PREFACE
The interruption of electric power circuits has always been an
essential function, especially in cases of overloads or short
circuits when immediate interruption of the current flow becomes
necessary as a protective measure.
In earliest times, circuits could be broken only by separation
of contacts in air followed by drawing the resulting electric
arc out to such a length that it can no longer be maintained.
This means of interruption soon became inadequate and special
devices called circuit breakers had to be developed.
The
basic problem has been to control and quench or extinguish the
high power arc, which necessarily occurs at the separating
contacts of a breaker when opening high current circuits.
Since arcs generate a great deal of heat energy, most often
destructive for the breaker's contacts, technology had to find
ways to limit the arc duration and develop contacts that can
withstand the arc effect time after time.
In the present article you will find a summarized description of
the different designs of make/break contacts found in the modern
power circuit breakers and the key factors influencing their
architecture and material choice.
INTRODUCTION
Since the beginning, scientists based their research on
exploring the capabilities of the available quenching mediums.
Hence the breakers are classified accordingly.
The arc quenching ambients are numerous.
Historically, it should be mentioned that pure water has had
some use in Europe. The arc produced gases, steam and hydrogen,
which are as effective as the vapor and hydrogen from oil in
quenching the arcs, but insulation problems have limited the use
of this medium and at present no breakers are being built that
use this technique.
For the purpose of the present article, we will limit our quest
to those most popular: Oil, Compressed air, SF6 and Vacuum.
BREAKER CATEGORIES
Breaker
development research has explored the available mediums and has
come up with a lot of breaker designs but most of them fall into
four major categories:
OIL CIRCUIT BREAKER
In oil circuit breakers, the arc is drawn in oil inside a
special compartment of the interrupting chamber called the
explosion pot. The intense heat of the arc decomposes the oil
and produces gases, mainly composed of hydrogen, generating high
pressure that produces a fluid flow through the arc and out of
the explosion pot through vents situated on its walls. Thus
extending the arc’s column and carrying its energy away until
its total extension see Fig 3.
At transmission voltages below 345 kV, oil breakers used to be
popular. They are increasingly losing ground to gas-blast
circuit breakers such as air-blast breakers and SF6 circuit
breakers.
In air-blast circuit breakers, air is compressed to high
pressures. When the contacts part, a blast valve is opened to
discharge the high-pressure air to the ambient, thus creating a
very-high-velocity flow near the arc to dissipate the energy.
SF6
CIRCUIT BREAKER
In SF6 circuit breakers, the same principle is employed, with
SF6 as the medium instead of air. In the “puffer” SF6 breaker,
the motion of the contacts compresses the gas and forces it to
flow through an orifice into the neighborhood of the arc. Both
types of SF6 breakers have been developed for EHV (extra high
voltage) transmission systems.
VACUUM CIRCUIT BREAKER
The vacuum breaker uses the rapid dielectric recovery and high
dielectric strength of vacuum. A pair of contacts is
hermetically sealed in a vacuum envelope.
Actuating motion is transmitted through bellows to the movable
contact. When the contacts are parted, an arc is produced and
supported by metallic vapor boiled from the electrodes. Vapor
particles expand into the vacuum and condense on solid surfaces.
At a natural current zero the vapor particles disappear, and the
arc is extinguished. Vacuum breakers of up to 242 kV have been
built.
MAKE/BREAK CONTACTS
The breaker’s heart is the switching element. It is where the
arc quenching takes place. It mainly contains the make/break
contacts and the interrupting medium. The make/break contacts’
functions can be reduced to:
• Conduct the electric current when the breaker is closed.
• Withstand the arc’s destructive effect while interrupting.
Generally, the make/break contacts have a stationary part and a
moving part. By bringing the moving part to touch the stationary
one, electric current flows and the breaker is closed.
By driving the moving contact away from the stationary contact
the electric arc develops and by quenching it the current stops
flowing and the breaker is open.
Contact design and choice of materials are greatly affected by
the arc's energy, duration and the chemical reactions that may
occur with the ambient medium under the arc’s effect.
To understand these crucial elements, it is necessary to review
the electric arc’s characteristics.
ELECTRIC ARC
The electric arc is a natural phenomenon. Despite its
destructive nature it is of great use to current switching in
circuit breakers. It acts as variable impedance from zero value
when the breaker is closed to infinity when the breaker is open.
HIGH-PRESSURE ARC
Found in blast type circuit breakers (air-blast, SF6 and oil
circuit breakers). Great heat generation and relative long
durations characterize them. They also generate deposition of
solid by-products that may affect the conductivity of the
contacts.
VACUUM ARC
Found in vacuum circuit breakers. They are limited and of short
duration. They cause no deposition of by-products.
CONTACT ARCHITECTURE
FOR HIGH-PRESSURE ARCS:
The contacts have to withstand the arc’s heat without excessive
damage. They also need to have good conduction properties.
Tungsten and tungsten alloys have good resistance-to-arc
properties but less conductivity. Copper and silver have great
conductivity but relatively poor resistance-to-arc properties.
