During disturbances on July 30 and 31, there was heavy power flow of 1000 MW on the Bina-Gwalior-Agra line
The grid collapse on two consecutive days — the Northern Grid on July 30 and the Northern, Eastern and North Eastern grids on July 31 — has been widely attributed to overdrawing of power by the Northern States, Uttar Pradesh in particular. But is that the real reason? An examination of relevant data would suggest that something else, and not overdrawing, was the real reason for the collapse. And that the Power Grid Corporation of India Ltd. (PGCIL), which owns and manages the power grids, failed to prevent the collapse in spite of several expensive equipment that are supposed to be in place to monitor the grids.
Power frequency reflects the balance between load and power generation in the grid at a given instant and the frequency would drop if there is an imbalance caused by overdrawing somewhere along the grid. This can affect the quality of power supply and the stability of the grid. To maintain the security of the grid, the permissible frequency band specified by the Indian Electricity Grid Code (IEGC) of the Central Electricity Regulatory Commission, effective May 3, 2010, is 49.5 to 50.2 Hz.
Let us see what the grid frequency situation was just before the grid disturbance actually occurred on July 30 at 2.35 a.m. According to the data available from the Power System Operation Corporation Ltd. (PSOC) of the PGCIL, the grid frequency at 2.32 a.m. on July 30 was 49.68 Hz. This is as good as it can get against the ideal frequency of 50 Hz.
In fact, data obtained from the NTPC Dadri thermal power plant from 12 midnight on July 30 up to the tripping instant shows that the frequency was extremely stable till the grid collapsed, with a frequency minimum of 49.72 Hz at 12.25 a.m. and a maximum of 50.09 Hz at 12.50 a.m. This range is well within the tight norm specified by the IEGC. The same was, in fact, true on July 31 as well. When the disturbance occurred at 1 p.m., the antecedent frequency was 49.84 Hz.
Also, it was not a case of generating stations pumping in more power to the grid to restore the balance and the grid frequency. Supply from Dadri during that time suggests that generation was actually being reduced. In fact, it would seem quite odd to say that there was overdrawing of power at 2.30 a.m. on a rainy day. It is true that farmers do use power at night to pump water because of low tariff, which also allows the grid to maintain a certain balance, but it cannot be imagined that there would have been overdrawing by the farming community on a rainy night.
According to experts, however, grid frequency is not the only parameter that warns you of an impending grid disturbance. There can be other reasons such as a single-line overloading, which is quite distinct from general overdrawing from the grid. This can be due to poor load management by the load despatch centre concerned. This is what seems to have happened.
As per the information available from the PSOC, during both the disturbances, there was heavy power flow of 1000 MW on the 400 kV Bina-Gwalior-Agra line. Though there are two circuits available on this line, one circuit has been under outage since July 28 for upgrade to the 765 kV level. The loading capability or the permissible limit, known as Surge Impedance Loading (SIL), on this single circuit line is 691 MW. So the overloading was by more than 300 MW. Similarly, overflow was seen in many circuits in the Eastern sector as well though not as drastic. They were of the order of 550 MW against an SIL of 515 MW.
Interestingly, there was a warning of an impending disaster the previous day itself when a similar power overflow occurred on the same line at 3.10 p.m. on July 29 and there was a “near-miss” situation. Clearly, the load despatch centre did not heed this warning, trace the cause and be prepared for such an eventuality.
The immediate response of the load despatch centre in such a case of power overflow should be to make other parallel lines on that corridor available for diverting the increased flow or trip an operational line off the grid or ask some utility to increase generation so as to restore the balance. Clearly, none of these was implemented, which reflects on the poor load management system in place.
Even more interesting is the fact that these two instances are not the first such occurrences on this line. This line seems to have some problem and has been acting like some kind of a blockage in the network. A similar near-miss situation occurred on November 28, 2009, when the same circuit carrying around 1000 MW tripped on one of its phases. This had led to subsequent cascade tripping of other parallel lines in the corridor. However, the system survived because of support from generators and quick operator action.
In the current incident, which also led to the cascade tripping of other lines in the Northern grid on July 30 and, because of synchronous interconnectivity with the Eastern and North-Eastern Grids, tripping of the entire North-East-North-Eastern Grid on July 31, this could not be managed and collapse prevented. The reasons for this are what need to be established.