Diagnostics and Testing of HV Cables

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Diagnostics and testing of high voltage cablesystems. Suurjännitekaapelien eristystendiagnostiikat ja testaus (KaDiat)

Petri Hyvönen, Bolarin Oyegoke, Martti Aro Report TKK-SJT-63

Final Report

ISSN 1237-895XISBN 951-22-6550-8

Helsinki University of TechnologyHigh Voltage Institute

Espoo, Finland 2003



2 Diagnostics and testing of high voltage cable systems

Preface

Research of diagnostics of high-voltage power apparatus insulation systems was started at

Helsinki University of Technology High Voltage Institute some years ago with studies on

application of Transfer Function (TF) as diagnostic tools for power transformers. The studieswere performed in close co-operation and funding of Finnish electric energy industry. First

diagnostic project KaVika on cable systems was carried out during years 2000 – 2001. Project

was a preliminary study to cover wide variety of techniques used in cable diagnostics. Project

based mainly on literature studies. Some experimental studies on laboratory was done.

International research on diagnostic techniques of electrical insulation systems has been

increasing for decades. Electric power plants as well as transmission and distribution grids

include large number of important and valuable components of different age, with low or no

knowledge of there actual condition. On the other hand, condition based maintenance strategy is

becoming more and more common with power utilities and companies. With this view, theFinnish electric energy industry donated in 1999 to HUT a five-year senior researchership for

strengthening the research and teaching of diagnostics of electrical insulation systems.

The present study is experimental part of research dealing with diagnostics of power cable

systems with on-site measurements of partial discharges and dielectric response. Based on results

from previous cable project partial discharge and dielectric response measurements were

choosen. Extensive on-site measurement program was carried out during the project. Additional

verification test on cables were made in laboratory. This report summarises literature and

conference publications during the project.

In addition to the University, this study was funded by the National Technology Agency(TEKES), Foundation for development of electric power engineering, Pirelli Cables and Systems

Oy, Fortum companies and City Electrical Company group EK-12. Lauri Nyyssönen (Pirelli

Cables and Systems Oy) acted as chairman and Markku Hyvärinen (Helsinki Energy) acted as

vice-chairman of the project board The other members were Jukka Leskelä (Finergy ry.), Kirsi

Nousiainen (TUT), Martti Torikka (Pirelli Cables and Systems Oy), Aimo Kukkonen (Fortum

Sähk önsiirto Oy), until 30.12.2002 Kari Heinonen (Fortum Service Oy), from 1.1.2003 Matti

Kuussaari (Fortum Service Oy). Martti Aro is responsible for the project. Petri Hyvönen acted as

secretary of the project board. Petri Hyvönen and Bolarin Oyegoke acted as senior researchers in

the project.The next step will be further development of cable diagnostic systems and procedures. Similar

research activities was found very common world wide, and good international contacts and co-

operation especially within CIGRE WG 33.03 and with universities in Nordic Countries and

Middle Europe will support the research effectively.

Espoo Finland, June 2, 2003 Martti Aro Petri Hyvönen



Diagnostics and testing of high voltage cable systems 3

Table of Contents

Preface ............................................................................................................................................ 2

Table of Contents............................................................................................................................ 3Tiivistelmä ...................................................................................................................................... 4

Summary......................................................................................................................................... 5

1 Introduction.............................................................................................................................. 6

1.1 Background....................................................................................................................... 6

1.2 Aim of the work................................................................................................................ 6

2 Summary of Publications......................................................................................................... 6

2.1 Dielectric Response Measurement as Diagnostic Tool for Power Cable Systems........... 6

2.2 Application of dielectric response measurement on power cable systems. Literaturereview............................................................................................................................. 8

2.3 Diagnostics of MV XLPE power cable systems using polarisation and depolarisation

current measurement method ......................................................................................... 9

2.4 Detecting degree of water treeing in XLPE power cable systems using polarisation and

depolarisation current method...................................................................................... 10

2.5 Experience with the application of time domain dielectric response method in condition

assessment of distribution oil-paper cables.................................................................. 11

2.6 Dielectric response as diagnostic tool for power cable system Laboratory and On-site

measurement................................................................................................................. 122.7 Selectivity of DAC and VLF Voltages in After Laying Tests of Extruded MV Cable

Systems. Literature review........................................................................................... 13

2.8 On-site partial discharge measurements on medium voltage cable systems. ................. 15

2.9 Condition assessment of MV power cables based on practical measurements.............. 16

2.10Diagnosing the Condition of Medium Voltage Covered Counductors .......................... 17

3 Acknowledgements................................................................................................................ 17

4 List of symbols and abbreviations ......................................................................................... 17

5 Definition of basic quantities................................................................................................. 18

6 List of KaDiat-project publications ....................................................................................... 18

Address of the authors:

Petri Hyvönen Email: [email protected]

