AN ELECTRICAL ISOLATOR
Background of the Invention
This invention relates to an electrical isolator and an associated electrical switch.
Description of the Prior Art
The reference in this specification to any prior publication (or information derived from it),
or to any matter which is known, is not, and should not be taken as an acknowledgment or
admission or any form of suggestion that the prior publication (or information derived from
it) or known matter forms part of the common general knowledge in the field of endeavour to
which this specification relates.
The use of sulfur hexafluoride (SF6) gas in the electrical industry as a gaseous dielectric
medium for high-voltage circuit breakers, switchgear, and other electrical equipment is
known. However, SF6 gas insulated switches are no longer preferred due to the greenhouse
gas effect of SF6 (approximately 23,900 times that of C02). In addition, switches
incorporating SF6 gas require sealing and such sealed switches generally attract higher
maintenance costs to ensure proper operation through the lifetime of the switch. A further
issue is the recent introduction of reporting requirements associated with such switches,
requiring that the switching apparatus is checked annually to determine any leakage, which
must then be reported. This reporting places a significant burden on the operators of any
such switch gear.
There are generally two types of electrical switches used at medium voltage. The first type is
fault make and load break switches. A typical application for such switches is overhead line
load break switches and load break switches in a Ring Main Unit (RMU). The second type is
fault make and fault break switches. A typical application for these switches is Ring Main
Unit (RMU) circuit breakers, e.g. indoor and metal enclosed switchgear, or the like.
An electrical isolating switch generally comprises three main components, namely an
interrupter, an isolator, and a mechanism for actuating the interrupter and isolator. A vacuum
interrupter is one type of interrupter that is widely used in a wide range of electrical switches
that is SF6 free. Their design is well known in the art; however they are unsuitable for use as
an isolator due to the very high internal electrical field strength that exists between the open
contacts and the fact that, as a result of the shape of the internal electric field, the highest
electrical stress occurs at the conducting contact surface. Small asperities and surface
imperfections caused by its operation will give rise to so-called "stress raisers" that will result
in degradation of such a vacuum interrupter's isolation capacity, typically resulting in a
flashover at a lower voltage than designed.
Npn Sustained Disruptive Discharges (NSDD) are also a problem with such vacuum
interrupters. This phenomenon of NSDD is generally caused in part by impurities in the
vacuum switch contact material. Refer to "Peculiarities of non-sustained disruptive
discharges at interruption of cable/line charging current" A. M. Chaly, L.V. Denisov, V.N.
Poluyanov, I.N. Poluyanova, Tavrida Electric, 22, Vakulenchuka Str., Sevastopol, 99053
Ukraine. For these reasons, it is generally necessary to use an isolator in series with a vacuum
interrupter to provide a safe means of isolation.
Some electrical switches are required to make onto a faulted line and then to break the short
circuit fault current, whilst other switches are only required to break load currents. This
making and breaking of fault currents, or the breaking of load currents, can be carried out by
any suitable interrupter such as a vacuum interrupter, solid state electronic interrupter, or air
blast interrupter. Other technologies may also be suitable. However, all of these known
interrupters require an additional isolator that is able to reliably withstand the maximum
voltages that are likely to be seen in service in order to provide safe isolation.
There are a number of prior art documents relating to different types of isolators. For
example, U.S. patent no. 4,484,044 teaches a load switch which includes a vacuum switch in
series with an air disconnecting switch. The vacuum switch comprises a fixed electrode, a
movable electrode attached to one end of an axially movable control rod and a retaining
spring which exerts a resilient force on the control rod tending to separate the electrodes. The
air disconnecting switch comprises a conically shaped male contact and an opposing female
contact shaped to permit insertion of the male contact therein. The male contact has a
relatively large diameter base portion attached to the other end of the control rod and forming
a step with the control rod. The female contact has spring loaded locking projections for
releasably engaging the step of the male contact and a stopper for exerting a force on the
control rod sufficient to close the electrodes of the vacuum switch when the male contact is
moved against the stopper after engagement with the female contact. The spring loading of
the locking projections of the female contact, the shape of the male contact and the spring
constant of the retaining spring are selected such that the force on the control rod during
engagement of the male and female contacts is not sufficient to close the electrodes of the
vacuum switch, while the force on the control rod during disengagement of those contacts
acts to fully separate the electrodes of the vacuum switch prior to the release of the male
contact.
This is a typical design of a prior art isolator, as shown in Figure 1 (Figure 3 of U.S. patent
no. 4,484,044). It consists of moving contact 12, fixed contact 7, and isolating distance L.
This type of isolator is used in medium voltage electrical switchgear, both in air and in SF6.
