АвтомобилиАстрономияБиологияГеографияДом и садДругие языкиДругоеИнформатикаИсторияКультураЛитератураЛогикаМатематикаМедицинаМеталлургияМеханикаОбразованиеОхрана трудаПедагогикаПолитикаПравоПсихологияРелигияРиторикаСоциологияСпортСтроительствоТехнологияТуризмФизикаФилософияФинансыХимияЧерчениеЭкологияЭкономикаЭлектроника
Read the text. Should the voltage on a circuit fall below a specified level for some reason, a device called a capacitor can momentarily maintain the voltage at line value
Should the voltage on a circuit fall below a specified level for some reason, a device called a capacitor can momentarily maintain the voltage at line value. Basically, a capacitor serves the same purpose as a storage tank in a water system. By maintaining the water in a storage tank at a definite level, the pressure on the water supplied by the system connected to it is maintained evenly.
It is the job of capacitors to keep the power factor as close to 1 as possible. The power factor is an important essential of electricity keeping the power factor close to 1 is a considerable economic advantage to the utility company and to the consumer. Inductance is the element in the circuit which is pulling the power factor below 1. Capacitance is the enemy of inductance. Therefore, capacitors counteract inductance, keep the power factor close to 1, and save money for the utility company. The capacitor usually consists of two conductors separated by an insulating substance. Among other materials which may be used, a capacitor can be made of aluminum foil separated by oil-impregnated paper, or synthetic insulating materials.
Capacitance is the property of a capacitor. Capacitance depends on the area of the conductors, on the distance between the conductors and on the type of insulating material used.
Introducing capacitors into a circuit causes the current to lead the voltage in phase. Introducing inductance (or an inductor) into a circuit causes the current to lag the voltage in phase. The capacitor counteracts this loss of power and makes power production more economical.
Capacitors are mounted on cross arms or platforms and are protected with lightning arresters and cutouts, the same as transformers vacuum or gas (SF6) switches. As their names imply, air switches are those whose contacts are opened in air, while the other type switches are those whose contacts are opened in oil, vacuum, or gas. Oil switches are usually necessary only in very high-voltage, high-current circuits.
Air switches are further classified as air-break switches and disconnect switches.
The air-break switch has both the blade and the contact equipped with arcing horns. These are pieces of metal between which the arc resulting from opening a circuit carrying current is allowed to form. As the switch opens, these horns are spread farther and farther apart and the arc is lengthened until it finally breaks.
Air-break switches are of many designs.
A modern development of the air-break switch is the load-break switch which breaks the arc inside a fiber tube. As in the expulsion lightning arrester, the fiber tube produces a gas which helps to confine the arc and blow it out. There is a possibility that the unconfined are associated with the horn-type switch might communicate itself to adjacent conductors or structures causing damage and possible injury. But the load-break switch eliminates this potential hazard.
The disconnect switch is not equipped with arc quenching devices and, therefore, should not be used to open circuits carrying current. This disconnect switch isolates one portion of the circuit from another and is not intended to be opened while current is flowing. Air-break switches may be opened under load, but disconnect switches must not be opened until the circuit is interrupted by some other means.
Oil Switches. The oil switch has both the blade and the contact mounted in a tank filled with oil. The switch is usually operated from a handle on the outside of the case. As the switch opens, the arc formed between the blade and contact is quenched by the oil.
Oil switches may be remote controlled as well as manually operated. They are used for capacitor switching, street lighting control and automatic disconnect in case of power failure.
Recloser. A recloser consists essentially of an oil switch or breaker actuated by relays which cause it to open when predetermined current-values flow through it. Reclosers are usually connected to protect portions of primary circuits and may take the place of line fuses. The switch or breaker is arranged to reclose after a short interval of time and re-open again should the fault or overload which caused the excess current-flow persist. In a recloser, there is an operating rod actuated by a solenoid plunger which opens and closes the contacts, whereas the repeater fuse works only when the metal has been melted by overheat.
However, should it be desirable to delay the action of the recloser, it can be done by an ingenious timing device.
All of the materials and equipment, assembled properly, make up the primary circuits that serve a particular area. The operation of the circuits include such things as the switching of circuits or portions of circuits (termed sectionalizing) for transferring loads or emergency restoration of service, for controlling voltage regulators, for switching capacitors and other manipulations of lines and equipment-operations usually performed manually under centralized control. Where circumstances generally involving the importance of service continuity, and where justified economically permit, these operations may be accomplished automatically. The relays that actuate these operations may be controlled by preprogrammed computers that impart “instructions” via radio, by telephone lines, or by independent circuits established for that purpose.
Дата добавления: 2014-11-13; просмотров: 13; Нарушение авторских прав