JP2009300273A - Diagnosis method of power cable when power cable is repaired - Google Patents

Diagnosis method of power cable when power cable is repaired Download PDF

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JP2009300273A
JP2009300273A JP2008155501A JP2008155501A JP2009300273A JP 2009300273 A JP2009300273 A JP 2009300273A JP 2008155501 A JP2008155501 A JP 2008155501A JP 2008155501 A JP2008155501 A JP 2008155501A JP 2009300273 A JP2009300273 A JP 2009300273A
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cable
repair
power cable
harmonic
repaired
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JP5120085B2 (en
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Toshiyuki Kobori
敏之 小堀
雅直 ▲広▼崎
Masanao Hirosaki
Toshifumi Shirono
寿文 城野
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JFE Steel Corp
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Abstract

<P>PROBLEM TO BE SOLVED: To provide a diagnosis method of a power cable for correctly determining whether the power cable is properly repaired when the power cable has been repaired. <P>SOLUTION: After a repairing agent is injected into the cable through a cable conductor so as to repair the power cable, an AC voltage is applied to the cable after a predetermined period, and a third harmonic is measured in a flowing loss current. If a measurement value is five times or more of a measurement value before the power cable is repaired, it determines that the power cable is properly repaired. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

本発明は、電力ケーブル修復時における電力ケーブルの修復方法に関し、電力ケーブルなかでもＣＶケーブル（架橋ポリエチレンビニルシースケーブル）に施した修復の是非を短期間で的確に診断しようとするものである。   The present invention relates to a method for repairing a power cable at the time of repairing the power cable, and intends to accurately diagnose whether or not the repair is performed on the CV cable (crosslinked polyethylene vinyl sheath cable) among the power cables in a short period of time.

電力ケーブルの中でも、図１に示す構造になるＣＶケーブルは、電気的特性、熱耐性および強度等に優れ、また経済的にも優れているため、現在、広範囲で使用されている。
図中、符号１は導体、２は内部半導電層、３は絶縁体（架橋ポリエチレン）、４は外部半導電層、５はクッション層、６はアルミ被（遮水層）、７は防食層である。 Among power cables, the CV cable having the structure shown in FIG. 1 is excellent in electrical characteristics, heat resistance, strength, and the like, and is economically excellent.
In the figure, reference numeral 1 is a conductor, 2 is an internal semiconductive layer, 3 is an insulator (cross-linked polyethylene), 4 is an external semiconductive layer, 5 is a cushion layer, 6 is an aluminum sheath (water-proof layer), and 7 is an anticorrosion layer. It is.

かようなＣＶケーブルの劣化の一つとして、「水トリー」と呼ばれる絶縁体である架橋ポリエチレンに生じる劣化現象があり、これによりケーブルの絶縁低下が惹起される。
ここに、水トリーとは、架橋ポリエチレン内部の隙間や電界平滑化のための半導電層の欠陥部分等に電界が集中し、その部分を起点として樹の枝状（tree）に隙間が伸展していき、そこに水が入り込んだものをいう。 As one of the deteriorations of such a CV cable, there is a deterioration phenomenon that occurs in crosslinked polyethylene, which is an insulator called “water tree”, and this causes a decrease in insulation of the cable.
Here, the water tree means that the electric field concentrates on the gaps in the cross-linked polyethylene and on the defective part of the semiconductive layer for smoothing the electric field, and the gap extends into a tree branch from that part. It refers to something that has water in it.

水トリーは、図２に示すように、その発生場所によって、内導トリー８、外導トリー９、ボウタイトリー10に分別され、内導トリー８は内部半導電層の欠陥、外導トリー９は外部半導電層の欠陥、ボウタイトリー10は架橋ポリエチレン内部の隙間（ボイド）が原因である。これらの水トリーは、最終的には絶縁体を橋絡することになる。
かような水トリーが発生したケーブルは、過電圧等をきっかけとして絶縁破壊を起こすため、ケーブルの更新を余儀なくされていた。 As shown in FIG. 2, the water tree is classified into an inner guiding tree 8, an outer guiding tree 9, and a bottling tree 10 depending on the location of the water tree. Defects in the outer semiconductive layer, votletree 10, are caused by voids in the cross-linked polyethylene. These water trees will eventually bridge the insulator.
The cable in which such a water tree was generated had to be renewed because the insulation breakdown was caused by an overvoltage or the like.