SF6 isolators are substantially smaller than air insulated devices since SF6 gas has 2.5 times
the dielectric strength of air, therefore an SF6 insulated device is normally 40% of the size of
and air insulated device in each linear dimension resulting in a device which may be only 10
to 20% of the volume of an air insulated device. However, these isolators have the
disadvantage of requiring large isolating distances in air as can be seen from the attached
electrical field plots of Figure 2. Figure 2 shows the electrical field plot of the isolator of
Figure 1. It can be seen that for an isolating distance L of 172 mm the estimated maximum
electrical stress will be 2,800 volts/mm. Thus, as air has a breakdown of 3,000 volts/mm,
this means that 172 mm is the minimum separation that can be provided for this arrangement
to function as an isolator.
Similarly, U.S. patent no. 3,598,939 relates to an isolating switch having large metallic
electrodes presenting substantially smooth surfaces facing one another, with at least one of
the electrodes being movable by means of a moving carriage to which it is secured. The
electrodes in the open gap position have a relatively high withstand or insulation strength on
switching voltage surge, impulse voltage, and with a relatively small gap space. The
movement of the carrier to contact both electrodes corresponds to the closed position of the
switch while movement of the carriage to break the contact between the electrodes
corresponds to the open position. In that latter position, a substantially uniform electrostatic
field is produced in the gap between the electrodes.
U.S. patent no. 3,624,322 discloses an isolating switch which employs semispherical-type
electrode shielding energized parts which are mounted on the top of a pair of tilted insulator
columns. The columns are mounted to a support frame by means of rotor bearings, which,
when rotated by an appropriate mechanism, cause the tops of the insulator columns to move
in a circular path. Linkages are employed and are responsive to column rotation in a first
direction to electrically contact the blade and jaw of the switch arrangement, and to withdraw
the blade and jaw in response to column rotation in a second direction to break contact. The
smooth surfaces of the electrodes employed face one another in this second instance and
provide an open gap condition which produces a substantially uniform electrostatic field
between facing surfaces.
U.S. patent no. 3,592,984 describes an isolating switch having spherical, ellipsoid, toroid or
spheroid electrodes and a retractable switchblade. The electrodes in the open gap position
have a relatively high withstand on switching voltage surge, impulse voltage and with a
relatively small gap space. The extension of the retractable switchblade to contact both
electrodes corresponds to the closed position of the switch while retraction of the switchblade
into one of the electrodes corresponds to the open position. In that latter position, an open gap
is produced between the electrodes and results in a substantially uniform electrostatic field in
the gap. This has the advantage that the switch open gap may be made substantially shorter
than the distance from the electrodes to ground and yet insure that any flashover will be
between the electrodes and ground rather than across the switch open gap.
U.S. patent no. 5,237,137 teaches, in an isolating switch for metal-clad, compressed-gas
insulated high-voltage switchgear, a mechanical control unit containing a rotatably supported
lever arrangement. The lever arrangement locks automatically in a neutral position and
retains an auxiliary contact pin until it is released by a guide surface connected to the main
contact pin. A mating contact of the auxiliary contact pin is also spring-loaded and follows
this auxiliary contact pin somewhat after being released, initially while maintaining the
equipotential bonding.
U.S. patent no. 4,591,680 provides for an isolating switch, which is suitable for electrically
isolating and connecting components of gas-insulated encapsulated switching stations under,
at the most, low load conditions, wherein a fixed contact member is provided with a central
trailing contact which ends in a contact member. It is coaxially surrounded by a circle of
rated-current fingers and a fixed contact shielding electrode. The central contact rod of the
movable contact member is coaxially surrounded at a distance by a shielding electrode which
is also movable. In order to prevent undesirable flash-overs, in particular flash-overs at the
encapsulation, the rated-current fingers are in contact with the contact rod in the area
surrounded by the shielding electrode which is also movable. They are mounted to be
rotatable and have forces applied to them which press their end members radially inward.
The contact member is constructed as a shield-like plate having a front face which is domed
forward towards the movable contact arrangement. When the trailing contact is pushed back,
the rated-current fingers, which are located behind the front face when the trailing contact is
pushed forward, project through openings in the contact member. The contact rod and the
shielding electrode which moves along with the former are provided with circumferential
grooves.
The above prior art switches are generally focused on convex electrical field control
electrode shapes. There currently exists a requirement for a compact and low cost air
insulated unsealed electrical isolator to be used either alone or in combination with an
interrupter to create an SF6-free electrical isolating switch.
Summary of the Present Invention
In a first broad form the present invention seeks to provide an electrical isolator which
includes:
a) a body defining an aperture therethrough;
b) a first electrical contact arranged at a first end of the aperture;
c) a second electrical contact movably arranged at a second end of the aperture, said
second contact configured to be operatively movable through the aperture to
electrically connect to, or disconnect from, the first contact; and
d) at least two concave electrical field control screens fixed to the body at respective
ends of, and about, the aperture such that the screens lie transverse to the aperture and
an open-end of each concave screen is directed towards the other.
Typically the body is manufactured from a solid dielectric insulating material.
