Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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TITLE
Rotary switches
DESCRIPTION
Technical Field
The present invention relates to rotary switches, and in particular to double
pole (or
multi pole) double break rotary switches for current interruption.
Background Art
Double break rotary switches having any convenient number of poles are well
known
as disconnect switches or off-load switches and typically include_a bridging
member
that is rotatable to make direct physical contact with fixed contacts or
busbars at its
opposite ends.
Summary of the Invention
The present invention provides an improved rotary switch comprising first and
second
poles, each pole including a rotatable bridging member and a pair of fixed
busbars
having at least one primary contact, wherein the rotary switch is adapted such
that the
direction of current flow through the first pole is opposite to the direction
of current
flow through the second pole such that arcs established in the first pole are
deflected
away from arcs established in the second pole.
The rotary switch is particularly useful when applied to do distribution
systems (e.g.
as an off-load switch) that are designed to have a low L/R time constant and
where
high power density is a dominant objective. However, the high arc voltage that
is
achieved by the rotary switch will also be effective in interrupting dc and ac
circuits
with significant L/R time constants.
A rotary actuator of any suitable construction is preferably used to rotate
the rotatable
bridging members between a closed position and an open position. In the closed
position the opposite ends of each rotatable bridging member are in direct
physical
contact with the associated fixed busbars so that current can flow between the
busbars
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through the rotatable bridging member. In the open position the opposite ends
of the
rotatable bridging member are spaced apart from the associated busbars. The
rotary
actuator will preferably be able to rotate the rotatable bridging members in
both
directions (i.e. in a first direction to move from the closed position towards
the open
position to open the rotary switch and in a second, opposite, direction to
move from
an open position towards the closed position to close the rotary switch). The
rotary
actuator may rotate the rotatable bridging members under the control of any
regulator
or control means such as an electronic control unit, for example. As in other
conventional rotary switches, a spring-detent or other snap action rotary
drive
coupling mechanism can be inserted between the rotary actuator and the
rotatable
bridging members in order to maximise the speed of rotation of the rotatable
bridging
members at primary contact "making" and "breaking" times.
The rotatable bridging members are preferably adapted to rotate in tandem and
may
therefore be rotated by the same rotary actuator. In practice, the rotatable
bridging
members can be mounted to a common drive shaft so that the respective contact
systems of each pole are synchronised in phase with respect to the common
rotary
actuation.
The rotatable bridging member and the fixed busbars of each pole will
preferably be
axially spaced apart to avoid any potential flashover between poles.
Full arcing between the rotatable bridging member and the fixed busbars of
each pole
is initiated when respective arcing contacts physically separate. The
interaction of
generated magnetic flux with the current loop flowing through each pole
creates a
radial repulsive force which deflects each arc away from the centre of the
current
loop. This increases the arc length and increases the arc voltage in the
process. It
also causes the rotatable bridging member of each pole to be accelerated away
from
the associated busbars towards the open position. This increases the
separation
between the respective arcing contacts which further increases the arc length.
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The current loops flowing through the two poles are in opposite directions.
This can
be achieved by appropriate connection of the fixed busbars to the external
circuit.
The current loops are mutually coupled and if the current flows in opposite
directions
then resulting magnetic flux will interact with the current loops in a manner
that
generates a repulsive force between the current loops. This causes arcs in the
contact
systems of the two poles to be repelled from each other, hence reducing the
risk of
flashover between poles.
The whole of the rotary switch is preferably immersed in a liquid dielectric.
It will be
readily appreciated that the term "liquid dielectric" is not just intended to
cover
proprietary liquids that are specifically marketed as such, but any liquid
that has a
sufficient dielectric withstand. This would include de-ionised water,
FLUORINERT
and other equivalent perfluorocarbon fluids, mineral transformer oils,
silicone
transformer oils, synthetic oils and esters, methylene chloride etc. A
particularly
preferred coolant fluid is a proprietary transformer insulating fluid such as
MIDEL
and its equivalents. The liquid dielectric improves the cooling of the rotary
switch
and the generation of arc voltages as described in more detail below. The
liquid
dielectric may be stationary or, in some arrangements, may flow past the
rotary
switch.
