The present invention relates to an electric insulator of the kind
described in the preamble to claim 1 and a method for manufacturing the same as
described in the preamble of claim 7. The insulator comprises an elongated, hollow
insulant provided at each end with an attachment or connection flange. The insulator
may comprise a contact device with cooperating fixed and movable main and arcing
contacts. The insulator may also be in the form of a support insulator for a circuit
breaker and then comprises a continuous operating rod for the circuit breaker.
The invention is primarily intended for rated voltages of the order
of magnitude of 100 - 300 kV but may advantageously also be used at voltages both
below and above this range. Preferably, an insulator according to the invention
is filled with an electrically insulating gas, preferably SF6, but may
also be filled with other types of insulating and arc-extinguishing media. In the
following, the designations insulation, insulant, and the like, relate to insulation
against electrical flashover.
An insulator usually comprises two attachment means retained by an
insulant. Its primary task is to electrically separate two poles with different
voltage potential. For applications within high-voltage engineering, insulants of
porcelain or of polymeric material are preferably used. To attach them to each other
or to another object, such as a base, a mechanism housing, a circuit-breaker unit,
or a conductor, the poles of the insulator are equipped with attachment flanges.
As dimensioning criterion for an insulator, there is primarily used that distance
between the poles which provides sufficient safety against flashover, but also other
aspects, such as resistance to mechanical forces, may occur. In the case of hollow
insulants, also the thickness of the material in relation to the length and the
diameter is of importance. In gas-filled insulators, also the tightness of the material
and in pole connections must be taken into consideration. The attachment flanges
must be able to be connected tightly to the insulants and also withstand a certain
overpressure of the enclosed insulating gas.
A gas-filled insulator of the above-mentioned kind with one attachment
flange at each end is previously known from US-A-4 827 373. A support insulator
here supports an extinguishing chamber insulator comprising an interrupting element,
whereby a hermetically sealed space, filled with SF6 gas, extends through
the units. The insulant of both the support insulator and the extinguishing chamber
insulator is in the form of a hollow, circular-cylindrical porcelain body, the outer
surface of which is enamelled. The porcelain body exhibits a number of water-repellent
grooves along the main part of its length. At each end the porcelain body has a
smooth cylindrical portion, each of which being surrounded by an annular pole of
electrically conducting material provided with an attachment flange.
The support insulator and the extinguishing chamber insulator are
attached to each other by a screw joint, a connection flange for connection to the
network being placed between the insulators. The connection flange is first attached
to the pole ring of one of the insulators, in which case good tightness against
the leakage of gas must be ensured. After this, the insulator, with the connection
flange mounted thereon, is attached to the pole ring of the second insulator. Also
with this attachment, the tightness against leakage of gas must be ensured. In this
connection, a problem with the known insulator is that, during mounting, two cut
surfaces must be sealed and that the time of mounting may be considerable.
To achieve sufficient safety against electric flashover, the distance
between the poles must be made large. This distance is counted between two adjacent
electrically conducting parts of the poles. In this case, these consist of the pole
rings externally fixed around the ends of the porcelain body. The total overall
height of the insulator is thus the safety distance plus the height of each pole
ring. In this way, the insulator will have a height which is larger than that which
is defined by the safety distance. A problem arises in that the insulant, to be
able to resist mechanical transverse forces in relation to the overall height, must
be dimensioned stronger than what would be necessitated by the transverse forces
in relation to the safety distance.
An additional gas-filled insulator according to the preamble of claim
1 is known from SU 1557590. This insulator comprises an insulant wound or cast onto
two end poles. The poles are designed with radially projecting elevations which
engage into radial slots formed in the insulant during the casting. By the overpressure
in the insulator, the elevations are brought to be pressed against the sides of
the slots such that a self-sealing joint is formed by way of a coating. A disadvantage
with the known insulator is that, during casting of the insulant, a mould must be
arranged on the inside of the insulator. This mould must either be left or be able
to be removed in some other way afterwards. The possibilities of achieving an insulant
with good insulating properties, which may be controlled afterwards, are thus not
good. In case of outdoor mounting, an insulator must be provided with grooves to
increase the leakage path and reduce the risk of flashover. A further disadvantage
of the known insulator is that such an outer insulant, provided with grooves, must
be manufactured in an additional process, which increases the cost and increases
risk of the occurrence of air pockets which may jeopardize the safety.
SUMMARY OF THE INVENTION
The object of the invention is to achieve an insulator of the above-mentioned
kind, which has a lower overall height for a given rated voltage and which entails
a saving of material and time during mounting. The lower overall height permits
the thickness of the material and the length of the insulant to be made smaller,
thus reducing the production cost. These properties are achieved according to the
invention by means of an insulator which exhibits the features described in the
characterizing part of the claims 1-6.
According to the invention, a flanged annular pole is inserted and
attached with its annular part on the inside of a hollow insulant. In this way the
better insulating capacity, in relation to that of the external atmosphere, of the
insulating gas enclosed in the insulator is utilized. The safety distance in relation
to flashovers is smaller with an insulating gas between two poles than with air
between the poles. By placing the annular parts of the poles on the inside of the
gas-filled insulant, the distance between the poles can be reduced without jeopardizing
the safety with respect to flashover. On the outside of the insulant, only the flanges
of the poles are exposed, which flanges thus constitute the conducting parts between
which the safety distance in air is to be defined. The saving in height may amount
to 10 % of the overall height of traditionally designed insulators.
