The present invention relates to circuit breakers.
Circuit breakers typically utilise a pair of electrical
contacts, maintained normally in contact with each other, through which an electrical
contact is made. In order to break the circuit, eg. upon detection of a fault condition,
one contact is moved relative to the other to separate the two contacts. As the
contacts are moved apart, due to the potential gradient between them, an electrical
arc is created. Where high voltages are involved it is necessary to arrange for
this arc to be extinguished in order to prevent excessive damage to the circuit
breaker and other attendant hazards.
It is well known, in order to extinguish the arc, to place
the contacts in a sealed vessel filled with a background gas consisting of sulphur
hexafluoride (SF6) at high pressure (typically in the region of 600kPa
(six atmospheres)). The gas is chosen for its dielectric properties, enhanced by
its pressurisation, by virtue of which arcing is reduced. Such circuit breakers
are in use in, for example, the substations and switching stations used in commercial
electricity supply networks.
In some examples, the effect of the gas is further enhanced
by arranging, through a "puffer" arrangement of a piston coupled to the circuit
breaker's movable electrode, that as the electrodes are separated a flow of gas
passes over them. US Patent No. 4339641 (General Electric Corporation) discloses
such an arrangement.
The same document illustrates the provision of a shield
or nozzle around the electrodes, formed of dielectric material, by means of which
the arc is to some degree confined. The design of this component is intended among
other objects to maximise gas pressure for arc extraction and minimise ablation
of the nozzle material.
Sulphur hexafluoride is recognised as a highly potent greenhouse
gas (several orders of magnitude more potent than carbon dioxide) and there are
consequently both official recommendations and important commercial incentives to
dispense with it. One approach which is the subject of currently active research
is to seek a substitute dielectric gas. Such research has been based on the use
of elevated pressure, as in the known circuit breakers using sulphur hexafluoride.
An option known in the literature is to use a proportion of sulphur hexafluoride
in combination with some other less harmful gas, but clearly the goal of dispensing
with SF6 is not thereby achieved.
High voltage circuit breakers are known which do not utilize
a dielectric gas for arc extinction but instead have electrodes in an evacuated
housing. However in such devices the electrical arc typically generates temperatures
sufficient to cause an undesirable degree of ablation of the electrodes themselves,
reducing the electrode's working lifetime.
An example of a circuit breaker which operates at low pressure
is provided by UK patent application 2087651 (Westinghouse Electric Corporation
et al). It appears that this is a device having low current density at the contacts
and the low gas pressure serves "to minimise contact erosion". Annular shields around
the perimeters of the contacts serve to intercept hot, eroded material.
US Patent 2167665, assigned to the Detroit Edison Company,
describes a circuit breaker in which horn fibre or other organic matter is placed
adjacent the arc that is decomposed. It also describes an arrangement in which movement
of a rod extension, upon opening of the contacts, tends to create decreased pressure
which operates to draw the arc through a space between the rod extension and a fibre
tube, helping to confine the arc within an arcing space.
In experiments, the inventors have unexpectedly observed
that arc extinction can be enhanced when the pressure of background gas is reduced
below atmospheric pressure.
In accordance with a first aspect of the present invention
there is a circuit breaker comprising first and second electrodes which are contactable
with each other to complete an electrical circuit, a withdrawal mechanism for moving
one electrode away from the other to break the circuit, and a shield arranged in
proximity to the electrodes such as to be subject to ablation by the aforementioned
arc, the material and arrangement of the shield being such that its ablation by
the arc causes it to release arc-extinguishing gas, the device further comprising
means for providing, at least in the vicinity of the electrodes and at the instant
of striking of an arc between them during breaking of the circuit, a gas pressure
below 101325 Pascals, and being characterised by provision of a sealed enclosure
containing the electrode and the shield, the enclosure containing a background gas.
For the avoidance of doubt, atmospheric pressure in this
context is 101325Pa.
It is found by experiment that an effective circuit breaker
can be constructed in accordance with the present invention despite, and in fact
by virtue of, the low gas pressure utilized. This is contrary to expectation.
The shield may form a cavity within which the arcing takes
place. In this way the desired ablation and also the arc extinguishing effect of
the gas can be increased. Pressure within the cavity may be transiently increased
by the effects of the arc, further improving arc extinction.
Preferably the shield comprises electrically insulating
Sub-atmospheric pressure in the vicinity of the electrodes
may be provided by providing a suitable gas pressure in the enclosure.
The background gas need not comprise a dielectric gas such
as SF6. Currently the favoured gas is nitrogen. Argon, carbon dioxide
and air are potential alternatives.
It is preferred that the background gas pressure inside
the enclosure is 60 kPa or below. 34 kPa (5psi) is believed to be still more favourable.