Phone: +358 9 451 5874

Fax: +358 9 451 2395

Helsinki University of Technology, High Voltage Institute

Postal Address: P.O. Box 3000, FIN-02015 HUT, Finland

Street Address: Otakaari 5 L, Espoo Otaniemi

National Metrology Institute, High Voltage Measurements

http://www.hut.fi/Units/HVI




TiivistelmäSähk ön tuotanto-, siirto- ja jakelujärjestelmille asetettavat luotettavuusvaatimukset ovat

lisääntyneet huomattavasti viime aikoina. Energiamarkkinoiden vapautuminen sek ä erilaiset

viranomaisvaatimukset ja kiristyvät korvausvaatimukset ovat asettaneet energiayhtiöille uusiahaasteita. Sähk önjakelun luotettavuus on nousemassa entistä tärkempää rooliin. Toisaalta

varsinaiseen huoltotoimintaan k äytettävät resurssit ovat pienentyneet. Nykyisin on entistä

tärkeämpää tehdä oikeita asioita, oikeassa paikassa, oikeaan aikaan. Korjaavasta ja ennakoivasta

kunnossapidosta ollaan siirtymässä entistä enemmän kuntoon perustuvaan kunnossapitoon.

Kuntoon perustuvan kunnossapidon onnistumisen edellytyksenä ovat hyvät ja luotettavat

työkalut laitteiden nykykunnon määrittämiseen.

Keskijännitejakeluverkoissa on k äytetty kaapeleita 1900-luvun alkupuolelta lähtien.

Vanhimmat nykyisin Suomessa k äytössä olevat kaapelit ovat 1940 – 50 lukujen taitteesta..

Käytössä on laaja kirjo erityyppisiä kaapeleita, joiden kunnosta tai vikahistoriasta ei ole tarkkaatietoa. Nykyisin uusien kaapelijärjestelmien k äyttöönottotestaus on vähäistä. Toisaalta

ulkoistamisten seurauksena on olemassa perusteltu huoli kaapeleiden asennustekniikkaan

liittyvän ammattitaidon häviämisestä.

Teknillisen korkeakoulun suurjännitetekniikan laboratoriossa tehdyn projektin puitteissa

toteutettiin laaja keskijännitekaapelijärjestelmien kenttätestausohjelma. Esiselvitysten perusteella

osittaispurkausten mittaus ja dielektrisen vasteen mittaus ovat varteenotettavia menetelmiä

kaapelijärjestelmien nykykunnon määrittämiseen. Osittaispurkausmittauksilla on mahdollista

havaita, tunnistaa ja paikantaa kaapelieristyksille haitalliset paikallisesti esiintyvät pienet

sähk öiset purkaukset. Osittaispurkaukset aiheuttavat kaapelieristyksessä eroosiota, joka voivat johtaa yllättävään vikatilanteeseen. Dielektirisen vasteen mittauksen avulla voidaan havaita

öljypaperieristyksen kostuminen sek ä polymeerieristyksissä mahdollisesti olevat vesipuut.

Kenttämittausten sek ä tulosten jatkok äsittelyn avulla on pyritty luomaan alustavat raja-arvot

kaapeleiden kunnon luokittelulle. Saatuja tuloksia voidaan hyödyntää myöhemmin tehtävissä

kenttämittauksissa sek ä erilaisissa jatkotutkimusprojekteissa. Tuloksia on kuitenkin syytä

tarkastella uudestaan ja tarkentaa, kun mittausten lukumäärä kasvaa.

Kenttämittausten tulosten perusteella on oletettavaa, että mitatut polymeerikaapelit ovat

hyvässä kunnossa. Öljypaperieristeisten kaapeleiden osalta voidaan havaita selvästi heikentyneitä

kaapeleita. Tulosten perusteella on selvää, että kattava kuntoarvio ei voi perustu vain yhdellämittausmenetelmällä saatuihin tuloksiin. Yhdistämällä osittaispurkausmittauksen ja dielektrisen

vasteen mittaustulokset voidaan kaapelin nykykunnosta antaa huomattavasti kattavampi arvio.

Jatkotutkimusta tarvitaan useilla eri osa-alueilla. Tulosten merkitys kaapelin odotettavissa

olevan eliniän kannalta on epäselvää. Tämän seikan selvittäminen vaatii runsaasti lisätutkimusta

sek ä laajoja testaussarjoja.




Summary

Reliability demands on electricity generation and distribution are increasing. Power utilities have

faced new challenges due to liberalisation of electricity markets, increasing authors regulation

and new claims for compensation of unexpected power shut downs. As a result power utilities

have to concentrate more on asset management to reduce costs, to postpone investments, to

optimise technical management keeping at the same time reliability and power quality at high

level. Nowadays it is important to do right things at right place at right time. Maintenance

strategy is changing from predictive or repairing maintenance to the condition based maintenance

(CBM). Success of CBM depends partly on how good and reliable tools are available to

determine condition of electrical apparatus.