ケーブル内の水トリーによるケーブルの劣化を診断する方法としては、特許文献１〜３に各種の方法が開示されている。
上記の従来技術により水トリーが発見され、絶縁抵抗が低下したケーブルは更新し、引き換えを実施してきたが、かかる更新には、多額の費用と時間が必要となる。
特開２００４−３５４０９３号公報 特開２００３−２７０２８６号公報 特開２００３−２４０８１５号公報 Various methods are disclosed in Patent Documents 1 to 3 as a method of diagnosing cable deterioration due to a water tree in the cable.
The water tree has been discovered by the above-described conventional technology, and the cable having a reduced insulation resistance has been renewed and exchanged. However, such renewal requires a large amount of cost and time.
JP 2004-354093 A JP 2003-270286 A JP 2003-240815 A

一方、水トリー等により劣化し、絶縁抵抗を低下したケーブルに対して、シリコン製の薬剤（ケーブルキュアと呼ばれる）をケーブル導体部に注入し、ケーブルを修復する技術が知られている（特許文献４）。
この技術は、水トリー等の発生により修復が必要と判断されたケーブル中の所定領域に対し、ケーブル導体部の隙間を利用してケーブル内にケーブルキュアを充填することによって、水トリー等を補修しようとする技術である。
特開昭６３−１７４２２１号公報 On the other hand, a technique for repairing a cable by injecting a silicone chemical (called cable cure) into a cable conductor portion of a cable that has deteriorated due to a water tree or the like and has reduced insulation resistance is known (Patent Literature). 4).
This technology repairs water trees, etc., by filling the cable with cable cure using the gaps in the cable conductors for a predetermined area in the cable that is determined to be repaired due to the occurrence of water trees, etc. It is a technique to try.
JP 63-174221 A

上記の技術の開発により、ケーブルの修復が可能となった。
しかしながら、この技術により修復したケーブルの評価は、修復後のケーブルの絶縁抵抗値を測定し、その値が一定値以上であれば適正に修復がなされたものと見なしていた。 The development of the above technology has made it possible to repair the cable.
However, in the evaluation of the cable repaired by this technique, the insulation resistance value of the cable after the repair was measured, and if the value was a certain value or more, it was considered that the cable was properly repaired.

上述したとおり、上掲特許文献４に開示の技術では、修復後のケーブルの絶縁抵抗値を測定し、その値が一定値以上であれば適正に修復がなされたものと診断していた。
しかしながら、絶縁抵抗値は、測定日の気温や湿度と相関が深いため、一回の測定ではその値の信頼性が低く、必ずしも適正に修復がなされたかどうかは明確ではなかった。 As described above, in the technique disclosed in the above-mentioned Patent Document 4, the insulation resistance value of the cable after restoration is measured, and if the value is equal to or greater than a certain value, it is diagnosed that the restoration has been properly performed.
However, since the insulation resistance value has a strong correlation with the temperature and humidity on the measurement day, the reliability of the value is low in one measurement, and it is not always clear whether the value has been properly repaired.

本発明は、上記の問題を有利に解決するもので、電力ケーブルを修復した場合に、その修復が適正になされたか否かを的確に判定することができる電力ケーブルの診断方法を提案することを目的とする。   The present invention advantageously solves the above problem, and proposes a method for diagnosing a power cable that can accurately determine whether or not the power cable has been properly repaired when the power cable is repaired. Objective.

電力ケーブルの修復の適否を判断する手段として絶縁抵抗値を利用する場合、絶縁抵抗値は、測定日の気温や湿度と相関が深いため、一回の測定ではその値の信頼性が低く、必ずしも適正に修復がなされたかどうかは明確ではなかった。
また、ケーブル導体部に注入されたケーブルキュアは、ケーブル内での化学反応により時間と共に特性が変化するため、一回の測定では信頼性の高い診断を行うことが難しい。 When using the insulation resistance value as a means to determine the suitability of power cable repair, the insulation resistance value has a strong correlation with the temperature and humidity on the measurement day, so the reliability of the value is low in one measurement, not necessarily It was not clear whether the restoration was done properly.
In addition, since the characteristics of the cable cure injected into the cable conductor portion change with time due to a chemical reaction in the cable, it is difficult to perform a highly reliable diagnosis with a single measurement.

