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By Jack McCall
. While the addition of each Plug-in Hybrid Electric Vehicle (PHEV) to the street is a victory for the environment, their widespread adoption will present another potential challenge to the power distribution system, particularly in urban networks. With as few as one car in five being PHEVs, smaller cities can see hundreds of MW of load added to their urban grids, with large cities seeing over 1 GW as the table below highlights.
Urban distribution networks are often designed so that customers receive electric power from a single isolated substation. Upgrading space-constrained urban substations to accommodate significant amounts of new load is often difficult or impossible, whether the load is from PHEVs or new commercial developments. And while urban networks are generally highly reliable, this single substation dependency makes them highly vulnerable to transformer failure events. . Electrically paralleling urban distribution substations can simultaneously address both load growth and increase network resiliency. Cable connections between the medium voltage busses of adjacent substations can be used to transfer transmission levels of power between them. This increases resilience to equipment failures and other events. It also increases the load-serving capacity of the stations without the need for adding or upgrading transformers. Depending on the substation configuration, such a connection can theoretically increase load servicing capacity by 50-100% or more.
Conventional cables are not practical for this connection. The amount of power that must be transmitted is usually 4 – 10x the power capacity of traditional medium voltage cables. Installing multiple sets of cables is difficult in urban areas where
Superconductor cables, only recently available for utility applications, uniquely solve these issues. A single distribution voltage superconductor cable can carry amounts of power normally associated with transmission voltage levels, therefore eliminating the need for multiple cables and greatly simplifying placement issues. Superconductor cables also have a unique dual-personality; under normal conditions they conduct power very efficiently, but during faults they actually limit the amount of current that can flow through them. This eliminates the risk of substation equipment damage from excessively high fault currents when paralleling substations. The installation of superconductor cable-powered bus ties between distribution substations serve as an efficient means to utilize more effectively and fully the existing power delivery infrastructure while simultaneously increasing reliability.
Jack McCall is Managing Director for Superconductor Power Systems at AMSC.
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