Typically the aperture is tubular.
Typically the electrical isolator includes a sliding contact for connecting the first contact to
the second contact in the aperture.
Typically the electrical isolator includes a mechanism configured to actuate the second
contact through the aperture into, or out of, contact with the first contact.
Typically the body includes an external conductive screen.
Typically the external conductive screen includes a conductive paint or a sprayed metal
coating.
Typically the external conductive screen is earthed, in use.
Typically said screens are configured to modify the electrical field in the aperture to thereby
maintain a desired electrical stress profile between the contacts.
In a second broad form the present invention seeks to provide an electrical isolator which
includes:
a) a body defining an aperture therethrough;
b) a first electrical contact arranged at a first end of the aperture;
c) a second electrical contact movably arranged at a second end of the aperture, said
second contact configured to be operatively movable through the aperture to
electrically connect to, or disconnect from, the first contact; and
d) at least two electrical field control screens extending outwardly from respective ends
of the aperture, the screens modifying the electrical field in the aperture to thereby
maintain a desired electrical stress profile between the contacts.
Typically the body is manufactured from a solid dielectric insulating material.
Typically the aperture is tubular.
Typically the electrical isolator includes a sliding contact for connecting the first contact to
the second contact in the aperture.
Typically the electrical isolator includes mechanism configured to actuate the second contact
through the aperture into, or out of, contact with the first contact.
Typically the electrical isolator the body includes an external conductive screen.
Typically the electrical isolator the external conductive screen includes a conductive paint or
a sprayed metal coating.
Typically the electrical isolator the external conductive screen is earthed, in use.
Typically said screens are configured to modify the electrical field in the aperture to thereby
maintain a desired electrical stress profile between the contacts.
In a third broad form the present invention seeks to provide an electrical switch which
includes:
a) a housing;
b) an interrupter inside the housing for interrupting an electrical current;
c) an isolator inside the housing and arranged in electrical communication with the
interrupter, the isolator having:
d) a body defining an aperture therethrough;
e) a first electrical contact arranged at a first end of the aperture;
f) a second electrical contact movably arranged at a second end of the aperture, said
second contact configured to be operatively movable through the aperture to
electrically connect to, or disconnect from, the first contact; and
g) at least two concave electrical field control screens fixed to the body at respective
ends of, and about, the aperture such that the screens lie transverse to the aperture and
an open-end of each concave screen is directed towards the other; and
h) a mechanism configured for actuating the interrupter and the isolator.
Typically the interrupter includes a vacuum interrupter.
Typically the mechanism includes an insulating pushrod entering the housing through a
passage in a portion of the housing having at least two concave electrical field control screens
fixed to the portion at respective ends of, and about, the passage such that the screens lie
transverse to the passage and an open-end of each concave screen is directed towards the
other, said screens configured to distribute an electrical field in the passage in order to
provide an area of low electrical stress.
Typically said screens are configured to modify the electrical field in the aperture to thereby
maintain a desired electrical stress profile between the contacts.
In a third broad form the present invention seeks to provide an electrical isolating chamber
for electrically isolating first and second regions, the isolating chamber including:
a) a passage extending between the first and second regions;
b) a member extending through the passage;
c) at least two concave electrical field control screens provided about the passage such
that the screens lie transverse to the chamber and an open-end of each concave screen
is directed towards the other, said screens being configured to distribute an electrical
field in the chamber in order to provide a third region of low electrical stress, the
member extending through the third region.
Typically at least one of the first and second regions is provided inside a housing for
electrical equipment.
Typically said screens are configured to modify the electrical field in the chamber to thereby
maintain a desired electrical stress profile along the member.
Typically the member includes at least one of:
a) a mechanical actuator;
b) optical fibres; and,
c) fluid pipes.
Brief Description of the Drawings
An example of the present invention will now be described with reference to the
accompanying drawings, in which: -
Figure 1 shows a type of prior art isolator described in U.S. patent no. 4,484,044;
Figures 2a and 2b show electric field plots in air for the prior art isolator of Figure 1;
Figure 3a shows an example of an isolator having the two flat parallel plate electrical field
control screens;
Figures 3b and 3c show general electric field plots in air of the two flat parallel plate
electrical field control screens;
Figure 4a shows an example of an electrical isolator in accordance with the current
arrangement;
Figures 4b and 4c show typical electric field plots of two flat parallel plate electrical field
control screens partially embedded in a solid dielectric, without an external conductive
screen;
Figures 5a and 5b show further electric field plots of the isolator of Figure 4 having two flat
parallel plate electrical field control screens partially embedded in a solid dielectric, without
external conductive screen;
Figures 6a and 6b show typical electric field plots of two flat parallel plate electrical field
control screens partially embedded in a solid dielectric with grounded external conductive
screen;
Figure 7 shows an example of an electrical isolator according to the current arrangement,
without an external conductive screen;
Figure 8 shows an example of an electrical isolator according to the current arrangement,
with external conductive screen;
Figures 9a and 9b show an electric field plot of the electrical isolator of Figure 7;
Figures 10a and 10b show a further electric field plot of the electrical isolator of Figure 7;
Figures 1 a and 1 b show an electric field plot of the electrical isolator of Figure 8;
Figures 12a and 12b show an electric field plot of the electrical isolator of Figure 8 having a
grounded external earth screen;
Figure 13 shows an example of a switch disconnector in accordance with the current
arrangement; and
Figure 1 shows a further example of a switch disconnector in accordance with the current
arrangement.
