Dielectric Response Power Cable System

Dielectric Response Measurement as Diagnostic Tool for Power Cable Systems

Bolarin Oyegoke, Petri Hyvönen, Martti Aro

Literature review

ISSN 1237-895X ISBN 951-22-5396-8

Helsinki University of Technology High Voltage Institute Espoo, Finland 2001

Report TKK-SJT-47

2

Dielectric respose as diagnostic tool for power cable systems

Preface This report summarises a particular part of literature review of research on diagnostic testing and measurements of power cable systems on-site. The two basic topics partial discharge measurements and dielectric response measurements are reviewed in separate reports. The survey on dielectric response theory in Finnish language is based mainly on course given by prof Roland Eriksson and his colleagues at KTH Sweden in August 2000. The space charge measurement methods are reviewed as a possible diagnostic tool for high-voltage DC cable systems in future. An experimental part with tests and measurements on medium voltage cables on-site is planned to follow still in 2001. In addition to the University, this study was funded by the National Technology Agency (TEKES) and Foundation for development of electric power engineering. Risto Harjanne (Helsinki Energy) acted as chairman of the project board. The other members were Jarmo Elovaara (Fingrid), Jari Eklund (TEKES), Kari J Heinonen (Fortum Service), Olli Lindgren (Fortum Technology Centre), Erkki Kemppainen (ABB Transmit), Jukka Leskelä (Finergy), Kirsi Nousiainen (TUT), Lauri Nyyssönen (Pirelli Cables and Systems) and Antti Vähämurto (Empower). Summarising Report TKK-SJT-49: Advanced diagnostic test and measurement methods for power cable systems on-site Partial Reports TKK-SJT-45: Partial discharge measurements as diagnostic tool for power cable systems TKK-SJT-46: Basic theory for dielectric response measurements (in Finnish). Dielektrisen vasteen mittausmenetelmien teoreettinen perusta. TKK-SJT-47: Dielectric response measurements as diagnostic tool for power cable systems TKK-SJT-48: Space charge measurement as possible diagnostic tool for high-voltage DC cable systems in future Address of the authors: Bolarin Oyegoke Email: [email protected] Phone: +358 9 451 5875 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

Dielctric response as diagnostic tool for power cable systems

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Table of Contents Preface...........................................................................................................................................................2 Table of Contents..........................................................................................................................................3 Summary .......................................................................................................................................................4 1

Introduction.............................................................................................................................................4 1.1 Background....................................................................................................................................4 1.2 Frequency and time domain methods ............................................................................................5 1.3 Comparing time and frequency domain DR measurements ..........................................................5

2

Off-line methods .....................................................................................................................................6 2.1 Measurement of tanδ and total harmonic distortion in the loss current at power frequency .........6 2.2 Measuring dissipation factor tanδ..................................................................................................6 2.3 Measuring DC leakage current ......................................................................................................7 2.4 Measuring the polarisation current and DC transient response current.........................................8 2.5 Measuring depolarisation current ..................................................................................................8 2.6 Measuring return voltage...............................................................................................................8 2.7 Measuring potential decay after charging......................................................................................9

3

On-line methods....................................................................................................................................10 3.1 Measuring DC component in AC charging current .....................................................................10 3.2 Measuring DC superposition current...........................................................................................11 3.3 Measuring the insulation resistance.............................................................................................11 3.4 Measurement of dielectric dissipation factor and DC component...............................................12 3.5 Method of locating water treeing deterioration in XLPE cable insulation on-site ......................12

4

Commercial dielectric response measuring systems.............................................................................13 4.1 Insulation diagnostic system IDA 200.........................................................................................13 4.2 Cable diagnostic system KDA 1..................................................................................................13 4.3 Cable diagnostic system CD30/31...............................................................................................14 4.4 Cable testing and diagnostic system PHG TD.............................................................................15

5

Examples of measurement of dissipation factor in function of frequency..........................................15

6

Discussion.............................................................................................................................................19

References...................................................................................................................................................21 Annex 1. Measurement of Dielectric Response ..........................................................................................23

4

Dielectric respose as diagnostic tool for power cable systems

Summary 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. For high voltage XLPE cables the major problem is electrical trees. Increased moisture content will be harmful to the oil-paper insulated cables. Existing diagnostic methods for detecting water tree deterioration and for evaluation of moisture content are reviewed. Diagnostic criteria are based on the non-linearity of the DR with respect to the charging voltage. Measurement of one parameter e.g. tanδ alone, even in function of frequency, may not be sufficient to reveal the status of the cable insulation. Therefore, its measurement is often combined with measurement of another parameter e.g. DC leakage current. Measurement of return voltage alone may not, either, reveal the status of the cable insulation sufficiently enough. In this respect its combination with some other diagnostic parameters such as self decay voltage and/or polarisation and deparisation current are proposed and used. Dielectric response gives an overview of average condition of the insulation system under study, but no localisation of the possible degraded areas. Further research is needed for more detailed conclusions regarding the status of a particular insulation. Predicting the remaining life of the insulation system requires still further research.

1 Introduction This report deals with dielectric response measurements on insulation of medium voltage power cable systems.

