SWITCHGEAR & PROTECTION
PREPARED BY:Md.Foyez Ahammad
Dept:EEEID:13205100
Air-Blast Circuit Breaker
Air-Blast Circuit Breaker (Cont. 1)* The breakers employ a high pressure air-blast as quenching medium.
* Air blast is obtained by opening the valve of high pressure air reserver.
* During opening the contacts high pressure air blast is forced through the contacts.
* Air blast cools the arc and enhances rapid increase of dielectric strength.
* Air blast sweeps away the arc and arcing products to the atmosphere.
* Restriking voltage can not breakdown the dielectric strength of the medium and arc is extenguished.
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Air-Blast Circuit Breaker (Cont. 2)
* Risk of fire is eliminated as no oil is used.
* As dielectric strength development is very rapid, final contact gap requirement is smaller, resulting reduced size of circuit breaker.
* As the arcing time is much less than that of oil CB, less damage is done to the contacts due to tremendous arcing heat.
* Air-blast CB is suitable in a place of frequent operation as arc energy dissipated is smaller than that of oil CB.
Advantage :
* Arc extinction is independent of fault current as required blast is obtained from external source than arc energy.
* Arcing products are completely removed to the air, so medium is never deteriorated as happens in oil circuit breaker.
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Air-Blast Circuit Breaker (Cont. 3)
* Air has relatively inferior arc extinguishing quality in comparison with Sulphur hexaflouride (SF6).
* Considerable maintenance is required for high pressure air tank.
* Installation area :
* Widely used in high power installation.
Disadvantage :
* Mainly used where voltage level is beyond 11 KV.
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Types of Air-blast Circuit Breaker
i. Axial-blast type : Air blast is directed along the arc path.ii. Cross-blast type : Air blast is directed at right angle to the arc path.iii. Radial-blast type : Air blast is directed radially.
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Description of Air-blast Circuit Breakersi. Axial-blast air circuit breaker :
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* Air blast is directed along the arc path.
* Describe construction and common features of Air-blast Circuit Breakers
* Contact separation required for arc extinction is smaller (1.75 cm or so).
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Axial-blast air circuit breaker : (Cont.)8
* This small gap constitutes inadequate clearance for normal voltage operation.
* An isolating switch is incorporated in series as a part of this type of CB.* Just after arc extinction isolator is opened providing necessary clearance for insulation.
Course : switchgear, Ref: Principal of power system by V. K. Mehta, Prepared by : Md. Aziz ul huq, Faculty, EEE, IUBAT, [email protected]
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Description of Air-blast Circuit Breakers
ii. Cross-blast air circuit breaker :
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* Air blast is directed at right angle to the arc path.* The cross-blast lengthens and forces the arc into a suitable chute for arc extinction.
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* The chute consists of arc splitter and baffle.
ii. Cross-blast air circuit breaker : (Cont.) 10
* The splitters serve to increase the length of the arc and the baffles gives improved cooling.* As a result, arc is extinguished and flow of current is interrupted.
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* Arc extinction is independent of fault current as required blast is obtained from external source than arc energy.* After arc extinction contact gap is big enough to provide required clearance .
* So, series isolating switch is not required to ensure insulation like axial-blast air circuit breaker.
Sulphur Hexaflouride (SF6) Circuit Breaker 12
* Sulphur Hexaflouride (SF6) gas is used as arc quenching medium.* Sulphur Hexaflouride is an electronegative gas and has strong tendency of absorbing free electrons.
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* Free electrons of arc is absorbed by SF6 and become immobile negative ions.
Sulphur Hexaflouride (SF6) Circuit Breaker: (Cont.) 13
* This loss of conducting electrons in the arc builds up enough insulating properties in the medium and arc is extinguished .* Construction and working procedure.
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* Fixed and moving contacts are enclosed in a chamber, called interruption chamber, containing SF6 .* The interruption chamber is connected to a high pressure SF6 gas reservior through valve with opening and closing mechanism.* Both the contacts are hollow inside and equipped with horns to safeguard from flashover.* At normal condition SF6 gas pressure in interruption chamber is 2.8 Kg/cm2 and in gas reservior is 14 Kg/cm2.
Sulphur Hexaflouride (SF6) Circuit Breaker: (Cont.)14
* When arc is struck, valve is opened, blast of SF6 gas flows between the contacts through the hollow passage to the recycling chamber and arc is extinguished .
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* After each extinction exhaust SF6 gas is recycled and fed into the high pressure gas reservior.* The tips of contacts and arcing horns are coated with arc resistant copper-tungsten material to prevent damage.* Due to the high degree of arc quenching capabilities of SF6 gas, this circuit breaker is widely used in high voltage and high power service.* To prevent leakage of expensive SF6 gas, the whole circuit is built in air tight mechanism.
