Training_note_2010-09-29

Importance of On-site testing and Diagnosis of Transmission Power Cables Prof.dr.hab.ir. Edward Gulski, FIEEE onsite.hv.solutions AG, Switzerland Poznan University of Technology, Poland Delft University of Technology, The Netherlands

In co-operation with:

Grid companies

International manufactures

Universities and not-forprofit organizations

IEEE HK Joint Chapter of PES/IAS/PELS/IES Seminar: Importance of On-site testing and Diagnosis of Transmission Power Cables

1.

Basic Aspects of Condition Assessment of Transmission Power Cables

2.

International Trends in Testing and Diagnosis

3.

Principles of using sinusoidal damped AC voltages for on-site energizing of power cables

4.

Examples of Modern Condition Assessment of HV Power Cables


1.

Basic Aspects of Condition Assessment of Transmission Power Cables

2. 2.

International International Trends Trends in in Testing Testing and and Diagnosis Diagnosis

3. 3.

Principles Principles of of using using sinusoidal sinusoidal damped damped AC AC voltages voltages for for on on--site site energizing energizing of of power power cables cables

4. 4.

Examples Examples of of Modern Modern Condition Condition Assessment Assessment of of HV HV Power Power Cables Cables


On-site testing and diagnosis of (E) HV power cables

‘’Quality and cable system integrity of the cable circuit’’

New installed / repaired cables

On-site testing to: exclude poor workmanship (installation defects)

‘’Availability /reliability of the cable circuit’’

Serviced aged cables

On-site diagnosis to: detect local aging and check insulation degradation status


Asset management classification of the cable condition

On-site testing and diagnosis voltage withstand

PD detection and diagnosis

New installed, diverted cables

quality and integrity

dielectric-loss diagnosis

Service aged cables

reliability and availability

maintenance and replacement

Asset management decision support


Asset management decision

C

Status

Condition

Maintenance goal

A New connection, very good condition

New or aged: no defects or aging symptoms

Minimal maintenance, go for optimal availability.

B Good condition

Aged: degradation observed, possibly harmful defects present

No impact on reliability. Keep in status B, against minimal cost and maintain optimal availability.

Significantly aged: very degraded, serous defects present

Very low reliability. Instable situation, end as quick as possible.

C Stable, but poor condition

D Instable situation E End of life cycle

Reliability decreased. Refurbish to status B, or if this is not possible, stabilise to prevent quick aging. Maintain optimal availability, by periodical condition assessment.

No operation recommended. Maintenance aimed to prevent environmental pollution and safe conservation. Extension of life time or availability is no issue.


Integral approach: non-destructive insulation condition assessment

Cable termination

Cable insulation

Cable joint

Weak -spots insulation condition assessment by: Ø PD detection and localisation at voltages up to 1.7 U0 -PDIV -PD magnitudes at voltages up to 1.7xU0 -PD location -PD patterns

Local defects

Integral insulation condition assessment by: ØDielectric losses measurement up to 1.7U0 Ø Ø Return voltage measurement RVM Ø Voltage Withstand Diagnosis up to 1.7 U0

Global degradation


Typical defects in a power cable joints

Ref: NEETRAC


Typical defects in a power cable insulation

Ref: NEETRAC IEEE HK Joint Chapter of PES/IAS/PELS/IES Seminar: Importance of On-site testing and Diagnosis of Transmission Power Cables

Dielectric loss factor and PD magnitude

[Source: PD in Electrical Power Apparatus; ISBN 3-8007-1760-3] IEEE HK Joint Chapter of PES/IAS/PELS/IES Seminar: Importance of On-site testing and Diagnosis of Transmission Power Cables

Partial discharges: quantities

Discharge test. Any particular discharge larger than q0 measured below test voltage Ut leads to rejection of the object under test, see shaded area.


Partial discharges: PD behavior of cable termination [Source: PD in Electrical Power Apparatus; ISBN 38007-1760-3]


Dielectric properties of HV insulation

I.

