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POSITION PAPER

POWER CABLE SYSTEM TESTING Ensuring the reliability of power transmission and distribution

SAFER, SMARTER, GREENER


CONTENTS 1 EXECUTIVE SUMMARY__________________________________________________________ 4 2 INTRODUCTION _______________________________________________________________6 2.1 Power cable, accessories and cable system ................................................................................... 6 2.2 DNV GL - Energy services for power cable system ........................................................................ 6 3 AC POWER CABLE AND ACCESSORIES __________________________________________ 8 3.1 Test requirements verification .......................................................................................................... 9 3.1.1 Standards for AC cable and accessories ........................................................................... 9 3.1.2 Scope of testing ................................................................................................................... 10 3.2 Test failure statistics ............................................................................................................................ 10 3.2.1 Population data of cable system type test ......................................................................... 10 3.2.2 Test failure statistics data ...................................................................................................... 11 3.2.3 Analysis of the results of cable system type test ............................................................. 12 3.2.4 General test results of cable system test after installation ........................................ 13 3.2.5 Combination of test voltage and duration ........................................................................ 14 3.2.6 Failure statistics data and analysis results of cable system test after installation ......16 3.3 In-service failure investigation experience ................................................................................... 17 3.3.1 Failure statistics .................................................................................................................. 17 3.3.2 Root cause investigation ................................................................................................... 19


4 DC POWER CABLE AND ACCESSORIES ________________________________________________20 4.1 Introduction ........................................................................................................................................... 20 4.2 Standards for HVDC extruded cable system ..................................................................................... 21 4.3 Two DC specific requirements ............................................................................................................. 21 4.3.1 Temperature drop across the insulation ................................................................................. 21 4.3.2 Superimposed impulse voltage test ...................................................................................... 23 5 RECOMMENDATIONS TO IMPROVE STANDARDS___________________________________ 24 5.1 New insights into prequalification test ............................................................................................... 24 5.1.1 Standards ................................................................................................................................. 24 5.1.2 Recommendations ................................................................................................................... 24 5.2 Measurement of conductor temperature during type test ..................................................... 26 5.3 Recommendation for testing of outer protection for buried joints ................................................. 26 5.4 Experience with test after installation .................................................................................................. 27 5.4.1 Series resonant testing ............................................................................................................ 27 5.4.2 Additional tests of testing after installation .......................................................................... 27 6 CONCLUSION __________________________________________________________________________ 28 7 REFERENCES _____________________________________________________________________________ 30


1 - EXECUTIVE SUMMARY In recent years, significant quantities of land and submarine cable systems have been installed to satisfy the ever-increasing demand for electricity. And this will increase: society continuously demands more electricity from the grid; from today to 2050 the electricity consumption will increase by about 140% [1]. The overall European network investments is estimated to increase another 80% by 2050 [2]. Although undergrounding increases the initial costs of electric power transmission and distribution, power cable systems are typically less susceptible to outages due to external forces than overhead systems and fulfil aesthetic and environmental purposes.

04 ENERGY Power cable system testing


I

nternational studies have established a failure rate of AC XLPE land cable system (60 to 500 kV): for cables of 8,8% per 100 km per year, for joints of 0,8% per 100 components per year and for terminations of 1,3% per 100 components per year [3]. The failure of cable systems incurs enormous costs – any outage is expensive in terms of reputation as well as of service disruption. Subsea cable damages are common and extremely costly, and can jeopardize an entire windfarm's output with a single malfunction. In 2015 alone, insurance claims from cable failures totalled EUR 60 million [4]. Therefore, it is essential to install and operate a reliable power cable system in the power grid. As a world-renowned independent testing, inspection, certification and advisory organization, DNV GL including KEMA Laboratories and its Energy Advisory Team, aims to safeguard the quality assurance process of power cable systems at all stages from design, manufacturing, installation to operation by the following: Type tests verify that certain designs are adequate for normal operation  Prequalification (PQ) tests simulate the complete life cycle  Test after installation (TAI) verify the correct installation 

Monitoring during operation assists to detect early warning signs for failure  Failure analysis aims to find the root cause of a failure 

This paper summarizes the results of 905 type tests and PQ tests on AC cable and accessories carried out over a 25-year period (1993-2017) by KEMA Laboratories, and 1291 tests after installation on AC cable circuits over a 21-year period (1997-2017) by on-site testing teams of KEMA Laboratories. This paper also covers exclusive root causes based on 227 individual power cable failure investigations over a 20-year period (1997-2016) by DNV GL Energy Advisory. From the test-failure statistics it was found that around 25% (type- and PQ tests) and 10% (TAI) of tested systems initially fail to pass standardized tests. Post-mortem studies show that roughly two thirds of the failures involve cable joints and terminations, whereas in one third of the failures of cable itself is involved; the studies account for 58% of total investigated failures in (extra) high voltage.

This paper shares experiences and recommendations on interpretation and guidance of the relevant standards.

Power cable system testing ENERGY 05


2 - INTRODUCTION 2.1 Power cable, accessories and cable system A power cable is used for electric power distribution and transmission. A cable system consists of a cable and its accessories. Cable accessories (joints and terminations) are designed to control the high-voltage stress to prevent breakdown of the insulation. Based on the power system they serve, a power cable system is for AC or DC application. Several types of AC and DC cables exist based on the design of the cable and insulation material used. The insulation of most cable types are extruded cross-linked polyethylene (XLPE) and paper. Since the year 2000, XLPE cable has been the preferred cable type (87%) for almost all installed AC cables. The trend towards the use of XLPE will continue and its use will grow towards higher voltage levels [3]. Therefore, this paper mainly focuses on XLPE type of AC and DC cable. Generally, XLPE cables consist of a solid cable core, a metal screen and/or metal sheath and a non-metal oversheath (figure 1). The cable core consists of the conductor, the inner semi-conducting layer (conductor screen), the solid main insulation and the outer semi-conducting layer (insulation screen). These three

06 ENERGY Power cable system testing

layers are extruded in a single process. The conductor of power cables can be made of copper or aluminum. It can be either a solid or stranded round conductor or a segmented conductor (Milliken conductor) in order to reduce losses caused by skin effect. The cable can be designed to have longitudinal water blocking by means of barrier tapes or powder. The metal screen or sheath must be capable of carrying the short-circuit current in case of failure. It can be optionally equipped with fibers for temperature monitoring. The oversheath protects the cable during installation, serves as an anti-corrosion layer and insulates the screen from earth. Optionally it can be applied with a functional layer, such as semi-conducting skin to enable a sheath test after installation or a flame-retardant skin for installations in tunnels or buildings if required.

2.2 DNV GL - Energy services for power cable system DNV GL is well positioned as a global player within the Maritime, Oil & Gas and Energy industry. DNV GL - Energy business area is a world-leading testing, consulting and certification company for the global energy sector.


