- The function of a circuit breaker is to isolate the faulty part of the power system in case of abnormal conditions.
- A protective relay detects abnormal conditions and sends a tripping signal to the circuit breaker.
- A circuit breaker has two contacts :
- Fixed contact
- Moving contact
- Under normal conditions these two contacts remain in closed position.
- When the circuit breaker is required to isolate the faulty part, the moving contact moves to interrupt the circuit. On the separation of the contacts, the flow of current is interrupted, resulting in the formation of arc between the contacts.
- The voltage drop across the arc is called arc voltage.
- As the are path is purely resistive, the arc voltage is in phase with the arc current. the magnitude of the arc voltage is very low.
High Resistance Interruption
- In this method of arc interruption, its resistance is increased so as to reduce the current of a value insufficient to maintain the arc.
- The arc resistance can be increased by cooling, lengthening, constraining and splitting the arc.
- When current is interrupted the energy associated with its magnetic field appears in the form of electrostatic energy. A high voltage appears across the contacts of the circuit breaker.
- If this voltage is very high and more than the withstanding capacity of the gap between the contacts, the arc will strike again.
Remember: High res istance int errup tion m ethod is employed for both ac and dc circuit breakers.
Current Zero Interruption
- This method is applicable only in case of ac circuit breakers.
- In case of ac supply, the current wave passes through a zero point. This feature of ac is utilised for arc interruption.
- The current is not interrupted at any point other than the zero current instant, otherwise a high transient voltage will occur across the contact gap. The current is not allowed to rise again after a zero current occurs.
- There are two methods of zero current interrupting arc.
Recovery Rate Theory
- The arc is a column of ionised gases. To extinguish the arc, the electrons and ions are to be removed from the gap immediately after the current reaches a natural zero.
- In this method, the rate at which the gap recovers its dielectric strength is compared with the rate at which the restriking voltage (transient voltage) across the gap rises.
Remember: The arc will extinguished if the dielectric strength increases more rapidly than the restriking voltage.
Energy Balance Theory
- The space between the contacts contains some ionised gas immediately after current zero and hence, it has a finite post-zero resistance. At the current zero moment, power is zero because restriking voltage is zero. When the arc is finally extinguished, the power again becomes zero, the gap is fully de-ionised and its resistance is infinitely high.
- If the rate of removal of heat is faster than the rate of heat generation the arc is extinguished.
Restriking Voltage The transient voltage which appears across the breaker contacts at the instant of arc being extinguished is known as restriking voltage.
Recovery Voltage The power frequency rms voltage, which appears across the breaker contacts after the arc is finally extinguished and transient oscillations die out is called recovery voltage.
Expression for Restriking Voltage and RRV
- Figure shows a short circuit on a feeder beyond the location of the circuit breaker.
- W her e L a nd C ar e t he ind uc t ance and capacitance per phase of the system up to that point of circuit breaker location
- When the circuit breaker is closed, the short circuit current flows through R, L and the contacts of the circuit breaker, the capacitance C being shotcircuited by the fault.
- When the circuit breaker contacts are opened and the arc is extinguished, the current i is diverted through the capacitance C, resulting in a transient condition.
- The inductance and the capacitance form a series oscillatory circuit.
- The natural frequency of oscillation is
- Voltage across the capacitor or restriking voltage
- The maximum value of restriking voltage = 2Epeak = 2 × peak value of the system voltage
- The Rate Rise of Restriking Voltage (RRRV)
- The maximum value of RRRV (RRRV)max = wn Epeak
- To reduce the restriking voltage, RRRV and severity of the transient oscillations, a resistance is connected across the contacts of the circuit breaker. This is known as resistance switching.
- The resistance is in parallel with the arc. A part of the arc current flows through this resistance resulting in a decrease in the arc current and increase in the deionization of the arc path and resistance of the arc
- The resistance switching helps in switching out capacitive current or low inductive current.
- The voltage equation in s-domain
- The capacitor voltage in s-domain
- The frequency of damped oscillation
- If the value of the resistance connected across the contacts of the circuit breaker is equal to or less than
there will to be notransient oscillation.
