FIELD OF THE INVENTION
The present invention relates to an improved contact system for dual motion
high voltage gas circuit breakers.
BACKGROUND OF THE INVENTION
A Circuit Breaker (CB) is primarily used to interrupt normal /fault /capacitive/
inductive currents of high voltage power transmission and distribution
systems. When the CB is in closed condition it allows flow of normal electrical
charge (current) through a closed electrical system (circuit). The magnitude
of 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 circuit breakers.
When fault current is interrupted by a circuit breaker, an 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.
In conventional 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 (Fig.1 (a)). 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, promoting 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.
Additionally the conventional interrupters use static field profiling electrodes
resulting in varying filed intensities as the contact separate. More clearly,
when shields are fixed the electrostatic field and voltage withstand
capabilities of the inter-electrode gap are limited. To overcome this constraint
and to achieve desired interrupter performance excess quantity of gas and
higher differential pressures are employed by designers.
In conventional circuit breakers, only one arcing contact has been operated
by drive and second arcing contact is fixed during breaker operation. In such
conditions, the engagement of both the contacts (arcing and current
carrying) cannot be continued for long time during opening as it hampers the
uniform electric field distribution across contact system and results to
withstandability of lower TRVs. In case of dual motion circuit breakers, it is
possible to engage both the contacts (arcing and current carrying)
independently for long time without affecting the uniform electric field
distribution across the contact system. However, design of contact system
shall be critical in case both the contacts travel at different speeds. The
novelty of the proposed patent is to meet design requirements even in such
contact systems.
In general, and according to prior art, to limit the voltage appearing across
the contacts during interruption, multiple breaks are preferred. The multiple
break system is operated by same drive which requires higher energy drive
for its operation. To overcome this problem, a dual motion contact system
has been identified as an alternative solution (Fig 1.(b)). However, in all
these systems, the second movable contact i.e., other than primary moving
contact takes sufficient portion of energy from the operating mechanism. In
some of the conventional breakers, such system have relatively high energy
requirements and are difficult to operate with low energy mechanisms (ref:
US 20080257866A1).
The electrostatic field, between movable and fixed contacts, is non-uniform
for various reasons like electrode profile and relative position of contacts. The
field intensification adversely affects voltage withstanding capabilities of the
gas gap. The availability of hot and conducting gas further complicates the
situation preventing successful interruption. In most of the interrupters, the
arced gas after leaving the nozzle may spill out into the contact system due
to improper channeling and this may lead to thermal failures. Beyond
particular instant of time during current interruption, minimum arcing time is
decided by the effectiveness with which arcing gas is being removed across
inter-electrode gas gap and uniformity of the electrostatic field between
arcing contacts. To overcome this problem, creepage length of nozzle is
increased by different ways. Nevertheless performance of the system is
limited and none of the interrupters is utilized to full capabilities. 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 (refer Fig. 2). 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 cannot be controlled to the extent of requirement by means of arcing
and current carrying contact shields (refer US patent 4937406). 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 (refer Fig. 2).
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. To address some of these requirements,
optimized contact system has been innovated.
OBJECTS OF THE INVENTION
The main objective of invention is to improve interrupting performance of
higher voltage circuit breakers by:
• Reduction of breaks per pole for circuit breakers of higher voltage
class.
• Timed relative motion / dual motion contact system and accelerated
contact separation.
• To achieve lowest electrical stress levels on second movable contact by
introducing dynamic field controlled electrode.
• To achieve lowest electrical stress levels on second movable contact by
moving itself with predefined speed.
• Accomplishment of pre-defined speed travel characteristics of dual
motion contacts during opening and closing
• Pre-defined location of arcing contacts with respect to current carrying
contacts.
• Effective discharge of arced gas through nozzle outlets
• Elimination of hot gas across main current carrying contacts.
• Elimination of hot arced gas in the dual motion mechanism
compartment and early discharge into main gas volume.
SUMMARY OF THE INVENTION
Accordingly there is provided an improved contact system for dual motion
high voltage gas circuit breakers. The improved circuit breaker interrupter
exhibits the following novel characteristics:-
1. A novel dual motion mechanism.
2. By introducing movable arcing contact at optimum position with
reference to fixed current carrying contact.
3. By introducing socket at optimum position with reference to Moving
current carrying contact.
4. By introducing insulating shroud over socket at optimum position with
reference to Moving current carrying contact and Nozzle.
5. By optimizing the exhaust passage for the arced gas from the outlet of
nozzle at all instants of circuit breaker opening operation.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The invention is described with the help of Figures 1 to 6, where:
Figure 1: Conventional interrupters and with dual motion mechanism.
Figure 2: Conventional arcing contact system
Figure 3: First Movable contact Assembly
Figure 4: Second Movable arcing Contact Assembly
Figure 5: Dynamic Field controlled Electrode Assembly.
Figure 6: Invented contact system with hot arced gas exhaust.
DETAILED DESCRIPTION OF THE INVENTION
In the present invention, the socket [01], made of a high conductivity and
low erosion material is held on a socket support or protection volume [02].
