Technical Paper: Enhancement of Existing Transmission Line/Corridor Capacity – The Necessity & Methods
India’s power grid has been in a state of rapid evolution due to increased load demand. The past decade has seen substantial changes in the Indian power ecosystem. Going forward, the future, which is dominated by urban evolution, evolving demographics, and expanding renewable will need to adopt innovative means of solving energy delivery challenges.
High rate of growth requires increasing transmission infrastructure at a rapid pace, which is turn needs more and more space on the ground for right of way (ROW) for transmission lines and substations. Moreover, the faster pace of addition in Grid of Renewable Generation (RE) has put additional pressure on reducing timelines for enhancing transmission infrastructure.
The solution lies in maximizing power carrying capacities of existing transmission corridors. This will help to not only meet the growing demand faster and more economically but also have a positive impact on the environment overall, as existing infrastructure use is maximized before deploying additional resources.
Hence, transmission system owners, operators and planners need to consider sufficiently wide range of solutions that can exploit additional capacity of the existing system without compromising safety margins. The possible means to achieve this is with help of Uprate and Upgrade of the existing infrastructure of Transmission.
Solutions for Uprate, help to increase and/or unlock the latent potential capacity of existing Transmission lines, in the same Right of Way (ROW) by means of reconductoring, use of Dynamic Line rating (DLR), use of Power Flow Controller and by simultaneously enhancing the life of existing infrastructure with tower coating solution.
Solutions for Upgrade help to transform the complete transmission backbone of the system with help of voltage upgradation of existing transmission lines with Insulated Cross Arms and or other solutions, with reduced additional Right of Way (ROW) in existing corridors.
These technologies, which are enablers of power unlock, can help Indian Utility & planners to build a robust & resilient Grid. The details of these technologies are explained here with their specific advantages and differentiation.
Dynamic Line Rating:
Transmission lines should ensure safe, reliable and assured power delivery. In India, renewable energy sources are being added rapidly while transmission lines traditionally had longer lead times to be built to evacuate the power. It may also happen that load on a transmission line has increased while alternate infrastructure to share higher load is still under development. In such situations, it would be great to have a solution which temporarily allows higher power to be safely carried on existing transmission lines.
Dynamic line rating (DLR) is an overhead transmission line real time monitoring system. DLR sensors are becoming popular globally with the utilities for enhancing effective line capacity. DLR once installed on the line conductor, provides real-time situational awareness of conductor temperature, sag, current, and weather conditions. Additionally, it provides predictive load model to effectively enhance the line ampacity, under favourable weather conditions.
Depending upon the weather, transmission line capacity can be enhanced, and reliability can be increased by using DLR devices across the lines. The ampacity of conductor is usually calculated at following ambient conditions:
- Ambient temperature: 45°C
- Solar radiations: 1045 W/m2
- Wind Speed: 0.56 m/s
In reality, weather conditions are never simultaneously equal to above values, which provides opportunity to take benefit of favourable weather condition to allow higher current on the line, without violating the operating temperature and sag limits.
The DLR is a simple device which along with a weather station (see figure-1) is easy & quick to install, voltage agnostic and is relocatable.
Figure-3 shows the changing weather conditions for any year at one of the pilot installations of DLR in India. Figure-2 shows the actual line current (blue curve) against the allowable line current (orange curve). The static design limit of line ampacity was about 200 A. It is clear from these curves that for all periods of observation, the allowable line loading was much higher than the static limit. Experience from various installations indicates that on average 15-20% additional power can be unlocked.
Understanding the transmission line with help of DLR real time data brings in intelligence and can also help to identify if there is a real need of CAPEX investment for new lines.
Uprate or Reconductoring:
While DLR provides a quick solution to increase the power flow on a line, a more permanent solution to increase capacity is by reconductoring the transmission line. A number of high-performance conductors (HPC) are available, which have higher current limits and operating temperatures as compared to ACSR conductor. The line capacity can be increased by 2 to 2.5 times using such conductors. HPC conductors can be chosen such that they have lower weight and better sad performance as compared to existing conductor on the line. This allows the conductor to be changed without need of any modification in the existing tower and foundations of the transmission line. Figure-5 shows the comparative performance of these HPC conductors with respect to ACSR conductor. The conductors on the extreme right on the curve are from the HTLS (high temperature low sag) subset of HPC conductors. They offer highest increase in ampacity with better efficiency in terms of A/mm2 of Aluminium. ACCC (or Aluminium Conductor with Carbon Composite Core) are the most efficient conductors with best performance n terms of increase in ampacity, sag performance as well as better losses.
Figure-4 compares the diameter, weight, resistance, ampacity and sag of various HPC conductors, equivalent to ACSR Zebra conductor. It is clear that ACCC conductor provides lowest sag and highest ampacity and the best power loss for same current. Its weight is also less as compared to corresponding ACSR conductor, thereby ensuring suitability to replace existing ACSR conductor on a line on the same tower. In many cases with old lines, due to growth of habitation, the available ground clearances below lines are no longer safe. Reduced sag achieved post reconductoring can help to improve these clearances, thereby improving the safety for the population near the line.
Reconductoring of a line can be carried out much faster as compared to building a new line for increasing the capacity. Reconductoring is done on existing corridor and does not call for additional ROW, thereby avoiding any related risk of execution delays. The reconductoring process requires few hours of shutdown daily and line can be temporarily charged (with due precautions) during remaining hours till work resumes the next day.
