ELECTRIC UTILITIES;

UTILITIES - ELECTRIC;

You asked for a discussion of the feasibility of building an underground direct current (DC) line as an alternative to the proposed Phase 2 (Norwalk to Middletown) transmission line.

SUMMARY

While the vast majority of the transmission system uses alternating current (AC), DC has been used in certain applications in above and underground lines. The feasibility of building an underground DC line as an alternative to the proposed phase 2 line (which would use AC and be primarily above ground) would depend on several factors. These include the role of the line (transporting power from one specific point to another vs. being part of a regional grid) and the route that the line takes.

DC TRANSMISSION LINES

The overwhelming majority of the electric transmission system in the U. S. uses alternating current (AC) technologies in which the current changes direction 60 times per second. On the other hand, DC transmission technologies, in which the current flows in one direction, have been used in certain applications. For example, an above ground DC line that has been in operation for decades connects the New England and Quebec power grids. The recently built Cross Sound cable, which links Connecticut with Long Island under the Long Island Sound seabed, also uses DC. The developer of this line considered using AC, but determined that DC would provide operational benefits and be less expensive. Recently, two underground DC lines have been built in Australia, one 37 miles long and the other 110 miles long.

DC lines have several advantages over AC lines that make them preferable in certain circumstances. DC lines are controllable and can function as the equivalent of power plants, while power on an AC line automatically follows the path of least resistance. DC lines require two cables, while AC requires three. Partially for this reason the DC lines, in and of themselves, can be less expensive per mile than AC lines. In addition, the ability to move power at high voltages for long distances on underground AC lines is limited by engineering constraints, which is less of an issue for underground DC lines. Also, most underground AC lines carrying 230 kilovolts or more have used fluids to dissipate the heat produced by the transmission cables, raising concerns about possible leaks and damage to aquifers. In contrast, most DC cables have used non-draining paper for insulation.

On the other hand, DC systems are subject to several limitations. They are primarily designed for point-to-point transmission of power, and it is expensive to build the converter stations needed to connect a DC line to a power plant or substation, as well as to the AC transmission grid. Each converter station costs up to $ 50 million and can use up to three or four acres of land. In addition, the unavailability of DC circuit breakers restricts the feasibility of using DC in a grid. All of the connected DC lines in the grid must be taken out of service when an outage occurs or when a segment needs to be turned off for repairs or modifications. Unlike AC lines, power does not automatically reroute itself to avoid blackouts when there is a fault on a DC line.

Some of the above information was taken from a Department of Energy Website, http: //www. eere. energy. gov/climatechallenge/cc_options4. htm#ref42

PHASE 2 PROPOSAL

Connecticut Light & Power (CL&P) and United Illuminating (UI) have proposed building a 345-kilovolt AC transmission line from Norwalk to Middletown, which would connect with the recently approved Phase 1 (Bethel-Norwalk) line and an existing 345-kilovolt line that runs through the central part of the state. The proposal calls for 45 miles of the line to be above ground and 24 miles to be underground. In developing the proposal, CL&P and UI considered using DC, but determined that AC

would provide more flexibility and better promote system reliability. The companies estimate that the proposed line would cost approximately $ 500 million.

The issue of whether a completely underground DC line is a feasible alternative to the CL&P/UI proposal involves two separate questions, the use of DC vs. AC technologies and the burial of all vs. some of the line. As discussed above, DC is a useful technology in moving power from one point to another, and can be less expensive than AC technologies. In addition, because DC lines can function like power plants they can improve the reliability of service in load pockets. These are areas, such as the southwestern third of the state, where the demand for power exceeds locally available supply and the existing transmission system is limited in the amount of power it can bring in. On the other hand, DC technologies are less useful in network grid applications, such as bringing power to a series of substations (the phase 2 proposal would connect five substations). They are also less flexible, for example in their ability to provide for interconnections with new power plants.

The feasibility of placing the entire line underground is affected by several considerations, including the route of the line. The cost of placing a line underground is significantly affected by the underlying geology of the area. Because the phase 2 line crosses a major traprock ridgeline, placing the entire line underground could significantly increase its costs, regardless of whether it uses AC or DC. The actual cost of burying the line in a specific area cannot be determined until extensive tests are conducted to determine its geology. In addition, there are engineering constraints on the ability to move large amounts of power for long distances on underground AC lines.

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