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WEEK 13: DISTRIBUTION: SUBSTATION: COMPONENTS Distribution: Substation Design: Switching Schemes Sections: Functions | Types | Switching Schemes | Components 1. Switching Equipment and Devices include circuit breakers, disconnecting switches, and grounding switches. 2. Transformers include metering transformers [current transformers, potential transformers] or capacitor voltage transformers, and power transformers. 3. Line Traps are discussed fully on Plant Operation and Maintenance. They are used to make the transmission line appear as a simple two-terminal line used in telephone circuits. 4. Capacitors and Reactors include coupling capacitors, series and shunt capacitors; shunt and current-limiting reactors. 5. Station Buses and Insulators carry high amounts of energy in a confined space. Design factors involve utilization of electric energy adequately and economically and provide sufficient structural strength to withstand the maximum stresses that maybe imposed on the conductors. Two designs are: the (1) strain-bus design, used primarily on high-voltage, is similar to a transmission line and consists of a conductor usually ACSR, copper, or high-strength aluminum strung between substation structures; and (2) rigid-bus design used for extra-high-voltages. The design of station buses depends on the following: (a) Current-carrying capacity, (b) Short-circuit stresses, and (c) minimum electrical clearances. The current-carrying capacity of a bus is limited by the heating effects produced by the current. The permissible temperature rise for plain copper and aluminum buses is limited to 30 degrees Centigrade above an ambient temperature of 40. Many factors enter into the heating of a bus, such as the type of material used, the size and shape of the conductor, the surface are of the conductor and its condition, skin effect, proximity effect, conductor clearance, ventilation, and inductive heating caused by the proximity of magnetic materials. Bus Materials, aside from the two previously mentioned, with hard-drawn aluminum in tubular shape are most widely used in HV and EHV open-type outdoor stations. Skin Effect in a conductor carrying an alternating current is the tendency toward crowding if the current into the outer layer, or "skin" of the conductor due to the self-inductance of the conductor increasing the effective resistance and lower current rating for a given temperature rise. Proximity effect is the distortion of the current distribution caused by the induction between the out and return conductors causing a concentration of current in the parts of the buses nearest each other, thus increasing their effective resistance. Thermal expansion and contraction of bus conductors is limited through the use of expansion joints and clamps permitting the tubing to slide. Bus Spacing is standardized for respective BILs. Clearances should be adequate for both line-to-ground and phase-to-phase values. Coordinate kV class and BIL when choosing minimum values but maybe decrease in line with good practice, depending on local conditions, procedures. etc. 6. Lightning Arresters and/or Gaps are discussed fully on Plant Operation and Maintenance: Protection Systems. 7. Grounding Systems provides (a) the ground connection for the grounded neutral for transformers, reactors and capacitors, (b) the discharge paths for lightning rods, arresters, gaps and similar devices, (c) safety to operating personnel by limiting potential differences, (d) means of discharging and deenergizing equipment for maintenance practices, and (e) a sufficiently low resistance path to ground to minimize rise in ground potential with respect to remote ground. The basic substation ground system takes the form of a grid of horizontally buried conductors, usually AWG 4/0 bare stranded copper cable buried 12 to 18 in below grade and spaced in a grid pattern of about 10 to 20 feet. Methods of testing ground resistance fall into three general groups; (a) Triangulation or three-point methods, in which two auxiliary test grounds and the point to be measured . 8. Support Structures are usually made of steel, aluminum, or wood, and concrete foundations. The typical open-type substation requires strain structures to support the transmission line conductors and other substation components. When the structures are made of steel or aluminum they require concrete foundation, however, when they are made of wood, concrete foundations are not required. Fabricated aluminum or steel equipment support structures may consist of single wide-flange or tubular-type columns. Rigid-frame structures are composed of wide flanges or tubular sections. Lattice Structures are made of angle members. The design of supporting structures is affected by phase spacings and ground clearances required, by the type of insulators, by the length and weight of buses and other equipment, and by the wind and ice loading. Other structural and concrete work required includes site selection and preparation, roads, control houses, manholes, conduits, ducts, drainage facilities, catch basin and fences. Site Selection includes the study of the topography and drainage patterns of the area together with a subsurface soil investigation. |
