Circuit Breaker Arc Phenomena - copy PDF

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Circuit Breaker Arc Phenomenon notes...

Description

CIRCUIT BREAKER Circuit breakers are mechanical devices designed to close or open contact members, thus closing or opening an electrical circuit under normal and abnormal conditions. The interrupting capacity of a circuit breaker is the maximum value of current which can be interrupted by it without any damage.

Functions The function of a circuit breaker is to break a circuit when various abnormal conditions arise and create a danger for the electrical equipment in an installation. Circuit breakers are required to interrupt short circuit currents which may reach a value of several tens of thousands. At the same time, in order to quickly eliminate the source of the fault, a circuit breaker must open the circuit with least possible delay. A circuit incorporates manual as well as automatic control for switching operations. Automatic circuit breakers are equipped with a trip coil connected to a relay, designed to open the breaker automatically under abnormal conditions An automatic circuit breaker performs the following duties;    

It carries the full load current continuously without overheating or damage It opens and closes the circuit on no load It makes and breaks the normal operating current It makes and breaks the short circuit currents of magnitude upto which it is designed for.

Operating principle A circuit breaker essentially consists of fixed and moving contacts, called electrodes. Under normal operating conditions, these contacts remain closed and will not open automatically until and unless the system becomes faulty. Of course, the contacts can be opened manually or by remote control whenever desired. When a fault occurs on any part of the system, the trip coil of the circuit breaker gets energized and the moving contacts are pulled apart by some mechanism, thus opening the circuit. When the contacts of a circuit breaker are separated under fault condition, an arc is struck between them. The current is thus able to continue until the discharge ceases. The production of arc not only delays the current interruption process but it also generates enormous heat which may cause damage to the circuit breaker itself as well as to the system. Therefore, the main

problem in a circuit breaker is to extinguish the arc within the shortest possible time so that heat generated by it may not reach a dangerous value.

Arc Phenomenon When a short circuit occurs, a heavy current flows through the contacts of the circuit breakers before they are opened by the protective system. At the instant when the contacts begin to separate, the contact area decreases rapidly and large fault current causes increased current density and hence rise in temperature. The heat produced in the medium between contacts is sufficient to ionize the air or vaporize and ionize the oil. The ionized air or vapour acts as conductor and an arc is struck between the contacts. The p.d. between the contacts is quite small and is just sufficient to maintain the arc. The arc provides a low resistance path and consequently the current in the circuit remains uninterrupted so long as the arc persists. During the arcing period, the current flowing between the contacts depends upon the arc resistance. The greater the arc resistance, the smaller the current that flows between the contacts. The arc resistance depends upon the following factors; 

Degree of ionization: The arc resistance increases with the decrease in the number of ionized particles between the contacts

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Length of the arc: The arc resistance increases with the length of the arc

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Cross-section of arc: The arc resistance increases with the decrease in area of crosssection of arc.

Arc extinction The factors responsible for the maintenance of arc between the contacts are; 

p.d. between the contacts When the contacts have a small separation, the p.d. between them is sufficient to maintain the arc. One way to extinguish the arc to separate the contacts to such a distance that p.d. becomes inadequate to maintain the arc. However, this method is impracticable in high voltage system where a separation of many meters may be required.

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ionized particles between contacts The ionized particles between the contacts tend to maintain the arc. If the arc path is deionised, the arc extinction will be facilitated. This may be achieved by cooling the arc or by bodily removing the ionized particles from the space between the contacts.

Methods of Arc Extinction There are two methods of arc extinction in circuit breakers. A. High resistance method In this method, arc resistance is made to increase with time so that current is reduced to a value insufficient to maintain the arc. Consequently, the current is interrupted or the arc is extinguished. The principal disadvantage of this method is that enormous energy is dissipated in the arc. Therefore, it is employed only in d.c. circuit breakers and low capacity a.c. circuit breakers. The resistance of the arc may be increased by; 

Lengthening the arc: The length of the arc can be increased by increasing the gap length between the contacts but it is not practicable to draw the arc out to such a length (may be in meters for ht system) that the voltage available becomes insufficient to maintain the arc.

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Cooling the arc: Cooling helps in the deionization of the medium between the contacts. This increases the arc resistance. Cooling is brought about by bringing the arc in contact with cool air.

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Reducing X section of the arc: If the area of cross section of the arc is reduced, the voltage necessary to maintain the arc is increased. In other words, the resistance of the arc path is increased. The cross section of the arc can be reduced by letting the arc pass through a narrow opening or by having smaller area of contacts.

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Splitting of arc: The resistance of the arc can be increased by splitting the arc into a number of smaller arcs in series. Each one of these arcs experiences the effect of lengthening and cooling. The arc may be split by introducing some conducting plates between the contacts.