DNV GL Energy covers two service lines for power cable systems: KEMA Laboratories and Energy Advisory.  KEMA Laboratories

focus on testing, inspection and certification of electrical transmission and distribution equipment. KEMA Laboratories are located in the Netherlands, United States and Czech Republic. Power cable system testing services are mainly carried out at KEMA Laboratories in the Netherlands. It is a well-recognized conformity assessment body accredited per ISO/IEC 170201 and ISO/IEC 170252. The testing services are physically located at KEMA High-Voltage and High-Power Laboratories. Their facilities perform electrical tests on cable systems rated up to 550 kV AC and 320 kV DC (expanding investment ongoing), as well as short-circuit withstand and various non-electrical tests. Experienced test-engineers and inspectors have the extensive knowledge in the testing that customers need. KEMA Laboratories are entitled to issue Type Test Certificates.  Energy Advisory supplies services in transmission and distribution grids, power systems and markets, renewable energy, energy efficiency, energy storage, measurement and cyber security. Its services include advisory on all types of power components, such as power cable systems, transformers, switchgear, lines, insulators, bushings, rotating equipment etc. DNV GL is fullfilling the role of independent and impartial party for many years already and has therewith built up a wealth of experience in failure investigations.

1 2 3 4

DNV GL - ENERGY SERVICES  Laboratory testing services for HV, MV and LV cables

and accessories

 On-site testing services for HV and MV cable circuits

including SAT3

 Witnessing of tests, inspection services and

manufacturing follow-up including FAT4  Type test certification  Advisory services for HV, MV power cable systems including general support, problem solving, courses, training, review project documentation, independent analyses  The Smart Cable Guard system provides information of incipient discharges and failures (including their location) in MV cables [5]  Failure investigation

Conductor Conductor

Metal Metal sheaths and/or sheaths metal screen and/or Metal screen

Conductor Conductor screen screen

Insulation Insulation

Non-metal Non-metal (over)sheaths (over) sheaths

Insulation Insulation screen screen

Figure 1 - Diagram of an XLPE cable

ISO/IEC 17020: Conformity assessment – General criteria for the operation of various types of bodies performing inspection. ISO/IEC 17025: General requirements for the competence of testing and calibration laboratories. SAT: Site acceptance test. FAT: Factory acceptance test.

Power cable system testing ENERGY 07


3 - AC POWER CABLE AND ACCESSORIES The rationale for high-voltage testing is the verification of absence of defects in insulating materials due to inadequate design and/or production, verification of proper electrical field management at discontinuities like joints and terminations but also to reveal deficiencies in overall design and assembly. It is assumed that adequate high-voltage dielectric tests represent most possible HV stresses in service. For example, a lightning impulse test represents external over-voltages which are a consequence of lightning strokes; a switching impulse test represents internal over-voltages which are caused by switching operations in the power system.

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3.1 Test requirements verification As an accredited testing and certification organization of equipment for transmission and distribution of electricity, KEMA Laboratories follow various standards which exist worldwide, such as IEC (International Electrotechnical Commission), CENELEC (European Committee for Electrotechnical Standardization), international and national standards for low-voltage (LV), medium-voltage (MV), high-voltage (HV) and extra-high-voltage (EHV) cables, accessories and cable systems. 3.1.1 Standards for AC cable and accessories  IEC 60502 is applicable to cables and accessories in the 1 to 30 kV voltage range. This standard consists of three separate volumes, IEC 60502-1 for LV cables (1 and 3 kV), IEC 60502-2 for MV cables (6 to 30 kV) and IEC 60502-4 for MV accessories (6 to 30 kV). The cable part describes the construction and testing of single- or three-core cables and contains guidelines or, where appropriate restrictions. The accessory part covers the testing of indoor terminations, outdoor terminations, joints etc.  IEC 60840 is dedicated to cables and accessories above 30 kV and up to 150 kV. This international standard describes the various tests performed as routine test, sample test and type test.

In the latest version of IEC 60840 (2011), some tests previously included only in IEC 62067 have been introduced into IEC 60840, e.g. a prequalification test for cable designs with a high electrical stress. Also, cables and accessories which fall into this high electrical stress category5 can now only be tested as part of a system. Cables and accessories which do not fall into this category can still be type tested on an individual basis, i.e. as separate items.  IEC 62067 covers cables and their accessories above 150 kV and up to 500 kV. This standard describes the various tests performed for routine test, sample test and type test. This standard assumes high-stressed cable designs and consequently only accepts testing on a system basis. In addition to type tests, a prequalification test is mandatory.  CENELEC HD 620 and HD 629.1 are dealing with extruded cables and their accessories in the 6 to 36 kV range. These standards can be seen as the European counterparts of IEC 60502-2 and IEC 60502-4. HD 620 (extruded cable) consists of a common part, general requirements, and parts related to particular types of cables. All parts related to particular types of cables are a collection of national sections from the participating countries. Unlike HD 620, HD 629.1 (accessories for extruded cables) does not contain specific national parts.

5

High electrical stress category mentioned in this paper is cables where the calculated electrical field at the conductor screen is higher than 8,0 kV/mm and at the insulation screen higher than 4,0 kV/mm.

Power cable system testing ENERGY 09


3.1.2 Scope of testing The application of power cables continues to increase in circuit length, normal current and system voltage. However, with higher voltage and current rating comes higher stress, which can affect reliability and lifespan. The following tests stand out:  Type test: tests performed before supplying on a general commercial basis a type of cable system or cable or accessory, to demonstrate satisfactory performance characteristics to meet the intended application. Tests are also carried out to prove conformity with the specifications. These are intended to prove general quality of material and design of a given type of manufactured cable, accessories and cable systems. A successful type test demonstrates the adequate design in accordance with the standards. Certification applies to the subcomponent of the cable system, which implies that validity of the certificate is guaranteed only for the accessories in combination with the applied cable. Alternatively, certification of a system applies to the system only when it includes the (type of) subcomponents installed in the test.  Prequalification test: test made before supplying on a general commercial basis a type of cable system, to demonstrate satisfactory long term performance of the complete cable system. Test is simulating the complete life cycle through 180 heating cycles at 1,7 times the rated voltage.  Test after installation: test made to demonstrate the electrical integrity of the cable system as installed.  Special tests: including water tree6, quasi water-tightness test7, short-circuit test of cable and/or accessories, conductor DC and AC resistance measurement etc.

3.2 Test failure statistics This paper summarizes the results of 905 type tests and prequalification tests on cable and accessories carried out over a 25-year period (1993-2017) at KEMA Laboratories [6]. In addition, it describes the experience with series resonant testing8 of cable circuits after installation from 1997 to 2017. During the last 21 years, 1291 cable circuits have been tested, resulting in valuable information regarding e.g. the breakdowns, partial discharges that occur. It also gives insight in the combinations of test voltage and duration prescribed by the utilities. 3.2.1 Population data of cable system type test Based on 25 years test results of cable system type test, for this survey, tests are grouped as follows:  Tests per IEC 60840 and IEC 62067 have been grouped as “HV”  Tests per IEC 60502 and CENELEC HD 620 and HD 629.1 are grouped as “MV”  The group “termination MV” consists of both indoor and outdoor terminations  The “termination HV” group comprises outdoor and GIS terminations  The “joint HV” group comprises joints with and without cross-bonding 10% 16%

10%

11%

16% Cable HV

11%

CableCable HV MV

13%

CableTermination MV HV

13%

Figure 2 shows part of the test set-up for a type test on a 400 kV cable system.

