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Sectionalizing and special equipment accelerate the upgrade of 138-kV lines with robotic arms.
After the summer of 1999, customer sensitivity and reliability concerns became the No. 1 priority at ComEd, an Exelon company in Chicago, Illinois, U.S. Consequently, the transfer capability of the transmission system became a major concern.
As with most utilities, many of ComEd's transmission lines had been in service for more than 60 years and had been regularly maintained and updated. However, most of the lines had not been modified to increase capacity capability.
ComEd identified two existing 138-kV circuits in its power grid that could increase both customer reliability and transfer capability. Supported on double-circuit lattice tower structures, these lines constitute the main transmission system interconnection between Midwest Generation's (formerly ComEd's) Joliet Generating Station and ComEd's Dresden Station. The two lines also provide service to several large industrial customers that operate 24 hr a day, 7 days a week. These customers opted for two-line 138-kV service. Interruptions are nearly impossible and require customers to prepare for numerous hours or even days to avoid major process disruptions and associated costs.
Structure and Line History Built in the late 1920s, the structures have been maintained regularly, and the tower lines have been updated several times. However, the recent influx of independent power producer generation in the area and the sale of the Joliet facility have increased the need for transmission-line capacity.
A study revealed that with minimal tower-structure modifications to each of these lines, the transfer capability could be increased by at least 100 MVA. To accomplish this, ComEd would have to replace the lines' 300-kcmil (19-strand copper) conductor with 1113-kcmil (45/7), steel-reinforced (ACSR), aluminum conductor.
ComEd also determined that installing two additional circuit switchers in the lines would improve operating flexibility (Fig. 1). These switches, plus the customers' two-way feed switches, allowed the line to be divided into sections that could be de-energized one at a time. Thus, if one section was deenergized, customers could still have feed...