[PDF] Remnant Static Mechanical Stresses and Water Tree Ageing of

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ȈJicable'15,21Ǧ25June 2015ǦVersaillesǦFranceȈJicable'15,21Ǧ25June 2015ǦVersaillesǦFrance

Remnant Static Mechanical Stresses and

Water Tree Ageing of XLPE Power Cables

Erling ILDSTAD , Kurt-Albert GROV PLASSEN

Norwegian University of Science and Technology

Department of electric Power, Norway

Hallvard FAREMO

SINTEF Energy Research, Norway

ȈJicable'15,21Ǧ25June 2015ǦVersaillesǦFrance

Enhanced water tree degradation of

Dynamic Subsea Cables?Caused by mechanical stress and water ingress Typical applicationsOff Shore Power grid connection of: - Wind Turbines - Oil/gas production units

Can such cables be designed

without metallic water tight sheath barriers?? ȈJicable'15,21Ǧ25June 2015ǦVersaillesǦFrance

From previous examinations:

1. High dynamic mechanical tension enhance

water treeing in a similar manner as static strain?

2. Compressive strain suppress water tree growth.

Will frozen-in residual strains critically affect

initiation and growth of water trees?Main question adressed in this paper: ȈJicable'15,21Ǧ25June 2015ǦVersaillesǦFrance

Enhanced watertreeing due to

bending: ȈJicable'15,21Ǧ25June 2015ǦVersaillesǦFrance Test rig for ageing of cable samples. Water, voltage, static and dynamic mechanical stress • 3 m long samples of triple extruded 12 kV cable cores. • Copper conductor removed between the mechanical clamping. • Dynamic tension at 0, 0.1 and 1 Hz , 6% strain , static 2% • Kept in water filled tubes at 30ºC , 20, 40 and 60ºC • The cable samples were aged at 2U 0 , and water tree examined after 3 weeks. ȈJicable'15,21Ǧ25June 2015ǦVersaillesǦFrance

1-2 tons

applied during removal of the copper conductor ȈJicable'15,21Ǧ25June 2015ǦVersaillesǦFrance

Result 1: No significant different water

treeing during ageing at static and dynamic mechanical strain. ȈJicable'15,21Ǧ25June 2015ǦVersaillesǦFrance

Result 2: Radial distribution of Bowtie

trees (after 3 weeks at 6% static strain)

• Nearly constant density of bow-tie in the innermost 2/3 of theinsulation, but reduces sharplytowards the insulation screen.

• Almost no bow-tie trees close to the insulation screen. • Vented water trees from the

conductor screen only=> Strong indication of residual compressive stresses in theoutermost insulation region.

051015202530

0,4 0,8 1,2 1,6 2 2,4 2,8 3,2 3,6 4

Tree density [cm

-3

Distance from inner semiconductor [mm]

ȈJicable'15,21Ǧ25June 2015ǦVersaillesǦFranceȈJicable'15,21Ǧ25June 2015ǦVersaillesǦFrance

High thermal expansion of XLPE compared to that

of the metallic conductor - adhesion prevent the insulation from shrinking back to its equilibrium dimension.

Cooling from the outside results in compression

forces at the outer insulation surface

Origin of remnant Frozen-in mechanical stress:

ȈJicable'15,21Ǧ25June 2015ǦVersaillesǦFrance

Main purpose of this work:

• Experimentally examine possible enhancement of water treeing caused by remnant mechanical stresses, frozen-in during manufacturing of extruded XLPE power cables • Examine possible temperature effects ȈJicable'15,21Ǧ25June 2015ǦVersaillesǦFrance Result 3: Measured longitudinal shrinking after having removed the cable conductor; at 20 C ȈJicable'15,21Ǧ25June 2015ǦVersaillesǦFrance

Examples of watertrees:

