In the past three years, Exelon Corp. (Chicago, Illinois, U.S.) has made great strides in upgrading its transmission and distribution in-frastructure in the Chicago metropolitan area. This includes both the upgrading of existing facilities and the building of new lines and substations.

This article is the second in a series addressing Exelon's efforts to upgrade transmission infrastructure. The first article, “Casting for a Low-Cost Solution” (T&D World, June 2001) addressed the first phase of Exelon's System Reinforcement Project, the goal of which was to meet the load demands on the north side of Chicago, where the utility installed a new transmission substation.

The TSS 40 Diversey Substation was fed through two existing static 138-kV pipe cable lines (11413 and 11418), which passed in front of the new substation. These lines were separated into four individual lines: 11413, 11418, 4013 and 4018. Lines 11413 and 11418 feed the station from the north, while lines 4013 and 4018 supply power to the station from the south. These lines provided the new Diversey TSS 40 with transmission capacity and increased reliability.

The goal of the second phase of the System Reinforcement Project was to increase the load capacity at Diversey TSS 40, which could be done several ways. The quickest and most cost-effective way was to convert the existing static lines to a forced cooled system. Lines 11413 and 11418 feeding from the north were selected to become the forced cooled lines. These lines, which begin at Exelon's Northwest TSS 114 Overhead to Underground Terminal at Addison Street and the Chicago River, were installed initially in the 1970s with a single 6-inch (15-cm) fluid return line for future forced cooling use.

After the new forced cooling equipment was installed, lines 11413 and 11418 and the return pipe contained more than 66,000 gal (249,837 liters) of dielectric fluid. Since the return pipe was installed nearly 30 years ago and never used, its overall condition needed to be assessed. The condition of the manholes, cable pipes, dielectric fluid and associated equipment were analyzed. The cable pipe inspection included a review of the history of the pipe, cathodic protection survey and inspection of the pipe inside the manholes. The history of the pipe showed no leaks through loss of dielectric fluid. This testing revealed that the cable pipe was in an acceptable condition, so the only necessary task was to grease tape the pipe inside the manhole to preserve integrity.

The return line pipe inspection included a visual inspection inside the manholes, a cathodic protection survey, a video analysis inside of the pipe plus pressure and vacuum testing. Although the return pipe passed the pressure and vacuum test, the video exposed a few questionable areas. However, no action was taken to repair the spots because isolation valves were installed in each manhole on the return line. Future repairs on the return line can be made with the cable lines in service by putting the lines in static mode of operation.

Checking the Fluid Condition

The fluid samples from these lines revealed extra-high dissolved gas contents, mainly due to previous cable failures and past use of methane blankets to keep the moisture from coming in contact with the fluid while in transit. Cable fluid containing large amounts of gasses raises some flags. Dissolved Gas Analysis (DGA) tests determine how much high gas is in the dielectric fluid and the basic condition of the cable. The DGA includes the following gases: nitrogen, oxygen, carbon dioxide, carbon monoxide, hydrogen, methane, ethane, ethylene and acetylene. Failures of cables joints or terminations produce combustible gases in the fluid. These gases will remain in the fluid in cases where repairs are made that do not require complete replacement of the cable. Thermal cycling of the line and the replacement of cables and joints all contribute to the migration of fluid throughout a static cable system.

Prior to beginning the forced cooling project, transmission line engineering had to reduce the level of gasses in the fluid. In this case, DGA did not pinpoint any cable degradation trends, so Exelon looked for a way to develop a new base line and asked, What could ComEd do to establish a new fluid base line and reduce the gases? The utility investigated two options.

Option 1

Draining the fluid and replacing it with new fluid would be the normal practice and would entail replacing thousands of gallons of fluid. It would require three complete changes to remove most of the residue from the interior of the pipes and on the cable inside the pipe.

Option 2

De-gas the existing dielectric fluid inside the cable system. This new process would save the cost of replacement dielectric fluid, fluid disposal and labor.

Because of the high cost of the labor and necessary material to replace the dielectric fluid, ComEd elected to process the fluid inside the cable pipe system. Processing the fluid inside the cable system required new equipment and procedures. ComEd engineers modified an existing Absolute Vacuum (ABVAC) fluid-processor and designed a new high-pressure portable pump. The new high-pressure portable pump and fluid-processing equipment needed to be synchronized to balance the flow and pressure inside the pipe.

With the line out of service for approximately three days, the plan was to remove the fluid from the line at 200 psi, heat it, put it under vacuum to remove the gases, repressurize to 200 psi and return the fluid to the cable system. The project was split into two phases. Line 11413 was processed during the week of May 20, 2002, and Line 11418 was processed during the week of May 27, 2002, resulting in significant reduction of dissolved gases

Up and Running

Diversey TSS 90 was put into service in June 2000, with four 138-kV bulk power transmission lines as a source of power. A second phase was planned to increase the power source by adding a forced cooling system to the two transmission lines that feed the station from the north. The forced cooling substantially increased the capacity of the new substation. This fluid processing to remove the gasses from the dielectric fluid produced excellent results. This new technique reduced the gas levels in the dielectric fluid of total combustible gases, in some cases by more than 100%.

The fluid in these lines was processed with the lines out of service. In the future, fluids could be processed with the lines energized. This would require careful monitoring to prevent any line trips or failures of the line due to sudden pressure changes.

Donald Johnsen is a senior engineer in the transmission department of Exelon (ComEd). He is responsible for the design and installation of pipe-type, self-contained fluid-filled, and solid-dielectric high-voltage underground transmission cable systems. In the last three years, he has project managed the design of more than six transmission projects. He has worked for 29 years in the design, repair and maintenance of more than 20 underground transmission cable projects, and has served as underground construction superintendent in Chicago for ComEd.

James Gburek, a senior engineer in the transmission department of Exelon, has worked for ComEd for more than 34 years. He is responsible for the design and installation of pipe-type, self-contained fluid-filled, and solid-dielectric high-voltage underground transmission cable systems. In the last six years, he has managed the design of more than 16 major transmission projects. He has been active in troubleshooting, repair and maintenance of underground transmission 69-, 138- and 345-kV lines and apparatus.