The crucial 765 kV Raichur-Solapur line that was declared commissioned by the state-owned Power Grid Corporation (PGCIL) on New Year’s Eve, marking the integration of the southern region with the electricity grid for the rest of the country, has tripped twice in the first week of operation.
This resulted in the accidental separation of the southern grid from that for the rest of the country on both occasions, as a second line between Raichur and Solapur, which was to come up alongside the newly commissioned line and act as a crucial buffer, is not yet ready.
The separation of grids took place on January 2 and January 7, resulting in the sudden loss of about 800 MW that would have flowed to the southern grid. For the comparatively small southern grid, a sudden loss of 800 MW means a big jerk. The impact on January 7 at 9.30 pm was the grid frequency nosediving from 50 Hz to 49.3 Hz, the frequency report of the southern region shows (see graph). This triggered oscillations that endangered the stability of the grid. These days, the load of the southern region is in the range 27,000 to 33,000 MW.
In addition to the two serious instances of line tripping, there have been four incidents of tripping of the interconnecting transformers at Raichur substation. On January 5, the newly commissioned single-circuit line had to be opened for technical reasons, with the southern grid intentionally separated.
Data on the southern region load dispatch centre website shows that the tripping happened on account of “over-current” and problems in the “auxiliaries” of the interconnecting transformer of PGCIL. For two instances of transformer outage, no reason has been provided by PGCIL on the grid operator’s website (see chart).
According to experts, the two grids have been joined through a weak link capable of carrying only 800 MW. A special protection scheme should have been put in place to initiate corrective action in case of excessive power flows but apparently this has not been done and the southern grid is being subjected to such shocks.
Experts involved in the commissioning of the single-circuit line have described it as a hurried exercise by PGCIL to be eligible to recover the requisite transmission tariff, without there being any tangible benefit for stakeholders in either the south or the rest of the country. Plus, many PGCIL transmission lines leading up to Solapur and going beyond Raichur are not ready either, and without these the single line cannot be optimally utilised.
The commissioned line, despite a 2,500-MW wheeling capacity on paper, will be allowed to carry “equalising power” — electricity required to even load fluctuations out — to the tune of 800 MW only. If power flows were to exceed this, the line could trip. Even when the second circuit line is ready, experts say it will not be possible to increase power flows beyond 800 MW unless the associated lines of PGCIL on either side of Raichur and Solapur are commissioned.
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