1.1 Background 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. Diagnostic tests aim at detecting any reduction of the electrical strength due to degradation process. Thermal and mechanical degradation are two major deterioration mechanisms affecting oil-paper insulation. Physical, chemical and electrical degradation are the major deterioration mechanisms affecting polymeric insulation. These degradation mechanisms cause changes of structure, increase generally the intensities of polarisation and the intensities of conduction of the cable insulation. A change in structure increases generally the dielectric losses. For investigation of the changes in structure many tests are used. Based on electrical diagnostic parameters measured or derived from the measured data these tests give information on stages of destruction as well as causes of stressing or degradation of the cable insulation.

Dielctric response as diagnostic tool for power cable systems

5

'LHOHFWULFUHVSRQVH'5LQIUHTXHQF\GRPDLQLVWDQ/ II ,QWLPHGRPDLQWKH'5DSSHDUVDV polarisation current behaviour in application of DC voltage on cable system, and as return voltage and depolarisation current behaviour after disconnecting and short-circuiting the cable for a certain time periods. Measurement of DR is an advanced non-destructive tool for diagnostic testing of different insulation systems, such as paper-oil and polymeric insulation. DR gives an indication of insulation condition e.g. of high-voltage cable systems. Changes in insulation such as water trees and electrical trees or other deterioration change the DR.

1.2 Frequency and time domain methods Dielectric response can be measured in different ways. Relevant parameters of DR shall be known when considering and comparing the DR’s of different insulations or DR’s of the same insulation after certain periods in service. Preferably, certain parameters should be kept constant. In time domain the DR appears as depolarisation current [5, 9, 10], return voltage (also called residual, recovery and build-up voltage) [2, 3, 9, 10], polarisation current, discharge voltage [11, 12] and isothermal relaxation current [3, 4]. In frequency domain the DR appears as dissipation IDFWRU WDQ/ DW FHUWDLQ IUHTXHQFLHV RU IUHTXHQF\ UDQJH >    @ DQG DV WRWDO KDUPRQLF distortion in the loss current at power frequency [13, 16, 17]. Diagnostic criteria are based on the non-linearity of the dielectric response in the time and frequency domain with respect to the charging voltage. In frequency domain non-linearity is characterised by a voltage dependent dissipation factor, whereas in the time domain an over proportional increase of the response with higher charging voltage occurs. Non-linearity in the dielectric response has been subject of study in many doctoral theses [9, 18, 19, 20]. For on-site application very low frequency (0.1 Hz) voltage tests in combination with tanδ measurement have proved as a good diagnostic tool for service aged XLPE cables [1, 21]. Measurement of tanδ at 50 Hz was without information about the condition of polymer-insulated cables under investigation [2]. Furthermore, tanδ measurement at 50 Hz will involve large capacitive currents compared to VLF (0.1 Hz). Major problem with tanδ measurement at 0.1 Hz was the sensitivity of the measuring device. However, improved 0.1 Hz tanδ measurement system are commercially available [1].

1.3 Comparing time and frequency domain DR measurements Time domain measurement of DR detects the final conductivity of insulation faster than the frequency domain measurement. However, the time domain measurement has the weakness of being unable to detect fast polarisation processes. In addition, interpretation of measurement result in time domain is more difficult. DR measurement as a function of frequency has the advantage of being able to detect fast polarisation processes and, interpretation of result in frequency domain is relatively easy. The equipment for DR measurement in frequency domain are more expensive.

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Dielectric respose as diagnostic tool for power cable systems

2 Off-line methods Dielectric response measurements as diagnostic method to detect deterioration of insulation can be performed off-line or on-line. Some methods may be used both off-line and on-line.

2.1 Measurement of tanδ and total harmonic distortion in the loss current at power frequency The method deals with the 50/60 Hz insulation loss current measurements in high voltage cable insulation containing water trees. In order to isolate the small insulation loss current from the significantly larger, quadrature (capacitive) current, a current-comparator-based (CCB) high voltage capacitance bridge is needed (Fig. 1). The loss current waveform is measured and the harmonic distortion of the loss current is correlated with the length of water trees. This method is still at the laboratory research level.

L ine 60 H z

P hase-L ock Lo op (P .L.L.)

D igital W aveform G enerato r

20 48 x 60 H z

H igh V oltage A m p lifier V = 0 -2 0 kV f= 0 -15 kH z Cs

D etecto r/ R ecorder

Filter

Cx H igh V oltage C C B C ap acitance B rid ge

D igital Scop e

Fig. 1. Block diagram of the measurement set-up.

2.2 Measuring dissipation factor tanδ Dielectric dissipation factor measurement is an effective method in detecting insulation deterioration. However, it has the drawback that the increase in tanδ due to local deterioration does not show up in the measurements. That is to say, the measured value for the whole cable length is an averaged value and smaller than the value of tanδ for the locally deteriorated section. Contrary to the DC leakage current, tanδ can in principle also be measured on-line. However, the apparatus for that needs redesign for greater ease of use and more compactness. The apparatus for measuring dielectric dissipation factor comprises three sections, the tanδ measurement section, the voltage divider and the current transformer [7] (Fig. 2).

Dielctric response as diagnostic tool for power cable systems

Voltage Divider

tanδ Measurement section

Test Cable i

7

is Current Voltage Converter

Current Transformer

o

90 Phaseshift Circuit

Automatic Balanced Circuit

tanδ % % 0.25 C nF 38.5

Current Voltage Converter

Fig. 2. Schematic representation of the principle of dielectric dissipation factor measurement.

