Higher education, medical and corporate multi-building organizations have long taken advantage of the cost savings, design flexibility and improved reliability that result from owning and operating their own primary distribution systems. In contrast to the above-grade outdoor philosophy followed by most utilities, customer-owned primary switching equipment and substation transformers are usuall...
Higher education, medical and corporate multi-building organizations have long taken advantage of the cost savings, design flexibility and improved reliability that result from owning and operating their own primary distribution systems. In contrast to the above-grade outdoor philosophy followed by most utilities, customer-owned primary switching equipment and substation transformers are usually located inside the building they serve or in below-grade vaults at the building perimeter. This provides protection from the elements and accidental damage, and accommodates the aesthetics of campus landscaping. However, placing equipment indoors, where it has to compete for space with program requirements, means designs must be as compact as possible while still allowing for safety, function and reasonable provisions for future expansion.
New technology, from insulator components to meters and protective relays, reduces the footprint, improving the safety of switchgear and providing space-saving benefits in new installations when existing systems are upgraded for greater reliability, increased capacity or compliance with current codes.
In recent years, there has been an increase in the availability of switchgear that uses alternatives to the standard air-insulated design. These new technologies reduce the required spacing between energized parts and decrease the size of equipment enclosures. Some of this technology has long been used in the domestic utility industry and is now being applied to products for commercial and industrial application. Some has been adapted from European switchgear designs, which have traditionally been more compact than those in the United States. This selection of available compact switchgear products allows the electrical engineer new flexibility in electrical room design and meeting tight space constraints.
SF6 Insulating Gas
Perhaps the most significant transfer of technology from the utility market to commercial application has been the use of sulfur hexafluoride (SF 6 ) gas to replace air as the primary insulating medium in switchgear. SF 6 is a non-toxic, non-flammable and non-reactive gas with excellent electric insulation and arc-quenching properties. Its insulation strength allows reduced spacing between energized parts, and its arc-quenching ability permits load-break switching with very short contact separation distances. These designs allow switchgears to fit more voltage and current capacity into a smaller space.
In most applications, the equipment operates with an internal SF 6 pressure higher than atmospheric to attain the required electrical ratings. This also assures that a small leak will not result in contamination of the interior of the tank. Pressure switches or density sensors can be used to detect any loss of gas and initiate a remote alarm before the internal pressure drops to an unacceptable level. Conveniently sized cylinders of gas and regulators are available as maintenance accessories to permit recharging if needed. Although SF 6 has been identified as a greenhouse gas, I am not aware of any current regulations affecting its use in electrical equipment. Because the quantities involved are small and sealed within the tank, environmental impact should be minimal compared to other uses, such as in manufacturing processes.
In dense downtown areas where overhead distribution lines are not practical, utilities have long used underground distribution feeders, switching equipment and transformers. These vaults are usually open to the elements through ventilating grills and subject to dampness, and even flooding, during heavy rains. This has led to the development of switchgear and transformers of compact design that can continue to operate under adverse environmental conditions, including complete submersion. All of the energized components are contained within a welded steel tank, with cable connections made in such a way that they are also sealed from the environment. The primary insulation in this equipment has traditionally been mineral oil, which possesses excellent insulation and arc-quenching characteristics, but is limited in indoor applications because of flammability.
Conversion of existing oil-insulated designs to SF 6 gas has brought the advantages of vault-style switchgear inside, where it has been found to be an excellent solution to many space-constrained retrofit projects. This is probably the most compact load-interrupter switchgear design available and has the additional advantage of being fully sealed and submersible, making it immune to water leaks and other environmental impacts that are responsible for many switchgear failures. This type of switchgear is available in switch-only, switch-and-fuse or switch-and-vacuum interrupter configurations. Figure 2 (p.22) shows an installation of a six-switch sectionalizing line-up that replaced standard air interrupter switches as part of a generator system upgrade. This installation occupies one-fourth the floor space of the equipment it replaced, which allowed acceptable code working clearances to be maintained in this tight electrical room.
Use of vault-style switchgear requires modifying some operating and maintenance procedures to accommodate differences from traditional equipment. This switchgear is designed to use fully shielded, dead-front connectors such as elbow terminators, universal bushings or taped-and-shielded terminations. Due to the close spacing of cable terminals, standard air termination kits cannot be used. Because the connections are fully insulated and shielded, and the switches and bus are sealed within the tank, circuits are not readily available for voltage testing or application of protective grounds (see "If it Ain't Grounded, It Ain't Dead). These issues can be addressed by specifying the switchgear with test points and integral grounding switches, or by identifying other locations in the distribution system where these functions can be performed.
When electrical insulation between energized conductors breaks down and flashes over, it usually occurs at a point where the electric field stress is concentrated by the geometry of the conductors. By careful design of the physical shape of energized parts, the electric field between them can be controlled and their spacing reduced. One manufacturer has combined this concept with the benefits of SF 6 gas in a hybrid switchgear design. The switching takes place in a sealed tank, but cable connections and fuses are air-insulated in a compartment whose size has been reduced through field shaping. This allows traditional cable termination kits to be used, but special lugs and fuses unique to this switchgear are required to maintain control of the electric field.
