Once it is firmly bonded, gaps between the insulator and conductor will be eliminated and unwanted discharges will most less likely to happen. Insulation – there are four main points to remember when insulating the conductor. There must not have any spaces between the insulators and screens, there must be a lowest possible level of contamination, the insulations and screens must be smooth to obtain minimal stresses and the circularity and concentricity must be of the best value. Insulation screen – this is to ensure that the electrical field will only go through the insulator Outer Covers – metallic sheets may be applies to the cables so as to prevent moisture and water going in to the conductor VIII. Techniques for Increasing Current-Carrying Capacity Jao A.Perseverancia It has become a practice for a long time to match the continuous cable ratings to its short term post-fault rating. Although being relatively safe ,since The cable is being rated for the worst conditions such as short term faults, There are certain techniques that can be used to provide higher ratings to your system and prevent further cable failure. These certain PE4| GROUP 5
UNIVERSITY OF SANTO TOMAS FACULTY OF ENGINEERING España, Manila techniques involve the following Special Backfills, Sheath bonding type, low dielectric loss insulating materials and artificial cable cooling. A. Special Backfills -Aspects such as increase in soil thermal resistivity and cable overheating may lead to thermal runaway and cable failure. To remedy this , two types of speacialized backfills were formulated to prevent these kinds of events from happening, Selected Sands and Stabilised backfills, a. Selected sands - Sands to be used which were obtained from approved sources are selected so that the in situ thermal resistivity or on site thermal resistivity in its dry state is is not greater than 2.7 Km/W. It is preferred that the sand selected is coarse with a mixture of particle sizes but with good cohesion.The figure below shows the relation ship between the density of materials and thermal resistivity Therefore sands must be as compact as possible. b. Stabilized backfills -Stablilized backfills are special backfills so that the thermal resistivity of the backfills is less than or equal to 1.2 Km/W.There are two types of stabilized backfills used, Cement- bound sand and a Sand-gravel Mixture. Cement-bound sand uses sand mixed with cement in proportions of 14:1.On the other hand, sand- gravel mixtureuses a 1:1 proportion wherein gravel must have a nominal particle size of 10 mm. In a backfill, The space between a cable and the inner surface of a duct is made up of air and exhbits poor thermal transmission properties which may cause the cable to fail.
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- Spring '18
- Kenneth Sinson
- UNIVERSITY OF SANTO TOMAS, power cable