CIRCUIT BREAKERS & THEIR OPERATIONS

 Circuit  Breakers

A circuit breaker is equipment that can open or close a circuit under all conditions viz. no-load, full load, and fault conditions.  It is so designed that it can be operated manually (or by remote control) under normal conditions and automatically under fault conditions. For the latter operation, a relay circuit is used with a circuit breaker.

A circuit breaker is a piece of equipment which can 

(i) make or break a circuit either manually or by remote control under normal conditions 

(ii) break a circuit automatically under fault conditions 

(iii) make a circuit either manually or by remote control under fault conditions

Thus a circuit breaker incorporates manual (or remote control) as well as automatic control for switching functions.  The latter control employs relays and operates only under fault conditions. Under normal operating conditions, the contacts remain closed and the circuit breaker carries the full-load current continuously.  In this condition, the e.m.f. in the secondary winding of the current transformer (C.T.) is insufficient to operate the trip coil of the breaker but the contacts can be opened (and hence the circuit can be opened) by manual or remote control.  When a fault occurs, the resulting overcurrent in the C.T. primary winding increases the secondary e.m.f.  This energizes the trip coil of the breaker and moving contacts are pulled down, thus opening the contacts and hence the circuit. The arc produced during the opening operation is quenched by the oil.  It is interesting to note that relay performs the function of detecting a fault whereas the circuit breaker does the actual circuit interruption.

 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 coils of the circuit breaker get 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 conditions, 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 system or to the circuit breaker itself.  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 breaker 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 (usually the medium is oil or air) is sufficient to ionize the air or vaporize and ionize the oil.  The ionized air or vapor acts as a 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 : 

(i) Degree of ionization— the arc resistance increases with the decrease in the number of ionized particles between the contacts. 

(ii) Length of the arc— the arc resistance increases with the length of the arc i.e., separation of contacts. 

(iii) Cross-section of arc— the arc resistance increases with the decrease in the area of X-section of the arc.

Principles  of arc Extinction

Before discussing the methods of arc extinction, it is necessary to examine the factors responsible for the maintenance of arc between the contacts. 

These are : 

(i) p.d. between the contacts 

(ii) ionized particles between contacts 

Taking these in turn, 

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

(ii) The ionized particles between the contacts tend to maintain the arc.  If the arc path is deionized, 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 extinguishing the arc in circuit breakers viz. 

1. High resistance method. 2.  Low resistance or current zero method 

1. High resistance method.  In this method, arc resistance is made to increase with time so that the 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 : 

(i) Lengthening the arc.  The resistance of the arc is directly proportional to its length.  The length of the arc can be increased by increasing the gap between contacts. 

(ii) Cooling the arc.  Cooling helps in the deionization of the medium between the contacts. This increases the arc resistance.  Efficient cooling may be obtained by a gas blast directed along the arc.

(iii) Reducing X-section of the arc. If the area of X-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 a smaller area of contacts. 

(iv) Splitting the 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. 

2. Low resistance or Current zero method.  This method is employed for arc extinction in a.c. circuits only.  In this method, arc resistance is kept low until the current is zero where the arc extinguishes naturally and is prevented from restriking in spite of the rising voltage across the contacts.  All modern high power a.c. circuit breakers employ this method for arc extinction. In an a.c. the system, current drops to zero after every half-cycle.  At every current zero, the arc extinguishes for a brief moment.  Now the medium between the contacts contains ions and electrons so that it has small dielectric strength and can be easily broken down by the rising contact voltage known as restriking voltage.  If such a breakdown does occur, the arc will persist for another half-cycle.  If immediately after the current zero, the dielectric strength of the medium between contacts is built up more rapidly than the voltage across the contacts, the arc fails to restrike and the current will be interrupted. 

The rapid increase of dielectric strength of the medium near current zero can be achieved by : 

(a) causing the ionized particles in the space between contacts to recombine into neutral molecules.

(b) sweeping the ionized particles away and replacing them by un-ionized particles Therefore, the real problem in a.c. arc interruption is to rapidly deionize the medium between contacts as soon as the current becomes zero so that the rising contact voltage or restriking voltage cannot breakdown the space between contacts. 

The de-ionization of the medium can be achieved by:

 (i) 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, the higher dielectric strength of the medium can be achieved. 

(ii) 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 a higher rate of de-ionization and consequently, the dielectric strength of the medium between contacts is increased. 

(iii) cooling.  The natural combination of ionized particles takes place more rapidly if they are allowed to cool.  Therefore, the dielectric strength of the medium between the contacts can be increased by cooling the arc.

(iv) 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 with the discharge or by forcing oil into the contact space.

Important  Terms

The following are the important terms much used in the circuit breaker analysis : 

(i) Arc Voltage.  It is the voltage that appears across the contacts of the circuit breaker during the arcing period. As soon as the contacts of the circuit breaker separate, an arc is formed.  The voltage that appears across the contacts during the arcing period is called the arc voltage.  Its value is low except for the *period the fault current is at or near zero current points.  At current zero, the arc voltage rises rapidly to a peak value and this peak voltage tends to maintain the current flow in the form of an arc. 

(ii) Restriking voltage.  It is the transient voltage that appears across the contacts at or near current zero during the arcing period. At current zero, a high-frequency transient voltage appears across the 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 system.  This transient voltage is known as the restriking voltage. The current interruption in the circuit depends upon this voltage.  If the restriking voltage rises more rapidly than the dielectric strength of the medium between the contacts, the arc will persist for another half-cycle. On the other hand, if the dielectric strength of the medium builds up more rapidly than the restriking voltage, the arc fails to restrike and the current will be interrupted. 

(iii) Recovery voltage.  It is the normal frequency (50 Hz) r.m.s. the voltage that appears across the contacts of the circuit breaker after final arc extinction.  It is approximately equal to the system voltage. When contacts of the circuit breaker are opened, current drops to zero after every half cycle.  At some current zero, the contacts are separated sufficiently apart and dielectric strength of the medium between the contacts attains a high value due to the removal of ionized particles.  At such an instant, the medium between the contacts is strong enough to prevent the breakdown by the restriking voltage. Consequently, the final arc extinction takes place and circuit current is interrupted.  Immediately after the final current interruption, the voltage that appears across the contacts has a transient part.  However, these transient oscillations subside rapidly due to the damping effect of system resistance and normal circuit voltage appears across the contacts.  The voltage across the contacts is of normal frequency and is known as recovery voltage.