SUITABILITY OF DIFFERENT TEST VOLTAGES FOR ON-SITE TESTING OF XLPE CABLE SYSTEMS

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HIGHVOLT Prüftechnik Dresden GmbH

SUITABILITY OF DIFFERENT

TEST VOLTAGES FOR

ON-SITE TESTING OF

XLPE CABLE SYSTEMS

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Suitability of different test voltages for on-site testing of XLPE cable systems 2

Content

 Introduction

1

 Comparison of test methods

2

3

 IEC Standard

 Test parameters and their significance

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Introduction

Cable Life

Manufacturing

Damage

Mi

st

ake

T

est

ing

Transportation

Installation

T

esting

Power-freq.

50/60

Hz

Operation

50/60 Hz

Cable life



Re

pair

Ageing

T

est

ing

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Introduction

Commissioning and Diagnostic Testing

Commissioning test

Quality control after installation of the cable system

Detecting all faults which would lead to a breakdown during

service time of the cable system

Aim

Diagnostic test

Check of specific values of the cable system

Condition assessment for remaining life time estimation

of the cable system

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Content

 Introduction

1

 Comparison of test methods

2

3

 IEC Standard

 Test parameters and their significance

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6 Suitability of different test voltages for on-site testing of XLPE cable systems

Comparison of test methods shall be based on

parameters describing

1) The harmfulness for healthy insulation

2) The usefulness of a test method for an application, i.e. the

capabilities to distinguish between healthy and defective

insulation

3) The applicability (for example expressed by the equipment

size, weight and its power consumption)

Numbers 2 and 3 are only worth discussing if the previous

numbers are answered satisfactorily

Test parameters and their significance

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A Cable System

 Will be used with constant AC stresses (with occasional

switching and lightning overvoltages)

 Quality depends only on performance under these

stresses –

A good cable is one that works well a long

time, not one that passed a certain test

A Test should therefore



“Produce the same dielectric effect the insulation as

overvoltages ... in service” (IEC 60071-1)

 Thereby be sensitive to those faults dangerous during

operation

Test parameters and their significance

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Purpose of test

 Initiate discharges in defects so that they can be found

 Withstand test: Cause sufficient growth in the defects to

lead to breakdown

Influential test parameters:

 Test voltage value

 Test voltage duration (both for PD inception as for growth)

 Test voltage wave shape (type of stress)

Type and amount of stress is important for

significance of test

Test parameters and their significance

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Test parameters and their significance

What is a Significant Test?

Cable system without faults:

Test result  cable system is healthy

Test result  cable system is not healthy

Cable system with faults:

Test result  cable system is not healthy

Test result  cable system is healthy

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Content

 Introduction

1

 Comparison of test methods

2

3

 IEC Standard

 Test parameters and their significance

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H

V

C

R

XLPE Cable System

AC Conditions:

 tan

d

= 

 i

_c

>> i

_r

 Capacitive field control

DC conditions (without consideration of space charges):

 C becomes irrelevant

 Resistive field control

 Defects usually have lower resistance and lower field

Differences between AC/DC and MV/HV cables

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Examples for Typical Cable Geometry

 MV Cable: 24 kV, insulation thickness 5.5 mm, mean

operational field strength at voltage peak 3.1 kV/mm

 HV Cable: 138 kV, insulation thickness 17.8 mm, mean

operational field strength at voltage peak 6.3 kV/mm

 EHV Cable: 400 kV, insulation thickness 26 mm, mean

operational field strength at voltage peak 12.6 kV/mm

Differences between AC/DC and MV/HV cables

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Technical Consequences

 Field strength increases with cable voltage

 Recommended limit for test field strength (XLPE):

