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| The
circuit breaker |
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The
circuit breaker is the most important and complicated
of all types of power circuit interruption equipment.
This
is due to its highly important capability of interrupting
the powerful short circuit current, over and above its normal
role of conducting, isolating and interrupting nominal load
currents. |
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2.1
General description
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Its
structure can be divided to three major parts: |
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2.1.1
Power circuit
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It
is where the main current flows or is interrupted; and
it includes: |
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2.1.1.1
Arcing chamber
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The
arcing chamber is a closed volume containing a fixed contact,
a moving contact and the interrupting medium. The current
is established when the moving contact touches the fixed
contact and interrupted when they part.
An
arc is created when the contacts part. The interrupting
medium is responsible for quenching the arc and establishing
the nominal level of isolation between the open contacts.
Several
chambers may be connected in series to serve higher voltage
levels; in this case a grading capacitor is installed
in parallel with each chamber to balance the voltage across
the contacts when parting. |
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2.1.1.2
Insertion resistor
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The
sudden modification of circuit characteristics, when circuit
breakers operate, produces peak voltage impulses where
the level is determined by the circuit characteristics.
These impulses may reach very high levels and must be
reduced.
A
well-known method is closing or opening in two or three
steps on resistors. |
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| 2.1.1.2.1
On Trip |
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voltage impulse levels are generally acceptable when interrupting
nominal or short circuit currents, but they can be dangerously
high when interrupting small capacitive or inductive currents.
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| 2.1.1.2.2
On close |
| Sudden
energizing of a circuit always generates voltage impulses
with usually moderate levels, with the exception of closing
or reclosing on long unloaded lines where the impulses,
function of the line length, of the instant of closing or
reclosing and of the discrepancy of the three poles, can
reach extremely high levels. |
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2.1.1.2.3
Description
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A
resistor with a predetermined value is placed in series
with an auxiliary contact. Both are installed in parallel
with the main arcing chamber.
The
auxiliary switch is programmed to close few milliseconds
before the main contacts on closing and to open few milliseconds
after the opening of the main contacts on tripping. The
programmed delay is called the insertion time. |
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2.1.2
Operating mechanism
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It
is where the needed energy to part the contacts and to
extinguish the arc is developed.
It
includes devices, called energy accumulators, to store
the needed energy.
Examples
of accumulators are:
- Springs
- Nitrogen-charged cylinders
The
most common operating mechanisms in circuit breakers are:
- Spring operated
- Hydraulically operated
- Pneumatically operated |
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2.1.3
Control
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The
order to operate the breaker is launched in the control
part of the circuit breaker, as an electric impulse of
a fraction of a second duration. The order is then amplified
in the operating mechanism to a complete circuit breaker
operation capable of interrupting short circuit currents.
The
control includes:
- Closing and tripping coils
- Control relaying system
- Pressure switches and gauges
- Surveillance and alarm system
- Re-inflating system to restore the energy spent on the
operation. |
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2.2
Functioning characteristics
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The
circuit breaker has special functioning characteristics.
It is normally either closed or open for long periods
of time, is asked to change state on occasion and rarely
sees a short circuit current. |
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2.2.1
Reliability
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It
needs to change state efficiently after long periods of
inactivity. |
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2.2.2
Correct function
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The
circuit breaker control must ensure correct closing action,
whatever the closing current value, and ensure breaking
(opening) at the required moment by releasing, by mechanical
action or via a relay, the energy stored in the accumulators.
This
energy has to face opposing forces when closing or breaking
circuits under load or not, and even stronger forces with
short circuit currents.
This
means an excess of energy, when operated without load,
has to be damped with a proper damping system. |
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2.2.3
Operation cycles
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The
circuit breaker has to be capable of executing different
operation cycles and achieve fast breaking of short circuit
currents, the faster the better for the network. Recent
progress has reduced the response time from 5 to 3 cycles,
and down to 2 cycles. It is already planned to have response
times of 1 cycle.
Operation
has to be reliable, robust and easy to maintain. |
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2.3
Circuit breaker types
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The
main problem of circuit breakers stems from the nature
of their existence.
A
circuit breaker has to interrupt weak capacitive or inductive
currents, up to high short circuit currents, and as a
result, to extinguish powerful electric arcs.
