UNIVERSITY OF SANTO TOMAS FACULTY OF ENGINEERING España, Manila POWER CABLE DESIGN AND SYSTEMS – MV, HW, AND EHV I. Basic Electric Theory Applicable to Cable Design Jose Lorenzo Z. Ladisla In engineering, every option must be considered to be able to meet a certain standard. The point where practical, economical, and technical aspects meet is the optimum solution to a problem. For instance, the utilization of underground power distribution system isn’t the best option for every areas in the country. Though the underground power distribution has its advantages, it is still better to use the overhead distribution system in some places. Cables have a wide variety of designs to meet various requirements. Since cables are placed in different locations, different properties are also needed so that the performance of the cable is at its best. Different insulations, armors, and protective finishes are incorporated to the cable to protect it from harsh environment. Current Carrying Capacity There are various factors that affect the current carrying capacity or ampacity of a cable. This section of the journal will discuss some of the factors affecting the ampacity of air cables and underground cables. Conductor Size - The cables are installed and spaced from a wall on a tray in trefoil formation. The size of the cable is directly proportional to its ampacity. The graph below shows how conductor size greatly affects the ampacity of the cable. Figure 1.1 Ampacity ratings with varying conductor sizes Solar Radiation Intensity PE4| Christian L. Ranada
UNIVERSITY OF SANTO TOMAS FACULTY OF ENGINEERING España, Manila - Exposure to solar radiation results to a higher temperature of the cables thus increasing the resistance and decreasing its ampacity. According to IEEE standards, solar intensity heavily relies on the type of atmosphere the area has. The table below shows how exposure to sunlight affects the cable’s ampacity. Table 1.1 Influence of solar radiation intensity on ampacity when surface absorption coefficient is 0.8 Ambient Air Temperature - Ambient air temperature has a huge influence on the cable’s ampacity. This factor is primarily considered in both IEC and IEEE standards. Figure 1.2 Ampacity ratings with varying air temperature Conduit - Enclosing a cable inside a conduit results to added thermal resistance of the conduit wall and raised temperature of enclosed air, this leads to lower ampacity ratings for cables. The graph below shows the effects of enclosing a cable inside a conduit and also how cable size plays a role on the effects of the conduit. PE4| Christian L. Ranada
UNIVERSITY OF SANTO TOMAS FACULTY OF ENGINEERING España, Manila Figure 1.3 Effect of enclosing a cable inside a conduit to its ampacity with varying conductor size Groups of Cables - Grouping cables results to hotter cables, and lesser cable ampacity. The cable diameter and the distance between circuits influences this effect, if the distance between groups overrides the critical ratio then the thermal proximity effect can be ignored.
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- Spring '18
- Kenneth Sinson
- UNIVERSITY OF SANTO TOMAS, power cable