JPH09243685A - Diagnostic apparatus for insulation degradation of hot-line cable - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an apparatus by which the degradation of a cable under a hot-line can be diagnosed precisely by excluding the influence of a zero-phase voltage. SOLUTION: The other end of a cable-shielding grounding conductor 3 one end of which is passed through a transformer for a zero-phase current and which is connected to a cable shield 3' is not grounded, and it is connected to the neutral point N of a high-voltage system. A zero-phase voltage V0 is applied to the cable shield via the cable-shielding grounding conductor. Thereby, a measuring error based on the zero-phase voltage is removed. In addition, a ground fault current which flows in the cable-shielding grounding conductor is detected by a sensor. When the ground fault current reaches a definite value, switches 13a, 13b are changed over, and the cable-shielding grounding conductor is grounded.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a device for diagnosing the degree of insulation deterioration of a high-voltage power cable during hot-line operation without the need to prepare a power source for measurement in advance.

[0002]

2. Description of the Related Art As a device for diagnosing the degree of insulation deterioration of a high-voltage power cable during hot-line operation, a ZC attached to a cable end without preparing a measurement signal power source in the high-voltage system in advance.
By measuring the AC terminal voltage for each of the three sets of elements having different frequency-impedance characteristics, which are switched and connected to the secondary side of T (zero-phase current transformer), the equivalent center noise frequency generated inside the cable is measured. An apparatus for knowing the magnitude of electromotive force of the above and diagnosing the degree of insulation deterioration based on this has been proposed by the applicant as Japanese Patent Application No. 7-143412.
This device will be described with reference to FIGS.

1 is a high voltage bus bar, 2 is a cable to be measured, 3
Is a cable shield ground wire. The measurement set 5 is connected to the secondary side of the ZCT 4 inserted into the cable end together with 3. 6 is the anticorrosion layer capacitance of the cable 2 to be measured, the value of which is C _S. One end of the ZCT 4 on the secondary side is grounded for safety. Inside the measurement set 5 there is a fixedly installed capacitance C ₀ and three sets of elements C ₁ , R ₁ and R ₂ selected by a changeover switch. FIG.
FIG. 3 is an equivalent measurement circuit diagram in which is converted into the primary side of ZCT4 and rewritten. Assuming that the transformation ratio of ZCT4 is n, between the shield terminal S and the ground G, the measurement set 5 has a first combination of n ² C ₀ + n ² C _{1 and} a second combination of n ² C ₀ + R ₁ / n ² . , N ² C ₀
Three sets of elements having different frequency-impedance characteristics, which are composed of the third combination of + R ₂ / n ² , are prepared. Looking at the cable side from the measurement set 5, the anticorrosion layer capacitance C _{S of the} cable and two sets of electromotive force arms are present. Fhz
Electromotive current capacity limb is generated limb of the noise current caused current capacity X under basic frequency Fhz strains, the electromotive force e _x, made internal impedance Z _X, its value is _X = e _x / Z X. NF
The hz electromotive force limb is a generation limb of the measurement target electromotive force Y under the equivalent central noise frequency NFhz which is N times the fundamental frequency.
It consists of an electromotive force e _Y and an internal impedance Z _Y , the value of which is Y = e _Y / Z _Y. The impedance of C _S is also added to n ² C ₀ as a fixed element, but n ² C ₀ >> C _S. DV
M (Digital Volt Meter) is a voltmeter capable of measuring a true AC effective value, whereby the values of X, Y and N can be calculated by measuring the AC terminal voltage for each of the three sets of elements. The Y value is a value that is an index for deterioration diagnosis, and the magnitude of the Y value can be used to know the degree of insulation deterioration of the cable.

