The Transmission Company of Nigeria (TCN) on Thursday said restoration of the national grid was almost completed following its collapse on Wednesday.

Mrs Ndidi Mbah, TCN General Manager, Public Affairs, said this in a statement made available to newsmen in Abuja.

Mbah said that the national grid experienced system disturbance at about 11:27a.m. on Wednesday, but that its restoration was almost completed as at 11:00p.m.

"The incident was a result of sudden drop in system frequency from 49.94 Hertz (HZ) to 47.36Hz, which created system instability," she said.

Hertz is a unit of frequency which replaces the earlier term of one cycle per second (cps).

Mbah said that reports from the National Control Centre (NCC), revealed that the collapse was precipitated by the tripping of a unit with a load of 106 Megawatts (MW) in one of the generating stations due to "Exhaust over Temperature"

She said the tripping pulled out other grid-connected units in the plant, which resulted in aggregated generation loss of 457MW.

"A train of events ensued, culminating in the collapse of the national grid.

"As obtainable in all systems, when a component of the electric power system is defective, the entire configuration is vitiated," she said

Mbah said investigation was going on to establish the cause of the failure.

She said the Nigerian Electricity Supply Industry (NESI) appreciated the understanding of the government and consumers of electricity within and outside the country.

Mbah said that NESI was committed to leveraging on interventions instituted thus far to enhance power supply reliability.