JP7127730B2 - voltage transformer - Google Patents

Description

The present invention relates to transformers for measuring voltage in transmission lines or power installations, and more particularly to voltage transformers used in transmission lines and power equipment for transmitting high voltage power.

2. Description of the Related Art Conventionally, in voltage transformers, a winding transformation method and a capacitor voltage division method have been adopted. In the capacitor voltage division method, as shown in FIG. 6, a conventional voltage transformer 800 includes a plurality of capacitors 804 and 806 connected in series between a power line 802 placed in air and ground 810. An insulator is placed and, for example, the voltage across a capacitor 806 closest to the ground 810 (hereinafter referred to as capacitor voltage) is measured via a voltage transformer 808 with a measuring instrument (voltage measuring device or the like). Since the capacitor voltage of the capacitor 806 is the voltage of the transmission line 802 divided by the capacitors 804 and 806 , the voltage of the transmission line 802 can be measured (calculated) by measuring the capacitor voltage of the capacitor 806 . The insulator is filled with insulating oil, for example, for insulation. In addition, by connecting the measuring equipment via the voltage transformer 808, the measuring equipment is insulated from the high voltage side.

Devices such as gas insulated switchgear (GIS: Gas Insulated Switchgear) used in conduit-type power transmission lines also measure the voltage divided by capacitors in the same manner as above in electronic transformers. A method is proposed. In the case of GIS, an intermediate electrode is provided in a form insulated from the container inside a container formed in a conduit that accommodates a transmission line (hereinafter also referred to as a bus), and a capacitor is placed between the intermediate electrode and the ground. and measure the capacitor voltage of the capacitor.

For example, Patent Document 1 below discloses a voltage transformer for a gas-insulated switchgear with reference to FIG. This voltage transformer is arranged in a pipeline forming a part of the outer shell container of the gas insulated switchgear so as to share the central axis with the tubular containers 902 and 904 made of metal. It has a combined cylindrical metal voltage dividing electrode container 903 . _SF6 gas is sealed at a predetermined pressure inside the pipeline and inside other containers or the like connected to the pipeline. A main circuit conductor (bus line) 905 extending linearly along the central axis of the conduit is housed in the conduit. A cylindrical voltage-dividing electrode (intermediate electrode) 916 surrounding the main circuit conductor 905 is arranged in the voltage-dividing electrode container 903 , and the voltage-dividing electrode 916 is electrically connected to the upper end of the connection conductor 915 . A flange provided at the upper end of a metal connection pipe line 906 is connected via an insulating spacer 907 to the lower flange of the voltage dividing electrode container 903 .

The other end of the connection conductor 915 not connected to the voltage dividing electrode 916 is connected to the primary coil of a voltage transformer (not shown in FIG. 7), and the signal generated in the secondary coil is measured. 7, C11 is the stray capacitance (electrostatic capacitance) between the main circuit conductor 905 and the voltage-dividing electrode 916, C12 is the stray capacitance between the voltage-dividing electrode 916 and the voltage-dividing electrode container 903, and C12 is the connection conductor 915. and the connection conduit 906 is indicated by C13.

The electrical connection relationship of the stray capacitances C11 to C13 is as shown in FIG. FIG. 8 shows portions corresponding to the main circuit conductor 905, connecting conduit 906 and connecting conductor 915 of FIG. Assuming that the potential of the main circuit conductor 905 is E1, the potential (divided potential) E2 of the connection node of the stray capacitances C11 and C12 is E2=E1×C11/(C11+C12+C13). In the gas-insulated switchgear voltage transformer, this divided potential is used as a primary voltage, and after conversion by a voltage transformer (transformer) 909, voltages at output terminals 921 and 922 are measured. In FIG. 8, L is a resonance reactor for improving the transformation ratio, and ZD is a burden (load) for suppressing ferroresonance.

JP-A-5-90048

However, the conventional voltage transformers shown in FIGS. 6 to 8 have the problem that the linearity and frequency characteristics of the detection signal are insufficient due to the use of the voltage transformer. In particular, when detecting a low-level signal, the nonlinearity of voltage conversion by a voltage transformer poses a problem.