In a preferred construction then the rotatable bridging members have opposite
ends,
each end including an arcing contact. Each of the fixed busbars has at least
one
primary contact and a contact arm that includes an arcing contact. The
rotatable
bridging members are then rotatable between a closed position where the
opposite
ends of each rotatable bridging member are in direct physical contact with the
contact
arm and the primary contact of the associated busbars, and an open position
where the
opposite ends of the rotatable bridging members are spaced apart from the
contact
arm and the primary contact of the associated busbars. Between the open and
closed
positions the rotatable bridging members will adopt an intermediate position
where
the arcing contacts on the opposite ends of the rotatable bridging member are
in direct
physical contact with the arcing contact on the contact arm of the associated
busbars
but where there is no longer any direct physical contact with the primary
contacts.
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The contact arm of each fixed busbar is preferably formed as an integral part
of the
busbar and can be suitably shaped and sized to allow the associated rotatable
bridging
member to move past it, in sliding contact with it, as it rotates between the
closed
position and the open position.
The primary contacts of the fixed busbars of each pole preferably include at
least one
resilient contact member that is in sliding contact with a respective end of
the
associated rotatable bridging member when the rotatable bridging member is in
the
closed position. The primary contacts represent the main flow path for current
between the fixed busbars of each pole and opposite ends of the associated
rotatable
bridging member when the rotatable bridging member is in the closed position.
The fixed busbars of each pole may include a plurality of primary contacts. It
is
therefore important to note that when the associated rotatable bridging member
is in
the intermediate position then it is preferably spaced apart from all of the
primary
contacts of the busbars such that the only flow path for current between the
busbars
and the opposite ends of the rotatable bridging member is between the
respective
arcing contacts that are still in direct physical contact with each other.
When a
plurality of primary contacts is employed by a particular busbar then they
will
preferably form a group of parallel connected electrical circuits between that
busbar
and the respective end of the associated rotatable bridging member. As such,
the
continuous current rating of the rotary switch is influenced by the degree of
current
sharing between parallel connected electrical circuits and it is beneficial if
the degree
of contact wear, erosion and resultant contact resistance can be as uniform as
possible.
Moreover, the "making' (closing) and "breaking" (opening) current ratings of
the
rotary switch are influenced by any corresponding sequential "making" and
"breaking" behaviour. To this end, the rotatable bridging member of each pole
is
preferably shaped so as to allow approximately synchronised "making" and
"breaking" of all the parallel connected circuits. The speed of rotation of
the rotatable
bridging member of each pole is also made to be as fast as is technically
practical so
as to minimise the elapsed time associated with any non-synchronisation. It
will be
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noted that the respective arcing contacts also operate as an electrical
circuit that is in
parallel with the primary contacts at their respective "making" and "breaking"
times.
This synchronisation feature is particularly beneficial when arcing contact
wear and
erosion has occurred in service and arcing contact resistance has increased as
a result.
This is because arcing contact resistance directly influences the "making' and
"breaking" currents that are experienced by the respective primary contacts.
The contact system that is formed by the rotatable bridging member and the
primary
contacts of each pole benefits from a sliding or "wiping' contact action and
significant contact pressures may be employed within the primary contact to
ensure
that mating contact surfaces are cleaned during relative movement. Moreover,
these
mating contact surfaces are not subjected to extremes of electrical erosion
when
parallel connecting arcing contacts are employed. These two factors result in
the
rotary switch having a low contact resistance and resultant voltage drop when
in the
closed position.
The rotary switch is preferably a double pole (or multi pole) double break
rotary
switch.
