In 2019, when Cyclone Fani hit Odisha, 200,000 electric poles, 84,000 km of low tension wires and 11,000 distribution transformers were damaged, plunging more than 2.5 million households into darkness for a fortnight to two months. This cyclone served to demonstrate the superiority of underground cabling for transmission and distribution systems. Underground cabling can be considered a fundamental component of the electricity grid. It ensures a robust electricity supply even in extreme weather conditions such as cyclones, floods and lightning. Further, underground cabling allows the expansion of the transmission network through forests and environmentally fragile areas with fewer right-of-way (RoW) roadblocks given their unobtrusive nature.
Drivers and use cases
In dense urban agglomerations, where grid expansion using overhead wires would entail RoW problems, using underground cabling can help in the timely commissioning of projects. Connecting essential facilities such as water supply systems, hospitals, railways and road crossings through underground cables will ensure reliable and robust power supply even during natural calamities.
In circles/zones that have irreconcilably high aggregate technical and commercial losses, distribution companies can consider utilising underground cabling since it reduces the chances of grid manipulation and lowers I2R losses given the absence of steel wires. However, such a decision should be taken after a thorough analysis of the cost factors.
From a technical point of view, the underground cables require lower O&M expenditure for the first 10 years, are much more reliable, and are capable of absorbing emergency power loads. Moreover, they support real-time monitoring of the system and occupy very limited space in comparison to overground cables.
Underground cabling projects
Underground cabling by distribution utilities is primarily being done under the government’s Integrated Power Development Scheme (IPDS). The scheme has earmarked Rs 20 billion for executing these projects. It envisages the implementation of underground cables aggregating 18,008 km across various states.
The government completed underground cabling under the IPDS in Varanasi in 2018. The project entailed the conversion of overhead lines to underground cables, the establishment of two 33 kV substations, and the augmentation of 11 existing substations at an estimated cost of Rs 4.32 billion. Similarly, the Bangalore Electricity Supply Company has approved the conversion of 7,250 km of overhead electricity lines at a cost of Rs 14 billion. The cables will be laid on the existing overhead line corridors with a total length of around 7,241 km. The project is estimated to be completed by mid-2021. The Asian Development Bank has cleared a $190 million loan for the project.
In Mumbai, Adani Electricity Mumbai Limited’s 220 kV Aarey EHV substation, expected to be commissioned in 2022, will be the first-of-its-kind underground high voltage DC cable system that can get an additional 1,000 MW of power. Apart from this, underground cabling projects are under way at Kumarapalayam, Congress Nagar division, Hudkeshwar, Jaipur, Gorakhpur, Bhubaneswar (90 per cent of cabling completed) and Amravati. Also, the Odisha government is planning to develop a calamity-proof power network in the coastal districts of the state that are prone to summer cyclones in phases, at an estimated cost of around Rs 190 billion, which includes underground cabling in town areas of these districts.
Issues and challenges
The installation of underground cabling systems is constrained by the high upfront cost of their installation in comparison to the alternative means of cabling, since underground cables have to be insulated with several layers to protect them from moisture-related damage. The estimated cost of an 11 kV overhead single-circuit line with a dog conductor is Rs 0.5 million-Rs 0.6 million per km while the estimated cost of a 3×300 square mm 11 kV underground cabling system is around Rs 2 million per km. The cost of the underground cabling system varies depending on the area (urban or rural) of road restoration required.
There are concerns associated with the irreversible nature of these cables. These cables cannot be uprated once they are commissioned. Further, O&M issues related to these cables are difficult to identify and repair. From a practical perspective, the underground clutter of internet fibre optic cables, water pipes, sewage pipes, etc., leads to several issues, necessitating careful planning and technically astute management on the part of contractors during its installation.
These issues notwithstanding, underground cabling for distribution and transmission projects can be executed on a priority basis in areas that are vulnerable to extreme climatic contingencies. They prevent the loss of lives through electrocution during floods and allow for faster repair of the grid after a cyclone or a flood.