電力ケーブルを修復した場合、その補償は通常20年が要求される。しかしながら、これほどの長期にわたって修復の適否を診断することは現実的でない。
そこで、発明者らは、修復が適正になされたか否かの判断をできるだけ早い時期に行うための方策について種々検討を重ねた。 When power cables are repaired, the compensation usually requires 20 years. However, it is not realistic to diagnose the suitability of repair over such a long period.
Therefore, the inventors have repeatedly studied various measures for determining whether or not the repair has been properly performed at the earliest possible time.

その結果、ケーブル導体中にケーブルキュアを注入後、一定期間経過後に、ケーブルに交流電圧を印加し、そのとき流れる損失電流中の第３高調波を測定し、この測定値を修復前の第３高調波測定値と比較することにより、信頼性の高い診断ができることの知見を得た。
また、上記した交流電圧の印加を一定の電圧範囲にわたって行い、そのとき流れる損失電流中の第３高調波と共に損失電流中の基本波を測定し、基本波（Ｉ1）に対する第３高調波（Ｉ3）の比で示される第３高調波電流発生率（Ｉ3／Ｉ1）の増加率を検出することによっても、信頼性の高い診断ができることの知見を得た。
本発明は上記の知見に立脚するものである。 As a result, after injecting the cable cure into the cable conductor, an AC voltage is applied to the cable after a certain period of time, the third harmonic in the loss current flowing at that time is measured, and this measured value is the third before the restoration. The knowledge that a highly reliable diagnosis can be made was obtained by comparing with the harmonic measurement value.
Further, the application of the AC voltage is performed over a certain voltage range, the fundamental wave in the loss current is measured together with the third harmonic wave in the loss current flowing at that time, and the third harmonic wave (I 1 ) It was also found that a highly reliable diagnosis can be made by detecting the increase rate of the third harmonic current generation rate (I 3 / I 1 ) indicated by the ratio of I 3 ).
The present invention is based on the above findings.

すなわち、本発明の要旨構成は次のとおりである。
１．電力ケーブル修復のために、該ケーブル内にケーブル導体を介して修復剤を注入後、一定期間経過後に、ケーブルに交流電圧を印加し、そのとき流れる損失電流中の第３高調波を測定し、該測定値が修復前の測定値の５倍以上であった場合に適正な修復が達成されたと判断することを特徴とする電力ケーブル修復時における電力ケーブルの診断方法。 That is, the gist configuration of the present invention is as follows.
1. In order to repair the power cable, after injecting the repair agent into the cable through the cable conductor, after a certain period of time, an AC voltage is applied to the cable, and the third harmonic in the loss current flowing at that time is measured. A method of diagnosing a power cable at the time of power cable repair, wherein it is determined that proper repair has been achieved when the measured value is 5 times or more of the measured value before repair.

２．電力ケーブル修復のために、該ケーブル内にケーブル導体を介して修復剤を注入後、一定期間経過後に、ケーブルに一定電圧範囲にわたって交流電圧を印加し、そのとき流れる損失電流中の基本波および第３高調波を測定し、当該電圧範囲において、損失電流（Ｉloss）中の基本波（Ｉ1）に対する第３高調波（Ｉ3）の比で示される第３高調波電流発生率（Ｉ3／Ｉ1）の増加率が50％以下の場合に適正な修復が達成されたと判断することを特徴とする電力ケーブル修復時における電力ケーブルの診断方法。 2. For power cable repair, after a repair agent is injected into the cable via a cable conductor, an AC voltage is applied to the cable over a certain voltage range after a certain period of time. The third harmonic current is measured, and the third harmonic current generation rate (I 3 ) indicated by the ratio of the third harmonic (I 3 ) to the fundamental wave (I 1 ) in the loss current (I loss ) is measured in the voltage range. / I 1 ) A method for diagnosing a power cable at the time of power cable repair, wherein it is determined that proper repair has been achieved when the rate of increase is 50% or less.

３．上記１または２において、損失電流中の第３高調波の測定を、ケーブル修復時から少なくとも120日後に行うことを特徴とする電力ケーブル修復時における電力ケーブルの診断方法。 3. 3. The method for diagnosing a power cable at the time of power cable repair, wherein the third harmonic in the loss current is measured at least 120 days after the cable repair.