Sulphur Hexaflouride (SF6) Circuit Breaker: (Cont.) 15
* Advantages :
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* Due to the superior arc quenching properties of SF6, such circuit breakers have very short arcing time. * Since dielectric strength of SF6 gas is 2 to 3 times greater than that of air, such breakers can interrupt much larger fault current.* Provides noiseless service due to close gas circuit.* The closed gas enclosure keeps the interior dry so there is no moisture problem.* There is no risk of fire as SF6, gas in non-inflammable. * Requires low maintenance, low foundations and auxiliary equipments. * Suitable to use in an explosion hazard environment like coal mine as it is completely enclosed and sealed from atmosphere.
Sulphur Hexaflouride (SF6) Circuit Breaker: (Cont.)16
* Disadvantages :
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* SF6 breakers are costly due to high cost of SF6.* Additional equipments are to be incorporated for reconditioning of SF6.
* Applications :* A typical SF6 unit is capable of dealing 60 KA and 50-80 KV.* Number of units are incorporated in series to serve higher ratings.* Conventional ratings of SF6 breakers are 230 KV, 10-20 MVA and interrupting time less than 3 sec.
Note on Flashover & Horn17
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* Flashover is defined as high intensity arc between two conductors over insulation .* Horn is a mechanical arrangement of metallic conductor to perform flashover in a guided way in order to protect insulation.
Vacum Circuit Breaker (VCB) 19
* Vacum is used as arc quenching medium.
* Range of vacum is from 10-7 to 10-5 torr. 1 torr = 1 mm Hg = 1/760 atm. pressure
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* Since vacum offers highest insulating strength, it has highest arc quenching capabilities than any other medium.
Vacum Circuit Breaker (Cont.) 20
* Dielectric strength of vacum builds up thousands time faster than any other medium.* So, arcing current interrupts at first current zero.
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Vacum Circuit Breaker (Cont.) 21
* Principal :* When contacts are opened in the vacum, arc is produced through the ions of metallic vapours as per Field emission theory.
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* Metal vapours condense rapidly at the contacts and at inner surface of the breaker and ensures instant arc extinction.
* Construction :* It consists of fixed contact, moving contact and arc shield mounted inside a vacum chamber. * The movable contact is mounted on stainless steel bellows and the whole chamber is sealed preventing leakage . * A glass or ceramic vessel is used as the outer insulating body.
* At very short contact separation ( 0.625 cm) arc is extinguished as dielectric strength development of vacun is very high.
Vacum Circuit Breaker (Cont.) 22
* Advantages :* They are compact, reliable and have longer life.
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* There is no fire risk and hence suitable to use in fire hazard .* They can interrupt low as well as very high fault current.
* They require little maintenance and are quiet in operation.
* They can successfully withstand lightning surge.* Low arc energy is dissipated in VCB.* VCB has low inertia and hence require smaller controlling mechanism.* Due to little maintenance requirement VCBs are used in outdoor and rural areas handling around 66 KV and 100 MVA.
Switchgear components 24
i. Bushings
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ii. Circuit breaker contacts iii. Instrument transformersiv. Bus-bars conductors
Bushing :
Switchgear components(Cont.) 25
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* Bushing is the arrangement of insulating material between or around high voltage conductor and surrounding earthed metallic plate. * The function of Bushing is to prevent breakdown between high voltage conductor and surrounding earthed metallic plate.
* Breakdown occurs in the form of puncture or Flashover. * puncture damages the insulating material and renders it incapable of withstanding even the normal working voltage.
* Flashover occurs between the exposed portion of conductor and earthed metallic plate over the insulation causing little damage and remains the bushing serviceable.
* So bushing is so designed that flashover happens before puncture.
ii. Circuit breaker contacts 26
* Circuit breaker contacts handle both normal current and short circuit current.