2.3 Measuring DC leakage current DC test voltage is applied between the conductor and insulation shield of a cable (Fig. 3). Magnitude of the DC leakage current is used to judge the situation of the insulation. The undeteriorated section of cable does not affect the value of DC leakage current, but local deterioration can be known as an absolute quantity[7]. This implies that since sound part of the cable insulation do not contribute much to the measured value of DC leakage current, then the degree of cable insulation deterioration can be estimated by measuring the DC leakage current. he local deterioration causes a current significantly larger than the sound insulation. This method involves application of high DC voltage, and it is applicable for on-site off-line measurement. The combination of the methods of DC leakage current and dielectric dissipation factor provides an effective means for diagnosing insulation deterioration of cable off-line. Current limiting resistor

High DC Voltage Generator

Test Cable Guard

A

Leakage Current Detector

Fig. 3. Circuit for DC leakage current measurement.

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Dielectric respose as diagnostic tool for power cable systems

2.4 Measuring the polarisation current and DC transient response current Current that is flowing through the cable during charging is measured in time domain. This current is called as polarisation current. During the charging period the cable is charged with direct voltage. Different mechanisms of polarisation and conduction that are of great importance are activated during this period. Polarisation current is not measured during transient period after applying charging voltage. Variable parameters during the measurements include the charging voltage and period and the period for polarisation current measurement. Values of the polarisation current during the measurement and voltage nonlinearity of polarisation current carries information about the insulation condition. The peak of the current that is flowing through the cable immediately after applying charging voltage is called as DC transient response current. Its value carries information about the insulation condition as well. Measurement of polarisation current Ip and DC transient response current is performed during the charging period (Fig. 4).

2.5 Measuring depolarisation current The procedure of measurement of depolarisation or discharge current can be divided into two parts (Fig. 4). During the charging period the cable is charged with direct voltage. Different mechanisms of polarisation and conduction of great importance are activated during this period. After the charging period the cable is disconnected from the direct voltage source and shortcircuited with current measuring system. Different mechanisms of depolarisation are activated. During this period the depolarisation (discharge) current Idp is measured and integrated to obtain the absorption charge Q. Ratio of Q to capacitance C of the cable is used as index of the deterioration. Variable parameters include the charging voltage and period, and the period for discharge current measurement. Usually, the large transient discharge current immediately after the short-circuit is not measured, although it also may include some information on insulation condition.

2.6 Measuring return voltage Four different terms are used in literature on the same quantity, i.e. return, residual, recovery and build-up voltage. The procedure can be divided into three parts (Fig. 4). During the charging period the cable is charged with direct voltage. After that, the cable is disconnected from the direct voltage source and short-circuited for a definite period. After that, the return voltage is measured under open circuit conditions. The source of the return voltage is the relaxation processes in the dielectrics, given rise to an induced charge on the electrode of the test object. In this method variable parameters include the charging voltage and the charging period, short circuit period and the return voltage period. The maximum of return voltage is the diagnostic parameter to be evaluated. The three methods mentioned above can be combined as a one measurement. In this method variable parameters include the charging voltage and the charging period, short circuit period and the return voltage period. The characteristic parameters are the maximum return voltage value, the polarisation current and the depolarisation current.

Dielctric response as diagnostic tool for power cable systems

9

urrent (A)

oltage (V)

ch

Return voltage Short circuit p

Time (s) Charging period

Return voltage period

dp

Fig. 4. Polarisation, depolarisation and return voltage method.

2.7 Measuring potential decay after charging The voltage discharge rate, i.e. the initial steepness of the self-discharge voltage is used as the parameter for the diagnosis. In this method only the decay voltage Ud is measured (Fig 5). In the combined method (voltage response method), also the return voltage is measured (Fig. 5). By measuring the initial steepnesses of the two voltage curves the two dielectric processes, conduction and polarisation, can be investigated separately. Combined measurement of the potential decay after charging (Fig. 5) together with the depolarization (discharge) current (Fig. 4) is also used for diagnosis. Voltage (V)

U ch

Ud Sd

Return voltage Sr

Time (s) Charging period Inner discharge period

Return voltage period Short circuit period

Fig. 5. Measurement of decay Ud and return Ur voltages.

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Dielectric respose as diagnostic tool for power cable systems

3 On-line methods Water trees has long time been recognised as the most hazardous factor in life of XLPE distribution cables and the major cause of insulation failure. The existing methods for cable diagnosis such at the measurement of the DC leakage current and or tanδ require an interruption in electrical service and needs extensive installation work. For these reasons, in Japan some hotline diagnostic methods are developed and used to detect water tree deterioration. These methods include the DC current in AC charging current method, the DC superposition method, a method to measure insulation resistance, and a method of detecting electrical tree deterioration in XLPE cable insulation on-site. Accuracy of the DC component current method and the DC superposition method is compared in [8].

3.1 Measuring DC component in AC charging current When high AC voltage is applied to cable insulation, a DC component may be detected in the AC charging current within a short time. It is known that the magnitude and polarity of the DC component are closely related to deterioration of the cable insulation. Accordingly, the degree of cable insulation deterioration can be estimated by measuring the DC component [6, 7]. A switch is connected between the other end of the metal shield for the purpose of disconnecting it from the ground during the measurement [6]. For the DC component measurement the switch is opened (Fig. 6). A closed circuit is formed by connecting the grounding potential transformer, distribution line, cable under measurement, measuring device and ground in series.

