Select Committee on Northern Ireland Affairs Minutes of Evidence


Further Memorandum submitted by Northern Ireland Electricity plc

  Thank you for your letter of 24 May 1999 in which you requested further information relating to:

    —  customers experiencing multiple interruptions of supply;

    —  how circuits are prioritised for refurbishment; and

    —  network performance comparisons with other companies.

  We reply as follows:


  As regards customers experiencing multiple interruptions of supply you requested information on the extent to which records are kept and a statistical analysis of the frequency of interruptions.

1.1 Records

  As described in our earlier submission to the Committee, the 11 kV overhead network is principally comprised of a number of arterial lines with normally open points running between adjacent 33/11 kV substations together with multiple subsidiary radial spur lines. Each arterial line is controlled by a circuit breaker which operates automatically in response to network faults, thereby limiting an interruption of supply to those customers supplied via that circuit breaker.

  Spur lines are generally controlled and protected by fuses. Spur line faults do not affect the integrity of the mainline and therefore interruption of supply is limited to those customers connected to the faulted spur.

  Where part of the network is required to be taken out of service to facilitate maintenance work a planned outage is arranged which requires the manual operation of circuit breakers, switches or fuses. Where possible, the network is normally reconfigured manually to provide an alternative source of supply to as many customers as possible and thus minimise the number of customers affected by the outage. The lack of a re-supply facility on radial spurs means that there is generally little scope to minimise supply interruptions during planned outages on those parts of the network.

  The National Fault and Interruption Reporting Scheme (NAFIRS) is used to record historical data relating to supply interruptions on the transmission and distribution networks. The NAFIRS database is structured to record outages from a network perspective and facilitates assessment of network performance on either a circuit by circuit basis or by a comparison of the performance of the NIE network against those GB RECs also subscribing to NAFIRS. Supply interruptions are categorised as either fault related" or planned outages".

  NAFIRS records the number of customer interruptions associated with each network incident, relates each incident to the associated network device and also records the cause of the interruption. Each network device is linked within the database hierarchy to the appropriate circuit, the supplying substation and district. This allows flexible and detailed analysis of network performance.

  However, NAFIRS does not record outages against the specific customers affected and accordingly cannot be used to provide a definitive analysis of the frequency of supply interruptions against specific customers. Such an arrangement would require the tracing of the electrical connectivity of each customer through the complete transmission and distribution network and the summation of all incidents recorded against each interruption device. In addition, a strict account would also need to be taken of all occasions when the network configuration was altered from its normal running state.

  While this constraint makes the analysis requested in your letter difficult, it is possible to provide some indication of the likely range of interruptions experienced by customers by limiting the analysis to the 11 kV network only. The omission of the relatively small number of interruptions contributed by the low voltage, 33 kV and the transmission networks likely to be experienced by the same customers should not invalidate the general conclusions derived.

1.2 Analysis of fault-related supply interruptions

  NAFIRS can provide information relating to the frequency of faults cleared by circuit breakers and fuses and the associated number of customer interruptions. Table 1 below provides an analysis for the 11 kV urban and rural networks. The figures represent annual performance averaged over the three years from 1995-96 and the two sets of data are not necessarily mutually exclusive.

  The table shows, for example, that 292 HV circuit breakers and 132 HV fuses experienced an annual average of one fault over the three year period.


Average annual frequency of faults cleared by circuit breakers and fuses and the associated number of customers affected
No. of faultsFault clearing device 11 kV Circuit Breaker11 kV Fuses
NumberCustomers NumberCustomers
1292184,336 Data
2 7645,446 not
3 2010,405 readily
4 64,145 available
5 63,164 5244
6 1 851 181
7 1 350 1110
8 1 121 172
9 1 108 00
10 0 0 130
11 0 0 00
12 0 0 113
13 0 0 141

  NAFIRS reports customer interruptions" rather than the actual number of customers affected. In some fault situations where manual switching is involved two or more customer interruptions are reported although only one customer was involved. The above analysis derives the number of customers affected from the number of customer interruptions reported in NAFIRS. To account for double counting" within customer interruptions, the estimate of customer numbers is based on 75 per cent of the customer interruptions reported. This is considered to represent a reasonable indication of the number of customers involved.

