In this article we will discuss about:- 1. Meaning of Circuit Breakers 2. Operating Principle of Circuit Breakers 3. Arc Phenomena 4. Arc Extinction 5. Resistance Switching 6. Ratings.
Meaning of Circuit Breakers:
Circuit breakers are mechanical devices designed to close or open contact members, thus closing or opening of an electrical circuit under normal or abnormal conditions.
Automatic circuit breakers, which are usually employed for the protection of electrical circuits, are equipped with a trip coil connected to a relay or other means, designed to open the breaker automatically under abnormal conditions, such as overcurrent.
The automatic circuit breakers perform the following duties:
(i) It carries the full-load current continuously without overheating or damage,
(ii) It opens and closes the circuit on no load,
(iii) It makes and breaks the normal operating current and
(iv) It makes and breaks the short-circuit currents of magnitude up to which it is designed for.
The circuit breaker performs first three duties satisfactorily but in performing fourth duty i.e., when it is to make or break short-circuit currents, it is subjected to mechanical and thermal stresses. The circuit breakers are rated in terms of maximum voltage, number of poles, frequency, maximum continuous current carrying capacity, maximum interrupting capacity and maximum momentary and 4-s current carrying capacity.
The interrupting or rupturing capacity of a circuit breaker is the maximum value of current which can be interrupted by it without any damage. The circuit breakers are also rated in MVA which is the product of interrupting current, rated voltage and 10–6.
Operating Principle of Circuit Breakers:
A circuit breaker is a switching and current interrupting device. It consists, essentially, of fixed and moving contacts, which are touching each other and carry the current under normal conditions i.e., when circuit breaker is closed. When the circuit breaker is closed, the current carrying contacts, called the electrodes, engage each other under the pressure of a spring.
During the normal operating condition the circuit breaker can be opened or closed by a station operator for the purpose of switching and maintenance. To open the circuit breaker, only a small pressure is required to be applied on a trigger. Whenever a fault occurs on any part of the power system, the trip coils of the breaker get energized and the moving contacts are pulled apart by some mechanism, thus opening the circuit.
The separation of current carrying contacts produces an arc. 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 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.
The basic construction of a circuit breaker requires the separation of contacts in an insulating fluid which serves two functions:
1. Extinguishes the arc drawn between the contacts when the circuit breaker opens.
2. Provides insulation between the contacts and from each contact to earth.
The insulating fluids commonly used for this purpose are as follows:
i. Air at atmospheric pressure.
ii. Compressed air.
iii. Oil producing hydrogen for arc extinction.
iv. Ultra high vacuum.
v. Sulphur hexa-fluoride (SF6).
The fluids used in circuit breakers should have the properties of high dielectric strength, non- inflammability, high thermal stability, arc extinguishing ability, chemical stability, and commercial availability at moderate cost.
Of the simple gases air is the cheapest and most widely used for circuit breaking. Hydrogen has better arc extinguishing property but it has lower dielectric strength as compared to air. Also if hydrogen is contaminated with air, it forms an explosive mixture. Nitrogen has similar properties as air. CO2 has almost the same dielectric strength as air but is a better arc extinguishing medium at moderate currents. Oxygen is a good extinguishing medium but is chemically active. SF6 has outstanding arc-quenching properties and good dielectric strength. Of all these gases SF6 and air are used in commercial gas blast-circuit breakers.
Arc Phenomena of Circuit Breakers:
The arc consists of a column of ionized gas having molecules which have lost one or more electrons. The electrons being negatively charged are attracted towards the positive contact (i.e., anode) with a high velocity and on the way they detach more electrons by impact. The positive ions are attracted towards the negative contact (i.e., cathode), but as they comprise almost the entire weight of the atom, they move towards it relatively slowly. Thus current flow is caused due to movement of electrons.
Initiation of an Arc:
For the initiation of an arc it is necessary that the electrons are emitted from the cathode as soon as the contacts begin to separate on occurrence of fault.
Initiating electrons are thought of produced by the following two processes:
(i) By High Voltage Gradient at the Cathode Resulting into Field Emission:
As the moving contact is withdrawn the contact area and the pressure between the separating contacts decreases and due to decrease in contact area the resistance increases (but it is still much less than an ohm). Although contact resistance is quite small but due to large magnitude of fault current a sufficiently high potential drop, of the order of 106 V/cm, is caused between the separating contacts so as to dislodge the electrons from the cathode surface.
(ii) By Increase of Temperature Resulting into Thermionic Emission:
As the contacts apart, the decrease in contact area causes increase in current density to very high values, of the order of 106 A/cm2. These very high current densities raise the temperature of the contact (cathode) surface resulting into thermal emission.