[0004]

In the conventional device described above, the zero-phase voltage V ₀ is small when the grounding system of the high voltage system is the non-grounding system or the direct grounding system, and there is no particular problem. However, in the case of the resistance grounding method in which a large-capacity generator is independently operated in the high-voltage system, or external power reception or external power transmission is performed in parallel, a problem of large measurement error occurs. To some extent, a large-capacity generator generates a zero-phase electromotive force whose main component is the third harmonic, so that a zero-phase voltage, so-called V, is always present between the neutral point of the system and the ground. _{A 0} appears, which penetrates the measurement circuit and is a harmonic having the third component as the main component, so that it is difficult to distinguish it from the Y value, which is the target to be regarded as the deterioration index. This phenomenon is not generally known. However, it became empirical during the progress of the degradation diagnosis under the live line using the conventional device.

FIG. 6 is an explanatory view of the introduction of a measurement error based on the zero-phase voltage V ₀ appearing between the neutral point N and the ground G in the case of the conventional diagnostic device. In FIG. 6, the cable shield grounding wire 3 is pulled out through the ZCT 4 penetrating the cable end. Reference numeral 3'denotes a cable shield, and in the penetration portion of the ZCT 4, the cable shield grounding wire 3 connected to the terminal S of the cable shield 3'is in a folded-back condition with 3 '. This is because the ZCT4 measures the vector sum of the three-phase conductor currents of the cable, and therefore the other shield ground current I _S needs to be removed by folding back inside the ZCT4. The ground current I _S leading to this is not measured by ZCT4. 5 is a secondary connected measurement set of the ZCT4, the interior of which has already been described in FIGS. Although H is the high-voltage conductor end of the cable to be measured, the high-voltage conductor end H is equal to the neutral point N in the zero-phase circuit. 7
Is the zero-phase electromotive force whose main component is the third harmonic that is assumed inside the generator, and its value is e _G. 8 is the NGR of the generator
(Neutral point ground resistance), 9 is the main transformer or GTR
It is the NGR of (grounding transformer). Reference numeral 10 is a zero-phase impedance of the entire system excluding NGR 8 and 9, and is mainly composed of the sum of the capacitances of the respective phases of the cables connected to the system.

The zero-phase voltage V ₀ is substantially determined by the electromotive force 7, the value of the resistor 8, the value of the resistor 9, and the value of the impedance 10. However, since the electromotive force 7 has the third harmonic as a main component, the zero-phase voltage V ₀ also has a large third harmonic component. Reference numeral 11 is a virtual electromotive force existing in the cable insulator of the measurement object, and its value is e _C. 12 is the zero phase impedance of the cable, the value of which is Z _C. Note that e _C ,
Z _C is not only e _Y and Z _Y shown in FIG. 5, but also equivalent to the integrated form of e _X and Z _X. 6 is the anticorrosion layer capacitance, which exists between the cable shield 3'and the ground, and its value is C _S
It is. In FIG. 6, the end of the cable shield ground wire 3 is dropped to the ground G. Therefore, the zero-phase voltage V ₀ is equal to the electromotive force 1
Since most of them are applied to the zero phase impedance 12 in series with 1, the electromotive force value e _C is approximately proportional to V ₀ if it is smaller than the zero phase voltage V ₀ , and inversely proportional to the zero phase impedance value Z _C. The current in the form of a circle will be measured through ZCT4. The influence of the electromotive force 11 is important because it is masked and difficult to detect.

The above example is shown in the following table.

[Table 1] Case 1 Cable to be measured 11 KV 3 x 500 mm ² CV 780 m a. Measured value during generator operation NGR: 63.5Ω 100A Operated as a resistance ground system X _mA (60HZ) NFhz YmA judgment 18.5 201 99.8 YmA value (30 to 300) I'm sorry. B. Although the generator continues to operate, measured value XmA (60HZ) NFhz YmA judgment 9.4 315 6.8 YmA value when the NGR was opened and the system was set to a non-grounded system. It fell to 7% of the case of (3 to 30) and entered the light caution area. C. V ₀ and its frequency measurement value (by digital multimeter) V ₀ V frequency hz a. In the case of 59.9 180 b. In the case of 12.3 60