Cables have been used in electricity distribution since beginning of 20th

century. Oldest still in

use cables in Finland has been installed late 1940’s. Nowadays almost all new distribution

systems in urban areas are done using cables. Wide variety of cables are in use. Knowledge of

condition and history of cables is not so well documented. Professional skill to install cable

systems is disappearing due to outsourcing of maintenance groups. There is a worry about

increasing amount of after installation faults due to loosing of professional skills to install cable

systems. Wide variety of cable insulation systems and large variety in ages of cables still in use

will place hard challenges for cable condition assessment.

Earlier studies showed that measurement of insulation resistance is almost only diagnostic

measurement performed to cable systems. Knowledge and experience of new diagnostic method

is not so widely distributed. Based on previous project, dielectric response (DR) measurement

and partial discharge (PD) measurement seemed to be most promising tools for the condition

assessment of cable systems. Increased moisture content in oil-paper insulated cable or water

trees in polymer insulation can be detected with DR measurement. Harmful localised electrical

discharges can be observed, recognised and located with PD measurement.

As a result of previous findings a new on-site measurement project was started. Aim of the

project was to arrange extensive on-site measuring program, analysing the measurement results

and generate a basis on knowledge rules to assessment of cable condition.

On-site measurement results showed that all measured polymer insulated cables seemed to be

in good condition. No clear evidence of degradation could be detected on XLPE-cables. Few

clearly degraded oil-paper insulated MV-cable could be found during the measurements.

Results showed clearly that reliability of only one diagnostic method for instance dielectric

response is not sufficient to assess condition of cable insulation. It is therefore recommended that

final decision of the cable insulation condition should be based on combined result of the

dielectric response and partial discharge measurement.

Future research is needed in several fields of condition monitoring of cable insulation.

Significance of measurement results to the remaining life expectation of cable insulation is not

clear. Clarification of this aspect requires lot of future research and implementation of large test

series.




1 Introduction

This report deals with diagnostics of insulation of medium and high voltage power cable

systems, including paper-oil and extruded cables as well as their joints and terminations. Studies

on diagnostic possibilities on covered conductor overhead line (CC-line) are also covered.Diagnostic measurements in connection of both after laying commissioning tests and periodic

diagnostic tests on cable systems are discussed.

1.1 Background

Electric power systems include a large number of expensive and important power cable systems

of different age manufactured and mounted during decades. For reliability reasons, preventive

measures need to be taken on considered components. Quality requirements are increasing and

outages of electric power distribution are expensive. Condition based maintenance is becoming

more and more common for economic reasons. Repair and replacement of important cablesystems are expensive and correct timing in these would give large savings of costs.

Insulation systems of high-voltage power cables and their accessories are subject to different

kinds of stresses during their service life and thus, to degradation and deterioration. These can

lead to a reduction of life and so to a lower reliability of electrical power systems. Therefore, a

lot of research efforts and activities and many publications are directed towards a better

understanding of degradation phenomena and the finding of tools for insulation diagnosis and

remaining life estimation techniques. In order to check the quality and steadiness of a cable

system, it is important to perform diagnostic tests on this before setting into operation and after a

defined period of operation. On-site insulation diagnosis to determine the degradation state of high voltage equipment is of great interest within the power and grid companies and utilities.

1.2 Aim of the work

The aim of this experimental project was carry out large on-site test program. Aim of the on-site

test program was to collect large amount of cable condition information from Finnish cable

networks. Partial discharge and dielectric response measurements were used as tools to collect

cable condition information. Additional supporting tests in laboratory were done. Literature

studies were also continued. One of the aims was also to realise on-site cable diagnostics service

for future needs of cable owners.

2 Summary of Publications

Short summaries of findings in individual publication written during the project is presented in

the following.

2.1 Dielectric Response Measurement as Diagnostic Tool for Power Cable

Systems

Oyegoke, Hyvönen, Aro. “Dielectric Response Measurement as Diagnostic Tool for Power CableSystems”. Wire & Cable technical symposium, Chicago, USA, June 2002.




Oyegoke, Hyvönen, Aro. “Dielectric Response Measurement as Diagnostic Tool for Power Cable

Systems”. Wire Journal International. April 2003 p 106 – 110. 2003.

Increase in moisture content is one of the most common causes for deterioration of insulation

properties and premature ageing of oil-paper insulated system. Preliminary results on dielectric

response measurements as diagnostic tool for power cable systems are presented. Among several

other non-destructive dielectric response methods, loss factor or the tangent delta measurement

over certain frequency range, also known as frequency domain dielectric spectroscopy is

investigated and analysed.

Measurement of dielectric response (tanδ) in frequency domain has been done on short cable

samples embedded in a plastic tube filled with water. The cable sample under investigation was

prepared with a small drill through the metallic sheath to allow water penetration as shown in

Figure 1.

60 100 70 79040

Conductor

Paper insulation Al-sheathPE-jacket

Plastic tube

Water hole

Heat-shrinkable tube

Figure 1 Schematic view of the tested cable.

Measurement of the dielectric response as a function of frequency was done by using

Programma®

IDA 200 insulation diagnostic system. Measurement example of tanδ as a function

of frequency at six different time instance is shown in figure 2.