All HV power cables have high voltage life expectancy;

II.

During the whole service life > 30 years they are subjected to thermal, mechanical and environmental stresses;

III.

All these influences raise the working temperature of the HV power cable;

IV.

Higher temperature accelerates chemical reactions resulting in degradation of dielectric characteristics;

V.

Increase of degradation results in increases of loss factor and future degradation;

VI.

Measurement of dielectric losses is important issue to avoid failures.


Dielectric losses: physical origin of losses

Conductive losses

Dipole losses

Interface losses Partial discharges losses


Dielectric loss factor of an oil-paper-insulated cable [Source: PD in Electrical Power Apparatus; ISBN 3-8007-1760-3]


Dielectric loss factor and PD magnitude of an oil-paper-insulated cable [Source: PD in Electrical Power Apparatus; ISBN 3-8007-1760-3]

1.

healthy cable

2.

insulation screen damage 1cm2

3.

insulation screen damage 6cm2


Dielectric losses: tan δ as function of the conductor temperature


Tan δ of a 150kV as function of the conductor temperature


Dielectric losses

1.

The tan δ measurement can be applied for the determination of the loss factor of the insulation material. This factor increases during the ageing process of the cable.

2.

The tan δ measurement should be regarded as a diagnostic and/or supporting measurement.

3.

In practice, in HV insulation is known that in addition to absolute value of tan δ at certain test voltage also the increment of tan delta as measured at two designated voltages so called Δ tan δ or tip-up is important for condition assessment.

Se ction Alkm r-Oter3

L1 L2

0.5

L3

dielectric losses [%]

0.45 0.4 0.35 0.3 0.25 0.2 40

45

50

55

60

65

70

test voltage [kVpeak ]


1. 1.

Basic Basic Aspects Aspects of of Condition Condition Assessment Assessment of of Transmission Transmission Power Power Cabl Cables es

2.

International Trends in Testing and Diagnosis

3. 3.


4. 4.



Norms (voltage testing)

Standards/norms/guides

Description

IEC 60840

Power cables with extruded insulation and the accessories for rated voltages above 30kV up to 150kV Test methods and requirements;

IEC 62067

Power cables with extruded insulation and the accessories for rated voltages above 150kV;

IEC 60141

Tests on oil-filled and gas-pressure cables and their accessories up to and including 400kV;

IEC 60060-3

High Voltage test techniques –Part 3: Definitions and requirements for on-site testing;

IEEE 400

Guide for Field Testing and Evaluation of the Insulation of Shielded Power Cable Systems Rated 5KV and Above.


International recommendations

Cable type MV (6…30 kV)

XLPE

PILC

(E) HV (30 – 400 kV)

XLPE

Oil-filled

Internationally not recommended

DC

VLF 0.1 Hz

AC 50 Hz

DAC 20-300 Hz

Installation test / Acceptance test

No

Yes*

Yes

Yes

Maintenance test / Diagnostic test

No

Yes*

Yes

Yes


Yes

Yes

Yes

Yes


Yes

Yes

Yes

Yes


No ***

No **

Yes

Yes


No ***

No **

Yes

Yes


Yes

No **

Yes

Yes


No

No **

Yes

Yes

Internationally recommended

* Partial discharge (PD) inception voltage (PDIV) can be different as compared to continuous and damped AC stresses ** according to [1] IEC 60840 Power cables with extruded insulation and their accessories for rated voltages above 30 kV (Um = 36 kV) up to 150 kV (Um = 170 kV) Test methods and requirements. [2] IEC 62067, Standard Power cables with extruded insulation and their accessories for rated voltages above 150 kV (Um = 170 kV) up to 500 kV (Um *** according to [3] Recommendations for a new after laying test method for HV extruded cable systems CIGRE 1990, Paris, paper 21-105 [4] CIGRE WG 21-09; After-laying tests o HV extruded cable systems; Elektra 173, August 1997


Effectiveness of routine HV withstand testing

Type of insulation defect A.

B.

C.