As a countermeasure, it is conceivable to directly input the capacitor voltage (divided voltage) to an electronic circuit including a semiconductor operational amplifier (hereinafter also referred to as an operational amplifier) without passing through a voltage transformer. However, in this case, the high-voltage side and the measuring instrument side are not insulated, and there is a problem that the operational amplifier is damaged by switching surges of the system circuit breaker or the like. Even if an isolation amplifier is used as an operational amplifier, there is a limit to the insulation performance of the isolation amplifier, and there is a high possibility of damage due to surges.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a voltage transformer capable of realizing insulation between the high voltage side and measuring equipment and improving the linearity and frequency characteristics of the measured voltage.

A voltage transformer according to a first aspect of the present invention includes a ground wire having one end grounded, a capacitor formed between the other end of the ground wire and a transmission line, and AC power supplied from the transmission line. and an integration unit that integrates the signal detected by the detection unit. The integration result of the integration unit is the voltage of the transmission line. used to generate measurements of

As a result, it is possible to achieve insulation between the high voltage side (transmission line side) and the measuring equipment, and since a voltage transformer is not used, the linearity and frequency characteristics of the detection signal can be improved.

Preferably, the voltage transformer further includes a tubular member surrounding the transmission line, and a conductive member disposed between the transmission line and the tubular member and connected to the other end of the ground wire, and the capacitor is , is the stray capacitance formed between the transmission line and the conductive member.

As a result, the voltage of the transmission line (bus line) can be measured using the intermediate electrode (conductive member) used in the conventional GIS or the like. The current flowing through the ground line is a value obtained by dividing the voltage of the transmission line by the impedance of the capacitor, which is the stray capacitance, and is a current source having the characteristics of a voltage differential signal.

The detector includes a coil (current transformer) having a core with high relative permeability. The core is desirably a core formed of a member having a relative magnetic permeability of 8000 or higher.

As a result, the current flowing through the ground line can be detected more accurately. A core with a high relative magnetic permeability has a small excitation current, so it is suitable for sensing minute currents.

More preferably, the integrating section includes an operational amplifier that amplifies the signal detected by the detecting section, and a capacitor that connects an output terminal of the operational amplifier and an inverting input terminal of the operational amplifier.

Thereby, the current signal detected by the detection unit can be integrated and output as a voltage signal.

A doubling plan for this scheme is also presented. The detection unit includes a first detection unit and a second detection unit, the first detection unit being arranged between the other end of the ground line and an intermediate node on the ground line, and the second detection unit being located between the intermediate node and the ground line. placed between the earth.

As a result, even if one of the measurement systems including the detection unit and the integration unit fails, the other measurement system including the detection unit and the integration unit can continue to measure the voltage of the transmission line.

More preferably, the voltage transformer further includes a first switch capable of short-circuiting the other end of the ground line and an intermediate node on the ground line, and a second switch capable of short-circuiting the one end of the ground line and the intermediate node. include. As a result, during maintenance or the like, one measurement system can be bypassed and only the other measurement system can be operated, and each measurement system can be individually removed and inspected.

Next, a method of voltage conversion of a differential signal as it is without using an integration circuit will be shown. The voltage transformer includes, in place of the integrating section, an operational amplifier to which the signal detected by the detecting section is input, and a resistor connecting the output terminal of the operational amplifier and the inverting input terminal of the operational amplifier. The non-inverting input terminal is grounded. Thereby, the signal (current) detected by the detection unit can be converted into a voltage and output. In this case, integration processing is performed analogously or digitally in the latter stage.

According to the present invention, high insulation between the high-voltage side (transmission line side) and measuring equipment can be achieved, and since a voltage transformer is not used, the linearity and frequency characteristics of the measured voltage can be improved as compared with the prior art.

In addition, in a capacitor voltage dividing type transformer, basically a charge trap (when the power supply via the transmission line is cut off, the accumulated charge remains in the ungrounded capacitor, and the charge is released from the grounded capacitor). attenuates, there is a problem that the voltage divided by the capacitor cannot correctly reflect the voltage of the transmission line for a certain period of time after the power supply is restarted. Since traps do not occur, the voltage of the transmission line can be measured more accurately than the capacitor voltage division method. For reference, the charge trap phenomenon will be described with reference to FIGS. 9 and 10. FIG. The upper graph in FIG. 10 shows the waveform of the output voltage (Up) of the power supply shown at the left end of FIG. It is a waveform when re-inputting at a negative peak. The lower graph in FIG. 10 shows the waveform (corresponding to the measured waveform) of the voltage (Us) of the resistor R2 in FIG. 9 when the voltage is supplied as shown in the upper part of FIG. As described above, due to the charge trap phenomenon, a transient output waveform that deviates from the actual waveform (supplied voltage) is measured.