Drawings
Figure 1 is a plan view of a double pole double break rotary switch according
to the
present invention;
Figure 2 is a side view of the rotary switch of Figure 1;
Figure 3 is an end view of the rotary switch of Figure 1;
Figure 4 is a schematic view of the rotary switch of Figure 1;
Figure 5 is a plan view of the rotary switch of Figure l in a closed position;
Figure 6 is a schematic view of the rotary switch of Figure 5;
Figure 7 is a plan view of the rotary switch of Figure l in a partially open
position;
Figure 8 is a schematic view of the rotary switch of Figure 7;
Figure 9 is a plan view of the rotary switch of Figure l in a closed position
and
showing the current loop in one pole;
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Figure 10 is a plan view of the rotary switch of Figure l in an intermediate
(pre-
arcing) position and showing the current loop in one pole;
Figure I l is a plan view of the rotary switch of Figure 1 in a partially open
(arcing)
position and showing the current loop in one pole;
Figure 12 is a plan view of the rotary switch of Figure 1 in a partially open
(arcing)
position and showing the current loop in both poles;
Figure 13 is an end view of the rotating switch of Figure 1 in a partially
open (arcing)
position and showing the current loop in both poles; and
Figure 14 is a schematic view show how the rotary switch of Figure 1 can be
integrated into external electrical circuitry for unidirectional dc current
flow between
a voltage source and a load.
The basic construction of a double pole double break rotary switch will now be
described with reference to Figures 1 to 3. A rotary switch having three poles
can be
used in a three-phase ac circuit and the present invention would include a
rotary
switch having such a construction.
The rotary switch of Figure 1 to 3 is intended to be used with an external
circuit such
as a dc distribution architecture and provides an off-load switch for
switchgear that is
extremely compact and reliable. The rotary switch may also operate as an off-
load
isolator while the external circuit is being de-energised. The dc distribution
architecture may form part of a marine power and propulsion distribution
system or a
transmission system for renewable energy devices such as wind turbines or
subsea
turbines, for example. The rotary switch would typically be opened after the
current
and the prospective open circuit voltage have been reduced to acceptable
levels by
other means (e.g. external electronic means and/or the application of a
foldback
characteristic). For example, the rotary switch might be opened after the
current and
the prospective open circuit voltage have been reduced to <20A and <50V,
respectively. Once the rotary switch has interrupted the circuit then the
prospective
current and voltage can be increased by other means. Preferably, sufficient
time will
be allowed between opening and closing the rotary switch for arc extinction to
occur.
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Such a rotary switch might easily be capable of carrying 3kA when closed and
withstand 300kV when open. The principle benefits of the rotary switch are its
inherent separation of arcs through the liquid dielectric and the consequent
reduction
of the risk of flashover between poles during the opening, recently opened and
subsequent open phases. The rotary switch also has inherently high arc voltage
and
rapid and complete arc extension behaviour which are a result of the
electromagnetic
repulsions between arcs and contact systems, further aided by the cooling
effect of the
liquid dielectric.
The rotary switch includes positive and negative poles 2, 4 that are axially
spaced
apart from each other as most clearly shown in Figures 2 and 3. Figure 1 is a
plan
view of the positive pole 2 but it will be readily appreciated that the
negative pole 4
has precisely the same construction.
Each pole 2, 4 includes first and second fixed busbars that are spaced apart
in the
same plane. More particularly, the positive pole 2 includes first and second
fixed
busbars 6a, 6b and the negative pole 4 includes first and second fixed busbars
8a, 8b.
Each busbar is generally L-shaped. A first part 10 of each busbar defines the
entry or
exit point of a current loop and a second part 12 defines a contact arm that
is shaped
and sized to be in direct physical contact with an end of the rotatable
bridging
member (see below) when the rotary switch is in the closed position.
The first part 10 of each busbar can be connected to an external dc circuit
(external
circuitry) as shown schematically in Figure 14 and described in more detail
below.
