การเปลี�ยนวธิีการทดสอบสายใตด้ินแรงสูงจาก DC เป็น AC

ตามมาตรฐาน IEC 60840 (2004)ตามมาตรฐาน IEC 60840 (2004)

IEC 60840 (2004)

No DC voltage is No DC voltage is No DC voltage is No DC voltage is No DC voltage is No DC voltage is No DC voltage is No DC voltage is

specified for specified for specified for specified for

Insulation test Insulation test Insulation test Insulation test

Defects in Cable & Accessories

Defects in Cable & Accessories

Why Not DC ?

� Stress created by DC field (resistivity) is different from 50 Hz AC field (permittivity).

� Even massive insulation defects massive insulation defects massive insulation defects massive insulation defects in extruded dielectric insulation cannot be detected with DC at the recommended voltage levelsdielectric insulation cannot be detected with DC at the recommended voltage levels

� Space (trapped) charge Space (trapped) charge Space (trapped) charge Space (trapped) charge development at the sites of water trees or other defect sites

� Only finds conductive type gross workmanship conductive type gross workmanship conductive type gross workmanship conductive type gross workmanship errorserrorserrorserrors in extruded dielectric cable systems

Example of Failure

สายส่งสถานีย่อยทววีฒันา

Example of Failure

สายส่งสถานีย่อยเสาระหง

Example of Failure

สายส่งสถานีย่อยแสนแสบ

24/02/588888

Example of Failure

สายส่งวภิาวด ี– สามย่าน MH 76

24/02/589999

Example of Failure

สายส่งวภิาวด ี– สามย่าน MH 8

24/02/5810101010

Conductive Type Defect Revealed by DC Test

สายส่งแจ้งวฒันะ – เมอืงทอง

24/02/5811111111

Space Charge

DC hipot output negatively charge up void. These trapped space charges remain after test. When AC is reapplied,

there’s a high difference of potential across very little insulation. Leads to electrical trees – cable fails.

Partial Discharge

� Localized electrical breakdown taking place in the electrical insulation system under voltage stress that only partially bridged the insulation between conductors and which may or may not occur adjacent to a conductorbetween conductors and which may or may not occur adjacent to a conductor

Partial Discharge

Electric Stress Distribution With No Void

( 12.7 kV rms on the cable conductor )

Electric Stress Distribution With a Void

( 12.7 kV rms on the cable conductor )

Partial Discharge

Partial Discharge

What Choices Do We Have ?

� AC voltage at power frequency

� AC resonance voltage

� ACRL test voltage

� ACRF test voltage� ACRF test voltage

� AC oscillating wave voltage (OWTS)

� Very Low Frequency voltage (VLF)

IEC 60840 Test Voltage

Rated voltageHighest voltage for equipment

Value of U0 for Determination of test voltages

Voltage test after

installation

U (kV) U (kV) U (kV) (kV)U (kV) Um (kV) U0(kV) (kV)

45 to 47 52 26 52

60 to 69 72.5 36 72

110 to 115 123 64 128

132 to 138 145 76 132

150 to 161 170 87 150

Power Frequency Test Voltage

� A voltage of U0 applied for 24 h (IEC 60840-2004)

� The cable to be tested is connected to the

network in normal operation by means of a

power transformer of which the neutral point

network in normal operation by means of a

power transformer of which the neutral point

of the secondary wye is not grounded. Upon

connecting one phase of the cable to ground

during a previously determined period, the

other two phases will be exposed to √3 times the nominal voltage with respect to earth (from

IEEE Transaction 1992).

AC Resonant Voltage

�AC voltage testing uses alternating current at

frequencies between 20Hz and 300Hz is an

effective method to energize onsite all types of

cable systems. As compared to AC voltage

stresses during factory testing and service stresses during factory testing and service

operation it is recommended for withstand testing.

�HVAC withstand test: HVAC withstand test: HVAC withstand test: HVAC withstand test: The cable section can be

accepted if after application of a selected HVAC

voltage stress for a recommended duration no

breakdown has occurred.

�DiagnosisDiagnosisDiagnosisDiagnosis: At certain voltage levels partial

discharges, dielectric losses can be measured in

function of time/voltage and used for diagnostic

AC Resonant Voltage

AC Resonant Voltage

AC Resonant Voltage

On-site test of a 245 KV cable system on Los Angeles Water

and Power Administration /USA

Testing of the 400 kV cable system of London with a

length of more than 20 km

Damped AC Voltage

� Damped AC voltage testing uses damped alternating

current at frequencies between 20Hz and 500Hz. In

combination with partial discharge measurement it is an

effective method in testing on-site all types of cable

systems. Due to similarity in partial discharges

occurrence at AC voltage stresses during factory testing

systems. Due to similarity in partial discharges

occurrence at AC voltage stresses during factory testing

and service operation it is recommended for on-site

testing and PD measurements.

