JPH0425659Y2 - - Google Patents

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention relates to a current detection circuit that accurately detects the current of high voltage equipment.

[Technical background of the invention] For example, in high-voltage equipment such as withstand voltage testing equipment, current detection is performed by grounding one end of the secondary winding of a high-voltage transformer via a current detection resistor.

FIG. 10 is a circuit diagram showing an example of such a conventional high voltage circuit.

That is, an AC power source 2 such as a commercial power source is connected to the primary side 1a of the high voltage transformer 1, and a high voltage is obtained on the secondary side 1b.

One end (COM) of the secondary winding is grounded via a current detection resistor 3.

One high voltage output terminal 4 is grounded, and the other high voltage output terminal 5 is connected to the other end (HV) of the secondary winding 1b.

Therefore, by connecting a load 6, that is, an appropriate device under test, between the high voltage output terminals 4 and 5, the withstand voltage can be tested.

The current flowing through the load 6 is measured by taking out the voltage between the terminals of the current detection resistor 3 via an amplifier 7 and using this output value.

[Problems with Background Art] By the way, in such a withstanding voltage testing apparatus, for example, a high voltage is applied to a device under test via a high voltage cable.

Inside the withstanding voltage testing apparatus, a stray capacitance (c in the figure) exists between the high voltage output terminals 4 and 5, and a stray capacitance also exists in the high voltage cable. Since current also flows through this stray capacitance, the voltage obtained from the amplifier 7 becomes larger than the current actually flowing through the load 6, causing an error in the measured value of the current.

FIG. 11 is an equivalent circuit of the conventional withstand voltage test device shown in FIG. 10.

If we set up a node equation with one end (COM) of the high-voltage transformer at a common potential, the following equation (1) holds true.

(jωC+1/R _L )(V1−Vs)=1/RSVs (1) Then, jωC of the first term on the left side of equation (1) becomes the error.

here V1 is the output voltage of the high voltage winding Rs is the resistance value of the current detection resistor Vs is ground potential C is the value of stray capacitance R is the resistance value of the load resistor.

Errors caused by such stray capacitance do not pose much of a problem when the output voltage is low and the detected current is large, but conversely, when the output voltage is high and the detected current is small, a large error occurs. For example, when the stray capacitance is 40Pf, the detection current is 0.5mA,
If the load resistance value is 2MΩ, the error is 0.03%, but if the detection current is 0.01mA and the load resistance value is 500MΩ, the error becomes 536%. In order to reduce such stray capacitance, for example, inside a withstand voltage test equipment, it is considered to provide an electrostatic shield to the other end (HV) of the secondary winding and connect it to one end (COM) of the secondary winding. It will be done. However, such a device has a complicated structure, is expensive, and significantly increases in weight. Furthermore, it is almost difficult to reduce the stray capacitance outside the withstand voltage test device.

[Purpose of the invention] This invention was made in view of the above circumstances.
The object of the present invention is to provide a current measuring device that can accurately measure load current by canceling the current flowing through stray capacitance generated between the output end of a high-voltage winding and ground with a simple configuration. .

[Summary of the invention] That is, the invention provides a high voltage winding and a low voltage winding on the secondary side of a high voltage transformer, inserts a current detection resistor between one end of the high voltage winding and ground, and detects this current. The current flowing through the load connected between the terminals of the high-voltage winding was measured from the voltage between the terminals of the resistor, and one end was given the output voltage of the low-voltage winding, and the other end was connected to the terminal of the current detection resistor. The current flowing through the current detection resistor via the stray capacitance between the high-voltage output end at the other end of the high-voltage winding and ground is canceled by a canceling means having a compensation capacitor.

[Embodiment of the invention] An embodiment of the invention will be described in detail below with reference to the block diagram shown in FIG.

AC power supply 1 on the primary side 11a of the high voltage transformer 11
Connect 2. A common terminal (COM) is led out from the middle of the secondary winding of this high voltage transformer 11, and a high voltage winding 11b and a low voltage winding 11c are provided.

Then, the high voltage output end (HV) of the high voltage winding 11b is connected to one high voltage output terminal 13a, and the other high voltage output terminal 13b is grounded.
A load R is connected between a and 13b.

And connect the common terminal (COM) to the current detection resistor 1
4, and the low voltage output end (LV) of the low voltage winding 11c is grounded through a compensation capacitor 15.

This compensation capacitor 15 is connected to the stray capacitance between the high voltage output end (HV) of the high voltage winding 11b and the ground (C shown by the broken line in the figure). , that is, the capacity is multiplied by the output voltage ratio of the low and high voltage windings.

And common terminal (COM) and current detection resistor 1
4 is connected to the input of an amplifier 16, and a voltage corresponding to the voltage across the terminals of the current detection resistor 14, that is, the current flowing through the load R, is obtained at the output of the amplifier 16.

In this way, the block diagram shown in FIG. 1 is given by the equivalent circuit shown in FIG.

If we set up a node equation with one end (COM) of the high-voltage transformer at a common potential, the following equation (2) holds true.

(jωc+1/R _L )(V1−Vs)=1/RsVs+jωNC(VS−V2) …(2) Here, V1 is the output voltage of the high voltage winding V2 is the output voltage of the low voltage winding Rs is the resistance value of the current detection resistor Vs is the ground potential, N is the turns ratio of the low-voltage and high-voltage windings, and C is the stray capacitance value. R _L is the resistance value of the load resistance.

Here, V2=-V1/N, and by substituting this into equation (2), the above equation (2) can be transformed into the following equation (3).

