Field of the Invention
The present invention relates to an improved insulated arcing chamber assembly
for SF6 gas circuit breakers. In particular it relates to an improved insulated
arcing chamber with integrated shielding electrodes.
Up to 245kV single- break circuit breaker designs are common, beyond this
rating two or more breaks are used in series to form a circuit breaker. The
multiple breaks demand higher operating energy, voltage equalizing devices etc
impacting circuit breaker reliability.
Background of the Invention
A Circuit Breaker (CB) is primarily used to interrupt abnormal currents in a
transmission/ distribution system. When the CB is in closed condition it allows
flow of electric current through a closed electrical system (circuit) to the load.
The magnitude of load current is governed by the system characteristics and
state. Short circuits, causing abnormal flow of current, are sensed by current
sensors and prevented by isolating the source and the load by the circuit
breaker.

When fault current is interrupted by an interrupter, arc is struck between the
arcing contacts. The energy content of the arc depends on the current
magnitude, length of the arc and similar other parameters. As the temperature of
the arc is quite high it decomposes the insulating medium (gas) and materials
exposed to arc. Byproducts of the chemical reaction at elevated temperature also
accumulate in the vicinity destabilize insulation and shall be removed for
sustaining the dielectric properties of the inter-electrode gap for subsequent
interruptions. The circuit breaker experiences electrostatic stresses during
interruption operation, the intensity if exceeded results in failure of insulation
leading to undesirable performance of CB.
In interrupters one of the two contacts is fixed, during interruption the moving
contact is driven by operating mechanism and an arc is struck on contact
separation. At current-zero the arc is extinguished naturally exposing the
developed inter-electrode gap to system and transient recovery voltages. The
gap reignites or the arc is re-struck should the gap fail to withstand these
voltages. For mechanical constraints limited inter-electrode gaps only can be
generated in conventional interrupters, necessitating use of multiple breaks for
EHV circuit breakers. Up to 245kV single- break circuit breaker designs are
common, beyond this rating two or more breaks are used in series to form a
circuit breaker. The multiple breaks require voltage equalizing devices like
grading capacitors etc, affecting circuit breaker reliability and cost.

In general, to limit the voltage appearing across the contacts during interruption,
multiple breaks are preferred. The multiple break system is operated by single
drive which requires higher operating energy. The electrostatic field, between
movable and fixed contacts, is non-uniform and time-varying for various reasons
like electrode profile and relative position of contacts. The field intensification
adversely affects voltage withstanding capabilities of the insulating gas gap. The
availability of hot gas further prevents successful interruption.
For successful interruption, the primary design requirements are: sufficient inter-
electrode gap; optimal dielectric properties of the gas and field uniformity in
inter-electrode gap. In some of the existing designs, the contact system is
arranged in metallic enclosure without arcing chamber assembly. This
arrangement may cause premature gas breakdowns during current interruption
as the arced gas is not contained and may directly make contact with metallic
enclosure. In conventional designs of insulated arcing chamber, when contacts
are in open condition, the electrostatic field across the insulator may be close to
uniform. In the absence of shields integrated to insulated arcing chamber the
surface stress can not be controlled to the extent of requirement by means of
arcing and current carrying contact shields. Such an arcing chamber is disclosed
4, 937,406. However, during opening operation (that is from closed condition to
open condition) the arcing contacts are out of their shields and the electrostatic
stress across the contact system and insulated arcing chamber increases
enormously and may cause flashover at lower recovery voltages.

There is therefore a need for an insulated arcing chamber with integrated shields
keep the electrostatic field across the arcing contacts, current carrying contacts
and on the surface of insulated arcing chamber all the time close to uniform
irrespective of moving contact position.
Summary of the Invention
The main object of the present invention is therefore, to provide an improved
insulated arcing chamber assembly for SF6 gas circuit breaker with integrated
shielding electrodes for improving the interrupter, performance in terms of
capacity and interruptions.
Another object of the invention is to control post-arc field intensity by means of
arcing chamber and integrated shielding electrodes.
Yet another object is to specify profile of integrated shielding electrodes.
One more objective is to ensure safe electrostatic field intensity across contact
system and insulated arcing chamber during all instants of contact separation.