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* During normal current flow temperature rise and voltage drop at the junction of contacts should not exceed limit.* During fault current tremendous heat due to arc should not melt and vaporize contact tips.
a) Tulip type contacts b) Finger and wedge contactsc) Butt contacts
* Following three types of contacts have been designed to serve the purpose and They are :
ii. Circuit breaker contacts 27
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a) Tulip type contacts
* Moving contact moves inside the fixed contact.* Arc is produced between the tips of the contacts and hence arcing damage remains confined at the tips.* During normal operation electricity flows throughout the whole body i.e. non damaged portions of the contacts.* This technique results in less heating and less voltage drop across the contacts during normal operation.
ii. Circuit breaker contacts 28
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b) Finger and wedge contacts
* Spring pressure keeps the contacts tightly closed results in less heating and less voltage drop across the contacts during normal operation.* As per construction this type of contacts are not suitable for use with arc controlling devices.* This type of contacts are largely used in low voltage oil circuit breaker.
ii. Circuit breaker contacts 29
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c) Butt contacts
* Spring pressure is used for making forced contacts ensuring less heating and less voltage drop across the contacts during normal operation.* Use of higher quality materials, 85% silver and 15% nickel, at contact tips ensures less arcing decay.* End-to-end mating eliminates the affects of frictional wear and allows for quick-break (15-millisecond).* Widely used for dealing with higher short-circuit current.
iii. Instrument Transformers 30
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* Modern power system carries very high voltages with thousands of amperes of current.* Measuring instruments can not be used directly in the power line of such high ratings to have accurate readings.* To overcome this problem instrument transformers are used which provide accurate fractional output at the secondary ensuring accurate readings and suitable operation.
iii. Instrument Transformers 31
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* Instrument transformers are of two types. They are :a) Current transformer (C.T.)b) Potential transformer (P. T.)
* In high power system C.T. is connected in series while P.T. is connected in parallel rendering suitable fraction of input parameter.* Use of Instrument transformers help in following ways: ■ permits isolation between measuring instruments and high voltage. ■ wires with minimum insulation and cross section in secondary.
iv. Bus-bars and conductors* In some cases of outdoor installation CBs are connected directly to the overhead lines while for some structural outdoor and indoor arrangement CBs are connected to Bus-bars.
Problem of Circuit Interruption 32
* Inductive and capacitive loads In electric system stores and releases energy at each cycle.
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* When circuit breaker is tripped due to fault current, this stored energy partly dissipates at CB and rest of the energy is dissipated at the system as oscillatory surge.
* For efficient design of circuit breaker some topics should be understood clearly. They are:
* Circuit breaker should be capable enough to withstand dissipated energy and oscillatory surge.
i. Rate of rise of re-striking voltage (R.R.R.V.)ii. Current chopping
iii. Capacitive current breaking
Problem of Circuit Interruption 33
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* Fig. (i) shows generator, transformer, CB and fault point and Fig. (ii) represents equivalent circuit. Fig. 19.18 shows restriking.
i. Rate of rise of re-striking voltage (R.R.R.V.)
* It is rate of increase of re-striking voltage and is abbreviated by (R.R.R.V.). Conventional unit : KV/µ sec.* Consider fig (ii) up to fault point L represents inductance an C represents capacitance of single phase transmission line.
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Course : switchgear, Ref: Principal of power system by V. K. Mehta, Prepared by : Md. Aziz ul huq, Faculty, EEE, IUBAT, , [email protected]
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* Before interruption , The capacitor is considered shorted due to fault current.
i. Rate of rise of re-striking voltage (R.R.R.V.) (Cont.)
* The fault current is limited only by inductance L and the whole generator voltage appears across the inductor .
* Fig . 19.18 shows system voltage e, current i, arc voltage ea where i lags e by 900.
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Course : switchgear, Ref: Principal of power system by V. K. Mehta, Prepared by : Md. Aziz ul huq, Faculty, EEE, IUBAT, , [email protected]
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i. Rate of rise of re-striking voltage (R.R.R.V.) (Cont.)
* When the contacts are opened and the arc finally extinguishes at some current zero, the generator voltage e is suddenly applied to the inductance and capacitance in series.* This L-C combination forms an oscillatory circuit with transient frequency fn. Where
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Course : switchgear, Ref: Principal of power system by V. K. Mehta, Prepared by : Md. Aziz ul huq, Faculty, EEE, IUBAT, , [email protected]
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* The transient voltage known as re-striking voltage reaches instantaneous peak voltage of 2Em, double the system peak voltage.
i. Rate of rise of re-striking voltage (R.R.R.V.) (Cont.)
* The transient voltage appears across C and hence across contacts.
* The system losses decreases transient peaks but initial overshoot may cause restrike for arc continuation.* If rate of rise of re-striking voltage (R.R.R.V.) is greater than rate of rise of dielectric strength, only then arc continues otherwise not.
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Course : switchgear, Ref: Principal of power system by V. K. Mehta, Prepared by : Md. Aziz ul huq, Faculty, EEE, IUBAT, , [email protected]
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i. Rate of rise of re-striking voltage (R.R.R.V.) (Cont.)
R.R.R.V. depends on (a) recovery voltage (b) natural frequency of oscillation.