30%

Termination HV

Termination MV

Termination MV

Joint HV

20%

30%

Joint HV

Joint MV

Joint MV 20%

Figure 3 - Quantity distribution of type tests per component

Figure 2 - Part of the test set-up for a type test on a 400 kV cable system

Figure 3 shows the quantity distribution of tests with respect to the various components. A distinction is made between MV components (53%) and HV components and systems (47%). Almost half of all tests were on cables (46%), either as a separate component or as part of a system. Terminations, again either tested separately or in a system, represent exactly one third (33%). For consistency in data, test results are given for the individual component, even when it has been tested as part of a system.

Long duration test (accelerated ageing test related to water treeing): accelerated ageing at increased power-frequency voltage, one 60 m active test length shall be energized at 500 Hz voltage for not less than 3000 h, followed by the cable breakdown test see Appendix G of HD 620. 7 Longitudinal and quasi radial water-tightness (lqwd): the degree of quasi radial water-tightness is determined by means of three tests on water content of the swellable tape, water absorption capacity of the swellable tape and water permeability coefficient of the non-metallic sheath, see Appendix F of HD 620. 8 Series resonant testing: the test system usually consists of a reactor which is connected in series with the cable under test, creating a series resonant circuit. 6

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400 400

Accessory

350 350

Cable

300 300

Accessory HV

100%

Cable HV

80%

250 250 60% 200 200 40%

150 150 100 100

Accessory 20%

50 50 Cable 00

IEC 60502

IEC 60840

IEC 62067

HD620, 620,HD HD HD629.1 629.1

Other HV

OtherMV MV Other

Figure 4 - Number of type tests grouped by standard

1993

1995

1997

1999

2001

2003

2005

2007

2009

2011

2013

2015

2017

Figure 5 - Failure data of HV cable and accessories per year

Accessory HV

100%

0%

60%

Cable HV

50%

80%

40%

60% 30%

40% 20%

20%

10% 0%

0% 1993

1995

1997

1999

2001

2003

2005

2007

2009

2011

2013

2015

2017

Cable HV HV Cable

Cable MV Cable MV

Termination Termination HV HV

Termination Termination MV MV

Joint HV HV Joint

Joint MV MV Joint

Figure 6 - Failure data of MV cable and accessories per year

Figure 7 - Failure rate by component

As can be seen in figure 4, most tests were performed per IEC 60502 and IEC 60840. Type tests on cable and accessories per European standards (HD 620 and HD 629.1) represent only a limited number of tests.

accessories. A typical HV test loop consists of a cable, two outdoor terminations (porcelain and/or composite), two GIS terminations in a back-to-back configuration and a (cross-bonding) joint. This means three types of accessories against only one cable, which explains why there are more accessory tests than cable tests in the HV group.

Over the 25 years, the majority of these tests are performed on extruded MV cables. This is not surprising as MV cables are considered a commodity and are made by many manufacturers around the world. Also in this MV range, a few cables were tested per the CENELEC standard. In the HV range, tests per IEC 60840 amount to an average of roughly 5 cables each year. Once again, the number of tests performed for the IEC 62067 standard is limited (average 2 to 3 per year) by the fact that this standard was only published in 2001. KEMA Laboratories have tested an average of 21 cable accessories per year. Most HV test are based on

3.2.2 Test failure statistics data This paper covers type tests on 905 components during the last 25 years. These components come from a wide variety of manufacturers, from emerging to well-established. Figure 5 shows failure data of HV cable and accessories from 1993 to 2017. Figure 6 gives failure data of MV cable and accessories at the same period. Test failure rates of cable system type tests per component are displayed in figure 7. It shows that just over 10% of all type tests on MV cables result in failure whereas for HV cables this number is 26%.

Power cable system testing ENERGY 11


No.

Type of cable

Rated voltage U09/U10[kV]

Conductor cross-section [mm2]

Ei [kV/mm]

Eo [kV/mm]

1

Cu/XLPE

19/33

800

2,78

1,91

2

AL/XLPE

38/66

800

4,50

2,85

3

Cu/XLPE

64/110

1200

5,25

3,12

4

Cu/XLPE

76/132

1200

5,67

3,23

5

Cu/XLPE

76/132

2500

4,53

2,79

6

Cu/XLPE

220/380

2500

10,7

5,92

Table 1 - Calculated electrical stress for different cables11

Regarding terminations, 53% of all MV terminations fail, against 16% of all HV terminations. Of the tested MV joints 32% fail in a test, against 42% of the HV joints. In the HV category, the failure rate of terminations is just half that of joints (16% versus 32%, the latter mostly due to failure during tests of outer protection for joints), while in the MV category the opposite is true but both numbers are higher at 53% for terminations and 42% for joints. 3.2.3 Analysis of the results of cable system type test Based on the information from figure 5 to 7, the following analysis can be made:  HV cables display a significantly larger failure rate than MV cables. Based on formulas of the calculated nominal electrical stress at conductor screen (Ei) and at insulation screen (Eo), this is to be expected, because the electrical stresses are generally higher in HV cables than in MV cables (e.g. see table 1).  The higher stresses applied on HV cables make them more sensitive to improper material handling and processing during manufacturing. The lower stresses imposed on MV cables make them more tolerant to irregularities at the screens or insulation contaminants.  Another reason for a lower test failure rate of MV cables might be that they are not exposed to combined electrical and thermal stresses as opposed to HV cables. For example, the electrical type tests in both standards differ with respect to the heating cycle application: (MV) cables in the IEC 60502-2 range are subjected to heating without applied voltage, while IEC 60840 and IEC 62067 range (HV and EHV) cables are subjected to both heating current and continuous voltage during the heating cycle test. The absence of voltage during the MV cable heating cycle test is somewhat compensated by performing a 4-hour, 4 U0 test after the lightning impulse test.

9 10 11

The 4-hour voltage test is at ambient temperature only, so the combined electrical stress and thermo-mechanical effects are not tested. However, the cable is included as part of the test loop when testing MV accessories and it is thus subjected to heating cycles under voltage. So far there have been no failures in the cable during type testing of MV accessories. Therefore, the difference in failure rate between MV and HV cables may be attributed to the lower stress in MV cables.  Accessories consistently show a higher failure rate than cables. Regarding accessories, figure 7 indicates a lower failure rate for HV compared to MV components. This is probably due to accessories being designed and installed more carefully to handle the higher stresses. An accessory must handle the high electrical stress in the cable insulation, i.e. to avoid a local increase of stress. Also, stresses parallel with the interface should be kept to a minimum. Next to this, accessories must cope with thermo-mechanical forces exerted by cables. This interaction between cable and accessory can place significant demands on accessories.  Several type tests, especially in the HV range (IEC 60840), have been performed on cable systems comprised of components that had been type tested successfully before on an individual-component basis. However, the combination of the components into a cable system, not previously tested, may show negative results. Experience demonstrates that a successful system type test is not guaranteed, when the individual components passed. This is because of the demanding interaction between cable and the accessories.  MV terminations show a much higher failure rate than HV ones. MV terminations are subjected to additional tests that HV cables do not have to undergo, such as salt fog test for outdoor termination and humidity

U0: rated r.m.s. power-frequency voltage between each conductor and screen or sheath for which cables and accessories are designed. U: rated r.m.s. power-frequency voltage between any two conductors for which cables and accessories are designed. Source from cables tested by KEMA Laboratories; calculation values (Ei and Eo) based on formulas in clause 6 of IEC 60840.

