Application of High-Capacity Conductors for Uprating Transmission Lines Capacity in Nepal

Abstract

Electricity demand and market penetration in Nepal is increasing day by day due to increased industrialization, expand urbanization, large population density, change in energy consumption habits of the consumers etc. Utility has to face multiple challenges to expand new transmission line in urban and semi-urban areas due to lack of availability of corridors triggered by the high real-estate cost, infrastructure development, forest/ecology conversation etc. Additionally, transmission line projects usually have limited execution schedule to comply with the increased load demand and project completion to match with new hydropower projects. To maximize the power transfer per unit RoW, replacing the old ACSR conductors with newly developed HTLS conductors would be the best solution which have higher power transfer capacity in the order of 2 to 3 times. Moreover, it can be completed in lesser time span without any significant work and budget plan vis-à-vis a new line. The same transmission line footprint might be used without any major modification in the existing tower. Load flow analysis of the INPS using the tool: DigSilent PowerFactory 15.1. is carried out at different generation scenario considering the maximum load constant throughout the study. Total internal generation of 2438.72 MW and maximum load of 2054.59 MW is recorded from secondary sources which are interconnected in INPS. At 100% generation scaling, load flow result is noted with the surplus power of 230.07MW and the grid losses of 154.06 MW which accounts the loss of 6.31% of the total system in-feed. At that scenario, ten (10) 132kV line sections (515.82 ckt.-km length of ACSR Bear and 28 ckt.-km of ACSR Panther) is found to be overloaded by more than 100%. Maximum line loading of 167.22% is noted at Dhalkebar-Mirchaiya section (ACSR Bear) followed by 154.44% at Kusaha-Kataiya (ACSR Bear) section. Gandak-Bardaghat line section having ACSR Panther conductor is also noted to be overloaded by 116.03%. At decreased internal generation, power deficit is fulfilled by the external in-feed which caused the total grid power loss to increase gradually. Critical 132kV line sections found at 100% generation scaling are considered for further analysis to use equivalent HTLS conductor. Replacement of insulators along with erection accessories are also considered in the study. ACCC OSLO conductor having equivalent cross section, weight per unit length and meeting required ultimate tensile strength is considered as equivalent conductor and found the most compactible for both ACSR conductor types. ‘IEEE 738 Standard’ is applied to calculate the derated current carrying capacity of the candidate HTLS conductor under NEA service condition which is found to be 2.45 times higher than ACSR Bear and 2.84 times higher than ACSR Panther conductor for the value of maximum allowable current which is quite satisfactory to consider as an equivalent conductor. Line loading is reduced drastically at all critical line section after conductor uprating viz. a maximum value of 62.78% at Lamosanghuv- Khimti section and 54.83% at Dhalkebar - Mirchaiya section. Line loading value of 25.57% is minimum at Gandak- Bardaghat section. It is also remarkable that the surplus power in INPS generation is found to be 245.67 MW which is more by 15.60 MW than the case before. It means grid power loss is decreased by 15.60 MW which is only 5.67% of total internal generation and it was 6.31% before conductor replacement. vii Sag-tension analysis of old ACSR and HTLS ACCC OSLO conductors is also carried out using PLS-CADD tool to study the behavior at different weather cases. It’s all complied the required criteria for not violating the existing sag at any operating temperature condition of each individual conductor. Total cost estimated for conductor replacement is found to be USD 25.798 Million which is almost equivalent to set up a new 15.60 MW hydropower plant and it is considered as an analogy of grid power loss reduction after conductor replacement. The test of old ACSR conductors, insulators, erection hardware and fitting accessories etc. is recommended as per prevailing IS/BS at accredited laboratory to find the remaining useful life and its technical viability to re-use in new/old transmission lines.