The
problem is then, essentially, an arcing problem.
Another
problem is overvoltage impulses; this is related to the
nature of the circuit where it is installed.
One
of the major factors influencing the capacity of circuit
breakers is the interrupting medium. It affects circuit
breakers' concept and design.
By
this principle, circuit breakers are classified in families
according to the type of interrupting medium used. |
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2.3.1
Interrupting medium
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A
good number of substances have acceptable qualities to
be interrupting mediums.
Three
of them are widely preferred by circuit breaker designers
around the world. This is due to their excellent breaking
and insulating properties that lead to high performance
and economic designs. They are:
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Mineral oil
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Compressed air
-
Sulfur hexafluoride, or SF6 |
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2.3.1.1
Mineral oil
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Mineral
oil was, until recently, the interrupting medium of choice.
It
has excellent breaking and insulating quality especially
when it is very pure, as is the case when it is used in
certain devices such as capacitors or transformers, which
are airtight devices.
However,
circuit breakers have breathing holes and the oil is in
contact with the arc. Thus, one finds in the breaker's
oil a certain amount of impurities, in the form of moisture
and miscellaneous dust, including carbon particles. This
decreases its isolation properties significantly.
It
is imperative to monitor the state of the oil inside breakers
in service, and to replace it periodically in function
of the number of breaks performed by the device.
The
criteria for oil replacement depend on the structure of
the breakers and are indicated by the manufacturer. |
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2.3.1.2
Compressed air
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Air at atmospheric pressure has the following advantages:
- Good insulation quality
- Always available
- Costs nothing
The
insulating quality of air rises rapidly with its pressure.
In
practice we can count on a disruption voltage of up to
90 kV between contacts separated by 1 cm at 10 bars pressure,
and 1.5 times this value for the same distance at 20 bars
pressure.
Compressed
air was mainly used for interruption in the earlier pneumatic
circuit breaker designs. Later on it was used for insulation
between the contacts after they opened, the latter being
placed inside an insulating chamber designed to resist
the air pressure. This reduced significantly the distance
between the open contacts.
Air
quality for pneumatic circuit breakers:
It
should be noted that the excellent quality of air is greatly
affected by the humidity. Indeed, it is important that
any condensation in the insulators and air conduits be
avoided, or internal tripping may occur. Installing the
costly drying compression stations greatly raises the
cost of operating air blast circuit breakers. |
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2.3.1.3
Sulfur hexafluoride, SF6
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A certain number of gases, called electronegative, have
better insulating qualities than air. Among them is sulfur
hexafluoride, SF6, has seen a great deal of success in
electrical apparatus design because of its excellent insulating
properties and remarkable arc quenching abilities.
It
is five times heavier than air, odorless, colorless, nonflammable
and non-toxic when new. Its dielectric strength is 3 times
the air's dielectric.
When
subjected to an electric arc, it partially decomposes.
In the presence of moisture and impurities it produces
acid by-products that attack metal and the insulating
envelopes. An efficient way to reduce by-products is to
use activated alumina inside the chambers containing the
gas.
SF6
being a gas at normal temperatures, and at atmospheric
pressure it liquefies at -60 °C, and at 20 bars
it liquefies at 20 °C, which is detrimental to its insulating
qualities. For applications at very cold temperatures,
it must be heated or mixed with other gases like Nitrogen
or CF4. |
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2.3.2
Oil circuit breaker
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The
first high voltage breakers were the bulk oil circuit
breakers, followed by the minimum oil circuit breakers.
In
an oil circuit breaker, the arc decomposes part of the
oil into gases composed of 70% Hydrogen and 20% Acetylene,
and also produces carbon particles. |
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| 2.3.2.1
Bulk oil circuit breaker |
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It
consists of a steel tank partly filled with oil, through
the cover of which are mounted porcelain or composite
insulating bushings.
Contacts
at the bottom of the bushings are bridged by a conducting
cross head carried by a wooden or composite lift rod,
which in common designs drops by gravity following contact
separation by spring action, thus opening the breaker.
An
air cushion above the oil level serves as an expansion
volume to prevent pressure from building up inside the
chamber after the interruption of the short circuit current.
