ESSENTIAL
QUALITIES OF HV CIRCUIT BREAKER
High-voltage circuit breaker
play an important role in transmission and distribution systems. They must
clear faults and isolate faulted sections rapidly and reliably. In-short they
must possess the following qualities.
" In closed position they
are good conductors.
" In open position they
are excellent insulators.
" They can close a shorted
circuit quickly and safely without unacceptable contact erosion.
" They can interrupt a
rated short-circuit current or lower current quickly without generating an
abnormal voltage.
The only physical mechanism
that can change in a short period of time from a conducting to insulating state
at a certain voltage is the arc.
HISTORY
The first circuit breaker was
developed by J.N. Kelman in 1901. It was the predecessor of the oil circuit
breaker and capable of interrupting a short circuit current of 200 to 300
Ampere in a 40KV system. The circuit breaker was made up of two wooden barrels
containing a mixture of oil and water in which the contacts were immersed.
Since then the circuit breaker design has undergone a remarkable development.
Now a days one pole of circuit breaker is capable of interrupting 63 KA in a
550 KV network with SF6 gas as the arc quenching medium.
THE NEED FOR
TESTING
Almost all people have
experienced the effects of protective devices operating properly. When an overload
or a short circuit occurs in the home, the usual result is a blown fuse or a
tripped circuit breaker. Fortunately few have the misfortune to see the results
of a defective device, which may include burned wiring, fires, explosions, and
electrical shock.
It is often assumed that the
fuses and circuit breakers in the home or industry are infallible, and will
operate safely when called upon to do so ten, twenty, or more years after their
installation. In the case of fuses, this may be a safe assumption, because a
defective fuse usually blows too quickly, causing premature opening of the
circuit, and forcing replacement of the faulty component. Circuit breakers,
however, are mechanical devices, which are subject to deterioration due to
wear, corrosion and environmental contamination, any of which could cause the
device to remain closed during a fault condition. At the very least, the
specified time delay may have shifted so much that proper protection is no
longer afforded to devices on the circuit, or improper coordination causes a
main circuit breaker or fuse to open in an inconvenient location.
TESTING OF
CIRCUIT BREAKERS
The design of circuit breaker
is not only a science but also an art. Because of the complex phenomena
involved, circuit breaker prototypes have to be verified by practical tests in
the laboratory. There are two types tests of circuit breakers, namely Routine
tests and Type tests. Routine test are performed on every piece of circuit
breaker in the premises of the manufacturer. The purpose of the routine test is
to confirm the proper functioning of a circuit breaker. Type tests are
performed in a high voltage laboratory; such tests are performed on sample
pieces of circuit breaker of each type to confirm their characteristics and
rated capacities according to their design. These tests are not performed on
every piece of the circuit breaker. All routine and type tests are performed
according to Indian Standard (IS) codes, or International Electromechanical
Commission (IEC) codes or British Standard (BS) codes.
In High-power laboratories the
ability of the circuit breakers to interrupt the circuit currents is verified
in test circuits which is infact the lumped element representation of the power
system. These test circuits must produce the corrected forms of the short
circuit current as well as the final voltage that strikes the circuit breaker
immediately after the breaker has interrupted the test current. The forms of
voltage and current to which the test object must be subjected are laid down in
ANSI and International Electromechanical commission (IEC) standards. These
standardized waveform represent 90% of the possible conditions in the real
system.
CIRCUIT
BREAKER SWITCHING AND ARC MODELLING
The switching action, the basic
function of the circuit breaker refers to the change from conductor to
insulator at a certain voltage. Before interruption, the short circuit flows
through the circuit breaker arc channel. Because of non zero resistance of the
channel , the short circuit current causes a voltage across the contacts of the
circuit breaker; the arc voltage. The arc behaves as a non-linear resistance.
Thus both are voltage and arc current cross the zero value at the same time
instant. If the arc is cooled at the time current goes through zero the circuit
breaker interrupts the current because the electrical power input is zero.
During current interruption, the arc resistance increases practically from zero
to almost infinity in microseconds. Immediately after current interruption, the
transient recovery voltage builds up across the circuit breaker .As the gas
mixture in the interelectrode space does not change to a completely insulating
state instantaneously, the arc resistance is finite at that time and a small
current can flow ;the post –arc current.
Black box arc models are
mathematical description of the electrical properties of the arc .This type of
model does not simulate the complicated physical processes inside the circuit
breaker but describes the electrical properties of the circuit breaker.
Measured voltage and current traces are used to extract the parameters for the
differential equations describing the nonlinear resistance of the electrical
arc for that specific measurement.
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