本発明によれば、電力ケーブルを修復した場合に、その修復が適正になされたか否かを短期間で的確に判定することができる。   According to the present invention, when a power cable is repaired, it can be accurately determined in a short period whether or not the repair is properly performed.

以下、本発明を図面を用いて具体的に説明する。
まず、本発明で修復の適否の判断の際に利用する損失電流中の第３高調波について説明する。
ケーブルの導体とシース間は、架橋ポリエチレンという誘電体を挟んだコンデンサといえる。そのため、ケーブルに電圧を印加した場合、電流がケーブルに充電される。理由的なコンデンサは、充電電流のみで損失電流は発生しない。しかしながら、実際には、コンデンサ中で消費される電流があるので、充電される電流中に損失電流が含まれる。 Hereinafter, the present invention will be specifically described with reference to the drawings.
First, a description will be given of the third harmonic in the loss current that is used when determining whether repair is appropriate or not in the present invention.
It can be said that the capacitor between the cable conductor and the sheath sandwiches a dielectric called cross-linked polyethylene. Therefore, when voltage is applied to the cable, current is charged to the cable. The reason capacitor has only a charging current and no loss current. However, in reality, since there is a current consumed in the capacitor, a loss current is included in the charged current.

ところで、健全なケーブルでは、電圧を印加したとしても、損失電流中に第３高調波電流は生じないが、水トリーが発生したケーブルでは、損失電流中に第３高調波電流が含まれるようになる。   By the way, even if a voltage is applied to a sound cable, the third harmonic current does not occur in the loss current. However, in the cable in which the water tree is generated, the third harmonic current is included in the loss current. Become.

図３に、損失電流（Ｉloss）中に含まれる第３高調波（Ｉ3）を示す。
損失電流中に含まれる第３高調波とは、印加している電流の３倍の周波数の電流であり、その大きさと基本波（Ｉ1）との相違のずれによって、水トリーの数と長さを検出することができる。
すなわち、第３高調波の大きさは、水トリーの数に起因し、水トリーの発生が多ければ多いほど、第３高調波の大きさは大きくなる。また、第３高調波と基本波との相違のずれは、水トリーの長さに起因し、位相が小さければ小さいほど、長い水トリーが発生していることを表す。 FIG. 3 shows the third harmonic (I 3 ) included in the loss current (I loss ).
The third harmonic included in the loss current is a current having a frequency three times that of the applied current, and the number and length of water trees are different depending on the difference between the magnitude and the fundamental wave (I 1 ). Can be detected.
That is, the magnitude of the third harmonic is due to the number of water trees, and the greater the number of water trees generated, the greater the magnitude of the third harmonic. Further, the difference in difference between the third harmonic and the fundamental wave is due to the length of the water tree, and the smaller the phase, the longer the water tree is generated.

上述したとおり、水トリーが発生したケーブルには、損失電流中に第３高調波電流が含まれるようになる。
従って、この損失電流中の第３高調波を利用することによって、電力ケーブルの修復の適否を判断することが可能となる。 As described above, the third harmonic current is included in the loss current in the cable in which the water tree is generated.
Therefore, by using the third harmonic in the loss current, it is possible to determine whether or not the power cable is repaired.

図４に、修復後のケーブルについて、120日目に種々の大きさの交流電圧を印加し、そのとき流れる損失電流中の第３高調波を測定した結果を示す。なお、同図には、修復前における第３高調波の測定結果も併せて示す。
同図に示したとおり、修復後の第３高調波の値は修復前のそれに比較して高い値になっていた。 FIG. 4 shows the results of measuring the third harmonic in the loss current flowing at the 120th day, with the AC voltage of various magnitudes applied to the cable after repair. The figure also shows the measurement result of the third harmonic before the repair.
As shown in the figure, the value of the third harmonic after the repair was higher than that before the repair.

なお、発明者らは、当初、修復が適正になされた場合には、この第３高調波の値はゼロになるものと予想していた。
というのは、水トリーの発生がない健全なケーブルでは、かような第３高調波は現れないからである。
しかしながら、実際には、上述したとおりかなり高い値の第３高調波が観察された。 Note that the inventors initially expected that the value of the third harmonic would be zero when the repair was properly performed.
This is because such a third harmonic does not appear in a healthy cable with no water tree.
However, in practice, a considerably high value of the third harmonic was observed as described above.

