Note: Descriptions are shown in the official language in which they were submitted.
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TITLE OF THE INVENTION
OPTICAL VOLTAGE MEASURING APPARATUS
BACKGROUND OF THE INVENTION
The present invention relates to an optical
voltage measuring apparatus which can improve the
measurement precision of a main circuit voltage of a
switching device which is used, for example, in a
generating/transforming station.
In a high-voltage switching device of several kV
or more, a main circuit voltage is measured by
electrostatic capacitance voltage division or
resistance voltage division. In this kind of ineasuring
deice, there is known a cone-shaped insulation spacer
which is used in a gas-insulation switching device as
shown in FIG. 1(see, e.g. Jpn. Pat. Appln. KOKAI
Publication No. 2000-232719 (page 4, FIG. 1).
As shown in FIG. l, a main circuit conductor 1 is
supported by a cone-shaped insulation spacer 2, and is
insulated from a cylindrical tank 3 which is at a
ground potential. The insulation spacer 2 is made up
of a first dielectric body 4 which is formed by
injecting an epoxy resin, and a second dielectric body
5 which has a lower resistivity than the first
dielectric body 4 and is formed in a layer shape on the
surface of the first dielectric body 4. An annular
buried electrode 6 is buried in the first dielectric
body 4 on the tank 3 side. An annular buried metal 7
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is provided at both ends of an outer peripheral end
portion, and is airtightly fixed to the end of the tank
3.
A secondary-side capacitor 8 is connected to the
buried electrode 6, and the buried electrode 6 is
grounded via the buried metal 7. A detection impedance
9 is connected in parallel with the secondary-side
capacitor 8. A primary-side electrostatic capacitance
and a primary-side volume resistance 11 by the
10 second dielectric body 5 are formed between the main
circuit conductor 1 and the buried electrode 6.
Thereby, a voltage, which is divided by the
primary-side electrostatic capacitance 10 and the
secondary-side capacitor 8 including the detection
impedance 9, can be measured. The second dielectric
body 5 is provided in order to improve the time
constant. In the case of high frequencies, voltage
division is executed by the primary-side volume
resistance 11, and the main circuit voltage can
precisely be measured.
On the other hand, there is known a voltage
measurement technique using an electro-optic element
(Pockels effect element) (see, e.g. Jpn. Pat. Appln.
KOKAI Publication No. 2000-258465 (page 3, FIG. 1)).
However, the voltage that can be applied to the
electro-optic element is about 1 kV or less, and a
voltage divider has to be used in order to measure a
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high voltage. Thus, a voltage divider with high
precision is required and, if dielectric strength, etc.
is considered, the size of the voltage divider becomes
large, and the gas-insulation switching device itself
becomes large in size.
In the main circuit voltage measurement of the
above-described conventional high-voltage switching
device, there are the following problems. If an
electro-optic element is to be used, a dedicated
voltage divider is required, and the size of the gas-
insulation switching device becomes large. Then, a
voltage dividing circuit may be fabricated by using the
insulation spacer 2 to measure the main circuit
voltage. In this case, the primary-side electrostatic
capacitance 10 and primary-side volume resistance 11 of
the primary-side voltage dividing circuit are present
within the tank 3, and the secondary-side capacitor 8
and detection impedance 9 of the secondary-side
voltage-dividing circuit are present in the air outside
the tank 3. A temperature rise occurs due to power-on
current within the tank 3, and a temperature variation
occurs outside the tank 3. It is difficult to make
similar the temperature characteristics of the primary-
side and secondary-side voltage dividing circuits. In
addition, although the humidity in the tank 3 is low,
there is an effect of humidity on the outside of the
tank 3.
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Under the circumstances, there is a demand for a
measuring apparatus which can measure a main circuit
voltage with high precision, with the fabrication of a
voltage dividing circuit which can make similar
environmental characteristics such as temperature and
humidity by using, e.g. the insulation spacer 2 that is
an insulating structure of a gas-insulation switching
apparatus. In addition, there is a demand for a
measuring apparatus which can prevent, in the case of
using an electro-optic element, a large variation in
secondary-side impedance due to the connection of the
electro-optic element, and which can obtain a stable
voltage division ratio.
BRIEF SUMMARY OF THE INVENTION
It is an object of the present invention to
provide an optical voltage measuring apparatus which
measures a main circuit voltage with high precision,
without being affected by environmental
characteristics.
