EP1022749B1

EP1022749B1 - Electrostatic capacitive divided-voltage transformer

Info

Publication number: EP1022749B1
Application number: EP20000101081
Authority: EP
Inventors: Toshio c/o Kabushiki Kaisha Meidensha Sohde; Akira c/o Kabushiki Kaisha Meidensha Kobayashi; Takashi c/o Kabushiki Kaisha Meidensha Sakurai
Original assignee: Meidensha Corp; Meidensha Electric Manufacturing Co Ltd
Current assignee: Meidensha Corp; Meidensha Electric Manufacturing Co Ltd
Priority date: 1999-01-22
Filing date: 2000-01-20
Publication date: 2006-09-06
Anticipated expiration: 2020-01-20
Also published as: DE60030496D1; DE60030496T2; EP1022749A1; JP2000277363A

Description

BACKGROUND OF THE INVENTION:

a) Field of the invention

The present invention relates to an electrostatic capacitive divided-voltage transformer utilizing a power cable or insulating bus bar and applicable to a voltage detector.

b) Description of the related art

A voltage transformer (so-called, VT) includes an inductive VT having a transformer structure and an electrostatic capacitive VT having serially-connected capacitors.
A usage division between the inductive VT and the electrostatic capacitive VT will be made generally in the following way according to a line voltage (system voltage).
In a case of an open-type power substation and a power generation station, the inductive VT has been used up to the line voltage of 36 kV and the electrostatic capacitive divided-voltage VT has been used for a high voltage application equal to or higher than 72.5 kV.
In addition, in a case of a power generation station or a power substation in a GIS (Gas Insulated Switchgear), the inductive VT has been used up to 245 kV and the electrostatic capacitive divided-voltage VT has been used for a voltage application higher than 300 kV.
A boundary between the usage division between the inductive VT and capacitive VT is not strictly determined but is determined for an economical reason such that which one of the two type transformers is cheaper under the same specification. In general, as the voltage is increased, the electrostatic capacitive divided-voltage transformer is more economically advantageous than the inductive VT.
Japanese Patent Application First Publication No. Heisei 8-51719 published on February 20, 1996 (corresponds to a United States Patent No. 5,493,072 issued on February 20, 1996) exemplifies a previously proposed series-connected capacitive graded high-voltage cable terminator and suspension insulator. Japanese Patent Application First Publication No. Heisei 10-79205 published on March 24, 1998 exemplifies a previously proposed power cable.

SUMMARY OF THE INVENTION:

When a direct current voltage (DC voltage) is applied to the inductive VT described above such as a DC insulating withstanding voltage test or a DC insulating test and, then, a frequency is zeroed, the inductive VT becomes zeroed after generating the impedance of ωL₀ due to a presence of a reactance (L₀). Hence, an insulating check in a true meaning of the term cannot be made unless the inductive VT undergoes a test with the VT separated from the line of the system. Consequently, a specially installed disconnecting device or drawing device is needed.
When the inductive VT is mistakenly tested with its winding terminals connected directly to the line voltage, there is a great possibility of the inductive VT being destroyed with a short-circuit current being caused to flow due to the zero impedance. Hence, there are many cases where a power fuse is attached onto a primary winding of the inductive VT having the voltage specification of being equal to or below 3.6 kV.
On the other hand, in the case of the electrostatic capacitive divided-voltage transformer (VT), such a problem as described above does not occur. Therefore, as compared economically with the electrostatic capacitive divided-voltage transformer itself and its accessories, it is expected that a usage range in the electrostatic capacitive divided-voltage transformer can be expected.
Then, along with a development in a digital meter and/or a digital relay, a secondary load of the VT (voltage transformer) is extremely lowered. The VT has a load which conventionally indicates 200 VA but which is recently reduced to 30 VA. In addition, the VT equal to or higher than the voltage 72.5 kV has the load which indicates conventionally 500 VA but which recently indicates 50VA. In the future, a reduction rate of the secondary load will be increased.
In the GIS (Gas Insulated Switchgear), there are many cases where the inductive VT has been used.
If a voltage detection can be made without an increase in space or if the voltage detection can be made without use of SF6 gas, the VT may possibly be the capacitive VT. A limit of manufacturing the capacitive VT can further be expanded.
It is an object of the present invention to provide an electrostatic capacitive divided-voltage transformer utilizing a power cable or an insulating bus bar requiring no installation space.
According to one aspect of the present invention, there is provided with an electrostatic capacitive divided-voltage transformer according to claim 1.
This summary of the invention does not necessarily describe all necessary features so that the invention may also be a sub-combination of these described features.