1 is a schematic diagram showing a schematic configuration of a voltage transformer according to a first embodiment of the present invention; FIG. 2 is a circuit diagram showing a configuration example of a signal matching unit in FIG. 1; FIG. It is a schematic diagram which shows schematic structure of the voltage transformer based on 2nd Embodiment of this invention. It is a circuit diagram which shows the structure provided with multiple measuring devices which concern on a modification. FIG. 4 is a circuit diagram showing a circuit that converts current, which is a detection signal, into voltage; 1 is a schematic diagram showing a schematic configuration of a conventional voltage transformer; FIG. FIG. 4 is a cross-sectional view showing another configuration of a conventional voltage transformer; 8 is a circuit diagram showing an equivalent circuit of the voltage transformer of FIG. 7; FIG. FIG. 3 is a circuit diagram showing an equivalent circuit of a voltage transformer of a capacitor voltage dividing system; It is a graph which shows a charge trap phenomenon. 4 is a graph showing an example of load characteristics of a coil (current transformer);

In the following embodiments, identical parts are provided with identical reference numerals. Their names and functions are also the same. Therefore, detailed description thereof will not be repeated.

(First embodiment)
Referring to FIG. 1, a voltage transformer 100 according to the first embodiment of the present invention includes a capacitor string 102 connected to a power transmission line 200 having one end placed in the air, the other end of the capacitor string 102 and the ground. 202, a detection coil 106 wound around the ground wire 104, a signal matching unit 108 to which a signal (current) generated in the detection coil 106 is input, and a protection element 110. include. A series of capacitors 102 is generally formed by connecting a plurality of capacitors in series.

The ground line 104 is made of a conductive member and formed into a line (bar shape), and has one end connected to the capacitor array 102 and the other end connected to the ground 202 . Any method may be used to electrically connect the ground line 104 to the capacitor array 102 . It may be fixed by welding, screws, bolts, or the like.

A detection coil 106 is for detecting the current flowing through the ground line 104 . The power (voltage) transmitted by the power transmission line 200 is alternating current with a predetermined frequency (eg, 50 Hz, 60 Hz, etc.), and surrounding electrical elements are affected electromagnetically according to the voltage fluctuation. By providing the capacitor string 102 and the ground line 104 between the transmission line 200 and the ground 202 , a fluctuating current flows through the ground line 104 according to the voltage change applied to the transmission line 200 . This current value is a value obtained by dividing the transmission line voltage by the impedance of the capacitor string 102 . A secondary current is generated in the output of the detection coil 106 due to the varying magnetic field formed by the varying current (primary current) flowing through the ground line 104 . As described below, the secondary current can be integrated by the signal matching unit 108 to measure (calculate) the voltage of the transmission line 200 .

Since the current detected by the detection coil 106 is relatively small, it is preferable to provide the detection coil 106 with a core formed of a member with high magnetic permeability (for example, a member with a relative magnetic permeability of 1000 or more). For example, silicon steel or the like can be used for the core. It is more preferable to use permalloy or the like having a relative magnetic permeability of 8000 or more for the core.

The protective element 110 is for releasing current to the ground 202 when an abnormal voltage occurs, and is also called a lightning arrester. Protective element 110 is provided for safety and is not essential to the functioning of voltage transformer 100 and may be omitted.

Referring to FIG. 2, signal matching unit 108 includes operational amplifier 112, resistor R1, and capacitor C1. FIG. 2 shows the ground wire 104 and sensing coil 106 of FIG. One end of the detection coil 106 is connected to the inverting input terminal (the terminal marked with "-") of the operational amplifier 112, and the other end is grounded (connected to the ground 202).

The non-inverting input terminal (the terminal marked with "+") of the operational amplifier 112 is grounded, and the output terminal and the inverting input terminal of the operational amplifier 112 are connected by a parallel-connected capacitor C1 and resistor R1. With this configuration, the operational amplifier 112 functions as an integrator, and the output signal OUT is a signal obtained by integrating the signal input to the inverting input terminal. Note that the resistor R1 is for stabilizing the circuit and is not essential for integration.

