PD Measurement for HV Cable System

8/2/2019 PD Measurement for HV Cable System

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Proceedings of the 14th International Middle East Power Systems Conference (MEPCON10), Cairo University, Egypt, December 19-21, 2010, Paper ID 128.

112

Investigation of Partial Discharge Measurement for HV

Cable System with Variable FrequencyA.ELFaraskoury *, F.Tahoun, M. Awad O.E.Gouda

Egyptian Electricity Holding Company

Extra High Voltage Research Centre*

Faculty of Engineering,

Cairo UniversityCairo, Egypt Cairo, Egypt

a.elfaraskoury @yahoo.com [email protected]

Abstract The main principle of the insulation co-ordination and HV testing is that the test voltage simulates the

stresses which occur during the operation of the HV apparatus.

High voltage tests should provide the information for decision

whether a defect in the insulation is dangerous or not for the

later operation. That means the failure mechanism (caused by

the kind of internal cavity defect and kind of the voltage stress)

during the HV test and the later operation should follow the

same physical process. To accelerate this process, the test voltage

is usually higher than the corresponding stress during

operation.This paper is an over view covering best practices for

cable testing using variable frequency test system (20-300Hz) and

applying predictive diagnostic programs to aging cable systems.

It contains also the results of many tests carried out on XLPE

cable systems, withstand voltage test is completed with partial

discharge (PD) measurements.

Index Terms - High voltage cables - Variable frequency-

Partial discharge- Cross bonding links- High frequency current

transformers.

I. INTRODUCTION

The variable frequency test system is used to improve

cables performance by using up to date technology for On-site

withstand tests of power cable systems. The tests are mainly

performed to check the quality of the accessories and their

assembling. Damages of the cable during lying can be

detected too by using the frequency tuned resonant test

systems. The withstand test may be combined with PD

measurements check the performance of cable systems

according to IEC standards. High voltage tests should provide

the information for decision whether a defect in the insulationis dangerous or not for the later operation [1-4]. The most

important stress of a XLPE cable in service is the stress with

the operational alternating voltage. If an on-site test is

completed with a partial discharge (PD) measurement, all the

experience of the various tests can be transferred to the

various factory tests to improve the high voltage cable

performance.

II.CHARACTERISTICS OFVARIABLEFREQUENCY

TESTSYSTEMS

The tests are carried by the support of Extra High Voltage

Research Center, Egyptian Electricity Holding Company,

Cairo, Egypt. Resonance is achieved by tuning the frequency

of the converter unit to the natural frequency (f) of the

oscillating circuit formed by the reactor (L) and the cableunder test (C). The test voltage is pure sine-shaped in the case

of series resonant circuit. Its frequency depends on the load

capacitance according to the equation:

( )CLf

=

2

1 (1)

The frequency increases by decreasing the load capacitance,

the test current and the (reactive) test power are given by

L

CVI = (2)

L

CVP = 2 , with V= test voltage (3)

For the constant inductance L, the minimum resonant

frequency fmin is at the maximum load capacitance Cmax.. In

addition to the test voltage V, the maximum load capacitance

Cmax and the acceptable frequency range fmin to fmax is a

decisive design criteria for the fixed HV reactor of variable

frequency test system:

( ) max2

min2

1

CfL

=

(4)

maxminmax 2 CfVI = (5)


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maxmin

2

max 2 CfVP = (6)

The maximum reactive test power Pmax decreases with

decreasing fmin [1].

The IEC 60840 /2004 and IEC 62067/2006 [2, 3] recommends

a frequency range from 20 to 300 Hz, corresponding to

15min

max==

f

f (7)

The relation between maximum and minimum load is

according to equation (4)

= (8)

and the characterizes the very wide range of testable cable

lengths.

The quality factor (q) of a resonant test system is the ratio

between test powerPand required feeding power PS

q = p / ps (9)

In this work the withstand voltage tests and PD measurements

are carried out on the cable system by a variable frequency

test system (20Hz-300Hz) with 4.8 F maximum test

capacitance, which corresponds to cable length of 16.4 Km at

a cable capacitance of 0.26 F/Km at 220 kV XLPE cables

and with a cable length of 24.3 Km at a cable capacitance of

0.32 F/Km. The tests are mainly performed to check the

quality of the accessories and their assembling. Damages ofthe cable during lying can be detected too by using the

frequency tuned resonant test systems. The withstand test may

be combined with PD measurements to check the performance

of cable systems according to IEC.