According to an aspect of the present invention,
there is provided an optical voltage measuring
apparatus comprising: a main circuit conductor, a
dielectric body which insulatingly supports the main
circuit conductor and is fixed to a grounding member, a
buried electrode buried in the dielectric body, and an
electro-optic element which is connected to the buried
electrode and measures a voltage of the main circuit
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conductor, wherein a voltage, which is divided by an
electrostatic capacitance ratio between an
electrostatic capacitance, which is created between the
main circuit conductor and the buried electrode, and an
electrostatic capacitance, which is created between the
buried electrode and the grounding member, is applied
to the electro-optic element.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
The accompanying drawings, which are incorporated
in and constitute a part of the specification,
illustrate embodiments of the invention, and together
with the general description given above and the
detailed description of the embodiments given below,
serve to explain the principles of the invention.
FIG. 1 is a cross-sectional view of an insulation
spacer which functions as a conventional voltage
divider;
FIG. 2 is a cross-sectional view of an insulation
spacer which functions as a voltage divider according
to Embodiment 1 of the present invention; and
FIG. 3 is a cross-sectional view of a post spacer
which functions as a voltage divider according to
Embodiment 2 of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will now be
described with reference to the accompanying drawings.
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[Embodiment 1]
To begin with, an optical voltage measuring
apparatus according to Embodiment 1 of the present
invention is described with reference to FIG. 2.
FIG. 2 is a cross-sectional view of an insulation
spacer which functions as a voltage divider according
to Embodiment 1 of the invention. In FIG. 2, the
structural parts common to those in the prior art are
denoted by like reference numerals.
As is shown in FIG. 2, a main circuit conductor 1
of a gas-insulation switching device is supported by
and fixed to a cone-shaped insulation spacer 2, and is
insulated from a cylindrical tank (grounding member) 3
which is at a ground potential. The insulation spacer
2 has a first dielectric body 4 which is formed by
injecting an epoxy resin. An annular buried electrode
6 is buried in the first dielectric body 4 on the tank
3 side. An annular buried metal 7 is provided at both
ends of an outer peripheral portion, and is airtightly
fixed to the end of the tank 3. An insulation gas is
filled in the tank 3.
One end of an electro-optic element 20, which is
formed by using a single crystal of BGO, BSO, etc., is
connected to the buried electrode 6, and the other end
of the electro-optic element 20 is grounded.
Measurement light (optical signal) for measuring a
voltage is made incident on the electro-optic element
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20 via a light source driving device 21, a light source
22 such as a light-emitting diode, an optical fiber 23,
a light guide collimator unit 24, a polarizer 25 for
converting incident light to linearly-polarized light,
and a 1/4 wavelength plate 26 which converts linearly-
polarized light to circularly-polarized light.
The electro-optic element 20 converts the incident
circularly-polarized light to elliptically-polarized
light in accordance with the intensity of electric
field, and emits measurement light. The measurement
light passes through an analyzer 27, and only one
polarization component is emitted. The emitted light
is guided to an optical fiber 29 via a light reception
collimator unit 28, and is sent to a detector 30. The
detector 30 converts the measurement light to an
electric signal, and a measured voltage is calculated
by an electronic circuit 31. The components from the
light guide collimator unit 24 to the light reception
collimator unit 28 are accommodated in a shield case 32
which eliminates the effect of electric field.
In the insulation spacer 2, a primary-side
electrostatic capacitance 10 is created between the
main circuit conductor 1 and the buried electrode 6,
and a secondary-side electrostatic capacitance 33 is
created between the buried electrode 6 and the tank 3.
Accordingly, a voltage, which is divided by a compound
capacitance of the primary-side electrostatic
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capacitance 10, the electrostatic capacitance of the
electro-optic element 20 itself and the secondary-side
electrostatic capacitance 33, is applied to the
electro-optic element 20.
In this case, the electrostatic capacitance of the
electro-optic element 20 is much smaller than the
secondary-side electrostatic capacitance 33, and the
voltage division ratio is substantially determined by
the secondary-side electrostatic capacitance 33. The
reason for this is that despite the specific dielectric
constant of the electro-optic element 20 being higher
than that of the epoxy resin, the electrode disposition
between the buried electrode 6 and the tank 3 is a
coaxial electrode arrangement, and the mutually opposed
electrode area becomes much greater than the area of
the electro-optic element 20. In the insulation spacer
2 with a diameter of about 300 mm, the electrostatic
capacitance ratio between the electro-optic element 20
and the secondary-side electrostatic capacitance 33 is
100 or more.
Thereby, the voltage that is divided by the
electrostatic capacitance ratio between the primary-
side electrostatic capacitance 10 and secondary-side
electrostatic capacitance 33 is applied to the electro-
optic element 20, and the main circuit voltage can be
measured. The primary-side electrostatic capacitance
10 and secondary-side electrostatic capacitance 33 are
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formed of the same epoxy resin, and have similar
variations in electrostatic capacitance due to
temperature variations. Since a predetermined low
humidity is constantly kept within the tank 3, the
inside of the tank 3 is not affected by humidity.
