BACKGROUND OF THE INVENTION
Field of the Invention
[0001] The present invention relates generally to a DC bushing employed in an electric power
transmission system. More particularly, the invention is concerned with an improved
structure of a DC bushing equipped with a shield barrier enclosing a lower insulated
shield disposed at a lower portion of the DC bushing.
Description of the Related Art
[0002] Not only in Japan but also in other countries, electric energy demand goes on increasing
steadily, which is accompanied with a trend of developing an extended scale of high-rated
transmission system as well as complexity or intricacy in the system configuration.
Further, in view of the situations which the electric energy enterprises have been
confronting in recent years, the power generation plants tend to be installed at locations
remote from the areas where the consumers are resident. Besides, the scale of the
transmission system is increasing. Such being the circumstances, there are required
stability and reliability of the transmission system inclusive of stability of voltage,
enhanced short-circuit capacity and so forth. To this end, a DC (Direct Current) power
transmission is considered as a means which can promise effective solution meeting
the requirements mentioned above. In this conjunction, structurization of a DC transmission
system rated 500 kV (hereinafter also referred to as the DC-500 kV transmission system)
is being planned. For realization of such DC-500 kV transmission system, DC bushings
for DC apparatuses and machines are indispensable.
[0003] Figure 1 of the accompanying drawings shows a DC bushing in the state mounted in
transformer equipment. Referring to the figure, in the transformer equipment provided
in a transmission line, a transformer 20 composed of a core 21 and a coil 22 is disposed
within a tank 11 which is filled with an insulating oil 23. In such transformer equipment,
a DC bushing 24 is employed for insulating an output or input line of the transformer
20 from the tank 11.
[0004] In conjunction with DC insulation, it is noted that distribution of the electric
filed as making appearance is determined primarily by voltage apportionment which
in turn is determined by resistivities of the insulating oil and oil-impregnated paper.
Accordingly, for realization of the satisfactory DC insulation, it is required in
addition to the insulation techniques adopted heretofore in the insulation for AC
apparatuses and/or instruments that the dielectric strength of a solid insulation
member such as the oil-impregnated paper having higher resistivity as compared with
the insulating oil has to be increased.
[0005] Under the circumstances, there has been proposed such an insulation structure as
shown in Fig. 2 of the accompanying drawings. More specifically, Fig. 2 shows an insulation
structure disclosed in JP-A-56-81909. Referring to the figure, a porcelain tube 1
supported in a tank 11 filled with insulating oil is provided with a barrier 6 at
a lower portion of the porcelain tube 1, the barrier 6 being formed by oil-impregnated
paper, wherein a shielding electrode 3 which is so disposed as to enclose a metal
flange 10 mounted at a bottom end of the porcelain tube 1 is covered with an insulating
cover or layer 4 formed of oil-impregnated paper to realize a lower insulated shield
5. With the insulation structure described above, voltage as applied is apportioned
among the solid insulators with the DC dielectric strength being thus enhanced. Generally,
in the case of a solid insulator formed by radially laminated oil-impregnated paper
layers, a breakdown electric field of high intensity makes appearance in the direction
thicknesswise of the oil-impregnated paper layers. Thus, in order to prevent the dielectric
breakdown from occurring, starting from the conductor 2 and the lower insulated shield
5, outer peripheries of these portions are enclosed by a pulp-molded shield barrier
8 formed of oil-impregnated paper, while an oil gap 7 is provided between the shield
barrier 8 and the lower insulated shield 5 for allowing the insulating oil to flow
through the oil gap 7.
[0006] In the conventional DC bushing known heretofore, a mineral oil is used as the insulating
oil, while the solid insulator is formed of kraft paper or press-board impregnated
with mineral oil. In this conjunction, it is noted that volume resistivity of the
mineral oil is lower than that of the oil-impregnated paper by one order of magnitude.