In case of circuit breakers the contacts used are usually of copper, the thermionic emission from such a metal is quite low and so for initiation of arc, the field emission is mostly responsible.
Maintenance of Arc:
The electrons so emitted from the cathode make many collisions with the atoms and molecules of gases and vapours existing between the two contacts during their journey towards the anode. Such collisions cause ionization of atoms and the molecules, thus dislodging more electrons.
The ionization is further facilitated by:
(i) High temperature of the medium around the contacts caused by high current densities, with high temperature the kinetic energy gained by the moving electrons is increased.
(ii) The field strength or voltage gradient which increases the kinetic energy of moving electrons and increases the chances of detaching electrons from neutral molecules.
(iii) An increase of mean free path—the distance through which the electron moves freely. As the contacts apart the mean path increases and the number of neutral molecules increases, also, the increase in mean path decreases the density of gas which further increases the free path movement of the electrons.
All the above three processes (thermal emission, ionization and field emission) may start either one after the other or almost simultaneously and enable the arc to be initiated and maintained and finally if the arc current is high, the arc may attain a temperature high enough for thermal ionization to become the main source of electrical conductivity.
As the contacts of the circuit breaker apart, an arc is formed. The voltage that appears across the contacts of the circuit breaker is called the arc voltage.
For moderate values of current and voltage, the arc characteristic can be expressed by Ayrton’s equation –
ea = A + B/ia …(6.1)
Thus with the increase in arc current, the voltage drops as a hyperbola. The constants A and B vary linearly with the arc length l
A = α + ϒl
and B = β + δl …(6.2)
Average values of α, ϒ, β and δ for arcs in air between copper electrodes are as follows –
α = 30 V; ϒ = 10 V/cm; β = 10 VA; δ = 30 VA/cm
From above Eq. (6.1) it is obvious that the volt-ampere characteristic of an arc voltage is negative i.e., arc voltage is high when the arc current is low and vice versa. This is, of course, a well-known property of arcs.
Figure 6.1 illustrates the characteristics of ac current and voltage with respect to time. It is seen from Fig. 6.1 that the arc voltage is almost constant during the time when the current is near its peak values. At current zero, the arc voltage rises rapidly to peak value and this peak value tends to maintain the current flow in the form of arc.
The voltage across the arc is in phase with arc current as the arc current is predominantly resistive. The magnitude of arc voltage increases in each successive current loop. This is because the circuit breaker contacts are assumed to be separating thereby increasing the arc length and therefore the arc voltage.
Arc Extinction in Circuit Breakers:
When current carrying contacts of a circuit breaker are parted, an arc is formed, which persists during the brief period after separation of contacts. The arc provides a gradual transition from the current-carrying to the voltage isolating states of the contacts, but it is dangerous on account of the energy generated in it in the form of heat which may result in explosive force.
The circuit breaker should be capable of extinguishing the arc without causing any damage to the equipment or danger to personnel. The arc plays a vital role in the behaviour of the circuit breaker. The interruption of dc arcs is relatively more difficult than ac arcs. In ac arcs, as the current becomes zero during the regular wave, the arc vanishes and it is prevented from re-striking.
Before discussing the methods of arc extinction, it is necessary to examine the factors responsible for the maintenance of arc between the contacts.
(i) Potential difference between the contacts, and
(ii) Ionised particles between the contacts.
The potential drop between the aparting contacts is just sufficient to maintain the arc and is quite small. One way to extinguish the arc is to separate the contacts to such a distance that the potential drop becomes inadequate to maintain the arc. However, this method is impracticable in high voltage systems where a separation of many metres would be required for this purpose.
The conductance of the arc is proportional to the number of electrons per cubic centimetre produced by ionization, the square of the diameter of the arc, and the reciprocal of the length. We cannot do much by increasing the length of arc to any reasonable value. What can be done is to reduce the density of free electrons, i.e., reduce the ionization, and decrease the diameter of the arc. The arc extinction can, therefore, be facilitated by deionizing the arc path. This may be achieved by cooling the arc or by bodily removing the ionised particles from the space between the circuit breaker contacts.
Resistance Switching in Circuit Breakers:
A deliberate connection of a resistance in parallel with the contact space (or arc) is called the resistance switching. Resistance switching is employed in circuit breakers having high post zero resistance of contact space (i.e., air-blast circuit breakers).
Severe voltage oscillations occur due to:
(i) Breaking of low inductive currents (i.e., current chopping) and
(ii) Breaking of capacitive currents.