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tests in climatic chambers for indoor termination. Yet, the individual failure cause suggests that these additional tests are not solely responsible for the higher failure rate.  Compared to HV terminations, HV joints show a twice as high failure rate mostly due to tests of outer protection for joints. These tests are performed on HV joints which are claimed to be suitable for direct burying. Requiring the joint to be immersed in water, appears to be quite demanding. Around 40 - 50% of test objects fail. For MV joints, the test sequence contains heating cycles while immersed in water.

Most of the testing is on newly installed or replaced cable circuits. From the year 2000 testing after installation using series resonant test sets has become common practice. Figure 10 shows the distribution of the cumulative circuit lengths as a function of time. Over 21 years the average circuit length is 150 km per year. In the last five years, the average per year is 258 km. What this suggests is that in recent years, the total length of cable circuits is increased due to land cable connected to sea cable. The individual lengths of the tested cable circuits show a large variation. Not only short cable circuits inside a substation have been tested, also long cables covering a large (20 - 50 km) distance between two substations. Most tested circuits are below 15 km. The longer lengths (above 40 km) are prominent on 132 kV and 150 kV circuits.

31%

31% U≤110 kV

U ≤ 110 kV

58%

110 kV<U≤ 150 kV

110 kV < U ≤ 150 kV

58%

U > 150 kV

Figure 8 - Population of distribution of test after installation per voltage range

U ≤ 110 kV

110 kV < U ≤ 150 kV

U > 150 kV

100

100 80 80 60 60 40 40

U>150 k

20 20

0

110 kV< kV

1997 1997 1998 1998 1999 1999 2000 2000 2001 2001 2002 2002 2003 2003 2004 2004 2005 2005 2006 2006 2007 2007 2008 2008 2009 2009 2010 2010 2011 2011 2012 2012 2013 2013 2014 2014 2015 2015 2016 2016 2017 2017

When looking more in detail to the number of tested circuits through the years, the distribution as given in figure 9 can be made.

11%

Figure 9 - Number of tested circuits from 1997 to 2017

U ≤ 110 kV

110 kV < U ≤ 150 kV

U > 150 kV

350 350 300 300 250 250 200 200 150 150 100 100

U>150 kV

50 50

110 kV<U kV

00 1997 1997 1998 1998 1999 1999 2000 2000 2001 2001 2002 2002 2003 2003 2004 2004 2005 2005 2006 2006 2007 2007 2008 2008 2009 2009 2010 2010 2011 2011 2012 2012 2013 2013 2014 2014 2015 2015 2016 2016 2017 2017

3.2.4 General test results of cable system test after installation In total, 1291 tests have been performed within the period of 21 years with a mobile voltage source on cable systems following installation or repair: test after installation (TAI). For this paper, tests are grouped as three voltage ranges: U ≤ 110 kV, 110 kV < U ≤ 150 kV and U > 150 kV respectively. Figure 8 shows the population distribution of tests with respect to the three voltage ranges. More than half (58%) of all tests were performed on cables (110 kV < U ≤ 150 kV). The cables in the range (U ≤ 110 kV) comprise almost one third (31%) of the total tests. Only a relatively small number of all tests (11%) were on cables above 150 kV because of the generally limited number of projects compared to the lower voltage ranges.

11%

Figure 10 - Total length (km) of tested circuits from 1997 to 2017

Power cable system testing ENERGY 13


120

Duration [minutes]

100 80 60 40 20 0 0,5

0,9

1,3

1,7

2,1 2,5 2,9 Test voltage [ x U0]

3,3

3,7

4,1

Test voltage [x U0]

Figure 11 Distribution of the combination of test voltage and duration for all voltage ranges (in total 1291 tests)

80

Duration [minutes]

70

IEC

60 50 40 30 20 10 0 0,5

1

1,5

2 2,5 Test voltage [ x U0]

3

Test voltage [x U0]

3,5

4

Figure 12 Distribution of the combination of test voltage and duration for cables (U â&#x2030;¤ 110 kV) (in total 383 tests)

3.2.5 Combination of test voltage and duration This paper describes the experience with series resonant testing of cable circuits after installation. It also gives insight in combinations of test voltage and duration prescribed by the utilities.

It should be mentioned here that the large variety of test conditions shown in these figures, reflects local (national) ideas about test after installation. Nevertheless, it gives an impression about the various combinations of test voltage and duration.

In figure 11 the distribution of combination of test voltage and duration for all voltage ranges from 1997 to 2017 are shown. The circles show the number of tests with particular combination of voltage and duration.

In figure 12, figure 13 and figure 14 the various combination of test voltage and duration are shown, based on the three voltage ranges.

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120

Duration [minutes]

100 80

IEC

60 40 20 0 0,5

0,7

0,9

1,1

1,3

1,5

1,7

1,9

2,1

2,3

2,5

Test voltage [x U0]

Figure 13 Distribution of the combination of test voltage and duration for cables (110 < U â&#x2030;¤ 150 kV) (in total 721 tests)

120

Duration [minutes]

100 80 60

IEC

IEC IEC

IEC

40 20 0 0,5 0,6 0,7 0,8 0,9 1 1,1 1,2 1,3 1,4 1,5 1,6 1,7 1,8 1,9 2 2,1 Test voltage [ x U0]

Test voltage [x U0]

When varying the combinations, one should expect a longer test duration when choosing lower test voltages. Most of the circles in figure 11 fulfils this expectation. However, apart from a few small circles, some large circles except one based on IEC in figure 12 are exceptions. On the other hand, these circles relate to specific projects. For this reason, the choice of the combination of voltage and duration in practice, also depends on the on-site circumstances, local requirements, age of the circuits, etc.

Figure 14 Distribution of the combination of test voltage and duration for cables (U > 150 kV) (in total 187 tests)

For figure 13 the emphasis is significantly around 1,7 U0 during 60 minutes which is in line with IEC 60840:2011. This standard also allows to apply a voltage of U0 for 24 h as an alternative option. For figure 14 the emphasis is 1,1 U0, 1,3 U0, 1,4 U0 and 1,7 U0 during 60 minutes. These are all in line with the recommendations (based on a particular voltage level) given in clause 16.3 of IEC 62067:2011.

Taking the above into account, for the test voltage and duration in figure 12 for cables (U â&#x2030;¤ 110 kV) the majority is specified at 2 U0 during 30 to 60 minutes and 2,5 U0 during 30 minutes. In IEC 60840:2011, it recommends testing at 2 U0 during 60 minutes for these cables.

