Of the Substations that ITCTransmission Initially Acquired in 2003, four were mid-1970s vintage gas-insulated substations (GIS). The SF6 gas used as the primary insulating medium for the 120-kV transmission bus and circuit breakers had been leaking since just a few years after the original installation. The gradually increasing rates of SF6 gas emissions from the GIS bus and the problematic dual-pressure gas circuit breakers had been a black eye against the overall performance and expectations of the small-footprint GIS that were installed.
ITCTransmission, a subdivision of ITC Holdings Corp. (Novi, Michigan, U.S.), had one substation in particular, the St. Antoine site, that had developed numerous breaches, which resulted in weekly service call-outs to raise the pressures back to the normal level. When these problems began occurring, the gas leaks were not considered an environmental issue; rather, they were viewed as helping to contribute to a service man's job security and paycheck. After several attempts to repair the variety of leak sources and welded bus construction (with limited success), it simply became routine to either react to frequent alarms or schedule gas fills.
The St. Antoine site was not alone in being problematic. The three other sites within a 10-mile (16-km) perimeter feeding into the city of Detroit, Michigan, presented nearly similar challenges during the course of their lifetimes. In retrospect, the empirical evidence suggests the OEM designs of the 1970s era were not robust enough for the outdoor environment and seasonal climate changes of Michigan. The four existing GIS sites did not have the advantage of being housed in the climate-controlled indoor environment that other utilities with GIS decided was prudent.
Because the four GIS sites were located in Detroit and fed important loads, including General Motors' world headquarters, a reliable transmission supply throughout the city was deemed an important issue, as was heightened international awareness of greenhouse emissions affecting climate.
To a lesser degree, but still important, was that there were very few critical spare parts for the 30-year-old GIS equipment. The substation designs had been out of production for a long time, and in the event of a serious failure, ABB (Zurich, Switzerland) and AREVA T&D (Philadelphia, Pennsylvania, U.S.) anticipated long lead times in support of the OEM equipment. Quite simply, given the frequent maintenance of the vintage GIS, their effective life cycle had ended.
So, by early 2004, the ITCTransmission engineering group took steps to address the 120-kV GIS infrastructure issues at the St. Antoine, Caniff, Midtown and Cato substations.
AN ALLIANCE PARTNERSHIP
Implementing a business strategy, ITC Holdings Corp. — including its subdivision ITCTransmission — entered into an alliance partnership with Mitsubishi Electric Power Products Inc. (MEPPI; Warrendale, Pennsylvania). Although there is nothing revolutionary about the company's tight manufacturing tolerances, simple proven designs and attention to detail during assembly (especially with single O-ring seals and sealant products), MEPPI has developed a reputation for high-quality products with the lowest overall cost of ownership.
Choosing to partner with MEPPI made sense to ITC for a variety of reasons. An important design feature required for new circuit breakers was that the mechanisms driving the interrupters not rely on pneumatic or hydraulic pressure. ITC's past experience with the inherited air-blast and hydraulically driven circuit breakers was that these systems required frequent maintenance and repairs. Also, earlier versions of gas breakers and other equipment using SF6 had a less-than-stellar record of being leak-free, producing moisture ingress and refilling problems. MEPPI indicates that its equipment will lose less than 1% gas volume per year, or it will perform the repair to make it right under warranty. Consistent with ITC's vision and goal, a prominent sign within the Mitsubishi factory indicates that the company takes leak controls seriously: "No Leak Too Small … We Catch 'em All."
The alliance partnership established for outdoor switchyard transmission-class circuit breakers served as the impetus for ITC to further explore the abilities of MEPPI to supply GIS replacements. A separate substation department within MEPPI handles GIS, and conceptual design meetings began in March 2005. At the first and subsequent design meetings, discussions revolved around technical details of the state-of-the-art replacement for the failing GIS at the St. Antoine substation.
A major challenge of the St. Antoine GIS replacement was the requirement that, at most, only one-third of the normal 120-kV load could be removed at any given time. This challenge alone required several custom interfaces between the existing and new GIS to keep two-thirds of the 120-kV station load fed at all times.
The existing bus was of the welded design, which increased the difficulty of the fieldwork that would be necessary to install the interfaces. The schedule was fairly aggressive, such that within the three-month construction phase, there would be the replacement of a three-circuit-breaker ring bus connected to three 24/32/40-MVA transformers, a solid dielectric underground cable and two high-pressure gas-filled underground cables.
In addition, the congested vicinity of St. Antoine GIS in downtown Detroit afforded little construction lay-down space for staging and storing materials. Therefore, the scheduling of materials for just-in-time delivery was another challenge MEPPI had to meet.
In performing due diligence of other GIS suppliers' economics and abilities, ITC found that only MEPPI had the experience of replacing GIS in part, while keeping portions energized and serving load during the construction process. While it was a calculated risk to attempt such an endeavor, MEPPI had recently completed two similar replacement in-part projects for Southern California Edison (Rosemead, California, U.S.), which included replacing failing 230-kV and 550-kV GIS at Serrano. The 230-kV substation replacement resulted in a total of 14 MEPPI breakers, making this the largest 242-kV GIS installation in North America.
After weighing all of its options, ITC decided that given the risk and visibility of its project, and in support of its newly forged alliance partnership, MEPPI was the logical choice. ITC and MEPPI also agreed to use M.J. Electric, LLC (Iron Mountain, Michigan) as the installation contractor. This was another risk-reducing measure for the St. Antoine GIS project, because MEPPI and M.J. Electric had worked together previously and recently completed a GIS project for Commonwealth Edison of Chicago, Illinois, U.S. (see T&D World, July 2008).