Consequently, when a DC voltage is applied, a major proportion of the voltage will
be borne by the oil-impregnated paper forming the insulating cover 4 of the lower
insulated shield 5, resulting in that the electric field which is higher by one order
of the electric field appearing in the oil gap 7 acts on the oil-impregnated paper
layer. Besides, due to structural or geometrical factors, the electric field appearing
at an upper end portion of the lower insulated shield 5 immersed in the oil exhibits
highest intensity. For these reasons, the insulation certainly can be ensured for
the DC voltage of 250 kV with the structure of the conventional DC bushing described
above. However, when DC voltage of 500 kV is applied, dielectric breakdown may take
place, starting from the upper end portion of the lower insulated shield 5.
SUMMARY OF THE INVENTION
[0007] In the light of the state of the art described above, it is an object of the present
invention to provide a DC bushing which can ensure an increased DC dielectric strength
in the vicinity of the lower insulated shield and which can thus assure improved reliability
of the DC bushing.
[0008] In view of the above and other objects which will become apparent as the description
proceeds, the invention is directed to a DC bushing which is comprised of a porcelain
tube constituting a lower portion of the DC bushing and immersed in an insulating
oil contained in a tank, a lower insulated shield provided at a lower end portion
of the porcelain tube and including a shielding electrode covered with an insulating
cover, and a shield barrier disposed around an outer periphery of the lower insulated
shield with an oil gap being defined between the shield barrier and the lower insulated
shield.
[0009] In the DC bushing of the structure described above, it is taught according to a first
aspect of the present invention to realize the shield barrier with a thickness which
is greater than a width of the aforementioned oil gap as viewed in a direction widthwise
of the oil gap.
[0010] Further, according to another aspect of the invention, it is taught to form the shield
barrier by using a solid insulator which exhibits a higher volume resistivity than
that of the oil-impregnated paper.
[0011] According to yet another aspect of the present invention, it is taught to implement
the shield barrier in such a structure that upon application of a DC voltage, a proportion
of the DC voltage to be borne by the shield barrier is larger than 15 % inclusive.
[0012] The above and other objects, features and attendant advantages of the present invention
will more easily be understood by reading the following description of the preferred
embodiments thereof taken, only by way of example, in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] In the course of the description which follows, reference is made to the drawings,
in which:
Fig. 1 is a view showing schematically an apparatus in which a DC bushing known heretofore
is mounted;
Fig. 2 is a sectional view showing a DC bushing known heretofore;
Fig. 3 is a sectional view showing a DC bushing according to an exemplary embodiment
of the present invention;
Fig. 4 is a fragmentary enlarged sectional view showing a major portion of the conventional
DC bushing together with a map of electric field making appearance upon application
of a DC voltage;
Fig. 5 is a fragmentary enlarged sectional view showing a major portion of the DC
bushing shown in Fig. 3 together with a map of electric field making appearance upon
application of a DC voltage; and
Fig. 6 is a sectional view showing a DC bushing according to another exemplary embodiment
of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0014] Now, the present invention will be described in detail in conjunction with what is
presently considered as preferred or typical embodiments thereof, by referring to
the drawings. In the following description, like reference characters designate like
or corresponding parts throughout the several views. Also in the following description,
it is to be understood that such terms as "upper", "lower", "top", "bottom" and the
like are words of convenience and are not to be construed as limiting terms.
[0015] Figure 3 is a sectional view showing a DC bushing which is designed for use in an
electric power transmission system rated DC 500 kV according to an exemplary embodiment
of the invention.
