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
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.
11. Comparative data published
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 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.
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;
· 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.
14. The distribution system
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
· the low voltage network, which delivers
the final supply from an 11kV/low voltage transformer to customers'
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
11kV arterial line
11kV subsidiary line
Low voltage line
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.
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