MODERN GIS ADVANTAGES
The St. Antoine GIS design remained in a three-breaker-bay ring-bus configuration, with single-phase insulated construction, rated at 63 kA. The primary design focused on safety, reliability and low maintenance. MEPPI had a reputation for very low SF6 gas leak emissions, and it increased its stake in the project with a leak rate guarantee of less than 0.1% per unit volume per year for the GIS. An extended warranty that guarantees this leak rate for a long time provided ITC with assurance that leaks should no longer be a problem at St. Antoine.
The heart of any GIS is its circuit breaker, and the heart of a circuit breaker is its mechanism. The St. Antoine circuit breakers used the BM-2 torsion bar spring mechanism. This mechanism requires no maintenance whatsoever over the course of the in-service lifetime. A proprietary self-lubricating process is used for the bevel gearing, sliding and latching surfaces that comprise the load-bearing surfaces of the mechanism.
The gas-density monitors were supplied by Mitsubishi of Japan, using the company's preferred Saginomiya-brand temperature-compensated device for alarm and control. ITC agreed to use this and other standard components supplied by Japan at MEPPI's recommendation, so as not to introduce unnecessary changes to the proven designs and disrupt the tight schedule for the project. The GIS compartments, other than the circuit-breaker zones, used Trafag-brand temperature-compensated density monitors. The Trafag gas monitors do not have any control function on the breakers to isolate sections of the GIS; instead, they only supply both a low and low-low alarm indication.
The theoretical design limit of the GIS bus and breakers is that at zero SF6 pressure, the components' insulation systems will withstand 1.0 per unit line-to-ground voltage. Throughout the design process, this fail-safe philosophy was employed when practical. Applying the fail-safe concept, the native pressure-switch contacts were used to depend on the made-up (closed) condition to signify normal pressure, whereas abnormal low pressure resulted in (opened) contacts to the local alarm annunciator and supervisory control and data acquisition system (SCADA).
Industry-accepted Dilo-brand gas fill/vacuum valves were used throughout, mounted on custom manifolds to facilitate future maintenance tasks. AZZ/CGIT Inc. (Medway, Massachusetts, U.S.) worked with MEPPI to supply the aluminum bus and enclosure connecting the breakers, switches, line feeders and transformers. CGIT also supplied the temporary bus interfaces needed during the construction phases and interfaces of the GIS to the underground cables.
MEPPI selected Phoenix Electric Corp. (Boston, Massachusetts), with which it has a long-standing relationship, for design and supply of the local control cabinets. ITC had thorough involvement during the design phase of these cabinets in order to tailor them to its specifications. In addition to the hard-wired standard annunciator, one of the notable features was a state-of-the-art fiber-optic SCADA monitoring system.
REPLACEMENT SUCCESS
Retrofit projects combining old and new assemblies together always present a particular challenge to the substation owners and their contractors. While ITC and MEPPI considered the first project a success, there were lessons learned that apply to all GIS installations, retrofit and new.
ITC involved its own engineers, operating field agents and other support staff throughout the project's design phase as one key to success. A Black & Veatch (Overland Park, Kansas, U.S.) consultant experienced with GIS specifications was retained to assist with the first GIS project. Particular attention was given to places where MEPPI supplied equipment touched by another vendor, existing equipment and existing structures. Special focus in these areas eliminates the majority of all design mistakes, and this is especially true for retrofit projects.
During the bulk of commissioning tests, key engineering, operating and maintenance personnel would witness MEPPI's site personnel as they completed the functional tests of the new GIS. Witnessing and participating in these tests provided invaluable equipment training that would be impossible to replicate later in a classroom or a factory setting. It is best to limit the reliance on existing drawings, but rather verify in the field as much as possible. Field verification using modern measurement techniques and surveyors confirmed that not all of the existing drawings for St. Antoine were accurate and up to date. In particular, the potheads where underground lines connected to the new GIS received special scrutiny.
ITC had to set the outage dates far in advance of the installation. The installation schedule was created and reviewed thoroughly with ITC and the installation contractors, and the sequence of events that would be taken during the installation was rehearsed prior to the outage. ITC built various delays and recovery time into the schedule. Twelve-hour shifts were the norm, with second shifts added when needed.
The Midtown GIS replacement was completed and placed in service in June 2008, in similar successful fashion as the previous two projects. ITC has other GIS projects in its design queue, and MEPPI will remain an alliance partner on those projects, as well.
ABOUT ITC HOLDINGS CORP.
ITC Holdings Corp. invests in the electricity transmission grid to improve electric reliability, improve access to markets and lower the overall cost of delivered energy. ITC is the nation's largest independent electricity transmission company. Through its subsidiaries — ITCTransmission, Michigan Electric Transmission Co., LLC and ITC Midwest, LLC — ITC operates regulated high-voltage transmission systems in Michigan's Lower Peninsula and in portions of Iowa, Minnesota, Illinois and Missouri. It serves a combined peak load in excess of 25,000 MW.
Through its subsidiaries ITC Grid Development, ITC Great Plains and ITC Panhandle Transmission, ITC is also focusing on new areas where significant transmission system improvements are needed.
Joseph Jasinski ([email protected]) is a senior engineer at ITC Holdings Corp. His focus is circuit breakers, circuit switchers, GIS and predictive maintenance pertaining to transformers and substation thermography. He supports ITC's field and operating agents' daily activities across all ITC Holdings Corp. subsidiaries and serves as a chief liaison with Mitsubishi on technical matters. He previously worked at Detroit Edison, where he held a position in the predictive maintenance program. Jasinski earned his BSET degree from Wayne State University in 1995.
