The CGIT Bus System Consists of Four Key Elements:

Bus Assemblies
The basic CGIT bus system consists of three parallel isolated phases of coaxial transmission line. Each phase consists of a grounded aluminum alloy tube, which encloses the concentric tubular aluminum alloy conductor. The inner conductor is supported within the outer enclosure by solid dielectric insulators. The interior of the bus is filled with SF6 to provide electrical insulation between the inner conductor and the outer enclosure.

The various components of the bus assembly including straight sections, elbows, tees and crosses are factory assembled in shipping units up to 60 feet (18 meters) long. The sections are shipped with dry nitrogen at 5psig (35kPag) to keep the interior of the bus clean and dry.

In the field, center conductors are joined together using plug-in contacts. The outer enclosure is either welded or bolted together using flanges with double o-ring seals. The joints are then leak checked after assembly. In the case of underground joints, a coating of corrosion protection material is applied. When an electrical proof test and hookup of accessory systems is complete, the bus is ready for final commissioning.

A section of bus may be in straight, elbow, tee or factory-joined combination configurations. Bus sections are constructed with one fixed insulator, which anchors the conductor to the enclosure. On long sections, one or more moving insulators may be included to support the conductor. These insulators are attached to the conductor and are allowed to move within the enclosure to compensate for thermal expansion.

The fixed insulator may be either Tripost or conical. The conical insulator is used to separate the system into gas compartments. The fixed insulator is located on one end of a straight bus section. To insure that the conductor will remain centered, the fixed insulator is always located near elbow or tee elements.

The moving insulator is always a Tripost type. Between sections there is a set of conductor contacts which compensate for differential thermal expansion between enclosure and conductor. HM type finger contacts are used throughout the system to provide a low-resistance and compact current transfer path.

Support Insulator
The conductor is centered and supported in the enclosure by a Tripost epoxy insulator. The insulator is cast directly to an aluminum sleeve which is then fixed onto longer lengths of conductor as required. The Tripost insulator is shown in the figure below.

Conical Insulator
A conical insulator, as shown below, is used when it is necessary to have a gas or contamination barrier in the bus.

Particle Traps
Both the Tripost and conical system include a Tri-Trap particle trap. Basically, the particle trap provides a region with a very low field between itself and the enclosure. The simple but effective design consists of an aluminum shield with slots that is electrically connected to the enclosure. Any conducting particle is moved by the electrical field through the slots into the very low field region. Here the particle is effectively trapped as the field is so low that the particle will not be elevated or moved. During the high-voltage field acceptance test, the voltage is raised in steps specifically designed to move any contamination into the particle traps, thus insuring a completely reliable system. The trap is also installed at low points in the system to trap particles moving under the influence of gravity.