1/R _L (V1-Vs) = {1/Rs+jωC(N+1)}Vs...(3) In this case, the equivalent circuit shown in Fig. 2 can be expressed by the equivalent circuit shown in Fig. 3, for example. , and the stray capacitance C can be eliminated.

The current 1/R _L (V1-Vs) flowing through the load R _L and the current {1/Rs+jωC (N+1)} Vs flowing through the current detection admittance {1/Rs+jωC(N+1)} are equal.

It should be noted that the present invention is not limited to the above-mentioned embodiment, but can also be applied to a case where an insulation resistor R is connected between the high voltage output terminals, as in the equivalent circuit shown in FIG. 4, for example.

That is, in this case, a compensation resistor having a value obtained by dividing the value of the insulation resistance R by the winding ratio of the low-voltage and high-voltage windings is connected in parallel to the compensation capacitor NC.

The node equation at this time is given by the following equation (4).

(jωC+1/R+1/R _L ) (V1−Vs) = 1/RsVs+(jωNC+N/R) (Vs+V1/N)…(4) Therefore, this equation (4) can be transformed into the following equation (5). .

1/R _L (V1-Vs) = {1/Rs+1/R(N+1)+jωC(N+1)}Vs...
(5) The equivalent circuit of equation (5) is shown in FIG.

Therefore, in the equivalent circuit of FIG. 5, stray capacitance C and insulation resistance R can be eliminated in equation (5). Furthermore, at this time, the current flowing through the load R _L is equal to the product of the current detection admittance {1/Rs+1/R(N+1)+jωC(N+1)} and the voltage across it.

Furthermore, as shown in Figure 6, the present invention connects the output end of the low voltage winding to an amplifier AMP that operates with an amplification factor k and a compensation capacitor with the COM terminal as a common.
It may be grounded via NC.

The node equation at this time is given by the following equation (6).

(jωkC+1/R _L )(V1−Vs) =1/RsVs+jωNC(Vs+V1/NK) (6) Therefore, this equation (6) can be transformed into the following equation (7).

1/R _L (V1-Vs) = {1/Rs+jωC(k+N)}Vs (7) The equivalent circuit of this equation (7) is shown in FIG.

Therefore, if the stray capacitance C can be eliminated and the amplification factor k of the amplifier AMP is varied, then
Even if the value of stray capacitance changes, correction is possible.

In addition, as shown in FIG. 8, the present invention uses an autotransformer as the high voltage transformer, and connects one end of the current detection resistor 14 to its common terminal (COM).
The other end of this current detection resistor 14 is connected to the high voltage output terminal 13.
b, and connect the output of the low voltage output terminal (LV) to
Inverter 2 that operates with the COM terminal as a common
1 and the compensation capacitor 15 in series to the other end of the current detection resistor 14.

In this case, the capacitance of the compensation capacitor 15 is multiplied by the amplification factor of the inverter 21, so
A capacitor with a value obtained by dividing the necessary compensation capacitor capacity by the amplification factor of the inverter may be used.

Furthermore, as shown in FIG.
An autotransformer is used as an autotransformer, and its low voltage output terminal (LV) is connected to a buffer 22 and a compensation capacitor 15 that operate with the high voltage output terminal 13b as a common.
It may also be connected to one end of the current detection resistor 14 via.

In particular, in the case of the embodiment shown in FIG. 9, since the detection current does not flow through the compensation capacitor, it is possible to perform compensation with high accuracy.

[Effects of the invention] As described above, according to the invention, with a relatively simple configuration, it is possible to offset the error caused by the current flowing in the stray capacitance between the output end of the high-voltage winding and the ground, and to obtain accurate load current. A current detection circuit that can measure current can be provided.

[Brief explanation of drawings]

Figure 1 is a circuit diagram showing one embodiment of the present invention;
The figure is an equivalent circuit diagram of Figure 1, Figure 3 is an equivalent circuit diagram when appropriate correction is made to the circuit of Figure 1, and Figure 4 is an equivalent circuit diagram of Figure 1.
5 is an equivalent circuit diagram showing other embodiments of the present invention, FIGS. 6 to 9 are equivalent circuit diagrams showing other embodiments of the present invention, and FIG. 10 is an equivalent circuit diagram showing other embodiments of the present invention. A circuit diagram showing an example of the position of the detection circuit, Fig. 11 is the 10th
FIG. 3 is an equivalent circuit diagram of the current detection circuit shown in the figure. 11...High voltage transformer, 12...AC power supply, 1
3a, 13b...High voltage output terminal, 14...Current detection resistor, 15...Compensation capacitor.

Claims (1)

[Scope of Claim for Utility Model Registration] (1) A high-voltage transformer with a high-voltage winding and a low-voltage winding on the secondary side, a current detection resistor interposed between one end of the high-voltage winding and ground, and the current detection means for measuring the current flowing through the load connected between the terminals of the high-voltage winding from the voltage between the terminals of the resistor, one end of which is supplied with the output voltage of the low-voltage winding, and the other end of which is the terminal of the current detection resistor. A current comprising a canceling means for canceling the current flowing to the current detection resistor via a stray capacitance between the high voltage output terminal at the other end of the high voltage winding and the ground, by a compensation capacitor connected to the current detecting resistor. detection circuit. (2) The current detection circuit according to claim 1 of the utility model registration, characterized in that the high voltage transformer is an autotransformer in which a tap for a low voltage winding is derived from the middle of a high voltage winding.