This may endanger the operation of the system. This can be avoided by employing resistance switching (by connecting a resistor across the contacts of the circuit breaker).
On occurrence of fault, the contacts of the circuit breaker open and an arc is struck between the contacts. With the arc shunted by the resistance R a part of arc current is diverted through this resistance. This results in the decrease of arc current and an increase in the rate of deionization of the arc path. Thus the arc resistance is increased leading to a further increase in current through the shunt resistance R. This build up process continues until the current becomes so small that it fails to maintain the arc. Now the arc is extinguished and the circuit current gets interrupted.
Alternatively, the resistance may be automatically switched-in by transference of the arc from the main contacts to the probe contact as in the case of an axial blast circuit breaker, the time required for this action is very small (usually less than one half-cycle of the current wave). Having the arc path substituted by a metallic path, the current flowing through the resistance is limited and then easily broken.
Typical resistor connections are shown in Fig. 6.18. In Fig. 6.18 (a) a second break is provided to break the resistor current. In Fig. 6.18 (b) the gaps are so arranged that the moving contact finally breaks the resistor elements. In Fig. 6.18 (c) the arc first appears across fixed and moving contacts F and M which is then transferred across fixed and probe contacts F and P and then broken there.
The shunt resistor also helps in limiting the oscillatory growth of re-striking voltage transients. It can be proved mathematically that the natural frequency of oscillations of the circuit shown in Fig. 6.17(a) is given as –
The effect of shunt resistor R is to prevent the oscillatory growth of re-striking voltage and cause it to grow exponentially up to recovery voltage. This is being most effective when the value of R is so chosen that the circuit is critically damped. The value of R required for critical damping is 0.5. √L/C. Fig 6.17 (b) shows the oscillatory growth and exponential growth when the circuit is critically damped.
To sum up, resistors across breaker contacts may be used to perform any one or more of the following functions:
1. It reduces the RRRV and thus reduces the burden on the circuit breaker.
2. It ensures the damping of the high frequency re-striking voltage transients during switching out inductive or capacitive loads.
3. In a multi-break circuit breaker it helps in distributing the transient recovery voltage more uniformly across all the contact gaps.
The resistors employed may be either nonlinear or wire wound. Nonlinear resistors are suitable both from space and reliability considerations for small shunt currents where wire-wound resistors tend to be less satisfactory from mechanical considerations. Where heavy currents are involved there may be difficulty in accommodating the relatively large volume of required resistor material.
Nonlinear resistors are not suitable for modification of the RRRV and of the voltage peak as are linear resistors, but they are especially suited to voltage equalization and overvoltage suppression applications in which relatively small currents of the order of 1-10 A at normal peak voltage are adequate.
In the plain break oil circuit breakers (tank type) the post-zero resistance of the contact space is low. Hence resistance switching is not necessarily required. The performance at low currents can, however, be improved by employing resistance switching and it is sometimes employed; when interrupting a small current, the value of reactance in the circuit will tend to be so high that the inductance L in the expression for the critical resistance will be larger, resulting in resistors of the order of thousands of ohms.
The post-zero resistance of air-blast circuit breaker is high. This may result in severe voltage transients due to current chopping. Hence the resistance switching is employed. The auxiliary contacts here are replaced by isolating contacts, which are parts of air circuit breakers.
Circuit Breaker Ratings:
The rating of a circuit breaker is given according to the duties that are performed by it. For complete specifications, standard ratings and various tests of switches and circuit breakers IS 375/1951 may be consulted.
Apart from the normal working of circuit breakers the circuit breaker is required to perform following three major duties under short-circuit condition:
1. A circuit breaker must be capable of breaking the circuit and isolating the faulty section in case of a fault. This is described as breaking capacity of a circuit breaker.
2. Since in practice a circuit breaker is put on 2-3 times in order to ensure the permanency of the fault i.e., it must be capable of making circuit in the greatest asymmetrical peak in current wave. This refers to making capacity of a circuit breaker.
3. When a circuit breaker works in conjunction with the other circuit breakers and in case of a fault on any one section the breakers in the sound sections should not trip i.e., a circuit must be capable of carrying fault currents safely for a short time while another circuit breaker (in series) is clearing the fault. This refers to short-time capacity of a circuit breaker.
In addition to the above ratings, a circuit breaker should be specified in terms of (i) the number of poles (ii) rated voltage (iii) rated current (iv) rated frequency and (v) operating duty. The number of poles per phase of a breaker is a function of the operating voltage.