Homogenous

Weak inhomogeneo us Strong inhomogeneo us

Example

Failure type

Breakdown test effectiveness / PD presence effectiveness

AC

DAC

24Uo

DC

e.g. less or no insulation, moisture

Breakdown, No partial discharges

/ --

/ --

/-

/ --

e.g. missing field grading

Not necessary a breakdown; possible predischarges

/ -+

/ -+

/-

/-

/ -+

/-

e.g. sharp edges, cavities, small installation defects

Partial discharges + breakdown

NO PD -> No breakdown

/ ++

++

NO PD -> NO breakdown


Norms (PD detection)

TEST TYPE

DESCRIPTION

I.

AC or DAC voltage test & PD measurement in accordance to IEC60270, IEC885-3 and IEEE 400.3

a. b. c.

PD level measurement in [pC] localization of PD in cable insulation localization of PD in cable accessories

II.

AC or DAC voltage test & PD measurement (unconventional) not standardized (IEC 62478 under preparation, Cigre WG D1.37)

a.

RF (up to 500 MHz) PD measurement in [μV] detection of PD in cable accessories only (not applicable for direct burred cables)

b.


On-site electrical tests using AC and damped AC voltages

New installed cables

Repaired / refurbished cables

Service aged cables


Maintenance test

Diagnostic test

Object

Test type

Evaluation method

Voltage withstand test Diagnostics: e.g.: partial discharges (PD) dissipation factor (tan δ)

Monitored Evaluation of the diagnostic data

Acceptance thresholds

Non-monitored Criteria Criteria Pass/Fail



Installation / Acceptance test

Maintenance / Diagnostic test

DAC excitations at max. test voltage Over -voltage not failed PD -free -> increase

failed

PD present -> evaluate PD observed

PD not observed Repair

Evaluation of PD data

Harmful PD

Harmless PD

Test passed



DAC excitations at max. test voltage* Over-voltage

(*) the time duration will be in accordance to IEC recommendations or resp. to IEEE 400 document (50 excitations)

PD-free -> increase PD present -> evaluate

Power cable rated voltage

Ref

(*) AC/DAC voltage test level Acceptance test

< 30kV

IEC 60502

Max. 3.0….2.0xUo

30kV-150kV

IEC 60840

Max. 2.0…1.7xUo

150kV-500kV

IEC 60067

Max. 1.7…1.1xUo

(*) AC/DAC voltage test level Maintenance test In general i has to be lower than acceptance test e.g. 80%


1. 1.


2. 2.


3.

Principles of using sinusoidal damped AC voltages for on-site energizing of power cables

4. 4.



DAC voltage type equivalence to continuous AC voltage

continuous sinusoidal AC (20-300Hz)

damped sinusoidal AC (20-300Hz)

dielectric losses

PD inception voltage PDIV

≈

PD inception voltage PDIV

PD extinction voltage PDEV

≈

PD extinction voltage PDEV

PD level in [pC]

≈

PD level in [pC]

PD pattern information

< PD pattern information

Test over-voltage destructiveness > Test over-voltage destructiveness Dielectric losses ≈ Dielectric losses


What is damped AC voltage ?

1. The damped alternating voltage (DAC) is an oscillating switching impulse voltage (OSI) of quite low damping and a frequency in the range of 20Hz up to 400Hz.

2.

Damped AC voltages are accepted test voltages for on-site testing: i. IEC 60060-3 High Voltage test techniques –Part 3: Definitions and requirements for on-site testing ii. IEEE 400 Guide for Field Testing and Evaluation of the Insulation of Shielded Power Cable Systems iii. IEEE 400.3 Guide for PD Testing of Shielded Power Cable Systems in a Field Environment. IEEE HK Joint Chapter of PES/IAS/PELS/IES Seminar: Importance of On-site testing and Diagnosis of Transmission Power Cables


50kV

100kV

Example: PD as measured on a termination of a 220kV power cable

150kV


International survey about using damped AC voltages (2007) Purpose of testing using DAC voltages

partial discharge measurement

voltage testing 23%

52% 25% dielectric losses measurement Relevance of combining PD test and the voltage test Yes: 50%