Regardless
of improvements, the bulk oil circuit breaker presents
many disadvantages:
- Great weight and bulk
- Risk of fire
- Strong reaction to ground
- Frequent bushing failure, etc. |
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2.3.2.2
Minimum oil circuit breaker
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These
breakers were developed for 170 and 245 kV systems, the
highest voltages at the time, where the inherent problems
of bulk oil breakers were the most severe, also to eliminate
oil as insulating medium and thus reduce the quantity
of oil in switchgear installations to an amount that would
not cause any hazard. The excellent arc-quenching properties
of oil, however, were used later in specially developed
oil- and pressure-tight arc-interrupting chambers.
Minimum
oil circuit breakers for high voltage are single-interrupter
up to 170 kV and multiple-interrupter breakers for 230
kV and higher.
Contacts
are placed in a cylindrical, insulating envelope, with
connection terminals at either end, and placed on an insulating
support.
Compared
to a bulk oil breaker, ground isolation is considerably
improved by the elimination of the vulnerable isolating
bushings and of the metal tank in proximity of the arc.
The oil no longer insulates to ground, and oil volume
is reduced by a factor of 10 to 20.
They
use an arc-control device in the arcing chamber which
physically shortens the arc and the arcing time, thereby
reducing the arc energy.
When
the breaker interrupts small currents, the arc is extinguished
by a forced axial flow of oil. In short-circuit range
the arc is quenched as a function of current. The arc
is blown by a jet of oil at right angles to its axis,
and extinguished.
Most
of minimum oil circuit breakers are designed for fast
reclose. They have to be able to interrupt short circuit
currents twice in a row with 0.2 sec to 0.3-sec interval,
therefore the arc-control device has to contain enough
oil to succeed in performing the second interruption. |
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2.3.3
Air blast circuit breaker
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Until
recently, the air blast circuit breakers have dominated
the high and very high voltage applications. From 170
kV to 800 kV and breaking capacity from 20 kA to 100 kA.
Over
100 kV the breaker has multiple chambers connected in
series. Each element is optimized to around 80 kV. At
first, 800 kV breakers had 12 chambers in series per phase,
now they have only 8 chambers per phase.
Although
increasing the air pressure increases the speed of dielectric
regeneration, it is still relatively slow. Insertion resistors
are often used to reduce voltage surges.
Air
blast circuit breakers adapt well to the high-voltage
power network's new demands.
For
example, adding insertion resistors, single or double
step, to reduce closing voltage surges, is easily done.
Also, it is capable of achieving very fast breaking times,
2 cycles and even less, improving the network's
stability.
In
general, air blast circuit breakers are high tech and
robust equipment, with great electrical and mechanical
endurance. Contact wear is low due to the short arc duration
and low arc voltages.
The
compressed air circuit breaker has two major disadvantages:
- Installation of expensive compression stations
- High noise levels on operation |
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| 2.3.4
SF6 Circuit breaker
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Sulfur
hexafluoride (SF6) gas has proved to be an excellent arc-quenching
and insulation medium for circuit breakers.
SF6
breakers are available for all voltages ranging from 14.4
kV to 800 kV, continuous current up to 4000 A, and symmetric
interrupting ratings up to 63 kA.
SF6
circuit breakers are either the dead tank design, or the
live tank design and the GIS design.
During
recent years SF6 circuit breakers have reached a high
degree of reliability, and they cope with all known switching
phenomena.
Their
completely closed gas system eliminates any exhaust during
switching operations and thus adapts to environmental
requirements. They may be installed horizontally or vertically,
according to the structural requirements of the substation.
The quick dielectric regeneration of the arc plasma in
SF6 makes insertion resistors unnecessary, simplifying
the apparatus.
Their
compact design considerably reduces space requirements
and building and installation costs. In addition, SF6
circuit breakers require very little maintenance. |
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| Conclusion |
An accurate analysis makes it possible to make decisions
that are profitable to the breaker, the network and to
the maintenance personnel. In order to achieve this, knowing
the timing machine and the significance of the operating
times is important but not enough.
Knowing
well the breaker itself, the reference values (timing
chart) and the network characteristics is necessary.
All
of this backed with the experience and sense of judgment
of the testing personnel. |
