principles of circuit breakers
TRANSCRIPT
Principles of Circuit Breakers
By D M. Karkare
3/27/2010 BMS Evening College of Engineering, Bangalore
Current Zero Interruption Theories
The recovery rate theory ( Slepian’s theory)
The energy balance theory( Cassie’s theory)
3/27/2010 BMS Evening College of Engineering, Bangalore
Recovery Rate Theory
The rate at which the gap recovers its dielectric
strength, is compared with the rate at which
restriking voltage across the contacts rises.
If the re-striking voltage rises more rapidly
than the dielectric strength, the space breaks
down and the arc persists.
3/27/2010 BMS Evening College of Engineering,
Bangalore
Energy Balance Theory
Immediately after the current zero, the contact space contains
ionized gas and therefore has a finite post-zero resistance.
The rising restriking voltage causes a current to flow between the
contacts so that power is dissipated in the contact space.
Initially when the restriking voltage is zero, the power is zero and it
is again zero when the space has become fully de-ionized and the
resistance is infinitely high; between these two limits the power
dissipated rises to maximum.
3/27/2010 BMS Evening College of Engineering, Bangalore
Contd…. If the heat so generated exceeds the rate at which heat can be
dissipated from the contact space; ionization will persist and breakdown will occur, giving an arc for another half cycle.
3/27/2010 BMS Evening College of Engineering, Bangalore
Contd…. A rapid increase of dielectric strength or post-zero resistance is necessary
for successful interruption and this can be achieved by: Lengthening of the arc Cooling Blast Effect.
Dielectric strength or post-zero resistance is proportional to the length of the gap so that lengthening by rapid opening of the contacts is obvious procedure. Natural combination of ions and electrons takes place more rapidly if they are allowed to cool. Cool non-ionized gas or liquid can be forced into the contact space thereby cooling the ions and electrons and sweeping them away. These methods are applied in oil and air-blast breakers.
3/27/2010 BMS Evening College of Engineering, Bangalore
Recovery Voltage
The normal frequency r.m.s voltage that appears
across the breaker poles after final arc extinction has
occurred, is termed as the recovery voltage.
The instantaneous value of the recovery voltage at the
instant of arc extinction is called is called the active
recovery voltage.
3/27/2010 BMS Evening College of Engineering,
Bangalore
Contd….
The active recovery voltage depends on the following
factors:
The power factor
The armature reaction
The circuit conditions.
3/27/2010 BMS Evening College of Engineering, Bangalore
Effect of Power Factor on recovery voltage
With low p.f, i.e when the ratio of reactance to resistance of
the circuit is high, the active recovery voltage will be high.
With High p.f, i.e when the resistance is high as compared to
the reactance, the active recovery voltage would be
considerably lower.
In general the active recovery voltage equals the maximum
value of the system voltage multiplied by sin , where is
power factor angle.
3/27/2010 BMS Evening College of Engineering, Bangalore
Effect of armature reaction on recovery voltage
The recovery voltage is less than the normal system
voltage because of demagnetizing effect of armature
reaction.
The fault currents flowing in the generator windings
are of lagging power factor.
They have demagnetizing armature reaction. Hence
they reduce the terminal voltage 3/27/2010 BMS Evening College of Engineering,
Bangalore
Effect of circuit conditions on recovery voltage
Three phase faults that are insulated from
earth, either at neutral or at fault, produce
recovery voltages in the first phase to clear
which are normally more severe than those
produced by single-phase or three phase faults
on systems with earthed neutrals, 3/27/2010 BMS Evening College of Engineering,
Bangalore
Restriking Voltage
• The transient voltage that appears across the contacts
at the instant of arc extinction is called the re-striking
voltage.
3/27/2010 BMS Evening College of Engineering, Bangalore
3/27/2010 BMS Evening College of Engineering, Bangalore