No: 50%

[Ref] IEEE PES Insulated Conductor Committee Spring 2008 Meeting, Minutes F05D Damped AC Voltages Testing


International survey about using damped AC voltages (2007) Voltage testing

Max. Utest ≤ 3xU0 18%

82%

Max. Utest ≤ 2xU0

Breakdown during voltage testing:

0% …. 2%

Before breakdown PD has been measured:

81%

[Ref] IEEE PES Insulated Conductor Committee Spring 2008 Meeting, Minutes F05D Damped AC Voltages Testing


International survey about using damped AC voltages (2007) Test voltage level as used for PD measurements Max. Utest ≤ 3xU0 18% 9%

Max. Utest ≤ 1xU0

73%

Max. Utest ≤ 2xU0 The present standards IEEE 400 is using the name Oscillating Waves (OW). The present IEC 60060-3 is using the name damped alternating voltages (DAC). To avoid any confusion between oscillating voltages in the range of kHz and damped AC voltage in the range of 20Hz-500Hz as there are in use 100% of responders support the proposal to change the name to DAC. [Ref] IEEE PES Insulated Conductor Committee Spring 2008 Meeting, Minutes F05D Damped AC Voltages Testing


Damped AC On-site testing of (E) HV power cables

has been introduced by Seitz Instruments AG in Switzerland end of 90’s. Since than worldwide almost 200 systems are successfully implemented for testing and diagnosis of distribution and transmission power cables using different OWTS® (oscillating wave test system) solutions: OWTS® 25 since 1999 OWTS® HV 250 since 2004 OWTS® M-series 28 since 2005 OWTS® M-series 60 since 2006 OWTS® HV 150 since 2007 OWTS® HV 350 since 2008 OWTS® HV 250 system



Example of a continuous AC voltage source

On-site testing and PD measurement on a 150 kV XLPE cable at KEMA, The Netherlands, 1994 [Ref] ISBN 90-73192-5-6

Example of a damped AC voltage source

On-site testing and PD measurement on a 150 kV external gas pressurized cable at a Dutch utility Nuon, The Netherlands, 2007

technological progress IEEE HK Joint Chapter of PES/IAS/PELS/IES Seminar: Importance of On-site testing and Diagnosis of Transmission Power Cables

On-site testing of (E) HV power cables

Example of a continuous AC voltage source

Example of a damped AC voltage source

Weight: 20.000kg

Weight: 1.000kg

Supply voltage: 3 x 400V (200kVA)

Supply voltage: 8kW

Load: 20 nF - 5000 nF

Load: 20 nF – 13000 nF

Output: AC 220kVrms (25 Hz - 300 Hz)

Output: DAC 250kVrms (20 Hz - 300 Hz)

Test object: power cables 220kV / 8km

Test object: power cables 220kV / 30km



Applicability for testing different power cable ratting voltages Power cable rated voltages [kV]

Uo [kV]

HV 150 x Uo

HV 250 x Uo

HV 350 x Uo

66

38,1

2,8

110

63,5

1,7

2,8

132

76,2

1,4

2,3

150

86,6

1,2

2.0

2,8

220

127

0.8

1,4

1,9

330

190,5

0.9

1,3

380

219,4

Type HV150

Type HV350

1,1



Applicability for HV and EHV power cables Results of several successful comparison tests as published by Cigre (1990), TU Delft/Kema (1999), Cesi (2002), EuroTest (2007), TU Helsinki (2007), Prysmian Cables and Systems BV, EPRI State Grid, China (2010), have demonstrated the applicability of damped ac (dac) voltages.

