There are very few pieces of technology from the 1970s that we are still using today – and for good reason. Up until recently, however, medium voltage switchgear designs had only been minimally altered in the past fifty years.
The electrical distribution industry is undergoing a massive transformation. The industry’s workforce is changing as experienced engineers and electricians retire creating a workforce shortage, budgets are being tightened to deal with a tougher economy, and industrial and regulatory standards have become stricter as safety practices are shared and implemented. Additionally, equipment space is at a premium in facilities across many industries that require high power density from their limited floor space. For these and many other reasons, it is time to reevaluate medium voltage switchgear applications.
Traditionally, medium voltage switchgears in the United States have been Air Insulated Switchgears, meaning that the principal method for achieving dielectric stability is sufficient air gaps between phase conductors. Metal enclosed fused switches and metalclad vacuum circuit breaker switchgear designs fall into this category. While they sometimes incorporate thin layers of epoxy coating insulating tape, phase gap distances are all determined using air gap. While Air Insulated Switchgear has been the only option for many years, the widespread use of bus differential protection schemes shows that the design community is somewhat untrusting of air insulation’s abilities to prevent phase-to-phase faults.
The latest design of medium voltage switchgear addresses these issues through an innovative insulation system called Solid Insulated Switchgear. Rather than air separating the bus bars, the system uses a molded epoxy insulation as the source of dielectric strength. A key benefit of this design is that solid insulation allows for a decreased air gap between the phase conductors, which reduces the total equipment size. Additionally, exposure to live parts is eliminated.
In Shielded Solid Insulation Switchgear (2SIS) not only are there no exposed parts, but every interior portion is at ground potential. This enhanced version includes a ground shield layer on the outside of the insulating layer. This ground layer minimizes the phase-to-phase interaction, reduces likelihood of arc flash, and helps to protect the insulating material itself. With the ground shield behaving like a Faraday cage, it’s able to minimize the propagation of electric field into the ambient areas of the switchgear. This helps prevent insulation break down phenomena such as tacking and partial discharge.
2SIS offers incredible benefits including a drastic reduction in switchgear footprint. Traditional metal-clad switchgear has a standard section width of 36 inches, and depths and heights averaging 92 and 95 inches respectively, whereas 2SIS can deliver section widths of less than 15 inches and depths as small as 36 inches. When comparing traditional metal-clad switchgear to 2SIS switchgear, the footprint can be reduced by as much as 60 percent. Additionally, the decreased footprint has allowed for a front-accessible switchgear design. This allows the switchgear to be placed against walls or pushed into corners of electrical rooms. This is a particularly attractive benefit for space-constrained facilities.
Budget-wise, 2SIS can also impact the bottom line. While traditional metal-clad switchgear only offers 1200A and above ratings, 2SIS switchgears offer circuit breakers with ratings of 200A, 600A and 1200A. This allows designers to select breakers which more accurately reflect the load requirements and reduce cost.
Improving both financial savings and peace of mind, shielded solid insulation significantly increases the reliability of the medium voltage switchgear. The ground shield paired with the molded epoxy insulation acts to increase the life expectancy of the insulation itself. By protecting not just the conductor, but the insulation as well, the switchgear’s life expectancy is increased.
In air insulated models there is space for the intrusion of dust, rodents or vermin into the live switchgear components resulting in damage and equipment replacement. With 2SIS, the switchgear’s live parts are enclosed in solid dielectric material and better protected from harsh operating environments. This robust switchgear design means a reduced maintenance interval. The maintenance cycle can be up to ten years for 2SIS switchgear.
Reducing the likelihood of arc flash incidents is perhaps the greatest benefit of 2SIS technology. By separating each phase with two insulating epoxy layers, two ground shields and an air gap the probability of any kind of phase-to-phase interaction is minimized. By ensuring that all exposed parts on the interior of the switchgear are at ground potential, there is little to no room for phase to phase or phase to ground faults. Additionally with reduced maintenance requirements, personnel are in and around power distribution equipment less, thereby reducing human exposure to medium voltage power. Through safety interlocks and intuitive operation, inadvertent operation or misuse of the switchgear can be prevented, adding a level of safety by design.
When considering an electrical upgrade, it’s important to compare different models and benefits. Shielded solid insulated switchgear addresses issues of modern electrical design, whether it be space, budget, safety or maintenance.
Joe Richard is the US Launch Manager for Schneider Electric’s Premset Switchgear. Joe graduated from the Georgia Institute of Technology with a BS degree in Electrical Engineering in 2007, and has been with Schneider Electric since 2008.