In a preferred embodiment an improved arcing chamber assembly for SF6 gas
circuit breakers comprising a first HT shield or shielding electrode (11), and a
second HT shield or shielding electrode (12) integrated to said insulated arcing
chamber (10); wherein a dynamic field electrode (7) is provided between a first
movable contact assembly (1) and a static current carrying contact shield (9),
thereby improving electrostatic filed between the moving contact system and the
static current carrying contact assembly (8, 9) of said circuit breaker.
Brief Description of the Accompanying Drawings:
The invention is described with the help of Figures 1 to 6, where:
Figures 1(a) and (b): show conventional interrupters without and with
insulated arcing chamber.
Figure 2: shows contact system of dual motion interrupter.
Figure 3: shows contact system and insulated arcing chamber assembly.
Figure 4: shows insulated arcing chamber assembly of the present invention.

Figure 5: shows support insulator assembly of the present invention.
Figure 6: shows gas circuit breaker with insulated arcing chamber and
integrated shielding electrodes of the present invention.
Detailed Description
Conventional interrupters with or without an insulated arcing chamber are shown
in Figure 1(a) and Figure 1(b).
As shown in Figure 2 the first movable contact assembly of gas circuit breaker 1
of the present invention, comprises a first movable contact 3, made of a high
conductivity material with low erosion refractory material tip is held on a contact
support 2. The first movable contact 3 of the moving contact assembly is
covered by a movable contact shield 3' made from low erosion refractory
material. Movable current carrying (CC) contact 4 is fixed to nozzle 5 and to the
first movable contact assembly. The first movable contact 3, movable contact
shield 3' and movable current carrying contact 4 are in total called as the first
movable contact assembly. A second movable / fixed contact or pin 6 is
surrounded by a dynamic field electrode 7, (Fig. 3) when CB is in fully open
condition. The second movable contact is located inside the dynamic field

electrode 7 and the arrangement is again inside the fixed current carrying (CC)
contact assembly. The static current carrying contact assembly comprising static
current carrying contact 8 and static CC contact shield 9. In open condition of
circuit breaker, the dynamic field electrode 7 projects out from the static CC
contact shield 9 and the gas gap between movable shield and first movable CC
contact 4 decide withstandable voltage. The first movable CC contact is
separated from the static CC shield by a distance proportional to the system
voltage and the SF6 gas density. The entire contact system is placed in an
insulated arcing chamber 10 to contain and to guide arced gas during CB
operation.
Fig. 3 shows the contact system and circuit breaker insulated arcing chamber
assembly. The entire contact system is insulated from the grounded metallic
enclosure with designed SF6 gas density and by means of an insulated arcing
chamber 10. This chamber isolates the arced gas developed across the contact
system from the clean / cold gas near the coaxial grounded enclosure. The
novelty of the proposed design is that the insulated arcing chamber improves the
electrostatic field across the contact system during all instants of the contact
separation unlike conventional designs where insulated chamber affect the
electrostatic field distribution adversely. The insulated arcing chamber assembly
consists of a first HT shield 11 and a second HT shield 12 (shielding electrodes)

integrated to insulated arcing chamber 10 on either side. The insulated chamber
is made of Aramid / Kevlar or a combination of these fibers or equivalent
material wound and impregnated with epoxy resin in vacuum/ pressure. The
internal and external surfaces of insulated chamber are resistant against arced
SF6 gas. The profiles of HT shields are to maintain uniform E-field between the
following components during all the instants of contact separation:
1. First movable contact shield 3' and second movable / fixed contact 6.
2. Dynamic field electrode 7 and first movable CC contact 4.
3. Static CC contact shield 9 and first movable CC contact 4.
In the present invention, dynamic field electrode 7 is introduced between first
movable contact assembly 1 and static current carrying contact shield 9.
Dynamic field electrode 7 improves electrostatic field between moving contact
system and static CC contact assembly. First movable contact assembly is
coupled to dynamic field electrode through nozzle 5 by means of a self locking
coupling arrangement. In this design, the electrostatic field between the contact
systems is highly influenced by the following parameters:
1. Distance to which dynamic field electrode 7 has to be moved.
2. Instant of presence of dynamic field electrode 7 between CC shield and
first movable CC contact.

