"The details about what led to tripping in the system will be known only when data from important sub-stations and power stations in the grid is received by the National Regional Load Despatch Centre (NRLDC) under PGCIL," says the initial report submitted on grid disturbance in the northern region on Tuesday.

As per the report, a sudden dimming of control room lighting was noticed in NRLDC at 0436 hours on Tuesday and it is likely that some tripping in the system took place at that time. About two minutes later, at 0438 hours, the northern regional grid separated in two parts, with the tripping of two lines - the 400 kV Kanpur-Agra and 400 kV Kanpur-Ballabgarh lines.

Thereafter, with excess generation in the eastern part, the frequency rose to above 52.5 Hz in the Singrauli-Rihand complex. The eastern Uttar Pradesh region, as per the report, survived for about five minutes after which all the generating units (including Singrauli, Rihand, Anpara and Obra) tripped on high frequency and the eastern part of the northern region collapsed at 0444 hours. On the western side, with a generation deficit, frequency started going down and as is recorded through the Badarpur frequency, the western part also collapsed at 04.40 hours. This led to a near total collapse of the northern regional grid and a loss of generation of around 15,500 MW.

Giving details about the sequence of events before Tuesday's grid collapse in the report, the NRLDC has stated that the northern grid was operating normally till 2300 hours on January 1, except for outage in Rihand-Dadri High Voltage Direct Current (HVDC) pole-2, caused due to the failure of converter transformers supplied by Bharat Heavy Electricals Limited (BHEL).

"Due to inclement weather, there were transient faults on the operating pole-1 of the HVDC Rihand-Dadri line starting from 2321 hours on January 1.

This line continued operation with successful auto-re-starts till 0002 hours on January 2," says the report.

To restrict such transient faults, the Rihand-Dadri pole-1 was put on a reduced voltage mode of operation at 0017 hours on January 2 and the power flow on this line was consequently reduced from 750 MW to 500 MW. As per the report, some power stations had already backed down due to a prevailing high frequency situation in the northern grid and the NTPC's stations at Singrauli and Rihand were also asked to back down.

Moreover, it was also not possible to transfer surplus power to the western region because the latter also had a high frequency situation.

The 400 kV Obra-Panki line of the Uttar Pradesh Power Corporation Ltd (UPPCL) tripped at 0130 hours and could not be brought back, apparently due to a persistent fault. Since the frequency continued to be above 51.0 Hz, the Singrauli and Rihand power stations were again given backing instructions at 0110 hours.

The 400 kV bus `A' at Panki substation of UPPCL had a fault at 0310 hours. Three 400 kV lines, namely the Panki-Muradnagar, the Panki-Muradnagar and the Panki-Kanpur II tripped at Panki due to a fault in the Obra-Panki line.

"Due to the consistent load throw-off, the frequency rose from 50.6 Hz to 51.2 Hz. Also, as a consequence of the tripping of the Panki-Muradnagar line, other lines along this corridor got overloaded," says the report.

Once again, orders were issued to generating stations to back down generation to lower levels. "With these actions, line loadings were brought within permissible limits by about 0420 hours," says the report. At that time, the 400 kV Kanpur-Agra loading was about 700 mw and the 400 kV Kanpur-Ballabgarh loading was below 500 mw. The 400kV Kanpur-Ballabgarh loading was below 500 mw. The 400 kV Kanpur-Panki-II had already been restored and the system was perceived to be under control when the sudden tripping took place at 0438 hours on January 2.

Details for this are being collected from various sub-stations and a clear picture is expected to emerge by this week-end, sources said.