Further Memorandum submitted by Northern
Ireland Electricity plc
Thank you for your letter dated 23 April 1999
in which you sought our response to two additional points, with
a third point included in a cover note.
We are pleased to reply as follows:
1. THE STRUCTURE
You asked for an indication of the number of
customers whose supplies might be at risk as a result of failures
on various parts of the distribution system.
The 33kV network is essentially a sub-transmission
system providing supply to approximately 200 33/11kV substations.
The 33kV network is designed so that the majority of 33/11kV substations
can be resupplied from an alternative 33kV circuit in accordance
with standards required under NIE's licence. The system redundancy
so provided reduces the effect of 33kV faults on customers' supplies
by either minimising the duration of supply outages or, where
a substation is supplied by duplicate 33kV circuits, ensuring
that customers fed from such substations are unaffected by a fault
on one 33kV circuit.
Faults on the 33kV network do have the potential,
however, to result in the loss of supply to one or more 33/11kV
substations potentially affecting several thousand customers.
The 11kV network as described in our earlier submission,
is made up of main arterial lines with normally open points, running
between adjacent 33/11kV substations and multiple subsidiary radial
lines which have progressively developed to facilitate new customer
connections. Faults on the 11kV and low voltage network therefore
impact more directly on customers' supplies. Approximately 70
per cent of all customer interruptions are due to faults on the
You asked for a breakdown of 11kV faults between
arterial lines and spur lines. As an indication of the split,
we would advise that the 11kV overhead network comprises over
800 circuits and some 20,000km of overhead line and experiences
typically 3,000 faults annually. Approximately 30 per cent of
these faults affect 11kV arterial lines.
The approximate range of customers affected
by a single fault on either of a 33kV, 11kV or low voltage line
is provided in Table 1.
Scale of customers per fault by network
|11kV arterial line||1-3,000
|11kV subsidiary line||1-300
|Low voltage line||1-300
2. ACCOMMODATING THE
You asked to what extent could the accelerated refurbishment
programme be accommodated without deferral of other projects.
Our 14 April submission to the Committee
illustrated the benefits for customers in maintaining a system
of capex regulation which incentivises the company to invest efficiently
to achieve the required outputs at lowest cost. Whilst the capex
allowance for the second regulatory period was determined after
detailed and careful scrutiny by the MMC, it would be reasonable
to assume a modest level of savings in the capex programme by
the end of this period on the assumption that the efficiency incentives
To date, firm efficiency savings have been possible in respect
of that part of the transmission plan which relates to the reinforcement
of the north and west of the transmission network by the decision
to establish a new 275kV substation at Dungannon as an alternative
to the previously proposed 275kV substation at Omagh. In the normal
course of events, NIE would have argued that it should retain
the benefit of the savings in financing costs which occurred over
the period (so as to maintain the ongoing incentive for efficient
investment) and customers would get the benefit in the form of
lower prices at the next review.
However, in the light of the storm experience, NIE proposes
to use these savings as a contribution towards the funding of
the accelerated refurbishment programme where they will be sufficient
to cover around 50 per cent of the funding required.
The remaining 50 per cent must be covered through our review
of investment priorities which has identified areas where investment
can be deferred with a level of risk which is minimal when measured
against the significant risk which arises if the network's resilience
to storm conditions is not enhanced.
As explained in Annex A5 of our April submission,
we have re-prioritised our performance improvement strategies
in the context of the identified risks and, consequently, we have
rephased the programme by bringing forward additional line refurbishment
into the current regulatory period at the expense of deferring
some 11kV automation to the next period. A similar reappraisal
of our priorities for the undergrounding of overhead lines has
resulted in the deferment of part of the high voltage undergrounding
programme to allow for the undergrounding of low voltage lines
during the current period. A review of asset replacement priorities
has facilitated a rephasing of the meter replacement programme
in a way which enables NIE to remain compliant with our meter
As regards your question on technical resources, the amendment
of the transmission plan to an alternative more cost-effective
reinforcement project will not, in itself, release any significant
technical resources to cover the accelerated refurbishment programme.
Also the resource capacity released by the deferral of 11kV automation,
HV undergrounding and meter replacement would be insufficient
to cover the expanded refurbishment programme. Additional technical
resources for design, management and operational switching will
be required. We have therefore taken the necessary steps to provide
the required resources, as detailed in Annex A3 of our April submission.
You requested clarification of the initials AAAC in your
cover note. AAAC refers to All Aluminium Alloy Conductor.
4 May 1999
Appendix 23. Back
Appendix 21, p. 99. Back
Appendix 21, p. 98. Back