[Table 2] Case 2 Cable to be measured 11KV3 × 325mm ² CV 540m a. Measured value during generator operation Generator NGR: 318Ω Main transformer NGR: 63.5Ω / 2, 220A Operated as a resistance ground system XmA (60HZ) NFhz YmA judgment 9.6 251 41.2 Remark as a medium caution area. Main generator NGR: 63.5Ω, operated as a 120A resistance grounding system even though the generator continued to operate XmA (60HZ) NFhz YmA judgment 8.6 192 73.5 YmA value almost doubled c. V ₀ and its frequency measurement value (by digital multimeter) V ₀ V frequency hz a. In case of no measurement b. 88.7 180 In addition to these cases, the analytical conclusions obtained are as follows.

[0008] 1. In a resistance grounding system during generator operation, the generator effectively acts as a zero-phase electromotive force source of the third harmonic, and the zero mainly composed of the third harmonic is between the system neutral point and the ground. Generate a phase voltage V ₀ . The value varies depending on the characteristics of the generator, the impedance of the grounding equipment, and the impedance of the zero-phase circuit, but it is 1 in the 11KV system.
There is also an example that exceeds 00V. But it not recognized in general because it is 1.6% slightly as V ₀ = 100 V with respect to 100% ground voltage 6350V lineages. 2. However, if there is a zero-phase voltage of 100 V and the third-order harmonic is the main component, enormous errors will occur in the live-line cable deterioration diagnosis according to the prior art as shown in the case, and the reliability of the measured value will be high. Disappears. A calculation example shows that the capacitance for three phases of the cable is 1 μF (approximately 1 km
(Corresponding to the length), when 180 HZ and 100 V are applied, a current of 113 mA flows, which is unrelated to the insulation deterioration of the cable, but is mixed in the YmA measurement value. 3. Under such an environment, the larger the capacitance of the cable is, the larger the YmA value is observed, and the deterioration diagnosis is meaningless. Therefore, it is highly desired to develop a device that eliminates the influence of the zero-phase voltage V ₀ .

It is an object of the present invention to provide an apparatus which can eliminate the influence of zero-phase voltage and perform accurate deterioration diagnosis under a live cable. Another object of the present invention is an apparatus capable of performing accurate deterioration diagnosis of a cable under a live line by eliminating the influence of a zero-phase voltage, which is provided with a safety measure against a ground fault occurring during a cable deterioration diagnosis. Is to provide.

[0010]

The insulation deterioration diagnosing device of the present invention is characterized in that the other end of a cable shield grounding wire passing through a zero-phase current transformer and connected at one end to a cable shield is not grounded. By connecting the neutral point voltage to the neutral point of the high voltage system and applying the neutral point voltage to the cable shield through the cable shield grounding wire, the zero phase voltage is prevented from entering the cable, and based on the intrusion of the zero phase voltage. The feature is that the measurement error is removed.

Further, the insulation deterioration diagnosing device of the present invention includes a cable shield ground wire whose one end is connected to the cable shield passing through the zero-phase current transformer and a neutral wire existing in the high voltage system of the live cable. An impedance and a first switch connected in series between a point and the other end of the cable shield ground wire; and a second switch having one end grounded and the other end connected to the other end of the cable ground wire. A sensor for detecting a ground fault current flowing through the cable ground line, and a control for opening the first switch and closing the second switch when the ground fault current detected by the sensor reaches a certain value. And means.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION In the insulation deterioration diagnosing device of the present invention, the cable shield ground wire, which has been dropped directly to the ground in the conventional device, is disconnected from the ground and connected to the neutral point of the high voltage system.
With this configuration, the zero-phase voltage V ₀ generated between the neutral point and the ground is driven out of the measurement circuit,
The mixing of the error current based on V ₀ is eliminated. However, although the current passing through the cable anticorrosion layer capacitance increases based on the zero-phase voltage V ₀ , this current is not measured due to the folding effect in the ZCT.