The test system can also be designed in advance for use as two

independently running systems with single reactor which also

can be operated in series or parallel. Such system is available

in Extra High Voltage Research Centre Laboratory.

The on-site testing of cables has to check the insulation

condition after-laying and assembly of cable system, as well

as ageing of cables and accessories, since the performance ofthe cables and accessories was tested during the type and

routine tests in the factory. The after laying test of new cables

fills the quality assurance gap between the type and routine

tests of the cable at the manufactures site and the

commissioning of the complete cable system on-site. During

the assembly or repair of a cable system, defects of the cable

sheath and misassembled of joints and terminations can occur

[4].

The test system block diagram in parallel operation mode is

given in Figure (1).

Fig.1 Block diagram of test system in parallel operation

mode of HV reactors.

The resonant circuit must have both capacitance and

inductance when the resonance occurs, the energy absorbed at

any instant by one reactive element within the system. The

control of test system searches for the resonant frequency

automatically and the HV test is carried out at this frequency

as shown in Figure (2). The voltage applied for 1 h, either

with a voltage according the Table I, depending on practical

operational conditions.


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Fig.2 withstand voltage of 220 kV XLPE cable after resonant

frequency automatically.

TABLE I

STANDARDS AC VOLTAGE TESTS FOR AFTER

INSTALLATION OF EXTRUDED HV CABLES

III.TEST ARRANGEMENT

A.WITHSTAND VOLTAGE TESTS

The tests have been carried out on-site according to IEC

for HV cables according to Table I, the test cable have

different lengths to determine the faulty joints and cable

defects sample of these are defects shown in Figures (3, 4, and

5). The results of the on-site withstand testing of 66/220 kV

XLPE cables by the use frequency tuned resonant test system

(20- 300 Hz) are in use since early 2007. To date more than

16 circuits and about 113 km long can be tested by withstand

voltage test with duration of 1 hour and eight faults during test

, three joints , one termination as shown in Figure (6). Sources

of partial discharges in extruded cables include breakdowns in

voids, cavities, along an interface between an energizedelectrode and floating conductor, in an electrical tree etc. It is

known, that HV power cables failure can occur as a result of

the normally applied operational voltage or during a transient

voltage lightning or switching surges. The failure can occur if

localized electrical stresses are greater than the dielectric

materials in the area of localized stress or the bulk dielectric

material degrades to the point where it cannot withstand the

applied voltage.

Fig.3 Breakdown during withstand voltage for 66 kV cable

after 00:31:55 with 27.05 Hz


after 00:47:40 with 26.94 Hz


after 8 second with 42.8 Hz


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Fig.6 220kV cable termination failures during

withstand voltage test.

B.

PD MEASUREMENTS

Partial discharges measuring at Cross-Bonding (CB)

links by using inductive sensors are especially designed as an

inductive sensor, such installation is even possible under on-

line conditions, as the sensor is a clamp-on type high

frequency current transformer sensor (HFCT) that can be

opened and clamped around a cross-bonding link cable.

Figure (7) shows the installed HFCT inside a cross-bonding

links, also used PD gating unit MPD540 for gating purposes

in order to provide the possibility to filter certain external

background noise. Also HFCT sensor will be connected

around the core of each phase of the cable as shown in the

Figure (8) to measure both phase-to-phase and phase-to-earthPD activity in the cable and termination. The calibrated HFCT

sensor uses inductive coupling to detect PD pulses flowing

between phases and earth, converter the high frequency

current pulses from the discharges into high frequency voltage

pulses on the PD test unit. The PD sensitivity using HFCT the

central measuring frequency is recommended to lie between 2

MHz and 10 MHz in a flat zone of the frequency spectrum.

The spectrum is obtained from FFT of calibration PD pulses.

Furthermore, the measuring frequency must be set in order to

obtain the greatest possible PD signal/noise ratio. In addition,

an ''on-site performance check" must be carried for the

selected measuring frequency before the PD measurement

starts [5].

PD sensors work based on detection of high frequency current

pulses that occur during PD in the cable system. The PD

pulses occur in very short time, the width and rise time of the

pulses are in the nanosecond region. Consequently, PD pulses

with energy frequency up to hundred MHZ are generated [6].