OWTS® HV technology uses damped sinusoidal ac voltages in the range of 20Hz up to 500Hz


Important Standards

IEC 60060-3: High Voltage test techniques –Part 3: Definitions and requirements for on-site testing; IEEE 400: Guide for Field Testing and Evaluation of the Insulation of Shielded Power Cable Systems; IEEE 400.3: Guide for PD Testing of Shielded Power Cable Systems in a Field Environment; IEC 60270: Partial discharges measurements; IEC 885-3 Test methods for partial discharges measurements on lengths of extruded power cable; IEC 60840: Power cables with extruded insulation and the accessories for rated voltages above 30kV up to 150kV Test methods and requirements;

OWTS® HV 150 system

IEC 62067: Power cables with extruded insulation and the accessories for rated voltages above 150kV;. IEC 60141: Tests on oil-filled and gas-pressure cables and their accessories



has been introduced in 1998 as a consequence of experience in on-site ac testing on the one hand and the technological progress in power electronics and advanced signal processing on the other hand and it is characterized by lightweights measured with max. 6kg/MVA compactness versus output voltage system assembling and voltage erecting effort necessary power demand for long lengths possibility of sensitive PD detection and tanδ measurement

After-laying testing of new installed HV and EHV cables using OWTS® HV system



The damped alternating voltage (dac) is an oscillating switching impulse voltage (OSI) of quite low damping and a frequency in the range of 20Hz up to 400Hz

IEC 60060-3 Damped alternating voltage


Damped AC On-site testing of (E) HV power cables Principles of damped sinusoidal ac (dac) voltage generation

350kV HV divider and PD measuring unit including measurement and control system

350kV external inductance build up with a coil base, 3 coils and 1 coil with top electrode

350kV switch unit

350 kV power unit, 2 parallel units in master slave configuration which deliver 16 mA



Principles of damped sinusoidal ac (dac) voltage generation

HV Source

HV Solid-State Switch

Inductor L Test Object: Power Cable

Cc

S

HV Divider Process Control Unit

PD Coupling Capacitor

Data Storage

PD detector

PD Analysis Dielectric losses estimation

For the generation of dac voltages the capacitance of the HV power cable in series in a large air-core inductor L in the range of few H’s is charged with a continuously increasing HV voltage with a low current in the range of 10mA up to 30mA. IEEE HK Joint Chapter of PES/IAS/PELS/IES Seminar: Importance of On-site testing and Diagnosis of Transmission Power Cables



HV Source

HV Solid-State Switch

Inductor L Test Object: Power Cable

Cc

S

HV Divider Process Control Unit

PD Coupling Capacitor

Data Storage

PD detector

PD Analysis Dielectric losses estimation

By linear charging with continuously increasing HV voltage no ‘‘steady state’’ DC conditions occur in the test object. After the L•C loop is closed in a time e.g. < 1μs by HV solid-state switch in the loop a sinusoidal damped ac voltage with a frequency between 20Hz and few hundreds of Hz is produced. IEEE HK Joint Chapter of PES/IAS/PELS/IES Seminar: Importance of On-site testing and Diagnosis of Transmission Power Cables



OWTS® HV 350 system the dac voltage frequency is approximately the resonant frequency of the circuit L•C. the maximum capacitance which can be tested using dac system can be calculated in dependence of maximum test voltage applied and the permissible ac current in the resonant circuit and is in the range up to 13μF.



Examples of typical damped ac voltage frequencies for different lengths of two typical 230kV power cables Length [km]

XLPE (C=200pF/m) [Hz]

Oil filled (C=350pF/m) [Hz]

0.25

258

195

0.5

183

138

1

129

98

2

91

69

4

65

49

8

46

35

16

32

24

20

29

22


Damped AC On-site testing of (E) HV power cables User steps De-energizing and disconnection of the HV power cable circuit Connection of the OWTS® HV system

Test object definition and IEC60270 PD calibration Energizing the cable with dac voltages

Partial discharges detection Voltage testing

Dielectric loss measurement Voltage withstand test

Disconnection of the OWTS® HV system Analysis and report generation IEEE HK Joint Chapter of PES/IAS/PELS/IES Seminar: Importance of On-site testing and Diagnosis of Transmission Power Cables

Damped AC On-site testing of (E) HV power cables User steps Step 1: test object definition