Source High voltage (6.6 kV) distribution line

To load

GPT

SW

XLPE cable

M.D.

Fig. 6. Measuring circuit of DC component in the field. SW switch. GPT grounding potential transformer. M.D. measuring device of DC component.

Dielctric response as diagnostic tool for power cable systems

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3.2 Measuring DC superposition current In this method a DC voltage is superimposed over the AC operating voltage (Fig. 7). The superimposed voltage is applied between nodes 1 and 2. The DC superposition current Ids is obtained by calculating the difference between two current values measured with a superimposed voltage of different polarity applied between nodes 1 and 2 (Ids = Ids+ - Ids-). The DC current component Idc is also measured without DC superimposed voltage [8]. Transformer

Cable

AC Supply

M easuring Device

1

DC Supply

Capacitor 50 µ F

2

Fig. 7. Set-up for the DC current component method and DC superimposed method.

3.3 Measuring the insulation resistance The method is applicable to a distribution system with a grounding potential transformer GPT (Fig. 8). A DC source of about –50 V is connected between the neutral point and the ground through a blocking coil and switches for applying the negative DC voltage to an AC cable without turning off the AC. The measuring system is mainly composed of a measuring circuit for the resistance and a device for discriminating the stray ground current. The resistance value measured with this method has close correlation with the insulation resistance obtained by measuring the DC leakage current as a conventional method [6]. Source High voltage distribution line

I1

L

GPT

Cable

G1 E

Fig. 8.

C

G2 I2

C2

Measuring circuit of insulation resistance in the field.

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Dielectric respose as diagnostic tool for power cable systems

3.4 Measurement of dielectric dissipation factor and DC component Combined measurements of DC leakage current and dielectric dissipation factor give an accurate diagnosis of insulation deterioration. The diagnostic system was designed to consist of three separate units, measurement section, charging current detection section, and circuit breaker section (Fig. 9). [7] Transformer High-voltage lead Measurement section Control and computing section DC component measurement section Tanδ measurement section

Termination 6kV XLPE Cable Charging current Detecting box DC component detector

Shield

Grounding lead

Current transformer

Voltage divider High-voltage interrupting circuit Circuit breakers

Fig. 9. Schematic circuit of dielectric dissipation factor and dc component measurement.

3.5 Method of locating water treeing deterioration in XLPE cable insulation on-site Presently, there is no tool for locating the place of water tree in the cable. However, the philosophy of detecting the water tree position may be to study the link between water tree development and partial discharge ignition at the tip of water tree. The philosophy will work on the condition that PD actually occurs at the tip of water tree after a sufficiently high voltage (few times the nominal voltage). Such attempt has been advocated by the Instrument manufacturing company under the directorship of Professor Matthew S. Mashikian. There was a strong criticism on this method. Some researchers have tried to use it in the laboratory and found it destructive rather than nondestructive. In addition, it is doubtful if all water trees actually lead to PD.

Dielctric response as diagnostic tool for power cable systems

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4 Commercial dielectric response measuring systems 4.1 Insulation diagnostic system IDA 200 Insulation Diagnostic System IDA 200 is a system measures the complex impedance of a cable at a variable voltage and frequency (capacitance and tanδ at 0.0001-1000 Hz). A digital signal processing unit (DSP) generates a test signal with the desired frequency (Fig. 10). Sinewave generator

Hi sint

Asint A

Test object

cost Real(Ch 0)

Imag(Ch 0)

Real(Ch 1)

Imag(Ch 1)

∫

X Ch0

∫ ∫ ∫

X X

V

Lo A

Ch1 X

Principle of the sine correlation technique.

Guard Ground Schematic block diagram of the IDA 200-system.

Fig. 10. Schematic block diagram of the IDA 200-system and the principle of the sine correlation technique. The signal is amplified with an internal amplifier and then applied to the cable. The voltage over and the current through the specimen are measured with high accuracy using a voltage divider and an electrometer. For the measuring input, IDA 200 uses a DSP unit that multiplies the input (measurement) signal with a reference sine voltage, and then integrates the results over a number of cycles. With this method, noise and interference is rejected-allowing IDA 200 to work with voltage levels up to 200 V and still achieve high accuracy and detail of analysis. (Programma).

4.2 Cable diagnostic system KDA 1 The Cable diagnostic instrument KDA 1 (Seba-dynatronic) is based on the measurement of the depolarisation current. The cable under test is charged at 1kV DC for 30 minutes. Then the cable is short-circuited for 5 seconds, and the depolarisation currents are measured for the fooling 30 minutes (Fig. 11). The measured data are saved and processed with Isothermal Relaxation Current (IRC) analysis. The depolarisation current measured is described as the sum of three experimental functions given by 3

i (t ) = I 0 + ∑ a j e −t / τ i i =1

where parameters aj, τi are strongly correlated with the material properties. The time constant τ3 is related to water tree degradation of the cable insulation.