Detailed Description of the Preferred Embodiments
With reference now to the accompanying drawings, by way of background, Figure 3a shows
an example o an electrical isolator 9 with a first electrical contact 4 and a second movable
electrical contact 5 which is generally configured to be operatively movable to electrically
connect to, or disconnect from, the first contact 4. Sliding contact 6 typically facilitates
contact between electrical contacts 4 and 5. The isolator 9 also includes two parallel electrical
field control screens 3 and 32 each arranged, as shown, proximate the respective electrical
contacts 4 and 5. The screens 3 1 and 32 lie transverse to the contacts 4 and 5 and the screens
3 and 32 are configured to evenly distribute an electrical field in order to reduce electrical
stress between said screens 31 and 32 when the contacts 4 and 5 are disconnected.
Figure 3b and 3c show the electrical field plot of another example of two parallel plate
electrical field control screens 31, 32 in air, displaced from each other by a distance of 68
mm. As shown in the graph of Figure 3c, this conductor arrangement results in an estimated
maximum electrical stress of 2,800V/mm just before the contacts 4 (by means of sliding
contact 6) and 5 electrically connect to each other.
In accordance with an example of the current arrangement, Figure 4a shows an electrical
isolator 9 having a body 1 defining an aperture 2 therethrough, as shown. The isolator 9 also
includes a first electrical contact 4 arranged at a first end of the aperture 2, and a second
electrical contact 5 movably arranged at a second end of the aperture 2. The second contact 5
is generally configured to be operatively movable through the aperture 2 to electrically
connect to, or disconnect from, the first contact 4 by way of sliding contact 6. The isolator 9
also includes at least two electrical field control screens 3 1 and 32 extending outwardly from
respective ends of the aperture 2, as shown. The two opposing parallel plate electrical field
control screens 3 1 and 32 are typically partially embedded in a solid dielectric 33. The
screens 3 and 32 are configured to modify the electrical field in the aperture 2 to thereby
maintain a desired electrical stress profile between the contacts 4 and 5.
The aperture or central hole 2, preferably round, provides an aperture for the second or
moving contact 5 to pass through. The moving contact 5 is typically driven from a suitable
mechanism. It may be manually or electrically operated by any one of many suitable
operation mechanisms that persons skilled in the art would be familiar with. In one example,
the moving contact 5 typically connects with the first or fixed contact 4 by way of a sliding
contact 6 so that an electrical circuit is completed. The sliding contact 6 may be a "Multilam"
or similar contact.
Figures 4b and 4c show an electrical field plot of the two opposing parallel plate electrical
field control screens 31, 32 partially embedded in the solid dielectric 33. As shown, an
applied voltage of 135kv creates an estimated maximum electric stress of 2800 volts/mm at
an internal air to solid dielectric interface A-A. Note that the area of high stress associated
with air as dielectric between the screens 3 1 and 32 in Figure 3c is now embedded in the
solid dielectric 33 and the separation between the screens can be reduced to 47.5mm from the
initial 68mm.. A comparison of the upside-down shape of the electrical stresses of Figure 3c
and Figure 4c show that the electric field gradient is reduced in the arrangement of Figure 4a
in the region of the contact 4 (with associated sliding contact 6), s that as the contact 5
approaches the contact 4, the electrical stress will be reduced compared to the arrangement of
Figures 3b and 3c. .
The electrical stresses at air to dielectric interfaces is important in order to predict the
reliability over the lifetime of the product., Figures 5a and 5b show a further electrical field
plot of the two opposing parallel plate electrical field control screens 3 1 and 32 partially
embedded in the solid dielectric 33. An applied voltage of 135kv creates an estimated
maximum electric stress of 2,525 volts/mm at the external air to solid dielectric 33 interface
C-C, which is less than the air break down stress of 3,000 Volts/mm.
Figures 6a and 6b show an electrical field plot of two opposing parallel plate electrical field
control screens 31, 32 partially embedded in a solid dielectric with a grounded external
conductive screen 10 added about the dielectric 33, as shown. An applied voltage of 135kv
creates an estimated maximum electric stress of 3,000 volts/mm at the internal air to solid
dielectric interface A-A.
