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

SUITABILITY OF DIFFERENT TEST VOLTAGES FOR ON-SITE TESTING OF XLPE CABLE SYSTEMS Michael Hensel HIGHVOLT Prüftechnik Dresden GmbH

2 3 Content 1 Introduction Test parameters and their significance Differences between AC/DC and MV/HV cables Comparison of test methods IEC Standard Suitability of different test voltages for on-site testing of XLPE cable systems 2

Testing Testing Testing Damage Mistake Ageing Repair Introduction Cable Life Manufacturing Transportation Cable life Installation Operation 50/60 Hz Powerfreq. 50/60 Hz Suitability of different test voltages for on-site testing of XLPE cable systems 3

Introduction Commissioning and Diagnostic Testing Commissioning test Quality control after installation of the cable system Aim Detecting all faults which would lead to a breakdown during service time of the cable system Diagnostic test Check of specific values of the cable system Aim Condition assessment for remaining life time estimation of the cable system Suitability of different test voltages for on-site testing of XLPE cable systems 4

2 3 Content 1 Introduction Test parameters and their significance Differences between AC/DC and MV/HV cables Comparison of test methods IEC Standard Suitability of different test voltages for on-site testing of XLPE cable systems 5

Test parameters and their significance Characteristics of Test Methods to be Assessed 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 Suitability of different test voltages for on-site testing of XLPE cable systems 6

Test parameters and their significance Why test with same Type of Stresses as in Service? 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 Suitability of different test voltages for on-site testing of XLPE cable systems 7

Test parameters and their significance Which parameters are interesting? 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 Suitability of different test voltages for on-site testing of XLPE cable systems 8

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 Suitability of different test voltages for on-site testing of XLPE cable systems 9

2 3 Content 1 Introduction Test parameters and their significance Differences between AC/DC and MV/HV cables Comparison of test methods IEC Standard Suitability of different test voltages for on-site testing of XLPE cable systems 10

Differences between AC/DC and MV/HV cables Field Distribution in Layered Dielectrics XLPE Cable System AC Conditions: tan d = C R H V 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 Suitability of different test voltages for on-site testing of XLPE cable systems 11

Differences between AC/DC and MV/HV cables Differences between MV, HV and EHV cables 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 Suitability of different test voltages for on-site testing of XLPE cable systems 12

Differences between AC/DC and MV/HV cables Differences between MV, HV and EHV Cables 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 Suitability of different test voltages for on-site testing of XLPE cable systems 13

2 3 Content 1 Introduction Test parameters and their significance Differences between AC/DC and MV/HV cables Comparison of test methods IEC Standard Suitability of different test voltages for on-site testing of XLPE cable systems 14

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 Suitability of different test voltages for on-site testing of XLPE cable systems 15

Very Low Frequency (VLF) (1) Continous voltage with a frequency between 0.01 to 1 Hz, typically used for testing 0.1 Hz (Europe) (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 (3) Due to the few voltage cycles PD measuring is not as sensitive as with power frequency (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) (6) VLF testing in a well defined test procedure for MV applications For MV cable systems well established For EHV cable systems not standardized Suitability of different test voltages for on-site testing of XLPE cable systems 16

Very Low Frequency (VLF) Suitability of different test voltages for on-site testing of XLPE cable systems 17

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 Suitability of different test voltages for on-site testing of XLPE cable systems 18

OWTS/DAC (Oscillating wave) Suitability of different test voltages for on-site testing of XLPE cable systems 19

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 Suitability of different test voltages for on-site testing of XLPE cable systems 20

Continuous AC 20-300 Hz Suitability of different test voltages for on-site testing of XLPE cable systems 21

24 h rated voltage (1) Voltage form and frequency identical to service conditions (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 Commissioning testing with 24 h of rated voltage is like a 1 day guarantee Well suitable for diagnostic testing of MV/HV/EHV cable systems Not recommended for commissioning testing Suitability of different test voltages for on-site testing of XLPE cable systems 22

2 3 Content 1 Introduction Test parameters and their significance Differences between AC/DC and MV/HV cables Comparison of test methods IEC Standard Suitability of different test voltages for on-site testing of XLPE cable systems 23

IEC Standard IEC 60502 old version Suitability of different test voltages for on-site testing of XLPE cable systems 24

IEC Standard IEC 60502 new version IEC 60502-2 Suitability of different test voltages for on-site testing of XLPE cable systems 25

IEC Standard IEC 60502 new version IEC 60502-2 Suitability of different test voltages for on-site testing of XLPE cable systems 26

IEC Standard IEC 60840 latest version IEC 60840 Suitability of different test voltages for on-site testing of XLPE cable systems 27

IEC Standard IEC 60840 latest version IEC 60840 Suitability of different test voltages for on-site testing of XLPE cable systems 28

IEC Standard IEC 62067 latest version IEC 62067 Suitability of different test voltages for on-site testing of XLPE cable systems 29

IEC Standard IEC 62067 latest version IEC 62067 Suitability of different test voltages for on-site testing of XLPE cable systems 30

THANK YOU FOR YOUR ATTENTION Michael Hensel HIGHVOLT Prüftechnik Dresden GmbH

MV cable test van 36 kv / 10 A Suitability of different test voltages for on-site testing of XLPE cable systems 32

HV cable test system 260/80 kv 83 A Suitability of different test voltages for on-site testing of XLPE cable systems 33

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 Cable type: Cable length: Cable capacitance: Test voltage: Test duration: 400kV XLPE 16km 2360nF 260kV 1 hour Suitability of different test voltages for on-site testing of XLPE cable systems 11