PA8 Conference Proceedings of ISEIM 2014

Partial Discharge Measurement for Medium Voltage

Cables Using Different Voltage Wave Forms

EI-Sayed M. EI-Refaie, Mohi Besheir, M. K. Abd Elrahman, and Ramy Saad Helwan University - Cairo, Egypt

drelsaymr@yahoo.com, mohybeshir@yahoo.com, m _kamaI1963@yahoo.com, eng.ramy _ ets@yahoo.com

Abstract-Partial Discharge (PD) is the most important issue that arise when testing service aged and non-aged cables. For long time, High Voltage Direct Current (HVDq was used to detect the occurrence of PD activity in cables. Due to technical reasons, the HVDC was replaced by the Very Low Frequency (VLF). Wide range of frequencies and wave forms were used for this purpose. This paper presents an evaluation to the ability of VLF to detect successfully the presence of PD. The study was based on laboratory testing data collected from medium voltage cable having artificial defects. PD was measured on healthy and unhealthy samples of 12/20 kV cables with VLF as well as with power frequency. Different wave forms such as sinusoidal and rectangular with different frequencies in the range from O.OIHz to 0.1 Hz were used.

Keywords: Partial discharge, Very low frequency, XLPE medium voltage cables, Cable testing.

I. INTRODUCTION

Medium voltage distribution cables and their accessories

form an essential part of the power delivery systems. Many of these systems employ insulating materials that have superior insulation properties such as Cross Linked Poly Ethylene

(XLPE). With aging of the insulating system, its dielectric properties change so that they could be used to provide a

convenient way for monitoring the degradation of the system insulation [1]. In the past, it was generally not practicable to conduct the testing of cables using AC voltage as large capacity is needed to charge up the cable to the required level, so cables were tested by DC voltage. This type of voltage leads to growth of water trees, especially with cables

insulated with XLPE. This constraint had since been overcome with the introduction of Very Low Frequency (VLF) testing technique [2]. Very low frequency (VLF) systems seem to provide results that are more accurate than those obtained using higher frequencies [3].

II. WHY SHOULD WE USE VLF?

Several reasons can be mentioned to test the underground cable networks with VLF

A. Security Requirements and Standards

New standards like IEC60060-3 defme the VLF voltage source as an adequate waveform for high voltage testing. It

represents today's state of art of different high voltage sources. In fact, the VLF cable tests based on standard

mentioned before has become a worldwide accepted field test and diagnostic method for commissioning and maintenance work within medium and high voltage applications. Regular diagnostics protect the user of incipient failures on underground distribution systems.

B. Technical Reasons for Using VLF

• Weight and volume of the test equipment.

• Mobility for field applications.

• Higher efficiency in finding insulation defects.

• Higher sensitive and precision on PD measurements compared to power frequency.

• Diagnostic efficiency, using true sinusoidal high voltage source for PD measurements.

• VLF testing is far more effective than DC.

• DC may produce space charges in the dry cable insulation with long term damage to the cable .

C. General Strategic Reasons for Using VLF

• Improve system reliability.

• Reliable system for life time considerations and system assessment data evaluation.

• Condition based and preventive maintenance.

III. TEST VOLTAGE SOURCE

To secure the distribution networks on a long term view, reliability and performance tests using VLF high voltage field tests related to the recommended standards, avoid incipient

faults in the underground residential distribution networks [4, 5]. Today adequate portable VLF test equipments for field use are available on the market. Latest research findings regarding power frequency, VLF testing and diagnostic results supports the idea ofVLF. Newly designed state of the

art VLF high voltage sources uses solid state high precision

amplifiers. A technique producing a true sinusoidal output voltage allowing high precision partial discharge and

harmonic free high voltage sources to secure partial discharge and tan delta requirements for precision diagnostic

measurements [6, 7].

The VLF test set can work in manual and in automatic mode. In automatic mode the test set select the suitable

frequency according to the insulation resistance and charging capacitance of the cable under test without taking into account the ability of this frequency to detect hidden defects in the cable to give accurate values of PD. "Fig. 1 " shows a particular load diagram for a commercial VLF source. Since the rated power of the VLF source determines the frequency

of testing waveform regardless any other technical data, the motivation 0 f this work was to investigate the effect of the frequency as well as the wave form of the applied voltage on detecting the PD presence.

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Figure 1 Load diagram of portable VLF test set [8].