Specifically, the primary-side electrostatic
capacitance 10 and secondary-side electrostatic
capacitance 33 are exposed in the same environment.
Although a floating electrostatic capacitance in the
insulation gas is added to the primary-side
electrostatic capacitance 10, there is no influence by
environmental characteristics, similarly with the
above-described case.
According to the optical voltage measuring
apparatus of Embodiment 1, the buried electrode 6 is
buried in the first dielectric body 4 that is formed of
the epoxy resin, and the voltage that is applied to the
electro-optic element 20 is divided by the primary-side
electrostatic capacitance 10 and secondary-side
electrostatic capacitance 33, which are formed of the
same epoxy resin. Therefore, the electrostatic
capacitance ratio is not affected by the environmental
characteristics such as temperature and humidity, and
the main circuit voltage can be measured with high
precision.
[Embodiment 2]
Next, an optical voltage measuring apparatus
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according to Embodiment 2 of the present invention is
described with reference to FIG. 3. FIG. 3 is a cross-
sectional view of a post spacer which functions as a
voltage divider according to Embodiment 2 of the
invention. Embodiment 2 differs from Embodiment 1 with
respect to the insulator that functions as the voltage
divider. In FIG. 3, the structural parts common to
those in Embodiment 1 are denoted by like reference
numerals, and a detailed description thereof is
omitted.
As shown in FIG. 3, a main circuit conductor 1,
which is coupled by a coupling 35, is supported and
fixed by a post spacer 36 which has a first dielectric
body 4 formed of an epoxy resin. A main-circuit-side
buried metal 37 is buried in the first electric body 4
on the coupling 35 side. A cylindrical ground-side
buried metal 38 is buried in the first electric body 4
on the tank 3 side, and the buried metal 38 is fixed to
the tank 3. A columnar buried electrode 39 for
dividing the main circuit voltage is buried in a
substantially central part of the ground-side buried
metal 38. An electro-optic element is connected to the
buried electrode 39.
Thereby, a primary-side electrostatic capacitance
40 is formed between the main-circuit-side buried metal
37 and the buried electrode 39, and a secondary-side
electrostatic capacitance 41 is formed between the
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buried electrode 39 and the ground-side buried metal
38, and thus the main circuit voltage is divided. The
buried electrode 39 and the ground-side buried metal 38
are disposed in a coaxial electrode arrangement,= and
the secondary-side electrostatic capacitance 41 is
greater than the primary-side electrostatic capacitance
40. Since these electrostatic capacitances 40 and 41
are created by the first dielectric body 4 of the same
insulation material and are used in the same
environment, there is no influence by the environmental
characteristics.
According to the optical voltage measuring
apparatus of Embodiment 2, the same advantageous
effects as in Embodiment 1 can be obtained.
The present invention is not limited to the above-
described embodiments, and the invention can be
variously modified and implemented without departing
from the spirit of the invention. In the above-
described embodiments, the first dielectric body 4 has
been described as being formed by using a general epoxy
resin. If inorganic material, such as silica, is
added, the temperature characteristics can be relaxed.
In addition, the specific dielectric constant can
easily be adjusted by the mixture ratio, and the range
of choices of electrostatic capacitance can be
increased. Moreover, other insulation materials, which
are used in electric appliances, such as polycarbonate
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resin, polyester resin and phenol resin, can be used.
In the case where use is made in a managed
electric room and a surface creeping leak current due
to contamination and wetting is negligible, the use in
the air is applicable. Specifically, the main circuit
voltage can precisely be measured in the case where the
primary-side electrostatic capacitance 10, 40 and the
secondary-side electrostatic capacitance 33, 41 are
formed of the same insulation material, use is made in
the same environment and surface creepage insulation
strength is high.
As has been described above in detail, according
to the present invention, the primary-side
electrostatic capacitance and the secondary-side
electrostatic capacitance are formed of the same
insulation material and the voltage that is divided by
the electrostatic capacitance ratio therebetween is
connected to the electro-optic element. Therefore, the
main circuit voltage can be measured with high
precision, without the electrostatic capacitance ratio
being affected by environmental characteristics such as
temperature and humidity.
Additional advantages and modifications will
readily occur to those skilled in the art. Therefore,
the invention in its broader aspects is not limited to
the specific details and representative embodiments
shown and described herein. Accordingly, various
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modifications may be made without departing from the
spirit or scope of the general inventive concept as
defined by the appended claims and their equivalents.
