ComEd engineers have added an innovative tool to their reliability toolbox. The new technology provides an alternative to conventional automatic circuit reclosers that re-energize a line section to see if a fault is temporary (caused by tree or wildlife contact) or permanent (resulting from a downed wire or cable failure). The new tool uses pulse-closing technology to verify that a line is clear of faults before initiating a close operation. S&C Electric Co. developed the PulseCloser in 2008 for its pole-mounted IntelliRupter. ComEd, however, needed a pad-mounted version of the unit, which would prove critical in an effective pilot of the technology in the company's suburban distribution system.
In June 2011, ComEd completed the first U.S. installation of a pad-mounted IntelliRupter PulseCloser. This installation takes ComEd a step further in smart grid deployment, which will provide customers higher service-reliability levels.
One of the major reasons not to close into a fault is to reduce wear and tear (induced thermal and mechanical stresses) on electrical distribution system components. By not having to deal with the shock of typical recloser operation on a permanent fault, it is estimated the let-through energy the system must experience is reduced by 98%. This means significantly less damage to equipment, which should pay huge future dividends in extending the life of underground cable and other assets. Equally important, voltage sags experienced by customers during conventional recloser operation are eliminated.
After interrupting a fault, pulse closing injects a low energy current pulse into a phase to determine whether the fault has cleared. Analysis of the resulting waveform determines whether a fault still exists downstream. If the fault is clear, the subsequent phases are pulsed until all three phases are closed. If a fault persists, the circuit remains open. The IntelliRupter PulseCloser is designed to operate with new and existing distribution automation functions, including supervisory control and data acquisition (SCADA) systems, and provides better segmentation and coordination than conventional reclosers. The device also offers innovative protection features, such as intelligent fuse saving, which can further improve service reliability and minimize the costs associated with a fault.
ComEd has had a long and successful working relationship with S&C and partnered with the company on a pilot project to validate the cost and benefit of this technology. This pilot installation enables ComEd to evaluate features such as pulse closing, intelligent fuse saving, automatic restoration system and single-phase tripping.
Traditionally, distribution systems have relied on overcurrent coordination to ensure that the correct device clears a fault. However, properly coordinating a large number of devices in series has practical limitations. Pulse finding can expand the number of protective devices that can be coordinated in series by overcoming the coordination constraint since faults are detected without using time-current characteristic curves. Pulse finding hunts for the fault location and isolates it without the need for peer-to-peer communication.
For a fault downstream of an IntelliRupter, each device will trip open. The first upstream device then pulses the downstream section of line since it has voltage on the source side. If the pulse reveals there are no problems on that line section, the segment is re-energized. The next downstream device follows the same process until the line segment with the fault is identified. That unit then stays open, isolating only the faulted section and impacting the minimum number of customers.
The inherent flexibility of the IntelliRupter permits the use of an IntelliTEAM II application, which allows load-qualified feeder reconfiguration using distributed intelligence and communications between the source and remote switches while, at the same time, ensuring there is sufficient capacity to transfer so an adjacent feeder will not be overloaded.
A New Wrinkle
As originally envisioned, the pilot program was designed for only overhead PulseCloser applications. However, ComEd engineers introduced a challenge by wanting a pad-mounted version that did not exist at the time. Additionally, ComEd engineers wanted the overall size to be comparable to other pad-mounted switchgear in use on the system.
ComEd's suburban distribution system is a hybrid of overhead wire and underground cable. Faults on the underground distribution system are typically permanent in nature. However, underground cable is routinely connected to additional downstream overhead lines that can experience temporary faults. The pad-mounted pulse-closing units had to protect the hybrid circuits. This provided installation flexibility and expanded potential sites for application of the technology.
In the 1980s, an Electric Power Research Institute (EPRI) research project characterized faults on feeders with predominantly overhead exposure. Approximately 40% of faults in this study occurred during periods of adverse weather, which included rain, snow and ice. The EPRI research project found 85% of faults were temporary in nature and could be restored with one or two reclose operations.
The safe integration of new smart grid technology into ComEd's electric distribution system required the development of construction standards, operating procedures, tools, training and acquisition of material.
Construction standards were developed for the overhead units in fall 2010 and the pad-mounted units in spring 2011. Operating procedures were developed with the support of work methods, dispatch and training personnel. S&C supported the training of ComEd's craft and dispatch employees. A portable demonstration unit was used to familiarize employees with the new equipment.
The pad-mounted IntelliRupter was installed as part of a new full-loop (bi-directional) automation scheme with overhead IntelliRupters. The location was selected as part of ongoing outage analysis efforts by ComEd's distribution automation department. The suburban underground area was a good fit for the pilot project and was integrated into a focused reliability plan developed by regional reliability engineers to harden service to both residential areas and the central business district.
ComEd engineers wanted a self-powered pad-mounted unit so external power would not be required. An external power source would add cost, complexity and, likely, further congest rights-of-way or easements. To meet the self-powered requirement, S&C added two integrated power modules to provide unit power for open, close and pulse-closing operations as well as Wi-Fi and SCADA communications.