IV. PARTIAL DlSCAHRGE MEASUREMENT

The term partial discharge (PD) refers to a discharge that does not completely bridge the space between the electrodes. It is not always possible to prevent minor manufacturing

faults, cavities or in homogenates in the insulation material

causing weak points. These weak points can cause over stressing by the electric field. This can lead to extremely

rapidly progressing local electric discharge, i.e. electric partial

discharge. For decades the PD measurement and the location was based on time base propagation fault location and it has

been the most important and efficient non-destructive method [9]. The quality and the lifetime of the insulation can be assessed. PD measurement on-site using adequate filtering and sensitivity starting at several Pico Coulombs (PC) today are commercially available. For PD measurement onsite, compared to laboratory, different requirements have to be

fulfilled. The absolute magnitude of PD level itself is not as important as that one from laboratory measurements. Even

more interesting is the location of PD sources. In most cases the PD does not result from the internal cable insulation, but

from the accessories like joints and terminations. Therefore the source location becomes more important. With partial discharge measurement one can prevent local mounting faults or electrical trees to prevent incipient faults [10].

V. PARTIAL DlSCAHRGE MEASURING SYSTEM

The measurements of PD have been executed using coupling capacitor ( 0.6 nF, 100 kV) and two channel digital storage oscilloscope with sampling rate of 100 MHz. By

using the software provided with the oscilloscope we had the ability to transfer the data from the oscilloscope to laptop to make the analysis to the measured quantities. "Fig. 2" presents the classical straight method for measuring the partial discharge.

VI. TEST SAMPLES

In order to investigate the ability of VLF to detect hidden defects in cables, 3 m cable length of 12/20 kV, 1 *70 mm2,

XLPE were used as test objects. The first sample was healthy

Conference Proceedings of ISEIM 2014

without any defects. The second sample was not healthy and it has an artificial cavities made by drill at the middle of the

cable. The cavity was with different diameters and different depths .

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Figure 2 Electrical equivalent test circuit.

VII. RESULTS AND DISCUSSION

A. Effect of Wave Form and Frequency

In this case the cable SUbjected to VLF voltage with wave form sinusoidal and rectangular with frequency range of 0.01 Hz to 0.1 Hz. "Fig. 3" shows the measured values of PD in case of healthy cable, whereas the unhealthy case is shown in

"Fig. 4". These results was obtained at 20A kV ( 1.7 Vo) according to IEC 60270 by using artificial cylindrical void

having a diameter of Imm and height of 3. 5mm.

For the case of unhealthy, with high measured values of PD, it is clear that the sinusoidal wave form is more sensitive

than rectangular wave for the full range of frequencies. However in the healthy case, it is only superior for limited

range of frequency. Although, in both healthy and unhealthy cases, the maximum difference between the measured values was only at the left portion of the curve, it could be concluded

generally that the sinusoidal wave form is more effective in detecting the presence of PD especially at higher values of

PD pulses.

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...... Sinusoidal _Rectangular

................ .....

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0.02 0.04 0.06 Frequency (Hz)

Healtby

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Figure 3 Effect of wave form and frequency of the applied

voltage on the magnitude of PD in healthy case.

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Diameter =1 mm and depth = 3.5 mm

. . • . . Sinusoidal ___ Rectangular •••• " . . ...... ... . .... ...... "" ..

............. . . ...

·nhealthy

O+-----�----�----�----�----� o 0.02 0.04 0.06 0.08 0.1

Frequency (Hz)

Figure 4 Effect of wave form and frequency of the applied voltage

on magnitude of PD in the unhealthy case.

In the same time, the effect of the applied voltage frequency on the ability to detect the presence of PD is also investigated in the last two figures. It is clear that there is no clear trend to be used to conclude a general notice on the effect of frequency.

B. Effect of Voltage

In this case, the ability of the applied voltage with power

frequency ( 50 Hz) as well as VLF ( 0.01 to 0.1 Hz) to detect

the PD occurrence was investigated. In both cases the PD

magnitude increases with increasing the applied voltage. The increasing in the Pd magnitude is the same for both the cases of the sinusoidal voltage as shown in "Figs. 5 and 6". The measured values of PD magnitude in case of rectangular wave are less than the sinusoidal wave voltage in power frequency and VLF, see "Fig. 7". With increasing the applied voltage in case of VLF the effect of the frequency becomes more noticeable.

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Figure 5 Effect of power frequency applied voltage

on the magnitude of PD.

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6

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Conference Proceedings of ISEIM 2014

-.SkY ··. · 9 kr �1Jk) - 1 kY �21 k\,

........................ .......................... .... ...........

.-------�-------�-------.. -------. O+-----�-----r----�------r-------,

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Figure 6 Effect of sinusoidal wave voltage

on the magnitude of PD.

