Figure 8.2 Example of stabilized backfill PE4| Group 5_5EEB
UNIVERSITY OF SANTO TOMAS FACULTY OF ENGINEERING España, Manila B. Sheath bonding type When Installing Distribution voltage cables, it usually is installed with solidly bonded sheaths to minimize sheath circulating currents produced by magnetic flux on single core cables that links the conductors and sheaths. To remedy this, A trefoil formation is usually made, however this proves futile since it has poor heat dissipation and there is considerable heating effect upon each cable. Although this is not a problem for cable ratings up to 33 kV, Ratings above 33 kV may have a very tough time. In exchange for the trefoil formation, specially bonded cable systems are used. The way these systems work involves earthing the single core sheaths at a single point and insulating all other points. By doing this, sheath losses are eliminated, and space can be available between the phase cables and reduces the mutual heating effect. To protect the sheaths from transient voltages arising from lightning and switching transients, sheath voltage limiters are fit at all joint and sealing end points where the sheath is insulated. The graph below shows the induced sheath voltages in proportional with the size and spacings of cables. There are three variations of specially bonded systems that are used which are end-point bonding, mid-point bonding and cross-bonding. Figure 8.3 Sheath Voltage and Spacing /Diameter relationship a. End-point bonding - This system earths one termination and insulates the other with inclusion of Sheath voltage limiters. A separate earth continuity conductor is needed to account for the fault currents that usually passes through the cable sheaths. PE4| Group 5_5EEB
UNIVERSITY OF SANTO TOMAS FACULTY OF ENGINEERING España, Manila Figure 8.4 End-point bonding schematic b. Mid-point bonding When the length of the cable is too long, A mid-point bonding system is used wherein the middle of the cable is earthed and all other terminations is insulated and fitted with sheath voltage limiters. Figure 8.5 Mid-point bonding Schematic c. Cross-bonding - In this system, the route is split into major section sections each of which contains three drum lengths fitted with insulated flanges. At every third joint the sheaths are connected, then at all other positions they are joined which ends in all sheaths being connected in series. The combination of sheaths at the third joint is made inside a link disconnecting box. PE4| Group 5_5EEB
UNIVERSITY OF SANTO TOMAS FACULTY OF ENGINEERING España, Manila Figure 8.6 Cross-bonding process Figure 8.7 Cross-bonding Schematic C. Low Dielectric Loss Insulating Materials - Simply changing the type of insulating material your cable uses may have profound effects on the rating your cable can handle. Having Low dielectric Loss Cables offers advantages over the conventional paper insulated, fluid-filled cables. Cables such as PPL or Polypropylene Laminated cable and XLPE or Cross-Linked Polyethylene. The
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- Spring '18
- Kenneth Sinson
- UNIVERSITY OF SANTO TOMAS, power cable