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RECOMMENDATIONS FOR TESTING OF LONG AC SUBMARINE CABLES WITH EXTRUDED INSULATION FOR
SYSTEM VOLTAGE ABOVE > 30 (36) TO 500 (550) KV
Convener: Anders GustafssonSecretary: Johan Karlstrand
TUTORIAL B1.27 – TB 490
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Copyright
DISCLAIMER
Ownership of a CIGRE publication, whether in paper form or on electronic support only infers right of usefor personal purposes. Are prohibited, except if explicitly agreed by CIGRE, total or partial reproduction of the publication for use other than personal and transfer to a third party; hence circulation on any intranetor other company network is forbidden.
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Why new test recommendations?
• Power ratings of submarine cable links increase, which need higher rated voltages. Electra 189, covered a voltage range of > 36 - 170 kV
• The rationale and background behind the tests needed to be clarified
• The longitudinal water tightness test needed a revision
• Tests of rigid repair joints should be taken into account
• The Technical Brochure aims to provide stakeholders with an improved understanding and quality assurance of testing long AC submarine extruded cable systems
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• Examination of relevant IEC standards / CIGRE recommendations and documentation
• The work should adopt a system approach. Particular attention should be paid to repair joints as part of the submarine cable system and the WG should consider tests with external water pressure, heat cycling and mechanical handling (during installation of a repair joint)
• The work should propose development and prequalification tests for the EHV submarine cable system and requalification in case of minor or major changes and define the range of prequalification and type approval for EHV submarine cable systems
• The work should propose tests for long submarine cable lengths - both in the factory and after installation and explain clearly the basis for the recommended tests and the range of application
• The work should include a review of the currently available technologies for submarine cable and joint design and consider possible implications for testing
• A Technical Brochure should be prepared for publication• The work should also include an update of the existing recommendations
in Electra 189 for U > 36-170 kV. (added in 2010)
Terms of Reference
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Working Group B1.27: Deliverables
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• Anders Gustafsson SE Convener• Johan Karlstrand SE Secretary• Anders Jensen DE• Christian Remy FR• Geir Clasen NO• Gianni Miramonti IT • Harry Orton CA• Juan Prieto ES • Robert Donaghy EI• Ronald Gruntjes NL • Søren Krüger Olsen DK • Takenori Nakajima JP
• Work started in Feb 2008 and was finished in Sep 2011
Member list of WG B1.27
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Recommendations for AC submarine cables - History
• Recommendations for mechanical tests on submarine cableso Electra 68 – 1980 Electra 171 – 1997 WG B1.43 (2011)
• Recommendations for tests of long lengths of extruded cableso Electra 189 – 2000 (170 kV - extruded)
• Standards for land cables and systemso IEC 60840, IEC 62067, TB 303
• TB490 follows a System Approach for the whole voltage range> 36 - 550 kV
TB490 - WG B1.27
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Submarine cable testing – Overview
SYSTEM TEST Pre-Qualification Test
Type Test
Routine Tests Sample Tests
Water Penetration Test
Mechanical TestsSYSTEM TEST
COMPONENT TESTS
After Installation TestSYSTEM TEST
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High voltage submarine cable system
Termination
Prefabricated Joint Factory Joint (Flexible)
Repair Joint
ON-SHORE OFF-SHORE
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Table of Contents
• Executive Summary
1. Introduction
2. Definitions
3. Current technologies for submarine cable designs
4. Current technologies for submarine joint designs
5. General aspects on submarine cable testing
6. Routine Test
7. Sample Test
8. Type Test on Cable System
9. Prequalification Test
10. Extension of Qualification Test
11. After Installation Test
12. References
13. Appendices
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1. Introduction: Selected projects ≤ 170 kV
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1. Introduction: Reference Projects > 170 kV
> 200 km for U ≥ 245 kV installed in the next few years
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2. Definitions: What is a “long length”?
• A ‘long’ length is considered to be:
o a delivery length which includes factory jointsor
o a delivery length for which the carrying out of high voltage tests and partial discharge tests strictly in accordance with IEC is impracticalor
o a delivery length which cannot be accommodated on an individual transportable drum
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2. Definitions: Internal/External design of joint
• Internal design of joint - The joint’s electrical function based on the design principleso to transfer the currento to control and withstand the electrical stresseso to screen the joint electricallyo to protect the insulation system from moisture ingress.