Power cable system testing ENERGY 15


3.2.6 Failure statistics data and analysis results of cable system test after installation This paper covers test after installation on 1291 tests during the last 21 years from 1997 to 2017. In figure 15, it is shown that almost one third of the failed tests is on cable systems with voltage less than or equal to 110 kV. For HV cable systems (U > 150 kV) the figure is 25%. The failure rate between these two categories (110 kV < U ≤ 150 kV)is nearly half of total failed tests. The average failure rate is almost 10% of three voltage ranges. The voltage range above 150 kV has the highest failure rate of 16% (see figure 16).

The failure statistics information gives an indication of test results; it cannot be used to draw conclusions on general performance of newly installed cable systems, because the various circuits have been tested with various combinations of test voltage and duration (see clause 3.2.5). Failure statistic results are an indication of the value of this test after installation and may help utilities to determine what tests might be appropriate to be confident about newly installed or replaced high voltage cable circuits. Figure 17 shows the impression of test set-up of a test after installation.

16% 14%

25% 25% 25%

32% 32% 32%

12% 10% 8%

U≤110 U≤110kV kV 43% 43%

43%

4%

110 110kV<U≤ kV<U≤150 150kV kV

2%

U>150 U>150 kV kV

0%

Figure 15 - Distribution of failure per voltage range

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6%

Total

U≤110 kV

110 kV<U≤ 150 kV

Figure 16 - Failure rate of testing after installation

U>150 kV


Figure 17 - Impression of a test set-up of test after installation

3.3 In-service failure investigation experience Since KEMA was established in 1927, with the main aim to improve reliability of underground cable systems, this service is increased and improved. On a regular basis, failures of underground and submarine power cables in service are being investigated at DNV GL's dedicated power failure investigation facility. 3.3.1 Failure statistics Failures as mentioned here, can be a full breakdown, but can also be a defect found by a diagnostic test or a general failure of any of the cable's components such as mechanical deformation, optical fiber anomalies, etc. Note that the information presented here is based on the failure investigations performed by DNV GL only [7].

Cable 36%

Joint 36%

In figure 18, the distribution of investigated failures (total 227) between the different components is shown. This represents the investigation period 1997 to 2016. Roughly two thirds of the failure investigations involved cable joints and terminations, and one third involved failures of the cable itself. Thus, the cable accessories (joints and terminations) are the components investigated most. Figure 19 shows the distribution of the failures over the voltage ranges. The majority of investigated failures concerns (E)HV cables and accessories (U > 36 kV). The number of cases for low-voltage application is very small.

(E) HV 58%

LV 3%

MV 39%

Termination 28%

Figure 18 - The distribution of failures between components investigated

Figure 19 - The failure rate per voltage range involved

Power cable system testing ENERGY 17


KEMA High-Voltage Laboratory - the Netherlands Dimension of main test hall: overall dimension 85 x 50 x 19 m (L x W x H) with 16 MV bays, 8 HV bays and 4 EHV bays. Large Faraday Cage with very low PD noise level. Outdoor test bays: Three independent large outdoor test facilities for prequalification tests on power cable systems up to 400 kV. Mobile test equipment: Ready to go test trailers for on-site tests for cables systems, transformers, generators and complete substations. Testing capabilities: Various AC voltage sources up to 900 kV, DC voltage sources up to 900 kV, three LI and SI impulse generators up to 2600 kV, heating current transformers up to 50 kA. All relevant state-of-the-art measuring systems for AC, LI, SI, PD, RIV, temperature rise tests (cycled), capacitance, tan delta and others. Specialized in type tests for:  AC and DC Cables and accessories, LV, MV up to EHV  Insulators – porcelain, glass and composite  Power and distribution transformers  Instrument transformers, CT, VT and CVT  Switchgear – all types MV, HV and up to EHV cable testing:  Type testing as per international standards or special test requirements on cable systems up to and including 550 kV  Accelerated 500 Hz ageing test for water treeing of XLPE insulating materials  Prequalification tests as per (IEC 62067) up to 400 kV cable systems. Three outdoor test facilities with cable gallery and directly buried for outdoor terminations, joints and GIS terminations  Two transportable on-site cable commission test sets for AC voltage tests up to 400 kV / 80 A (18 – 300 Hz) with series resonant voltage source  Field test facilities for generators, transformers and substations  Test facility for HVDC-cables up to rated voltage of 320 kV DC Calibration of HV measuring systems: Accredited for calibration of high voltage DC, AC, impulse and current measuring systems (Certified Reference Laboratory)

Inspection Services – the Netherlands Accredited for witnessing of type tests and factory acceptance tests of Medium- and High-Voltage components at non-KEMA laboratories all over the world.

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Unknown 11%

Unknown 8%

Ageing 9% Design 11%

External damage 17%

Production 33%

Installation 19%

Ageing 8%

Production 8%

Design 13%

External damage 6% Installation 57%

Figure 20 - General failure causes for cables (from failure investigation studies)

Figure 21 - General failure causes for accessories (from failure investigation studies)

3.3.2 Root cause investigation The reason to conduct power failure investigation can be following [7]:

5. Production: the failure occurred because of an error during production (handling or material) 6. Unknown: the cause of the failure remained unclear Note that experience learns that also failures that happen after decades of operation can still be caused by errors in design, production and installation. The distribution of failure causes identified in DNV GL studies for cable failures and for accessories are depicted in figure 20 and figure 21 respectively.

 Technical

reasons: avoidance of future failures, knowledge where to improve: design, manufacturing, testing, installation and operation  Economical/political reasons: penalties, indirect costs, reputation damage, rebuild trust, etc.  Insurance reasons: the party responsible for the failure must be identified, settle liability claims  Safety reasons: knowledge on risk and failure of similar components of installation, precaution to reduce safety risk

In practice, it is not a certainty that a failure investigation yields one clear root cause. Sometimes it is a combination of several causes and sometimes only the likelihood of the various possible root causes can be evaluated. Root causes of failures have been categorized into 6 generalized groups, being: 1. 2. 3. 4.

Ageing: the failure occurred because of natural ageing of the component Design: the failure occurred because of an inadequate design of the component External damage: the failure occurred because of an external foreign object damaging the component Installation: the failure occurred due to an inadequate installation (technique)

Most failures in (investigated) cables have their origin in the production process itself. Some typical causes that are seen include mechanical damage during handling of the cable and defects in the insulation. After production related issues, installation and external damage are the most likely causes of failures. Obviously, cable accessories heavily rely on the way the installation is done. A poorly executed installation may significantly shorten the life of a cable system. Knowledge obtained on these investigations, relevant information has been collected and used to create insight into the causes that lead to failures and, where possible, to identify common factors. This has led to improvements in design, production, testing, installation and operation of power cable systems, as knowing the root cause of failures is crucial for improvement of the complete chain. Nowadays, DNV GL also has a 24/7 service a framework agreement possibility to ascertain even more that the added value of such investigations is maximized.

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4 - DC POWER CABLE AND ACCESSORIES 4.1 Introduction High Voltage Alternating Current (HVAC) power transmission is currently the primary means of transmission of electricity, but such a power system reaches certain limits for long distances, especially in cable connections. Therefore, transmission infrastructures operating as High Voltage Direct Current (HVDC), which are suitable for long distance bulk power are getting more attention. European Offshore Grid Infrastructures Study estimates that there will be 20,000 km of HVDC cable installed in Northern Europe by 2030. Studies show the failure rate of an average of 0,5 â&#x20AC;&#x201C; 2,0 faults per 1,000 km of installed subsea cable per year [8].

