JPH09269352A - Cable diagnosis method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable two simultaneous measuring by measuring tan δ of a cable from the relation of cable application voltage and ground wire current, mounting a current transformer and measuring partial discharge generated in the cable from its output. SOLUTION: Alternating voltage V is impressed on the conductor 17 of a cable 11, passes capacitance between the conductor 17 and a sheath 12 between edge-cut parts 16, and current of a difference in phase caused by insulation characteristic between the voltage V and the cable 11 is flowed in a ground wire 13. A tan δ measuring instrument 14 is inserted in the ground wire 13, and tan δ is calculated from the current flowing in the ground wire 13 and reference current flowing between the conductor 17 and an ideal capacitor C. When the voltage V is increased to measure tan δat each voltage and local insulation breakage occurs in the sheath 12 together therewith, current by partial discharge flows in the ground wire 13. A current transformer 18 is mounted on the ground wire 13, and pulse current is taken out in a partial discharge measuring apparatus 15 via a lead 19 without changing the phase of measuring current. Discharge start voltage and tan δ are simultaneously found by the observation.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for diagnosing cable deterioration and the like, and more particularly to a cable diagnostic method capable of simultaneously measuring tan δ and partial discharge of a cable.

[0002]

2. Description of the Related Art The dielectric loss tangent, that is, the value of tan δ based on the phase difference between voltage and current when an alternating voltage is applied to an insulating material is an important index for the evaluation of the overall insulating property of the insulating material. Therefore, the overall insulation characteristics of the cable can be evaluated by measuring the tan δ of the cable. Moreover, the dielectric breakdown of the insulating material is likely to occur at a defective portion such as an external damage or a void in the insulator, and the partial discharge generated at the defective portion often causes the breakdown. Therefore, by measuring the partial discharge inception voltage of the cable and the charge amount of the partial discharge pulse, it is possible to evaluate a local insulation characteristic such as a defective portion of the cable.

In one of the tan δ measuring devices, a tan δ is obtained by comparing a reference current flowing through an ideal capacitor whose tan δ is almost zero with a ground wire current flowing through a ground wire of a cable.
There is something to ask for. On the other hand, one of the partial discharge measuring instruments,
A detection impedance is inserted into the ground wire of the cable, an AC voltage is applied to the cable, and the partial discharge signal caused by the local insulation breakdown of the cable insulator due to this applied voltage is detected as a change in the voltage across the detection impedance. There is. However, in the tan δ measuring device, if the impedance element is inserted in the ground wire of the cable,
Due to the influence of this impedance element, the phase of the ground wire current flowing through the ground wire is deviated.
Is impossible to measure.

From this, it is possible to accurately measure the tan δ measuring instrument and the partial discharge measuring instrument which requires the insertion of the detection impedance to the ground line at the same time.
It was not possible to simultaneously perform an δ measurement and partial discharge measurement. For this reason, the test voltage is increased stepwise to increase the ta
When performing both the nδ measurement and the partial discharge measurement, a measuring instrument or the like for either one of the measurements is connected to perform a measurement up to a prescribed test voltage, and then another prescribed measuring instrument for the other measurement. Then, the measurement was conducted again until the predetermined test voltage was reached.

Therefore, it is necessary to redo the connection work of each measuring device for the tan δ measurement and the partial discharge measurement, which sometimes makes quick measurement work difficult. Further, for example, if the cable is destroyed by the test voltage in the process of performing the partial discharge measurement while gradually increasing the test voltage, it becomes impossible to measure tan δ for this cable.

In order to more accurately determine whether the cause of this cable breakdown is due to, for example, local insulation breakdown or the overall insulation characteristics of the cable insulation,
Both measurement results of tan δ measurement and partial discharge measurement at an applied voltage until breakdown are indispensable. However,
If the cable breaks, it becomes impossible to perform either one of the tan δ measurement and the partial discharge measurement. Therefore, it is difficult to accurately find the cause of the breakage.

Ta up to the point of destruction of the cable
To obtain both the measurement results of nδ measurement and partial discharge measurement,
It is conceivable that the tan δ measurement and the partial discharge measurement are alternately repeated for each stepwise increase in the test voltage. In this case,
Each time the test voltage rises, it is necessary to reconnect the tan δ measuring device and the partial discharge measuring device, which complicates the measurement work.

[0008]

Therefore, an object of the present invention is to provide a cable tan without complicating the measurement work.
It is an object of the present invention to provide a cable diagnostic method capable of simultaneously performing δ measurement and partial discharge measurement without requiring reconnection of devices.