• External design of joint - The joint’s mechanical function based on the design principles o to withstand the impact from the surroundingso to withstand (in some designs) the mechanical bendingo to withstand the mechanical tension and torsion during laying
and operation.
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3. Current Technologies for Submarine Cable Designs
• Single-core designs
• Three-core designs
• Cable componentso Conductor (water tight)
o Insulation system
o Metal screen/sheath
o Armour
o Outer protection
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3. Submarine Cable Design: Water Tightness
• Dry versus Wet design:
o Dry – no water/moisture is in direct contact with the XLPE core
o Wet – water/moisture is in direct contact with the XLPE core
• Dry designs are mostly used for HV/EHV submarine cables; i.e. a metallic moisture barrier is mostly used
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3. Submarine Cable Design: Water Tightness
• Water tightness in both radial and longitudinal directions is crucial for HV/EHV cables
• Water tightness of a conductor is the ability to limit longitudinal water penetration at the maximum depth of the cable
• Water tightness may be improved by proper selection of conductor design and water blocking technologies
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3. Submarine Cable Design: Conductor
• Conductor designs can be solid, key-stone shaped, stranded-compacted or stranded - Milliken
• Conductor design contributes to the water penetration rate or the level of water blocking ability
o Water swelling (powders), swellable yarns or tapes, blocking compounds
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3. Submarine Cable Design: Conductor
• Solid Conductor – Water tightness inherent in design
• Compacted, profiled and segmental conductors – powdersand/or tapes are swelling when in contact with either fresh or saline water
o The swelling ability is worse for saline water
• Compacted, profiled and segmental conductors – different types of water blocking filling compounds may also be used tofill up the interstices between the strands
o The compatibility with inner semi-conducting screen should be checked
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3. Submarine Cable Design: Insulation System
• Extruded HVAC submarine cables consist predominantly of cross-linked polyethylene (XLPE)
o The cross-linking of polyethylene (LDPE) forms a three-dimensional network which implies higher temperature and higher mechanical strength
o The insulation system consists of an inner conductor screen, insulation and an outer insulation screen
• EPR insulation may also be used for U < 150 kV
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3. Submarine Cable Design: Insulation System
• The quality of XLPE materials and manufacturing processes has continuously improved
• While the electrical stress level on the XLPE insulation has continuously increased, the failure rate and unavailability of XLPE cable systems has decreased
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3. Submarine Cable Design: Metal Sheath
• An extruded lead alloy sheath covered by an extruded anticorrosion polymeric sheath or a semiconducting tape are the most common radial water barriers used today
• The polymeric anticorrosion oversheath is either insulating or semi-conducting
o To avoid dielectric breakdown of the insulating oversheath, a semi-conducting oversheath is often used
• For wet designs, an ordinary copper wire or tape screen are commonly used
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3. Submarine Cable Design: Armour
• The armour is applied:
o to provide enough tensile strength during installation
o to protect the cable from external impact
o to make the cable torsional-free during installation
• Copper, steel or stainless steel wires are used as armour materials
o In single-core submarine cables the steel armour creates a large iron loss from the magnetic field copper or stainless steel are used
o In three-core cables, the resulting magnetic field will be significantly lower steel wires are normally used
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4. Current Technologies – Flexible Joint
• Flexible joints are fully flexible; i.e. designed with the same limitations on bending radius and tensile strength as the cable
• Flexible joints have the same electrical and thermal performance as the cable
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4. Current Technologies – Flexible Joint
• The flexible joint is needed due to the following reasons:o limitation in continuous extrusion lengths, due to filter cleaning, scorch
formation etc.
o limitation of storage capacity (e.g. process platforms) of the individual lengths
o limitation of degassing equipment and processes
o limitation in the laying-up baskets volumes (for three-core cables)
o power limitation of the AC testing equipment
o damage to the cable core during handling
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4. Current Technologies- Repair Joint
• Repair joints can be dividedinto the following types:
o A1: Fully flexible
o A2: Flexible with somemechanical restrictions
o B: Rigid joint
R
T
R
T
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4. Current Technologies - Repair Joint
• A repair joint is made on the complete armoured cable and usually onboard on a repair vessel or barge. Suitable equipment and sufficient space have to be planned for.