0,001

0,01

0,1

1

0,001 0,01 0,1 1 10 100 1000

Freq. Hz

t a n

d e l t a

dry

77 hrs

177 hrs

182 hrs

198 hrs

207 hrs

Figure 2 Measured tanδ as a function of frequency at six different time instance

Information about moisture content contained within oil-paper insulated cable can be found at

high frequencies where the minimum of the response occurs. Increase in moisture content not




only affects the value of tanδmin but also the frequency at which this minimum value occur. Apart

from moisture, temperature is another factor that affects the frequency at which tanδmin value

occurs. Interestingly, magnitude of tanδmin remains stable with temperature. Using some part of

previously work an expression that relate moisture content and time is derived. Further work is

required on the practical applicability of this formula. Only seriously affected by moisture cable

can be detected by measuring tanδ at low frequencies.

2.2 Application of dielectric response measurement on power cable systems.

Literature review

Bolarin Oyegoke, Petri Hyvönen, Martti Aro, Ning Gao 2003 “Application of dielectric response

measurement on power cable systems. Literature review”. Paper accepted for publication in IEEE

Transactions on Dielectric and Electrical Insulation.

Thermal and chemical deterioration of XLPE cable insulation does not pose a great problemand electrical deterioration can be prevented through a controlled manufacturing. However, one

of the major problems associated with the medium voltage XLPE cables is deterioration by water

trees, and it sometimes is the main reason for insulation failures in XLPE cables in long service.

Increased moisture content will be harmful to the oil-paper insulated cables.

In general, water trees (vented or bow-tie trees) in XLPE and increased moisture content

inside oil-paper cable cause a reduction of 50 Hz, 0.1 Hz, DC or impulse withstand capability.

When a given electric field is applied over a dielectric material, different mechanisms of

polarisation and conduction are activated. The presence of water trees in XLPE cable or moisture

in oil-paper cable generally lead to increase of the intensities of polarisation and the intensities of conduction. A change in insulation structure will generally lead to an increase in the dielectric

losses. In order to investigate a change in structure, techniques based on dielectric response DR

are used. The DR is the memory effect in a dielectric material and can be measured in different

ways.

Dielectric response is an advanced tool for insulation diagnosis. Insulation deterioration and

degradation change the DR. Measurement of DR at different frequencies or, in time domain with

different time parameters, give some picture of insulation condition

Return voltage and polarisation spectrum are specially developed methods to characterise the

moisture content in transformer insulation. Their application to diagnosis of polymer insulatedcable shows that measurement of a single polarisation spectrum is not sufficient to detect bad

cable with water trees. Consequently, division spectrum technique is introduced. A lot of time

might be needed to construct two required spectrums. A more time saving technique is the use of

Quotient of initial slopes of two tests at different voltages. This Quotient of initial slopes is a

temperature independent parameter. However, since the quotient is calculated from the return

voltage measurement, the resistance in parallel with the cable can affect the value of this quotient

and the D factor. Maximum of the return voltage depends on the cable length.

Measurement of slopes of two voltage curves (return voltage and decay voltage after charging)

gives valuable information about thermal and moistening process inside the cable insulation. Themethod is less sensitive to environmental electrical noise because it measures voltage of the order

of magnitude of volts. However, because the method involves measurement of return voltage all




weaknesses attributed to this measurement are eventually applied to measurement of slopes of

voltage curve.

IRC is a non-destructive method that uses depolarisation current measurement data. A method

for evaluating the ageing status of insulation based on A-factor may be insufficient for diagnosis

purpose. Presently, residual dielectric strength of insulating material evaluated by soft computing

(Fuzzy-method) is proposed. This has improved the diagnosis by this method. However,

calculated value of residual strength depends on the reference voltage values. Determination of

the degree of non-linearity as additional diagnostic criterion is not possible with this method.

Environmental electrical noise may affect the depolarisation current measurement.

Measurement of both polarisation and depolarisation currents is not much discussed in

literature. However, measurement of both currents can improve diagnosis of cable system.

Measurement at various voltage levels offers possibility and a particularly easy way of testing for

non-linearity. However, environmental electrical noise may affect the current measurement.

Discharge method combines both the slope of decay voltage and depolarisation or discharge

current value. It is a promising method that requires further investigation. As there are not many

literatures on this method it is difficult to discuss its full advantages and weaknesses.

Measurement by this method will require long time as by polarisation spectrum method.

Measurement of tanδ (complex permittivity or complex capacitance) as a function of

frequency and its combination with non-linearity effect gives possibility to distinguish between

the response due to water tree and that of accessories. However, factor such as temperature can

influence the measurement result.

Measurement of tanδ at power frequency and its combination with DC leakage current or DCcomponent in AC charging current is another combination that is quite promising. Temperature

remains an interesting factor that might affect measurement result also in this method.