The following cases can be considered for selecting the neutral point of the system. 1. When the secondary of the main transformer is a Y connection and the NGR for the main transformer is installed between the neutral point and the ground. In this case it is quite obvious and its neutral point is used. 2. The secondary of the main transformer is a Δ connection, but a grounding transformer GTR is provided, and an NGR (neutral point grounding resistance) for GTR is installed between the neutral point of the primary Y connection and the ground. If. In this case, too, it is obvious to use the neutral point. 3. If the secondary of the main transformer is Δ connection and the NGR is provided only at the neutral point of the generator, the neutral point of the generator is used. Alternatively, a new GTR is provided. 4. If the GTR is of a type that has an NGR on the secondary side, the secondary side drains the NGR and short-circuits, and a new N
Only after installing the GR can the primary neutral point be used.

Next, the following means are required as a countermeasure against a ground fault occurring in the target cable during measurement. 1. Current transformer CT penetrating the cable shield ground wire
As a sensor, if the passage of a ground current above the specified level is recognized, the cable shield ground wire is immediately dropped to ground, and the switch connected to the neutral point via the protective resistance is opened. Without this protective resistor, as far as the cable concerned is concerned, a grounding accident will occur in the direct grounding state. It is necessary to set the value of the protection resistance to a resistance value equivalent to the value of NGR so that the protection system of the system is not changed. Although the sensor has been described to operate with the current element, the same applies to the case where the sensor is operated with the voltage element, for example, the voltage drop of the protection resistor. A transformer with a turn ratio of 2.1: 1 is used, the primary side of which is connected in parallel with the NGR, and indirectly from the secondary side this N
When the cable shield ground wire is dropped to the ground through the secondary winding with the GR functioning as a protective resistance, the current transformer CT penetrating the cable shield ground wire is used as a sensor to pass the ground current above the specified level. Immediately after recognizing, the first switch connecting the primary of the transformer and the neutral point is opened, and then the cable shielding ground wire is directly grounded. The ground current path is secured for a slight time delay during this period by a varistor or arrester having a large energy resistance provided between the open contacts of the second switch for directly dropping the cable shield ground wire to the ground. Compared with the case of 1, the sequence to reach the direct grounding of the cable shield grounding wire is changed because the direct short circuit of the secondary side of the transformer reflects on the primary side and the extreme decrease of the impedance of the NGR occurs. This is to avoid it.

Next, FIG. 1 shows an outline of an insulation deterioration diagnosing device of the present invention using the constituent elements required from the results of the examination as described above. In FIG. 1, the same symbols as those in FIG. 6 indicate the same components, and the description thereof will be omitted. In FIG. 1, the other end of the cable shield ground wire 3 is not directly dropped to the ground, and the first switch 13a and the protection resistor 14 are connected in series between the other end and the neutral point N of the high voltage system. Connected to. Further, a second switch 13b is connected between the other end of the cable shield ground wire 3 and the ground. Normally, the first switch 13a is closed and the second switch 13b is opened. Therefore, the zero-phase voltage V ₀ is mainly applied to the anticorrosion layer capacitance, and the zero-phase current I _S reverses its polarity from that in the case of FIG. 6 and generally its value increases. However, since the shield grounding wire 3 passing through the ZCT 4 and the cable shield 3'are folded back, ZCT
Reference numeral 4 is a loop circuit that does not detect the zero-phase current I _S and the zero-phase voltage V ₀ hardly enters the measurement circuit, that is, 11 is an electromotive force source, and the zero-phase impedance 12 is a cable shield 3 ′. Shield ground wire 3, protective resistor 14,
The current flowing back through the first switch 13a and the neutral points N and H can be measured, and the purpose of eliminating the influence of the zero-phase voltage V ₀ is achieved. In this case, the resistance value of the protective resistor 14 is the anticorrosion layer capacitance 6 and the impedance 1
The smaller the number is, the more effective the elimination is.