The connection with the external dc circuit can be made by any suitable means
such
as fixed busbars with slotted holes to allow bolted joints to be adapted to
suit
geometric tolerances. Alternatively, flexible links of laminated copper foil
or
multiple wire strand construction may be employed. External busbar connections
may be made using insulated bushings or moulded busbar seals where the
connections
must pass through the wall of a reservoir that contains the liquid dielectric
in which
the rotary switch is immersed.
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The second part or contact arm 12 of each busbar is angled slightly (i.e. it
is thicker at
its free end than where it joins with the first part 10) or otherwise profiled
so that
there is sufficient clearance to accommodate the opposite ends of a rotatable
bridging
member 24 as it moves past the contact arm with an overriding requirement that
the
corresponding arcing contacts 28, 30 shall be in sliding contact in the
intermediate
position when the contacts 14 are "making" or "breaking" contact with the
rotatable
bridging member 24 of the associated pole.
As most clearly shown in Figure 3, each fixed busbar includes a contact 14
with a pair
of resilient contact members (callipers) 16 that are axially spaced apart by
spacers 18
and biased inwardly by leaf springs 20. The end parts 22 of the contact
members 16
are designed to be in sliding contact with the rotatable bridging member 24
and can be
made of copper and plated or faced with silver or a copper-tungsten alloy to
provide
resistance to sliding contact wear and erosion. This in turn provides a low
and stable
contact resistance throughout the operating lifetime of the rotary switch.
Although it
is generally preferred that each contact 14 includes a pair of contact members
16, it
will be readily appreciated that a single contact member can also be used.
Each fixed
busbar may also include a plurality of co-located contacts (each having one or
two
resilient contact members) that can be connected together in parallel to
permit a
desired thermally limiting current rating for the rotary switch to be
achieved.
The components that are assembled to form the contacts 14 are secured to the
first
part 10 of each of the fixed busbars 6a, 6b by bolted joints. Clearance holes
at the
fixing end of each contact 14 are shown in Figure 1 and these may include
provision
for the use of an assembly jig or other assembly method that maintains the
precise
alignment of the contacts while they are being secured to the fixed busbars.
To improve the clarity of Figure 3, part of the drive shaft 26 and the second
fixed
busbars 6b, 8b have been omitted.
Each pole 2, 4 also includes a rotatable bridging member (moving blade) 24.
The
rotatable bridging members 24 are mounted to a drive shaft 26 in a manner that
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provides electrical insulation between the rotatable bridging members and a
common
rotary actuator and are mechanically rotated in tandem by the common rotary
actuator
(not shown) such that they maintain the same physical relationship with their
respective fixed busbars at all times. The rotatable bridging members 24 can
be
rotated between a closed position and an open position.
The rotatable bridging members 24 include a pair of notches 24a, 24b in the
edges
that face the first part 10 of the associated busbars when the rotatable
bridging
members are in the closed position. The notches 24a, 24b are designed to allow
approximately synchronised "making" and "breaking" of all parallel connected
circuits in the case where each fixed busbar includes a plurality of contacts
14.
The busbars and the rotatable bridging members 24 are typically made of
copper. The
busbars may be electroplated with a corrosion resistant metal and the
rotatable
bridging members may be silver plated in order to provide a low and stable
contact
resistance.
The general arrangement of the fixed busbars and rotatable bridging members 24
of
the two pole double break rotary switch is shown schematically in Figure 4.
When the rotatable bridging members 24 are in the closed position the opposite
ends
of each rotatable bridging member are in direct physical contact with an
arcing
contact 28 of the respective fixed busbar and also with the resilient contact
members
16 of the respective contact 14. This is shown in Figures 5 and 6.
When the rotatable bridging members 24 are in the open position the opposite
ends of
each rotatable bridging member are no longer in direct physical contact with
the
arcing contact 28 of the respective fixed busbar or with the resilient contact
members
16 of the respective contact 14. This is shown in Figures 7 and 8. When the
rotary
switch is fully open then the rotatable bridging members 24 will have normally
undergone a rotation of substantially 90 degrees from its closed position.