Combination of tanδ at power frequency and total harmonic distortion in the loss current is

another promising combined technique that requires additional efforts, especially to define the

significance of relative values of dielectric losses and the THD’s. Also here influence of

temperature on the measured values will need clarification.

Although cable systems undergo some overall deterioration but service failures occur at the

site of discrete defects. Combination of DR and PD measurements can improve diagnostic result.

However, it is doubtful whether PD test can identify the presence of water tree inside the cable ina non-destructive manner.

2.3 Diagnostics of MV XLPE power cable systems using polarisation and

depolarisation current measurement method

B. S. Oyegoke, P. O. Hyvönen, and M. M Aro 2003 “Diagnostics of MV XLPE power cable

systems using polarisation and depolarisation current measurement method”, Accepted to be

presented in ISH 2003, The Netherlands.

In time domain the dielectric response measurement requires an application of a dc voltage to

the cable for certain time often called charging period. The duration of this charging period is not

constant but varies depending on the researcher.




The paper presents the findings on the effect of various factors such as measurement set-up,

terminations and joint(s), temperature, and charging time that might lead to wrong diagnostic

conclusions.

Measurement on new XLPE cable without termination can not be made. Presence of heat

shrinkable resistive type of termination does not affect the measured currents on new XLPE

cable.

Depending on what measurement set-up is used there is likely-hood to obtained difference in

current values. Grounded test object arrangement gives slightly higher currents than the

ungrounded test object case.

Using the grounded test object arrangement the effect of temperature is investigated.

Temperature affects the polarisation and depolarisation currents values. This effect is clearly seen

within the first 100 seconds. It seems that temperature affects depolarisation current more than it

affects polarisation current. Henceforth, evaluation of current result within the first 100 secondsshould consider the temperature effect. Both currents show that the higher the temperature the

lower is the absolute value of the response.

Longer charging time produce higher currents (polarisation and depolarisation). Insufficient

charging time can lead to a wrong estimation of dielectric response function and conductivity of

the cable under investigation. Estimated conductivity at the same charging voltage level of 5 kV

is 1,3 10-17

S/m and 2,6 10-18

S/m respectively for 5 minutes and 10 minutes charging times.

Absolute value of response is more sensitive to temperature and changes in the cable circuit

for instance presence of resistive joint. The higher the temperature the lower is the slope of the

currents. Similarly, absolute value of the response is sensitive to the measurement arrangement(grounded or ungrounded test object).

Charging time affects both the polarisation and depolarisation currents, usually the longer the

charging period the higher the measured currents. In addition the longer the charging period the

lower is the slope of the dielectric response.

2.4 Detecting degree of water treeing in XLPE power cable systems using

polarisation and depolarisation current method

B. S. Oyegoke, P. O. Hyvönen, M. M. Aro 2003 “Detecting degree of water treeing in XLPE

power cable systems using polarisation and depolarisation current method”, Accepted to be

presented in ISH 2003, The Netherlands.

In time domain, the depolarisation current i(t) following DC charging at a particular voltage

can be measured. The quantity of absorption charge Q divided by the static capacitive charge of

the insulation produce a degradation index DI.

In more recent works, dielectric response in time and frequency domains indicated that

measurement of non-linearity in the dielectric response could become the basis for diagnosis of

water tree degraded cable. As such both the magnitude and the voltage dependence of the

response or degree of non-linearity are used as evaluation criteria for the prediction of the

insulation condition. Based on the same theory of dielectric response it is apparent that different




methods such as degradation index, aging factor and polarisation index are derived quantities

used to interpret the measured polarisation and depolarisation current.

The present paper describes another method that can be used to detect the degree of water

treeing in polymer insulated cable. The method is based on the depolarisation current

measurement.

The measurements were performed using AVO Megger S1-5010 (insulation resistance meter)

with a maximum output voltage of 5 kV and a current detection limit of 0.1 nA. Test voltage

levels were 1 kV, 2.5 kV and 5 kV. The instrument measured polarisation current during a pre-

set time of 10 minutes. Depolarisation current was measured for 1 minute irrespective of

charging time.

Condition assessment of cable insulation using degradation index DI necessitates computation

of the absorption charge Q. The DI value depends on temperature. Therefore, another method

called discharge rate (DCR) is used.In order to verify the accuracy of the DCR measurement was carried out on a new XLPE cable

with a resistive joint. Length of the cable is 154 m. The DI and DCR value obtained are

presented in Table 1. The measurement has been carried out in the laboratory at a constant

voltage of 2.5 kV but different temperature.

Table 1 Effect of temperature on DI and DCR.

TempoC DI ppm DCR

26 429 2,397

33 345 2,331From Table 1 it can be seen that effect of temperature on DCR is insignificant compared to

DI. Measurement on the same cable before introduction of a resistive joint gives DCR value of

3,401. Comparing this value with those in Table 1 it can be seen that presence of joint within the

cable system has caused a decrease in DCR value to 2/3.