Step 2: IEC60270 PD calibration


Damped AC On-site testing of (E) HV power cables User steps Step 3: voltage testing and diagnosis


Damped AC On-site testing of (E) HV power cables User steps Step 4: PD mapping generation

Step 5: report generation


Damped AC On-site testing of (E) HV power cables Tools Testing and diagnosis IEC60270 standardized PD detection PD-pattern generation Advanced PD pulse analysis TDR analysis of PD pulses PD mapping in cable insulation and accessories Multiple PD sites localization Dielectric loss estimation 2 1.8 1.6 D ie l.L o s s e s

Tan δ and Δtan δ in function of the test voltage

1.4 1.2 1 0.8 0.6 0.4 0.2 0 0.40 L1

L2

L3

0.80

1.00

1.36

x Uo



Experiences values of partial discharges (PD) evaluation criteria for on-site electrical tests on power cables using damped AC (DAC) voltage Cable type

PD site localisation

Condition assessment Safe

Extruded new-installed / service aged

cable insulation

Extruded service aged

accessories

Fluid-Filled (low pressure) service aged

Semi safe

Not save

PD free* PD free*

<100pC **

>100pC**

distributed; no local spot(s)

<100pC **

<500pC **

>1000pC **


spot(s) in cable insulation and accessories

<50pC **

<200pC **

>200pC **

Fluid-Filled (high pressure) / Gas cable service aged


PD free*

<300pC **

>500pC**



PD free*

(*) PD free (PDIV > 1.3 Uo) and with an acceptable background noise level e.g. 25..50pC (standardized PD detection) (**) only as an indication; depends on the PDIV, insulation type and condition of a individual cable


Damped AC On-site testing of (E) HV power cables Experiences values of dielectric loss (tan δ and Δtan δ ) evaluation criteria for on-site electrical tests on power cables using damped AC (DAC) voltage Cable type

PD site localisation

Condition assessment Safe

Semi safe

Not save

tan δ <0.1%

Extruded new-installed / service aged

cable insulation

Extruded service aged

accessories

tan δ <0.1% Δtan δ <0.1%***

tan δ <0.2% Δtan δ <0.1%***

tan δ >0.2% Δtan δ >0.1%***



tan δ <0.2% Δtan δ <0.1%***

tan δ <0.3% Δtan δ <0.2%***

tan δ >0.3% Δtan δ >0.2%***



tan δ <0.2% Δtan δ <0.1%***

tan δ <0.3% Δtan δ <0.2%***

tan δ >0.3% Δtan δ >0.2%***



tan δ <0.2% Δtan δ <0.1%***

tan δ <0.3% Δtan δ <0.2%***

tan δ >0.3% Δtan δ >0.2%***



tan δ <0.3% / Δtan δ <0.2%***

(***) tip-up value between 0.5Uo and 2Uo [IEC 60141]


1. 1.


2. 2.


3. 3.


4.

Examples of Modern Condition Assessment of HV Power Cables


Examples No 1: New 132kV XLPE Cable data: Location: Malaysia Installation year: 2009 Cable length: 7700m Operation voltage: Uo= 76kVrms Damped AC frequency during test: 44Hz Cable capacitance (one phase): 1.64uF History: Breakdown in two joints during soaking test


Examples No 1: New 132kV XLPE TDR analysis examples: L1 (Red)

L2 (Yellow)


Examples No 1: New 132kV XLPE cable PD mappings and histograms: Voltage

PD Mappings

PD Histograms

70kV

110kV


Examples No 1: New 132kV XLPE PD measurements results (after repair): 800 700

PD [pC]

600 500

L1

400

L2 L3

300 200 100 0 0

20

40

60

80

100

120

140

160

Voltage [kV]

Voltage Withstand Test @ 1.4xUo (50 excitations-30min per phase):

Voltage Withstand Test passed by three phases however in L1 PD were localized IEEE HK Joint Chapter of PES/IAS/PELS/IES Seminar: Importance of On-site testing and Diagnosis of Transmission Power Cables

Examples No 2: New 132kV XLPE cable

Installation year: 2009 Insulation type: XLPE Cable length: 1700m Voltage ratting: Uo =76 kVrms Measuring DAC frequency: 95Hz, Cable capacitance: 0.38uF

Up to 1.4Uo (106kVrms) no PD activity in the cable has been observed.