B. Low resistance or current zero method This method is applicable only in a.c. circuit interruption because there is natural zero of current 100 times in a second for 50 HZ 3ph supply system. This property of a.c. circuit is exploited for interruption purposes and the current is not allowed to rise again after a zero occurs. Also it is neither necessary nor desirable to cut off the current at any other point on the a.c. wave because this will induce high voltage in the system. In this method the arc resistance is kept low until the current is zero where the arc extinguishes naturally and is prevented from restriking after it has gone out at a current zero. This method of arc extinction is employed in all modern high power a.c. circuit breaker. The phenomenon of arc extinction is explained by two theories as follows; 

Energy balance or Cassie theory: This theory states that if the rate of heat dissipation between the contacts is greater than the rate at which heat is generated, the arc will be extinguished, otherwise it will restrike. The heat generated varies from time to time depending upon the separation of breaker contacts. Initially when the contacts are about to open, the restriking voltage is zero and, therefore the heat generated is zero. Again when the contacts are fully open, the resistance between the contacts is also infinite and hence the heat generated is zero. Between these two limits the heat generation reaches the maximum. Now if the heat so generated could be removed by cooling, lengthening and splitting the arc at a rate higher than that of generation, the arc is extinguished.

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Recovery rate or Slepian’s theory: This theory states that if the rate at which the ions and the electrons combine to form or are replaced by the neutral molecules i.e. the rate at which the gap recovers its dielectric strength is faster than the rate at which voltage stress rises, the arc will be extinguished: if otherwise the arc may be interrupted for a brief period but it again restrike. In an a.c. system current drops to zero after every half cycle. At every current zero, the arc extinguishes for a brief period. Now the medium between the breaker contacts contains ions and electrons so that it has small dielectric strength and can be easily broken down by the rising contact voltage called the restriking voltage. If such a breakdown does occur the arc will persist for another half cycle when the process will be repeated. If immediately after the current zero, the dielectric strength of the medium between breaker

contacts is built up more rapidly than the voltage across the contacts, the arc fails to restrike and current will be interrupted. The rapid increase of dielectric strength of the medium near current zero can be achieved by either causing the ionized particles in the space between contacts to recombine into neutral molecule or sweeping the ionized particles away and replacing them by unionized particles. The problem is, therefore, to remove the ions and electrons either by causing them to recombine into neutral molecules or by sweeping them away, as soon as the current becomes zero, so that rising contact voltage or restriking voltage cannot breakdown the space between the contacts . this can be achieved by the following methods; 

Lengthening of the gap: The dielectric strength of the medium is proportional to the length of the gap between contacts. Therefore, by opening the contacts rapidly, higher dielectric strength of the medium can be achieved.

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High pressure: If the pressure in the vicinity of the arc is increased, the density of the particles constituting the discharge also increases. The increased density of particles causes higher rate of deionization and consequently the dielectric strength of the medium between contacts is increased.

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Cooling: Natural combination of ionized particles takes place more rapidly if they are allowed to cool. Therefore, dielectric strength of the medium between the contacts can be increased by cooling the arc.

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Blast effect: If the ionized particles between the contacts are swept away and replaced by unionized particles, the dielectric strength of the medium can be increased considerably. This may be achieved by a gas blast directed along the discharge or by forcing oil into the contact space.

Pre-arcing and arcing time Pre-arcing time: It is the time between the commencement of the fault and the instant when cut off occurs. When a fault occurs, the fault current rises rapidly and generates heat in the fuse element. As the fault current reaches the cut off value the fuse element melts and an arc is initiated. The time from the start of the fault to the instant the arc is initiated is known as prearcing time. The pre-arcing time is generally small: a typical value being 0.001 second. Arcing time: This is the time between the end of pre-arcing time and the instant when the arc is extinguished. Total operating time: It is the sum of pre-arcing and arcing time. It may be noted that operating time of a fuse is generally quite low (say 0.002 second) as compared to a circuit breaker (say 0.2 second or so).

Current

Cutoff Current

Asymmetrical current

Currentzero (Arcfinished)

Time Fault occurs Pre‐arcing time

Arcingtime Totaloperatingtime

Fig: Pre-arcing and Arcing time

Restriking voltage and recovery voltage In ac circuit breakers, the current interruption takes place invariably at the natural zeroes of current wave. At current zero, a high frequency transient voltage appears across the breaker contacts and is caused by the rapid distribution of energy between the magnetic and electric fields associated with the plant and transmission lines of the power system. This transient voltage is known as the restriking voltage. This voltage appearing across the breaker contacts at the moment of final current zero has a profound influence on the arc extinction process. Under the influence of this voltage the arc tries to restrike and hence it is named as restriking voltage. After current zero, the arc gets extinguished if the rate of rise of restriking voltage between the contacts is less than the rate at which dielectric strength of the medium between the contacts gains. Thus, restriking voltage may be defined as the resultant transient voltage which appears across the breaker contacts at the instant of arc extinction.

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 Recovery voltage: It is the normal frequency (50 Hz) r.m.s. voltage that appears across the contacts of the circuit breakers after final arc extinction. It is approximately equal to the system voltage...