• Repair joints are necessary to have in stock for most long submarine cable systems
Rigid joint (Type B)
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5. General Aspects on Submarine Cable Testing
• Routine tests (Chapter 6)o Test on manufactured lengtho Test on factory installed jointso Factory acceptance test
• Sample tests (Chapter 7)o Sample tests on cableo Sample tests on joint
• Type tests (Chapter 8)o Mechanical tests on complete cableo Electrical tests on complete cableo Non-electrical tests on cable components and on complete cable
• Prequalification test (Chapter 9)
• Extension of qualification test (Chapter 10)
• After installation tests (Chapter 11)
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5. General Aspects: Conditions
• Follow IECo Ambient temperature: 5-35 ⁰Co 10-500 Hz for routine testing
• Additional design data needed to document tests accordingto IEC :
• Design of armour/tension/laying depth
• Method of water blocking technologies
• Maximum conductor temperature
• Ability to be coiled and relevant test parameters
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5. General Aspects: Development Tests
• Recommendations and standards are generally not sufficient to take all design and installations conditions into account
• Examples of issues that may need to be considered are:• Vibration and strumming of cable in water currents; free spans
• Ability to withstand deep water pressures and deformation
• Design of hang-offs
• Efficiency of water blocking tapes/compounds after years of operation
• Handling of optical fibre unit/cable integrated in cable
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6. Routine Test: Introduction
• General
o Routine tests are made to demonstrate the integrity of the manufactured cable and to verify that the product meets the design and manufacturing specifications within specified tolerances
• Routine tests are mainly performed on the following parts for submarine cable systems:
o Manufactured lengths
o Factory joints
o Complete delivery length
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6. Routine Test: Frequency Range
• Today IEC 62067 and ICEA S-108-720 stipulates 20 – 300 Hz for After Installation Testo … but new edition of IEC 60060/3 stipulates 10 – 500 Hz for onsite testing
• AC resonant equipment (ACRF) has been used below 20 Hz
• Longer lengths (high C) and EHV (high U) need some degree of freedom regarding frequency range WG B1.27: longer time duration at lower voltages for factory acceptance and after installation tests compared to IEC 62067 may be agreed
• VLF-testing is not considered mature for HV and EHV
• WG B1.27 recommends 10-500 Hz for routine, factory acceptance and tests after installation
• The lower frequency limit of 10 Hz is important for testing of “long” lengths
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Source: Gockenbach/Hauschild-2000
6. Routine Test: 10 Hz as lower frequency limit
IEC 62067 prescribes 20 Hz as lower frequency limit for AC resonanttests. IEC 60060/3 prescribes 10 Hz. The new lower frequency limit in TB490 is set to 10 Hz giving insignificant change in breakdown strengthbut a 4 times higher load capacity!