As is known in the art of electrical engineering, the most uniform electrical field distribution
is achieved by two parallel plates of infinite size. Figure 3 shows that a reasonably uniform
electrical field distribution can indeed be achieved with small parallel control screens
separated by an appropriate distance in air. In addition, by partially embedding such screens
in a solid dielectric as per Figures 4 and 5, the spacing between the contacts 4 and 5 can be
reduced. As the reduction in size of an isolator is generally desirable, this aspect is an
important feature of the current arrangement.
Without any external influences to the electric field, the electric field in dielectric 33 is
typically uniform. However, this arrangement is not suitable for electrical isolator design in
practice since the uniform electrical field between the parallel electrical field control screens
3 1 and 32 is easily disturbed by adjacent electrical fields and grounded structures. When the
electrical field becomes disturbed, it generally becomes non-uniform and the maximum stress
increases which can cause a significant loss in dielectric performance.
The application of a grounded external conductive screen 10 in Figure 6 shields the field
from such external influences, however it has the effect of causing an increase in the
maximum internal electrical stress at A-A. Further increasing the separation does little to
reduce the maximum internal electrical stress since it is mostly influenced by the location of
the external conductive screen 10. It is therefore seen that whilst uniform electric fields can
be achieved by parallel plate electrical field control screens there are several major
disadvantages.
Figure 7 shows an example of an electrical isolator 9, in accordance with the current
arrangement. The isolator 9 typically includes a body 1 defining an aperture or hole 2
therethrough. The isolator 9 also includes a first electrical contact 4 arranged at a first end of
the aperture 2, as well as a second electrical contact 5 movably arranged at a second end of
the aperture 2. The second contact 5 is generally configured to be operatively movable
through the aperture 2 to electrically connect to, or disconnect from, the first contact 4 by
way of sliding contact 6.
The isolator 9 also includes at least two concave electrical field control screens 3 1 and 32
fixed to the body at respective ends of, and about, the aperture 2 such that the screens 31 and
32 lie transverse to the aperture 2 and an open-end of each concave screen 3 1 and 32 is
directed towards the other, as shown. The screens 3 1 and 32 are configured to evenly
distribute an electrical field in the aperture 2 in order to reduce electrical stress between said
screens 3 1 and 32 when the contacts 4 and 5 are disconnected. T e screens are typically
concave and may include a similar bowl-shaped configuration, or the like.
The example of an isolator 9 of Figure 8 has an external conductive screen 0 applied where
in Figure 7 it does not. In some circumstances it is preferable to apply an external conductive
screen 10 by coating the external surface of the body 1 with a conductive coating as an
electrical field control measure. In some circumstances it may be preferable to earth this
conductive screen, in use. The external conductive screen 10 is preferably a conductive paint
or a sprayed metal coating.
The body 1 of the current arrangement is preferably, but not necessarily tubular or circular,
about the centerline and made of a suitable solid dielectric insulating material such as a
polymer. The preferred polymer is an electrical grade epoxy resin such as Huntsman
CW2229. If it is to be used in an outdoor environment, then a suitable cyclo-aliphatic epoxy
resin is preferred such as a Huntsman CY184 or CY5622. The dielectric strength of such a
polymer is approximately 20,000 Volts/mm whilst the dielectric strength of air is
approximately 3,000 Volts /mm. The preferred dielectric constant of the solid dielectric
insulating material is in the range of 1 to 6.
The aperture or central hole 2, preferably round, provides an aperture for the second or
moving contact 5 to pass through. The moving contact 5 is typically driven from a suitable
mechanism. It may be manually or electrically operated by any one of many suitable
operation mechanisms that persons skilled in the art would be familiar with. In one example,
the moving contact 5 typically connects with the first or fixed contact 4 by way of a sliding
contact 6 so that an electrical circuit is completed. The sliding contact 6 may be a "Multilam"
or similar contact.
As described above, the concave electrical field control screens 3 and 32 are arranged in an
opposing manner and are typically embedded in the body 1. These electrical field control
screens 3 and 32 serve to shape the electrical field in such a manner as to optimally shape
the lines of equipotential and distribute them evenly such that the resulting electrical stress is
as uniform as possible. This ensures the most compact design possible.
The isolators of Figures 7 and 8 are generally designed for application in a 12kV rated
system, rated continuous current of 630 Amps, and Lightning Impulse Withstand Voltage
(LIWV) of 1lOKv. In order to provide a reliable isolator, and to allow for statistical spread of
test results in production, the isolator 9 is typically designed to withstand a LIWV of 135,000
Volts. However, it is to be appreciated that different examples of the isolator 9 can be applied
to any rated voltage or current.
Figure 9 shows a prediction of the electric stress of the isolator 9 of Figure 7, without the
external conductive screen, at location of highest electrical stress 34 in the solid dielectric to
air interface A-A in the central hole 2. The maximum electrical stress is approximately 2,800
Volts/mm midway between the electrical field control screens 3 and 32. This has the desired
effect of providing stable isolator performance when the LIWV is applied.