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In addition to HVAC capabilities of KEMA laboratories, investments lead to the extension towards an HVDC test facility. Currently most tests are carried out in accordance with IEC standard, CigrĂŠ brochures (see page 21) or customer specifications.


4.2 Standards for HVDC extruded cable system  Cigré Technical

Brochure 496 (Recommendations for testing DC extruded cable systems for power transmission at a rated voltage up to 500 kV) is a worldwide technical guide for testing a DC extruded cable system. This technical document is prepared by working group B1.32 and published in April 2012.  IEC 62895 (Cables with extruded insulation and their accessories for rated voltages up to 320 kV for land applications: test methods and requirements) is the first international standard for DC extruded cable system testing and is issued in 2017.

4.3 Two DC specific requirements Two specific requirements in testing of an HVDC cable system, different from an HVAC cable system exist:

- max) specified maximum temperature drop (∆0 across the insulating layer of a cable  a superimposed impulse voltage12 test  a

The next paragraph focuses on these two topics based on practical testing experience.

12

4.3.1 Temperature drop across the insulation In an HVAC cable, the electric field distribution depends on capacitances which are dominated by the permittivity of the insulation material. Contrary to an HVAC cable, the field distribution in an HVDC cable is determined by the resistivity of the insulation material. When the conductor is at a high temperature, there will be a temperature gradient across the insulation. For HVAC cables, this will not influence the electric field distribution as the permittivity is rather constant in this temperature range. However, the conductivity of the insulating material is strongly depending on temperature and consequently has a big influence on the field distribution in HVDC cables. To avoid too high field stresses in the insulation of HVDC cables, the manufacturer normally declares a maximum temperature drop across the insulation (∆0-max) [9]. The maximum temperature difference across the -max ,(excluding the semiconducting screens) insulation, ∆0 is provided by a cable manufacturer and depends on the insulating material (e.g. XLPE) and cable design. When testing, this temperature difference needs to be maintained within a specified limit during a steady state of a load cycle. In practice, it is not possible to measure ∆0-max directly, because of inaccessibility of the insulation for temperature sensors (presence of the semiconducting layers). By application of heat transfer theory, a simple formula is derived to calculate ∆0-max [10]. Figure 22 shows a test set-up of HVDC cable type testing.

Superimposed impulse voltage: is a composite voltage test, which means that superposition of two different test voltages generated by the suitable connection of two separate test voltage sources, reference IEC 60060-1.

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CABLE SYSTEM (thermally insulated)

Termination Termination A-side A-side

CABLE SYSTEM (thermally insulated)

Termination Termination B-side B-side

Pos.11 Pos.

Pos. Pos.1515

Pos. Pos. 14 14

Pos. Pos. 22

Pos. Pos. 3 3

Pos.12 12 Pos.

Pos. 11

Pos. 11

Pos. Pos.13 13

Heating Heatingtransformers transformers Pos. Pos.44

Pos. Pos.10 10

Pos. Pos.6 6

Pos.55 Pos.

Pos. 7 7 Pos.

Pos.88 Pos.

Joint Joint22

Pos. Pos. 9 9

Joint 1 Joint 1

REFERENCE REFERENCECABLE CABLE (thermally insulated)

(thermally insulated) Thermal Thermalinsulation insulation

Pos. 16-17 Sheath,Sheath, Pos. 16-17

Conductor, Pos. 18-20 Conductor, Pos. 18-20

Insulation screen, Pos. Pos. 21-26 Insulation screen, 21-26

Figure 22 - A test set-up of HVDC cable type testing: indicated position (Pos.) of temperature sensors

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4.3.2 Superimposed impulse voltage test An HVDC cable in operation experiences switchingand lightning impulse (SI/LI) overvoltages, that are superimposed on the DC service voltage. In fact, DC insulation systems are prone to space charges. This needs to be taken into account when performing impulse tests and requires the superposition of two different voltages: DC and impulse. This unique test requirement is introduced for HVDC cables only. In the following a test circuit is described with two separate test systems (a DC generator and an impulse generator), with a test cable system in between. The test cable system consists (in this case) of three pieces of HVDC XLPE cable (50 m in total) assembled with two outdoor terminations and two joints. A major complexity is to prevent damage of each HV test system due to the presence of the other. Therefore, a sphere gap is installed to block DC voltage from the impulse generator; a damping impedance protects the DC generator from the switching/lightning transients. A superimposed impulse voltage test is essential for an HVDC cable system to be qualified for two different HVDC converter technologies, Line Commutated Converter (LCC) and Voltage Source Converter (VSC). In figure 23, the laboratory test set-up is shown for switching/lightning impulse (SI/LI) voltage superimposed on an HVDC voltage. A broadband voltage divider measures the combined stresses for an HVDC cable system.

Figure 23 - The laboratory test set-up for superimposed voltage test

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5 - RECOMMENDATIONS TO IMPROVE STANDARDS 5.1 New insights into prequalification test 5.1.1 Standards With the first edition of IEC 62067 (EHV cables) in 2001 the PQ test was introduced. This test is to prove the long-term reliability of a cable system. The test subjects cable designs and their accessories to heating cycles under voltage. The cable is usually installed under various laying conditions, resulting in different conductor temperatures. The PQ test requires the voltage (at a level of 1,7 U0) to be applied during one year and at least 180 heating cycles, each with a minimum of 24 hours. The cables in this class require testing of a minimum of 100 m of full-sized cable including at least one of each type of accessory. In the 4th edition of IEC 60840:2011, the PQ test of the cable system was also introduced. According to this standard, PQ tests shall be performed on cable systems when the calculated nominal electrical stresses at conductor screen will be higher than 8,0 kV/mm and/or at the insulation screen higher than 4,0 kV/mm respectively. Tests shall be conducted on the complete cable system with a minimum of 20 m of full-sized cable including at least one of each type of accessory.

24 ENERGY Power cable system testing

5.1.2 Recommendations KEMA Laboratories have performed the PQ test since 2001 at their facility in Arnhem, the Netherlands, and have witnessed this test at various manufacturers and test facilities world-wide. Based on 16 years experience, a guideline is given to execute this PQ test in accordance with the requirements in the standard and in line with the background for this test. The standard requires the various laying conditions to be incorporated in the test set-up and at the same time requires the cable conductor to reach the maximum operation temperature. Because various laying conditions result in various external environments, the maximum operation temperature cannot be attained along the whole length of the test setup. The cable in a non-ventilated duct will reach a different temperature than a directly buried cable or a cable installed in a tunnel as they carry the same current. Temperature differences simply cannot be avoided, but the standard does not give guidance on what difference in (conductor/sheath) temperature is acceptable.