Next, an embodiment of the insulation deterioration diagnosing device of the present invention equipped with a ground fault accident countermeasure will be described with reference to FIG. FIG.
Is a resistor having a resistance value equal to that of NGR (neutral point grounding resistor) 9 as the protective resistor 14, and one end of the protective resistor 14 is connected to the neutral point N of the transformer 15 via the first switch 13. . The transformer 15 may be the secondary side of the main transformer or the primary side of the grounding transformer GTR. 7'is a three-phase generator in which the zero-phase electromotive force e _C exists. 8
Is a neutral ground resistance NGR for the three-phase generator 7 '. The other end of the protection resistor 14 is connected to the cable shield ground wire 3 via a cable selection ground switching device 17 as a second switch. The cable shield grounding wire 3 is drawn out from the cable shield end S of the measurement target cable 2 through the inside of the ZCT 4.

The cable selection / ground switching device 17 is a group of opening / closing mechanisms for selecting one of a plurality of cables to be measured and switching the ground from a constantly grounded state to a measurement circuit configuration state. It contains a cable grounding open switch group and a selection cable measuring circuit configuration switch group 19 shown in FIG. 16 is a ground current sensor, which is shown in FIG.
In the above, the use of the current transformer CT is shown, but the voltage across the protective resistor 14 may be checked as a voltage sensor. During the measurement, as described in FIG. 1, the switch 13, the protection resistor 14, the corresponding switch of the selected cable measuring circuit configuration switch group 19, the cable shield grounding wire 3, the cable 2 to be measured, and the neutral point N are circulated. A measurement circuit is configured to detect the current.

The ground current sensor 16 also serving as the control means is
A command is issued immediately when the ground current I _S exceeds a predetermined threshold value, for example, 1 A during measurement, and one switch (current measurement target) of the selected cable ground open switch group 18 that has been opened during measurement. The switch connected to the cable 2) is closed to drop the cable shield grounding wire 3 directly to the ground G, and one of the corresponding selected cable measurement circuit configuration switch group 19 is opened, and the switch 13 is opened to protect it. The resistor 14 is separated from the neutral point N. As a result, the cable fault current is the original neutral point ground resistance NG.
It will return to the power supply 7'through R9 and 8 and the protection of cables and grids will be fulfilled by grid-specific equipment (not shown). Since the switch 13 may generate a high voltage between its open terminals, it is necessary to use a switch having a sufficiently high breakdown voltage between the terminals.

The reason why the resistance value of the protective resistor 14 is made equal to the value of NGR9 is that the protection system of the system is maintained under the same conditions as in a normal accident when a ground fault occurs. It has been taken into consideration. However, the power capacity of the protection resistor 14 may be smaller than that of the NGR. For example, the value of NGR9 is 63.5Ω and the nominal value is 11
When used as a KV100A resistance grounding system, for example, 3
The power capacity is (11KV / √3) × even if it is rated for 0 seconds.
Although a large value of 100 A = 635 KW is required, the protection resistance used in the present invention does not have to last for 30 seconds, and for example, 1 A can withstand 1 second, so 63.5 V ×
1A = 63.5W and a rating of 1 second is sufficient.
However, in reality, it is sufficient to use a resistor having a larger size and a larger power capacity, for example, a continuous rating of 200 W and a resistance of 63.5Ω in consideration of a margin.