In addition, Figure 10 predicts the electric stress of the isolator 9, without the external
conductive screen 10, at the body 1 to air interface 5 at C-C. Note that the maximum
electrical stress is approximately 4,800 Volts/mm. This is undesirable since it will cause the
air to become conductive at the instant of the applied LIWV on the surface of the insulator,
which will lead possible electrical breakdown externally when the LIWV is applied. Electric
stress will also be present at 15 during normal service at the rated voltage and this may give
rise to premature failure of the solid dielectric body 1 due to partial discharges created by the
electrical stresses in the presence of pollution such as dust, cobwebs or other foreign matter.
Figure 11 predicts the electric stress of the isolator 9 of Figure 8, with the external conductive
screen 0 ungrounded (or at a floating potential) at the location of the highest electrical stress
in the central hole 2. The maximum electrical stress is approximately 2,800 Volts/mm
midway between the electrical field control screens 3 1 and 32. This also has the desired
effect of providing stable isolator performance.
Figure 12 predicts the electric stress of the isolator 9 of Figure 8, with the external conductive
screen 10 grounded, at the location of the highest electrical stress 34 in the solid dielectric to
air interface in the central hole 2. The maximum electrical stress is approximately 2,800
Volts/mm midway between the electrical field control screens and 32.
The isolator 9 generally controls the maximum electrical stress in air by two actions, namely
by the opposing concave shape of the electrical field control screens 3 and 32, and due to
the fact that the electrical field control screens 31 and 32 are partially encapsulated in a high
dielectric strength solid dielectric insulating material in the body 1 in such a manner as to
ensure that the areas of maximum electric stress are within the insulating material.
If the maximum electrical stress occurs at the conductor to air interface 8, then any
inconsistency in the conductor shape, or asperity, or surface imperfections or irregularities in
the metallic electrode surface will cause degradation of the isolation capacity. Such
irregularities and surface imperfections can be caused by wear during the life of the isolator
9.
By comparing Figures 9, 10, and 2 it can be seen that it makes negligible difference to
the electrical stress in the air filled central hole 2 whether the external conductive screen 10 is
present or not, and whether the external conductive screen 10 is grounded or not.
However the isolator 9 with the external conductive screen 10 grounded is advantageous
because the internal field is not influenced by external factors such as other electric fields or
other grounded objects; it eliminates any electrical field stress on the surface which may
cause long term surface degradation due to the presence of partial discharges that may
increase with the presence of dust and other foreign material; it shapes the electrical field
such that the maximum electrical stress occurs at the point midway between the electrical
field control screens which has the desired effect of providing stable isolator performance;
and it provides a grounded surface that is safe to touch.
Due to these improvements, it can be seen that the isolator 9 is generally much smaller and
therefore cheaper to manufacture than the prior art isolator shown in Figures 1 and 2. It is
regarded as advantageous that the isolator 9 has a reduced size compared to the prior art
isolators. In general, the isolator 9 has approximately 35% to 40% in the linear dimensions or
10 to 25% of the volumetric dimensions of the prior art isolators having comparable electrical
performance. The isolator 9 will therefore be of suitable size and cost to replace prior art
isolators that previously have utilized SF6 gas as an insulating medium, however the isolator
9 will not have the environmental consequences of SF6 gas-filled equipment.
It is known that air has a dielectric strength of approximately 3000 Volts/mm. Design work
for the isolator 9 assumed 2,800 Volts/mm and testing confirmed this assumption to be
reliable for both positive and negative polarities of lightning Impulse withstand voltage. In
order to prove an isolator design it is necessary to conduct design tests for each type (type
tests) and to prove its isolation capability Lightning Impulse Withstand Voltage (LIWV) tests
are required to be satisfied. These tests are specified in the appropriate international standards
that apply.
Figure 13 shows an example of a further arrangement wherein the isolator 9 is applied to a
specific arrangement of an electrical switch. The electrical switch includes an insulated
housing 2 , an interrupter 13 inside the housing 2 1 for interrupting an electrical current, and
the isolator 9, as described above. The switch also generally includes a mechanism 16
configured for actuating the interrupter and the isolator 9.
The switch includes an insulated housing 2 1 and the isolator 9 is moulded into this insulated
housing, as shown. In this implementation the isolator 9 is connected in series with a vacuum
interrupter 13. The vacuum interrupter 13 has a moving contact 17 and a fixed contact 12.
The isolator 9 has fixed contact 4 and a moving contact 5. The moving contact of the vacuum
interrupter 17 is electrically connected to the moving contact of the current arrangement 5 by
a flexible conductor 14. Both the moving conductors 5 and 17 are mechanically driven by the
mechanism 16. This mechanism is so designed to drive both the vacuum interrupter moving
contact 17 and the current arrangement moving contact 5 at the required velocities, the
required timing, and the required displacements to suit the switch ratings.