� DAC withstand testDAC withstand testDAC withstand testDAC withstand test: The cable section can be stressed

with a selected DAC voltage stress for selected time

duration and the cable section can be rejected if a

breakdown has occurred.

� DiagnosisDiagnosisDiagnosisDiagnosis: At certain voltage levels partial discharges,

dielectric losses can be measured in function of

time/voltage and used for diagnostic purposes.

Damped AC Voltage

Damped AC Voltage

Damped AC Voltage

Damped AC Voltage

PD diagnosis of a service aged 150kV XLPE cable

Damped AC Voltage

Very Low Frequency Voltage

�Very low frequency (VLF) voltage testing uses

frequencies down to 0.01Hz to test on-site the all

types of cable insulation. Due to much lower

frequency range as compared to AC voltage

stresses, VLF testing is based on the fact that stresses, VLF testing is based on the fact that

insulation defects have to breakdown during time

of testing.

�VLF withstand testVLF withstand testVLF withstand testVLF withstand test: The cable section can be

rejected if after application of a selected VLF

voltage stress for a recommended duration

breakdown has occurred.

�DiagnosisDiagnosisDiagnosisDiagnosis: At certain voltage levels partial

discharges, dielectric losses can be measured in

Very Low Frequency Voltage

Programmable voltage waveforms

Sinewave test voltage at 0,1 Hz:

Squarewave voltage at 0,1 Hz with defined slew -rate:

Very Low Frequency Voltage

VLF HV test and diagnosis (DF&PD) system up to VLF HV test and diagnosis (DF&PD) system up to VLF HV test and diagnosis (DF&PD) system up to VLF HV test and diagnosis (DF&PD) system up to 400400400400kVrms, kVrms, kVrms, kVrms,

600600600600kV DC up to kV DC up to kV DC up to kV DC up to 7777µFFFF

Utility Practice

� KOREAKOREAKOREAKOREA: 24 hours working AC voltage test or VLF with PD measurement. � VLF for lower voltage cable (distribution cables 22.9

kV).

� Working voltage for 154kV and above.

In future, OWTS may be applied for distribution � In future, OWTS may be applied for distribution cables. AC withstand voltage test will be applied for transmission cable.

� JAPANJAPANJAPANJAPAN:10 mins working AC voltage test with construction check for 154kV class or less, 24-hours AC withstand test (about 1.7 U0) with PD measurement for 220KV and above

From IERE technical meeting in 2007

Utility Practice

� EDFEDFEDFEDF high voltage insulation testing policy (2010):

� The cable shall withstand an ac test voltage applied between a conductor and sheath or all conductors with the sheath earthed. Where applied between a conductor and sheath or all conductors with the sheath earthed. Where possible a Partial Discharge (PD) test shall be carried out during the ac test.

� The test will normally be provided by a series resonant test set operating close to power frequency.

Rated Line Rated Line Rated Line Rated Line

VoltageVoltageVoltageVoltage

(kV)(kV)(kV)(kV)

Ac Test Voltage for Ac Test Voltage for Ac Test Voltage for Ac Test Voltage for

New Cable SystemsNew Cable SystemsNew Cable SystemsNew Cable Systems

(kV)(kV)(kV)(kV)

FrequencyFrequencyFrequencyFrequency

(Hz)(Hz)(Hz)(Hz)

DurationDurationDurationDuration

(min)(min)(min)(min)

66 7220-300 60

132 132

Utility Practice

� Energy AustraliaEnergy AustraliaEnergy AustraliaEnergy Australia: Network Standard – Specification for Testing of Underground Cable (2004) � High Voltage DC Tests shall not be applied routinely to XLPE

insulated sub-transmission cables (123 kV) and/or accessories.

� Prior to commissioning, sub-transmission XLPE insulated cables shall be subject to VLF test voltages (preferred), or alternatively energized only at power frequency system voltages for a period of 24 hours before carrying load current.energized only at power frequency system voltages for a period of 24 hours before carrying load current.

� The test duration may be reduced at the discretion of the System Operator

� A VLF test, if available, is preferred to a system voltage test.

� For XLPE cables, it is accepted that for the present, unipolarpulsed test voltages shall be required for fault location.

Which Direction Shall MEA Go ?

Shall MEA continue with DC Shall MEA continue with DC Shall MEA continue with DC Shall MEA continue with DC

Test ?Test ?Test ?Test ?Test ?Test ?Test ?Test ?

Which Direction Shall MEA Go ?

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