- If R > there will be oscillation.
- R = is known as critical resistance.
- When low inductive current is being interrupted and the arc quenching force of the circuit breaker is more than necessary to interrupt a low magnitude of current, the current will be interrupted before its natural zero instant.
- In such a situation, the energy stored in the magnetic field appears in the form of high voltage across the stray capacitance, which will cause restriking of the arc.
- The energy stored in the magnetic field is
if i is the instantaneous value of the current which is interrupted. This will appear in the form of electrostatic energy equal to
These two energies will be equal
Interruption of Capacitive Current
- The interruption of capacitive current produces high voltage transients across the gap of the circuit breaker. This occurs when an unloaded long transmission line or a capacitor bank is switched.
- C represents stray capacitance of the circuit breaker. CL represents line capacitance. The value of CL is much more than C. Figure shows transient voltage across the gap of the circuit breaker when capacitive current is interrupted.
- At the instant M, the capacitive current is zero and the system voltage is maximu. If an interruption occurs, the capacitor CL remains charged at the maximum value of the system voltage. After instant M, the voltage across the breaker gap is the difference of VC and VCL.
- At instant N, i.e., half-cycle from A, the voltage across the gap is twice the maximum value of VC.
At this moment, the breaker may restrike. If the arc restrikes, the voltage across the gap becomes practically zero.
- The voltage across the gap falls from 2Vcmax to zero. A severe high frequency oscillation occurs.
The voltage oscillates about point S between R and N, i.e. between –3emax and emax.
- When restriking current reaches zero, the arc may be interrupted again. At this stage, the capacitor CL remains charged at the voltage –3emax.
- There is a problem of high transient voltage while interrupting a capactivie current.
Classification of Circuit Breakers
- Depending on the arc quenching medium employed, the following arc important types of circuit breakers:
- Oil circuit breakers
- Air blast circuit breakers
- Sulphur hexafluoride (SF6) circuit breakers
- Vacuum circuit breakers
Oil Circuit Breakers
- Mineral oil has better insulating properties than air.
- Oil has also good cooling property.
- When are is formed, it decomposed oil into gases.
Hence, the arc energy is absorbed in decomposing the oil.
- The main disadvantage of oil is that is inflammable and may pose a fire hazard.
- The possibility of forming explosive mixture with air and the production of carbon particles in the oil due to heating which reduces its dielectric strength.
Plain-Break Oil Circuit Breakers
- Figure shows a double break plain oil circuit breaker. When contacts separate there is a severe arch which decomposes the oil into gases. The gas obtained from the oil is mainly hydrogen.
- The oil is pushed away from arc and the gaseous medium surrounds the arc.
- The arc quenching factors are as follows :
- Elongation of the arc.
- Formation o gaseous medium in between the fixed and moving contacts. This has a high heat conductivity and high dielectric strength.
- Turbulent motion of the oil, resulting from the gases passing through it.
- A sufficient level of oil have the contacts is required to provide substantial oil pressure at the arc.
- Certain gap between the contacts must be created before the arc interruption occurs.
- The two breaks in series provide rapid arc elongation without the need for a specially fast contact.
- This is arrangement has the disadvantage or unequal voltage distribution across the breaks.
- The plain-break circuit breakers are employed for breaking of low current at comparatively lower voltages. They are used on low voltage d.c. circuits and on low voltage a.c. distribution circuits.
- They require large amount of transformer oil. The are not suitable for auto reclosing. Their speed is slow. They can be used up to 11kV with an interrupting capacity up to 250 MVA.
- The advantages of air blast circuit breakers over oil circuit breakers :
- Cheapnes s and free availability of the interrupting medium, chemical stability and inertness of air.
- High speed operation.
- elimination of fire hazard.
- S hor t and cons iste nt a r cing ti me and therefore, less burning of contacts.
- Less maintenance.
- Suitability for frequency operation.
- Facility for high speed reclosure.
- The disadvantage of an air blast circuit breaker :
- An air compressor plant has to be installed and maintained.
- Upon arc interruption the air blast circuit breaker produces a high-level noise when air is discharged to the open atmosphere.