[0016] Referring to Fig. 3, a DC bushing 24 is fixedly mounted in a tank 11 which is filled
with an insulating oil, wherein a conductor 2 is secured to a bottom portion of a
porcelain tube 1 of the DC bushing 24 by using a metal flange 10. A lower insulated
shield 5 is provided in such disposition that outer peripheral portions of the metal
flange 10 and the conductor 2 are enclosed by the lower insulated shield 5. To this
end, the lower insulated shield 5 is composed of a shielding electrode 3 and an insulating
cover 4 formed by winding oil-impregnated crepe paper around the shielding electrode
3. A barrier 6 formed of crepe paper is mounted at a lower portion of the porcelain
tube 1 in such disposition as to close a gap formed between the porcelain tube 1 and
the lower insulated shield 5. Further, a shield barrier 8 made of oil-impregnated
paper and serving as a solid insulator is so disposed as to cover a lower end portion
of the barrier 6 and an outer peripheral portion of the lower insulated shield 5,
wherein an oil gap is defined between the shield barrier 8 and the lower insulated
shield 5 for allowing the insulating oil to move through the oil gap 7.
[0017] Figure 5 is a fragmentary enlarged sectional view showing a major portion of the
DC bushing together with a map of electric field which makes appearance in a top end
portion of the lower insulated shield 5 and the shield barrier 8 disposed in opposition
to the lower insulated shield 5, when a DC voltage of 500 kV is applied to the conductor
2.
[0018] As can be seen in Fig. 5, the shield barrier 8 has a thickness W2 which is greater
than a width W1 of the oil gap 7 defined between the lower insulated shield 5 and
the shield barrier 8 disposed in opposition to the lower insulated shield 5, as viewed
in the direction widthwise of the oil gap 7. For the purpose of comparison, Fig. 4
shows a map of electric field making appearance in the conventional DC bushing shown
in Fig. 2. As can be seen in Fig. 4, in the case of the DC bushing known heretofore,
the thickness W3 of the shield barrier 8 is smaller than the width W1 of the oil gap
7 defined between the lower insulated shield 5 and the shield barrier 8 for some reasons
from the standpoint of manufacturing. More specifically, the shield barrier 8 is provided
intrinsically for the purpose of preventing the dielectric breakdown which may occur,
starting from the lower insulated shield 5 and the conductor 2, and it has heretofore
been believed that there is no necessity of increasing the thickness of the shield
barrier 8 because of higher intensity of the electric field in the solid insulator
as viewed in the direction perpendicularly to the layers of the solid insulator. However,
when the thickness W2 of the shield barrier 8 is increased, as described above, a
portion of the voltage which is to be borne by the barrier 6 in the case of the conventional
DC bushing (see Fig. 4) is transferred to the shield barrier 8, as can be seen from
Fig. 5, as a result of which the intensity of the electric field internally of the
barrier 6 and the insulating cover 4 is correspondingly mitigated, whereby the dielectric
strength of the DC bushing as a whole can be enhanced.
[0019] Parenthetically, it has experimentally been established that when the shield barrier
8 is formed in a thickness W2 of 25 mm which is larger than the width of the oil gap
and when the DC voltage of 500 kV is applied to the conductor 2, the proportion of
the voltage appearing across the insulating cover 4 of the lower insulated shield
5 (i.e., the proportion of the voltage to be borne by the insulating cover 4) is 66
% while that of the shield barrier 8 is 34 %. By contrast, when a DC voltage of 500
kV is applied to the conductor 2 of the conventional DC bushing shown in Fig. 4 in
which the thickness W3 of the shield barrier 8 is 5 mm, proportion of the voltage
to be borne by the insulating cover 4 of the lower insulated shield 5 amounts as high
as 92 %, whereas proportion of the voltage to be borne by the shield barrier 8 is
8 %. From the comparison of these experimental demonstrations, it will readily be
understood that by virtue of the structure of the DC bushing according to the invention,
the intensity of electric field making appearance in the insulating cover 4 can significantly
be mitigated or reduced. Thus, according to the teaching of the invention incarnated
in the embodiment illustrated in Figs. 3 and 5, there can be realized a DC bushing
which ensures increased dielectric strength and hence a high reliability of the bushing
for use in the DC transmission applications.