The contacts have also to overcome the deposition of by-products
that may become a problem if not wiped off before an insulating
layer is built.
The type of contacts that are commonly found in these types of
devices are:
SLIDING CONTACTS
As their name indicates, the moving contact and the stationary
contact touch on closing and slide into each other to a certain
distance before stopping at closed position. On opening, they
slide out until their separation and the arc's ignition. The
sliding action helps to wipe off the deposited by-products, to
make better contact on closing.
This type of contact usually separates the resistance-to-arc
role from the current carrying role, by using tungsten alloy
based contacts called arcing contacts that are meant to close
first on closing and separate last on opening and are submitted
to the arc. The current carrying role is attributed to copper or
silver plated copper contacts called main contacts. These
contacts are not subjected to the arc and therefore not eroded
by it.
DESIGN EXAMPLES
Designers worked hard to reach effective designs exploit these
principles.
In the KSO type oil circuit breaker by the General Electric
company, the stationary contact is a ring of sprung copper (or
sliver plated) contact fingers where two of them have a tungsten
tip (see Fig 7).
The moving contact is a solid rod of copper (or silver-plated)
with the upper part made of tungsten (see Fig 7a).
In the Dell-Alsthom, PKV type air-blast circuit breaker, the
stationary contact is a tulip contact made of sliver plated
copper contact fingers where two of these contacts are arcing
contact fingers made of tungsten. The moving contact is a tube
of silver-plated copper with the upper part made of tungsten
(see Fig 8a).
The ABB, DLF Type air-blast circuit breaker uses butt contacts
which are particularly shaped to achieve these goals.
In SF6 ABB, HPL Type circuit breaker, for main contacts the
stationary contact is a tulip contact made of sliver plated
copper contact fingers, the moving contact is silver-plated
copper tube The arcing contact is completely separate from the
main contact. The stationary contact is a tungsten rod and the
moving one is a tulip consisting of tungsten contact fingers.
(see Fig 10a).
All of these contacts use a wiping action when the moving
contact gets inserted in the stationary contact as we see the
traces of this action in Fig 11.
FOR VACUUM ARCS: none of the
above conditions are present. Two plates of conductive material
can constitute the moving and the stationary contacts. Generally
we need to separate them to a short distance (3mm to 20mm).
In a vacuum circuit breaker, vacuum interrupters are used for
breaking and making load and fault currents. When the contacts
in vacuum interrupter separate, the current to be interrupted
initiates a metal vapor arc discharge and flows through the
plasma until the next current zero. The arc is then extinguished
and the conductive metal vapour condenses on the metal surfaces
within a matter of microseconds. As a result the dielectric
strength in the breaker builds up very rapidly.
The properties of a vacuum interrupter depend largely on the
material and form of the contacts.
Over the period of their development, various types of contact
material have been used. At the moment it is accepted that an
oxygen free copper chromium alloy is the best material for high
voltage circuit breakers. In this alloy, chromium is distributed
through copper in the form of fine grains. This material
combines good arc extinguishing characteristic with a reduced
tendency to contact welding and low chopping current when
switching inductive current. The use of this special material is
that the current chopping is limited to 4 to 5 Amps.
At currents under 10kA, the vacuum arc burns as a diffuse
discharge. At high current values the arc changes to a
constricted form with an anode spot. A constricted arc that
remains on one spot for too long can thermally overstress the
contacts to such a degree that the deionization of the contact
zone at current zero can no longer be guaranteed.
To
overcome this problem, the arc root must be made to move over
the contact surface.
In order to achieve this, contacts are so shaped, as in Fig 12,
that the current flow through them results in a magnetic field
being established which is at right angles to the arc axis. This
radial field causes the arc root to rotate rapidly around the
contact resulting in a uniform distribution of the heat over its
surface. Contacts of this type are called radial magnetic field
electrodes and they are used in the majority of circuit breakers
for medium voltage applications.
A new design has come in vacuum interrupters, in which switching
the arc from dif-fusion to constricted state is done by
subjecting the arc to an axial magnetic field. Such a field can
be provided by leading the arc current through a coil suitably
arranged outside the vacuum chamber. Alternatively the field can
be pro-vided by designing the contact to give the required
contact path. Such contacts are called axial magnetic field
electrodes.
This principle has advantages when the short circuit current is
in excess of 31.5 kA.
TESTING
Make/break contacts as presented need to be tested periodically
to assess their condition. These contacts have to keep their
good conductive properties when the contacts are fully closed.
The conductive properties may be affected by the mechanical wear
due to friction on operation or electrical wear caused by the
electrical arc.
CONTACT RESISTANCE MEASUREMENT:
The best method for testing contact resistance is to put in
application Ohm’s law. It consists of applying a current,
usually 100 Amps, and to measure the voltage drop in volts
across the closed contacts. The resistance is then calculated in
dividing the voltage by the current.
The resulting value is read in micro-ohms, 1 micro-ohm= 10-6
ohms.