  Although the two customer groups are not mutually exclusive the analysis indicates that of those customers who were subject to one or more faults around 90 per cent experienced no more than two interruptions per annum. In respect of a "worst case" situation, less than one hundred customers experienced in excess of 10 faults per annum.

  It is not possible to provide a similar breakdown for planned interruptions. As for faults, planned outages are recorded in NAFIRS against the relevant circuit. Unlike a main line fault which causes a source circuit breaker to open affecting all customers on that circuit, supply interruptions arising from planned outages are normally restricted to the customers connected to the line section on which work is being carried out. This is achieved through the manual reconfiguration of the circuit prior to the commencement of the planned work. Also, increasingly, mobile generators and live-line working are used to avoid any customer outages during programmed work. Therefore, although the number of customer interruptions is recorded in NAFIRS there is no correlation between the planned outages of a particular circuit and the number of supply interruptions experienced by individual customers.


  Circuits are ranked according to a composite performance index which takes account of the three main performance indices of each circuit:

    —  Reliability: annual faults (per 100 km) on each circuit.

    —  Security: annual customer interruptions (per 100 customers) on each circuit.

    —  Availability: annual customer minutes lost (per connected customer) on each circuit.

  The composite index gives the greatest weighting to circuit availability (customer minutes lost) and therefore emphasises the number of customers affected on the circuit, whilst also taking account of the duration of outages and the number of faults.

  The ranking of circuits by this method provides a priority list (the rogue circuit" list) of circuits to be refurbished. The priority list may be subsequently modified to take account of local knowledge of other relevant aspects which is not immediately apparent from the statistical analysis.


  We can confirm that the performance figures for NIE's network in the table attached to your letter are correct. As requested we have provided further information which facilitates comparisons with other companies on the basis of fault outages and pre-arranged outages. (See Table 2 below.)

  The fault outage CML figures for NIE are given on two basis—inclusive and exclusive of the effect of exceptional circumstances. We do not have sufficient data to allow us to separate out the exceptional faults for the GB companies. We have included data for Scottish Hydro Electric and we have added a column showing average annual performance over the seven year period.


Availability of supply: Supply minutes lost per connected customer per annum
1991-921992-93 1993-941994-951995-96 1996-971997-98Average
Fault149177 184131145 122118146
PA10491 79829590 9090
Exceptional Faults 1181 1112
Total A253 268381213 240212 319269
Total B (excluding exceptionals)253 268263213 240212208 237
Fault200288 181161293 124152200
PA7068 73727282 6772
Total A270 356254233 365206 219272
Fault133134 156160189 137132149
PA19278 44524452 5173
Total A325 212200212 233189 183222
Fault101120 1139485 759397
PA7564 54392628 1543
Total A176 184167133 111103 108140
Fault6283 786664 628471
PA4646 43362416 1332
Total A108 129121102 887897 103

  Exceptional Faults

  1. 118 CML due to major loss of generation (3 February 1994).

  2. 61 CML due to transmission system capacity restrictions following a transformer fire (4 November 1997);

32 CML due to storm damage to the distribution system (24 December 1997);

18 CML due to loss of generation capacity (13 June 1997).

  In making comparisons please note that NIE's exceptional events include transmission incidents and major generation losses, whereas the figures for Manweb, Sweb and Swalec do not include transmission incidents, and the potential for a loss of generation to result in customer interruptions is much lower on the GB system.

  It is also worthwhile noting the increase in NIE's prearranged CMLs since 1994-95. This reflects the extent of the very significant ongoing overhead line refurbishment programme that has been in place since that time.

4 June 1999

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