別紙41 (課程博士(英文))Date of submission (month day, year)

フ,,Z ,20/

Depadment
Naohiro Hozumi

Electrical and Electronic

StudentlD Number

D169202

Yoshinobu Murakami

Information Engineering
Pplicanfs nameSupervisors

Nhet Ra

Abstract (Doctor)
Development ofcondition Monitoring Method for po、ver cables by Means ofpaHial
Title ofThesis

Disch雛ge and 、Nater Tree Detections(部分放電と水トリーの検出による電カケーブルの状態モニタリング手法の開発)'︑ゞ4孝气

Approx.80o words

Electric power transmjssion and distribution depends on a vast and expensive net、vork of high and medium
Voltage cables for power delivery. TO suppod the system operates properly with high reliability; thus, it isnecessary to maintain the power cable in good condition. The reliability and availability of the power cable aredetermined by the conditions of its insulation system. Defects and degradations are kno、¥n to be the major factors
Which give harm to the insulation power cable leading to fai1山'e. Defects are mainly created bef01・e an operationis staded, and lead to the failure ofthe cable line in tl)e early stage. padial discharoe (PD) measurement is useful
to detect such defects and local breakdo、vn, on the other hand, water tree is a degradation mode thattakes place
Severaltens ofyears a介er the operation has been started (aged). The cable may cause electric breakdo、un 、vhen a
Watertree grows and bridges the insulation, as it is significantly conductive. Diagnostic testing by means ofwatertree detection is 0介en employed for preventive measurement
In early stage (aged stage as welD, PD measurement is very useful but conventional methods cannot a仟ord toゞ疹毛气

approach the discharge source a11hough the cable is usua11y as long as several kilometers.1n the 、vear、out stage,
Water tree is the most signiflcant degradation mode. As the cable is practicaⅡy long and water trees are not
Uniformly distributed along the length, the conventional diaσnosing methods cannot aa'ord t0 10cate thedegradation. Based on these backgrounds, t、vo components ofthe studies were performed. First, we pro osed aPD detection method that can eaSⅡy approach the origin ofthe discharge by newly proposed capacitive coupHng
electrode. As the pD takes place spontaneously under commercial voltaoe, this detection is classified as a passivediagnosis. second, water tree detection method that can 】ocate the deoradation by considering chal・ge behavior
accompanied by 、vater tree. As the behaviortakes place by applying several different waveforms ofhigh voltage,
this detection is classified as an active diagnosis、For a newly developed pD detection method, we assumed that the outer shield ls not a perfect conductor, theChange in potential due to the current discharge and shield resistance 、vould be seen as pD signal propagating
through the cable. This wi11 be more significant because of skin e仟ect 、¥hen higher 介equency component istargeted. The change in potential along the shield electrode can be detected by the capacitive coupling bet、veen
the foil electrode and shield electrode itself、 1n order to retain a ground potential, anou)er identical cable 、vas used
as refel'ent cable.1n pl'actice, a pail' of cables in three phases can be taken as the object and reference. Basicexperiment and numerical analysis 、vere carried out by using a coaxial cable (RG58A/U).1n this study, a mimic
PD with the width of20 ns was applied to the ta熔et cable line with 50 Ω C11aracteristic impedance. The intensity
Ofthe caHbration can be modified by adjusting the charging voltage. The change in potential alono the cable line
Was detected using a foil electrode made from aluminum. FI'om the input of the mimic pD pulse sional, thedetection point voltage signal appeared about 250 ns later.

A numerical simulation was performed in the same manner to measurement. As the pulse propagates, adisplacement current is generated in the electric shielding layer, and the distribution of the volta8e source

appears. The detection signalis obtained by integrating the signals given to the output points by the respective
Voltage sources. This equivalent circuit is regarded as a lossless line, and the reaection of the signalis notConsidered. The experimental result and simulation result were compared, and it sho、vs similar characteristics.1n

Order to compare the sensitivity ofthis method, the same experiment 、vas conducted on a mimic insulationjoint.
The detection intensity of the proposed method is almost 30 times lo、ver than the conventional pD detection atthe insulation joint, but the same intensity can be detected if an amplifier is insta11ed during actual measurement
On、site. precise maintenance is needed as the cable gets old with time. Therefore, constant monitoring of pD isnecessary.圦le decided to design and prototype an inexpensive pD monitoring instrumentto monitor pD along theCable lines at aⅡ time, and the performance 、Nas satisfactory.
For the new water tree detection method, several di行'erent 、vaveforms of high voltage were employed. AWater、tree can be located based on the delay time of the residual charge signa1 介om the pulse, and (2) By

Npeatedly applying the pulse and by averagin8 responses, the sN ratio can be impl'oved. we named the method
Which measures the residual char菖e including information of the location by applying the pulse as "charge radar
method" and we discussed the feasibility ofthis method.

In the active diagnosis measurement, a 、vater・tree degraded cable 、和ith 5 m in length 、Nas inserted the400、m、10ng communication coaxial cable in order to imitate a real cable line. changing the location of thedegraded section,、ve measured residual ch釘ge signals using the charge radar. Residual char8e signals areObserved at locations of degraded sections as time delay in every subtracted 、vaveform. By integrating eachresidual charge signal, a fe、¥ percents ofthe amount、和hich is measured by the conventional method is calculated、Based on the proposed method,the degraded 、vater・tree xLPE cable is detected.

The responses to high voltage pulse voltages were investigated in order to clarify the chal'ge behavior with
Which the signal of ne、vly proposed charge radar method is detected. The assessment of chal'ge behavior under
Pulse voltages 、vas studied by usin8 a thin xLPE 行lm sample contained 、vatertree. space charge measureme爪 isPerformed by using pulsed Electroacoustic method (PEA). A numerical simulation 、Nas also perfonηed.
Assuming the trapping and de‑trapping process, the di行'erences in pulse response 、Nere explained depending on
the degree of 、vater tree degradation.1t was suggested that the degradation signalis based on the dia'erence in atransient cunent through the 、vater trees depending on the polarity of the bias pulse voltage applied prior to thePulse voltage.

ゞ',ミーPI・inciple ofthis method is similar to the conventional residual charge method.1n this method, the bias voltage isUsed for charge accumulation and depolarizing pulse voltage is used to release charge instead ofAc voltage.1f a
Pulse voltage is applied instead of the Ac voltage in the residual charge method(conventional method),(1) a