JPS6156790B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field of the Invention) The present invention relates to a method for constantly monitoring a cable anti-corrosion layer.

(Technical background of the invention and its problems) Generally, the presence or absence of abnormalities such as deterioration or external damage in the cable protection layer is determined by measuring the DC insulation resistance of the corrosion protection layer on the cable protection layer, as shown in Figure 1. It is judged by that.

In other words, in a circuit in which a parallel resonant circuit consisting of a coil L ₁ and a capacitor C ₂ is connected between a cable sheathing layer ₁ grounded by a capacitor C 1 and the earth, a DC power supply 3 of several volts is connected between the parallel resonant circuit and the earth. The insulation of the cable anti-corrosion layer is measured by measuring the current value flowing through the defective part of the cable anti-corrosion layer (not shown) on the coil _L1 and the insulation layer 1 and the DC voltage at the time of application. The resistance can be calculated and the degree of deterioration of the anti-corrosion layer can be determined from this resistance value.

However, this method of measuring the insulation resistance of the cable anti-corrosion layer has the disadvantage that it cannot be detected unless the anti-corrosion layer has a hole and water or the like has entered and the insulation resistance has decreased considerably.

(Objective of the Invention) The present invention was made with attention to the above points, and uses a high frequency power source to reduce the tanδ of the cable anti-corrosion layer.
By measuring this, the above-mentioned drawbacks are attempted to be overcome.

(Embodiment of the Invention) The present invention will be described below based on the drawings of one embodiment. The second figure has the same numbers attached to the same parts as in Figure 1.
In the figure, a dielectric loss tangent measuring device 4 and a high frequency power source 5 on the order of kilohertz are connected in parallel between the live cable insulation layer 1 which is grounded through an inductance L ₂ and the ground.

Therefore, the connection between the cable shield layer 1 and the ground can be equivalently considered as a parallel circuit of equivalent capacitance C _x and equivalent resistance R _x as shown by the dotted line.
When the dielectric loss tangent measuring device 4 is balanced with respect to the high frequency power source 5, the apparent dielectric loss tangent to be measured, that is, the dielectric loss tangent tan δ' of the cable anticorrosion layer including the inductance _L2 is expressed as the following equation.

tanδ′=1/R _x /ωC _x −1/ωL ₂ ...(1) where ω is the angular frequency However, the so-called true value of the dielectric loss tangent tanδ of only the cable protection layer that does not include the inductance _L2
Since is 1/ωC _x R _x , the apparent tan δ' is as follows.

tan δ'=tan δ/1-1/ω ² C _x L ₂ ...(2) Here, the angular frequency ω and the inductance L ₂
is known and the equivalent capacitance C _x is a measurable quantity, so the true tan δ can be calculated from equation (2).

However, in reality, the charged current flowing from the cable conductor 6 to the insulation layer 1 flows through the inductance L ₂ , so a potential difference at the commercial frequency is generated between the ground and the insulation layer 1. Since the inductance _L2 has a small impedance, the potential difference can be almost eliminated. Even if a small potential difference occurs, no problem will occur if a detector that selectively amplifies only high frequency components is used.

Further, the capacity of the high frequency power source is determined by the current flowing through the cable anti-corrosion layer, but by selecting an appropriate value for the inductance _L2 , the current from the power source can be reduced.

Note that when lightning surge voltage enters, it is expected that a large potential difference will occur across the inductance L ₂ , so the inductance L ₂ used here is
As such, it is desirable to use a magnetic saturation type reactor such as a reactor so that no potential difference occurs above a predetermined voltage.

In the figure, A is an ammeter, G is a galvanometer, C ₄ is a variable capacitor, R ₄ is a fixed resistance, R ₃ is a variable resistance, and C _s is the capacitance of a standard capacitor.

(Effects of the Invention) As described above, in the present invention, since the tan δ of the cable anti-corrosion layer is measured using a high frequency power source, it is possible to eliminate minute defects that could not be measured conventionally and deterioration of the cable anti-corrosion layer due to water absorption, etc. can also be detected.

[Brief explanation of the drawing]

FIG. 1 shows a circuit diagram for measuring the insulation resistance of a cable anti-corrosion layer in the conventional method, and FIG. 2 shows a circuit diagram for measuring tan δ of the anti-corrosion layer in the present invention. 1... Cable insulation layer, 4... Dielectric loss tangent measuring device, 5... High frequency power supply, 6... Cable conductor,
C _x ... Cable equivalent capacitance, R _x ... Cable equivalent resistance, L ₂ ... Inductance.

Claims (1)

[Claims] 1. Connect a dielectric loss tangent measuring device and a high-frequency power source in parallel between the cable shearing layer grounded by inductance and the ground, and calculate the cable corrosion protection layer on the cable shearing layer from the equilibrium condition of the dielectric loss tangent measuring device.
A method for constantly monitoring a cable anti-corrosion layer, the method comprising measuring tanδ.