JP2568346Y2 - Cable loss tangent measurement device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial application field) The present invention relates to a dielectric loss tangent measuring device for diagnosing a cable, in which a current transformer is attached to a ground wire for grounding a cable shield to the ground.

(Prior Art) In order to perform maintenance and management on a cable after the cable is laid, the insulation properties of the cable are regularly tested. In this test, the dielectric loss tangent is measured in order to diagnose the insulation deterioration of the cable insulator. To this end, a test voltage is applied to a cable conductor, a current transformer is attached to a ground wire for grounding a shield layer of the cable, and the output current is measured. An output current corresponding to the charging current of the cable is obtained from the current transformer. The dielectric loss tangent of the cable depends on the loss current component of the charging current. Therefore, the loss tangent can be obtained by extracting the loss current component from the output current of the current transformer.

(Problems to be Solved by the Invention) By the way, in order to measure the current flowing in the ground wire by the current transformer, it is necessary to penetrate the current transformer through the ground wire.

FIG. 3 shows a connection diagram when a conventional current transformer is mounted on a ground wire.

As shown in the figure, the shielding layer 2 of the cable 1 is grounded via a ground wire 3. Here, in order to mount the current transformer 4, it is necessary to cut the ground wire 3 once and connect both ends of the lead wire 5 penetrating the current transformer 4 thereto. However, such an operation is complicated, and there is a danger in cutting the ground line 3 for a live cable line.

Therefore, as shown in FIG. 4, a clamp-type current transformer 6 that is frequently used for measuring the current of a distribution line or the like is used. As is well known, the clamp-type current transformer 6 has a configuration in which a part of the toroidal core 6a can be opened and closed, and has an advantage that it can be easily mounted without cutting the ground wire 3. ing.

In particular, in the three-phase transmission line, as shown in FIG. 5, a ground wire 3 is provided to connect the shielding layers of each layer and to ground them collectively. By mounting three clamp-type current transformers 6 on the ground wire 3, the efficiency of the measurement operation can be improved.

That is, in the example of FIG. 5, the output current (I ₁ ,
(Corresponding to I ₂ , I ₃ ) is received by the three-phase current separation circuit 7, and the charging currents (I _a , I _b , I _c ) of each layer are obtained from the following equation.

Ia = I ₁ Ib = I ₂ −I ₁ Ic = I ₃ −I _{2 The} measuring section 9 takes in the charging current signal of each layer thus obtained and the voltage signal obtained via the voltage dividing resistor 8, and
Measure δ.

However, when was measured as described above, in the three-phase equilibrium, the current I ₃ that is detected by the current transformer 6 becomes a value substantially close to "0".

On the other hand, in a current transformer using a toroidal coil, when the primary current is small, the detection sensitivity is reduced. In particular, in the case of a clamp type current transformer, the tendency is remarkable because the magnetic resistance of the toroidal coil is high. Accordingly, there has been a problem that precise measurement of tan δ becomes difficult.

The present invention has been made in view of the above points, and has as its object to provide a dielectric loss tangent measuring device which can perform measurement by operating a current transformer with high sensitivity.

(Means for Solving the Problems) The cable dielectric loss tangent measuring device according to the present invention includes a ground line that connects the shielding layers of the three-phase cable lines to each other and is grounded, and a ground line that connects the shielding layers. Current transformers respectively mounted on the ground side of the line portion and the ground wire, a bias current generating unit for supplying a preset alternating current for each current transformer, and an output current of each current transformer. A bias current subtractor for subtracting an AC current generated by a bias AC current, and a loss tangent for extracting a loss current component from each subtracted output current and measuring an induction tangent of a cable insulator of each cable line. And a measuring unit.

(Operation) The shielding layers of the three-phase cable lines are connected to each other and grounded by a single ground line. Therefore, it is sufficient to perform one grounding operation for each of the three-phase cable lines, and the ground line portion between the shield layers can be short, so that the total length of the ground line can be small.

On the other hand, an AC current for bias is supplied to each current transformer, and the AC current is subtracted from the output current to obtain a measured current.

Therefore, each dielectric loss tangent of the three-phase cable line can be measured with high accuracy by a simple grounding operation.

(Embodiment) Hereinafter, the present invention will be described in detail with reference to the embodiment shown in the drawings.

FIG. 1 is a block diagram showing the operation principle of the cable dielectric loss tangent measuring device of the present invention.

In the figure, a shielding layer 2 of a cable line 1 is
Grounded. As described above, the clamp type current transformer 6 is mounted on the ground wire 3. The output current of the clamp type current transformer 6 is output to the amplifier circuit 11.

Here, the device of the present invention is provided with a bias current generator 12 and a bias current subtractor 13.

The bias current generator 12 has a lead 15 wound around a toroidal core of the current transformer 6.
Supply AC current for bias to 15. Further, the signal corresponding to the bias AC current value of the bias current generation unit 12 is:
The signal is output to the bias current subtractor 13. The bias current subtractor 13 is a circuit that operates to subtract the output of the bias AC current from the output of the amplifier circuit 11. The bias current generator 12 supplies a preset alternating current to the lead 15
The bias current subtracting unit 13 is configured by an operational amplifier or the like that subtracts a signal voltage corresponding to the bias AC current value from the output of the amplifier circuit 11.