The detection coil 106 detects the current flowing through the ground line 104, and the operational amplifier 112, as described above, converts the detection signal of the detection coil 106 (the detection signal of the current flowing through the ground line 104 (input signal IN)) into It integrates and outputs a voltage (output signal OUT). The current I(t) flowing through the ground line 104 is I(t)=I(t) where C0 is the capacitance of the capacitor string 102 and V(t) is the voltage across the capacitor string 102 (that is, the voltage of the transmission line 200). It is represented by C0*dV(t)/dt (where t represents time and dV(t)/dt represents voltage derivative). Therefore, integrating the current I(t) yields the voltage V(t), the voltage of the transmission line 200 .

If the coil output current is converted using an operational amplifier in such a circuit system, the load can be equivalently minimized, and a great improvement can be expected in terms of frequency characteristics and the like. For reference, FIG. 11 shows an example of frequency characteristics according to load. The effective value I ₀ of the through current of the coil is I ₀ =1 (Arms). In this example, when the load RL becomes smaller, the frequency characteristics are improved especially in the low frequency band.

An output signal OUT from the operational amplifier 112 is output to a subsequent measuring instrument (not shown). In the subsequent measuring equipment, for example, the output signal OUT is sampled at a predetermined frequency, converted into digital data, and stored. This allows the voltage of the transmission line 200 to be measured by the signal matching unit 108 and its variations to be observed.

As described above, the voltage transformer 100 can achieve isolation between the high-voltage transmission line 200 side and the measuring device side including the signal matching unit 108 by the detection coil 106 . Therefore, the operational amplifier 112 may be a normal operational amplifier instead of an isolation amplifier. In addition, the voltage transformer 100 can improve the linearity and frequency characteristics of the measured voltage as compared to voltage transformers using conventional voltage transformers.

(Second embodiment)
In the above-described first embodiment, the case of measuring the voltage of a transmission line placed in the air has been described, but in the second embodiment, the voltage of a device such as a GIS is measured.

Referring to FIG. 3, a voltage transformer 120 according to the second embodiment of the present invention includes a voltage dividing electrode container 218 arranged between outer tubes 210 and 212 housing a transmission line 200, a ground line 104 and a , a detection coil 106 wound around a ground wire 104 , a signal matching unit 108 to which a signal (current) generated in the detection coil 106 is input, and a protection element 110 . The outer tubes 210 and 212 are made of a conductive member (such as metal), and the transmission line 200 is arranged on the central axis of the outer tubes 210 and 212 . Outer tubes 210 and 212 may be covered with an insulating coating.

The voltage dividing electrode container 218 includes an outer shell 220 , spacers 214 and 216 positioned at opposite open ends of the outer shell 220 , and a conductive member 222 positioned around the transmission line 200 . The outer shell 220 may be covered with an insulating coating.

Outer shell 220 is formed in a cylindrical shape by a conductive member so as to surround power transmission line 200 . The outer shell 220 may be a cylinder (having a circular cross-sectional shape perpendicular to the axis) or a polygonal tube (having a polygonal cross-sectional shape perpendicular to the axis). Outer shell 220 is preferably made of highly conductive metal (eg, iron, copper, aluminum, alloys thereof, stainless steel, etc.).

Spacers 214 and 216 are for supporting transmission line 200 and securing voltage dividing electrode container 218 to sheath tubes 210 and 212 . The spacers 214 and 216 are made of, for example, a non-conductive member (resin or the like). Spacers 214 and 216 are preferably formed to allow transmission line 200 to pass therethrough and allow transmission line 200 to be positioned axially of shell 220 .

A sealed space 230 is formed inside the voltage dividing electrode container 218 by the outer shell 220 and the spacers 214 and 216 . Space 230 (the space enclosed by shell 220 and spacers 214 and 216) may be filled with oil, gas, molding material, or the like for insulation. Spaces 226 and 228 formed by outer tubes 210 and 212 may also be filled with oil, gas, molding material, etc. for insulation.

Conductive member 222 is electrically insulated from power transmission line 200 and is formed of a conductive member in a cylindrical shape. For example, conductive member 222 is positioned such that its axis coincides with power line 200 . Conductive member 222 corresponds to an intermediate electrode in a conventional capacitor voltage divider type transformer.

One end of the ground wire 104 is connected to the conductive member 222 . The ground wire 104 is arranged through a through hole 224 formed in the outer shell 220 so as not to contact the outer shell 220 and to be electrically insulated from the outer shell 220 . Any method may be used to electrically connect the ground line 104 to the conductive member 222 . It may be fixed by welding, screws, bolts, or the like.