A temperature independent method of detecting water tree presence in XLPE cable insulation

is presented. The method is based on the discharge characteristics of the cable. The presented

results show that the method is a promising one, however, to fully convert the DCR value into

figure of merit additional research work will be necessary.

2.5 Experience with the application of time domain dielectric response method

in condition assessment of distribution oil-paper cables

Bolarin Oyegoke, Petri Hyvönen, Martti Aro. Experience with the application of time domain

dielectric response method in condition assessment of distribution oil-paper cables. Accepted to

be presented in NORD-IS-2003, 11-13-6-2003, Tampere.

The non-destructive electrical test based on the polarisation and depolarisation current

measurement is performed on samples from oil-paper cable. A new method called conductivity

factor is proposed for data evaluation in time domain. The proposed method in time domain is

compared with the measurement result in frequency domain. The results presented here are

preliminary one that required more experimental work on more cables, however it appears that




the condition of oil-paper cable could be assessed with respect to moisture content if the

proposed method is used.

Avo Megger S1-5010 insulation resistance meter was used during the measurements. DD test

mode of the Megger was used. Sampling rate during the polarisation current measurement was

one point per every three seconds. Test voltage level was 2.5 kV. Polarisation current

measurement time was 10 minutes and depolarisation current measurement time was 1 minute.

Analysis of DR measured data has been based on the conductivity σ (S/m) and polarisation

index (PI ) values. Measurement of the conductivity at long time may reflect the DC conductivity

and is thus most affected by localised defects in the insulation. However, computation of

conductivity necessitates knowledge of the geometry capacitance or dielectric permittivity of the

test object. In addition, conductivity is a temperature dependent parameter.

PI value is temperature independent parameter. Cable with PI greater or equal to 4 is rated as

very good cable and PI less than unity is regarded as bad cable that requires immediate action.For PI between 4 and 1 the cable can be regarded as good or poor.

600p

60p

60

600

I

I

R

RPI ==

where R60, R600 and I p60, I p600 are resistance and currents at 60 and 600 seconds.

Another method that combines both polarisation and depolarisation current data is

conductivity factor CF. Conductivity factor (CF) indicates the change of insulation conductivity.

)60()60(

)30()30(

dpp

dpp

60s

30s

sisi

sisiCF

−

−==

σ

σ

Conductivity factor is calculated from the ratio of conductivity evaluated from the polarisation

and depolarisation currents at 30 and 60 seconds. With this method of analysis it is possible to

avoid the use of parameter that is temperature and geometry dependent. In addition CF is

independent of the charging voltage. Another way of interpreting CF is to consider the

conductivity at 30 s as the conductivity that is related to the condition of the oil in insulation.

A new method of interpreting the dielectric response measured data in time domain ispresented. The method employees the slope of conductivity. It is a non-temperature dependent

approach in a similar way as polarisation index method. In addition it avoids the use of geometry

capacitance and allows measurement at any charging voltage. The result obtained look promising

as it shows good trend in a similar way as the tanδ minimum. However, more investigation is

required in order to define the range of CF for “Very good”, “Good”, “Poor”, and “Bad” cable.

2.6 Dielectric response as diagnostic tool for power cable system Laboratory

and On-site measurement

Bolarin Oyegoke. “Dielectric response as diagnostic tool for power cable system- On-sitemeasurement results and data analysis” TKK-SJT-62 Report. 2003.




Dielectric response (DR) is an advanced tool for insulation diagnosis. Insulation deterioration

and degradation change the DR. Measurement of DR at different frequencies or, in time domain

with different time parameters, give some picture of insulation condition. The major problem

associated with medium voltage XLPE cables is deterioration by water trees, and it sometimes is

the main reason for insulation failures in XLPE cables in long service. Increased moisture

content will be harmful to the oil-paper insulated cables. The polarisation index and the

conductivity factor are two diagnostic parameters used in the present studies to evaluate the

condition of oil-paper cables with respect to moisture content. For detecting water tree

deterioration in polymer insulated cables diagnostic criteria based on the current magnitude,

current trend, DC conductivity level and non-linearity of the DR with respect to the charging

voltage have been used. In addition a new method discharge rate (DCR) that is based on

discharge phenomenon is proposed for diagnosis of polymer cable with respect to water tree

contain in its insulation. Measurement of one parameter may not be sufficient to reveal the status

of the cable insulation. Therefore combination of several methods may be necessary toadequately and accurately diagnosed oil-paper, XLPE and mixed cables. Further research is

needed for more detailed conclusions regarding the capability and limitation of PI and other

criteria used in the present work to diagnose the cable insulation.

Cable types HPLKVJ and PYLKVJ are oldest of all oil-paper cables still in service at 20 kV

network. They are found with very high PI value. It is doubtful if the PI limit defined from

experimental work on APYAKMM can be applied to these cable types. However, these cable

appear to be good condition since none has PI less than 1.

Furthermore, it is interesting to see that cable types PLKVJ are also the oldest types of oil-

paper cable still in operation at 10 kV network. Most of them has PI value higher than 4.Therefore it is doubtful here as well if PI technique define for APYAKMM cables is an

appropriate way to diagnose this type of cables.