Also, new to the pad-mounted IntelliRupter is a low-voltage compartment and a separated high-voltage cable compartment. The low-voltage compartment houses the protection and control module, communications module, manual operation lever, manual hot line tag switch, manual ground trip switch and SCADA communications antenna. The high-voltage cable compartment houses the 600-A rated dead-front elbow terminations and a clear polycarbonate viewing window used to confirm the open/close position of each interrupter. All versions of the device use the same red target with an “I” to show an interrupter is closed and a green target with an “O” to show it is open.
A manual operating mechanism is provided to allow on-site operation with an insulated disconnect stick for those rare occasions when the device needs to be operated locally.
The typical 600-A rated terminations used by ComEd include the standard 600-A T-body elbow with a load break reducing tap plug installed with the elbow. The load-break-reducing tap plug provides an industry-standard 200-A load-break interface used for testing, grounding and mounting an arrester, where appropriate. A 36-inch (0.9-m)-deep custom polymer concrete foundation was chosen to provide adequate cable training space.
Adapting for Underground Applications
Since IntelliRupter is a unitized package of fault-interrupting and control components originally designed for overhead applications, S&C needed to modify the physical design to accommodate the same functionality within the footprint of a pad-mounted enclosure. This was no small feat. Substantial engineering time was devoted to integrate all of the design components in a total package optimized for application on overhead distribution systems. It took significant work and creativity to re-engineer this integrated product for an underground distribution application.
One necessary change was relocating the communications and control modules that house the electronics for pulse-closing and automation applications. These modules were relocated to a low-voltage enclosure, where they could be readily accessed in a pad-mounted configuration. This allows for access to and removal of these modules while energized. Importantly, the relocation allowed phase spacing to be reduced and overall footprint to be minimized.
One of the biggest design challenges was adapting the overhead design to provide the operating features required in pad-mounted applications. The overhead design allows for manual operation through an industry-standard insulated disconnect stick. However, additional manual operating features were needed for a pad-mounted design.
Collaboration between ComEd and S&C resulted in the development of a new operating handle and provided input on other requirements for operating personnel. The handle is located inside the low-voltage enclosure and can be operated either by hand or by a disconnect stick. It allows locking and tagging in the open position. Manually actuated hot line tag and ground trip block profiles can be enabled by toggle switches in the low-voltage compartment.
A viewing window in the cable compartment was developed to allow clear indication of open/close status. This is a dead-front elbow-connected switchgear design, and viewing windows are typically used on elbow-connected switchgear to obtain indication of the open/close status. Also considered in the design were features to reduce condensation on the window to enhance visibility. A clear removable barrier in the switch compartment was included to enhance operator safety during visual inspection when the switchgear is energized.
ComEd's final design requirement was to incorporate sufficient depth in the cable compartment to allow elbow surge arresters to be installed on the backside of the 600-A T-body elbow terminations. The primary bushings were realigned for ease of cable training.
The integrated design of the overhead IntelliRupter created simplified construction work in the field since there was no control cabinet to hang and no control cables to run down the pole.
On the pad-mounted IntelliRupter, design reviews between ComEd and the S&C team throughout the design process proved quite valuable. ComEd shared details on its work practices, which resulted in such design improvements as changing the switch compartment's removable barrier from red GPO-3 material to a clear polycarbonate to aid visibility. Adding a provision on the manual operating handle allowed hook-stick operation, and optimized bushing locations simplified cable training. Also, the low-voltage enclosure design provided additional space for larger batteries so control and communications modules can be powered for a longer period if power is lost.
ComEd is collaborating with S&C on enhancements to the current design, including additional features that will further simplify operation. For instance, the low-voltage enclosure door will be changed to facilitate easier access to the electronics. An integral disconnect also is being considered to easily provide a visible gap on the source side. An integral disconnect would save ComEd employees time by disconnecting source power at the unit without opening an upstream switch or removing 600-A dead-break bolted connections for testing underground cables.
Peter Tyschenko (email@example.com) is the manager of distribution standards and technical support at ComEd. Since joining ComEd, he has been involved in field engineering, work management and reliability engineering. Tyschenko holds a BSEE degree from the University of Illinois at Chicago.
James Stamatopoulos (firstname.lastname@example.org) is a manager in distribution standards at ComEd. Since joining ComEd, he has been involved in field engineering, capacity planning and new business designs. Stamatopoulos holds a BS degree in general engineering from the University of Illinois at Urbana-Champaign and a MBA degree from DePaul University.
Patrick O'Connor (email@example.com) is principal engineer in distribution standards at ComEd, which he joined in 2002 after a career as manufacturer's representative. His current responsibilities include material specifications, vendor approvals, material failure analysis, new product evaluation and field support. O'Connor holds a BSME degree from Marquette University and is a registered professional engineer in Illinois.
Electric Power Research Institute www.epri.com
S&C Electric Co. www.sandc.com