The detection coil 106, the signal matching unit 108 and the protection element 110 are configured and function in the same manner as in the first embodiment (see FIGS. 1 and 2). Therefore, redundant description will not be repeated.

As described above, the power (voltage) transmitted by the power transmission line 200 is an alternating current of a predetermined frequency, and surrounding electrical elements are affected electromagnetically according to the voltage fluctuation. Conductive members 222 disposed within shell 220 create stray capacitance 232 between transmission line 200 and conductive members 222 . Therefore, by providing the conductive member 222 around the power transmission line 200 and providing the grounding line 104 for grounding the conductive member 222, the voltage change applied to the power transmission line 200 can be suppressed as in the first embodiment. Accordingly, a fluctuating current (primary current) flows through the ground line 104 . This current value is the value obtained by dividing the transmission line voltage by the impedance of the stray capacitance 232 . A secondary current is generated in the detection coil 106 by the resulting varying magnetic field. Therefore, if this is detected by the signal matching unit 108, the voltage of the transmission line 200 can be measured (calculated) by integrating the detected signal, as in the first embodiment.

As described above, the voltage transformer 120 can achieve isolation between the high-voltage transmission line 200 side and the measuring device side including the signal matching unit 108 by means of the detection coil 106 . Therefore, the operational amplifier 112 (see FIG. 2) that constitutes the signal matching unit 108 may be a normal operational amplifier instead of an isolation amplifier. Also, the voltage transformer 120 can improve the linearity and frequency characteristics of the measured voltage as compared to voltage transformers using conventional voltage transformers.

In the voltage transformer 120, the current detected by the detection coil 106 is relatively small. It is preferred to provide a core formed of

(Modification)
In the above embodiment, a case was described in which a pair of detection coils and a signal matching unit were provided. Probability is high. Therefore, as a countermeasure, the measuring device is duplicated in the modified example.

Referring to FIG. 4, the voltage transformer according to this modification includes measuring devices 130 and 132 and switches 148 and 150. In FIG. The configuration of the transmission line, which is the object of voltage measurement, and its surroundings is the same as in FIG. 1 or FIG. 4 shows the ground line 104 of FIGS. 1 and 3. FIG.

Measurement device 130 includes sensing coil 140, operational amplifier 144, resistor R2, and capacitor C2. The inverting input terminal of operational amplifier 144 is connected to the non-grounded terminal of sensing coil 140, and the non-inverting input terminal of operational amplifier 144 is grounded. The output terminal and the inverting input terminal of the operational amplifier 144 are connected by a parallel-connected capacitor C2 and resistor R2. The detection coil 140 is arranged between a second node 154, which is an intermediate node on the ground line 104, and a first node 152, which is a node on the ground side.

Measurement device 132 includes sensing coil 142, operational amplifier 146, resistor R3, and capacitor C3. The inverting input terminal of operational amplifier 146 is connected to the non-grounded terminal of sensing coil 142, and the non-inverting input terminal of operational amplifier 146 is grounded. The output terminal and the inverting input terminal of the operational amplifier 146 are connected by a parallel-connected capacitor C3 and resistor R3. The detection coil 142 is arranged between a second node 154, which is an intermediate node, and a third node 156, which is a node on the transmission line side.

As can be seen from FIGS. 2 and 4, measurement devices 130 and 132, respectively, are constructed similarly to the circuit of FIG. 2 and function as integrators in the same manner as the circuit of FIG. Therefore, redundant description of the functions of measuring devices 130 and 132 will not be repeated.

Switch 148 is arranged to be able to short-circuit first node 152, which is a node on the ground side, and second node 154, which is an intermediate node. The switch 150 is arranged so as to be able to short-circuit a second node 154, which is an intermediate node, and a third node 156, which is a node on the transmission line side.

With such a configuration, the switches 148 and 150 are normally operated in an open state in a redundant manner. When one of the switches 148 and 150 is opened and the other short-circuited during maintenance or the like, the current flowing through the ground line 104 can be partially bypassed to flow through the ground line 104 . Therefore, only one of measuring devices 130 and 132 can be activated to output either output signal OUT1 or OUT2. At the time of maintenance or the like, each unit can be separated at the position indicated by the dotted circle in such a configuration. In this case, there is no effect on the actual operation side.