BRIEF DESCRIPTION OF THE DRAWINGS:

Fig. 1 is a laterally cross sectional view of an electrostatic capacitive divided-voltage transformer in a first preferred embodiment according to the present invention.
Fig. 2 is a schematic connection (wiring) diagram of three-phase type electrostatic capacitive divided-voltage transformer connected to a low-voltage voltage transformer.
Fig. 3 is a longitudinally cross sectioned view of an electrostatic capacitive divided-voltage transformer applicable to a GIS (Gas Insulated Switchgear).
Fig. 4 is a schematic side view of an electric power device representing an application example of the electrostatic capacitive divided-voltage transformer shown in Fig. 1.
Fig. 5 is a laterally cross sectional view of the electrostatic capacitive divided-voltage transformer in a second preferred embodiment.
Fig. 6 is a longitudinally cross sectional view of the electrostatic capacitive divided-voltage transformer in a third preferred embodiment applicable to the GIS.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS:

Reference will hereinafter be made to the drawings in order to facilitate a better understanding of the present invention.
Fig. 1 shows a cross sectional view of a first preferred embodiment of an electrostatic capacitance voltage division type voltage transformer (also called, an electrostatic capacitive divided-voltage transformer abbreviated as a CVT) utilizing a power cable or in an insulating bus bar.
The whole CVT denoted by a reference numeral of 1 includes: an inner conductor 2 having a circular shape in cross section; an inner semiconductive layer 3 to make relaxation of electric field and potential around the conductor 2; a main insulative layer 4; and an outer semiconductor layer 5 to make relaxation of electric field and potential. These layers 3, 4, and 5 enclosing the conductor 2 in this order are in the form of either a power cable or an insulating bus bar.
The CVT 1 further includes a metallic layer 6 for an electrostatic capacitance voltage division purpose (hereinafter called, a first metallic layer); an auxiliary insulative layer 7; a ground metallic layer 8 (hereinafter also called, a second metallic layer); and a protective layer 9 which serves as an outmost layer of the CVT 1 and which is arranged on the second metallic layer 8 according to its necessity. A voltage division tap T is led by means of an insulated wire 10 from the voltage division metallic layer 6 and a grounded tap E is led from the second metallic layer 8 by means of a conductive wire 11.
The protective layer 9 is installed with a mechanical stress elimination, an anti-weather characteristic, and a thermal dissipation taken into consideration.
Material and thickness of the auxiliary insulative layer 7 are selected so that a capacitance C₂ across the insulative layer 7 is derived with a ratio thereof to a capacitance C₁ across the main insulative layer 4 taken into consideration.
In addition, from the electrical capacitance voltage division (first) metallic layer 6, the voltage division tap T is led via the insulated wire 10 to any one of the cable connection points and from the ground (second) metallic layer 8, the conductive wire 11 is used to lead out the ground tap E.
For the voltage division capacitors C₁ and C₂, the ratio of C₁/C₂ is constant although each capacitance changes with the length of the power cable or the insulating bus bar. Hence, a minimum required electrostatic capacitance for each voltage is determined. If the cable size is determined, a shortest length of the cable is calculated so that a free application above the length of the cable can be made.
Fig. 2 shows an application example of the CVT 1 shown in Fig. 1 to a three-phase CVT.
In Fig. 2, three of the same CVTs (1_R, 1_Y, 1_B) as shown in Fig. 1 are star-connected with the grounded tap E as a neutral point.
Furthermore, three-phase taps T_R, T_Y, and T_B are led out and are connected to a three-phase low-voltage VT box 22 via corresponding low-voltage insulated cables (or low-voltage shielded wire) 21.
Each low-voltage insulating cable 21 is further connected to a corresponding primary winding 26 of a three-phase low-voltage VT 25 using a five-leg iron core 24 via its corresponding choke coil 23.