Most of the tested oil-paper cables are in good or excellent condition meaning that PI is

greater than 2. However, cables with PI between 1 and 2 need further investigation in a near

future. Diagnosis of cable insulation condition using dielectric response method could be

improved if several methods are combined.

On-site measured data on polymer insulated (XLPE) cables and its analysis shows no clear

evidence of degradation due to water tree. Possibility of other degradation phenomenon e.g.

thermal degradation cannot be ruled out. However, the currently studied method can not detect

such degradation phenomenon

Test set-up and other factors such as temperature, joint and charging time has been studied at

laboratory level. Measurement results indicate that effect these factor plays a crucial role in data

interpretation.

2.7 Selectivity of DAC and VLF Voltages in After Laying Tests of Extruded

MV Cable Systems. Literature review

Bolarin Oyegoke, Petri Hyvönen, Martti Aro, Ning Gao and Michael Danikas 2003 “Selectivityof DAC and VLF Voltages in After Laying Tests of Extruded MV Cable Systems. Literature






water, conductive contaminant, insufficient compression of interface falls within 0.9 and 2 times

the breakdown voltage at 50 Hz.

At VFL 0.1 Hz tests a higher voltage may be required for detection of mechanical defects in

the insulating material depending on the shape and size of defect.

Tests with oscillating and VLF voltages have different sensitivity in detecting different kinds

of defects. For the purpose of obtaining good test results consideration should be given to the

combination of DAC and VLF. Their combinations with PD measurements will improve the

defect detecting capability and an improved test result will be obtained.

Results of investigation indicate that the behaviour of PD and breakdown phenomena are not

the same and depend on the characteristics and size of the defect, the state of the interface where

defect exists, the presence of moisture in cavities and applied voltage.

2.8 On-site partial discharge measurements on medium voltage cable systems.Petri Hyvönen. Keskijännitteisten maakaapelijärjestelmien osittaispurkausmittaukset k äyttö-

paikalla. On-site partial discharge measurements on medium voltage cable systems. Licentiate

thesis, Teknillinen korkeakoulu, 2003. Written in finnish TKK-SJT-60 Report.

Small local electrical discharges, which maybe harmful to the cable insulation can be

observed with partial, discharge measurement. Partial discharges degrade the insulation and can

cause total loss of electrical insulation properties of insulation material. On-site cable testing

with 50 Hz transformer is not economical. On-site tests can be performed using alternative

voltage stresses. Questions related to the selectivity of testing and partial discharge

measurements are complex problems. Findings reported in literature are partly contradictory. Allalternative voltage stresses except DC-voltage can be used while partial discharge measurements

are done. Results will be depend on used voltage stress. Straight forward comparison between

measurement result with different voltage stresses should not be done.

On-site tests were made using damped AC-voltage. The procedure of measurement and data

analysis presented in this work can be used as a basis for future assessment of cable insulation

condition. All measured polymer insulated cables seemed to be in good condition. No evidence

of degradation could be detected on XLPE-cables. Analysis of measurement result showed that

partial discharges observed on polymer cables are caused some external reasons. Main reason

seems to be SF6-switchgear which was connected to the measured cables during measurement.

10 kV oil-paper insulated APAKM and PLKVJ cables seem to be more degraded than 10 kV

APYAKMM cables. Insulation structure of these cables is different. Results of measurements on

20 kV oil-paper insulated cable does not indicate as clear weakening trend as 10 kV oil-paper

cables showed. In several cables partial discharges were located near measurement end

terminations. Lowered oil level in terminations could lead to this kind of behaviour. Checking of

termination condition should be done more frequently in future to avoid unexpected termination

failures.

Tests on artificially damaged termination clearly indicated that test with nominal voltage is

not enough to detect harmful installation faults. Only a few faults could be detected with nominal

voltage stress. Tests with twice-nominal voltage stress including partial discharge measurement




could find almost all installation faults studied in this work. Acoustical partial discharge

detection system can be used to point out accurate location of fault in cable accessories.

Sensitivity of the acoustical system was 50 pC. Sensitivity depend on type of termination or joint.

2.9 Condition assessment of MV power cables based on practical

measurements

Petri Hyvönen, Bolarin Oyegoke , Martti Aro. Condition assessment of MV power cables based

on practical measurements. Accepted to be presented in NORD-IS-2003, 11-13-6-2003,

Tampere.

Knowledge of the condition of cable networks is very important to the cable network owner.

However condition assessment of cable networks is extremely hard without any specially

designed diagnostic measurements. Many times condition assessment of MV power cable system

is based on only results from one diagnostics measurement. The most commonly used and wellknown method is measurement of insulation resistance. This paper presents measurement results

from on-site measurement program on oil-paper cables. It shows clearly that reliability of only

one diagnostic method for instance dielectric response is not sufficient. This is because dielectric

response and partial discharge measurement results give different condition class. It is therefore

recommended in this paper that final decision of the cable insulation condition should be based

on combined result of the dielectric response and partial discharge measurement.