It is currently believed that a pressure above 7 kPa (1psi) is desirable although
the effect of pressures below 7 kPa (1 psi) have to date not been thoroughly studied.
An alternative, or additional, means for providing the
required pressure in the vicinity of the electrodes comprises means for withdrawing
gas from this vicinity during the .process of breaking the electrical circuit. Pressure
is thus transiently reduced in this vicinity. A piston/cylinder arrangement may
be used to withdraw the gas.
A specific embodiment of the present invention will now
be described, by way of example only, with reference to the accompanying drawings,
- Fig. 1 is a somewhat simplified section, in an axial plane, through an embodiment
of the present invention;
- Fig. 2 is a simplified section through the same embodiment in a radial plane;
- Fig. 3 is a graph of experimental data, showing the critical electrode gap (vertical
axis) against gas pressure (horizontal axis) for several different background gases
used in a circuit breaker:
- Fig. 4 is a graph of experimental data, showing critical electrode gap (vertical
axis) against peak alternating current (horizontal axis) in a circuit breaker embodying
the present invention and using several different background gases; and
- Fig. 5 is a graph of experimental data showing the magnitudes of extinction
and re-ignition voltage peaks for different gases, for a gas pressure of 25 kPa
(3.7 psi) and peak alternating currents of 20 kA.
As illustrated in Figs. 1 and 2, a circuit breaker embodying
the present invention comprises a tubular static electrode 2 coaxially mounted with
a cylindrical movable electrode 4. The movable electrode 4 is a sliding fit in the
fixed electrode 2. Fig. 1 shows the movable electrode to be withdrawn from the fixed
electrode, in order to break an associated electrical circuit indicated, purely
schematically, at 6. However when (as under normal operating conditions) the circuit
breaker is closed, the movable electrode contacts the fixed electrode to complete
the circuit 6. More specifically, in the present embodiment, an end portion of the
movable electrode 4 is received in and contacted by the fixed electrode 2.
The movable electrode 4 is coupled to a withdrawal mechanism
which is schematically indicated at 8. Suitable mechanisms are well known in the
art, their function being to rapidly withdraw the movable electrode 4 along the
direction of the electrode axis, and will not be described in detail herein beyond
noting that a standard type of hydraulic actuator may be used, and that pneumatic
or solenoid actuated devices are possible alternatives.
The electrodes are contained in an enclosure 12, formed
in the present embodiment as a metal tube. The enclosure serves to maintain around
the electrodes a background gas, introduced prior to use of the circuit breaker,
whose nature and purpose will be considered below. The withdrawal mechanism 8 is
in this embodiment disposed outside the enclosure 12, the movable electrode 4 emerging
from the enclosure through a sealing gland 14 (whereby passage of gas in this region
is prevented) to reach the withdrawal mechanism 8.
Also disposed within the enclosure 12, and in the vicinity
of the electrodes 2, 4, is an insulating shield 16. In the present embodiment the
shield is an annular body into whose interior the movable electrode 4 extends. When
the contact breaker is closed, the movable electrode 4 projects out of the shield
16 to contact the static electrode 2.
While other materials may be used, the inventors favour
polymeric material for the insulating shield 16. The currently preferred material
is polytetrafluoroethylene (PTFE). The shield lies closely around one of the electrodes,
in the present example the movable electrode 2, which it partly surrounds, and is
of a type referred to as a "close proximity shield".
The particular arrangement and configuration of the electrodes
and shield is presented merely by way of example and may differ in other embodiments.
The background gas of the present exemplary embodiment
is nitrogen (N2) at a pressure of 25 kPa (3.7 psi). It is found in experiment
that the illustrated circuit breaker performs well despite its lack of a background
gas (such as SF6) with high dielectric properties, and the fact that
the gas is at low pressure. This is contrary to expectation. It is believed by the
inventors that this good performance is due to the presence of both the shield and
the sub-atmospheric pressure background gas. The inventors have found that in the
illustrated circuit breaker ablation promoted arc extinction is enhanced by reducing
the background gas pressure below atmospheric pressure.