By designing the attachment flanges so that they can be directly connected
to each other, the flanged connection plate may be completely omitted. This results
in only one cut surface which has to be sealed as well as in one joint with fewer
parts. The attachment flanges are formed with plane attachment surfaces, in which
an O-ring placed in a slot is arranged for sealing. The plane flanges may be provided
with projecting portions or be arranged with an oval plane section to make possible
connection into the network.
Another object of the invention is a method for manufacturing such
a gas filled electric insulator as claimed in claim 7.
BRIEF DESCRIPTION OF THE DRAWING
The invention will be explained in greater detail by description of
an embodiment with reference to the accompanying drawing, wherein
DESCRIPTION OF THE PREFERRED EMBODIMENTS
- Figure 1
- shows a section through two insulators according to the invention which are
attached to each other, whereof the lower insulator is a support insulator and the
upper is an extinguishing chamber insulator.
Figure 1 shows the upper part of a lower support insulator comprising
a tubular inner insulant 1 of, for example, glass fibre-reinforced epoxy resin,
onto which is cast an outer insulant provided with grooves and being, for example,
of silicone rubber. Into the upper part of the tubular inner insulant 1, an upper
pole 3 of electrically conducting material is inserted. The pole 3 comprises an
annular part 9, the outer diameter of which is smaller than the inner diameter of
the inner insulant 1. The annular part 9 is fixed to the insulant 1 with an adhesive
6, such as glue or the like. The pole further comprises a flange 4, the outer portion
5 of which is rounded to avoid concentrations of the electric field. The mounting
surface 7 of the flange is plane and has a slot 8, in which an O-ring (for sealing
against an end surface) may be placed. The annular part 9 of the pole, facing inwardly
towards the insulator, has rounded edges to avoid electric field concentrations.
An extinguishing chamber insulator, comprising a circuit breaker and
the lower part of which is shown in Figure 1, is mounted against the support insulator.
The extinguishing chamber insulator comprises an inner insulant 1' of, for example,
glass fibre-reinforced epoxy resin, onto which there is cast an outer insulant 2'
of, for example, silicone rubber provided with grooves. Into the tubular inner insulant
1', an upper pole 3' of electrically conducting material is inserted. The pole 3'
comprises an annular part 9', the outer diameter of which is smaller than the inner
diameter of the inner insulant 1' and which is fixed to the insulant 1' with an
adhesive 6' such as glue or the like. Further, the pole comprises a flange 4', the
diameter of which is larger than that of the corresponding flange 4 of the support
insulator and the outer portion 5' of which is rounded to avoid concentrations of
the electric field. The mounting surface 7' of the flange 4' is plane and provided
with a circular slot 8' with a square cross section, in which is placed an O ring
10 for sealing between the flanges 4, 4' which are mounted against each other.
The extinguishing chamber insulator is fixed to the support insulator
by fixing their attachment flanges 4, 4' to each other with a screw joint 11. The
screws are threaded in the upper flange 4' and, upon tightening, press the two attachment
surfaces 7, 7' against each other. The O ring 10 located in the slot 8' is thereby
subjected to an elastic deformation which causes it to seal against the two flanges.
An upper current collector 12 is connected to the upper pole 3' with a screw joint
13 to be further connected to the network by means of a conductor (not shown) connected
to the flange 4'.
The insulators which are assembled together in the manner described
above have a common internal space which is filled with an insulating gas, for example
SF6. The safety distance with respect to flashover is therefore smaller
on the inside of the insulator than on the outside thereof. The poles inserted into
each end of the insulants make use of this effect. The distance between the inserted
parts 9, 9' of the poles and the parts inserted into the other end of the respective
insulant is shorter than the corresponding distance between the flanges 5, 5' exposed
on the outside of the insulant. Since a shorter safety distance may be allowed on
the inside of the insulator, the dimensioning safety distance is thereby defined
between those outer edge portions of the flanges which are exposed on the outside
at the two ends of the insulator. The effect thus utilized permits the insulant
to be made shorter. A saving in length of up to 10 % may thus be achieved, which
entails a corresponding saving in costs.
The shorter length gives the insulator greater mechanical resistance
to transverse forces. If, therefore, the same resistance to transverse forces is
desired as with a longer insulator, the thickness of the material may be reduced.
In this way a saving in costs of about 5 % may be utilized. The connection flange
which is dispensed with by the invention constitutes a further saving of material
and a reduction of the mounting time, which entails a saving in costs of about 5
%. In total terms, thus, a saving of about 20 % may be obtained when utilizing the
The invention is not only intended to be utilized for support insulators
or extinguishing chamber insulators, but may arbitrarily be used with all types
of hollow insulators, both for high voltage and at a lower voltage. The poles inserted
into the insulant may also be attached by means of a shrinkage fit.