Electricity: measuring and testing – Electromechanical switching device – Circuit breaker
Reexamination Certificate
2001-02-21
2003-08-19
Le, N. (Department: 2858)
Electricity: measuring and testing
Electromechanical switching device
Circuit breaker
C218S063000
Reexamination Certificate
active
06608481
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a pole of a circuit breaker for high- and/or medium-voltage transmission and/or distribution grids, i.e. for voltages greater than 1000 Volt, which comprises a current measuring sensor which is integrated in its structure and is realized by means of optical technologies. The pole according to the present invention is now described with reference to a pole of a high-voltage circuit breaker without thereby limiting in any way the scope of its application.
It is known that current measurements are usually performed in a pole of a high-voltage circuit breaker in order to ensure adequate control of said circuit breaker. Current measurements are generally performed by using measurement poles which are known in the art as current transformers. These measurement poles generally comprise windings on a core made of magnetic material and supporting and insulation structures. Said current measurement poles can be of various kinds and are used according to particular configurations which are described hereinafter.
A first configuration of current transformers is the one known in the state of the art as stand-alone transformer.
FIG. 1
schematically illustrates an example of a current transformer which is generally used in said configuration.
The transformer is mainly constituted by three structural components: an insulator
1
, generally constituted by a finned tube made of polymeric material or porcelain; a head
2
, made of aluminum or steel; and a base
3
which is also made of aluminum or steel and constitutes the structure for anchoring to a supporting surface, for example a supporting pillar.
The primary winding
5
of the transformer is positioned inside the head
2
, as shown in
FIG. 1
, and is constituted by a through bar
6
which is arranged horizontally and fixed to the head
2
in a suitable manner.
The secondary windings
8
of the transformer are arranged inside some toroidal shields
7
and are supported by a supporting tube
9
which is fixed by means of its lower end to the base
3
of the transformer. Inside the tube
9
, conductors
10
from the secondary windings
8
are conveyed and connected, at their terminals, to a terminal box
11
which is arranged at the base
3
of the transformer. A flange
12
between the base
3
and the insulator
1
has holes
13
which are required for the passage of the conductors
10
and for introducing the dielectric gas that arrives from a filling valve (not shown in the figure;) provided in the base
3
. The dielectric gas can be constituted, for example, by sulfur hexafluoride (SF
6
), nitrogen or a mixture of the two gases.
The above described current transformer has several problems due to the use of a transformer having a magnetic core.
Under high currents the magnetic core of the transformer is in fact affected by saturation effects which compromise the current measurement to be performed. These effects force to model the transformer core according to the intensity of the currents to be measured and to the precision with which the measurement is to performed. This entails considerable engineering problems and high manufacturing costs.
Further disadvantages arise from the fact that windings with a magnetic core generally have a limited frequency band and are potentially sensitive to external electromagnetic interference.
These disadvantages lead to high production and operating costs which increase as the operating voltages rise, due to the need to use high-quality magnetic cores in order to ensure adequate repeatability of the performance of the measurement pole.
The stand-alone transformer configuration has, as described hereinafter, considerable problems in terms of bulk and high costs both during installation and during operation.
FIG. 2
is a schematic view of an example of use of said stand-alone transformer configuration in a high-voltage substation in which the pole shown in
FIG. 1
can be used as a current transformer.
The line current flows, for example in the direction of the arrow
24
, across a disconnector
20
to a circuit breaker
1
and from there to a current transformer
22
, already described in FIG.
1
. Access to the remaining part of the substation is gained by means of the disconnector
23
.
The current transformer
22
can be arranged both upstream and downstream of the circuit breaker
21
but in any case it is arranged outside the circuit breaker
21
. In order to ensure adequate insulation for each electrical pole of the line the transformer
22
must be placed on a separate support and located at a suitable distance from the circuit breaker
21
. This entails a considerable overall space occupation of the substation. This fact leads to high installation and operating costs. The plurality of different and separate functional elements inside the substation furthermore entails considerable problems in terms of maintenance and reliability.
FIG. 3
is a schematic view of an example of configuration in which integration between the circuit breaker and the current transformer is provided in a single pole. In particular, as described in
FIG. 3
, said integration is performed inside the body of the circuit breaker. The circuit breaker/current transformer assembly is mainly constituted by three parts, respectively an interruption chamber
30
, shown partially in
FIG. 3
, a region
31
which accommodates primary windings and secondary windings
34
of the transformer (provided on a magnetic core), an insulator
33
and a housing
32
which accommodates means
35
for the actuation of the moving contact of the circuit breaker and secondary terminals
36
of the transformer. Conductors
37
which protrude from the windings
34
are conveyed through a metal tube
38
located inside the insulator
33
to the secondary terminals
36
. Said metal tube
38
also accommodates a rod
39
for actuating a moving contact
40
of the circuit breaker. The primary current flows from the moving contact
40
to an external primary contact
41
which is located at the region
31
that accommodates the windings
34
.
Although the pole of
FIG. 3
advantageously mutually integrates the current measurement pole and the circuit breaker, it still uses current transformers wound on a magnetic core. In this configuration as in others which can be found in the art, the technological problems arising from the use of these components therefore remain. As described earlier, said technological problems are essentially the large space occupation and high costs of the windings and the non-ideal magnetic behavior of the core of these transformers.
There are other known poles which allow to solve the problems that arise from the use of windings on magnetic cores. These poles use optical technologies and are based on the measurement of the rotation of the polarization plane of a light wave which propagates through a transmission medium in the presence of a magnetic field. The rotation is proportional to the intensity of the magnetic field. This property is commonly known as Faraday effect. For the sake of descriptive simplicity, poles of this type are termed hereinafter “optical current sensors”.
FIG. 4
schematically illustrates a first known constructive example of optical current sensor.
An optical fiber
53
is wound on a suitable support (not shown in the figure) around a primary conductor
51
through which there flows a current (represented by the arrow
52
) to be measured. A control system
54
sends a light wave (represented by the arrow
55
) which travels along the optical fiber
53
. Along its path, the light wave
55
emitted by the control system
54
is influenced by the magnetic field (represented by the dashed arrow
50
) generated by the current
52
. Said light wave
55
returns to the control system
54
with its polarization angle rotated by a certain extent. The control system
54
measures this rotation. As already noted the extent of this rotation is proportional to the magnetic field
50
and therefore to the current
52
that flows along the primary con
Bohnert Klaus
Braendle Hubert
Cameroni Roberto
Centenaro Emiliano
Gabus Philippe
ABB T & D Technology Ltd.
Kerveros James C
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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