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In the US, we own and operate an electricity transmission network of approximately 9,000 miles spanning upstate New York, Massachusetts, Rhode Island, New Hampshire and Vermont. Our US electricity transmission facilities operate at voltages ranging from 69 kV to 345 kV, utilising nearly 8,900 miles of overhead lines, 94 miles of underground cable and 496 substations. We are the largest electricity transmission service provider in the northeastern US by reference to the length of these high-voltage transmission lines. In addition, we own and operate a 139-mile direct current transmission line rated at 450 kV that is a key section of an interconnector between New England and Canada.
In New England and New York, our transmission business operates within two independent organisations, ISO New England, a regional transmission organisation (RTO), and New York ISO, an independent system operator (ISO). These non-profit entities are the system operators for the New England and New York networks. As system operators, the ISOs are responsible for operating wholesale markets for energy, operating reserves and capacity, maintaining the operating reliability of the New England and New York networks, coordinating the activities of the transmission owners, and managing transparent transmission expansion planning processes.
We are one of several transmission owners operating within each of these ISOs. The transmission owners are responsible for certain aspects of the operation of the facilities they each own, such as maintenance, equipment restoration and switching operations. National Grid works closely with ISO New England and New York ISO to support efficient market and network operations and transmission investment.
The revenue for our transmission business in New England and New York is collected from transmission customers, including our US electricity distribution network businesses, pursuant to tariffs approved by state utility commissions and by the Federal Energy Regulatory Commission (FERC).
In New York, efficient and reliable operations within our long-term rate plan are our primary business drivers.
In New England, efficient operations are also key; however, the rate structure is such that network availability, energy deliveries and operational expenditure spend are all ‘pass-through’ items. We neither gain nor lose from variability of any of these. The tariff allows for recovery of, and a return on, new investment entering service.