[0020] Figure 6 is a sectional view showing a DC bushing designed for use in a DC-500-kV
transmission system according to another embodiment of the invention.
[0021] The DC bushing shown in Fig. 6 differs from the structure shown in Fig. 3 in respect
to the material and the thickness of the shield barrier. More specifically, in the
DC bushing according to the instant embodiment of the invention now under consideration,
the shield barrier designated by reference numeral 9 is formed of a solid insulator
having a higher volume resistivity than that of the oil-impregnated paper with the
thickness of the shield barrier 9 being held substantially same as that of the shield
barrier 8 employed in the conventional DC bushing. Except for these differences, the
DC bushing according to the instant embodiment of the invention is substantially same
as the conventional one. Accordingly, components same as or equivalent to those of
the DC bushing shown in Fig. 3 are designated by like reference characters and repeated
description thereof is omitted.
[0022] In the case of the DC bushing shown in Fig. 6, the insulating cover 4 is formed of
oil-impregnated paper such as kraft paper, pressboard or the like which has a volume
resistivity ranging from of 10
15 to 10
16 Ωcm. In the DC bushing now of concern, the shield barrier 9 is formed of an insulation
material having the volume resistivity which is higher than that of the oil-impregnated
paper by one order of magnitude. As the preferred insulation material for forming
the shield barrier 9, there may be mentioned engineering plastic materials such as,
for example, PET (polyethylene terephthalate), PTFE (polytetrafluoride ethylene),
PPO (polyphenylene oxide), PPS (polyphenylene sulfide), PMP (polymethyle pentene),
PE (polyethylene) and the like. The shield barrier 9 may be formed by molding a sheet
of the material mentioned above or by winding a film of the material mentioned above.
The materials enumerated above have specific inductive capacities falling within a
range of "2" to "3". In other words, the dielectric constant of these materials is
lower than that of the oil-impregnated paper and can not give rise to concentration
of the electric field in the oil gap even when an AC voltage is applied to the conductor
2. For these reasons, the materials mentioned above is preferred for forming the shield
barrier 9.
[0023] With the structure of the DC bushing described above, the proportion of the voltage
borne by the shield barrier 9 upon application of DC voltage to the conductor 2 is
greater than that to be borne by the shield barrier of the conventional DC bushing.
Thus, the intensity of the electric field appearing in the insulating cover 4 can
be mitigated, whereby the DC dielectric strength of the DC bushing can be improved
with the reliability thereof being enhanced.
[0024] In this conjunction, it has experimentally been established that when the shield
barrier 9 is formed of PTFE having highest volume resistivity in a thickness of 5
mm which is substantially equal to the width of the oil gap 7 and when the DC voltage
of 500 kV is applied to the conductor 2, the proportion of the voltage appearing across
the insulating cover 4 of the lower insulated shield 5 (i.e., the proportion of the
voltage to be borne by the insulating cover 4) is 76 % while that of the shield barrier
9 is 24 %.
[0025] In the case of the DC bushing shown in Fig. 3, the thickness W3 of the shield barrier
8 is selected to be greater than the width W1 of the oil gap 7 formed between the
shield barrier 8 and the lower insulated shield 5, while in the DC bushing shown in
Fig. 6, the shield barrier 9 is formed of a solid insulator having a higher volume
resistivity than that of the oil-impregnated paper, to thereby realize reduction of
the proportion of voltage to be borne by the insulating cover 4. However, the thickness
and/or material of the shield barrier may be determined reversely on the basis of
predetermined proportion of voltage to be borne by the shield barrier. In this conjunction,
experiment conducted by the inventors of the present application has shown that unless
the proportion of voltage to be borne by the shield barrier is greater than 15 % inclusive
thereof, the electric field appearing in the barrier 6 in the direction along the
layers in the vicinity of the top end portion of the lower insulated shield 5 as well
as the in-oil electric field appearing in the lower end portion of the lower insulated
shield 5 becomes high to incur dielectric breakdown. The proportion of voltage to
be borne by the shield barrier which is greater than 15 % inclusive can easily be
realized by increasing the thickness of the shield barrier and/or forming it of a
material having a large volume resistivity.