However,
any reference herein to an "open position" does not necessarily imply a fully
open
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position, but will include any position where there is no direct physical
contact
between the rotatable bridging members and the respective fixed busbars.
There is also an intermediate (or pre-arcing) position where the opposite ends
of each
rotatable bridging member 24 are in direct physical contact with the arcing
contact 28
of the respective fixed busbar but not with the resilient contact members 16
of the
respective contact 14.
The opposite ends of each rotatable bridging member 24 include arcing contacts
30
that are designed to be in direct physical contact with the arcing contacts 28
of the
respective fixed busbars when the rotatable bridging members are in the
intermediate
position. The arcing contacts 28, 30 of the fixed busbars and rotatable
bridging
members are located at those extremities where arcing can be expected as the
rotary
switch moves from the closed position to an open position. Erosion of the
arcing
contacts can be minimised by the use of suitable sacrificial arcing members or
arcing
horns.
The rotary switch can be closed by rotating the rotatable bridging members 24
from
the open position to the closed position. This can be achieved by means of the
common rotary actuator (not shown) which is adapted to rotate the drive shaft
26 in a
first direction to move the rotatable bridging members 24 from the closed
position to
the open position and in a second direction to rotate the rotatable bridging
members
from the open position to the closed position. As the rotary switch moves to
the
closed position the opposite ends 32, 34 of each rotatable bridging member 24
come
into contact with the end parts 22 of the resilient contact members 16 and may
push
the contact members slightly apart against the inward bias of the leaf springs
20. (It
will be readily appreciated that the contact members 16 may move slightly
towards
each other under the inward bias of the leaf springs 20 when the sliding
contact with
the respective end of each rotatable bridging member 24 is lost as they move
from the
closed position towards the open position.)
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When the rotary switch is closed, the corresponding arcing contacts 28, 30
will make
contact with each other before the contacts 14 make contact with the rotatable
bridging members 24. Arcing may therefore occur between the arcing contacts
28, 30
in some applications that have a high inrush current, e.g. capacitive systems.
During
the closing of the rotary switch, the objective is for any inrush current and
associated
arcing to have subsided while the arcing contacts 28, 30 are in close
proximity with
each other.
The flow path of dc current when the rotary switch is closed will now be
described in
more detail with reference to Figure 9. When the rotary switch is closed then
the
rotatable bridging members 24 are in a closed position and dc current can flow
between the busbars 6a, 6b of the positive pole 2 as indicated by the arrow.
More
particularly, in the case of the positive pole 2 dc current may flow from the
first part
10b of the bulbar 6b that defines an entry point of the current loop and into
the
contact 14b. Within the contact 14b the dc current flows along the contact
members
16 and into a first end 32 of the rotatable bridging member 24 through the end
parts
22 of the contact members that are in sliding contact with the first end 32.
The dc
current flows along the rotatable bridging member 24 and into the contact 14a
through
the end parts 22 of the contact members 16 that are in sliding contact with
the second
end 34 of the rotatable bridging member. Finally, the dc current flows from
the
contact 14a to the first part I Oa of the busbar 6a that defines an exit point
of the
current loop. In practice, some dc current may also flow directly between the
contact
arms 12a, 12b of the busbars and the rotatable bridging member 24 but this dc
current
will be much lower than that flowing through the respective contacts l4a, 14b
as a
result of the relatively high contact resistance between the fixed busbars and
the
rotatable bridging member. There will be a similar current loop between the
busbars
8a, 8b of the negative pole 4 but the direction of current flow will be
opposite as
described in more detail below.
The flow path of dc current when the rotary switch is in the intermediate (or
pre-
arcing) position will now be described in more detail with reference to Figure
10.
When the rotary switch is in the intermediate position there is no longer any
direct
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physical contact between the contacts 14a, l4b and the rotatable bridging
member 24.