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. ..... .. . . ......... .................... ... .. . . ................ . .

.-------�-------.-------I--------I

0.01 0.04 0. ( IeqD.elCJtHz)

Figure 7 Effect of rectangular wave voltage

on the magnitude of PD.

0.1

C. Effect of the Number of Voids

The effect of voids number on the PD measurement was carried out by using one and two voids with the same size. These voids were made at equal distance from the cable

sample terminals. The sinusoidal VLF wave form was superior to detect the occurrence of PD comparing with the

rectangular wave form. Where, the measured values in case of sinusoidal wave form were much higher than the measured values in case of rectangular wave form for both the cases of one and two voids. However the difference between the one

and two voids was clearer in case of rectangular wave form. The ability of both wave form to discriminate between one and two voids decreases with increasing the used frequency.

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In case of sinusoidal wave, the ability to detect the presence of PD decreases with increasing the used frequency. However, this is not noticed in case of rectangular wave form.

25

20

0' 15 S 0'

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..... One void _Two voids

Sinusoidal wave, 20.4 k' .

.... ... 101•••• • ••••••• 11 •• • ,"., ••••

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0

14

12

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0 0.02 0.04 0.06 Frequency (Hz)

0.08

Figure 8 Sinusoidal wave form applied voltage VLF

in case of multi-voids

..... Onnoid _TWOfOids

........... ...... .. . ..

.. . ... .. , ........ . " . ...... ... ".

Or-----�----�----�----_r----� o 0.02 0. 0.06

Freqmcy (Hz) 0.1

Figure 9 Rectangular wave form applied voltage VLF

in case of multi-voids

CONCLUSIONS

0.1

The main important points given from this work could be

summarized as follows:

• Partial discharge is a diagnosis and nondestructive test for on-site tests of cables which can improve the

reliability and extend the life time of the whole cable system. PD measurement at VLF test voltage is more sensitive to detect hidden defects in cables.

• Changing the frequency of the applied voltage has no considerable effect on the measuring PD pulses.

• Using sinusoidal wave form with reduced frequency gives better results help to assess the condition of cable systems.

Conference Proceedings of ISEIM 2014

• Increasing the applied voltage increases the PD activities for all types of the applied voltage.

ACKNOWLEDGMENT The authors would like to express their great thanks to the

general manager of AI-Hlmya substation, Ministry of Electricity and Energy for providing his facilities during this

work.

REFERENCES [I] EI-Sayed M. EL-Refaie, M. E. Beshir, Mohamed Kamal,

Ramy Saad, "Diagnostics of Medium Voltage Cable Systems

Using Very Low Frequency", ClRED 2013, Stochholm.

[2] Shew Chong Moh, "Very Low Frequency Testing- Its

Effectiveness in Detecting Hidden Defects in Cables", ClRED

2003, Barcelona.

[3] Cavallini, A., 2012, "The Role of Supply Frequency in the

Evaluation of Partial Discharge Inception Voltage in XLPE­

Embedded Cavities", Electrical Insulation and Dielectric

Phenomena (CEIDP), 2012 Annual Report Conference, pp.

478-490.

[4] IEC Standards IEC 60060-3-2006, International Electro

Technical Commission, "High Voltage Field Testing" .

[S] CENELEC Standard, Harmonization Document HD 620 S 1

and HD 621 SI "Test after Installation Requirement for

Medium High Voltage Cables with PE", (Section SC-I) and

PVC (SC-2) shield, Part S : 1996.

[6] P. Mohaupt, W. Kalkner, Bergmann, "New Approach for HV

Cable On-Site Testing", ClRED 2007, paper A 4.6 .

[7] J. Hernandez, N. Hampton, R. Harley, R. Hartlein, "Practical

Issues Regarding the Use of Dielectric Measurements to

Diagnose the Service Health of MY Cables", Georgia Institute

of Technology (NEETRAC), JICABLE 2007, paper A 7.S.

[8] User manual of portable VLF test set type Frida, Baur pruf­

und messtechnik-GMBH.

[9] R. Bartnikas, October 2002, "Partial Discharges. Their

Mechanism, Detection and Measurement." IEEE Transactions

on Dielectrics and Electrical Insulation, Volume 9 Number S

ISSN 1070-9878, pp.763-808.

[10] C. Goy, E. Whittaker, M. Baur, Conference Pproceeding of

CMD 2010, "Benefits of a Combined Diagnostic Method,

Using VLF Partial Discharge and Dissipation Factor

Measurement on Medium Voltage Distribution Cables".

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