Transmission lines are used to deliver electrical energy from generation source to end-use location. The electrical energy travels along wires in overhead lines strung between towers or poles or in underground lines insulated from the earth. The voltage of the transmission line usually depends on the distance and the amount of energy being conducted.
Power lines in Beaver deliver electricity to
residential locations throughout the remote village.
Transmission lines can be deployed for several uses:
1) To deliver electrical energy to a distant location to increase coverage and reduce the overall cost of delivered energy. AVEC is developing a micro-grid system that allows for consolidation of generation. This consolidation can provide reduced overall costs through delivery of the single generator over a transmission line to a neighboring community. The communities of Toksook Bay, Tununak, and Nightmute are interconnected through the use of micro-grids.
2) To deliver excess energy to an area that can utilize it. An example is the Swan – Tyee transmission line, now under construction, that will deliver power from the Tyee Hydroelectric project for use in Ketchikan.
3) To reduce losses. If a single transmission line is delivering power between two points, adding another transmission line between those points, adding a second line in parallel, will reduce the overall transmission line losses. The Northern Intertie from Healy to Fairbanks is an example a second line in parallel (with an existing line constructed in the 1960s).
4) To increase reliability. Operating two lines in parallel allows for one line to disconnect from service while the remaining line continues to deliver power.
5) Routing a transmission line to interconnect a new resource. Routing a transmission line by a location that can produce energy will allow the energy to be delivered in either direction along that transmission line. If a transmission line in the planning stage is rerouted by a geothermal site, when the site is developed, the geothermal energy can be delivered to either end of the transmission line for use in the system.
Transmission lines can range in cost from $100,000/mile to $2,000,000/mile depending on the voltage, wire size, terrain, icing conditions, accessibility, and structure type. Lower voltage systems of 15,000 – 25,000 volts can run from $100,000/mile to $400,000/mile. Higher voltage systems of 69,000 – 230,000 volts can cost $300,000/mile to $2,000,000/mile.
In recent years, the use of Direct Current (DC) transmission lines to transmit electricity has increased. There are several trade-offs when using DC rather than traditional AC electric transmission. Inverter stations are required at each terminal to convert DC to AC, and that energy can be run through a transformer to increase or decrease voltage. The cost savings for reduced transmission line facilities may be masked by the increased cost of inverter stations and harmonic reduction. The described Battery Energy Storage System uses similar converter technology to convert the 5,000-volt DC source from the batteries to 13,800 volts AC which can be interconnected with the existing transmission system. DC systems are usually reserved for long transmission lines that deliver large amounts of power. ABB has a commercial HVDC system called HVDC Lite. It is for systems under 100,000 kilowatts of energy transfer, and typically used to reduce dynamic voltage and power swings to which AC power systems are susceptible. Converters on the receiving end must be force-commutated to provide the constant 60-hertz or cycles per second that are required of standard AC systems. DC systems are significantly complex and should not casually be applied to the distribution of electricity.
The goal of the current EET HVDC transmission project is to assess and demonstrate the technical and financial feasibility of low-cost small-scale HVDC interties for rural Alaska.