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WEEK 07: TRANSMISSION: COMPONENT SPECIFICATIONS Sections: Transformers | Circuit Breaker | Switchgear | Switchers | Fuses Switchgear Assemblies cover a wide range of low-voltage and high-voltage structures that are generally factory-assembled and are divided into three main groups (1) metal-enclosed power switchgear, (2) metal-enclosed bus and (3) switchboards.
Isolated-Phase Metal-Enclosed Bus is a type of design in which each phase is enclosed in an individual metal housing, and an air space is provided between housings, It is considered to be the safest, most practical, and most economical way of preventing phase-to-phase short circuits by means of construction methods. The bus may be self-cooled or forced-cooled by circulating air or liquid. Switchboards are classified according to service, framework, panel material and arrangement
Switchboards Classification [Panel Material] The materials generally used for switchboard panels are slate and asbestos ebony. Slates, usually natural black slate or black marine-finished slate, are commonly used. Natural black slate is more expensive but has better electrical properties. Black marine finished slate consists of cheaper grades of slate rubbed with oil to give a dull velvety black finish. Such panel does not show oil stains and hence is adaptable to boards that have oil circuit breakers mounted directly on the rear of the panels. Slate boards are not desirable for voltages over 600 to 1,200 volts. Asbestos Ebony, manufacture by Johns-Manville, Inc, is available in the form of sheets or simple molded forms made up of asbestos fiber thoroughly impregnated with binding cements under heat and pressure into a compound of very high dielectric strength. Specifications are high dielectric strength, good insulation, ability to withstand shocks and vibration, uniform density, and light weight. It is unaffected by rapid temperature changes; does not shrink, crack or buckle, is readily cut and drilled; and is not generally affected by chemical. Steel used when there is an absence of live parts, such as knife switches. Some of the outstanding features of steel panels are: lower cost in manufacture, shipment, erection and maintenance; about five times as light in weight; simplicity in erection, as the panel forms its own framework; easily maintained and unbreakable. When a knife switch or other live part are to be installed, the switch parts are mounted on an insulated base located behind the steel panel and actuated from the front by means of an insulated operating handle. Switchboards Classification [Arrangement] There are a large number of possible ways in which switchboards are arranged, but generally they fall under two types. Vertical panel where all essential meters are placed on the top, all relays and nonessential meters at the bottom, and the control equipment halfway between the top and bottom of the board. Control Desk used when there are only a few instruments which are of the flush type. Sometimes a vertical panel is combined with a control desk, where all the control equipment are placed on the desk and all essential meters are on the vertical panel.
Switching Equipment is the entire system which control and distribute power in a plant. Selection of such equipment is governed by (1) Maximum safety to life and property; (2) Continuity of service; (3) First cost; (4) Available space; (5) Desired operating features; (6) Voltage of plant and system; and (7) Capacity of plant and system. Types of Switching Equipment The heart of control system of a plant is the switchboard; hence it is expected that the type of switching equipment is determined by the class of switchboard used. The classification below is the same as that for switchboards B. Manual Remote - the oil circuit breakers and busbar structures are mounted at some distance from the switchboards, the circuit breakers being operated by hand through connecting rods, chains, bell cranks, etc. This type should be confined to switchboards with circuit breakers of 2,000-ampere capacity or less and of 50,000 volts or less, indoor service, and to stations whose capacity does not exceed 25,000 kVA, three-phase. In general, remote manual control is not recommended for outdoor stations. C. Electrical Remote - uses electrically operated equipment located apart from the switchboard and operated by means of control switches mounted on the panels. Usually this type cost 25 per cent more than the previous two. There is practically no limitation to the application of this type, if the necessary control source is available. |
Metal-enclosed station-type switchgear is used in industrial, commercial, and utility installations, generally for voltages of 14.4 to 69kV, and ratings of the heavy-duty range up to 5000 A continuous current.
