DIRECT DOWNLOAD! Pdf of switchgear and protection To study different switchgear equipments. 1 used in electricai power system I switchgear and protection. SWITCHGEAR AND PROTECTIVE DEVICES. 6 | Page. Fundamentals of Power System Protection. The purpose of an Electric Power System is to. IEEMA- Schneider Electric. MV Network Switchgear,. Protection and Control. Ravinder Negi – Manager, Services Execution – INDIA. Schneider Electric, New .
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Switchgear & bestthing.info - Download as PDF File .pdf), Text File .txt) or read online. PDF | On Apr 28, , Dr.V Balaji and others published Introduction to Protection and switchgear. Switchgear/PCC Fundamentals. Niel Miele, PE, LEED AP. PACS Industries System Monitoring. • Circuit Protection/Logic. • Feedback. • Metering.
Errors in the Instrument Transformer - Ratio Error Reduction of Ratio and Phase Angle Errors Advantages and Disadvantages of Instrument Transformers Introduction to Electromagnetic Relays Electromagnetic Attraction Relays - Solenoid and plunger type relay Operating Principle of Electromagnetic Attraction Relays Advantages, disadvantages and application of Electromagnetic Relays Directional Power Relay Directional Induction Type Overcurrent Relay Introduction to Differential Relays Current differential relay Voltage Balance Differential Relay For higher voltages over about 66 kV , switchgear is typically mounted outdoors and insulated by air, although this requires a large amount of space.
Gas-insulated switchgear saves space compared with air-insulated equipment, although the equipment cost is higher. Oil insulated switchgear presents an oil spill hazard.
Switches may be manually operated or have motor drives to allow for remote control. Circuit breaker types[ edit ] A switchgear may be a simple open-air isolator switch or it may be insulated by some other substance. An effective although more costly form of switchgear is the gas-insulated switchgear GIS , where the conductors and contacts are insulated by pressurized sulfur hexafluoride gas SF6.
Switchgear & Protection.pdf
Other common types are oil or vacuum insulated switchgear. The combination of equipment within the switchgear enclosure allows them to interrupt fault currents of thousands of amps. A circuit breaker within a switchgear enclosure is the primary component that interrupts fault currents. The quenching of the arc when the circuit breaker pulls apart the contacts disconnects the circuit requires careful design.
It consists of two contacts a fixed contact and a moving contact. A handle is attached at the end of the moving contact. It can be operated manually or automatically.
The automatic operation needs a separate mechanism which consists of a trip coil. The trip coil is energized by secondary of current transformer. The terminals of the circuit breaker are bought to the supply. Basic action of circuit breaker Under normal working conditions the e.
Thus the contacts remain in closed position carrying the normal working current. The contacts can be opened manually also by the handle. Under abnormal or faulty conditions high current in the primary winding of the current transformer induces sufficiently high e.
This will start opening motion of the contacts. This action will not be instantaneous as there is always a time lag between the energization of the trip circuit and the actual opening of the contacts.
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The contacts are moved towards right away from fixed contact. As we have seen already the separation of contacts will not lead to breaking or interruption of circuit as an arc is struck between the contacts.
The production of arc delays the current interruption and in addition to this it produces large amount of heat which may damage the system or the breaker. Thus it becomes necessary to extinguish the arc as early as possible in minimum time, so that heat produced will lie within the allowable limit.
This will also ensure that the mechanical stresses produced on the parts of circuit breaker are less the time interval which is passed in between the enervation of the trip coil to the instant of contact separation is called the opening time.
It is dependent on fault current level. The time interval from the contact separation to the extinction of arc is called arcing time It depends not only on fault current but also on availability of voltage for maintenance of arc and mechanism used for extinction of arc.
Phenomena of arc, properties of arc, initiation and maintenance of arc Formation of an Arc: Under faulty conditions heavy current flows through the contacts of the circuit breaker before they are opened.
As soon as the contacts start separating, the area of contact decreases which will increase the current density and consequently rise in the temperature.
The medium between the contacts of circuit breaker may be air or oil. The heat which is produced in the medium is sufficient enough to ionize air or oil which will act as conductor. Thus an arc is struck between the contacts. The p. So long as the arc is remaining between the contacts the circuit is said to be uninterrupted.