* Short circuit occurring near bus-bar of power station, C becomes low resulting high natural frequency fn.. R.R.R.V. becomes very high a worse situation for circuit breaker.
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Course : switchgear, Ref: Principal of power system by V. K. Mehta, Prepared by : Md. Aziz ul huq, Faculty, EEE, IUBAT, , [email protected]
M05_ ii. Current chopping
* Current chopping is the phenomenon of current interruption before current zero is reached.* This phenomenon is happened in air blast type CB.* The powerful deionising effect of air-blast causes the arc current to fall abruptly to zero before natural current zero is reached.* This phenomenon is called current chopping and produces high voltage transient across the contacts of circuit breaker.
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Course : switchgear, Ref: Principal of power system by V. K. Mehta, Prepared by : Md. Aziz ul huq, Faculty, EEE, IUBAT, , [email protected]
M05_ ii. Current chopping ( Cont. )
* Suppose, as per Fig. 19.19( ii), at point a arc current is i and chop occurs due to air-blast.
* At this moment energy stored in the inductor is Li2/2.
* This energy instantly transfers to Capacitor C charging it to a prospective voltage e. The relation is as follows :
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Course : switchgear, Ref: Principal of power system by V. K. Mehta, Prepared by : Md. Aziz ul huq, Faculty, EEE, IUBAT, , [email protected]
M05_ ii. Current chopping ( Cont. )
* The value of prospective voltage e is very high and causes restrike and first chop happens.* As current i decreases gradually, number of successive chop with lower intensity occurs and at last arc is extinguished far before current zero is reached.* Excessive surge voltage due to current chopping is limited by shunting the contacts of the breaker with a resistor of suitable value.
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Course : switchgear, Ref: Principal of power system by V. K. Mehta, Prepared by : Md. Aziz ul huq, Faculty, EEE, IUBAT, , [email protected]
M05_ iii. Capacitive current breaking
* Capacitive current breaking happens when a circuit breaker opens a capacitive load.* Though unloaded, current I flows through the circuit breaker due to capacitance C of transmission line with respect to earth. * Fig. 19.20 shows circuit diagram while Fig. 19.21 represents capacitive current breaking phenomenon.* Consider Fig. 19.21, at point 1 current I is zero and voltage across capacitor is Vgm, the maximum voltage of generator.
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Course : switchgear, Ref: Principal of power system by V. K. Mehta, Prepared by : Md. Aziz ul huq, Faculty, EEE, IUBAT, , [email protected]
M05_ iii. Capacitive current breaking (Cont.)
* Suppose, at this moment contacts of CB are opened and voltage Vr across the contacts of CB is zero.* As Vg lags I by 900, at next half cycle voltage at point ‘A’ will be - Vgm.
* Vr becomes 2 Vgm , big enough for restriking and arc happens.
* Instantly capacitor C1 discharges and transient voltage at point ‘B’ reaches -4 Vgm , Vr becomes 3 Vgm .
* The arc current quickly reaches its first zero as it varies at natural frequency and once again the two halves of circuit become separated.
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Course : switchgear, Ref: Principal of power system by V. K. Mehta, Prepared by : Md. Aziz ul huq, Faculty, EEE, IUBAT, , [email protected]
M05_ iii. Capacitive current breaking (Cont.)
* About half a cycle later the same phenomenon occurs again and voltage difference across CB, Vr becomes almost 5 Vgm .
* Theoretically this phenomenon should proceed infinitely increasing voltage Vr by 2Vgm at every half cycle rendering a very serious condition.
* But voltage increase limits at 5 Vgm , due to leakage and corona.
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Course : switchgear, Ref: Principal of power system by V. K. Mehta, Prepared by : Md. Aziz ul huq, Faculty, EEE, IUBAT, , [email protected]
M05_ Resistance switching
* Connecting a resistance of suitable value across the contacts of CB is called resistance switching.* Resistance switching minimizes voltage surge across open contacts during current chopping and capacitive current breaking.
* Part of arc current passes through the shunt resistance R, consequently voltage surge is minimized.
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Course : switchgear, Ref: Principal of power system by V. K. Mehta, Prepared by : Md. Aziz ul huq, Faculty, EEE, IUBAT, , [email protected]
M05_ Resistance switching (Cont.)
* With R natural frequency of oscillation reduces and can be calculated with the equation
* The effect of R causes the restriking voltage grow exponentially to recovery voltage .
* The circuit becomes critically damped if the value of R is so chosen that R equals .5(L/C)½ .