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Dielectric respose as diagnostic tool for power cable systems inner/outer semiconductor Conductor 1 2

Cable jacket 3

RC

Rd

Sheath

RM

A

UF

i(t)

Computer

A D

2: Discharging

1: DC Charging

3: Measurement

Fig. 11. Basic measurement circuit for the IRC-Analysis. An empirical ageing factor (A-factor) is calculated to classify the ageing condition of the cable. This factor is calculated from depolarisation current ID at time constants τ3 and τ2 as A=

I D (τ 3 ).τ 3 I D (τ 2 ).τ 2

4.3 Cable diagnostic system CD30/31 The Cable Diagnostic System CD30 is for evaluation of the ageing degree and the damage condition of 1 kV to 30 kV PE and XLPE cables. The model CD31 is for oil-paper cables. The devices base upon measurement of return voltages at different charging voltages (Fig. 12). The tested cable is charged with DC voltages (0.5, 1, 1.5, 2U0) for 5 minutes (switch S1). Then, the high voltage source is turned off and the switch S2 closed for two seconds to discharge the cable capacitance over a resistor RD. (Hagenuk) S1 Test Cable S2

A

HV Rd

U

Fig. 12. Block diagram of the return voltage method.

D

PC

Dielctric response as diagnostic tool for power cable systems

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After this time the return voltage is measured for 10 to 40 minutes, depending on the cable length. For that, the cable is connected to the high input impedance measurement receiver U (switch S1). The measured value of return voltage is digitised and forwarded to the PC. The maximum values of the return voltages are plotted as a function of the charging voltage. This relationship can be linear or non-linear. The linearity factor is calculated as the ratio between the maximum values of the return voltage at 2U0 and U0 and used as an indicator of the ageing condition. The factor grater than 2 is considered as a non-linear response and signifies ageing of the cable and the factor 3 indicates a strongly aged cable.

4.4 Cable testing and diagnostic system PHG TD The instrument PHG TD measures tanδ at different sine voltage levels maintained at 0.1 Hz. The tanδ at 2U0 and the difference between 2U0 and U0 values are used as diagnostic criteria. A tanδ value larger than 1.2 × 10 −3 at 2U0 or the difference of tanδ at 2U0 and U0 larger than 6 × 10 −4 signifies water tree deterioration. If the cable is very long, it is possible to reduce the measuring frequency to 0.01 Hz in order to reduce the capacitive current generated by the high voltage source. However, as a consequence the measuring time will increase. (Baur)

5 Examples of measurement of dissipation factor in function of frequency Three oil impregnated paper insulated cables were measured in laboratory. Measurement of dissipation factor as a function of frequency was performed with IDA-200 measuring system. T a n D e lta C a b le 1 1

0,1 T a n D e lta

P has e-A P has e-B P has e-C 0,01

0,001 0,01

0,1

1

10

100

F re q u e n cy / Hz

Fig 13. Dissipation factor as a function of frequency of cable 1.

1000

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Dielectric respose as diagnostic tool for power cable systems

Cable 1 is a 20 kV single-phase cable with aluminium conductor and sheath. Phase A contained a joint. Phases B and C were without joints. Samples of cable were removed from service due to external mechanical failure. During the measurement of the dielectric response of the cable, its metallic sheath was connected to ground. Voltage supply and measurement was connected to the phase conductor. Current measurement was connected to the ground conductor. Guard connector of the IDA-200 termination box was left open. (Fig 13). All three phases of cable 1 show different responses. Comparing the responses of phase A to the phases B and C one will notice the influence of the joint that is present in phase A. The main difference between response of phases B and C is that the response of phase B is shifted slightly towards higher frequencies. The minimum value of the response is believed to carry information about the moisture content in the cable. In this regard for phase B and C the tanδ minimum can be seen to occur at 50 Hz and 10 Hz, respectively. For phase B the magnitude of tanδ is slightly lower than that for phase C. Based on this finding one may conclude that phases B and C are practically under the same condition in terms of moisture content. On the other hand, the minimum of tanδ measured in phase A is not clearly indicated. The presence of a joint in this phase is the most likely factor that is affecting the response measured on this phase. It would be interesting to see the contribution of the joint on the measured result before taken decision on the condition of the cable especially on phase A.

T a n D e lta C a b le 2 1

0,1 T a n D e lta

P has e-A P has e-B P has e-C 0,01

0,001 0,01

0,1

1

10

100

F re q u e n cy / Hz

Fig 14. Dissipation factor as a function of frequency of cable 2.

1000

Dielctric response as diagnostic tool for power cable systems

17

Cable 2 is a 20 kV three phase cable with aluminium conductors. All phases have own aluminium sheaths and outer jackets. Phases are combined inside one outer jacket. The cable was taken to measurements from store-house. During the measurements, metallic sheaths were connected to ground. Voltage supply and measurement was connected to phase conductor under the test. Guard connector of the IDA-200 termination box was connected to the other phases. Current measurement was connected to the ground conductor. The main difference between response of phases of cable 2 is that the response of phases B and C is shifted slightly towards higher frequencies (Fig.14). The minimum of the response on phase A show up at 25 Hz. For phases B and C the minimum values show up practically at the same frequency 35 Hz, and their magnitudes are also practically equal. A slightly higher tanδ value minimum can be seen on phase A. The phases B and C of cable 2 are in the same condition with respect to the moisture content. However, phase A may have a slightly higher moisture content. Cable 3 is a 20 kV three phase cable with aluminium conductors. All phases are inside of one lead sheath. The phases have no separate metallic sheaths. The cable does not contain outer jacket. During the measurement, all phases were connected together. The lead sheath was connected to the ground. Voltage supply and measurement were connected to the phases. Current measurement was connected to the ground conductor. Guard connector of the IDA-200 termination box was left open. The minimum of the response on cable 3 occurs at about 4 Hz (Fig. 15). The magnitude of this minimum tanδ is almost equal to that measured on phase B and C of cable 2. TanDelta Cable3 1

0,1 Ta nD elt a

Phase A+B+C

0,01

0,001 0,01

0,1

1

10

100

Frequency / Hz

Fig 15. Dissipation factor as a function of frequency of cable 3.

