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Linking the nation’s power grids
by tying the east to the west
As the use of electricity spread across the country in the 20th century, three distinct alternating current power grids evolved. One grid connected utilities in the eastern United States, while the second grid spread across the western half of the country. A third grid developed in Texas. The sparsely populated Great Plains formed a divide between the separate power systems.
As American dependence on electricity grew, utilities needed to increase reliability and match generation to electrical demand by allowing the exchange of power between the eastern and western grids. The Miles City Converter Station in eastern Montana is one of eight sites that tie the power grids together.
The Miles City Converter Station increases system reliability by providing a connection between the separate grids. It can balance power resources between the regions and allows the eastern and western grids to share energy reserves, resulting in enhanced energy exchanges and system operating flexibility.
Solving the puzzle
Attempts in the 1960s and 1970s to “tie” the power systems together failed because interconnections resulted in disturbances in one grid affecting operations in the other grid. Facilities such as the Miles City Converter Station solved this problem by acting as a giant shock absorber, converting the alternating current power on one side to direct current, then back to AC on the other side. The DC converter allows the reliable flow of energy between the grids, but maintains their separation. Now when disturbances occur in one system, they do not affect the other system.
Converter stations are the most cost-effective way of connecting the east and west power systems. Access to power from the other grid allows utilities to delay construction of powerplants needed to meet peak power demands.
Miles CityConverter Station at a glance
Operator: Western Area Power Administration
Construction began: August 1982
Operations began: July 1985
Initial cost: $30 million
Transfer capacity:
• From east to west: 200 MW
• From west to east: 200 MW |
General Electric built the Miles City Converter Station for Western Area Power Administration and Basin Electric Power Cooperative. Western owns 60 percent of the project and Basin owns 40 percent. The Miles City Converter Station began operating in 1985. Western’s Upper Great Plains Region operates and maintains the station. Wholesale power customers throughout the Missouri River Basin benefit from a more flexible system.:
About DC and AC electricity
While DC, or direct current, energy remains constant, AC, or alternating current, energy changes magnitude and direction. To help visualize the difference, imagine DC energy as driving across the Great Plains of Nebraska, while AC energy resembles driving across the Rocky Mountains, where you climb a mountain and descend into a valley.
At the converter station, AC electricity from one grid enters the DC converter station, where it is changed to a constant value—direct current. Then, this DC energy is converted back to alternating current on the other side to match the peaks and valleys of the AC power on that grid. This allows power to pass between the east and west while still maintaining separate systems, preventing power problems on one grid from affecting the other grid. | 