Cable 1 is oil-paper insulated 10 kV (U 0=5.8 kV) core type cable. Length of the cable is

1866 m. Cross section is 240 mm2. Cable system contains five joints. Capacitances of phases L1,

L2 and L3 are 1171, 1160 and 1162 nF. Partial discharge measurement, dielectric response

measurement and combined results are shown in table 1 below.

Table 1 PD, DR and combined results of Cable 1

Phase PD-result DR-result PD+DR result Action

L1 Good Good Good No action


L3 Good Poor Poor Attention

Cable 2 is oil-paper insulated 10 kV (U 0=5.8 kV) belt type cable. Length of the cable is

200 m. Cross section is 50 mm2. Capacitances of phases L1, L2 and L3 are 52 nF. Partial

discharge measurement, dielectric response measurement and combined results are shown intable 2.

Table 2 PD, DR and combined results of Cable 2.

Phase PD-result DR-result PD+DR result Action

L1 Bad Good Bad Immediately action needed

L2 Bad Good Bad Immediately action needed


PD measurement is powerful diagnostic tool that can be used to observe, recognise and locate

harmful local defects along cable system. However, PD measurement cannot detect local defect

in form of moisture within the cable insulation. DR measurement can detect increase moisture

content in oil-paper cable system. A good oil-paper cable free of moisture from dielectric

response measurement point of view may indicate considerable PD level. Also a PD free cable




may as well show spectacular increase in moisture content. Consequently, as practical examples

show both methods, DR and PD measurements are needed to perform comprehensive

classification of cable status.

2.10 Diagnosing the Condition of Medium Voltage Covered Counductors

Janne Vehanen, Petri Hyvönen. “Päällystetyn keskijänniteavojohdon kunnon diagnosointi”.

Diagnosing the Condition of Medium Voltage Covered Counductors. Written in finnish. TKK-

SJT-61 Report.

This report discusses the problems concerned medium voltage covered conductor lines, along

with their extent and severeness. Also, methods in diagnosing their condition are shown.

In the type of line, an aluminium alloy conductor is covered with polymeric insulation. The

purpose of the system is to reduce the spacing between conductors and, on the other hand, to

decrease failure density and improve safety compared to conventional bare conductor lines.

The basis of this work are damages in covered conductors found by some grid companies and

their concernes in the durabitily of its insulation. New problems have arised, including punctures

in insulation and inexplicable breakings of conductors.

In the study, the construction of the covered conductor lines with their benefits and problems

are introduced. In addition CC-lines are compared with different types of lines. Also the aging of

insulators and some unexpected problems occurred in use is discussed. Subsequently, methods in

condition diagnosing are presented.

The results of the tests and measurements performed on samples of CC-lines show that the

problem is extensive. Punctures are common and they grow easily. Partial discharge tests, using

damped AC-voltage, proved to be an effective measurement method for evaluating the condition

of the CC-lines insulation.

The measurements in this report were carried out in the high voltage institute of Helsinki

University of Technology in spring 2003.

3 Acknowledgements

National Technology Agency (TEKES), Foundation for development of electrical power

engineering (SVK-pooli), Pirelli Cables and Systems, Fortum companies and City ElectricalCompany Group EK-12 are thanked for funding of the project. The project board and especially

the chairman Mr Lauri Nyyssönen are thanked for very constructive guidance and support for

fluent progress of the project.

4 List of symbols and abbreviations

AC alternative current

APYAKMM H-insulated 10 or 20 kV oil-paper cable

CC covered conductor

CDA complex discharge analysis

CF conductivity factor




DAC damped alternative voltage

DC direct current

DCR discharge rate

DD dielectric discharge

DI degradation index

DR dielectric response

EP ethylenepropylene

HPLKVJ H-insulated 20 kV oil-paper cable

HUT helsinki university of technology

IRC isothermal relaxation current

MV medium voltage

OLI oscillating lightning impulse

OSI oscillating switching impulse

PD partial dischargePE polyethylene

PI polarisation index

PLKVJ belt insulated 10 kV oil-paper cable

ppm parts per million

PYLKVJ H-insulated 20 kV oil-paper cable

Q absorption charge

SF6 sulphurhexafluoride

tanδ loss factor

TF transfer functionTHD total harmonic distortion

TUT tampere university of technology

WG work group

VLF very low frequency

XLPE cross linked polyethylene

5 Definition of basic quantities

i(t ), idp(t ) depolarisation current

σ conductivity

R resistance

I current

ip(t ) polarisation current

U 0 nominal phase to ground voltage

6 List of KaDiat-project publications

[1] Oyegoke, Hyvönen, Aro. “Dielectric Response Measurement as Diagnostic Tool for

Power Cable Systems”. Wire & Cable technical symposium. Chicago, USA, June 2002.5p. 2002.



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Diagnostics and Testing of HV Cables

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