27-30 kV/mm

 Allowed ratio between test voltages and operational voltages

for MV 4 times and for HV 2 times higher than for EHV XLPE

cables

 Test methods using higher overvoltage's can be applied for

MV cables

 Using these methods for HV and EHV cables test voltages

would have to be reduced, in turn reducing sensitivity of test

Differences between AC/DC and MV/HV cables

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Content

 Introduction

1

 Comparison of test methods

2

3

 IEC Standard

 Test parameters and their significance

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DC

(1) Mainly used for testing of oil-paper cables

(2) Experience and research showed that DC voltage creates space charges in solid, extruded insulations designed for AC applications (3) These space charges can lead to a breakdown when AC voltage is

applied to the test object after the DC test

Space charges endanger an otherwise healthy cable system

DC voltage is therefore not used for commissioning and

diagnosic tests on AC cable systems

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Very Low Frequency (VLF)

(1) Continous voltage with a frequency between 0.01 to 1 Hz, typically used for testing 0.1 Hz (Europe)

(3) Due to the few voltage cycles PD measuring is not as sensitive as with power frequency

(2) Due to the low frequency the capacitive field control is not longer dominant

 reduced field strength in the faults  higher voltage necessary in comparison with AC of power frequency

Limit is here the max. field strength for XLPE of approx. 30 kV/mm

For MV cable systems  well established For EHV cable systems  not standardized

(4) Experience shows that the detection/diagnostic of water trees with VLF works well (5) Comparability between different VLF test results is well given but the comparison to

a test with AC power frequency is difficult due to the different field control effects (capacitive/resistive)

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OWTS/DAC (Oscillating wave)

(1) Usage of the cable system as test energy storage by charging it with a DC current (2) The DC charging ramp poses the danger for creating space charges

Might lead to a breakdown of a otherwise healthy insulation

(3) Typical test procedure of 50 „shots“ equals ~ 10 s of continuous AC voltage (depending on the capacitance of the cable system)

Only faults with a PD inception time of ~ 10 s can be detected (4) Does not represent stress under service conditions

(5) Test time highly depends on the parameters of the cable and the test system  very bad repeatability and thus not comparable

(6) Due to DC charging different behavior of insulation compared to power frequency AC test and contiuous AC test  not comparable with factory test or other on-site tests

Not standardized and not recommended for commissioning testing

Typical usage for diagnostic testing of MV cables where the safety margin in the electrical field strength is bigger compared to HV and EHV cable systems

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Continuous AC 20-300 Hz

(1) Usually resonant circuit with fixed inductance and frequency converter as supply (2) Test voltage very similar to service voltage. Even at 20 Hz, 1 hour is equivalent with

72.000 oscillations  high probability to initiate discharges in faults  very good possibilities to detect faults / e.g. PD

(3) Electric field distribution is very close to that under service voltage  represents very well the stess under service conditions

(4) Very high comparability with tests done at power frequency in the factory (5) Test can be repeated even on other cables  very good repeatability

(6) Test is very well defined in the standards and the existing experience of ~ 20.000 test conducted worldwide with continous AC between 20 - 300 Hz shows similar performance like the tests done in the factories

Standardized and well suitable for commissioning and diagnostic testing of MV/HV/EHV cable systems

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24 h rated voltage

(2) Only faults which lead to a breakdown withhin 24 h of cable system service voltage can be found

(3) PD measuring increases the chance to find more defects, but still all the faults with a PD inception time >24 h remain undetected

(1) Voltage form and frequency identical to service conditions

Well suitable for diagnostic testing of MV/HV/EHV cable systems Not recommended for commissioning testing

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Content

 Introduction

1

 Comparison of test methods

2

3

 IEC Standard

 Test parameters and their significance

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IEC Standard

IEC 60502 old version

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IEC Standard

IEC 60502 new version

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IEC Standard

IEC 60502 new version

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IEC Standard

IEC 60840 latest version

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IEC Standard

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IEC Standard

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IEC Standard

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HIGHVOLT Prüftechnik Dresden GmbH

THANK YOU FOR YOUR

ATTENTION

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11 Cable type: 400kV XLPE

Cable length: 16km Cable capacitance: 2360nF Test voltage: 260kV Test duration: 1 hour

Parallel and Series Connections for long HV

Cable Test

L1 + L4 L2 + L5 L3 + L6

6 x WRV 83/260 T

L4 II L5 II L6