[0009]

The present invention adopts the following constitution in order to achieve this object. <Structure> The cable diagnosis method according to the present invention measures tan δ of the cable from the relationship between the voltage applied to the cable and the current flowing in the ground wire of the cable, while mounting a current transformer on the ground wire, The partial discharge generated in the cable is measured based on the output of the current transformer.

<Operation> In the cable diagnosing method according to the present invention, the ground wire through which the measurement current for tan δ measurement flows is
The detection impedance of the partial discharge measuring instrument is not directly inserted, and the electric signal containing the pulse current due to the partial discharge passes through the current transformer that does not affect the phase of the current flowing through the ground line. Taken out.

This partial discharge measuring device detects the discharge pulse signal by converting the electric signal taken out through the current transformer into a voltage change at the detection impedance, as in the conventional case, and displays it. Therefore, by observing the starting voltage of the discharge pulse signal detected by the partial discharge measuring device, the partial discharge starting voltage of the cable can be accurately measured. Further, the tan δ measuring device accurately measures tan δ based on the relationship between the voltage applied to the cable and the ground wire current flowing through the ground wire not provided with a phase conversion element such as an impedance element.

Therefore, the tan δ measuring device and the partial discharge measuring device are not connected to each other at the same time without being removed or reconnected to each other at the same time.
And it becomes possible to measure the partial discharge pulse.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the illustrated embodiments. <Specific Example 1> FIG. 1 is a connection diagram showing a specific example 1 to which the cable diagnosis method according to the present invention is applied. The cable diagnostic device 10 is used for diagnosing a cable 11 such as a CV cable. The cable diagnostic device 10 includes a tan δ measuring device 14 connected to a ground wire 13 connected to a sheath 12 of a cable 11 and a partial discharge measuring device 15. The cable 11 is formed with an edge cut portion 16 at which both ends of the sheath 12 expose the external conductive layer in order to eliminate the influence of errors at both ends thereof. The sheath 12 is grounded between the edge cut portions 16 via a ground wire 13.

An AC voltage V having a commercial frequency for testing is applied from one end of the conductor 17 of the cable 11. With respect to this AC voltage V, the conductor 17 of the cable 11 and the sheath 12 between the edge cut portions 16 form a capacitance. Therefore, a current having a phase difference θ with the AC voltage according to the insulation characteristics of the cable 11 flows through the ground wire 13. This ground wire 13
The ta of the cable 11 is
In order to measure n δ, the tan δ measuring device 1 is attached to the ground wire 13.
4 has been inserted. The tan δ measuring device 14 calculates tan δ by comparing the ground wire current flowing through the ground wire 13 with the standard current flowing through the ideal capacitor C from the conductor 17. The basic operation principle of the tan δ measuring device 14 will be described later.

The tan δ at each voltage value is measured as the voltage of the AC voltage V applied to the cable 11 rises. As the applied voltage rises, the sheath 12 of the cable 11 is locally insulated. When the breakdown occurs, a pulse current due to partial discharge flows from the sheath 12 through the ground wire 13. In order to detect this pulse current, a current transformer 18 is attached to the ground wire 13. The current transformer 18 is the ground wire 1
3 through the lead wire 19 of the partial discharge measuring instrument 1 without affecting the phase of the measuring current flowing therethrough.
Take out to 5.

The partial discharge measuring instrument 15 outputs a pulse signal taken in through the lead wire 19, and the discharge start voltage can be known by observing the pulse signal.

FIG. 2 is a current-voltage vector diagram showing tan δ of the cable 11. The current vector I shown in FIG.
Is a ground wire current flowing through the ground wire 13, and this current I
Has a phase difference of θ with the applied voltage V. Like the current flowing through the ideal capacitor C, this ground line current I is composed of a current component Io that is 90 degrees out of phase with the applied voltage V and a current component Ir that is in phase with the applied voltage V that causes dielectric loss. Become. tan δ is represented by (Ir / Io).

In order to obtain this tan δ, the tan δ measuring device 14 changes the magnitude of the value of the reference current taken in via the ideal capacitor C, and obtains the magnification α at which the current component Io ′ that matches the current component Io is obtained. Ask. Further, the magnitude of the current value Ir 'obtained by advancing the phase of the reference current taken in through the ideal capacitor C by 90 degrees is changed, and the sum I'of the current value Ir' and the current component Io 'is balanced with the current I. I
The scaling factor β from which r'is obtained is determined. This ratio (β / α) is
It is equal to (Ir / Io), which gives tan δ, as is well known in the art.

