoljenog strujnog opterećenja kabla usvaja konstantna vrednost rz. Potom su za dva slučaja, uzorka neporemećenog zemljišta sa kablovske trase i uzorka posebno pripremljene kablovske posteljice, primenom metode iglaste sonde u laboratoriji dobijene vrednosti rz. Ukazano je na razlike u izmerenim vrednostima specifične toplotne otpornosti i vlažnosti uzorka prilikom ispitivanja uzorka isušivanjem u termokomori i isušivanjem na temperaturi ambijenta. Za isušivanje u termokomori su, na osnovu iskustava za kabl sa izolacijom od umreženog polietilena, odabrane temperature u opsegu od 60°C do 90°C, dok je u drugom slučaju temperatura oko 20°C u dužem vremenskom periodu. Dobijeni rezultati su analizirani i prikazani tabelarno i grafički. Na kraju rada izračunata je vrednost vlažnosti uzorkovane kablovske posteljice, koja odgovara vrednosti rz od 1 (K m/W) i dat je predlog sastava odgovarajuće posteljice koja omogućava projektovanu vrednost trajnog dozvoljenog strujnog opterećenja kabla.
Abstract
This paper at first hand briefly presents examples of the maximum permitted current changes for one cable depending on the specific thermal resistance soil value rz. It is emphasized that the permanently permissible current load of the cable should be calculated using a constant value rz in design - Then, for two cases from field, two samples were examined. The first sample is undisturbed soil from the cable route and the second sample is specially prepared cable bed. The corresponding rz values were obtained using the needle probe method in the laboratory. The difference is pointed out between the measured values of specific thermal resistivity and moisture of the samples. Some samples were being dried in thermal chambers and the others were being dried on ambient temperature. For drying in thermal chambers, based on experience for cross-linked polyethylene cables, the temperatures of 60°C to 90°C were selected for the first case while in the second case the temperature of 20°C over a long period of time was applied. The results obtained were analyzed and shown in appropriate tables and graphs. At the end of this paper the moisture of the soil was calculated for sampled cable bed that corresponds to the value of rz of 1(Km/W). The composition of the cable bed that allows the designed value of the rated power of the cable to be achieved is proposed.
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