Vented tree at the insulation

screen ȈJicable'15,21Ǧ25June 2015ǦVersaillesǦFrance Results 4: Number of bow-tie trees after ageing at low static mechanical strain ȈJicable'15,21Ǧ25June 2015ǦVersaillesǦFrance Results 5: Length of bow-tie trees after ageing at low static mechanical strain ȈJicable'15,21Ǧ25June 2015ǦVersaillesǦFrance

CONCLUSIONS

•Remnant static mechanical strain, affects the number of initiation sites for water trees •Strains lower than 2% appears to have a minor effect on initiation and growth of water trees in XLPE cable insulation. ȈJicable'15,21Ǧ25June 2015ǦVersaillesǦFrance ȈJicable'15,21Ǧ25June 2015ǦVersaillesǦFrance

Low longitudinal mechanical

design stress of polymeric cable insulation: • In metals (Cu, Al) yield occurs at strains above approximately 0.1 %, • In XLPE this limit is at about 5-10%. =>in a practical cable installation the externally applied longitudinal strain of the insulation, will be limited to values fare below the yield point of the

polymer. (In case of sharply bent cables the outer parts of the insulation may, however, be stretched to values closer to its yield)

ȈJicable'15,21Ǧ25June 2015ǦVersaillesǦFrance

Mechanical damage

mechanisms of water treeing

Craze formation: a) Liquid water formation

b) AC voltage => Pulsating

Maxwell

stress at the tip of water filled regions ȈJicable'15,21Ǧ25June 2015ǦVersaillesǦFrance

Water tree initiation and

Growth

E XLPE

Water-filled

void

Maxwell

forces

Tensile strain

• Water trees require an AC electrical field and condensed water. • Mechanical damage theory: elongated water-filled voids ĺ local electric field enhancement ĺ

Maxwell forces on insulation ĺ

Strain causes crazing of the polymer ĺ

Water tree formation

ȈJicable'15,21Ǧ25June 2015ǦVersaillesǦFrance

Forces acting on a cable hanging in a

caternary loop:

Where:

is the stationary gravity force determined by the wet weight m w (weight - buoyancy) per meter cable, the sea depth d, acceleration of gravity g and the catenary angle Į. is the vertical acceleration force assuming a sinusoidal vertical wave movement with a peak to peak amplitude of h. The frequency is determined by the period time T between subsequent wave peaks. - In many cases the wave frequency is below 0.1 Hz. F D is the drag or friction force associated with moving the cable up and down in the water. (Low in magnitude)

Total tensile forces

ȈJicable'15,21Ǧ25June 2015ǦVersaillesǦFrance

Example:

Subsea cable with a wet weight of about 30 kg per meter at a sea depth of d=250 m. => The total tensional design force F T will typically be in the range of 100 kN.Strong tensile armor is needed to prevent insulation damage :Design criteria:

-The relative elongation of the conductor and the armor must be equal, to avoid shear forces in the insulation.

-The maximum strain/elongation must be lower than that of the yield point of the conductor, to prevent permanent elongation of the conductor.

ȈJicable'15,21Ǧ25June 2015ǦVersaillesǦFrance

Frozen in stresses due to

thermal shrinkage • Large thermal expansion of polymers compared to that of metals • Uneven cooling

Examples of mechanical stress

distributions in case of cooling from one side: ȈJicable'15,21Ǧ25June 2015ǦVersaillesǦFrance

Results from TMA analysis of XLPE cable

insulation:

H.Suzuki

(1988)

Typical maximum frozen strain measured in

XLPE cables:

ȈJicable'15,21Ǧ25June 2015ǦVersaillesǦFrance

Typical micrographs of

vented water trees:

Vented trees from conductor screen

after dynamic testing at İ=6 % and frequency f=1 Hz,

Almost no vented trees from insulation

screen after dynamic tension at İ=6 % and frequency f=0.1 Hz, ȈJicable'15,21Ǧ25June 2015ǦVersaillesǦFrancequotesdbs_dbs46.pdfusesText_46

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