According to a NLC press note, yesterday two units of NLC had been taken off generation at 5.30 pm and 6.30 pm "with adequate prior intimation to TNEB". However, due to "non-operation of the tripping settings in the TNEB grid and non-shedding of the load, the frequency in the grid came down to as low as 47.5 hz". Normally frequency should have been between 49.5 hz and 50.5 hz.

As a special dispensation to TNEB it has been agreed that the frequency would be retained at a minimum of 48.3 hz, the NLC statement said, adding that below this level load should have been shed by TNEB.

Since the frequency fell "drastically" below this level, NLC's units had to island themselves in order to save the equipments, the statement said, noting that TNEB had resorted to manual load sheddingas against the automatic load shedding which could have prevented such an occurrence.

Though NLC tried to restore the units, they tripped again due to continued under frequency in the grid. Unless the TNEB grid is maintained at a frequency of 48.3 hz and above such occurrence in future could not be avoided, the statement said.

However, all the units which had tripped in Neyveli had been restored in a phased manner by Friday noon and except for two units which had been taken up for planned maintenance works, all other units had been brought back to normal operation, the statement added.

Meanwhile, the power situation in the State limped back to normalcy yesterday with the units of the Neyveli, Mettur and North Madras Thermal Power Stations, which had tripped on Thursday evening, "lighting up" again.

According to officials in the TNEB, all the units, which had tripped due to separation of the State from the Southern grid, had been gradually restored from the early hours of Friday and by afternoon all ofthem were in operation.

Board officials attribute the snapping of link with the Southern grid to a "sudden loss of generation" of about 100 MW by any one of the Southern states which could have disrupted the frequency level in the Southern grid causing tripping of the power stations in Tamil Nadu.