In the cable diagnostic device 10 of the present invention, ta
An accurate tan δ is obtained because an impedance element that affects the phase of the current is not inserted in the ground line 13 through which the ground line current for measurement by the nδ measuring device 14 flows. Further, the partial discharge measuring instrument 15 displays a partial discharge pulse by an electric signal from the current transformer 18 attached to the ground line 13, and an accurate discharge starting voltage can be obtained by observing the partial discharge pulse.

From this fact, tanδ is measured with the tanδ measuring device 14 and the partial discharge measuring device 15 connected at the same time.
And partial discharge can be measured simultaneously. Therefore, even in a destructive test in which the applied voltage V is increased stepwise,
Both measurement results up to the breakdown of the cable 11 can be efficiently obtained without changing the connection of the measuring equipment, and the cause of the breakdown can be accurately determined. .

<Second Embodiment> FIG. 3 is a connection diagram schematically showing a second embodiment to which the cable diagnosis method according to the present invention is applied. In the specific example 2 shown in FIG. 3, the sheath 12 of the cable 11 has two portions 12A due to the three edging portions 16.
And 12B are electrically separated. Sheath part 1
Ground lines 13A and 13B are connected to 2A and 12B,
The ground line 13 is joined to the ground lines 13A and 13B to form a ground line 13, and a tan δ measuring device 14 is connected to the ground line 13. A current transformer 18 is attached to each of the ground wires 13A and 13B, and each current transformer 18 is connected to the partial discharge measuring instrument 15 via a lead wire 19.

In the cable diagnostic device 10 of the specific example 2, one of the insulating portions, that is, the one sheath portion 12A acts as a coupling capacitor, and each current transformer 18 is connected to the ground wire 1.
It does not affect the phase of the measured current flowing through 3.
Therefore, the measurement of tan δ by the tan δ measuring device 14
It can be performed accurately as in the first specific example.
Further, when a pulse current due to local dielectric breakdown flows in either one of the sheath portions 12A, this pulse current becomes
Since the other sheath portion 12B acts as a coupling capacitor, it flows so as to form a closed loop between the ground lines 13A and 13B. On the other hand, the noise component is the ground line 13
Flows in the same direction from A and 13B toward the ground side. Therefore, by differentially amplifying the electric signals taken in through the respective lead wires 19 from both the current transformers 18 so that the respective noise components cancel each other, the noise components are removed and only the discharge pulse signal is obtained. Since it can be taken out, the discharge pulse can be observed more easily.

[0023]

According to the cable diagnosis method of the present invention, t
The detection impedance of the partial discharge measuring instrument is not directly inserted in the ground line through which the ground line current for an δ measurement flows, and the electric signal containing the pulse current due to the partial discharge is
Since it is taken out by the partial discharge measuring instrument through a current transformer that does not affect the phase of the current flowing through the ground line, t
Accurate measurement of tan δ and partial discharge pulse is possible without removing and reconnecting the an δ measuring device and the partial discharge measuring device alternately.

Therefore, according to the present invention, tan is efficiently
It becomes possible to measure δ and partial discharge pulse, and by increasing the test voltage step by step and using it for the withstand voltage test that causes the cable to breakdown, the measurement results of tan δ and partial discharge pulse until the breakdown will be continuous. Since it can be obtained, it is possible to more accurately determine the cause of the dielectric breakdown.

[Brief description of drawings]

FIG. 1 is a connection diagram schematically showing a specific example 1 to which a cable diagnosis method according to the present invention is applied.

FIG. 2 is a current-voltage vector diagram showing tan δ of the cable.

FIG. 3 is a connection diagram schematically showing a second specific example to which the cable diagnosis method according to the present invention is applied.

[Explanation of symbols]

 10 Cable Diagnostic Device 11 Cable 12 Insulation Part (Sheath) 13 Ground Wire 14 tanδ Measuring Instrument 15 Partial Discharge Measuring Instrument 16 Edge Cutting Section 18 Current Transformer

Claims (1)

[Claims] 1. A tan of the cable is obtained from a relationship between a voltage applied to the cable and a current flowing through a ground wire of the cable.
While measuring δ, a current transformer is attached to the ground wire, and the partial discharge generated in the cable is measured based on the output of the current transformer.