Standards & Applications Of Medium Voltage Bus Duct

The bus duct is employed for the effective and efficient supply of electricity in most industrial locations. Copper or aluminum is employed for the conductor of the bus duct that is insulated and enclosed completely for cover against mechanical damage and mud accumulation. A bus duct system is an efficient method of distributing power to your switchgear and various loads. However, bus duct problems can cause catastrophic damage and extensive downtime.

There are basically three general categories of bus duct. The nonsegregated phase bus implies a duct where all phase conductors are during a common metal enclosure without barriers between phases. This definition applies to rigid and versatile conductors like the bar or cable duct.

Segregated phase buses may be a duct where phase conductors are during a common metal enclosure but are segregated by ground metal barriers between each phase conductor. Isolated phase duct is when each phase conductor is enclosed in its own metal housing and adjacent conductor housings are separated by an air space. This duct usually has higher amp ratings than other types. The housing and conductors are tubular in shape and are welded at the site together. This duct is force-air cooled above a 25000A continuous rating.

Applicable standards

ANSI C37.23 standard covers the planning, manufacture, testing, and ratings of the all-metal enclosed bus duct, including cable duct systems. When a part of the equipment, it contains the associated equipment like interconnections, enclosures, switches, supporting structures, and disconnecting links. This includes disconnects primarily used with isolated phase duct but is often used with all other sorts of duct, including cable duct.

Outside the scope of this standard is duct rated to 600V or open type conductor assemblies. The CSA C22.2 No. 201 standard is that the Canadian measure for metal-enclosed duct for voltages rated from 751V to 46kV, AC, or DC. This standard covers the planning, manufacture, testing, and ratings of all kinds of metal-enclosed bus duct including cable duct systems. It includes all accessories related to all kinds of duct, as does ANSU. For isolated phase duct, it's limited to amp ratings up to 5000A continuous.

Typical medium voltage Bus Duct

Application

All three duct systems are literally identical on the application and have an equivalent basic rating as previously listed. Price usually determines which application is most appropriate that sort of duct. One shouldn't consider the duct cost alone but include the value of installation, and thus the entire cost of the installed duct system.

In general, segregated bus ducts are often considered slightly more reliable than non-segregated duct supported the very fact that a-bus-to-ground fault could also be contained within the separate, grounded metal- barrier compartments. this is often particularly true for initially low-level ground faults that exist on an ungrounded, or high-resistance grounded, system. Rigid conductor bus duct is essentially factory manufactured and requires bolting sections together within the field and installing insulated conductor joint boots.

Accessories are generally factory assembled, including equipment like switchgear and transformers. On the opposite hand, the cable duct is generally supplied to the installing contractor on nonreturnable reels for field installation and termination.

Standard switchgear or transformer connections are direct via the cable with suitable stress kits or terminating kits, counting on the termination requirement. Special terminating boxes with rigid copper conductor connections are often supplied to the cable from which the duct is connected. With rigid ducts, tee arrangements are quite practical.

It also can be accomplished with cable duct but with far more difficulty. The tee arrangement is manufactured of rigid conductors in an enclosure during which the cable duct system is connected to all or any sides. this needs stress cone kits or cable terminating kits at each cable termination. Such an appointment requires considerable space and is expensive.

Hanger supports, wall supports, and freestanding floor supports are basically similar for all kinds of the duct. Isolated phase duct, which becomes a requirement for generator feeds thanks to the traditional continuous current rating and minimum BIL level of 110kV, is manufactured from aluminum tube type housings and conductors. The conductors are supported on porcelain, post-type insulators, and therefore the duct system is pre-manufactured in specified lengths counting on the duct system’s physical arrangement. The installing contractor welds the conductors and housing within the field.

Isolated phase ducts are used for top ampacity requirements and may be supplied up to 25000A continuous rating without forced-air cooling. Beyond this rating, forced-air cooling is required with all the associated equipment. All accessories like elbows, tees, the isolating switch, and termination to the equipment it feeds are specifically designed for the appliance and usually come factory assembled, apart from termination.

Isolated phase duct is sort of special compared to the opposite duct systems described and requires considerably more room. it's also significantly costlier to get and install, but deemed a singular system for specific applications and, especially, as a generator duct feed. generally, rigid conductor type bus duct length is measured from centerline to centerline, and therefore the equipment to which it's attached, to determine overall duct-length footage. Bar extensions within a termination cubicle are considered a part of the termination equipment. For cable duct, an equivalent criterion is employed to determine the length.

The only exception is that 60 inches of duct length must be added to determine overall length dimensions, thanks to the cable extension leads required at each end. Such cable lead extensions are considered a part of the duct length, instead of a part of the termination equipment.

Ratings

Non-segregated duct, which incorporates cable duct, basically has an equivalent continuous current rating, from 600A to 6000A. The duct of upper ratings is supplied up to 12000A, but this is often for special applications requiring special design, manufacturing, and installation considerations. Ampacities are normally supported 40°C ambient where conductors have silver-plated joints, and therefore the allowable temperature rise of the conductor is 65°C. Noncurrent carrying parts that will be readily touched are allowed a 40°C rise. Ambient temperature range limits are -30°C to +40°C, and altitude is up to 1000 meters (3300 feet). Other ambient conditions need special consideration and should contribute to derating.

Voltage range for these sorts of duct are basically from above 600V to 34.5kV to ANSI but to 46kV to CSA. In practical terms, the utmost rating is 38kV with a BIL rating of 150kV. This includes cable duct, albeit cable at nominally 38kV can have a better BIL rating and may certainly have a better insulating voltage class like up to 33 percent insulation on higher voltage ratings. To ANSI standards, corona extinction levels aren't specified, but they're to CSA standards and are considered a production type test.

Momentary ratings aren't laid out in the standards but the duct is predicted to be suitable and match the momentary rating of the equipment to which it's connected. generally terms, the momentary ratings offered are 82 kA, 100kA, or 150kA for 600V ratings; 60kA, 80kA, and 100kA for five and 15kV ratings; 6kA and 80kA for 25 and 38kV ratings. The DC bus duct nominal voltage rating is 300 to 3200 volt DC with continuous ratings up to 6000A. Isolated phase duct continuous ratings go from 1200 to 24000A (self-cooled) although tests are conducted to 25000A for self-cooled current ratings.

Voltage ratings are 24.5, 29.5, and 34.5kV with corresponding BIL ratings of 110, 125, and 150kV for direct generator connections. aside from generator connection, voltage levels are 15.5, 25.8, and 38kV with corresponding BIL ratings of 110, 125, and 150kV.