The power transmission network has been constantly expanding to maintain uninterrupted transmission of power, provide evacuation infrastructure for power generation plants, and address regional demand-supply mismatches. Several new technologies have been introduced and incorporated in the transmission sector to speed up the building and development of transmission lines and towers. High voltage direct current (HVDC) technology is a key focus area since it allows power to be sent across long distances with minimal losses. Furthermore, as network constraints and demands for high availability and capacity increase, power utilities are focusing on reconductoring and upgrading transmission lines. Meanwhile, new transmission tower designs are being incorporated to reduce right-of-way (RoW) requirements, assure project completion in a timely manner, and withstand multitude weather conditions. Besides, LiDAR-based aerial surveys and drones are being deployed for faster construction.
A look at some of the key technology trends shaping the transmission line and tower segment…
Transmission lines
HVDC is one of the country’s primary technical focus areas. It aids in the transmission of power across long distances with minimum loss. When compared to high voltage alternating current (HVAC), HVDC system can minimise transmission losses by roughly 50 per cent, and it is the only possible technique of transporting a significant amount of power through a cable system.
For instance, a bipolar HVDC line requires only two conductors compared with six conductors in a double-circuit AC line for the same transmission capacity, leading to smaller transmission tower configurations. As a result, construction costs of HVDC lines are lower than those of comparable HVAC lines after a break-even distance (for instance, 300 km for a 1,200 MW system) despite the additional converter costs. In addition, losses on HVDC lines are roughly 3.5 per cent per 1,000 km as against 6.7 per cent for comparable AC lines, improving cost-effectiveness in the long term. Newer HVDC technology such as voltage source converters can precisely control system voltages and frequency, enabling it to help restart the grid following a blackout. Furthermore, HVDC lines can operate at overloads (10-15 per cent higher than the rated capacity) for a limited period (less than 30 minutes). This increased capacity under contingency conditions gives system operators sufficient time to implement mitigation measures, thus improving system reliability and resilience.
Developers are also deploying advanced conductor technologies that provide enhanced performance over conventional overhead conductors. Some advanced overhead conductors include aluminium conductor composite reinforced, aluminium conductor composite core, and aluminium conductor carbon fibre reinforced. Utilities have used these conductors in a variety of applications to increase transmission capacity and improve line strength and robustness in harsh environments. Superconducting cables are another type of advanced tra-nsmission technology. They are composed of materials that have near-zero resistance at extremely low temperatures, offering little to no electrical losses if used in transmission. Superconducting cables can provide up to 10 times the maximum current-carrying capacity of conventional cables with the same cross-sectional area. Underground transmission cables are often used in dense urban areas where there is insufficient space or receptivity for overhead lines. In these areas, there is usually significant competition for limited underground space.
Dynamic line ratings (DLR) are also being deployed to give operators a greater ability to fully utilise capacity on transmission networks. DLR can provide greater insights into the performance of a line over time. Rather than relying on engineering assumptions and maintenance schedules, the real-time status of the line can be used for decision-making to mitigate component failures and improve reliability. Sensors are used to monitor, measure and transmit data on line conditions and ambient conditions that determine the maximum current-carrying capacity of the line in real time.
Reconductoring
As securing approvals to build a new transmission line is often very difficult, reconductoring existing transmission lines can double the capacity while using the existing transmission towers and RoW. Ampacity enhancement is the most prevalent method of increasing capacity in overhead transmission lines. To this end, a design change requires analysis of the line capacity with its existing conductor choice, taking into consideration various design constraints/ criteria including thermal limits. It is possible to quantify any untapped capacity in conductors in terms of ampacity by conducting a detailed analysis of a line. Modern conductors with improved strength-to-weight ratios, higher operating temperature limits and better high temperature sag performance can be deployed after a cost-benefit analysis. If the ampacity increase approach does not deliver desirable results, a voltage increase can be considered. However, the voltage increase approach may require suitable voltage options at the line’s ends, or a significant capex on end-equipment in order to move to a new voltage.
There are various policies, regulatory recommendations and transmission planning criteria for reconductoring and uprating of lines. The Central Electricity Authority’s (CEA) Manual on Transmission Planning Criteria (2013) also talks about reconductoring of existing AC transmission lines with higher ampacity conductors. As per the manual, the choice of reconductoring will be based on cost, reliability, RoW requirements, transmission losses, downtime, etc. In February 2019, the CEA also released guidelines for rationalised use of high performance conductors (HPCs). The guidelines state that HPCs are a reasonable and economical option for uprating short lines, which experience occasional high electrical loads because of an insignificant increase in electrical losses. For longer lines, reconductoring with HPCs may also be economical, if the frequency and duration of high current loads is less.
Towers
Pre-construction surveys are an important part of the transmission project development process. They help determine the precise and quickest transmission line path, as well as the number of towers required along the route. Since traditional surveying techniques such as walkover surveys are time-consuming and inaccurate, utilities are now adopting LiDAR technology and drones for surveying. Utilities are now deploying drones to analyse possible site locations, develop site layouts, generate 3D visualisations, and estimate RoW.
On the tower design front, utilities are adopting novel tower designs including pole structures, narrow base towers, and multicircuit towers to minimise RoW requirements, reduce visual impact, save cost, and ensure the ease of construction and installation. Monopoles offer faster installation and greater flexibility in design. They can survive adverse weather conditions and require far less space than lattice towers, thus lowering the RoW requirement considerably. Utilities are also making use of narrow-base lattice towers to save space. Multicircuit towers, which are intended to handle three, four, or even six circuits to efficiently transmit bulk electricity, are being used at high voltage levels, particularly in forest areas and at substation entries.
A strong and sturdy tower foundation is another important component of a tower, which helps it sustain strong winds, storms and other unfavourable weather conditions. Micropiling is a new tower foundation design. Micropile-based tower foundations are made up of piles having a diameter of less than 200 mm. Micropiles can be used in a variety of geotechnical conditions, making them an ideal choice for transmission projects in deserts, mountains and oceans. Precast foundations (used during limited construction durations), grillage foundations (used in firm soil locations) and reinforced cement concrete spread (used in a variety of soil conditions) are some of the other tower foundation designs. Utilities are also implementing online asset monitoring solutions to improve transmission network availability. Digital tools are being used for tower patrolling and problem correction.
Summary
There are many drivers for the uptake of new technologies in the transmission segment. The delays in issuing forest approvals and obtaining RoW are a key challenge faced by developers. Utilities are increasingly adopting new technologies to overcome this. Also, given the benefits of competitive transmission tariffs and faster execution, new technologies can help developers overcome execution challenges and complete projects in a shorter time span.
The government’s ambitious plan to expand renewable energy to 500 GW by 2030 is a key driver for grid expansion. To meet the future peak load, huge investments are required to strengthen and ramp up the country’s transmission infrastructure. The National Infrastructure Pipeline has estimated a capital expenditure of about Rs 3,040 billion for the power transmission segment between 2020 and 2025. Several transmission lines and towers technologies are available and emerging. They are cost efficient and environmentally friendly. Therefore, transmission utilities need to select the optimal technology based on their requirements.