A connection form of each primary winding 26, each secondary winding 27, and each third winding 28 is a star, a star, and an open delta form and star neutral points in the star connection forms are grounded.
It is not necessary to directly attach the low-voltage VT box 22 onto the CVT 1.
However, the low-voltage VT box 22 may be attached onto a position a slightly far away from the CVT 1, for example, a position outside of a tank of GIS (Gas Insulated Switchgear). Output ends of each secondary winding 27 and each third winding 28 of the low-voltage VT 25 from a terminal box 29 is supplied to corresponding input ends of a digital meter or a digital relay (not shown).
Fig. 3 shows an example of an application of the electrostatic capacitive divided-voltage transformer shown in Fig. 1 to a GIS device.
As shown in Fig. 3, the CVT 1 has one end 2₂ of the conductor 2 sealingly enclosed with each layer member, i.e., the inner semiconductive layer 3, the main insulative layer 4, the outer semiconductive layer 5, the first metallic layer 6, the auxiliary metallic layer 7, and the second grounded metallic layer 8.
The CVT 1 is covered with the protective layer 9 with a conductor 2₂ projected from the other end of the conductor 2.
On the other hand, a bushing 32 is provided with a conductor 2₁ used to connect a device vessel 31 such as GIS and a connector 34 is installed to connect the CVT 1 to an inner conductor 33 of the device vessel 31. The bushing conductor 2₁ is connected to the connector 34. A connector 2₃ is installed so as to enable a connection between the bushing-sided conductor 2₁ and the CVT-sided conductor 2₂.
Then, when the CVT 1 is fitted into the bushing 32, the CVT-sided conductor 2₂ can be connected to the inner conductor 33 of the GIS via the bushing-sided conductor 21. The divided voltage tap T and the grounded tap E are led out from the CVT 1 and projected from the outside of the GIS. It is noted that a hermetic sealing is provided between the bushing 32 and the device vessel 31 and between the bushing 32 and CVT 1 and CVT 1 should tightly be fitted into the bushing 32.
Fig. 4 shows another example of the application of the electrostatic capacitive divided-voltage transformer (CVT) shown in Fig. 1.
In Fig. 4, a power cable 1₁ connecting to a device 41 or an insulating bus bar 1₂ connected between the devices 41 and 41 is formed of the CVT in the same manner as shown in Fig. 1. Cable connectors 42 are attached to the corresponding devices 41 and 41. The tap T and the grounded tap E (not shown) are drawn out from one of the connectors 42. The power cable 1₁ functions as both of the cable and the CVT and the insulating bus bar 1₂ functions as the bus bar and the CVT. The tap T and the grounded tap E are always a pair and either of the respective side connectors is drawn out as the tap T and the grounded tap E.
If each insulative material of the main and auxiliary insulative layers 4 and 7 is made of a thermoplastic material, a flexible CVT as the power cable can be achieved.
If each insulative material of the main and auxiliary insulative layers 4 and 7 is made of a thermosetting material, the CVT having a high rigidity and a large mechanical strength such as an epoxy molded product can be achieved.
Next, Figs. 5 and 6 show second and third preferred embodiments of the electrostatic capacitive divided-voltage transformer.
The CVT 1 in each of the second and third preferred embodiments of the electrostatic capacitive divided-voltage transformer includes: the substantially cylindrical inner conductor 2 which is a bus bar used to connect the device to GIS and which is capable of carrying the current having the same values as in the case of Fig. 1; the inner semiconductive layer 3; the main insulative layer 4; the outer semiconductive layer 5; the first metallic layer 6; the auxiliary insulative layer 7; and the cylindrically grounded metallic layer 8, sequentially on the inner conductor in the same manner shown in Fig. 1.