10 Hz 20 Hz
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6. Routine Test: ”Long” Lengths
• Testing of long cables is on the agenda…WG B1.38
o AC and DC cables face the same challenges
o New equipment and new testing methodologies are under development
o Research is ongoing
Group No : WG B1. 38 Name of Convener : John Densley
TITLE of the Working Group : After laying tests on AC and DC cable systems with new technologies
Needs of Target Groups:
Background: Extruded (XLPE) insulation is rapidly becoming the insulation of choice in both new and replacement transmission class cable circuits. While the cable and accessories are tested in the factory, the
k hi i ll h i l b d f h i ll i h b l d d b f h
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6. Routine Test: Summary in Annex A (Example)
Manufactured Lengths
Test References
>36-170 kV >170 kV
Partial discharge (PD) test [TB § 6.3.1] IEC60840 § 9.2 IEC62067 § 9.2
High voltage test [TB § 6.3.2] IEC60840 § 9.3 IEC62067 § 9.2
Factory Joints
Test References
>36-170 kV >170 kV
See TB § 6.4 1 TB § 6.4 TB § 6.4
Repair Joints
Test References
>36-170 kV >170 kV
See TB § 6.6 TB § 6.6 TB § 6.6
Terminations
Test References
>36-170 kV >170 kV
See TB § 6.7 IEC60840 § 9.1 IEC62067 § 9.1
Complete Delivery Length (FAT)
Test References
>36-170 kV >170 kV
High voltage test [TB § 6.5.1] TB § 6.5.1 TB § 6.5.1
Partial discharge (PD) test [TB § 6.5.2] TB § 6.5.2 TB § 6.5.2
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7. Sample Tests
• Sample tests - In Electra 189 described as Special tests
• When routine tests are not possible
• Frequency of tests according to IEC except:
o If PD-test is not possible on ”long length” (as defined by the WG), impulse and PD-tests shall be performed on samplestaken out from start/stop of each extrusion run
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7. Sample Tests – New tests
• Volume resistivity of conductor screen, insulation screen and semi-conductive polymeric sheath
• Examination of completed cable
o A sample of completed cable (length: more than one pitch of wire armour) shall be subjected to a visual inspection
o The number of wires shall be counted in each armour layer and validated according to the design
o The pitch length(s) of the armour layer(s) shall be measured and validated relative to the declared value by a tolerance of ± 10%.
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8. Type Test: Overview
• The type test shall demonstrate the satisfactory performanceof the submarine cable system design
• Main components:
o Check on insulation thickness of cable
o Mechanical tests on complete cable system
o Water penetration tests
o Electrical test on complete cable system
o Non-electrical tests on cable components and on complete cable
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8. Type Test: Range of Type Approval
• The range of type approval for land AC cable systems is described in IEC 60840 and IEC 62067. Those and the additional conditions below must be met:
o less severe mechanical stress
o method of water tightness is unchanged
o the design of conductor connection for joints is unchanged
o electrical stress at the conductor screen in the flexible joint does not exceed 10 %
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8. Type Test: Mechanical Tests
• Refers to CIGRE recommendation Electra No. 171 (1997)
• WG B1.43 will update and add features in a new TB (ready 2014)
Group No : WG B1.43 Name of Convener : Marc Jeroense
TITLE of the Working Group: Recommendations for mechanical testing of submarine cables
Background:
Update of mechanical tests for submarine cables is needed since submarine cable installations are growing for higher powers and new applications (wind farm connections, dynamic power cables and deeper sea installations etc).
The existing recommendation is from 1997 (Electra 171) needs to be updated.
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8. Type Test: Mechanical Test of Repair Joints
Tensile Bending TestElectra 171 § 2.2
Tensile TestElectra 171 § 2.3
Handling / laying Simulation Test
Flexible Joint Mandatory.Same R and T as for cable. No limitations for application
For information only (on the same or other cable / joint assembly)
Not necessary
Flexible Joint with some mechanical restrictions
Mandatory with R ,T as appropriate for the intended application, i.e. different from cable.E.g. repair in reduced water depth only.
For information only (on the same or other cable / joint assembly)
Not necessary
Rigid Joint Bending test only with radius R without load, if applicable.
Mandatory.Straight tensile test at T on the same joint assembly subjected to bend test at radius R without load
Advisable.
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8. Type Test: Water Penetration Tests
• For long submarine cables the water penetration tests are divided into three tests:
o Conductor water penetration test LWPa
o Metal sheath water penetration test LWPb
o Radial water penetration test of joints RWP
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8. Type Test: Background of LWP Test
• Mechanical and thermal preconditioning
o simulate actual installation conditions
• Water propagation distance is not fixed
o B1.27 recommendation < 30 m
• LWPa - conductor
o Water ingress in the conductor during a cable fault at the deepest part
• LWPb - metal sheath
o Water ingress under the metal sheath (reference is made to lead) at the near shore area where the water pressure does not increase the ability to stop water penetration ( 30 m water head)
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LWPa on Conductor
+ 33% d1
• Mechanical test
o Tensile bending test (Electra 171)
o 3 x 24 h heating cycles
• Expose conductor to water
o Put into pressure vessel (at a pressure corresponding to the maximum laying depth) for 10 days (5-35°C water temp) – no heat cycling !
o Cut at distance d1
Water detected?