1000

18

Dielectric respose as diagnostic tool for power cable systems

Fig 16 presents a combined measurement result on three different cables. In view of the preliminary result of tanδ measurements performed in the laboratory the following remarks can be made. Generally, the minimum values of tanδ in cable 2 and cable 3 are lower than that of cable 1. This can be interpreted in term of moisture contents. The moisture contents in cable 2 and cable 3 appear to be lower that of cable 1. It was observed in this preliminary investigation that the minimum values of cable response do not generally occur at 50 Hz. In the cases studied the minimum value at 50 Hz was observed only in one phase of one cable. In all the other cases this minimum values occur at different frequencies below 50 Hz.

T an D e lta 1

C 1, P A

0,1 T an D elta

C 1, P B C 1, P C C 2, P A C 2, P B C 2, P C 0,01

C 3, P AB C

0,001 0,01

0,1

1

10

100

1000

F req u en cy / H z

Fig 16. Dissipation factor as a function of frequency of cables 1, 2 and 3. C1, PA: Cable 1, phase A; C1, PB: Cable 1, phase B; C1, PC- Cable 1 phase C. C2, PA: Cable 2, phase A; C2, PB: Cable 2, phase B; C2, P: Cable 2 phase C. C3, PABC: Cable 3 phases ABC.

Dielctric response as diagnostic tool for power cable systems

19

6 Discussion A degraded insulation system shows increase of losses and decrease of dielectric strength. Dielectric response in its all appearance is a tool which can indicate the degradation and hence condition of electrical insulation of any kind. Water trees initiate and grow under electric field after water has penetrated into polymeric insulation. Water trees have long time been recognised as the most hazardous factor in life of XLPE distribution cables and the major cause of insulation failure. Water trees increase the tanδ and capacitance and decrease the electric strength of polymerinsulated cable. In addition, water and water trees modify leakage currents, DC absorption current, polarisation and depolarisation current as well as discharge voltage decay and return voltage. Field measurements of some of these parameters have proven to be a suitable means to detect degradation and presence of water trees. However, many measurement techniques have disadvantages, which have prevented their widespread application. For instance, tanδ measurement gives overall condition of the cable system and not that of the deteriorated part of the cable. Also leakage current in joint and termination appear in the leakage current of the cable system. [24]. The existing methods for cable diagnosis such at the measurement of the DC leakage current and or tanδ require an interruption in electrical service and needs extensive installation work. For these reasons, in Japan some on-site on-line diagnostic methods such as the DC component current method and the DC superposition method, are used to detect water tree deterioration. Accuracy of the DC component current method and the DC superposition method is compared. As a conclusion the on-line diagnostic methods are considered as efficient as the DC leakage current method. However, the method based on the DC superposition may not be applicable to all cables on-site. This is because with a low voltage (< 100 V), water tree can be detected in some cable, while in others superimposed voltage of 10 kV or more is necessary. At these relatively high DC voltages one must expect breakdown. Combination of the measurement of tanδ and the total harmonic distortion in the loss current, is a new method for diagnosis of power cable systems. However, this method is still on the laboratory level. Moreover, the significance of the relative values of tanδ and the total harmonic distortion current in the insulation is not yet understood. Results of accelerated ageing studies show that tanδ and water trees of polymeric cable increase with acceleration time and voltage, which both are important. However, as an example, acceleration at 16 kV for 2000 h increased tanδ more than acceleration at 20 kV for 1000 h. Even with 2000 h acceleration at 12 kV, the water treeing is more pronounced than with 1000 h at 20 kV [14]. The tanδ and capacitance of water-treed cable (e.g. at 70 oC), measured at power frequency (50 Hz) but variable voltage seems to decrease with increasing voltage. This is mainly due to heating of water in the trees due to long lasting measuring voltage (hand balanced Schering bridge). Reason for this is that relative permittivity of water decreases with temperature (εr = 80 at 20 oC and e.g. 60 at 60 oC), and long lasting measuring voltage application heats the water. Thus, this effect is not real but result of measuring conditions, and it is reversible. Also the water tree canal diameters decrease due to heating thus dicreasing the capacitance and tanδ. Independent of conditions, tanδ and capacitance have very good correlation. [14, 22, 23].