In a recent project, multiple lineups of this switchgear were used to provide isolation of substation transformers from primary feeders within a high-rise building. This provided a floor space saving of approximately 40% over traditional switchgear. We have also used this type of equipment successfully as fused primary disconnect switches on unit substations.
Circuit Breaker Switchgear
Although the discussion to this point has focused on metal-enclosed, load-interrupter switchgear, metal-clad, circuit-breaker switchgear are also now available in more compact designs. This type of equipment is often applied at the main service point to a campus, in places where current ratings exceed the capability of fuses, or in places where the greater flexibility provided by protective relays is required. Single-high, air-insulated, vacuum-interrupter switchgear that use narrower sections than traditional designs and require front access only is now available from multiple manufacturers. While originating with International Electotechnical Commission (IEC) ratings for the European market, this equipment can now be provided in the United States with American National Standards Institute (ANSI) ratings and Underwriters Laboratories (UL) listing.
Another significant advance in switchgear technology is enclosure construction that reduces personnel exposure to the effects of internal arcing faults. Rapid expansion of air and vaporization of metal during high current faults creates pressure that may result in failure of traditional switchgear enclosures, exposing operators to flash and blast hazards. Arc-resistant switch-gear uses strengthened enclosure construction, improved latching mechanisms and automatic pressure relief devices. It contains arc energy and safely vents it away from the operator through the top of the enclosure. Some manufacturers also offer high-speed optical detection and switching systems that sense and shunt internal arcing before dangerous energy levels are reached.
Finally, new switchgear designs take advantage of microprocessor-based meters and protective relays, saving panel space by combining the functions of multiple discrete devices into a single package, while providing functionality far beyond that of older technologies. Multi-function electronic meters combine the ability to measure and record all of the standard power quantities, such as amps, volts, watts and VARs, with power quality functions that include harmonic analysis, event reports and waveform capture. Multi-function protective relays can combine the numerous relay functions required for complex generator or utility intertie protection into a single space- and cost-saving package while providing improved accuracy, self-diagnostic and fault location features.
In many cases, campus distribution systems contain vaults and electrical rooms in which maintenance access and working clearances are less than desirable and may not comply with current codes and OSHA regulations. These installations are usually considered to be "grandfathered-in" by code officials, and maintenance staff members have no choice but to live with them until equipment failure or system expansion provide a driver for an upgrade. Older systems are often 5-kV class, and as loads grow, it may be desirable to upgrade the system to a higher voltage to accommodate the load growth without increasing the size and number of feeders. We have worked on many systems in which all 5-kV cable failures are replaced with 15-kV cable to be ready for future voltage conversion.
Figure 6 shows the footprint and working clearance required by a 15-kV, 750-kVA duplex primary switch and dry-type transformer combination super-imposed on that of the 5-kV equipment it replaces. Clearly, if space is tight to begin with, it is unlikely that traditional 15-kV equipment, with its larger physical size and greater working clearances, can ever be installed in the existing spaces. Use of compact SF 6 -insulated primary switches with the reduced footprint shown makes it far likelier that this installation can be upgraded to 15 kV in the existing space.
A New Generation
A new generation of switchgear and protection technology is making it possible to fit more equipment in less space. This equipment provides opportunities to design safer, more reliable and more space-efficient distribution systems. It is particularly well suited for replacement of aging equipment in tight spaces and accommodating voltage upgrades. Expect to see more compact switchgear products enter the domestic market as additional manufacturers integrate their European and North American product lines.
IF IT AIN'T GROUNDED, IT AIN'T DEAD!
Safe electrical work practices for voltages over 600 volts require that circuits and equipment be verified to be de-energized and then grounded before work is begun. The protective grounding device must be capable not only of bleeding off any induced voltage or capacitive charge, but also of safely conducting the maximum current that could flow if the circuit were accidentally re-energized. It must be able to do this until an upstream circuit breaker or fuse opens.
With traditional air-insulated switchgear, this is accomplished by an electrician first using a hotstick voltmeter to verify the absence of voltage and then using large clamp-type connectors to attach temporary grounding cables directly to the bus or to attachment points known as ground bails. The rules say that until the grounds have been applied, the circuit must be treated as energized. The connections must be made using insulated hot-sticks or insulating gloves, sleeves and flash suits. This practice has led to the old adage "if it ain't grounded, it ain't dead," that every wise high-voltage electrician respects.
The fact that some of the newer switchgear designs restrict access to the bus and cable terminations does not relax this requirement to test and ground before working on medium-voltage circuits or equipment. One solution is to specify integral grounding switches, which have long been a standard feature of European switchgear designs, but have seen little use in the United States, outside of electric utilities.
Grounding switches must have the same short-circuit withstand capability as the switchgear, and should be interlocked with the associated power switch or circuit breaker to prevent closing the grounding switch until the circuit is open. Capacitively coupled voltage test points or indicator lights can provide verification that the circuit is de-energized prior to grounding without exposing the electrician to energized parts. In some circuit breaker switchgear, voltage-sensing interlocks are provided that will block closing of the grounding switch if the circuit is being backed from the load side of the power device.