The socket is covered by a insulating shroud [03] made from low erosion
insulating material. The nozzle [05] is fixed to dynamic current carrying (CC)
contact [04] and to the socket contact assembly. The socket [01], insulating
shroud [03] and current carrying contact [04] are termed as the socket
contact assembly. The insulating shroud [03] is located at a predefined
position with reference to current carrying contact [04]. Fig. 3 shows the first
movable contact assembly.
In interrupter fully open condition, the pin [06] is surrounded by a dynamic
field controlled electrode [07]. The pin [06] is located inside the dynamic field
controlled electrode [07] and the arrangement is again inside the stationary
current carrying (CC) contact assembly. The pin [06] is dimensioned such
that it promotes uniform electrostatic field between the two contacts and at
the same time arcing shall happen across arcing contacts during making
operation. The dimensions of the pin is also decided by the fault current to be
interrupted, pressure window required to interrupt all possible test duties.
The static current carrying contact assembly comprising static current
carrying (CC) contact [08] and static current carrying contact shield [09].
The location of static current carrying contact shield [09] with respect to CC
contact [08] decides the electrostatic field uniformity across the contact
system. Further, location of movable shield/ dynamic field controlled
electrode [07] with reference to CC contact shield [09] decides the highest
electric field across the contact system. For reliable operation, it is essential
that during making/breaking, pre-aring/arcing shall takes place between
arcing contacts only. The profile and location of CC contact shield [09] and
Movable current carrying contact / shield [04] shall be designed in such a
way that arcing takes place only between movable arcing contact [06] and
fixed movable arcing contact [06]. The inner diameter of CC contact shield
[09] is a parameter decides the location of maximum stress on cc contact
assembly. Similarly, the location of current carrying contact [04] with respect
to socket [01] decides the electric field level around first movable contact
assembly. If socket [01] is out of current carrying contact, between arcing
contacts the electric field level is very high and difficult to withstand for TRVs.
If movable current carrying contact [04] is out of socket [01], then there may
be possibility of arcing between pin [06] and movable current carrying
contact [04]. In view of this optimization of location is important and socket
[01] is fixed slightly below the movable current carrying contact [04] such
that electric field level on socket [01] is higher than movable current carrying
contact [04]. Design of ‘H’ is critical in achieving interrupter performance in
dual motion contact system. Novel feature of the design is that, socket [01] is
within movable current carrying contact [04] and arcing shall be ensured
across arcing contacts and electrostatic field around movable contact system
is minimized.
The novelty of the proposed contact system is that the arcing contacts can be
engaged till drive achieves good speed characteristics by overcoming initial
torque and at the same time current carrying contacts [04],[08],[09] can be
isolated to greater extent which helps in keeping movable shield [07] at
optimum location to achieve uniform electric field across contact system. The
location of movable arcing contact [06] with respect to CC shield [09] and
socket [01] is important in dual motion circuit breakers as second movable
arcing contact [06] and main movable arcing contact [01] speed
characteristics are different. Based on these characteristics only, it is
necessary to decide location of above. One more component which decides
the optimal design of gas circuit breaker is location of movable arcing contact
[06]. Dt is the depth of pin [06] with respect to CC contact [08]. Novel
feature is that arcing contact is also within the dynamic field controlled
electrode [07] and CC contact shield [09] at the time of arc interruption. Fig.
4 shows the Second movable arcing contact assembly.
The location of movable arcing contact [06] with respect to CC contact [08]
shall decide the electric field pattern between the contact systems. If this
contact is exceptionally out from CC contact [08], electric field level between
arcing contacts is high and difficult to withstand transient recovery voltage
(TRV). If it is restricted, the electric field level between current carrying
contacts [04],[08],[09] is more than arcing currents [01] [06] and is not an
accurate design. It is also important, movable current carrying contact [04]
has to be isolated from cc contact/CC contact shield [09] sufficiently, to
ensure no flashover across nozzle [05] outer surface. Gm is the gas gap
between moveable current carrying contact [04] and dynamic field controlled
electrode [07]. Ga is the gas gap between socket [01] and pin [06]. The
novelty of proposed design is the rate at which Ga and Gm increase during
opening is highly non-uniform. Entire opening operation is divided into three
phases to achieve interrupting performance. In first phase, it works with the
concept of Gm>Ga . In second phase, it works with the concept of Gm=Ga . In
third phase, it works with the concept of Gm Ga, Gm =
Ga and Gm < Ga at appropriate timings of opening;
- both the arcing contacts have different speed-travel characteristics.;
- the arced gas discharged through the nozzle [05] outlet arranged to be
routed through openings provided in the metallic housing immediate to
arcing chamber [10] known as exhaust slots [17] without any
reflection of arced gas back to arcing contacts [01],[06] and across
gas gap created by contacts;
- exhaust slots [17] provided to achieve increased area of opening for
arced gas discharge as time progresses during current interruption;
- an arced gas stopper [18] provided at the outlet of nozzle [05] to
restrict hot arced gas to flow into dual motion mechanism; and
- an arced gas stopper [18] provided to guide arced gas to leave
through exhaust slots [17] effectively.