O'Hare International Airport is One of the Busiest Airports in the World, with more than 880,000 flight operations in 2008. Behind the scenes, O'Hare Airport — one of the 3.8 million customers served by Commonwealth Edison (ComEd; Chicago, Illinois, U.S.) — also represents a fairly harsh environment for padmounted switchgear.

The approximately 110 15-kV-class padmounted switchgear presently installed at O'Hare Airport use air as the dielectric medium and provide in-air switching. This switchgear design has provided a reasonable level of reliability for general applications, but has proven to be susceptible to airborne contamination problems.

The switchgear came from two different vendors. The one vendor that is no longer in business manufactured a minority of the units. The majority, however, were live-front PMH-style switchgear manufactured by S&C Electric Co. (Chicago).

These four-bay switchgear are installed in a variety of 600-A disconnect and 200-A fused configurations, with the most common configuration being two 600-A disconnect bays and two 200-A fuse bays. Since 1996, ComEd has experienced nine switchgear failures or preemptive switchgear replacements attributed to airborne contamination. This is a problem rate of approximately 8%.

SWITCHGEAR TECHNOLOGY

Open-air insulation is one of four commonly used medium-voltage technologies for circuit interruption. The other options are oil, vacuum and sulfur hexafluoride (SF₆). Special care is taken to prevent flashovers in air-insulated switchgear during opening operations. During load-break operations, arcs are contained and extinguished within an interrupter.

Air and vacuum are the preferred options in the medium-voltage range on both technical and environmental grounds. SF₆ is more commoly used in higher-voltage applications. According to the Intergovernmental Panel on Climate Change, SF₆ is a potent greenhouse gas. SF₆ has a global warming potential 23,900 times that of carbon dioxide (CO₂) and an atmospheric lifetime of 3200 years.

SWITCHGEAR LIFE FACTORS

It is usually difficult to determine the exact cause of a switchgear failure. The life expectancy of padmounted switchgear can be influenced by several factors: environmental corrosion (rusting of the case); surface contamination (dust, dirt and other airborne contaminants); wildlife (burrowing under foundations or crawling through damaged foundations); vehicle accidents (cars, trucks, snowplows and lawn-mowing equipment); misalignment of live parts; frequency of mechanical switching operations; and frequency of exposure to fault current.

ENVIRONMENTAL PERFORMANCE

Because of the criticality of service to O'Hare Airport, it is important to use padmounted switchgear that minimizes the possibility of a contamination failure caused by salt spray, deicing agents and jet fuel residue. Additionally, the switchgear is often located in low-lying areas where water runoff can create continuously high humidity within the switchgear and foundation. It is this environment, in conjunction with an extended condensation cycle, that increases the likelihood of contamination failures.

The contamination failures of the switchgear from the vendor who is no longer in business occurred primarily on the push-pull insulator used to drive the switch blades. These insulators used several molded skirts but were prone to leakage currents (tracking) and eventual failure under even relatively light amounts of contamination.

The S&C PMH series switchgear were less susceptible to contamination failure. In fact, the contamination failures that occurred on the S&C switchgear were caused predominantly by tracking on the interphase barriers.

Internal components such as hardware, surge arresters and cable terminations were also susceptible to contamination in both manufacturer designs.

TYPICAL FAILURE SEQUENCE

Moisture, combined with dust and dirt, deteriorates insulation within switchgear. Leakage currents form paths across contaminated insulator and barrier surfaces, which result in a switchgear flashover.

A typical event sequence leading to surface-contamination failure generally starts with a buildup of airborne contamination on the barriers or other components of the switchgear. This can be exacerbated by the location of the equipment, which makes it more vulnerable to contamination.

Combining moisture with the accumulated contamination provides a medium for tracking. Repeated condensation cycles within the switchgear, caused by standing water in the switchgear foundation, can extend the duration and severity of tracking. Finally, tracking reaches a critical point and a flashover occurs. The flash develops a conductive gas cloud, which spreads the flash phase-to-ground and phase-to-phase.

SWITCHGEAR OPTIONS

ComEd engineers evaluated several options in the drive to identify the optimum replacement for live-front, air-insulated switchgear. Two options were immediately identified: dead-front switchgear using SF₆ as the dielectric medium and dead-front, elbow-terminated switchgear using air as the dielectric medium. Both styles seemed to offer significantly improved performance levels in contaminated areas.

Switchgear using oil as the dielectric medium were not considered because of a historic company preference to avoid oil switchgear. After considering several possible vendors, ComEd engineers decided to focus on two types of S&C switchgear: the PME series using air as the dielectric medium and the Vista models using SF₆ as the dielectric medium.