A Brief About Overhead Power Transmission Lines (OHTL) (Part 1 – OHTL supports)

Introduction

The electrical power system comprises of three main subsystems: generation, transmission, and distribution. The transmission system is responsible for connecting the generation side to the consumption or distribution side as shown by figure 1.

Most of the power transmission systems are operating in Alternating Current (AC) with several high voltage levels that increases with the increase of the line length to reduce the transmission losses.

AC power transmission lines might be designed to be Overhead Transmission Lines (OHTL) or Underground Transmission Cables (UGTC). The majority of AC transmission lines are overhead transmission lines and the main reason is that the cost of underground cable system is considerably more compared to overhead lines. In general, OHTL are generally used for power transmission at long distances in open county and rural areas in addition to that, OHTL are generally with longer lifespan, easier to install and repair and require lower manufacturing & construction costs in comparison to UGTC.

Sample OHTL and UGTC are shown by figures 1 & 2 respectively:

Overhead power transmission lines components

An OHTL comprises of many several components, the utmost important remain both: supports and conductors. Other components are insulators, insulator and conductor fittings, overhead ground wire, spacers and brackets. Following figure 4 shows samples of OHTL designs with supports and some other components indicated.

Types of OHTL Supports

The main function of the of the OHTL supports (also named: towers, structures or pylons) is to provide the necessary mechanical support for the transmission conductor at the design electrical clearance from ground and also from other phase conductors. In addition to that all towers have to withstand all kinds of environmental effects.

OHTL supports can be made using different types of materials such as wood, concrete, steel, metal alloys and recently fiber reinforced polymers were introduced but still rarely used.

OHTL supports according to their mechanical design are classified into two types:

1.     Self-supporting; which might be of the following types (examples are shown in figure 5)

-         Lattice steel towers.

-         steel poles.

-         Steel reinforced concrete poles.

-         Wooden poles

1.     Guyed or non-self-supported: although those types of supports were introduced at the second quarter of the 20th century, but their use declined with time for the transmission purposes compared to the self-supporting ones. However, guyed supports can be in the H, V or Y shapes. Another use of the guyed supports is for fast transmission system restoration in case of transmission supports collapse or for transmission circuits diversion during projects or modification works and for this purpose, some special designed guyed supports called Emergency Restoration Systems (ERS) that can be assembled in short time in different configurations are used. Respectively a sample H-shape guyed power transmission support and an ERS support are shown in figure 6.

Moreover, OHTL supports can be classified according to the mechanical support they are providing to the conductors into:

1.     Angle supports: used when the line is changing its direction with deviation angles varies from 30 to 90 degrees in a standardized value (e.g. 30, 45, 60, etc.).

2.     Suspension supports: used for sections were the line is running straight with some allowed deviation usually not to exceed 10 degrees.

3.     Terminal supports: usually used at substation entry as they are designed to take full mechanical tension on one side and no tension (or slack) on the other side.

Furthermore, OHTL supports might be classified based on the number of circuits they are carrying (supporting); and so, it might be single circuit support, double circuits support or quadrable circuits support and in some rare cases, OHTL support is designed to carry one or more transmission circuits in the upper part of the support and a distribution circuit/circuits or insulated fiber communication cable in the lower part but this kind of supports utilization will be only possible with OHTL with short spans between supports.  

General considerations for OHTL supports design

In this part, the basic steps for the design of an overhead line supports are determined in brief remembering that in the design process, and although it is to some how standardized process, the design is highly affected by the data and requirements provided by the utility itself which are to be as accurate as possible to reflect the local working conditions of the line to be met by the design.

Anyhow, before starting the design of the line supports, the following details shall be provided:

1.     The line electrical requirements such as:

• The minimum clearances between conductors, and between conductor and the support itself.
• Number, type and location of ground wire/wires with respect to the outermost conductor.
• The minimum required mid-span clearance of the lowest conductor above ground level or to other conductors at intersection with other lines conductors.
• Minimum Insulator required creepage path and hence the minimum length of the insulator assembly.
• Electrical loading requirements (this will determine the number of conductors per phase and number of circuits per support.
• Short circuits, Etc.

1.     The line minimum mechanical strengths requirements and required safety factors.

2.     Environmental & and climatic conditions including the proposed preliminary routes/ terrains.

Based on the above details, the supports design process is conducted to provide supports that accomplish the above requirements as below:

a)     Selection of the basic supports configuration and minimum height to meet electrical design requirements. The supports minimum height is governed by:

-         The minimum permissible ground clearance.

-         The maximum sag.

-         Phase conductors vertical spacing.

-         Ground wire to top conductor vertical clearance.

 b)     Determination of the mechanical strength of the selected support configuration: the selected support configuration shall be designed mechanically to withstand the sum of the mechanical loads resulting from electrical requirements, climatic effects, terrain and the added safety factor. It is always to bear in mind during support design that the design the design is meet the loading requirements within the economic requirements of the utility.

The mechanical loads to be determined for the purpose of OHTL support design are:

I. Wind Loads: this step includes the determination of wind loads affecting both, the support and conductors.

II. Conductors load: this step includes the calculation of mechanical loads resulting from conductor’s tensions (the maximum working tension and Every day stress), short Circuit conditions, ice accumulation in cold weather countries.

III. Induced vibration effects.

IV. Finalize the design based on combined Loads: this is necessary to achieve a reasonable design that matches the requirements without being oversized and achieved by the assumption that not all of the factors aforementioned will be at highest together.

V. Applying the safety factor.

VI. Design verification through prototyping and type testing of the supports.