Another auxiliary insulative layer 61, another first metallic layer 62, another semiconductive layer 63, another outer main insulative layer 64, another semiconductive layer 65, and another outer conductor 66 made of a cylindrical foil are arranged on the second grounded metallic layer in this sequence. Furthermore, another semiconductive layer 67, another outer main insulative layer 68, another semiconductive layer 69, another first metallic layer 70, another auxiliary insulative layer 71, another second grounded metallic layer 72, and another protective layer 73 are arranged in this sequence.
It is noted that the inner conductor 2 is connected with the outer conductor 66 via an insulated wire 74 to make the outer conductor 66 the same potential as the inner conductor. In addition, the first metallic layers 6, 62, 70 are connected together via another insulated wire 75 to produce the tap T. Furthermore, the second grounded metallic layers 8 and 72 are connected via the insulated wire 76 to produce a tap E.
Suppose that the electrostatic capacitance between the inner conductor 2 and the first metallic layer 6 is C₁, the electrostatic capacitance between the first metallic layer 6 and the second grounded metallic layer 8 is C₂, an electrostatic capacitance between the second grounded metallic layer 8 and the other first metallic layer 62 is C₂', an electrostatic capacitance between the other first metallic layer 62 and the outer conductor 66 is C₁', an electrostatic capacitance between the outer conductor 66 and the other metallic layer 70 is C₁", and an electrostatic capacitance between the first metallic layer 70 and the other second metallic layer 72 is C₂''.
An electrostatic capacitance between the inner conductor 2 and the tap T is C₁ = C₁' + C₁". An electrostatic capacitance between the tap T and the grounded tap E is C₂ + C₂' + C₂". A large CVT having a large electrostatic capacitance can be achieved.
It is further noted that in order to equally divide the voltage between the inner conductor 2 and the grounded tap E into the same voltages by means of the first metallic layers 6, 62, and 70, namely, to provide the following equalities C₁ = C₁' - C₁" and C₂ = C₂' = C₂", a thickness and permitivity (dielectric constant) of each insulative layer are designed.
The bushing 32 shown in Fig. 6 includes the bushing-sided conductor 2₁ in the same manner as shown in Fig. 3. The bushing 32 is attached onto the device vessel 31. A conductor 2₃ is installed on a lower end of the bushing-sided conductor 2₁. A large current can be supplied to a load through the inner conductor 2 of the CVT 1. The divided voltage can be outputted from the tap T.
In the second embodiment, the CVT having the relatively large electrostatic capacity and having the bus bar used to connect the device to the GIS can be achieved. If a low-voltage penetrating type current transformer or a low-voltage dividing type current transformer is arranged on an outside of this bus bar, the CVT shown in Figs. 5 or 6 serves as a bus bar functioning as a voltage-and-current transformer for an integrated instrument purpose.
It is noted that the CVT 1 in each of the second and third embodiments shown in Figs. 5 or 6 includes the single outer conductor. However, a plurality of outer conductors may concentrically be installed, the grounded metallic layer may be interposed between the respective outer conductors. The semiconductive layer, the main insulative layer, the semiconductive layer, the first metallic layer, and the auxiliary insulative layer may be installed between the respectively corresponding ones of the outer conductors and of the second grounded metallic layers. Then, the inner conductor and the plurality of outer conductors may be connected via an electric wire 74. The metallic layer and each grounded metallic layer are connected to the electric wires 75 and 76 to achieve the CVT having the further large electrostatic capacity.
Although the present invention has been described above by reference to certain embodiments of the present invention, the present invention is not limited to the embodiments described above.
Modifications and variations of the embodiments described above will occur to those skilled in the art, in the light of the above teachings. The scope of the present invention is defined with reference to the following claims.