However, dc current will continue to flow between the busbars 6a, 6b of the
positive
pole 2 as indicated by the arrow. More particularly, in the case of the
positive pole 2
dc current may flow from the first part I Ob of the busbar 6b that defines an
entry point
of the current loop, along the contact arm I2b to the fixed arcing contact
28b, then
through the sliding contact resistance between the arcing contact 28b and the
moving
arcing contact 30b into the arcing contact 30b, and then into the first end 32
of the
rotatable bridging member 24. (It will be readily appreciated that when the
rotary
switch is in the intermediate position then the arcing contacts 28, 30 are in
direct
physical contact at the opposite ends of the rotatable bridging member 24.)
The dc
current flows along the rotatable bridging member 24 and into the contact arm
12a
through the sliding contact resistance provided by the fixed and moving arcing
contacts 28a, 30a. Finally, the dc current flows along the contact arm 12a to
the first
part I Oa of the busbar 6a that defines an exit point of the current loop. The
flow of dc
current between the busbars 6a, 6b is determined by the external circuitry
that the
rotary switch is controlling and the voltage drop between the respective pairs
of
arcing contacts 28a, 30a and 28b, 30b while they are still in direct physical
contact
with each other has minimal influence on the current in the external circuitry
provided
the arcing contacts are not badly warn. There will be a similar current loop
between
the busbars 8a, 8b of the negative pole 4 but the direction of current flow
will be
opposite as described in more detail below.
The flow path of dc current when the rotary switch is in a partially open (or
arcing)
position will now be described in more detail with reference to Figure 11.
When the
rotary switch is in the partially open position there is no longer any direct
physical
contact between the contacts 14a, 14b and the rotatable bridging member 24.
Neither
is there any direct physical contact between the contact part 12a, l2b of each
busbar
6a, 6b and the ends of the rotatable bridging member 24 (or more particularly
between
the respective pairs of arcing contacts 28a, 30a and 28b, 30b). However, dc
current
will continue to flow between the busbars 6a, 6b of the positive pole 2 as
indicated by
the arrow. More particularly, in the case of the positive pole 2 dc current
may flow
from the first part l Ob of the busbar 6b that defines an entry point of the
current loop,
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along the contact arm l2b to the arcing contact 28b. From fixed arcing contact
28b
the dc current will flow, as an arc, to the moving arcing contact 30b on the
first end 32
of the rotatable bridging member 24. The dc current flows along the rotatable
bridging member 24 to the arcing contact 30a on the second end 34 of the
rotatable
bridging member 24. From arcing contact 30a the dc current will flow, as an
arc, to
the fixed arcing contact 28b on the contact arm l2a of the bulbar 6a. Finally,
the dc
current flows along the contact arm I 2a to the first part I Oa of the busbar
6a that
defines an exit point of the current loop. The flow of dc current between the
busbars
6a, 6b is now only partly determined by the external circuitry that the rotary
switch is
controlling and is increasingly determined by the arc voltages that develop
between
the facing pairs of arcing contacts 28a, 30a and 28b, 30b as their arc lengths
increase
with the separation of the arcing contacts. There will be a similar current
loop
between the busbars 8a, 8b of the negative pole 4 but the direction of current
flow will
be opposite as described in more detail below.
