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Bibek Rai: Supervising Engineer II (Transmission Design)

Abstract:

The study focuses on modeling the HVTLs in PLS-CADD and assessing the ROW requirements for standard design spans, considering both I-string and V-string suspension insulators. Further, the electric and magnetic field profiles below the HVTLs are generated at 1.8m height above ground to estimate the maximum values below the line and at the edge of ROW. The result suggests that the existing practice of 46m and 30m ROW for 400kV and 220kV TL for standard design spans are sufficient and the use of V-string insulators could help reduce the ROW requirement by as much as 17%. However, it is revealed that the existing practice of 18m ROW for 132kV TL is not sufficient for both I-string and with V-string insulators. The electric and magnetic fields during maximum sag no wind conditions are found to be within the recommended limits by the International Commission on Non-Ionizing Radiation Protection (ICNIRP) at 1.8m above the ground. The research results can provide a basis for determining ROW and estimating the electric and magnetic field of HVTLs in Nepal.

Introduction:

Overhead AC transmission lines are predominant in Nepal. With the increasing generation capacity, integration of renewables and distributed energy resources, increasing local energy demand and the possibility of cross-border energy sale, the requirements for high voltage AC transmission lines (HVTLs) will be more than ever. The Nepalese transmission network comprises 400kV, 220kV, 132kV, and 66kV single-circuit and double-circuit transmission lines. There are many transmission line projects under construction and many more under feasibility study and design phase.

Impact and Problem Statement:

Transmission line projects affect forest areas and local communities, particularly villagers whose lands are used for the construction of transmission towers and the installation of conductors. These projects also impact natural resources, such as crops and trees, especially when the transmission lines pass through agricultural or forested areas.

Right of Way (ROW):

The Right of Way (ROW), also called Transmission Corridor, is a minimum safety corridor around power lines to meet the requisite safety clearances as well as the electric and magnetic field exposure limits. This strip of land is also required by utilities for constructing, maintaining, and protecting its transmission lines. The ROW of transmission lines mainly depends upon voltage, span length, type and size of conductor, wind speed, structure configuration, altitude, and electric and magnetic fields.

Existing Practices:

The existing Right of Way (ROW) practices for high voltage overhead transmission lines (HVTLs) in Nepal are based on the voltage level and are considered the same values irrespective of design span lengths, type of conductor and tower configurations. There is not any established practice and approach for estimating the electric and magnetic fields of HVTLs.

Conclusion & Recommendations:

The study assessed the ROW requirements and electric and magnetic fields of HVTLs in Nepal. The results indicate that the existing practice of ROW for 400kV and 220kV TLs is sufficient, whereas the ROW for 132kV TL should be increased to 23m. And the use of V-string insulators could help reduce the ROW requirements for HVTLs. Further, it has been found that the electric and magnetic fields below the line and at the edge of the ROW of HVTLs are within the recommended safe limits by ICNIRP.

The following recommendations have been made for transmission line utilities in Nepal to make the HVTLs more resilient and compliant with international practices:

1. It is recommended that the minimum ROW width based on the voltage level of the transmission lines shall be established along with provision for flexible ROW width based on geographic, environmental and social settings, such as for rural, urban and forest areas.
2. It is recommended that provisions and approaches for estimating electric and magnetic fields below the line and at the edge of the ROW shall be introduced and made mandatory, to be in-line with international practice of public safety. This will help address the public health and safety concerns relating to the effects of electric and magnetic fields from HVTLs.

To read the full paper please visit: www.ieeexplore.ieee.org