Between April and June this year, well over a dozen high-capacity power transmission towers of 765 kV and 400 kV capacity, the backbone of the Indian electricity network, collapsed in the face of pre-monsoon winds, highlighting the fragility of the country’s grid infrastructure. The 765 kV double-circuit transmission towers hook up power lines that typically carry electricity to the tune of 3,000-5,000 MW and the collapse of more than one tower could potentially trigger a cascading grid failure, unless Special Protection Schemes are in place or the transmission system is spruced up to handle such a huge loss of power. The official estimate of the time taken to bring these towers back into operation ranges between a few days to well over a month in some cases.
In response to the inter-state transmission towers — most of which are set-up by state-owned transmission firm Power Grid Corporation (PGCIL) — toppling like ninepins, the country’s grid manager, POSOCO, has raised the red flag. POSOCO has underlined the fact that as compared with first quarter of last fiscal (April-June 2014), there is a marked increase in the 765 kV tower collapses and damage in the first quarter of this financial year (April-June 2015), forcing the grid manager to issue an alert that these failures be “investigated thoroughly” and remedial measures “undertaken immediately” across all utilities. “In the absence of this, the grid continues to remain vulnerable,” POSOCO has observed in an official note, where it underscores the point that it is “improbable that all the failures are due to reasons beyond control, considering the geographical spread of incidents.”
The spate of tower collapses are, by no means, isolated incidents. They have been a reccurent theme, despite these being subjected to wind speeds much below the expected design threshold. This year, no major storm has been reported across the country during the April-June period.
In May last year, a record 30 transmission towers either collapsed or were severely damaged across the northern hinterland after a freak dust storm, forcing the grid manager to raise concerns over the large scale toppling of transmission towers, all of which were technically supposed to be designed to withstand much higher gusts of wind than the 115 kmph peak winds experienced on May 30, 2014.
Theoritically, 765kV transmission towers are supposed to be designed with higher safety margins and have a design life of about 100 years. A PGCIL official defended the quality of the country’s grid infrastructure and maintained that the 765 kV double circuit towers have been designed keeping in mind the maximum wind speed encountered in the region in the past 150 years and that all of these tower collapses were “isolated events”.
While regulators have been alive to the dangers posed by a spate of tower collapses on the overall functioning of the electricity grid, no concrete action has been initiated from the regualtory standpoint so far. In May 2014, the Central Electricity Regulatory Commission (CERC) did take up the incident of a load crash in the Northern Region on May 30, 2014 as a suo motu petition, but is yet to deliver an order.
As per the Bureau of India Standards’ Code of Practice for Design of Wind Loads for Buildings and Structures (IS-875 Part III), the basic design of structures such as buildings in areas around the National Capital Region and most of the northern region should factor in wind speed of 47 metres per second, which works to around 169 kmph. Additionally, for installations in open areas and high structures such as transmission towers, a margin of 10-15 per cent is to be further built in. For key structures such as high-tension 765kV towers, an extra safety margin of 7 per cent is supposed to have been factored in.
Incidently, in October last year, 28 months after two successive grid collapses triggered cascading outages across most of northern and eastern India, the country’s grid manager had red-flagged at least half-a-dozen hotspots in the nation’s grid infrastructure that continue to be vulnerable to high voltage tower collapses. According to an earlier July 2014 report by POSOCO, the lack of System Protection Schemes at these locations to mitigate the resulting unplanned outages and systemic shock put at risk major power generation complexes at locations that included Singrauli, Rihand, Vindhyachal, Korba, Ramagundam, Dadri, Sipat, Mundra and Sasan, which depend on high capacity 400 kV and 765 kV AC transmission corridors for power evacuation.
The July POSOCO report too flagged that the CEA had revised the transmission planning security criteria in January 2013 after the July 2012 twin blackouts. The revised criteria required the grid to remain stable in case of two contingencies in the same area such as tripping of two transmission lines or the collapse of a tower carrying two circuits.