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Course : switchgear, Ref: Principal of power system by V. K. Mehta, Prepared by : Md. Aziz ul huq, Faculty, EEE, IUBAT, , [email protected]
M05_ Circuit breaker ratings
* A circuit breaker should be capable to provide service at both normal and faulty condition.
a) Opening fault circuit and consequently breaking fault current.
* During faulty condition a circuit breaker should be capable to provide following three services. They are :
b) Being closed at a virtual fault situation.
c) Carrying fault current for short time.
* Corresponding to the above three duties circuit breakers have following three ratings.
i) Breaking capacity.
ii) Making capacity.
iii) Short-time capacity.
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Course : switchgear, Ref: Principal of power system by V. K. Mehta, Prepared by : Md. Aziz ul huq, Faculty, EEE, IUBAT, , [email protected]
M05_ i) Breaking capacity
* Breaking capacity is defined as the value of current(r.m.s.) a circuit breaker is capable of breaking at given recovery voltage.
* Fault current may contains a little portion of DC current for some while.
* Adjacent picture shows the status of fault current.* Consider, D-D´ is the moment of contact separation.
x = maximum value of a.c. componenty = d.c. component
Symmetrical breaking current = r.m.s. value of a.c. component = x/√2
asymmetrical breaking current = r.m.s. value of total current = ((x/√2)2 + y2) ½
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Course : switchgear, Ref: Principal of power system by V. K. Mehta, Prepared by : Md. Aziz ul huq, Faculty, EEE, IUBAT, , [email protected]
M05_ i) Breaking capacity (cont.)
* It is common practice to express breaking capacity in MVA.
* If I is the rated breaking current in amperes and V is rated line voltage in volts then for a 3-phase circuit,
Breaking capacity = √3 x V x I x 10-6 MVA
* The peak value of current during the first cycle of current wave after the closure of circuit breaker is known as Making capacity.
ii) Making capacity
* Just after contact all the reactive elements of the circuit consumes huge energy generating virtual effect of fault current due to short circuit.
Making capacity = 2.55 x Symmetrical breaking capacity.
2.55 = √2 x 1.8, multiplication with √2 converts r.m.s. value of symmetrical current into peak value.
Multiplication with 1.8 is for doubling effect.
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Course : switchgear, Ref: Principal of power system by V. K. Mehta, Prepared by : Md. Aziz ul huq, Faculty, EEE, IUBAT, , [email protected]
M05_ iii) Short-time capacity
* The period for which circuit breaker can carry fault current safely is known as Short-time capacity.
* Some times in the system fault persists for short time, 1 or 2 seconds, and then removes automatically.* To maintain continuity in power flow, circuit breaker should be capable to carry fault current for some designated time.
* If fault persists after that time, circuit breaker should trip.
* The short-time capacity depends on (a) the electromagnetic force effects (b) temperature rise at contacts.
Normal current rating
* Normal current rating is defined as the r.m.s. value of current the circuit breaker can carry continuously at its rated frequency.
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Course : switchgear, Ref: Principal of power system by V. K. Mehta, Prepared by : Md. Aziz ul huq, Faculty, EEE, IUBAT, , [email protected]
M05_ Some problems
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Course : switchgear, Ref: Principal of power system by V. K. Mehta, Prepared by : Md. Aziz ul huq, Faculty, EEE, IUBAT, , [email protected]
M05_ Some problems (cont.)
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Course : switchgear, Ref: Principal of power system by V. K. Mehta, Prepared by : Md. Aziz ul huq, Faculty, EEE, IUBAT, , [email protected]
M05_ Some problems (cont.)
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Course : switchgear, Ref: Principal of power system by V. K. Mehta, Prepared by : Md. Aziz ul huq, Faculty, EEE, IUBAT, , [email protected]
M05_ Some problems (cont.)
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Course : switchgear, Ref: Principal of power system by V. K. Mehta, Prepared by : Md. Aziz ul huq, Faculty, EEE, IUBAT, , [email protected]
M05_ Damping in oscillation
Generally, damped harmonic oscillators satisfy the second-order : where ω0 is the undamped of the oscillator and ζ is a constant called the .The value of the damping ratio ζ determines the behavior of the system. A damped harmonic oscillator can be:Overdamped (ζ > 1): The system returns () to equilibrium without oscillating. Larger values of the damping ratio ζ return to equilibrium more slowly. Critically damped (ζ = 1): The system returns to equilibrium as quickly as possible without oscillating. This is often desired for the damping of systems such as doors. Underdamped (0 < ζ < 1): The system oscillates (at reduced frequency compared to the undamped case) with the amplitude gradually decreasing to zero. Undamped (ζ = 0): The system oscillates at its natural resonant frequency (ωo).