Examples No 3: Serviced aged 110kV XLPE cable Installation year: 1974 Insulation type: early XLPE (manufactured by Russian Cable Company) Number of joint: Unknown (due to repayments different number of joints in each cable phase) Length: 1500m Localization: Moscow History: 5 years after installation, unexpected random breakdowns in all phases due to electrical treeing. Investigation showed that cable samples taken from deteriorated phases showed a symptom of water treeing phenomenon and electrical treeing. Last breakdown occurred in 2009. Cable was repaired and tested with OWTS 150 HV before putting to operation service.


Examples No 3: Serviced aged 110kV XLPE cable Diagnostic (energizing up to Uo – 64kVrms/90kVpeak) due to service history L1

L2

L3

10kV

70/40/70kV 90kV


Examples No 3: Serviced aged 110kV XLPE cable

PD vs. Test Voltage PD vs. Test Voltage [kV] 500

[pC]

400 L1

300

L2 200

L3

100 0 0

10

20

30

40

50

60

70

80

90

100

[kVpeak]


Examples No 3: Serviced aged 110kV XLPE cable Dielectric loss estimation (Δ tan δ)



PD localization (TDR) phase L2



PD mapping L1,L2,L3 (40kVpeak)








Examples No 4: Serviced aged 110kV EGP

Testing 110kV external gas- pressure cable circuit (2.2 μF), length 8.5 km using damped AC voltages (40 Hz); power grid in Holland IEEE HK Joint Chapter of PES/IAS/PELS/IES Seminar: Importance of On-site testing and Diagnosis of Transmission Power Cables


L1

L2

L3

PD at Uo diagnostic results (left: IEC 60270 phase revolved PD pattern, right: HF PD pulse sequences



Phase 4 (yellow) shows different behavior as compared to other phases: at Uo voltage PD activity has been observed.



TDR analysis of data from phase 4 (yellow) shows that there are no significant concentrations of PD activity in the cable insulation and cable accessories. A number of PD-pulses have been localized in the total length indication distributed characteristics.


Examples No 5: 110kV XLPE cable Test Measurement procedure damped AC voltages 1. The test voltage is raised in steps of 10 kV and three excitations at each voltage level are performed. 2. At the selected maximum voltage level voltage withstand test up to 1.7Uo is performed: 50 excitations (IEEE 400) or 1hrs (IEC 60840). 3. Final maximum test voltage depends on PDIV and PD magnitude at PDIV. 4. During the voltage test PD and the dielectric losses is measured. No .

U rms [kV]

U peak [kV]

1

7.1

10

2

14.1

20

3

21.2

30

140

4

28.3

40

120

5

35.4

50

6

42.4

60

7

49.5

70

8

56.6

80

9

63.6

90

10

70.7

100

11

77.8

110

20

12

84.9

120

0

13

91.9

130

14

99.0

140

15

106.1

150

U peak [kV

160

100

80 60 40

0.1 0.2 0.3 0.4 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.6 1.7

x Uo


Examples No 5: 110kV XLPE cable Goal field investigation: 1. 2. 3. 4. 5. 6.