During the heating cycle voltage test, the voltage is to be applied during 8760 hours (365 days) with at least 180 heating cycles. A heating cycle requires at least 8 hours heating and at least 16 hours of natural cooling. The practice issue is that to decide an appropriate duration per cycle. Testing per a standard should be independent from the location where it is performed. To remove ambiguity in the present requirements for a pre-qualification test, the recommendations below should be incorporated in the standard [11].  Different

laying conditions may only be combined in a single test loop when these conditions have a similar thermal effect for the cable. Installation conditions with obviously largely different thermal environments, burial in the ground versus exposure to the sun, must not be combined in a single test set-up but must be tested separately.  The reference cable should be installed near the test loop to ensure an equal thermal environment. This is essential to obtain a reliable estimate of the conductor temperature of the test loop at the location where the hot-spot is assumed, provided the currents are kept equal at all times.

 An

interruption of the current in the heating period is only acceptable if the 2-hour stable period can still be reached within the same time as is the case when no interruption occurs. Otherwise, the particular heating cycle is invalid and must be repeated.  An interruption in the test voltage during the stable heating period is not acceptable. The particular heating cycle is invalid and must be repeated.  An interruption in the test voltage during the cooling period is not acceptable when the conductor temperature differs more than 15 K from the soil temperature near the test loop (undisturbed soil temperature).  An interruption of the test voltage during the cooling period is only acceptable when the conductor temperature is within 15 K from the soil temperature near the test loop (undisturbed soil temperature). The particular heating cycle is still valid and does not need to be repeated. In this case, the duration of the voltage interruption must be added to the total test time (initially one year) to compensate for this interruption.

Power cable system testing ENERGY 25


Figure 24 and figure 25 show the test layout of prequalification tests.

5.2 Measurement of conductor temperature during type test

Figure 24 - HV termination of the prequalification test set-up of PQ1

A heating cycle test is an essential part of type test procedures for cables and their accessories, usually combined with voltage application. The concept of two separate loops, one test loop and a reference cable, is maintained in the same conditions. The conductor temperature measurement is done in the reference cable, since the tested cable conductor is inaccessible because voltage is applied, whereas the reference cable only carries current. The authors' procedure to install the thermocouples is [12]:  A small

PQ3

PQ1

PQ2 HVL

Figure 25 - The test layout of PQ tests

section of the insulation is carefully taken out and some thermocouples are fixed between the wires of the stranded conductor or in the interface between a solid conductor and conductor screen (see figure 26).  The insulation and all other outer layers taken out are then inserted back which restores the original thermal characteristics.

5.3 Recommendation for testing of outer protection for buried joints Annex G of IEC 60840 and IEC 62067 describe the tests of outer protection for (buried) joints. The aim of this test is to check the water tightness of a joint when it is subjected to temperature cycling.  Based

Figure 26 - Conductor of cable exposed to insert thermocouples

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on many years experience [13], approximately 50% of the joints subjected to this test fail. This test is not only to check the water tightness of a joint, but also to stress sheath sectionalizing insulation with DC and impulse voltage tests. Therefore, if joints will be installed in sand, which is mostly dry condition, still this test is required because of a probability of (occasional) moisture ingress over time.  A buried joint is rarely surrounded by a dry environment. In case the cable and joint are not to be subjected to wet conditions in service (i.e. not directly buried intermittently or continuously immersed in water), water immersion tests may be omitted as stated in above mentioned IEC standards. But if the joint has its sheath sectionalizing insulation, the thermal pre-conditioning, DC and lightning impulse withstand tests across the sheath sectionalizing insulation as well as the earthed exterior of the joint are still required for the dry situation.


5.4 Experience with test after installation When the installation of the cable system has been completed, a DC voltage test of the oversheath and an AC voltage test of the insulation are recommended in IEC 60840:2011 and IEC 62067:2011. Oversheath testing checks if the cable system has suffered any harmful damage during laying or backfilling. AC testing helps to ensure that the different installation and civil work methods are correct. 5.4.1 Series resonant testing Test after installation is to check the correct installation of the circuit. The commissioning of new HV cable systems was originally performed with DC voltage and later with oscillating voltages13[14]. Nowadays DC voltage is recognized as being not effective and even harmful for XLPE cables and oscillating voltages have become obsolete for HV cables. Classical AC sets require a test transformer which is impractically big and heavy in order to test long cable lengths, because of the high capacitive currents involved [14]. The alternative, series resonant testing (the test set is lighter than a traditional transformer), is now widely accepted, which is not only reflected in GIGRE publications, but also in today’s standards and requirements. Energizing a HV cable requires a large amount of reactive power. To avoid the need of a transformer capable of supplying this reactive power, the cable is energized by means of a resonance circuit. Resonance is achieved when the following equation is fulfilled.

∫– =

1 2π √ LC

The capacitance (C) is naturally fixed by the specific capacitance of the cable and the length of the circuit. Resonance may be achieved by adjusting the inductance (L) when testing with a fixed frequency f (50/60 Hz) or by adjusting the frequency of a tuned series resonant test set. These sets are common for testing cables. The requirement of IEC 60840:2011 reads the waveform shall be substantially sinusoidal and the frequency shall be between 20 Hz and 300 Hz.

13

When resonance is achieved, the reactive power swings back and forth between the cable and the inductor [15]. Only the losses such as dielectric and resistive losses and the losses associated with corona must be supplied by the exciter transformer. This series resonant testing (AC test) is proven to be most effective and not harmful for sound circuits. Experience has shown that a proper combination of test voltage and duration can indeed detect incipient failures. The combination of such an on-site test with partial discharge measurements gives additional information about accessories installed in the circuit. 5.4.2 Additional tests of testing after installation  Partial discharge (PD) measurements combined with AC test: PD measurements are not only valid as a diagnostic tool for MV cables, but can also be applied for additional indication of weak spots during a commissioning test [16]. After more than a decade experience, it shows that on-site PD measurements can reveal additional defects in cable systems with an average background noise level between 20 pC and 30 pC. However, experience has shown that these PD measurements are not possible in all circumstances. In case of bad weather, i.e. continuous rain, and, in substations where space is a limiting factor and neighboring circuits are near, higher disturbances can be experienced. It is possible to overcome obstacles as fences by proper shielding measures and maintaining a sufficiently large distance by using sticks for support and/or ropes for suspension for the HV connection.  Other tests and measurements that may be performed in conjunction with an AC voltage test. They may give information of circuits, such as impedance measurement, contact resistance measurement, verification of cross-bonding, etc. In addition, the cable capacitance also can be measured.  The Smart Cable Guard system for MV network: this is an advanced monitoring tool that accurately provides information on (partial) discharges and locates defects in MV cable network [5]. It has the following functions: real-time detection and localization of the development of partial discharges from defects and monitor its development over time; fault detection and localization.

Oscillating voltages: this method involves charging the test object (the cable) with a DC supply, after a few seconds, when test voltage is reached, a solid-state switch with fast closure time creates a series resonant circuit from the test object and air-core inductor. The circuit produces a damped oscillation at resonant frequency according to selected inductance of the air-core inductor.

