Transmission Network Performance Report 2005 - National Grid

FY2005DevelopmentsSome of the additional criteria that will be employed are:• The projected fault rate calculated using the expected long-term exposurefrom the regional lightning density and the circuit’s Challenge/Fault Ratio• Soil Resistivity/Grounding• Structure Types• Elevation/Geography3.1.2.2 Lightning Mitigating StrategiesThis stage will involve the engineering of mitigation strategies to be used as addonsto existing projects or for future projects.Some possible solutions that will be considered:• Improved Grounding• Counterpoise• Insulator Upgrades/Replacement• Shield Wire• Lightning ArrestorsAll circuits where mitigation projects are implemented will continue to bemonitored to track their lightning performance improvement.3.1.3 Vegetation Management ProgramNational Grid’s Transmission Forestry Department implemented a plan to carryout a risk assessment study and develop a mitigation strategy on its NE (69 kV,230 kV and 345 kV) and NY (115 kV, 230 kV and 345 kV) transmission system.The risk assessment was focused on quantifying the size and characteristics ofthe utility forest outside the electric utility right-of-way, this being the primarysource of transmission system interruption/loss of supply across the system. Thegoal of the Transmission Forestry Department is to minimize this risk, balancedagainst financial resources, to improve overall system reliability as measured bythe number of tree caused incidents and loss of supply to customers. Within thisgoal, there is a particular focus on major transmission lines (230kV and 345 kV)where National Grid seeks a management regime that results in no tree-causedservice interruptions. The project was designed to provide the TransmissionForestry Department the data required to quantify the current level of tree risk.Risk of outages from the utility forest adjacent to cleared rights-of-way is primarilyrelated to three factors: cleared width (distance from conductor to tree line),conductor height above ground and tree height. The Optimal Width Calculatordeveloped by Ecological Solutions, Inc., a consulting firm contracted by NationalGrid, is used to calculate a Risk Factor to quantify risk of a tree falling intoNational Grid Performance Report 29