Next, another embodiment of the insulation deterioration diagnosing device of the present invention having a countermeasure against the occurrence of a ground fault will be described with reference to FIG.
FIG. 3 shows a transformer 2 having a turns ratio of 1: 1 as an equivalent protection resistor.
It intends to indirectly utilize NGR9 via 0. The primary side of the transformer 20 is connected in parallel with the NGR 9 between the neutral point N and the ground G of the transformer 15 via the switch 13. The secondary side of the transformer 20 has one end on the ground,
The other end is connected to the cable shield ground wire 3. In addition,
The polarity of the secondary side of the transformer 20 must be the same as the primary side. With this configuration, the NGR 9 is indirectly inserted into the cable shield grounding wire 3 as a protective resistance between the earth and the ground, and the cable shield end S has the _zero potential V ₀ between the neutral point N and the same potential, that is, the earth. Will be given. Therefore, although the same state as in the case of FIG. 2 is configured, it is possible to obtain a sense of security that the user is psychologically distant from the neutral point N. The transformer 20 does not need to have a large capacity. As a design example, a continuous rating of 200 VA, a primary and secondary rated voltage of 110 V, and a rated current of about 1.8 A are sufficient.

If a ground fault occurs in the cable 2 to be measured during measurement under such a condition, an increase in the ground current I _S is inserted in the middle of the cable shield ground wire 3 and the current sensor 16 (or the transformer 20). Voltage sensor reading the secondary voltage of
When the ground current I _S exceeds, for example, 1 A, the switch 13 is immediately opened and the delay circuit (control means) 22 delays for a predetermined time, and then a closing signal is output to the switch 18 ′. Then, the path of the ground current I _S is only on the secondary side of the transformer 20, which has high impedance, so that the varistor or arrester 21 is discharged.
At the same time, the switch 18 'is closed and the cable shield ground wire 3 is directly dropped to the ground. The reason why the switch 18 'is not closed before opening the switch 13 here is to avoid that the effective resistance value of the NGR 9 may be significantly decreased even in a short time due to a short circuit on the secondary side of the transformer. is there.
In order to avoid complication in this figure, the selective cable ground switching device 17 shown in FIG. 2 is not shown, and instead the switch 1
Only 8'is shown. Although not shown in FIG. 2 and FIG. 3, it is practical that one measurement set 5 can be used to switch and measure a plurality of cables 2 to be measured. Also, one ZCT4 must be attached to each cable. Penetration type ZCT when establishing from the beginning
However, normally, a split type ZCT will be inserted later.

The proof of the present invention is shown in Case 1 of Table 1 in the case of the measurement a in which the generator is operated by resistance grounding and the measurement b in which the generator is operated by non-grounding, that is, the zero-phase voltage V ₀ itself. Although the effect of reducing it has already been shown, in the case of this example, even if the measurement B was set to the non-grounded system, it did not stop the operation of the generator, but simply opened the NGR for the generator. However, a plurality of GPTs (zero-phase voltage transformers) still remain in the system, which is equivalent to a high resistance value of NGR, so it is not a completely ungrounded system. Therefore, the zero-phase voltage V ₀ is 59.9 V in the case of measurement B, and is 12.3 V in the case of measurement B, which is only 20%. However, the YmA value is reduced to 7% due to the power of the effect of shifting the main component from the frequency of 180 HZ to 60 HZ.

In the apparatus of the present invention, the calculation of the reduction effect based on the following assumption will be shown as an example.

[Table 3] Noise current cable capacitance based on V ₀ detected by hypothetical value ZCT 1 μF Anticorrosion layer capacitance 2 μF Conventional technology 11.31 mA (100%) Protective resistance 63.5 Ω Present invention 1.59 mA (14%) Frequency 180HZ V ₀ 10V That is, even if the frequency is 180HZ, it is possible to obtain a larger effect than the reduction effect when the resistance grounded system is changed to the non-grounded system shown in Case 1 of Table 1. Moreover, the system also has the effect of maintaining the resistance grounding system.