20

Dielectric respose as diagnostic tool for power cable systems

Many researh groups have carried out measurement of dielectric response of oil-paper insulation systems either in time domain or frequency domain. The dielectric response in both domains provides novel diagnostic methods for quality control of medium and high voltage cables. However, the information obtained in frequency and time domain is equivalent only if the insulation system is linear. In addition, dielectric response measurements in both domains indicated that measurement of non-linearity in the dielectric response could become the basis for diagnosis of water tree degradation in cable. Non-linearity in the dielectric response has been subject of study in many doctoral theses [9, 18-20]. Measurement of loss angle of oil-paper cables as a function of frequency is normally performed using a low voltage power supply. Higher moisture content of insulation will increase loss angle. Anyhow, this behaviour is not so clearly seen through whole frequency range. Loss angle curves representing different moisture contents can cross each other. The loss angle has a minimum value which tends to increase with higher moisture content. This means that the assessment of insulation condition for different mass impregnated cables regarding its moisture content can be based on the minimum of loss angle. Polarisation (charging) and depolarisation (discharging) currents of oil-paper insulation will increase with moisture content. In addition to dielectric response function, the time domain measurement of polarisation and depolarisation currents allow for estimation of the conductivity of the test object. Increase in moisture content will increase conductivity. It is important to observe that the conductivity of oil paper system is strongly dependent upon the temperature. Without knowledge of temperature no simple criterion based upon the conductivity can be used to estimate the moisture content. Dielectric response gives an overview of average condition of the insulation system under study, but no localisation of the possible deteriorated areas. Predicting the remaining life of the insulation system based on DR and/or other measurements requires still further research work.

Dielctric response as diagnostic tool for power cable systems

21

References [1]

M. Kuschel et al. 1995 “Dissipation Factor Measurement at 1 Hz as a Diagnostic Tool for Service aged XLPE- Insulated Medium Voltage cables.” 9th ISH Graz Austria, paper 5616.

[2]

M Sturm and R Porzel 1995. “Progresses By the Computing Dielectrical Diagnostic of High Voltage Insulation.” 9th ISH Graz Austria paper 5624.

[3]

G. Hoff and H. G. Kranz 1999. “Correlation between Return Voltage and Relaxation Current Measurement on XLPE Medium Voltage cables.” High Voltage Engineering Symposium, IEE Conference Publication No. 467 paper 5.102.514

[4]

M. Beigert et al. 1993. “ Computer-Aided Destruction free Ageing Diagnosis for Medium Voltage Cables.” 8th ISH Yokohama, Japan paper 67.11.

[5]

M. Kuschel et al. 1997. “Dielectric response-a Diagnostic Tool for High Voltage Apparatus.” 10th ISH Montreal, Quebec Canada paper 393-396.

[6]

K. Soma et al. 1986 “Diagnostic Method for Power Cable Insulation.” IEEE Transactions on Electrical Insulation Vol. EI-21, No. 6, pp. 1027-1032.

[7]

S. Yamaguchi et al. 1989 “Development of A New Type Insulation Diagnostic Method for Hot-Line XLPE Cables.” IEEE Transactions on Power Delivery, Vol. 4, No. 3, pp. 1513-1520.

[8]

M. Hotta et al. 1995. “A Consideration of the Efficiency of Hot-Line Diagnostic Methods for XLPE Power Cables.” 9th ISH Graz 1995 paper 5635.

[9]

S. Hvidsten 1999. “Nonlinear dielectric Response of Water Treed XLPE cable Insulation.” Dr Ing thesis, NTNU, Trondheim, Norway, ISBN 82-471-0433-4.

[10] S. Hvidsten et al. 2000. “Condition Assessment of water treed service Aged XLPE Cables by Dielectric Response Measurements.” Cigre 2000 Paris, paper 21-201. [11] M. Muhr et al. 1997. “Investigations of 30 kV Polyethylene-Cables with the Discharge Current Method.” 10th ISH Montreal, Quebec, Canada, pp. 409-412. [12] E. Nemeth 1999. “Measuring Voltage Response: A non-destructive Diagnostic Test Method of HV Insulation.” IEE Proc.-Sci. Meas. Technol., Vol 146, No. 5, pp. 249-252. [13] A. T. Bulinski et al. 2000. “Measurement of the Harmonic Distortion of the Insulation Loss Current as a Diagnostic Tool for High voltage Cable insulation.” IEEE Power Engineering Society, Winter Meeting, Singapore, pp. 1615-1620. [14] P. Romero et al. 1991. “The Influence of Water Trees on Loss Factor and Capacitance of Medium Voltage Cables.” 7th ISH, Dresden Germany, paper 23-07. [15] M. Kuschel et al. 1998. “Investigation of the Non-linear Dielectric Response of Water Tree-Aged XLPE Cables in the Time and Frequency Domain.” IEEE International Conference on Conduction and Breakdown in Solid Dielectrics, pp. 85-88. [16] M. Nagao et al. 1990. “New Approach to Diagnostic Method of Water Trees.” Conference record of the IEEE International Symposium on electrical insulation. Toronto Canada, pp. 296-299. [17] Y. Yagi et al. 1998. “Study on Diagnostic Method for Water Treed XLPE Cable by Loss Current Measurement.” 0-7803-5035-9/98, 1998 IEEE, pp. 653-656.