During opening operation, dynamic field electrode 7 moves in downward
direction at a predefined instant of its operation depending on anticipated
minimum arcing time and the instant at which pin contact crosses the dynamic
field electrode 7. More clearly, at current-zero, the voltage withstand capabilities
between contacts is improved in the presence of dynamic field electrode 7 and
insulated arcing chamber assembly. It is important that dynamic field electrode
7 controls E-field across the contact system when circuit breaker is in open
condition. When circuit breaker is under opening operation, initially dynamic
field electrode can not control the electrostatic field stress to the extent of
requirement. At this point of time, the field uniformity could be achieved by HT
shields or shielding electrodes 11,12 integrated to the insulated arcing chamber
10. Profile of insulated arcing chamber and integrated shielding electrodes to
optimize electrostatic surface stress on the arcing chamber to highly reliable level
during current interruption. These shields also improve arcing times for low
current interruption duties like capacitive switching, inductive switching, cable
charging duty etc.
Fig. 4 shows the insulated arcing chamber assembly of the present invention.
The surface stress on the insulated arcing chamber 10 is controlled to as low as
possible during current interruption due to the following:

1. The electrical stress across the insulating chamber is highly time-varying
and depends on position of the current carrying contact and the arcing
contact.
2. As the gas inside the nozzle which is inside the arcing chamber becomes
hot and withstand voltage of the same reduce significantly during current
interruption.
The concept of the insulated arcing chamber of the present invention is also
implemented to achieve uniform voltage distribution across insulated operating
rod which is used to drive the interrupter and as support insulator 13 i.e., to
support / insulate the interrupter high voltage terminal from the ground.
Fig. 5 shows the support insulator 13 proposed for the above application.
Fig. 6 shows the gas circuit breaker with the insulated arcing chamber and
integrated shielding electrodes of the present invention.
The insulated arcing chamber can also be used as support insulator i.e. to
support / insulate the interrupter high voltage terminal from the ground
potential.

WE CLAIM
1. An improved arcing chamber assembly for SF6 gas circuit breakers
comprising a first HT shield or shielding electrode (11), and a second HT
shield or shielding electrode (12) integrated to said insulated arcing
chamber (10); wherein a dynamic field electrode (7) is provided
between a first movable contact assembly (1) and a static current
carrying contact shield (9), thereby improving electrostatic filed between
the moving contact system and the static current carrying contact
assembly (8, 9) of said circuit breaker.
2. The improved insulated arcing chamber as claimed in claim 1, wherein
the first movable contact (3) of said moving contact assembly held on a
contact support (2) is covered by a movable contact shield (3').
3. The improved insulated arcing chamber as claimed in claim 2, wherein
said first movable contact (3) is made of a high conductivity material
with low erosion refractory material tip:
4. The improved insulated arcing chamber as claimed in claim 2, wherein
said movable contact shield (3') is made from low erosion refractory
material.

5. The improved insulated arcing chamber as claimed in claim 1, wherein
said dynamic field electrode (7) is provided surrounding a second
movable / fixed contact or pin (6).
6. The improved insulated arcing chamber as claimed in claim 1, wherein
said first and second HT shields (11, 12) are integrated to said insulated
arcing chamber (10) on either side thereof,
7. The improved insulated arcing chamber as claimed in preceding claims,
wherein said insulated arcing chamber is made of Aramid or Kelvar or a
combination thereof or their equivalent material, wound and
impregnated with epoxy resin under vacuum or pressure, so that the
internal and external surfaces of said insulated arcing chamber (10) are
resistant against arced SF6 gas.
8. The improved insulated arcing chamber as claimed in preceding claims,
wherein the profile of said first and second HT shields (11, 12) are such
that a uniform E-field can be maintained between first movable contact
shield (3') and second movable / fixed contact (6); between dynamic
field electrode (7) and first movable CC contact (4); and between static
CC contact shield (9) and first movable CC contact (4).

9. The improved insulated arcing chamber as claimed in preceding claims,
wherein the profiles of said insulated arcing chamber and said integrated
shielding electrodes are such that during current interruption the
electrostatic surface stress on the arcing chamber is at a highly reliable
level.
10. The improved insulated arcing chamber as claimed in the preceding
claims, wherein said insulated arcing chamber (10) is adapted for
providing insulation to the high voltage terminals of the interrupter from
ground potential.
11. An improved arcing chamber assembly for SF6 gas circuit breakers,
substantially as herein described and illustrated in the figures of the
accompanying drawings.

1. An improved arcing chamber assembly for SF6 gas circuit breakers
comprising a first HT shield or shielding electrode (11), and a second HT
shield or shielding electrode (12) integrated to said insulated arcing
chamber (10); wherein a dynamic field electrode (7) is provided
between a first movable contact assembly (1) and a static current
carrying contact shield (9), thereby improving electrostatic filed between
the moving contact system and the static current carrying contact
assembly (8, 9) of said circuit breaker.