FIG. 2 is a graph showing the relationship between the primary current of the current transformer and the ratio error. The horizontal axis shows the primary current I of the current transformer.
Is plotted, and the specific error ε is blotted on the vertical axis. The ratio error ε is inversely proportional to the primary current I of the current transformer.

Here, assuming that the allowable ratio error is ε ₀ , as evident from the graph, when the primary current of the current transformer is smaller than K amperes, the desired characteristics are not satisfied.

Therefore, the bias current generator 12 shown in FIG.
As described in the drawing, the bias AC current value is selected in advance so that the primary current of the current transformer becomes a predetermined value of K amperes or more. Thus, even when the current flowing through the ground line 3 is extremely small, the current can be accurately amplified in the amplifier circuit 11 and sent out to the dielectric loss tangent measuring unit 14. The dielectric loss tangent measuring unit 14 is not different from the conventional one.

The present invention is characterized in that the bias current generating section 12 and the bias current subtracting section 13 are applied when measuring each of the three-phase cable lines 1 shown in FIG.
Are connected to each other by a part of one ground wire 3 and one end is grounded, and the two clamp-type current transformers 6 and 6 between the shield layers and the clamp-type current transformer 6 on the ground side are connected to each other. Apply bias AC current.

First, the voltage applied to the cable line 1 located on the left side in FIG.
The hook at the tip of the voltage dividing bar provided with is electrically connected to the end of the cable and detected. The detected voltage is input to the measuring unit 9 and the output current of the current transformer 6 on the left side in FIG. The output current I ₁ obtained by subtracting the current is input to the three-phase current separation circuit 7. The circuit 7 is output to the measuring unit 9 the output current I ₁ as the charging current I _a of the cable line 1. Measuring unit 9, for example, has a known structure comprising a Schering bridge circuit for measuring tanδ based the inputted detection voltage to the charging current I _a.

Next, in the figure, the voltage is similarly detected from the cable line 1 located at the center using a voltage dividing bar, and the output current obtained by subtracting the bias AC current from the output current of the current transformer 6 at the center. I ₂ is input to the three-phase current separation circuit 7. The circuit 7 outputs the charge current I _b from the output current I ₂ obtained by subtracting the output current I ₁ to the measuring unit 9, thereby measuring the tan [delta.

Finally, as shown in the figure, the voltage is similarly detected from the cable line 1 located on the right side, and the current transformer 6 on the right side is detected.
The output current I ₃ obtained by subtracting the bias alternating current from the output current of the input to the three-phase currents separation circuit 7. This circuit 7
Outputs the charge current I _c from the output current I ₃ obtained by subtracting the output current I ₂ to the measuring unit 9, thereby measuring the tan [delta.

When the bias AC current is supplied to the current transformer and the bias AC current is subtracted from the output of the amplifier circuit, another known circuit such as a digital circuit may be used.

(Effects of the Invention) The above-described device for measuring a loss tangent of a cable according to the present invention uses a current transformer for applying a predetermined bias AC current to a current transformer even when the current flowing through the ground wire is extremely small. Current can be measured in the maximum sensitivity range of the instrument. For this reason, it is possible to measure the dielectric loss tangent of the cable in the live state with the required high accuracy.

[Brief description of the drawings]

FIG. 1 is a block diagram showing the operating principle of the dielectric loss tangent measuring device of the cable of the present invention, FIG. 2 is a graph showing the relationship between the primary current of the current transformer and the ratio error, and FIGS. FIG. 5 is a block diagram showing a method of measuring a dielectric loss tangent of a cable according to the present invention, which shows a method of attaching a current transformer to a ground wire. 1 ... cable, 2 ... shielding layer, 3 ... ground wire, 6 ...
Current transformer, 11… Amplifier circuit, 12… Bias current generator,
13 ... Bias current subtraction unit, 14 ... Dielectric loss tangent measurement unit, 15
……Lead.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Jiro Kawai 2-1-1 Oda Sakae, Kawasaki-ku, Kawasaki-shi, Kanagawa Prefecture Inside Showa Electric Cable & Cable Co., Ltd. (56) References JP-A-62-254078 (JP, A) JP-A-1-158365 (JP, A)

Claims (1)

(57) [Scope of request for utility model registration] 1. A grounding line connected to the shielding layers of three-phase cable lines and grounded, a grounding line portion connecting the shielding layers, and a transformer mounted on the grounding side of the grounding line. Current transformer, a bias current generating unit that supplies a preset bias AC current to each current transformer, and an AC current generated by the bias AC current from the output current of each current transformer. And a dielectric loss tangent measurement unit that extracts a loss current component from each of the subtracted output currents and measures a dielectric loss tangent of the cable insulator of each of the cable lines. Dissipation tangent measurement device for cables.