Although the case where the detected current is integrated by an integration circuit using an operational amplifier and converted into a voltage has been described above, the present invention is not limited to this. After converting the analog detection signal (current) into digital data without using an operational amplifier, integration may be performed by a semiconductor operator (DSP (Digital Signal Processor) or the like).

In the above description, as shown in FIG. 2, the case where the detection signal (current) is integrated to generate the voltage value has been described, but the present invention is not limited to this. A current-to-voltage conversion circuit (hereinafter referred to as a current-voltage conversion circuit) may convert the detection signal, which is a current, into a voltage. FIG. 5 shows an example of a current-voltage conversion circuit. This current-voltage conversion circuit includes an operational amplifier 160 and a resistor R4. The output terminal and the inverting input terminal of the operational amplifier 160 are connected by a resistor R4, and the non-inverting input terminal of the operational amplifier 160 is grounded. The inverting input terminal of operational amplifier 160 is also connected to the ungrounded terminal of sensing coil 106 . As a result, the detection signal (current) IN of the detection coil 106 input to the inverting input terminal of the operational amplifier 160 is converted into a voltage and output as the output signal OUT. If the current input to the inverting input terminal of the operational amplifier 160 is represented by Iin, and the voltage of the output signal OUT is represented by Vout, then Vout=-R4×Iin.

In the circuit of FIG. 5, the operational amplifier 160 is insulated from the transmission line 200 by the detection coil 106 as shown in FIGS. Just do it.

Note that the current-voltage conversion circuit is not limited to that shown in FIG. Any device may be used as long as it can convert the detection signal IN, which is a current, into a voltage and output it.

Although the present invention has been described above by describing the embodiments, the above-described embodiments are examples, and the present invention is not limited only to the above-described embodiments. The scope of the present invention is indicated by each claim after taking into account the description of the detailed description of the invention, and includes all changes within the meaning and scope of equivalents to the words described therein .

ADVANTAGE OF THE INVENTION According to this invention, the voltage transformer which can implement|achieve the insulation of a high voltage side and a measuring instrument, and can improve the linearity and frequency characteristic of a measured voltage can be provided.

100, 120, 800 voltage transformer 102 capacitor array 104 ground wire 106 detection coil (detection means)
108 Signal Matching Unit 110 Protective Element 112, 144, 146, 160 Operational Amplifier 130, 132 Measuring Device
140 detection coil (first detection means)
142 detection coil (second detection means)
148, 150 switch 152 first node 154 second node 156 third node 200, 802 transmission line 202, 810 ground 210, 212 outer tube 214, 216 spacer 218, 903 voltage dividing electrode container 220 outer shell (cylindrical member)
222 conductive member 224 through hole 226, 228, 230 space 232 stray capacitance 804, 806 capacitor 808, 909 voltage transformer 902, 904 tubular container 905 main circuit conductor 906 connection conduit 907 insulating spacer 915 connection conductor 916 voltage dividing electrode 921, 922 output terminal

Claims (4)

a ground wire with one end grounded;
a capacitor formed between the other end of the ground line and a transmission line;
a detection means for detecting the current flowing through the grounding line when AC power is supplied from the transmission line, using a coil to insulate and detect the current;
an integrating means for integrating the signal detected by the detecting means;
the result of integration by the integrating means is used to generate a measurement of the voltage of the transmission line;
The detection means includes first detection means and second detection means,
the first detection means is arranged between the other end of the ground line and an intermediate node on the ground line;
the second detection means is arranged between the intermediate node and ground;
a first switch capable of short-circuiting the other end of the ground line and an intermediate node on the ground line;
The voltage transformer, further comprising a second switch capable of shorting the one end of the ground line and the intermediate node. a cylindrical member surrounding the transmission line;
a conductive member disposed between the transmission line and the tubular member and connected to the other end of the ground line;
2. The voltage transformer according to claim 1, wherein said capacitor is a stray capacitance formed between said transmission line and said conductive member. 3. The voltage transformer according to claim 1, wherein said detecting means includes a coil having a core made of a material having a relative permeability of 8000 or higher. The integration means are
an operational amplifier for amplifying the signal detected by the detection means;
4. A voltage transformer according to any one of claims 1 to 3, further comprising a capacitor connecting an output terminal of said operational amplifier and an inverting input terminal of said operational amplifier.

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