この理由についてはまだ明確に解明されたわけではないが、水トリーは架橋ポリエチレン内に第３高調波を発生させる電気回路を構成し、修復剤はその回路の特性を変化させる作用があることが、第３高調波を増加させた原因と考えられる。   The reason for this is not yet clearly understood, but the water tree constitutes an electrical circuit that generates the third harmonic in the cross-linked polyethylene, and the repair agent has the effect of changing the characteristics of the circuit, This is considered to be the cause of increasing the third harmonic.

そこで、次に発明者らは、修復前の第３高調波の値に対して修復後の第３高調波の値がどれくらい以上であれば適正な修復がなされたと判断できるかについて検討を加えた。
すなわち、修復前の第３高調波の値に対する修復後の第３高調波が種々の値をとる種々のケーブルについて、その断面を観察した。
その結果、修復後一定期間経過後の第３高調波の測定値が、修復前の測定値の５倍以上の場合には、架橋ポリエチレン内に修復剤が浸透し、水トリーに修復剤が充填された状態になっており、修復が適正になされたことが確認された。 Therefore, the inventors next examined how much the value of the third harmonic after the restoration was greater than the value of the third harmonic before the restoration could be judged to have been properly repaired. .
That is, the cross sections of various cables in which the third harmonic after the repair takes various values relative to the value of the third harmonic before the repair were observed.
As a result, when the measured value of the third harmonic after a certain period of time after repair is more than 5 times the measured value before repair, the repair agent penetrates into the crosslinked polyethylene and the water tree is filled with the repair agent. It was confirmed that the repair was done properly.

また、修復後、修復の適否を判断する最短期間について検討した結果、少なくとも120日目であれば、適正な判断が下さることが判明した。   In addition, as a result of examining the shortest period for determining whether repair is appropriate after repair, it was found that an appropriate judgment can be made at least on the 120th day.

次に、図５に、損失電流（Ｉloss）中の基本波（Ｉ1）に対する第３高調波（Ｉ3）の比で示される第３高調波電流発生率（Ｉ3／Ｉ1）と付加電圧との関係について調べた結果を示す。
同図から明らかなように、修復前の状態すなわち水トリーが発生していた状態では、第３高調波電流発生率が付加電圧の上昇に伴って急激に増大するのに対し、修復後には、付加電圧を上昇しても第３高調波電流発生率の増大はわずかである。
従って、この第３高調波電流発生率の推移よっても修復の適否を判断することができる。
ちなみに、上記の例で、付加電圧を10.0kVから12.7kVまで上昇させたときの第３高調波電流発生率（Ｉ3／Ｉ1）の増加率が50％以下であれば、適正な修復がなされたと判断することができる。 Next, FIG. 5 shows the third harmonic current generation rate (I 3 / I 1 ) represented by the ratio of the third harmonic (I 3 ) to the fundamental wave (I 1 ) in the loss current (I loss ). The result of having investigated about the relationship with an additional voltage is shown.
As is clear from the figure, in the state before the repair, that is, in the state where the water tree was generated, the third harmonic current generation rate rapidly increases as the additional voltage increases, whereas after the repair, Even if the additional voltage is increased, the third harmonic current generation rate is only slightly increased.
Therefore, it is possible to determine whether or not the repair is appropriate based on the transition of the third harmonic current generation rate.
By the way, in the above example, if the increase rate of the third harmonic current generation rate (I 3 / I 1 ) when the additional voltage is increased from 10.0 kV to 12.7 kV is 50% or less, proper restoration is possible. It can be judged that it was made.

なお、本発明において、ケーブル導体部へケーブルキュアを導入する方法については特に制限はなく、従来公知の方法で行えばよい。
また、ケーブルキュアの種類についても特に制限はなく、従来公知の薬剤いずれもが使用できるが、代表的なものとして次のものが挙げられる。
・ケーブルキュアXL（絶縁体）
メチルフェニルジメトキシシラン（C9H14O2Si）：60％以上
トリメチメトキシシラン（CH3OSi(CH3)3）：５〜10％ In the present invention, the method for introducing the cable cure into the cable conductor is not particularly limited, and may be performed by a conventionally known method.
Moreover, there is no restriction | limiting in particular also about the kind of cable cure, Although all conventionally well-known chemical | medical agents can be used, the following are mentioned as a typical thing.
・ Cable cure XL (insulator)
Methylphenyldimethoxysilane (C 9 H 14 O 2 Si): 60% or more Trimethylmethoxysilane (CH 3 OSi (CH 3 ) 3 ): 5 to 10%

