Transmission Network Performance Report 2005 - National Grid

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Transmission ReliabilityFY20051.9 Index of Transmission Reliability• National Grid uses an index of transmission reliability (ITR) as a measurefor assessing overall transmission reliability. It represents the percentageof time transmission circuits were available for the transmission ofelectricity from one terminal to another (or others).• Thus ITR is a measure of unplanned unavailability of our transmissionsystem. It is reduced by outage time due to unplanned outages(disturbances) on the system but it does not include outage time due toplanned outages.• As with other metrics we can categorize ITR by voltage classes and thefollowing figure shows three ITR curves for FY2004 for Load (69 kV and115 kV), Bulk (230 kV and 345 kV) and System (all circuits).Figure 1-7: Bulk, Load and System ITR for FY2005100.00%99.95%99.90%99.85%99.80%ITR %99.75%99.70%99.65%99.60%99.55%99.50%Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb MarLoad ITR Bulk ITR System ITR• Major reductions in both Load and Bulk Systems ITR have been mostlydue to lightning. There were several events throughout the warmweather months when lightning caused significant damage totransmission structures, poles or conductors.• Not all of these incidents impacted customers. In FY2005, 11% of alloutages on the transmission system directly impacted customers.10 National Grid Performance Report
FY2005Transmission Reliability1.10 System Availability• Availability is calculated based on the total length of planned, unplanned,and disturbance durations and the total hours the system or circuitshould have been available in a given period. The availability of thesystem measures the increased reliability of the system and is importantin case of an 'unplanned' event. High availability provides feweropportunities and less compromising situations in the event of failurewhen back up (redundant circuits) are in service (available). Thepotential risk is higher for line and customer related outages when allcircuits are not available.• In order to maintain a high level of Availability, circuits are returned toservice as quickly as possible in the event of unplanned disturbances. Inthe case of planned outages which require work over several days,circuits are returned to service at the end of each work day.• The value of System Availability for FY2005 98.63 for New England and98.60 for New York. The following figure indicates the availability of theLoad, Bulk and the Transmission System in FY2005.Figure 1-8: Bulk, Load and System Availability for FY2005100.00%99.00%98.00%97.00%96.00%95.00%94.00%Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb MarLoad Bulk System System FYTDNational Grid Performance Report 11
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Page 25 and 26: FY2005DevelopmentsDevelopmentsNatio
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Page 29 and 30: FY2005DevelopmentsThe lightning den
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Page 33 and 34: FY2005HVDCHVDCNational Grid Perform
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Transmission Network Performance Report 2005 - National Grid

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