Lecture no 3 switchgears bt255
Download as PPTX, PDF0 likes109 views
The document discusses the characteristics and theories surrounding circuit breakers, focusing on electric arcs and interruption mechanisms. It covers the nature of arc discharge, modes of arc extinction, and two main theories of arc interruption—Slepian's and Cassie's theories. The document explains how various methods and factors influence the interruption of arcs in both AC and DC circuit breakers.
Lecture no 3 switchgears bt255
- 1. Switchgears and Protective Devices (Sunil S Rao) Presented By: Engr. FAZAL UR REHMAN Lecturer Electrical KPTEVTA Visiting Lecturer COMWAVE INSTITUTE ISLAMABAD
- 2. Today's Topics Chapter 4.Characteristics of Circuit Breakers The Arc Extinction 4.5 Electric Arc, 4.6 Arc formation in A.C. (C.B), 4.7 Modes at Arc Extinction, 4.8 Arc interruption theories
- 3. Electric Arc The self sustained discharge of electricity between electrodes in gas or vapour having voltage drop across it is called an electric arc. This spark / arc formation resulted due small clearances between electrodes (in order of few cm) is also termed as discharge. This discharge is classified on the basis of V-A zones; Glow discharge, Townsend discharge, Arc discharge.
- 4. In the case of an arc discharge, the discharge is sustained by thermo-field electron emission from the cathode and it is accompanied with the formation of a tiny cathode spot. In the case of a glow discharge, the discharge is sustained by secondary electron emission due to ion bombardment. The Townsend discharge is a gas ionization process where an initially very small amount of free electrons, accelerated by a sufficiently strong electric field, give rise to electrical conduction through a gas by avalanche multiplication. Townsend discharge is named after John Sealy Townsend.
- 7. Arc Discharge During opening of current carrying contacts in a circuit breaker the medium in between opening contacts become highly ionized. Through which the interrupting current gets low resistive path And continues to flow through this path even the contacts are physically separated. This is called arc discharge.
- 8. Separation of circuit breaker contact result in arc production between the contacts as shown (arc current). Due to high resistive air stream voltage drop takes place across the arc which is called Arc Voltage.
- 9. Arc Extinction Air / Vacuum Gas / Oil Used for Arc- Extinction. E.g. SF6 gas has been used over here.
- 10. Arc Formation in AC C.B During separation of contacts, contact surfaces locally got heated. Due to high temperature electron emission from the surface takes place. Emission of electron result in ionization of gas between the contact space due to thermal ionization and collision. Mixture of ions and gas is formed between the contact. (Plasma formation) The ionized gas carry the arc current between contact.
- 11. Modes at Arc Extinction There are two modes of arc interruption; 1. High Resistance interruption 2. Low Resistance / Zero point interruption. Note. Artificial current zero principle is used in HVDC system breakers
- 12. High Resistance Interruption As the name suggest ‘ In this method the arc is interrupted by a high resistance path. As increase in resistance decrease the current (Ohm’s Law), and hence the arc. This method is used in low and medium voltage AC and DC circuit breakers.
- 13. High Resistance interruption can be done in many ways, like; 1. Lengthening of arc by arc runner. Horn like blades of conducting material connected to arcing contacts. 2. Splitting of arc. Resin bonded fibre glass plates placed in the path perpendicular to the arc blown with the help of magnetic field. 3. Cooling of arc. By bringing the arc in contact with cooler air result in increase of arc resistance.
- 15. Low Resistance of zero point interruption AC signal of 50Hz passes through zero point 100 times every second. As arc is basically a current flow through ionized gases. So in each second current pass 100 times through zero point and hence arc vanishes for 100 times in each second. The re appearance of arc can be interrupted in each chance (zero point) by removing the ionized gases with the help of some dielectric medium of high dielectric strength.
- 16. Arc interruption Theories How arc interruption occur in a circuit breaker ? There are mainly two theories given by Two scientists. So let us know about it. These two main theory explain current zero interruption of arc. The two arc interruption theory is as follows:. 1. Slepian’s Theory. 2. Cassies theory
- 17. Slepian’s Theory It describes the process as a race between the dielectric strength and restriking voltage. After every current zero, there is a column of residual ionized gas. This may cause the arc to strike again by developing necessary restriking voltage and this voltage is stress is sufficient to detach electron out of their orbit due to which a great heat is released.
- 18. According to this theory rate at which positive ion and electrons combine to form neutral molecules is compared with the rate of rising of restriking voltage. Due to the recombination dielectric strength of the gap gets recovered. Hence the rate of recovery of dielectric strength is compared with RRRV(rate of rising of restriking voltage). If the restriking voltage rises more rapidly than the dielectric strength, gap space breaks down and arc strikes again and persist.
- 19. According to the graph here (a), point denote rate of dielectric strength is more than restriking voltage and point (b) denotes the rate of dielectric strength is less than the rate of rising of restriking voltage.
- 20. Drawbacks: This assumption made while developing this theory is that the restriking voltage and rise of dielectric strength are comparable quantities which are not quite correct. This theory does not consider the energy relations in arc extinction. The arcing phase is not covered by this theory so it is incomplete.
- 21. Cassie’s Theory Cassie has given an alternative explanation of above process. It is also called Energy Balance Theory. He suggested that the re-establishment of arc or interruption of an arc both is an energy balance process. This means that if energy input to an arc continues to increase, the arc restrikes and if not, arc gets interrupted.
- 22. The assumption made in this theory: Arc consists of cylindrical column having a uniform temperature at its cross-section. The energy distributed in the column is uniform. The temperature remains constant. The cross-section of arc adjusts itself to accommodate the arc current. Power dissipation is proportional to the cross-sectional area of arc column.
- 23. Energy equation given by Cassie is given as, dQ/dt=EI-N where Q= Energy content/ length of the arc in cm E= Volts/cm I=total current N=Total power loss/cm
- 24. Breakdown occurs if power fed to the arc is greater than power loss. This theory holds true for High current. When it reaches current zero immediately after that contact space contain ionized gas and therefore has a finite post zero resistance. Now there is RRV(rising restriking voltage). This causes current to flow between the contacts. Due to this, power gets dissipated in the form of heat.
- 25. Initially when RV(restriking voltage is zero), automatically current and power will be zero. It again gets zero when space has become fully deionize and resistance between the contacts is infinitely high. In between these two limits, the power dissipated rise to a maximum. If the heat so generated exceeds the rate at which heat can be removed from contact space, Ionization will persist and breakdown will occur, giving an arc for another half cycle.