Possibility of energizing the test circuit with damped ac voltages IEC 60270 Calibration of the test circuit in its full range PD Measurement according to IEC 60270 PD Localization according to TDR method (VHF mode) Voltage withstand test Phase: A B Dielectric losses measurement

C

48 Hz damped AC test set –up Cable circuit: 110kV XLPE (5.77km)

Far-end of the 110 kV XLPE cable in GIS Due to presence of non-disconnected VT in GIS phase A was not tested


Examples No 5: 110kV XLPE cable Phase C

Phase B 90kV (Uo)

140kV (1.55xUo)

90kV (Uo)

120kV (1.33xUo)


Examples No 5: 110kV XLPE cable

600

L2 B

Q-V graphics

L3 C

500

[pC]

400 300 200 100 0 0

20

40

60

80

100

120

140

160

[kV] peak

90kV (Uo)

DAC voltages withstand test of 50 excitations (18 minutes) at 1.1Uo


Examples No 5: 110kV XLPE cable PD mapping @ 140 kV (1.6 Uo)


Examples No 5: 110kV XLPE cable

0.25

Dielectric loss [%]

0.2 0.15 0.1

0.05 0 0.4 Uo

Uo

1.3 Uo

Ur Phase B

Phase C


Examples No 6: Serviced aged 50kV oil-filled cable

Installation year: 1958 Insulation type: oil filled Cable length: 9.6km Power cable capacitance: 2.8uF Damped AC frequency: 35Hz Service history: Oil-leakages in L1 and L2



Oil filled (1968); 6.8km

• Up to 1xUo no PD activity has been observed. Starting at 1.3Uo up to 1.7Uo PD activity has been observed close to the near cable termination and at the joint No 14 position. Due to the fact that PDIV > Uo during normal operation no PD occur on the above mentioned sites

•The measured values vary between 0.29% and 0.42%; •With the test voltage increase up to 1.7 Uo the max increase is 0.075%



2 1.8

Installation year: Insulation type: Cable length: Voltage ratting: Cable capacitance: On-site testing using damped ac voltages:

1976 Oil-filled 13.314 m Uo = 132 kVrms 4 μF

Diel. Losses

1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 0.40 L1

L2

L3

0.80

1.00

1.36

x U_0

(29 Hz) up to 1.36 Uo



2 1.8

Installation year: Insulation type: Cable length: Voltage ratting: Cable capacitance: On-site testing using damped ac voltages:

1982 Oil-filled 27.513 m Uo = 132 kVrms 8.5 μF

Diel. Losses

1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 0.40

(20 Hz) up to 1.36 Uo

L1

L2

L3

0.80

1.00

1.36

x Uo


Conclusions

1.

Electrical testing on-site is an important step of quality insurance of new/repaired and service aged cable systems.

2.

Enhanced voltage testing up to e.g. 2.5xU0 of defect-free /non aged insulation does not have destructive influence on the service life of the component.

3.

Enhanced voltage testing up to e.g. 2.5xU0 of defective/aged insulation may have destructive influence on the service life of the component. Even if no breakdown has occurred.

4.

The Utest/U0 ratio decreases for on-site testing of HV cables above 150kV and therefore it has to be compensated by additional measures e.g. PD detection.

5.

Service aged HV power cables are important assets and there is an increasing need to obtain information about actual condition by applying integral and defect related on-site diagnostics.

6.

Sinusoidal damped AC voltages (DAC) can be applied for voltage testing of power cables (up to 380kV cable systems) in combination with diagnostic measurements.

7.

Partial discharges, dielectric losses diagnostic measurements at damped AC voltages can be used for nondestructive on-site testing and diagnosis of new and service aged power cables.

8.

Combining on-site testing with PD detection provides information about discharging insulation defects and about the fact if the on-site test had a destructive impact on the insulation system.

9.

For HV and EHV power cables more attention is needed to implement on-site testing and diagnosis.


Contacts for further information

Seitz Instruments AG Mellingerstrasse 12 5443 Niederrohrdorf Switzerland phone 41 56 496 7480 fax +41 56 496 7485 www.seitz-instruments.ch

SebaKMT GmbH Dr. Herbert Iann Str.6 96148 Baunach Germany phone +49 9544 680 fax +49 9544 2273 www.sebakmt.com

onsite.hv.solutions AG Töpferstrasse 5 6004 Luzerne Switzerland phone: + 41 41 500 0550 fax: + 41 41 500 0551 www.onsitehv.com


Training_note_2010-09-29

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