Select Committee on Northern Ireland Affairs Second Report


THE NORTHERN IRELAND ELECTRICITY SYSTEM

  10. NIE currently serves around 680,000 customers and the total length of its system is in excess of 50,000 kilometres. Electricity supplies are delivered to customers through a system of overhead lines and underground cables. A dis tinctive characteristic of the NIE system is that a very large proportion of the high voltage distribution system [7] is made up of overhead lines; some 23,000 kilometres are overhead lines and 3,700 kilometres, mainly in Belfast and other urban areas, are underground cables. In all, about 450,000 customers in Northern Ireland (about 66% of the total) living in mixed rural/urban environments outside the main cities, or in rural areas, depend for their electricity supplies on o verhead distribution lines. [8]

  11. Comparative data published[9] by the MMC shows that NIE had, in 1994-95, the joint smallest customer base of all the electric utilities in the United Kingdom. It was the smallest, by a substantial margin, in terms of gigawatt hours of power distributed, and had the second lowest customer density[10] and number of customers per kilometre of overhead line, roughly a third and a quarter respectively of the Great Britain average (excluding London). It als o had the smallest average number of customers per transformer, again about a quarter of the Great Britain average. This largely reflects the fact that much of NIE's customer base is widely dispersed throughout a large rural community.[11]

  12. The transmission and distribution network operates in two parts:

      ·  the transmission network, operating at 275kV and 110kV, delivers bulk supplies of electricity from the power stations to bulk supply points; and

      ·  the distribution network, operating at 33kV, 11kV, 6.6kV (in parts of Belfast) and 400/230V, which delivers supplies from the bulk supply points to customers' premises. Lines operating at 6.6kV or above are referred to as the high voltage network and the remainder as the low voltage network.

  13. The transmission system was relatively unaffected by the storm: 13 faults arose from this cause, only one of which resulted in a loss of supply to 15,000 customers for a period of two hours.[12]

  14. The distribution system[13] can be regarded as having four distinct elements:

      ·  the 33kV network, which provides supplies to approximately two hundred 33/11kV substations. This network is designed so that the majority of substations can be supplied by more than one 33kV circuit to reduce the incidence and duration of supply failures caused by faults on the 33kV network;

      ·  11kV arterial lines running between adjacent 33kV/11kV substations;

      ·  11kV subsidiary radial lines, which have been progressively developed to meet the need for new customer connections;

      ·  the low voltage network, which delivers the final supply from an 11kV/low voltage transformer to customers' premises.

  15. We asked NIE for an estimate of the number of customers who might be affected by a fault in each type of circuit. The information is set out in the Table below:

Type of circuit Customers affected
Range
Average
33kV line

11kV arterial line

11kV subsidiary line

Low voltage line

0-7,000

1-3,000

1-300

1-300

1,000

300

30

15

  In the event, although the majority of faults attributed to the storm occurred on the low voltage circuits, it was faults in the 11kV circuits which caused loss of supply to the greatest number of customers, although they constituted only about 20% total faults.[14]


7  See paragraphs 12 and 14 below. Back

8  Ev. p. 29. The overhead distribution network also includes 6,000 kilometres of low voltage lines. See also Q 3. Back

9  MMC Report, p. 7. Back

10  Customers per square kilometre. The lowest in each case was Scottish Hydroelectric. Back

11  Ev. p. 2. Back

12  NIE storm report, p. 11. Back

13  For a fuller summary of the structure of the distribution system, see Appendix 27, p. 108. Back

14  They were also responsible for about two thirds of total Customer Minutes Lost. See NIE storm report, p. 12. Back


 
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Prepared 29 July 1999