 Conductor Material Copper andn Aluminum are the standard materials used for bus conductors. Aluminum has a conductivity approximately 60 percent that of copper. The lower conductivity favors aluminum, since the required larger dimensions give a higher section modulus and a slightly lower skin effect. Using aluminum conductors results in saving approximately one third of the weight of the conductor material used for the same current-carrying capacity. This offers savings in raw materials and in lighter field and factory handling. These factors result in a lower selling price of bus, using aluminum conductors plus purchaser savings in field handling costs. Surge Protection When bus structures are subjected to voltage surges, it is sometimes recommended that surge-protective equipment be used. The choice of a lightning arrester is based on the impulse strength of the equipment to be protected, and the maximum system voltage that can occur from line-to-ground at the arrester location. Capacitors are sometimes used to decrease steepness of the voltage wave front, in order not to exceed the maximum allowable voltage stress on the insulation of the equipment that is protected. Forced-Air Cooled Bus On a high current rated bus, consideration should be given to forced-air cooling. On a forced-air cooling bus, a standard-design, self-cooled conductor approximately one-half the forced-cooled rating is used. (This presents savings both in space and the purchase price of the bus). The resultant heat generated by increased losses is carried away by cooling air in the closed system of an air-to-water heat exchanger. A self cooled bus to the highest rating is now often the preferred one because of its high cost of losses. An economic study should be made to compare the increase in losses, versus the purchase price savings of using a smaller conductor. The value of losses is generally capitalized over the expected life of the equipment. The loss evaluation is usually stated in a flat rate capitalized value in dollars per kW of losses with the actual evaluation depending on the predicted efficiency of the station. Giving this flat-rate capitalized value to the bus manufacturer permits him to select an economically optimum conductor size. The power requirement of the blower fan should also be considered in system losses. A source of cooling water is required for the heat exchanger. Station condensate water has been applied to this purpose. Consideration should be given to operation desired in the event of loss of cooling air. Specifically, how much load must the bus carry, and for how long under these conditions? Without forced cooling, the bus is subject to overload based on its self-cooled rating. The temperature of the bus rises exponentially with time to an ultimate value where annealing begins. The time that a bus can be operated at full load without cooling air and without damaging effects can be predicted by the bus manufacturer. Automatic devices should be included to give alarm signals in case of inadequate functioning of any part of the system. Consideration should be given to purchasing a spare fan motor as a standby. Conductor thermometers that are visible through viewing windows can be provided to check the temperature rise under actual operating conditions. The headers, which transmit the cooling air from one phase to another, include air deflectors to reduce turbulence and air friction. Grounded deionizing barriers are built into the headers to prevent air transfer from phase to phase. Enclosure Design - Insulated or Bonded? There are three housing designs available. The conductor current that induces housing voltages in turn causes currents to flow in the housings. The current flow paths depend on the availability of closed circuits. The predominating circulating pattern is determined by housing interconnections. On one arrangement, the housing sections are grounded at one end, and insulated at the opposite end. This arrangement has the purpose of preventing the circulation of currents between housings. This is called the "Discontinuous" or "Insulated-Housing Arrangement." Gasketing is used to insulate the housings. Another arrangement consists of electronically bonding the housing together. This arrangement has the purpose of permitting the free circulation of currents between housings. This is called the "Continuous" or "Bonded-Housing Arrangement." Bolted or welded cover seams are used to bond the housings. The third design known as the "Discontinuous Bonded" arrangement combines the advantages of the first two. Grounding of Isolated Phase Bus A continuous ground conductor should be provided in parallel with the isolated-phase bus to ensure that all enclosures are grounded. The ground bus may take the form of a separate conductor or it may be the enclosure itself, or it may be the supporting structure, if a continuous electrical path can be provided of the same momentary rating as the main-bus conductors. The ground bus shall be capable of carrying the rated momentary current of the bus for a period of 2 seconds. The ground conductor is preferably connected to the station ground at one point only, although more connection points are satisfactory if induced-current loops have been avoided. Induced voltages across insulated joints in the housing should be kept as low as possible and preferably below 2 volts during rated current operation. Ventilation Self-cooled metal-enclosed bus that have sections located both inside and outside a building, (so that ambient temperatures may be appreciably different) should include sealing means or baffles to prevent an interchange of air between the sections. Further more, widely fluctuating ambient air temperatures for any one section of bus necessitates consideration of the resulting enclosure air temperature, condensation, and pressure change. Some form of ventilation with filtered breathers or means of air circulation should be utilized and provided. Filtered drains should be provided at the low point in vertical sections to prevent accumulation of condensation. Terminations Bus conductors are electrically attached to the studs of generators or transformer bushings, or switch terminal pads, usually with flexible braid or multiple laminations depending on the degree of flexibility required. Similar flexible connections may be required in long straight runs of the bus to provide for temperature expansion or contraction. The terminating equipment at the generator may require hydrogen seal-off bushings or baffles. Under these conditions, provisions for venting hydrogen leakage to the atmosphere are included. Using Switches or Links for Station-service Transformers A generator bus frequently has tap connections to station-service transformers through group-operated disconnected switches or disconnecting links. Switches should not be operated to close an energized bus. Links should be easily removable, but only when the bus is de-energized. Both switches and links shall be capable of carrying the rated momentary current in that section of bus. Disconnect links can be supplied in the main leads but as the current ratings get higher, the task of removing links gets more difficult. Telescopic disconnect switches are recommended for this application. Installation: Isolated Phase Bus and Economical to Erect Isolated Phase Bus Assembled As Required Factory assembled units may accommodate a combination of installation methods depending upon the conditions at the erection screen. The basic simplicity of the design contributes to the economy through successive stages of erection. All pieces are precision manufactured and marked for an easy identification. Markings correspond with those on assembly drawings. Small or Large Assemblies When the bus or portions of it must pass through areas that are crowded or too small for erection of pre assembled 3-phase units, small knocked down assemblies are supplied. When specified and when conditions permit, complete 3-phase sections are furnished, including bus supports, covers, conductors, ground bus, and supporting members. |