These PD pulses with travel through the cable earth conductor

and finally can be recorded by the sensors. These types of

sensor mostly used in practice due to the advantage that these

sensor do not disrupt the normal configuration of the

accessories and cable part.

As the total cable line is energised, a PD test must be carried

out simultaneously at all accessories per phase. This

requirement leaded to the development of a new synchronous

multi channel PD measurement system. Selective PD

measurements need a potential free connection from the

accessories to storage and visualisation unit, these achievedusing optical fibres.

Fig.7 Cross-Bonding links with mounted three HFCT

for 220 kV (PD Sensors)

PD sensors work based on detection of high frequency current

pulses that occur during PD in the cable system. The PD

pulses occur in very short time, the width and rise time of the

pulses are in the nanoseconds region. Consequently, PD

pulses with energy frequency up to hundred MHz are

generated These PD pulses will travel through the cable earth

conductor and finally can be recorded by the sensors. For

unconventional PD detection, internal and external capacitive

and inductive sensor can be used. Internal inductive sensorscan be placed in the cable accessories without disturbing the

cable insulation because they placed on the top of earth screen

of cable. However, this type of sensors has to be already

installed in manufacture of the cable accessories [7].

Fig.8 Measurement set-up for PD detection using HFCT

For 66 kV


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Fig.9 Block diagram of an acquisition unit MPD 540

The PD signals are filtered, amplified and digitized. Having

an amplitude quantization of 14 bit and a sampling rate of 64

MS/s, the time accuracy of detection of a PD signal is at about

2ns. The quasi-integration is realized by a digital band - pass

filter. The center frequency for digital filter can be chosen in a

frequency range from DC up to 20 MHz, the bandwidth

between 9 KHz and 3 KHz, respectively. Hence an optimal

frequency band can be chosen to avoid disturbances and to

reach a high signal-to-noise ratio (SNR) even under noisy

conditions on site. Furthermore, the test voltage signal is

digitized in acquisition unit to document the test voltage

during the PD measurement as shown in figure (9) [8].

C. TEST SET-UP FOR PD MEASUREMENTS

The test set-up for on-site PD measurements should be

corona free. Therefore, corona protection spheres and

metallic pipes of suited diameter have to be used. Sufficient

clearance from HV connections to any part of the construction

should prevent PD from earthed potential free components.

IV.TEST RESULTS AND EXPERIENCE

The tests have been carried out on-site according to IEC

for 66/220 kV cables at 2 U0 for 66 kV and1.7 U0 for 220 kVhaving different lengths to determine the faulty joints and

cable defects as shown in Table I. Therefore, the XLPE cable

insulation was subjected to AC tests after assembling and at

the same time partial discharge measurements were done on

all accessories simultaneously for three-phase PD

measurement on the relevant joint box would be possible, also

the cross-bonding can be changed to straight-through

connection, to minimize cross-talk between the three phases

and to clearly distinguish between the three joint of one group.

By using a computer which already installed with PD software

connected to the spectrum PD analyser. The amplitude and

number of PD pulses as a function of phase cable performed

and this pattern will be useful to recognize the type of defect

in the cable system. However, calibration on this PD detection

method cannot be applied as in conventional PD detectionwhich described in the IEC 60270 standard [9] due to several

reasons related with high frequency behaviour of the sensors

and the type and routing of the measurement cables [10].

The results of rechecking frequency dependency on damping

and phase to phase cross-talk are shown in Figure (10), led to

choice of 1 MHz mid frequency for PD measurements to have

approximately equal sensitivities for all joints. For higher

measurement frequencies, under or over estimation of PD

level of different phases is probable. Lower measurement

frequencies would cause the disadvantages of lower cable

damping, resulting in higher external interference from both

ends of the cable link.

The results of the on-site PD measurements with the

alternating voltage of variable frequency have been performed

in conjunction with test at cross-bonding links using HFCT

sensors reported the discharge activity ranged from 7 to 500

pC. The variation of noise level which is experienced during

all measurements resulting in higher external interference

from ends of the cable, also corona effect caused by floating

parts close to high voltage at the cables termination.

Fig. 10 PD measurement relation frequency


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The on-site PD measuring level for cable system doesnt limit

in the standards but depends primarily on the experience of

those involved in the measurements, and the experience

learned how to the diagnostics of the PD limit. We always ask

this question from the manufacturers and owner customers,

What is the safe level for PD activity in the cable systems?