[0026] In the forgoing description, it has been assumed that in the DC bushing shown in
Fig. 3, the shield barrier 8 is formed of oil-impregnated paper. However, it goes
without saying that when the insulation material of high volume resistivity such as
PET or the like as adopted in the DC bushing shown in Fig. 6 is employed for implementing
the shield barrier 8 shown in Fig. 3, the dielectric strength can further be enhanced.
[0027] Besides, in both of the DC bushings shown in Figs. 3 and 6, respectively, the proportion
of the voltage to be borne by the shield barrier 8 and hence the DC dielectric strength
of the DC bushing on the whole can further be increased by decreasing the width of
the oil gap within a range where convection of the insulating oil for heat dissipation
is not interfered.
[0028] As can be appreciated from the foregoing, in the DC bushings according to the invention,
the proportion of voltage to be borne by the insulating cover of the lower insulated
shield can be reduced by selecting the thickness of the shield barrier to be greater
than the width of the oil gap as viewed in the direction widthwise thereof and/or
by forming the shield barrier of a solid insulator having a higher volume resistivity
than that of oil-impregnated paper or alternatively by realizing the shield barrier
in such a structure that upon application of a DC voltage, the proportion of the DC
voltage to be borne by the shield barrier is higher than 15 % inclusive. With the
structures of the DC bushing according to the invention, the DC dielectric strength
in the vicinity of the lower insulated shield can significantly be improved to assure
high reliability of the DC bushing.
1. A DC bushing (24), comprising:
a porcelain tube (1) constituting a lower portion of said DC bushing (24) and immersed
in an insulating oil (23) contained in a tank (11);
a lower insulated shield (5) provided at a lower end portion of said porcelain tube
(1) and including a shielding electrode (3) covered with an insulating cover (4);
and
a shield barrier (8) disposed around an outer periphery of said lower insulated shield
(5) with an oil gap (7) being defined between said shield barrier (8) and said lower
insulated shield (5),
wherein said shield barrier (8) is realized with a thickness (W2) which is greater
than a width (W1) of said oil gap (7) as viewed in a direction widthwise of said oil
gap (7).
2. A DC bushing (24), comprising:
a porcelain tube (1) constituting a lower portion of said DC bushing (24) and immersed
in an insulating oil (23) contained in a tank (11);
a lower insulated shield (5) provided at a lower end portion of said porcelain tube
(1) and including a shielding electrode (3) covered with an insulating cover (4);
and
a shield barrier (9) disposed around an outer periphery of said lower insulated shield
(5) with an oil gap (7) being defined between said shield barrier (9) and said lower
insulated shield (5),
wherein said shield barrier (9) is formed of a solid insulator having a higher
volume resistivity than that of oil-impregnated paper.
3. A DC bushing according to claim 2,
wherein said shield barrier (9) is realized with a thickness (W2) which is greater
than a width (W1) of said oil gap (7) as viewed in a direction widthwise of said oil
gap (7).
4. A DC bushing (24), comprising:
a porcelain tube (1) constituting a lower portion of said DC bushing (24) and immersed
in an insulating oil (23) contained in a tank (11);
a lower insulated shield (5) provided at a lower end portion of said porcelain tube
(1) and including a shielding electrode (3) covered with an insulating cover (4);
and
a shield barrier (8; 9) disposed around an outer periphery of said lower insulated
shield (5) with an oil gap (7) being defined between said shield barrier (8; 9) and
said lower insulated shield (5),
wherein said shield barrier (8; 9) is implemented in such a structure that upon
application of a DC voltage, a proportion of said DC voltage to be borne by said shield
barrier (8; 9) is higher than 15 % inclusive.