During normal operating conditions the voltage at any point of the power system is not constant. Due to this the manufacturer guarantees perfect operation of the circuit breaker at rated maximum voltage, which as a rule is higher than rated nominal voltage.
The rated maximum voltage of a circuit breaker is the highest rms voltage, above nominal system voltage, for which the circuit breaker is designed and is the upper limit for operation. The earlier practice of specifying the rated voltage of a circuit breaker as nominal system voltage is no more followed. The rated voltage is expressed in kVrms and refer to phase-to-phase voltage for three phase circuit.
i. Rated Current:
The rated normal current of a circuit breaker is the rms value of the current which the circuit breaker shall be able to carry at rated frequency and at the rated voltage continuously, under specified conditions. Under the specific conditions, important is the temperature rise of the various components of the circuit breaker under normal loads. The important condition for normal working of an oil circuit breaker is that the temperature of oil should not be more than 40°C and that of contacts should not exceed 35°C.
ii. Rated Frequency:
The rated frequency of a circuit breaker is the frequency at which it is designed to operate. Standard frequency is 50 Hz. Applications at other frequencies need special considerations.
iii. Operating Duty:
The operating duty of a circuit breaker consists of the prescribed number of unit operations at stated intervals.
The operating sequence denotes the sequence of opening and closing operations which the circuit breaker can perform under specified conditions.
This term expresses the highest rms value of short-circuit current that the circuit breaker is capable of breaking under specified conditions of transient recovery voltage and power frequency voltage. It is expressed in kA rms at contact separation.
It can be seen from the short-circuit current wave shown in Fig. 6.19 that the rms value of the current varies with time on account of the presence of dc component of the current which decays with time.
It is known that in a particular phase the current is maximum at the instant of fault, after which the current decays. In addition, owing to relaying time the circuit breaker starts to open its arcing contacts only some time later, after the initiation of short circuit. Hence the actual current interrupted by the circuit breaker is less than initial value of short-circuits current I1.
Let at the instant of separation of contacts.
AC component of short-circuit current, Iac = x
DC component of short-circuit current, Idc = y
Now symmetrical breaking current –
= RMS value of ac component of short-circuit current at the instant of separation of contacts
= x/√2 … (6.19)
Asymmetrical breaking current –
= RMS value of the combined sums of ac and dc components
Now according to these two values of breaking currents there are two corresponding values of breaking capacities. Conventionally the breaking capacity of a circuit breaker in MVA is given as √3 x rated voltage in kV x rated breaking current in kA.
This practice of specifying breaking capacity in terms of MVA is convenient while determining the fault level. However, as per revised standards the breaking capacity is expressed in kA for specified conditions of TRV, and this method takes into account both breaking current and TRV.
The two breaking capacities can now be defined as follows:
(i) The symmetrical breaking capacity of a circuit breaker is the value of symmetrical breaking current which the circuit breaker is capable of breaking at a stated recovery voltage and a stated reference re-striking voltage under prescribed conditions.
(ii) The asymmetrical breaking capacity of a circuit breaker is the value of the asymmetrical breaking current which the circuit breaker is capable of breaking at a stated recovery voltage and a stated reference re-striking voltage under prescribed conditions.
There is always a possibility that the circuit breaker is closed under short-circuit condition. The making capacity of the circuit breaker depends upon its ability to withstand the effects of electromagnetic forces which are proportional to the square of the peak value of the making current. The making current of a circuit breaker when closed on a short circuit is the peak value of the maximum current wave (including dc component) in the first cycle of the current after the circuit is closed by the circuit breaker.
For determination of making current of a circuit breaker, we must multiply symmetrical breaking current by √2 to convert the rms value to peak value, and then by 1.8 to take into account the “doubling effect” of maximum asymmetry.
Thus rated making current = 1.8 x √2 rated short-circuit breaking current
= 2.55 rated short-circuit breaking current
or Making capacity = 2.55 x symmetrical breaking capacity …(6.21)
Short-Time Current Rating:
a circuit breaker is sometimes required to carry short-circuit current for short intervals without tripping. This happens in case of momentary faults like birdage on the transmission lines and the fault is automatically cleared and persists only for 1 or 2 seconds. For this reason the circuit breakers are short-time rated and they trip only when the fault persists for duration longer than the specified time limit.
The short-time current of a circuit breaker is the rms value of current that a circuit breaker can carry in a fully closed position without damage, for the specified time interval under prescribed conditions. It is normally expressed in terms of kA for a period of 1 second or 4 seconds, known as one-second rating and four-second rating respectively. These ratings are based on thermal limitations.
Low-voltage breakers do not have any such short-time rating because these are normally equipped with direct acting series overload trips.