water blocking

8/18/2019 Water Blocking

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WATER BLOCKED MV XLPE CABLE

Testing of water

blocked cables

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1. Introduction

2. Why Water Block Cables?

3. How to Water Block Cables

4. Proving Water Blocking

5. Water Penetration Tests

6. Aspects of a Water Blocked Cable Design

7. Impact of Water Blocking on Lugs and Ferrules

8. Aberdare's Test Rig

Introduction


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This conducts electricity, causing …

Safe surfaces to become live and dangerous.

Instead of insulating, insulators conduct electricity. In PILC electrical trees will form within the insulation.

Conductive path between phases or from phase to earth =

Why Water Block Cables …


2.1

Water + Any conductive parts

Remedies…

Ensure that cables are always dry inside.

Maintain good insulation radials.

At cable ends keep insulation surfaces smooth.

Ensure insulation and sheathing is free of holes!


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This will attack ANY polymer, causing …

WATER TREES.

A water tree is a regions of reduced insulation strength. This is the ONLY known ageing chemical process for XLPE.



2.2

Water + Electrical stress

Eventually a water tree will cause an electrical tree, and …

Remedies…

Exclude all impurities.

Ensure that the HUMIDITY within the cable is less than 65%.


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Oxidizing agents attack metals, causing …

Armour to corrode, leaving the cable unprotected and unearthed.

Screens to be interrupted, leading to failure.



2.3

Water + Oxidizing agents

Vs

Ci

Cs

(Insulation)

(Sheath)

Vp

Example: On A 300 mm² 11 kV single core unarmoured with an interruptedscreen, the screen voltage will be 2 kV. This will puncture the sheath, allowingwater access to the screen.

Tracking will take place along the outer semiconcuctor.

The cable will sizzle for a few minute to a few days, then …


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Radially water blocked below the impervious layer.

Use polyethylene for the impervious layer.

Use metal for the impervious layer.

Use both.

How to Water Block Cables:


3.1

Radial water blocking

Outer sheath

Metallic sheath

Insulation

Conductor

Gener

alized

cable:

3 2


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Block longitudinal water migration by layer .

Water swellable powders, yarns or tapes.

Bitumen.

Unvulcanized rubber .

Solid conductor.

How to Water Block Cables:


3.2

Longitudinal water blocking

Generalized

cable: Outer sheath

Metallic sheath

Insulation

Conductor


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Water Penetration Tests:


5.1

Standard tests

IEC 60502-2 SANS 60840 IEC 62067 SANS 1339

MV XLPE HV XLPE EHV XLPE MV XLPE

Initial sample size 6 m 6 m 8 m 6 m

Condition Bend test Bend test Bend test Bend test

Sample size 3 m 6 m 8 m 3 mStripped ring 50 mm 50 mm 50 mm 50 mm

Water head 1,0 m 1,0 m 1,0 m 1,0 m

Filling time 5 minutes 5 minutes 5 minutes 5 minutes

Standing time 24 hours 24 hours 24 hours 24 hours

Heat cycles 10 10 10 10

Cycle duration 8 hours 24 hours 24 hours 8 hours

Heating duration ≤ 5 hours 8 hours 8 hours ≤ 5 hours

Hold time ≥ 2 hours ≥ 2 hours ≥ 2 hours ≥ 2 hours

Cooling time ≥ 3 hours 16 hours 16 hours ≥ 3 hours

Conductor temp (min) 95 °C 95 °C 95 °C 95 °C

Conductor temp (max) < 100 °C < boiling point < 100 °C < boiling point

Pass criterion No leak No leak No leak No leak

Step


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6 2a


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Tapes and Yarns containing Polyacrylate water swellable material.

Tapes can be either semiconducting or non-semiconducting.

Yarns are available in different diameters.

Bitumen is often used to water block wire armouring and also providescorrosion resistance

Aspects of a Water Blocked Cable Design:


6.2a

Materials

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Requirements for water blocking materials:

Short term stability

Long term stability

Repeatability of water blocking action

Compatibility between cable materials



6.2b

Materials

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Performance of superabsorbent materials:

Swell Height

Swell speed



6.2c

Materials

Water Swellable tapes


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6 3a


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Water blocking of conductors is achievedusing a swellable yarn in the centre andswellable tapes over each layer of

wires. Aberdare has completed type tests with

this design.



6.3a

Conductors

6.3b


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Wetting water blocked conductor.



6.3b

Conductors


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6.6


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Water blocking of the armour is achieved by flooding the armour with bitumen which seals the interstices between the armour wires and enhances thecorrosion resistance of the armour wires.



Armour

6.7


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Special Manufacture

Lead time for materials

Cost increase



Other issues

7.1


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Water blocking tape between the layers of a stranded conductor preventscurrent from flowing freely between layers.

Over a long length of conductor this is no problem because there is no usefulradial component to current.

It is a concern in the case of lugs and ferrules where there is a significantradial component to current.

In the steady state the heat sink effect of metals in contact will prevent

dangerous temperatures.

In the transient adiabatic situation short duration high temperatures couldoccur.

Impact of Water Blocking on Lugs Ferrules:


Concern

7.2


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Comparative testing is carried out between bare conductors without waterblocking components and bare conductors with water blocking components.

Each sample is terminated using lugs, and jointed mid-span with a connector (ferrule).

The temperatures in the connector, adjacent to the contact between conductorand connector, as well as of the conductor away from the connector, ismeasured.

The conductors are loaded to specific conductor temperatures and theconnector temperatures are measured.

Any increase in the connector temperature for the same conductortemperature indicates that there is some resistance to radial current at theconnector.



Test method

7.4


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Early indications are that there may be a slight increase in resistance to radialcurrent.

The likelihood of this causing damage to the insulation is low during a fault,because no insulation should be present at hot spot.

Never-the-less water blocking materials should be removed from conductorsinserted into connectors and lugs, until proven that this is not necessary.



Conclusion

8.4


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Aberdare’s Test Rig



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Thank you

water blocking

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