The answer to this can only be, there is no safe level for

internal PD in the cable systems, all internal discharges will

be damaging.

The PD Guide line level given below for the on-line PD

testing condition assessment of polymeric-insulated XLPE

cables, cable accessories and cable sealing ends of operation

voltages from 66 kV up to 400 kV as shown in Table II [11].

Table II

PD LEVEL FOR ON-LINE MEASUREMENTS FOR

OPERATION VOLTAGE FROM 66 kV UP TO 400 kV

Most polymer based insulation now has standards which set

by IEC guidelines (at least in the factory/type test) to have a

PD level of better than 10pC. It is difficult to see that properly

installed plant which is discharging less than this level is

going to fail by insulation failure. All other failure modes can

be sorted with maintenance programs and hence the aim

should be to run any new system discharge-free (this can be

tested at commissioning stage to provide a base-line, at

installation PD level).

The results of on-site PD measurements on this paper compare between two 220 kV XLPE cable system have 8Km length

and 14 cross bonding box as shown in Figures (11, 12), the

PD magnitudes in pico-Coulombs versus the cable length. PD

activity of up to pC can be observed at the discharging joint. It

can also noted from Figures (13, 14) that there are some PD

event originating in the termination at the remote end of the

cable, in this way, PD pattern allows a view of the PD

activity on the cable in a non-destructive.

Fig.11 PD measurements results on circuit 1

Fig.12 PD measurements results on circuit 2


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Fig.13 Example of PD measurement pattern some noise and

corona

Fig.14 Example of PD measurement pattern with same corona

due to floating electrode

V. CONCLUSION

The experience of testing with variable frequency, and several

other laboratory tests on aged cables show the advantage of

testing cable with frequencies in the range of 20-300Hz. on-

site withstand test of 66/220 kV XLPE cables with variable

frequency test systems combined with un-conventional PDdetection is performed by using HFCT sensors after

installation of HV cable system reduces the risk from the

service, also after repair the joint reassembling was done

exactly in the same place given good results. Nevertheless,

besides all routine and type tests before installation and the

use of prefabricated and pretested accessories with

conventional PD detection.

ACKNOWLEDGMENT

The authors would like to express his great thanks to the

staff of the Extra High Voltage Research Centre for providing

their facilities during this work.

REFERENCES

[1] S. Schierig, D. Russwurm, HV On-site Testing on Cable by AlternatingVoltage of Variable Frequency, IEEE Insulated Conductors Committee

(ICC), October 2000.

[2] IEC Publ. 60840, 3rd ed., Power Cables with Extruded Insulation andtheir Accessories for Rated Voltages above 30 kV (Um =36 kV) up to 150

kV (Um=170 kV) Test Methods and requirements, 2004-4.

[3] IEC Publ.62067 Power cables with extruded insulation and theiraccessories for rated voltages above 150 kV (Um = 170 kV) up to 500 kV

(Um =550 kV) - Test methods and requirements, 2006-1.

[4] M. Awad, F.Tahoun, A.ELFaraskoury and O.Gouda On-siteCommissioning Test and Diagnostics of 220kV XLPE Cable System,

CIGRE, B1.304, 2010.

[5] F.Garnacho, I.Trasmonte et al., On-site measurements experiences ininsulation condition for medium and high voltage cables, CIGRE, D1-

201, 2008.[6] S. M.eijer, R.A.Jongen, et al, On-site VHF Partial Discharge Detection

on Power Cable Accessories,Proceedings Jicable, 2007.

[7] Jarot Setyawan Investigation of Partial Discharge Occurrence ANDDetectability in High Voltage Power Cable Accessories TU DELFT-

NOV.2009.

[8] K.Rethmeier, P.Mohaupt, et al, New Studies on PD Measurements onMV Cable Systems at 50Hz and Sinusoidal 0.1 Hz (VLF) Test Voltage

CIRED- May 2007.

[9] IEC Publ. 60270 High voltage test techniques- partial dischargemeasurements 2000-12.

[10]S. M.eijer, R.C.Ladde, R.A.Jongen, et al, Sensitivity VerificationProcedure of VHF PD Detection Systems,Proceedings Jicable, 2007.

[11]HVPD Technical Guide for PD Levels in MV and HV Cables and Joints-Manchester, UK- May2009.

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PD Measurement for HV Cable System