Limiting Factors for the Pul-e-Khumri to Chimtala Transmission Line

These factors include the thermal capacity of the conductors, the ground clearance limit, the structural limits of the towers, the voltage limit, and the dynamic stability limit. The thermal capacity of the conductors is a critical factor in determining the power transfer limit of the line. Tetra Tech has not been provided with the necessary design information to assess the thermal capacity of the Pul-e-Khumri Line, including the conductor size, type, and number of conductors per phase. However, conflicting data exists in some documents, suggesting that the thermal capacity of the existing conductors may be adequate for 450-MW. The ground clearance limit is another factor that must be evaluated to determine the power transfer limit of the line. As the amount of power transferred on the transmission line increases, the electrical current increases, and the conductor heats up, causing it to sag toward the ground. Tetra Tech has insufficient information to determine the ground clearance limit, and the line design information, including plan and profile drawings, conductor information, and conductor sag-tension information, is necessary to assess this limit. The structural limits of the towers are also a critical factor in determining the power transfer limit of the line. The structural system that is currently being utilized to support the transmission system may not be adequate to support an alternate conductor if required. Tetra Tech has no data to evaluate the limits for supporting the conductors and the forces imposed by both conductor tension and angular forces. The voltage limit is another factor that must be evaluated to determine the power transfer limit of the line. Voltage drop on long AC transmission lines is a relatively complex phenomenon, and the major factors affecting voltage drop are the length of the line, the series inductive reactance of the line, and the power transmitted on the line. Tetra Tech has determined that the maximum power transfer on the Pul-e-Khumri Line is likely to be voltage limited, based on studies performed by Siemens-PTI. The maximum power transfer, with shunt capacitors for voltage support, is approximately 160 MW. The dynamic stability limit is less of a concern than it typically is, as the Pul-e-Khumri line will not be connected to the Afghan electric transmission system. However, the addition of dynamic reactive power support in the Kabul area to increase power transfer across the Pul-e-Khumri Line may introduce the opportunity for dynamic stability issues. In conclusion, the replacement of the current conductors should be considered no more than one possible piece of the total effort required to achieve a 450-MW capacity of the transmission line. The thermal capacity of the existing conductors may prove adequate for 450-MW, but this cannot be confirmed without the line design information. Upgrading to high temperature/low sag conductors of similar size and weight may adversely impact the total electrical clearances between the conductors and ground, and the structural costs and practicality of upgrading cannot be estimated with confidence without the design tensions of the line or the tower structural data.