An insulating pushrod 18 passes through a second isolator assembly 9. The purpose of this
second isolator 9 is to provide an area of low electrical stress that allows a shorter insulating
pushrod 18 to be used than would otherwise be required. This insulating pushrod 18 is driven
mechanically from a mechanism 11. The mechanism 11 may be manually operated, or
electrically operated by any one of many suitable operation mechanisms that persons skilled
in the art would be familiar with. A controller (10) may be employed to control the
mechanism 11 either manually, remotely or automatically by any one of many means that
persons skilled in the art would be familiar with.
In one particular example, the second isolator 9 includes a chamber 9.1 having a passage 9.2
extending between the first and second regions. The passage can be provided in a dielectric
material or similar as previously described, and typically has a pushrod or other member
extending therethrough. At least two concave electrical field control screens 9.3, 9.4 are
provided about the passage such that the screens lie transverse to the chamber and an openend
of each concave screen is directed towards the other, said screens being configured to
distribute an electrical field in the chamber in order to provide a third region of low electrical
stress within the passage so that the member extends through the third region.
It will be appreciated that an isolator of this form can be used to electrically isolate any two
regions, and in particular can be used to isolate a region that is at a significantly higher
electrical potential than another region, such as the inside of electrical switchgear. Despite
this, the isolator allows an insulting member to extend between the regions, for example to
allow the member to pass into switchgear housing.
This is particularly useful for allowing first and second regions, such as the inside and outside
of high voltage switchgear, to be electrically isolated. In particular, this allows a member to
pass into a region with a high electrical potential, whilst still maintaining required levels of
insulation. Thus, the isolating chamber alters the electrical fields in such a way as to limit the
maximum stress on the air in the chamber (as described earlier) which permits any insulating
member that needs to enter into the high voltage region of the switchgear to be significantly
shorter than if the electrical stress was not controlled by the isolating chamber leading to a
more compact structure than would otherwise be possible. Examples of such members might
include, but are not limited to mechanical operating shafts, optical fibres or fluid pipes
circulating coolant.
Figure 14 shows a further example wherein the isolator 9 is used as part of an electrical
switch. The switch assembly is enclosed in an insulated housing 22 and the isolator 9 is
moulded into the insulated housing 22. In this implementation the isolator 9 is connected in
series with a vacuum interrupter 13. The vacuum interrupter 13 has a moving contact 17 and
a fixed contact 12. The isolator 9 has fixed contact 4 and a moving contact 5. The moving
contact of the vacuum interrupter 17 is electrically connected to the terminal of the switch
assembly 19 by a flexible conductor 23. The moving contact of current arrangement 5 is
electrically connected to the terminal of the switch assembly 20 by a flexible conductor 24.
The moving conductors 5 and 17 are independently mechanically driven by the mechanism
25 and 26 respectively. These mechanisms are so designed to drive both the vacuum
interrupter moving contact 17 and the isolator moving contact 5 at the required velocities, the
required timing, and the required displacements to suit the switch ratings.
These insulating pushrods 18 are independently driven mechanically from a mechanism 25
and 26. These mechanisms may be manually operated, or electrically operated by any one of
many suitable operation mechanisms that persons skilled in the art would be familiar with. A
controller 10 may be employed to control these mechanisms either manually, remotely or
automatically by any one of many means that persons skilled in the art would be familiar
with.
Many modifications or variations will be apparent to those skilled in the art without departing
from the scope of the present invention. All such variations and modifications should be
considered to fall within the spirit and scope of the invention broadly appearing and
described in more detail herein.
It is to be appreciated that reference to "one example" or "an example" of the invention is not
made in an exclusive sense. Accordingly, one example may exemplify certain aspects of the
invention, whilst other aspects are exemplified in a different example. These examples are
intended to assist the skilled person in performing the invention and are not intended to limit
the overall scope of the invention in any way unless the context clearly indicates otherwise.
Features that are common to the art are not explained in any detail as they are deemed to be
easily understood by the skilled person. Similarly, throughout this specification, the term
"comprising" and its grammatical equivalents shall be taken to have an inclusive meaning,
unless the context of use clearly indicates otherwise.
THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:
1) An electrical isolator which includes:
a) a body defining an aperture therethrough;
b) a first electrical contact arranged at a first end of the aperture;
c) a second electrical contact movably arranged at a second end of the aperture, said
second contact configured to be operatively movable through the aperture to
electrically connect to, or disconnect from, the first contact; and
d) at least two concave electrical field control screens fixed to the body at respective
ends of and about, the aperture such that the screens lie transverse to the aperture and
an open-end of each concave screen is directed towards the other.
2) The electrical isolator of claim 1, wherein the body is manufactured from a solid
dielectric insulating material.