Live Line Reconductoring:
Uprate/upgrade is generally done by taking power shutdown of the existing line. Such shutdown schedule depends on power demand and varies with every day, causing variation in the execution plan. Hence, project execution requires daily coordination with system operator.
It is possible to uprate the transmission line without power shutdown and ensure uninterrupted power to the customers. This is possible using live-line reconductoring techniques. The need for same arises where no ROW is available for building a new line, no parallel line is available for power diversion loads served are critical (such as industry, hospitals, other essential services, etc.) where availing shutdown is practically ruled out.
One such live-line reconductoring work was executed successfully by Sterlite Power Transmission Limited, a first in India. In the city of Bengaluru, a 66 kV transmission line, passing through the very congested corridor, was reconductored to double its power transfer capacity. Figure-6 depicts the congested corridor of the line on which live-line reconductoring was carried out.
Protective Tower Coating:
Reconductoring increases the power capacity of the corridor and makes the line good for increasing loads for many years, with generally no modification required on the line towers. However, in few cases old towers might be suffering from corrosion (see figure-7) due to exposure to elements over long years.
Continued corrosion deteriorates the health of the tower which may result in tower failure and consequent risk of disruption in the power supply. When the galvanized structure loses its zinc coating, the deterioration of the structure rapidly enhances due to exposed surface to the atmosphere. The life of such structure becomes unpredictable and prone to failures. In order to avoid such failures due to ageing or corrosion, regular painting of the towers is necessary. Many of such paint jobs do not last long enough.
An eco-friendly, low solvent based, high thickness coating with high solids, high-build paint for weathered galvanized and previously painted structures is recommended. This is a modified linseed oil, metallic and ceramic pigmented coating designed for maximum corrosion protection over minimal surface preparation which can be provided both above and below grade structures based on the necessity. This coating can easily last for upwards of 10 to 15 years and provides excellent corrosion prevention. This way, the utility can be assured of longer working life of the infrastructure with enhancement of the power carrying capacity with very low additional investment.
Power Flow Controller:
There are situations in the meshed transmission network where parallel circuits (either at different voltage or of different length – often a case due to one of the lines going through a LILO), see uneven loading. Thus, one of the circuits may see overloads while other circuit(s) remain underutilized. This happens as different circuits offer different impedance in the circuit, and current takes the path of least impedance. Such situations are very frustrating, as they do not allow the investments already made (e.g. new lines) to be utilized effectively, thereby rendering the investment useless.
Power flow controller (PFC) comes to rescue in such situations. Scalable power flow control technology enables each power line to dynamically vary the effective impedance of a line and provide control to transfer more or less power on the line, based on the real-time needs of the grid. This helps Grid operators to transfer higher power using the existing infrastructure they already have.
PFC is an SSSC, which is scalable, modular, voltage agnostic, bi-directional FACTS solution which can regulate the power flow of transmission line, by increasing or decreasing the line reactance and thus by pushing or pulling the power to the other connected lines in any meshed network.
The technology solution is deployable within substations, on towers, or on a mobile platform.
Voltage upgrading can help to achieve much higher power transfer capability with reduced electrical losses and increased utilization of the existing corridors. Challenges in securing new ROW corridors are well known, which lead to project delays and unplanned cost increases. Increasing the voltage rating of overhead lines is possible if necessary electrical clearance is achieved. This can be planned with proper survey and suitable measures such as insulated cross arm, monopoles, special design of towers. Addition of higher voltage bays at terminal substation can be done if space is available or new substation can be built in vicinity of existing substation. While upgrading a corridor, maximum utilization can be achieved by building MCMV lines as well. Thus, the power carrying capacity of a line corridor can be increased upwards from 4 times to 10 to 20 times. This can completely transform the corridor(s) and open new ways to look at and plan the networks. Another great advantage is that the uncertainties around securing new ROW are drastically reduced, which means that projects can be expected to get completed on time.
While, specific design and solution would depend upon existing situation and line details; a solution with insulated cross arm is explained in more detail hereinafter.
Line Upgrade with Insulated Cross Arm:
Use of insulated cross arm can help to upgrade the line voltage, while retaining the existing tower & foundation with few modifications. This is a great solution and can lead to multiplying of corridor capacity by multiples quick time.
Among the key benefits of composite insulated cross-arms is that insulated swing under windy conditions is reduced to a minimum. Insulated cross-arms replace existing steel cross-arms and suspended vertical insulator strings on traditional lattice towers, thereby enabling conductors to be attached directly to the cross-arm giving space and clearance margin to go for tower upgrade solutions. This combined with HTLS conductors with low sag performance help achieve voltage upgrade, thereby improving power transfer capacity while still not infringing ground clearances.
All the above solutions are already part of CEA’s transmission planning guidelines. Some of the solutions like reconductoring have been deployed better than other solution in India. It is however considered that all these solutions if considered during planning stage by network planners, can go a long way to maximize utilization of existing transmission assets and corridors, while giving the best in terms of time, space and cost, as compared to building new lines to meet growing loads.
Biswajit Mukherjee, Smruti Mohapatra, DK Ashok, Amitabh Singhal, Manish Agarwal