- Problem of current chopping.
- Problem of restriking voltage.
- Switching resistors and equalising capacitors are generally connected across the interrupters.
- The s witching r esis tor sr ed uce t ransient overvoltages and help arc interruption.
- Capacitors are employed to equalise the voltage across the breaks.
- The number of breaks depends upon the system voltage.
SF6 Circuit Breakers
- Sulphur hexafluoride (SF6) has good dielectric strength and excellent arc quenching property.
- It is an inert nontoxic, nonflammable and heavy gas. At atmospheric pressure, its dielectric strength is about 2.5 times that of air.
- One major disadvantage of SF6 is its condensation at low temperature. The temperature at which SF6 changes to liquid depends on the pressure.
- SF6 circuit breakers arc manufactured in the voltage range 3.6 kV to 765 kV. however, they are preferred for voltages 132 kV and above.
- The dielectric strength of SF6 gas increases rapidly after final current zero.
- SF6 circuit breakers can withstad sever RRRV and are capable of breaking capacitive current without restriking.
- Problems of current chopping is minimised.
- Electrical clearances are very much reduced due to high dielectric strength of SF6.
Puffer-Type SF6 Circuit Breakers
This type of circuit breakers are also impulse type SF6 circuit breakers.
- This type is available in the voltage range 3.6 kV to 765 kV.
- The dielectric strength and arc interrupting ability of high vacuum is superior to those of porcelain, oil, are and SF6 at atmospheric pressure.
- SF6 at 7 atm. pressure and air at 25 atm. pressure ave dielectric strengths higher than that of high vacuum.
- The pressure of 10–5 mm of mercury and below is considered to be high vacuum.
- In high vacuum, of the order of 10–5 mm of mercury, the mean free path of the residual gas molecules becomes very large. Therefore, when contacts are separated, an electron travels in the gap without collision. The formation of arc in high vacuum is not possible due to the formation of electron avalanche.
- The breakdown strength is independent of gas density. It depends only on the gap length and surface condition and the material of the electrode.
- Power required to close and open its contacts is much less compared to other types of breaker.
- It is capable of interrupting capacitive and smallinductive currents, without producing excessive transient overvoltages. Vaccum circuit breakers have other advantages like suitability for repeated operations, least maintenance, silent operation, long life, high speed of dielectric recovery.
- Vacuum circuit breakers have now become popular for voltage ratings up to 36 kV. Up to 36 kV they employ a single interrupter.
Rating of Circuit Breakers
- A circuit breaker has to perform the following major duties under short-circuit conditions.
- To open the contacts to clear the fault.
- To close the contacts onto a fault.
- To carry fault current for a short time while another circuit breaker is clearing the fault.
- Therefore, in addition to the rated voltage, current and frequency, circuit breakers have the following important ratings.
- Breaking capacity
- Making capacity
- Short-time capacity
- The breaking capacity of a circuit breaker is of two types :
- Symmetrical breaking capacity
- Asymmetrical breaking capacity
Symmetrical Breaking Capacity
- It is the rms value of the ac component of the fault current that the circuit breaker is capable of breaking under specified conditions of recovery voltage.
Asymmetrical Breaking Capacity
- It is the rms value of the total current comprising of both ac and dc components of the fault current that the circuit breaker can break under specified conditions of recovery voltage.
Asymmetrical breaking current.
- The breaking capacity of a circuit breaker is generally expressed in MVA. For a three-phase circuit breaker.
- Breaking capacity =√3 rated voltage in kV × rated current in kA Making Capacity
- The rated making current is defined as the peak value of the current (including the dc component) in the first cycle at which a circuit breaker can be closed onto a short-circuit. IP in fig. is the making current.
- The capacity of a circuit breaker to be closed onto a short-circuit depends upon its ability to withstand the effects of electromagnetic forces. Making current = √2 '1.8´ symmetrical breaking current
- The multiplication by √2 is to obtain the peak value and again by 1.8 to take the dc component onto account.
= √2 ´ 1.8´ symmetrical breaking capacity
= 2.55 × symmetrical breaking capacity.