USING CONVENTIONAL ELPIPES
FOR LONG DISTANCE TRANSMISSION
By Roger Faulkner, President; and Ron Todd, CTO; Electric Pipeline Corporation
Overhead transmission lines grow more controversial each
year, while at the same time, the economic need to transport
bulk power grows, fueled in part by the rapid development of
utility-scale wind farms remote from population centers. It is
difficult to site new overhead power lines of any capacity, and
it is particularly difficult to site new high-voltage overhead
lines, which are aesthetically overpowering (Figure 1).
Figure 1: Relative Scale of 745kV AC Line to buildings
Most of these objections can be overcome by placing long-
haul (100 km and over) transmission lines underground. These
are the options for achieving this:
Conventional cables (maximum 500kV at present)
Superconducting cables (maximum 200kV at present)
Gas Insulated Lines (GIL) (maximum 800kV)
Solid-insulated electric pipelines based on metallic conductors, “elpipes”; (max voltage 800kV)
Of these options, underground cables have a significantly
lower power transfer capacity, and cost many times more than
overhead power lines, so they are rarely used except in and
around cities. Cables can be used to deliver AC or DC power,
but AC runs are limited to 50 km or so (which is short in this
context) before capacitive charging currents rise to the point
that the cable requires expensive reactive compensators to
deliver useful power.
Cables can currently transmit DC power hundreds of kilometers, limited mainly by economics and the need to maintain
acceptable I2R losses. Cables need to be wrapped on a drum for
delivery from the factory to the installation site. The required
bending radius limits the cable diameter, and thus the conductor size and insulator thickness and, therefore, also the capacity. Waste heat removal is a problem for high power cables. All
the available flexible electrical insulation materials are also
good thermal insulators, so as the voltage goes up, the thicker
insulation required to withstand the voltage reduces the thermal
dissipation capacity of the line per meter. Thus, increasing the
voltage of a thermally-limited buried cable does not increase
the power capacity proportionately to the voltage increase.
Typically an overhead power line can carry four times as
much power for each doubling of its voltage (at constant transmission efficiency). For passively cooled cables, doubling voltage less than doubles capacity (because the reduced thermal
dissipation capacity means the current must be reduced at higher voltage). About 1. 1 GW per cable pair is the limit at present;
and such cables must be buried shallowly (< 30 cm), or in special thermally conductive sand to achieve these power levels. If
the cables are actively cooled by a circulating coolant fluid,
transfer capacity can be increased to about 1. 5 GW/cable pair.
At present, the maximum rated voltage for cables is 500kV,
though breakthroughs in nanocomposite insulation technology
(see US patent 7,579,397, now licensed to Dow by EPRI)
promise to allow for thinner insulation layers (capable of
20kV/mm as opposed to the present 10-12kV/mm design voltage gradient) which will in the future enable an 800kV poly-mer-insulated HVDC cable.
Superconducting power cables, which were described by
Jack McCall of American Superconductor in the
November/December issue of Electricity Today, have been getting a lot of attention and R&D funding. High power superconducting DC cables have yet to be deployed, though relatively
high power (~0.6 GW), high voltage AC superconductor cables
have been in operation for a number of years. The cryocooling
systems, though complex, are included in the cost of superconductor electricity pipelines as quadruple redundant systems
with a variety of backup and service features. Though such
extensive cryocooling has never been deployed, the engineering is sound and all of the core components have either been
fully simulated or demonstrated. Nonetheless, the complexity
per se of superconducting DC transmission makes proving reliability a difficult process that must occur in stages over
decades. Still, the unique property of zero electrical resistance
means that superconducting cables will eventually be important
for long distance applications.
Gas Insulated Lines (GIL) are a proven alternative for high
capacity underground power lines, but though they have been
available for 35 years, there has so far been no installation
longer than 3. 25 kilometers, due to the high cost per km. These
designs rely on sulfur hexafluoride (SF6) gas, which is a potent
greenhouse gas, for insulation. GIL lines are the only underground option that is feasible for long distance AC power transmission. This is because of their low capacitance per km;
though not quite as low as an overhead line, it is low enough