Power cable system testing ENERGY 27


6 - CONCLUSION Testing and certification of cable network components will mitigate risks of failure in operating cable networks and systems. A survey of 25 years of type tests statistics from KEMA High-Voltage Laboratory shows that on average 25% of tests on cable system components initially fails to pass standardized type- and prequalification tests. Over 40% of all type tests of MV accessories result in a failure. The testing after installation of cable systems shows that almost 10% of newly installed or replaced systems fail to withstand the test voltage applied for a defined duration. Improvements in materials and production techniques are ongoing. However, the improvements do not noticeably decrease the failure rate. Considering the competitiveness of the market, the improvements have probably led to the realization of more cost-effective designs for cables and accessories rather than enhanced performance. In relation to these developments, type testing, PQ testing and testing after installation are valuable to prevent future problems. Furthermore, despite the trend of manufacturing HV cables by manufacturers having solid experience in the MV, this does not necessarily lead to successful HV designs. Moreover, the availability of ready-made cable production facilities does not imply reliable products when manufacturing experience across the whole quality circle is lacking. Improvements or changes in materials and/or design, may result in seemingly insignificant modification of components. Nevertheless, a new type test may still be needed. Type testing must be an ongoing process and, industry wide, the validity of a type test certificate should be limited to 5 years, or shorter in case of a change in design or material. In case all prevention actions fail, and a component fails during testing or operation, a high quality and independent failure investigation is crucial to settle claims, rebuild trust, enable improvement of reliability and mitigate risks of re-occurrence.

28 ENERGY Power cable system testing


Power cable system testing ENERGY 29


7 - REFERENCES [1] DNV GL, "Renewables, power and energy use forecast to 2050: Energy Transition Outlook", 2017 [2] Eurelectric, "Power distribution in Europe facts & figures", 2013 [3] CIGRE Technical Brochure 379, "Update of service of experience of HV underground and cable systems", CIGRE WG B1.10, 2009 [4] M. Kurkowska, "Cable malfunctions: what lessons can be learnt from recent failures?", Offshore Cabling, 2017 [5] DNV GL, "Smart cable guard 2.0," 2016 [6] E. Pultrum, H. He and R. Gruntjes, "Type testing cable and accessories, a must", 23rd International Conference on Electricity Distribution, 2015 [7] B. van Maanen, C. Plet, P. van der Wielen, S. Meijer, F. de Wild and F. Steennis, "Failure in underground power cables - return of experience", Jicable conference, 2015 [8] M. Kurkowska, "Cable malfunctions: the offshore industry's burning problem", Offshore Cabling, 2018 [9] A. S. Horeth, "Demands of dielectric testing of HVAC and HVDC power cables with extruded insulation", CIGRE AORC Technical meeting and International Conference on Global Trends in the Development of Power T&D System including Smart Grid, 2016 [10] H. He and W. Sloot, "Testing a 320 kV HVDC XLPE Cable System", INMR World Congress, 2017 [11] E. Pultrum, W. Sloot, J. Fernandez and R. Smeets, "How to perform a prequalification test - interpretation of the standard," CEPSI, 2016 [12] E. Pultrum and W. Sloot, "New approach to measure conductor temperature during type test", Jicable conference, 2007 [13] E. Pultrum, P. Kuijpers and W. Sloot, "Tests of outer protection for buried joints, experience and recommendation", Jicable conference, 2011 [15] Electra No. 173, "After laying tests on high voltage extruded insulation cable systems", CIGRE working group 21.09.1997 [16] E. Pultrum, S. Verhoeven and N. van Schaik, "Experience with after laying test", Jicable conference, 2013

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THE AUTHORS

Hong He Innovation Engineer KEMA Laboratories Ms. Hong He received her MSc degree in electronic and information engineering from Xi’an Jiaotong University in China. She has gained experience of inspection services for electrical equipment after graduation. Since 2010, she joined DNV GL's KEMA Laboratories as an inspector for MV/HV cable and accessories testing. In 2013, she became a test engineer in high voltage laboratory, mainly focused on MV/HV cable, accessories or cable system type tests, power transformer and switchgear testing etc. Currently she is involved in the innovation projects for extending company testing activities including HVDC cable system type test etc.

Edwin Pultrum Senior Test Engineer and Inspector KEMA Laboratories Edwin Pultrum received his MSc degree in electrical engineering at the Technical University of Delft. In 1993 he joined the R&D department of legacy KEMA where his activities were focussed on topics as test after installation for HV cables and diagnostic field testing for both MV and HV cables. In 2000 he joined KEMA Laboratories. He has extensive experience in site acceptance tests on cables and type tests on various components for a wide variety of international manufacturers and utilities. He is active in CIGRE and standardization (IEC, Cenelec) and an international recognized expert in the field of cables and accessories. Next to his activities as a test engineer, he is a qualified Inspector with specialized background in medium- and high-voltage cables and medium- and high-voltage cable accessories and he is one of calibration experts for high voltage measurement equipment at KEMA Laboratories.

René Smeets Service Area Leader KEMA Laboratories René Peter Paul Smeets obtained a PhD degree for research work on switchgear. Until 1995, he was an assistant professor at Eindhoven University, the Netherlands. During 1991 he worked for Toshiba Co. in Japan in the development of vacuum interrupters. In 1995, he joined KEMA, the Netherlands. Currently, he is Service Area Leader with KEMA Laboratories, dealing with innovation and technology management. In 2001 he was appointed parttime professor at Eindhoven University, the Netherlands. In 2013 he became vice professor at Xi’an Jiaotong University, China. In 2008 he was elected Fellow of IEEE. He is convener, secretary and member of several CIGRE working groups, as well as convener of two IEC maintenance teams on high-voltage switchgear. Since 2008 he is the chairman of the ‘Current Zero Club’. In 2014, he published the book ‘Switching in Electrical Transmission and Distribution Systems’ with John Wiley UK. He got six international awards, authored more than 200 international papers on several aspects of power switching and testing technology, and presented many training courses all over the world.

Power cable system testing ENERGY 31


SAFER, SMARTER, GREENER

DNV GL - Energy Utrechtseweg 310-B50 6812 AR Arnhem The Netherlands Tel: +31 26 356 9111 Email: contact.energy@dnvgl.com www.dnvgl.com

DNV GL DNV GL is a global quality assurance and risk management company. Driven by our purpose of safeguarding life, property and the environment, we enable our customers to advance the safety and sustainability of their business. We provide classification, technical assurance, software and independent expert advisory services to the maritime, oil & gas, power and renewables industries. We also provide certification and supply chain services to customers across a wide range of industries. Operating in more than 100 countries, our experts are dedicated to helping customers make the world safer, smarter and greener. In the power and renewables industry DNV GL delivers world-renowned testing and advisory services to the energy value chain including renewables and energy efficiency. Our expertise spans onshore and offshore wind power, solar, conventional generation, transmission and distribution, smart grids, and sustainable energy use, as well as energy markets and regulations. Our experts support customers around the globe in delivering a safe, reliable, efficient, and sustainable energy supply.

The trademarks DNV GL and the Horizon Graphic are the property of DNV GL AS. All rights reserved. This publication or parts thereof may not be reproduced or transmitted in any form or by any means, including photocopying or recording, without the prior written consent of DNV GL. Š DNV GL 04/2018

Power Cable System Testing - position paper  

Ensuring the reliability of power transmission and distribution

Power Cable System Testing - position paper  

Ensuring the reliability of power transmission and distribution