3) The electrical isolator of either one of claims 1 or 2, wherein the aperture is tubular.
4) The electrical isolator of any one of claims 1 to 3, which includes a sliding contact for
connecting the first contact to the second contact in the aperture.
5) The electrical isolator of any one of claims 1 to 4, which includes a mechanism
configured to actuate the second contact through the aperture into, or out of, contact with
the first contact.
6) The electrical isolator of any one of claims 1 to 5, wherein the body includes an external
conductive screen.
7) The electrical isolator of claim 6, wherein the external conductive screen includes a
conductive paint or a sprayed metal coating.
8) The electrical isolator of either one of claim 6 or 7, wherein the external conductive
screen is earthed, in use.
9) The electrical isolator of any one of the claims 1 to 8, wherein said screens are configured
to modify the electrical field in the aperture to thereby maintain a desired electrical stress
profile between the contacts.
10) An electrical isolator which includes:
a) a body defining an aperture therethrough;
b) a first electrical contact arranged at a first end of the aperture;
c) a second electrical contact movably arranged at a second end of the aperture, said
second contact configured to be operatively movable through the aperture to
electrically connect to, or disconnect from, the first contact; and
d) at least two electrical field control screens extending outwardly from respective ends
of the aperture.
11) The electrical isolator of claim 10, wherein the body is manufactured from a solid
dielectric insulating material.
12) The electrical isolator of any one of claims 10 or 1, wherein the aperture is tubular.
13) The electrical isolator of any one of claims 0 to 12, which includes a sliding contact for
connecting the first contact to the second contact in the aperture.
14) The electrical isolator of any one of claims 10 to 13, which includes mechanism
configured to actuate the second contact through the aperture into, or out of, contact with
the first contact.
15) The electrical isolator of any one of claims 10 to 14, wherein the body includes an
external conductive screen.
16) The electrical isolator of claim 15, wherein the external conductive screen includes a
conductive paint or a sprayed metal coating.
17) The electrical isolator of either one of claim 15 or 16. wherein the external conductive
screen is earthed, in use.
18) The electrical isolator of any one of the claims 10 to 17, wherein said screens are
configured to modify the electrical field in the aperture to thereby maintain a desired
electrical stress profile between the contacts.
19) An electrical switch which includes:
a) a housing;
b) an interrupter inside the housing for interrupting an electrical current;
c) an isolator inside the housing and arranged in electrical communication with the
interrupter, the isolator having: .
i) a body defining an aperture therethrough;
ii) a first electrical contact arranged at a first end of the aperture;
iii) a second electrical contact movably arranged at a second end of the aperture, said
second contact configured to be operatively movable through the aperture to
electrically connect to, or disconnect from, the first contact; and
iv) at least two concave electrical field control screens fixed to the body at respective
ends of, and about, the aperture such that the screens lie transverse to the aperture
and an open-end of each concave screen is directed towards the other; and
v) a mechanism configured for actuating the interrupter and the isolator.
20) The electrical switch of claim 19, wherein the interrupter includes a vacuum interrupter.
21) The electrical switch of either one of claims 19 or 20, wherein the mechanism includes an
insulating pushrod entering the housing through a passage in a portion of the housing
having at least two concave electrical fi eld control screens fixed to the portion at
respective ends of, and about, the passage such that the screens lie transverse to the
passage and an open-end of each concave screen is directed towards the other, said
screens configured to distribute an electrical field in the passage in order to provide an
area of low electrical stress.
22) The electrical switch of any one of the claims 19 to 21, wherein said screens are
configured to modify the electrical field in the aperture to thereby maintain a desired
electrical stress profile between the contacts.
23) An electrical switch which includes an interrupter and an isolator as claimed in any one
of claims 1 to 8 .
24) An electrical isolating chamber for electrically isolating first and second regions, the
isolating chamber including:
a) a passage extending between the first and second regions;
b) a member extending through the passage;
c) at least two concave electrical field control screens provided about the passage such
that the screens lie transverse to the chamber and an open-end of each concave screen
is directed towards the other said screens being configured to distribute an electrical
field in the chamber in order to provide a third region of low electrical stress, the
member extending through the third region.
25) An electrical isolating chamber according to claim 24, wherein at least one of the first and
second regions is provided inside a housing for electrical equipment.
26) An electrical isolating chamber according to claim 24 or claim 25, wherein said screens
are configured to modify the electrical field in the chamber to thereby maintain a desired
electrical stress profile along the member.
27) An electrical isolating chamber according to any one of the claims 24 to 26, wherein the
member includes at least one of:
a) a mechanical actuator;
b) optical fibres; and,
c) fluid pipes.
28) An electrical isolator or electrical switch, substantially as hereinbefore described.
29) An electrical isolator or electrical switch substantially as hereinbefore described and
illustrated with reference to the accompanying drawings.