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Dielectric respose as diagnostic tool for power cable systems

[18] A. Helgesson 2000. “Analysis of dielectric Response Measurement Methods and Dielectric Properties of Resin-Rich Insulation During Processing.” Doctoral thesis, Kungliga Tekniska Högskolan, Department of Electric Power Engineering, Electrotechnical design Stockholm, Sweden. TRITA EEA-0002, ISSN 1100-1593, 210 p. [19] R. Neimanis 2001. “On Estimation of Moisture content in Mass Impregnated distribution Cables.” Kungliga Tekniska Högskolan, Department of Electric Power Engineering, Electrotechnical design Stockholm, Sweden. TRITA EEK-0101, ISSN 1100-1593, 195 p. [20] Vahe Der Houhanessian 1998. “Measurement and Analysis of Dielectric Response in OilPaper Insulation systems.” Dissertation for the degree of Doctor of Technical Science, Swiss Federal Institute of Technology Zurich, Diss. ETH No. 12832, 108 p. [21] G. Kaul et al. 1993. “Development of a Computerized loss Factor Measurement System for Different Frequencies, Including 0.1 Hz and 50/60 Hz.” 8th ISH Yokohama, paper 56.04. [22] S. Pöhler 1989. “Dissipation Factor Measurements on Water Treed and Non-Water Treed XLPE Insulating Material.” 6th ISH New Orleans, LA, USA, paper 13.28. [23] A. Paximadakis et al 1991. “Drying and Refilling of Water Trees in Medium Voltage Cables.” 7th ISH, Dresden Germany, paper 23-05. [24] G. Bahder et al. 1977. "In Service Evaluation of Polyethylene and Crosslinked Polyethylene Insulated Power Cables Rated 15 to 35 kV." IEEE Transactions PAS-96, No. 6, pp. 1754-1766.

Method

Description

Return Voltage

Return voltage is measured after a period of charging and discharging the cable. DC test voltage is applied between the conductor and insulation shield of a cable and the current that flows on application of test voltage is measured. Ac test voltages are applied between the conductor and insulation shield of a cable, and dielectric dissipation factor or WDQ/LVPHDVXUHGZLWKDFRQYHQWLRQDOPHDVXULQJDSSDUDWXVDW each application of test voltage. ,QWKLVPHWKRGWDQ/LVPHDVXUHGDWDIL[HGYHU\ORZIUHTXHQF\ (0.1 Hz) but variable voltage. ,QWKLVPHWKRGFDSDFLWDQFHDQGWDQ/LVPHDVXUHGDWDIL[HG voltage but variable frequency (1mHz - 1kHz).

DC Leakage Current 7DQ/

7DQ/DWIL[HG 0.1 Hz Capacitance and 7DQ/DWYDULDEOH frequency DC Leakage current and 7DQ/ Depolarisation Current Polarisation Current Polarisation Current, Depolarisation Current, Return Voltage.

Voltage

Commercial equipment

Test duration

On-site *Off-line On-site *Off-line

CD30/31 Manufacture by HAGENUK

1 h/phase

AC voltage up to the rated lineground voltage

Laboratory

Schering bridge

AC 24 kV (rms)

On-site *Off-line On-site *Off-line

PHG TD Manufacture by BAUR IDA 200 Manufacture by PROGRAMMA

10 min/phase

KDA 1 Manufacture by SEBA

1 hour/phase

DC up to 24 kV DC 2-10 kV

AC 20 kV (peak)

A DC voltage is applied to the cable in step for some time and dc leakage current is measured at each stage. The cable is charge or polarise with a dc voltage. Then grounded or circuited for short period. During the grounding, discharging or depolarisation current is measured. The cable is charge or polarise with a dc voltage. During the charging the charging or polarisation current is measure. The cable is charge or polarise with a dc voltage. During the charging the charging or polarisation current is measure. Then grounded or circuited for short period. During the grounding, discharging or depolarisation current is measured. Then open circuited, during this time return voltage is measured.

Place

30 min/phase

On-site *Off-line DC

On-site *Off-line

DC

On-site *Off-line On-site *Off-line

Dielectric response as diagnostic tool for power cable systems

Annex 1. Measurement of Dielectric Response

Not Available

23

24

Description

Total Harmonic Distortion in the Loss Current at 50/60 Hz 7DQ/DQG7RWDO Harmonic Distortion in the Loss Current at 50/60 Hz. DC Component in AC Charging Current DC Superposition Current

7DQ/DQG'& Component in AC Charging Current

Voltage

Place

Commercial equipment

In this method the total harmonic distortion in the insulation loss current is measured at a power frequency.

AC 35 kV

Laboratory

,Q WKLV PHWKRG PHDVXUHPHQW RI WDQ/ DQG WKH WRWDO KDUPRQLF distortion in loss current are carried out. Both parameters give information about cable ageing.

AC 35 kV

Laboratory

High AC voltage is applied to cable insulation. For a short period dc component of current can be detected if the cable has water tree. The magnitude of this dc component and its polarity is use to judge deterioration of the cable insulation. A DC-superposed voltage is imposed over the normal ac operating voltage. The dc superposition current is obtained by calculating the difference between two current values measured with a superposed voltage of different polarity. DC leakage current and dielectric dissipation factor are measure for the purpose of diagnosing XLPE cables for insulation deterioration. Combined measurements of these parameters give an accurate diagnosis of insulation deterioration. The diagnostic system was designed to consist of three separate units, measurement section, charging current detection section, and circuit breaker section.

AC operating voltage

On-site *On-line

Developed and used in Japan. Availability not known.

DC voltage

On-site *On-line

Developed and used in Japan. Availability not known.

Operating voltage

On-site *On-line

Test duration

Dielectric response as diagnostic tool for power cable systems

Method