While the intention is not to limit the present invention
by reference to any specific explanation of its performance, it is believed that
the effect of the low background gas pressure is to cause the plasma arc produced
upon breaking of the circuit to spread more widely, as compared with the arc created
in a conventional high pressure device, and thereby to increase ablation of the
shield 16. The shield comprises a material which ablates to gaseous form in the
presence of an electrical arc. In the exemplary embodiment the PTFE shield is known
to be capable of arc induced ablation and to produce in response fluorines and fluorides
with excellent arc extinguishing properties. Chemical reactions produce gases including
carbon tetrafluoride (CF4) and C2F6. The process
involves sublimation of the PTFE monomers and their dissociation, which processes
are in themselves endothermic. The inventors have calculated, based on the current
and duration of the arc and on the mass ablated from the shield, that roughly 30%
of the arc's energy can in experimental examples go into ablation of the shield
material, assisting extinguishing of the arc. The ablated material also provides
a "chemical puff" of arc-extinguishing gas. The effect is to provide effective arc
extinction without need of SF6 as a background gas. Following striking
of the arc, pressure in the region of the electrodes is temporarily increased by
the heat and the ablation products generated by the arc and this increased pressure
is also believed to assist arc extinction. Products of the ablation may be vented
through the open ends of the shield 16.
Certain of the gases produced by the arc induced shield
ablation are in themselves environmentally undesirable but it is believed that at
least some of the chemical species produced by arc ablation re-combine to leave
materials that are environmentally non-threatening. That is, the chemical species
required for arc extinction are, at least in part, only transiently produced. Following
arc extinction and with appropriate delays caused by chemical recombination time
scales the chemically reactive fluorine/fluorides recombine to form solid fluorides
which do not easily disperse to form an environmental threat as do halogenic gases.
In order to enhance dielectric recovery with gas pressure
while respecting the need for sub-atmospheric gas pressure for ablation induced
arc extinction, the illustrated embodiment utilises a "reverse puffer" principle.
Piston action of the moving contact 4 within the shield 16 is used, upon withdrawal
of the contact 4, to reduce the pressure within the cavity in the shield 16. This
enables the ablation to be maximised for the thermal recovery (including ablation
enhanced pressurisation) whilst subsequently providing sufficient gas pressure for
good dielectric withstand.
Test results are provided in support of the claim regarding
the efficacy of sub-atmospheric pressure operation and of gases other than SF6.
Figure 3 shows the shortest gap lengths between contacts required to interrupt an
alternating fault current of peak value 20kA for various gas pressure in the range
6kPa to 580 kPa (0.8 to 84 psi), the horizontal scale being logarithmic. Results
are provided for five different gases - SF6, N2, air, CO2
and Ar. Notable features are:
- (a) Relatively small dependence upon gas pressure with SF6
- (b) The improved interruption with N2 for p<48kPa (7psi)
- (c) The similar performance of N2 to SF6 for p<48kPa
- (d) The similar behaviour of CO2, air to N2, SF6
for p<48kPa (7psi)
- (e) The generally poorer performance of Ar but nonetheless showing a similar
trend as N2 and CO2.
The similar performance of the gases tested below 48kPa
(7psi) implies the dominance of a common feature believed to be ablation of the
shield and pressurisation due to arc heating of the products of ablation.
Weighing the PTFE shield used in the tests after some 250
test firings indicates on average a PTFE weight loss of 0.14 grams per firing (for
cylinder and moving electrode diameter 2.2cm). The erosion of the PTFE wall was
significant but not excessive and performance deteriorated only slightly over 250
tests at fault currents of 20KA max.
Fig. 4 shows the results of experiments to examine the
effect of peak alternating current on the critical gap length for current interruption
at a pressure of 3.7 psi. These show a trend for the interruption performance at
lower currents to be approximately as effective as at 20 KA, as judged by the critical
gap length criterion.
Tests have also been conducted on an 80:20 N2:SF6
mixture, which behaves in a similar manner to pure SF6 and N2.
At the present state of knowledge, there therefore appears to be no significant
advantage in utilising N2:SF6 mixtures in preference to pure
N2 unless the recovery of dielectric strength might be improved.
The critical gap length results of Figs. 3 and 4 are supported
by measurements of the magnitude of the voltage extinction peaks close to the critical
gap length for current interruption for the various gases at 20kA peak current and
a pressure of 25kPa (3.7 psi), Fig. 5. In this diagram the labels on the Z axis
are as follows:-
and the parenthesised labels:-
- XP1 = first half-cycle extinction peak;
- XP2 = second half-cycle extinction peak;
- RP = second half-cycle re-ignition peak;
- (1) denotes 1 x half cycle critical firing and
- (2) denotes 2 x half cycle pre-critical firing.
It should be noted that the requirement for sub atmospheric
pressure gas in the vicinity of the electrode and shield upon striking of the electrical
arc may be met, eg. by virtue of the illustrated "reverse puffer" arrangement, without
the ambient pressure of background gas in the enclosure 12 being below atmospheric.
Thus the background gas pressure may be atmospheric (or conceivably even higher)
with the required sub-atmospheric pressure around the electrodes being transiently
created when the circuit breaker is activated to break the circuit.
Furthermore the pressure in this vicinity is, as has been
noted above, increased by the action of the electrical arc and so is transiently
increased following striking of the arc.