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LWPb on Metal Sheath
d2
• Mechanical test:
o Tensile bending test (Electra 171)
• Expose outer semiconducting layer to water
o Put into pressure vessel (5-35°C water temp) for 10 days heat cycling
o Cut at distance d2
Water detected?
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8. RWP Test on Joint
• Mechanical test
o Mechanical test according to Electra 171
o 10 x 24 h heating cycles
• Put into pressure vessel (at a pressure corresponding to the maximum laying depth) for 2 x 24 h (5-35°C water temp)
o Only pressurized part needs to be tested
• Water detected?
• Shape irregularities?
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9. Prequalification (PQ) Test: Background
• The PQ-test for land cable systems described in IEC wereintroduced to demonstrate that the manufacturer is able to design, produce and install cables at the required voltage level
• A special focus was put on the cable and accessoryinsulation characteristics, the cable core/accessoryinterfaces and the thermo-mechanical behaviour over time
• Which are the specific issues needed for verification of a submarine cable system?
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9. PQ-Test: Land versus Submarine Cable Systems
• For an AC extruded submarine cable system there aremainly four parameters that are most often different compared to an AC extruded land cable system:
1. a flexible (factory) joint is needed
2. the cable is armoured
3. the repair joints have a mechanical housing
4. the cable conductor is not of segmented design
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9. PQ-Test: Land vs. Submarine cable systems
• When is then a PQ-test required?
1. Flexible joint: PQ-test needed for either a land – or submarine cable system
2. Armour: External design Electra 171, performed prior to the electrical type test
3. Repair joint: External design Electra 171+ RWP, performed prior (or after) to the electrical type test
4. Conductor design: May be qualified in a PQ-test for a land cable systema. An exception could be a special water blocking compound not earlier tested for a land
segmented design
• Thus, a land cable system PQ-test may prequalify a submarine cablesystem, if all relevant conditions are met
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9. PQ-Test: Range of Approval
• Refer to IEC 60840 and 62067
• If a land (or submarine) cable system already is prequalified, another submarine cable system is qualified if the following conditions and the conditions given in IEC 60840 and 62067 are met:
o electrical stress at insulation screen of a repair joint is equal or lower
o if a flexible joint already is PQ-tested on a larger conductor and the proposed flexible joint on the smaller conductor has more than 10% higher electrical stress, the flexible joint shall be subjected to a type test with a stress larger than that already prequalified
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PQ-Test: Example for Range of Approval of Joints
• Factory Joints - inner stress is decisive - but large conductorcross-sections may be more challenging to manufacture.
Therefore;
o +10% higher inner stress is allowed (for smaller conductor cross-sections than tested for)
o Larger conductor cross-sections than tested for are not allowed
• Pre-molded Joints – outer stress is decisive
o 0% higher outer stress is allowed
o Larger conductor cross-sections than tested for are not allowed
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9. PQ-Test: Example for Range of Approval of Joints
Inner Stress for Factory Joints (< 10%)
Cross-section Area
Electrical Stress
Outer Stress for Premolded Joints
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10. Extension of Qualification – EQ-Test
• Refer to TB303, IEC 60840 and 62067
• The EQ-test is mainly related to changes in accessories which already have undergone a PQ-test (TB303)
• The arguments for not introducing an EQ-test specifically for submarine cable systems are the same as for the PQ-test
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11. After Installation Test
• Test on new installations are made when the installation has been completed
• If the complete installed length is too long a reduced test voltage with a longer duration may be agreed. Alternatively, a voltage of Uo may be applied for 24 hours
• Time Domain Reflectometry (TDR) give a “fingerprint” of the cable system to be used if needed during the cable life
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Submarine cable testing – Overview
SYSTEM TEST Pre-Qualification Test
Type Test
Routine Tests Sample Tests
Water Penetration Test
Mechanical TestsSYSTEM TEST
COMPONENT TESTS
After Installation TestSYSTEM TEST
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