The current flowing between the contacts depends on the arc resistance. With increase in arc resistance the current flowing will be smaller. The arc resistance depends on following factors, a Degree of ionization: If there is less number of ionized particles between the contacts then the arc resistance increases.
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More the length more is the arc resistance. By high voltage gradient at the cathode, resulting in field emission by increase of temperature resulting in thermionic emission. By High Voltage Gradient As the moving contacts start separating from each other, the area of contact and pressure between the separating contacts decreases.
A high fault current causes potential drop of the order between the contacts which will remove the electrons from cathode surface. This process is called field emission. By Increase of Temperature With the separation of contacts there is decrease in contact area which will increase the current density and consequently the temperature of the surface as seen before which will cause emission of electrons which is called thermal electron emission.
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In most of the circuit breakers the contacts are made up of copper which is having less thermionic emission. Maintenance of an Arc In the previous section we have seen the initiation of the arc by field emission emission.
The electrons while travelling towards anode collide with another electron to dislodge them and thus the arc is maintained. The ionizing is lactated by, i High temperature of the medium around the contacts due to high current densities. Thus the kinetic energy gained by moving electrons is increased.
This will decrease the density of gas which will increase free path movement of the electrons. Arc Extinction It is essential that arc should be extinguished as early as possible.
There are two methods of extinguishing the arc in circuit breakers which are namely, a High resistance method b Low resistance or current zero method High Resistance Method In high resistance method the arc resistance is increased with time.
This will reduce the current to such a value which will be insufficient to maintain the arc thus the current is interrupted and the arc is extinguished.
This method is employed in only d. These methods will be discussed in detail later in this chapter. Low Resistance Method The low resistance or current zero method is employed for arc extinction in ac.
In this method arc resistance is kept low until current zero where extinction of arc takes place naturally and is prevented from restriking. This method is employed in many of the modern a.
So at each current zero point the arc vanishes for small instant and again it appears. But in auxillary circuit breakers the arc is interrupted at a current zero point. The space between the contacts is ionized quickly if there is fresh unionized medium such as oil or fresh air or SF, gas between the contacts at current zero point.
This will make dielectric strength of the contact space to increase such that arc will be interrupted and discontinued after current zero. This action produces high voltage across the contacts which are sufficient to reestablish the arc.
Thus the dielectric strength must be building more than the restricting voltage for faithful interruption of arc. Then the arc is extinguished at next current zero.
While designing the circuit breakers the care is taken so as to remove the hot gases from the contact space immediately after the arc. So that it can be filled by fresh dielectric medium having high dielectric strength. In summary we can say that the arc extinction process is divided in thee parts, a Arcing phase b Current zero phase c Post arc phase In arcing phase, the temperature of the contact space is increased due to the arc.
The heat produced must be removed quickly by providing radial and axial flow to gases. The arc can not be broken abruptly but its diameter can be reduced by the passage of gas over the arc. When ax. Current wave is near its zero, the diameter of the arc is very less and consequently arc is extinguished. This is nothing but current 7ero phase.
switchgear & Protection
Now in order to avoid the reestablishment of arc, the contact space must be filled with dielectric medium having high dielectric strength.
This is post arc phase in which hot gases are removed and fresh dielectric medium is introduced. After every current zero, there is a column of residual ionized gas. This may cause arc to strike again by developing necessary restriking voltage and this voltage stress is sufficient to detach electrons out of their atomic orbits which releases great heat.
Si in this theory rate at which positive ions and electrons recombine to form neutral molecules is compared with rate of rise of restriking voltage.
Due to recombination dielectric strength of gap gets recovered. So rate of recovery of dielectric strength is compared with rate of rise of restriking voltage. If the restriking voltage rises more rapidly than the dielectric strength, gap space breaks down and arc strikes again and persists. In the Fig. The assumption made while developing this theory is that the restriking voltage and rise of dielectric strength are comparable quantities which is not quite correct the second drawback is that the theory does not consider the energy relations in the arc extinction.
The arcing phase is not covered by this theory so it is incomplete. Cassie's Theory Alternative explanation of above process s afforded by Cassie's theory or also called Energy balance theory. Cassie suggested that the reestablishment of arc or interruptions of an arc both are energy balance process.This process is called field emission. Similar Threads: Plain break and controlled break all circuit breakers. The arc can not be broken abruptly but its diameter can be reduced by the passage of gas over the arc.
The medium could be oil, air, vacuum or SF6.
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