ＣＶケーブルの構造を示した図である。It is the figure which showed the structure of the CV cable. 水トリーの発生場所を示した図である。It is the figure which showed the generation | occurrence | production location of a water tree. 損失電流（Ｉloss）中に含まれる第３高調波電流（Ｉ3）を示した図である。Is a diagram showing the loss current (I loss) third harmonic current contained in (I 3). 修復前後における第３高調波電流を比較して示した図である。It is the figure which compared and showed the 3rd harmonic current before and behind repair. 修復前後における第３高調波電流発生率（Ｉ3／Ｉ1）を比較して示した図である。Repair is a diagram comparatively showing third harmonic current generation rate (I 3 / I 1) before and after.

符号の説明Explanation of symbols

１ 導体
２ 内部半導電層
３ 絶縁体（架橋ポリエチレン）
４ 外部半導電層
５ クッション層
６ アルミ被（遮水層）
７ 防食層
８ 内導トリー
９ 外導トリー
10 ボウタイトリー 1 Conductor 2 Internal semiconductive layer 3 Insulator (crosslinked polyethylene)
4 External semiconductive layer 5 Cushion layer 6 Aluminum cover (water shielding layer)
7 Anticorrosion layer 8 Inner guide tree 9 Outer guide tree
10 Bow Tightly

Claims (3)

電力ケーブル修復のために、該ケーブル内にケーブル導体を介して修復剤を注入後、一定期間経過後に、ケーブルに交流電圧を印加し、そのとき流れる損失電流中の第３高調波を測定し、該測定値が修復前の測定値の５倍以上であった場合に適正な修復が達成されたと判断することを特徴とする電力ケーブル修復時における電力ケーブルの診断方法。   In order to repair the power cable, after injecting the repair agent into the cable through the cable conductor, after a certain period of time, an AC voltage is applied to the cable, and the third harmonic in the loss current flowing at that time is measured. A method of diagnosing a power cable at the time of power cable repair, wherein it is determined that proper repair has been achieved when the measured value is 5 times or more of the measured value before repair. 電力ケーブル修復のために、該ケーブル内にケーブル導体を介して修復剤を注入後、一定期間経過後に、ケーブルに一定電圧範囲にわたって交流電圧を印加し、そのとき流れる損失電流中の基本波および第３高調波を測定し、当該電圧範囲において、損失電流（Ｉloss）中の基本波（Ｉ1）に対する第３高調波（Ｉ3）の比で示される第３高調波電流発生率（Ｉ3／Ｉ1）の増加率が50％以下の場合に適正な修復が達成されたと判断することを特徴とする電力ケーブル修復時における電力ケーブルの診断方法。 For power cable repair, after a repair agent is injected into the cable via a cable conductor, an AC voltage is applied to the cable over a certain voltage range after a certain period of time. The third harmonic current is measured, and the third harmonic current generation rate (I 3 ) indicated by the ratio of the third harmonic (I 3 ) to the fundamental wave (I 1 ) in the loss current (I loss ) is measured in the voltage range. / I 1 ) A method for diagnosing a power cable at the time of power cable repair, wherein it is determined that proper repair has been achieved when the rate of increase is 50% or less. 請求項１または２において、損失電流中の第３高調波の測定を、ケーブル修復時から少なくとも120日後に行うことを特徴とする電力ケーブル修復時における電力ケーブルの診断方法。   3. The method of diagnosing a power cable according to claim 1 or 2, wherein the third harmonic in the loss current is measured at least 120 days after the cable is repaired.
JP2008155501A 2008-06-13 2008-06-13 Diagnostic method of power cable at the time of power cable repair Active JP5120085B2 (en)

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Citations (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2004354093A (en) * 2003-05-27 2004-12-16 Furukawa Electric Co Ltd:The Deterioration diagnostic method of power cable

Patent Citations (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2004354093A (en) * 2003-05-27 2004-12-16 Furukawa Electric Co Ltd:The Deterioration diagnostic method of power cable

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