Compact Overhead Line Design

A compaction of an overhead power line can be considered as a reduction of the line’s cross-section. This reduction implies smaller horizontal and vertical distances of the phases or poles, which has consequences in different aspects. This chapter introduces the general aspects to be taken into consideration for line compaction and the issues that need to be covered for a reliable design.

Compaction of overhead power lines influences, inter alia, the surface gradient of the conductor, resulting in corona-related effects. These include Audible Noise (AN) and the Radio Interference (RI). The Electric Field (EF) and Magnetic Field (MF) at ground level are also impacted. The regulatory limits for these parameters can become the governing factor for a compact line design. Additionally, for AC overhead power lines, compaction also affects the Surge Impedance Loading which impacts the ability of the line to transmit power. This chapter analyses how the design decision (e.g. conductor size, bundle configuration, phase to phase spacing, height, etc.) influence the electrical parameters that can affect the design limits and the power flow in AC lines.

This section describes the voltages and overvoltages that impact a transmission line, such as switching and lightning surges. It discusses the changes in the insulation stress and withstand characteristics due to compaction of overhead power lines, highlighting the differences in the insulation design between AC and DC overhead power lines and proposing some mitigation measures, like the placement of surge arrester devices. The AC system overvoltage stresses are the input of the insulation coordination study for the design of clearances and of the insulator chain of the AC overhead power lines. On the other hand, for DC lines these designs are usually determined by the pollution performance requirements. Note this is not a complete description of the insulation coordination process but describes the voltages and overvoltages that impact a compact transmission line.

This section describes the different phase or pole configurations that can be used for compact lines. They need to be divided into Right of Way (ROW) reduction and Surge impedance Loading (SIL) improvement (only for AC lines). For ROW reduction examples such as vertical configuration, compact delta with composite post cross arms, etc., can be described. For SIL improvement, increased bundle diameter, asymmetrical shaped bundles in delta or inverse delta with composite cross arms are proposed for study. The DC overhead power lines do not improve the power flow by modifying pole spacing or the bundle diameters, unlike in AC overhead power lines, where these parameters affect the surge impedance loading of the line. Therefore, in DC compact lines, the different pole configurations pursue a reduction of the Right of Way, a reduction of the visual impact and/or an optimisation of the corona-related effects (electric field, ion current, audible noise, etc.). The limits to these effects may often be the governing aspects of the design of compact DC lines. The overall design of a compact line in general considers this configuration as a primary concern, which is complemented with the conductor selection process, tower and foundation designs and hardware solutions. All these aspects are included in this section, which also gathers a general discussion on galloping, as it may be one of the major issues in compact lines due to the reduced clearances between phases or pole conductors and/or between them and the groundwires.

The principle to perform live line work on compact lines is no different to a conventional one. However, actual live line experience with compact designs is still limited. This design should consider live line work requirements during the early stages, to maintain the minimum approach safety distance (MAD), as it might become one of the governing design parameters of a compact line. The major differences between compact and conventional line live work can be summarized as: (1) Reduced electrical clearance. (2) More complicated insulator configuration. (3) Extensive use of Non-Ceramic Insulators (NCI) or composite insulators. This chapter revises the methods and techniques used for live line work on a compact line, the tools, the theory, the control of overvoltages and the experimental practices used by different utilities around the world, analysing the differences between HVAC and HVDC lines and other considerations to be taken into account.

In general, it can be stated that there are no specific differences between the construction of conventional lines and that of compact lines. The same general techniques are used for stringing, erecting supports, or completing the civil works, irrespective of the pole spacing. The differences can be found in the particular design of the line components (tower/pole, bundle configuration, cross-arms, fittings…). It is thus suggested that each line construction be dealt with on a case-by-case basis, as is normally the case for a conventionally spaced line design.

The electrical parameters are subject to the line configuration. This chapter analyses different tower configurations for AC and DC line and determines the electrical parameters for each configuration. The sensitivity of the phase and pole configurations in relation to the electrical parameters is described. In addition the various standards employed in the world relating to electrical values and limits are described. The objective of this section is to get a measure or quantification of the influence of compaction (i.e. the reduction of horizontal and/or vertical distances) on the line electrical design. The sensitivity to various parameters is evaluated to quantify the influence of the parameter change. Different configurations and geometries can be analysed to fulfil the regulatory requirements or limits related to the electrical parameters. A summary of these limits in several countries around the world is also included for information.

This chapter comprises eighteen cases of line compaction designs from all around the world, both in AC and DC. These cases cover different practical issues faced by real projects, providing solutions and alternatives. Some of the issues discussed in these examples are phase or pole layouts and rearrangements, crossarms designs, insulator and accessories configurations, corona-related phenomena, mechanical loads and weather considerations, use of surge arresters, live-line maintenance works, compatibility with other infrastructures, etc.

This section is to present the general information and idea of voltage upgrading as it might be considered as compact overhead power line design in some cases. Voltage upgrading means to increase the operating voltage of an existing overhead power line already in operation. This is normally done in order to increase the transmission capacity of the line, considering that it is also necessary to maintain or enhance the required reliability after years of operation. The chapter revises the issues to be considered in voltage upgrading (electrical issues, mechanical performance, environmental concerns, etc.) covering both AC and DC lines. Also real case studies are included for AC and DC lines, explaining the main challenges and solutions adopted.