The entry and exit points 10a, 10b of the current loop for the positive pole 2
are
defined by the relatively closely spaced adjacent ends of busbars 6a, 6b and
the
corresponding busbar connections to the external circuitry. They contribute
minimal
magnetic flux density in the arcing regions (labelled "arcing" in Figures 11
to 13)
between the arcing contacts 28a, 28b and 30a, 30b of the busbars 6a, 6b and
the
rotatable bridging member 24, respectively. This means that the magnetic flux
density in each arcing region is dominated by that generated by the dc current
that
flows in the local current loop that includes the busbars 6a, 6b, the arcing
contacts
28a, 28b and 30a, 30b, the arcing regions and the rotatable bridging member
24. The
magnetic flux density interacts with the dc current in the arcs to create a
radial
repulsive force which, as a result of the relatively low mass within the arcs
and the
nature of the liquid dielectric that surrounds the arcs, deflects each arc
away from the
centre of the current loop. This lengthens the arc and increases the arc
voltage in the
process. The same electromagnetic behaviour causes the opposite ends 32, 34 of
the
rotatable bridging member 24 to be repelled from the fixed busbars 6a, 6b,
thereby
assisting the rotary actuator to move the rotatable bridging member towards
the fully
open position and still further increasing the arc length and arc voltage. The
arc
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voltage will rapidly become sufficient to interrupt the current in the dc
circuit. The
current interruption performance of the rotary switch is therefore improved
and the
rotary switch can be made physically smaller and more compact. As a result,
the
rotary switch can have an exceptionally high power density (volts x amps /
size).
The whole of the rotary switch is immersed in a liquid dielectric such as
MIDEL.
More particularly, the rotary switch can be located in a tank or reservoir of
liquid
dielectric (not shown) that may include some form of pressure relief system
for
accommodating the pressure wave that is generated by the opening of the rotary
switch, dielectric monitoring instrumentation and other related control
systems. A
series of rotary switches can be located in the same tank with interposing
insulation
barriers to minimise the risk of flashover between adjacent rotary switches.
Immersion of the rotary switch in the liquid dielectric enhances cooling of
metallic
conductors and more particularly enhances the cooling of the arc, de-
ionisation and
arc extinction performance.
The flow path of dc current in both poles 2, 4 when the rotary switch is in a
partially
open (or arcing) position is explained further with reference to Figures 12
and 13. To
improve clarity of Figure 13 the drive shaft 26 and the second fixed busbars
6b, 8b
have been omitted.
It will be seen that for the positive pole 2 the dc current flow is from the
second
busbar 6b to the first busbar 6a as described above and as indicated by the
solid
arrow. However, for the negative pole 4 the dc current flow is from the first
busbar
8a to the second busbar 8b as indicated by the broken arrow. The dc currents
in the
two poles therefore flow in opposite directions in parallel planes. The
electromagnetic interaction between the magnetic flux density and the dc
current
flowing in the current loops generates a repulsive force that is parallel to
the rotational
axis of the rotary switch and which deflects the arcs in the respective arcing
regions of
the positive and negative poles 2, 4 away from each other. This deflection is
in
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opposition to the electrostatic attraction that would otherwise result in a
flashover
between the poles 2, 4.
There is no practical reason why for the positive pole 2 the dc current flow
could not
be from the first busbar 6a to the second busbar 6b and why for the negative
pole 4
the dc current flow could not be from the second busbar 8b to the first busbar
8a. The
direction of dc current flow in the poles 2, 4 will depend on how the busbars
are
connected to the external circuitry.
All current loops are mutually coupled between the poles 2, 4 and they must be
separated by a sufficient distance if the axial repulsive force is to provide
effective
benefits. Some degree of separation is needed in any case for electrostatic
reasons if
flashover is to be avoided.
Figure 14 shows a dc circuit for unidirectional current flow between a voltage
source
V and a load L. The two pole double break rotary switch is represented in the
electrical circuit by four separate switches. It will be readily appreciated
that the
switches A and B correspond generally to the busbars 6a, 6b of the positive
pole 2 and
switches C and D correspond generally to the busbars 8a, 8b of the negative
pole 4.
When the rotatable bridging members 24 are rotated by the common rotary
actuator
(not shown) then all four switches will be opened simultaneously and interrupt
the dc
current flowing in each arm of the dc circuit. In practice, for a dc circuit
of the type
shown in Figure 14 then the rotary switch only needs to have a single pole
(i.e.
switches A and B or switches C and D) such that current is only interrupted in
one
arm of the dc circuit, but having two poles provides better performance as a
result of
the summation of four arc voltages. A rotary switch of suitable construction
could
also be used for bidirectional current flow or ac working.