This measurement is used on all types of contacts.
DYNAMIC CONTACT RESISTANCE:
For some breakers, especially those that use an arcing nozzle to
drive the flow of the quenching medium (puffer type SF6 breakers
for example, see fig 10b), the wear of arcing contacts, if
excessive, may affect the arc quenching capability of the
interrupter, resulting in its destruction.
So the assessment of the arcing contact cannot be done by simply
measuring the contact resistance in the classic way as described
previously. A new method is developed called the dynamic contact
resistance. It consists of measuring the contact resistance as
described above but continuously while the contact is moving
from the first contact touch until the fully closed position.
This method permits to measure the length of the arcing contact
and by comparing it to a value measured when new, helps
determine its state of erosion.
It is important to state that this method does not apply to
butt-type contacts.
SUMMARY
The electric arc plays an important role in the choice of
material and shape of the make/break contacts.
In high-pressure arcs, intense heat is generated, which the
contacts have to resist and keep their conductive properties.
This is achieved by using tungsten alloys and copper or
silver-plated copper and tulip contacts shape.
In vacuum arcs, the choice of materials is crucial to limit
vapor emissions and favor their condensation within microseconds
otherwise the contacts are destroyed. Oxygen-free copper
chromium alloy is the best material for high voltage circuit
breakers and butt contact shape is commonly used.
BIBLIOGRAPHY
The present article is based on our personal experience.
• Power circuit breaker theory and design, edited by C.H.
Flurscheim, revised edition 1982;
• Circuit Interruption theory and techniques, edited by Thomas
E. Browne Jr., edition 1984;
• Contact material, Proc. 9th Int. Conf. Electr. Contact phenom./24th
Holm Conf. Electr. Contacts, IIT, Chicago, September 1978, pp.
81-86.
• JK. Lemelson, The failure of closed heavy current contact
pieces in insulating oil at high temperature, Proc. 6th Int.
Conf. Electric contact phenom., IIT, Chicago, June 1972, pp.
252-258;
• R. Holm and E. Holm, Electric Contacts: Theory and
Application, Springer-Verlag, New York, 1967, pp. 89, 136, 161,
438;
• COMPARISON BETWEEN VACUUM AND SF6 CIRCUIT BREAKER FOR MEDIUM
VOLTAGE on
http://www.panickker.net/article6.htm
• Circuit breaker: Definition and Much More from Answers.com
Science and Technology Encyclopedia.
About the Authors
Stéphan Perron, Hydro-Québec
teacher, High Voltage breaker maintenance Stéphan Perron has
been teaching maintenance and trouble shooting of High Voltage
Circuit Breakers and Thermography for more than 7 years at the
Hydro-Quebec Competence Center based in Saint Antoine des
Laurentides, QC, Canada. He developped his expertise by working
on the maintenance side of the High Voltage Circuit Breakers of
Hydro-Quebec for 18 years. His specialties are the ABB (models
SFE, HPL, ELF and DLF) breakers, the GE (models KSO, AT)
breakers and all the related test instruments, as well as the
sectionnors Joslyn VBM, the handling and behavior of the SF6 gaz
and interpretation of thermographic readings (Level 1) on
breakers. Stéphan Perron holds a DEC degree in Electronics from
the CEGEP St Jérome, QC, Canada
Emile Nasrallah is an
electrical engineer specialized in Power circuit breakers
maintenance. Since graduation in 1984 he worked as a field
engineer. In1990 he joined the worldwide circuit breaker
manufacturer GEC ALSTHOM as a specialized field engineer. In
1997 he became the manager of MV & HV circuit breaker SF6
division of ALSTOM, responsible of technical support,
maintenance and training for SF6 circuit breakers. In 2001 he
became manager of Air blast circuit breaker division for AREVA.
He was in charge of the Air blast (PK and PKV) refurbishing
program in partnership with hydro-Quebec and introduced a unique
administration system for the program (average of 35, 735 kV PK
air blast circuit breaker per year). In 2005 he joined General
Electric Company of Canada as a senior circuit breaker
specialist and is in charge of the circuit breaker division of
the Montreal service centre, responsible of the remanufacturing
program for Oil circuit breakers.
Dr. Fouad Brikci is the
president of Zensol Automation Inc. He was the first to
introduce the concept of truly-computerized test equipment in
the field of circuit breaker analyzers. As a former university
teacher in Ecole Polytechnique — Algiers and CNRS -LAAS
researcher in France, Dr. Brikci has developed experience in the
fields of electronics, automation, and computer science. Most
activities were focused on the industrial application of
computers. Among his achievements are the development of fully
computerized measuring systems for quality control in circuit
breaker manufacturing, laboratories, and maintenance services of
electric utilities. Dr. Brikci holds a PhD in Electronics and a
Master in Sciences in EEA (electronics, electrotechnics, and
automation) from the University of Bordeaux, France.
http://www.zensol.com,
email : zensol@zensol.com
Fouad Brikci, Zensol Automation Inc.