Claims

An electrostatic capacitive divided-voltage transformer, comprising:
a conductor (2);

an inner semiconductive layer (3);

a main insulative layer (4); and

an outer semiconductive layer (5),
characterized in that the conductor (2) is enclosed with the inner semiconductive layer, the main insulative layer, and the outer semiconductive layer in this sequence, and the electrostatic capacitive divided-voltage transformer further comprises: a first metallic layer (6), the first metallic layer enclosing the outer semiconductive layer; an auxiliary insulative layer (7) arranged on the first metallic layer; a second metallic layer (8) arranged on the auxiliary insulative layer, a divided voltage from a whole voltage between the conductor and the second metallic layer being enabled to be led between the first metallic layer and second metallic layer; and a first tap (T) led out from the first metallic layer via an insulating wire (10) and a second tap (E) led out from the second metallic layer, the second tap being grounded.
An electrostatic capacitive divided-voltage transformer as claimed in claim 1, characterized in that the electrostatic capacitive divided-voltage transformer further includes a protective layer (9) enclosing the second metallic layer (8).
An electrostatic capacitive divided-voltage transformer as claimed in claim 2, characterized in that the conductor (2) includes: a conductor part (2₂), one end of which is sealed with the inner semiconductive layer (3), the main insulative layer (4), the outer semiconductive layer (5), the first metallic layer (6), the auxiliary insulative layer (7), the second metallic layer (8), and the protective layer (9), and the other end of which is connected with a connector (2₃), the connector (2₃) being connected to another conductor (2₁) connectable to a circuit conductor (33) of a gas insulated switchgear (31).
An electrostatic capacitive divided-voltage transformer as claimed in claim 3, characterized in that the electrostatic capacitive divided-voltage transformer further includes a bushing (32) having the other conductor (2₁) therewithin and the connector (2₃) is disconnectably connected between the other conductor (2₁) and the conductor part (2₂).
An electrostatic capacitive divided-voltage transformer as claimed in claim 1, characterized in that the conductor (2), the inner semiconductive layer (3), the main insulative layer (4), and the outer semiconductive layer (5) constitute a power cable.
An electrostatic capacitive divided-voltage transformer as claimed in claim 1, characterized in that the conductor (2), the inner semiconductive layer (3), the main insulative layer (4), and the outer semiconductive layer (5) constitute an insulating bus bar.
An electrostatic capacitive divided-voltage transformer as claimed in claim 1, characterized in that the conductor (2) is a cylindrically shaped inner conductor which constitutes a bus bar, an outer conductor (66) having the same potential as the inner conductor is concentrically arranged on the inner conductor, the second metallic layer (8) is concentrically arranged between the inner conductor and the outer conductor, and another second metallic layer (72) is concentrically arranged on the outer conductor, and the first metallic layer includes a plurality of first metallic layers (6, 62, 70), each of the first metallic layers being concentrically arranged between the inner conductor and the second metallic layer, between the second metallic layer and the outer conductor, and between the outer conductor and the other second metallic layer, the inner and outer conductors being electrically connected, the second and the other second metallic layers being connected together.
An electrostatic capacitive divided-voltage transformer as claimed in claim 1, characterized in that the conductor (2) is a cylindrically shaped inner conductor which constitutes a bus bar, a plurality of outer conductors (66), each of the outer conductors having the same potential as the inner conductor, are concentrically arranged between the inner conductor and the respective outer conductors, and another second metallic layer (72) is concentrically arranged on an outmost one of the outer conductors, and the first metallic layer includes a plurality of first metallic layers (6, 62, 70), each of the first metallic layers being concentrically arranged between the inner conductor and the respective second metallic layers, between the respective second metallic layers and the respective outer conductors, and between the outermost one of the outer conductors and the outmost one of the second metallic layers, the inner and outer conductors being electrically connected, the second metallic layers being connected together.
An electrostatic capacitive divided-voltage transformer as claimed in claim 7, characterized in that each of the second metallic layers is grounded via a grounded tap (E) and if an electrostatic capacitance between the inner conductor (2) and the first metallic layer (6) is C₁, an electrostatic capacitance between the grounded second metallic layer (8) and the first metallic layer (62) is C₂', an electrostatic capacitance between the first metallic layer (62) and the outer conductor (66) is C₁', an electrostatic capacitance between the first metallic layer (66) and the first metallic layer (70) is C₁", an electrostatic capacitance between the first metallic layer (70) and the second metallic layer (72) is C₂", an electrostatic capacitance between the inner conductor (2) and the tap (T) is expressed as C₁ + C₁' + C₁", an electrostatic capacitance between the tap (T) and the grounded tap (E) is expressed as C₂ + C₂' + C₂", and C₁ = C₁' = C₁" and C₂ = C₂' = C₂".