The supply of electricity via the interconnector between Northern Ireland and the Republic is facing disruption over the summer.

ESB National Grid, which operates the interconnector in the Republic, has warned the problems could take several months to address.

While some power will still be imported into the Republic from Northern Ireland, capacity on the interconnector will be seriously curtailed.

The interconnector has become an important source of electricity for the Republic in recent years and during peak winter months it often helps to plug gaps in national supply.

However, an ESB National Grid spokesman said yesterday householders would not have their supply interrupted in any way. The most serious problem relates to a fault which developed last month in a 300 tonne transformer. A replacement will now have to be sourced in Sweden.

In addition, several large electricity customers will have to be connected to the national grid over the next few weeks and this will also have an impact on the interconnector, a spokesman said yesterday.

He said the breakdown in the transformer was not a common event and "could not have been predicted". He said users of the interconnector, among them ESB, Bord Gaís, Airtricty and Viridian, were aware that power flows over the interconnector were not guaranteed.

The main energy companies have been told of disruption. The amount of power coming into the Republic via the interconnector will be reduced initially from 330 megawatts to 250 megawatts and then to 120 megawatts.

The ESB's new 400 megwatt power plant at Coolkeeragh, Co Derry, will be seriously affected by the disruption. It needs to export significant amounts of power from this station into the Republic to make the station a commercial proposition.

Last night a spokesman for Coolkeeragh said the development was very worrying. A Viridian spokesman said it was also monitoring the situation and any long-term disruption would give rise to concern.

Several electricity companies export power from Northern Ireland into the South using the interconnector and take advantage of a price difference.

The Republic offers a guaranteed floor or "spill" price higher than that of Northern Ireland. The energy regulator, Tom Reeves, recently pointed out that some companies were making large sums from this practice.

A spokesman for ESB National Grid said capacity on the interconnector had "always been subject to change from time to time and indeed the market rules state that it can change up to three days ahead".