[0024]

INDUSTRIAL APPLICABILITY According to the present invention, in a resistance grounding system in which a large-capacity generator is independently operated or externally received or transmitted to the outside, the generator is used as an electromotive force source. In the presence of a zero-phase voltage whose main component is the third harmonic, the current flowing into the cable anticorrosion layer capacitance due to the zero-phase voltage is used to diagnose the deterioration under the live line of the cable. Since it passes back inside the zero-phase current, no electromotive force based on the zero-phase voltage is generated on the secondary side of the zero-phase current transformer, and the zero-phase current transformer detects the insulation of the cable to be measured. Due to the electromotive force existing in the body, most of the current flows back through the zero-phase impedance and the protective resistance of the cable, so that noise current due to zero-phase voltage can be eliminated and accurate cable deterioration diagnosis can be performed.

Further, according to the present invention, when a ground fault occurs during the diagnosis of cable deterioration, a protective means for directly dropping the cable shielding ground wire to the ground is provided. Return to and the safety during the measurement is protected.

[Brief description of drawings]

FIG. 1 is a schematic system configuration diagram of a cable deterioration diagnosis device of the present invention.

FIG. 2 is a circuit configuration diagram showing an embodiment of the present invention.

FIG. 3 is a circuit configuration diagram showing another embodiment of the present invention.

FIG. 4 is a circuit configuration diagram illustrating a conventional cable deterioration diagnosis device.

FIG. 5 is an equivalent circuit diagram of FIG.

FIG. 6 is a schematic system configuration diagram of a conventional cable deterioration diagnosis device.

[Explanation of symbols]

 2 Cable to be measured 3'Cable shield 3 Cable shield Ground wire 4 Zero-phase current transformer 5 Measurement set 8 and 9 Neutral ground resistance 13 Switch 14 Protective resistance 16 Current sensor 17 Cable selection ground switching device 20 Transformer 21 Arrester 22 Delay circuit

Claims (4)

[Claims] 1. Three types of different frequency-impedance characteristics are provided on the secondary side of a zero-phase current transformer inserted in a live-line power cable terminal in the presence of a zero-phase voltage generated in a resistance grounding high-voltage system. By switching and connecting the elements of the above, and measuring the AC terminal voltage, the magnitude of the electromotive force at the equivalent center noise frequency generated inside the cable can be known, and the degree of cable insulation deterioration can be diagnosed based on this. A deterioration diagnostic device, wherein the other end of a cable shield grounding wire that passes through the zero-phase current transformer and has one end connected to a cable shield is connected to the neutral point of the high-voltage system without grounding. Applying the zero-phase voltage to the cable shield via the cable-shield ground wire eliminates measurement errors due to the zero-phase voltage. Le insulation deterioration diagnostic device. 2. Three kinds of different frequency-impedance characteristics are provided on the secondary side of a zero-phase current transformer inserted in a live-line power cable terminal in the presence of a zero-phase voltage generated in a resistance grounding high-voltage system. By switching and connecting the elements of the above, and measuring the AC terminal voltage, the magnitude of the electromotive force at the equivalent center noise frequency generated inside the cable can be known, and the degree of cable insulation deterioration can be diagnosed based on this. A deterioration diagnostic device, wherein a cable shield grounding wire having one end connected to a cable shield passing through the zero-phase current transformer, a neutral point existing in a high voltage system of the live cable, and the cable grounding An impedance and a first switch connected in series with the other end of the wire, one end grounded and the other end connected to the other end of the cable shield ground wire A second switch; a sensor for detecting a ground fault current flowing through the cable shield ground wire; and a second switch that opens the first switch when the ground fault current detected by the sensor reaches a constant value. A live-wire cable insulation deterioration diagnosing device comprising: a control means for closing a switch. 3. The live line according to claim 2, wherein the impedance is a protective resistance having a resistance value equivalent to a neutral point ground resistance connected between the neutral point and the ground. Cable insulation deterioration diagnosis device. 4. The impedance is a secondary side of a transformer having a winding ratio of 1: 1 in which a primary side is connected in parallel to a neutral point ground resistance connected between the neutral point and the ground, 3. The live-wire cable insulation deterioration diagnosing device according to claim 2, wherein the control means closes the second switch after a predetermined time has passed since the first switch was opened.