15 Godfrey 3

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March 26, 2008

Brief overview

MV Cable Construction/Design

March 26, 2008page 2/

Contents

• Requirements

• Design of Paper Cables

• Design of Polymeric Cables

• Water Trees

• Switchgear trends

• Experiences


Operation Requirements• Life time

Expected > 35 years

• Carry electrical stresses

Short circuit - Thermal impact seconds

Dynamic (mechanical strain)

Emergency Thermal impact

Voltage strikes – BIL

• Withstand mechanical stresses

Strain and impact during laying/installation

Dynamic forces during short circuit

Vibration

Expansion and contraction of each component (operation temp -200 to 900C)

• Protection against environmental impact

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Concentric Round

Compacted Round

SOLID

CIRCULAR CORE

SOLID 90 & 120 120°

SECTORAL CORE

Non Compacted Sector

Compacted Sector

Concentric Round

Compacted Round

SOLID

CIRCULAR CORE

Solid Sector

SECTORAL CORE

Non Compacted Sector

Compacted Sector

• PAPER CABLE• Single Core – Stranded concentric round

• Three Core - Stranded non-compacted sector

• XLPE CABLE

• Single Core - Stranded compact round

• Three Core - Stranded compact round

• - Stranded compact sector

• - Solid 900 & 1200 (Europe)

Basic Conductor Types used in MV Cables


Conductor Screen

The conductor screen is a semi-conductive layer applied between the

conductor and the insulation that compensates for air voids trapped between

the conductor and the insulation.

Without conductor screening, an electrical potential exists that will over stress

these air voids, causing the air to ionize and go into corona (often known as

“partial discharge”).

Conductor screening also eliminates any irregularities in the conductor by

smoothing out the electrical profile on the surface of the conductor.


Conductor Screens - Electrical Field Distribution

• Electrical field in a cable

“unscreened conductor”

• Electrical field in a cable

“screened conductor”

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MV – Paper Cable “Belted”

Multicore cables are assembled in two ways.

Belted construction which is used up to 11kV. The individual cores are

paper insulated, laid up, and a paper wrapping called belt papers applied

over the laid up cores. The electrical field is contained within the

combination of insulation and belting, and the thermal mechanical

movement between these layers is restricted by the cable operating

temperature of 65oC


Secondly, screened construction, patented by Hoshstadter is used for 11kV

and above for both single core and multi core cables.

The individual cores are paper insulated with a metallic paper screen, and laid

up with a conductive wrap to tie the screens electrically together. The

electrical field is now uniformly contained within the insulating papers and the

design is capable of higher voltage levels than the belted design. This

“screened” design increases the operating temperature to 70oC.

MV – Paper Cable “Screened”


Manufacturing Process

Paper insulated and oil impregnated cables

1. The carbon screen paper, insulation paper, and metallic paper screen are

lapped onto the conductor. During this process the cable is being spooled

into the impregnation tank.

2. The drying process of impregnation under vacuum and heat removes the

moisture out of the insulation papers.

3. The metal sheath is “extruded” pressed onto the cable and cut to desired

length.

4. Armour and corrosion protection is then applied.

5. Electrical testing.

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March 26, 2008page 10 /

Earlier designs of paper cables for higher voltages relied on thicker wrappings

of insulation papers around the conductors and large amounts of belting

papers over all three conductors. Carbon papers were applied over the belt

papers which are coupled to the lead sheath.

Belted cables up to 33kV were made, however, problems with internal

discharge has now restricted the use of belted cables to 17.5 kV.

Once the problems and limitations of “belted “ cables were realised, the

solution to eliminate discharging in the fillers was to electrically screen the

insulated cores. Patented by Hoshstatder in 1914.

Wrapped Paper Insulation

March 26, 2008page 11 /

Wrapped Paper Insulation

Cables with wrapped insulation owe their flexibility to the gaps left betweenadjacent paper turns.This is done by staggering the next layer to give a 65:35 registration (lowerdiagram).

Some tolerance in manufacture can be accommodated, while still providing higherinsulation security in a radial direction than 50:50 registration would do (upperdiagram).

March 26, 2008page 12 /

Design - Medium Voltage “Belted” Cable < 17.5kV

conductor screen

insulation paper

bedding

carbon paper filler

lead sheath

steel wire armour

oversheathconductor

belt insulation

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March 26, 2008page 13 /

Electrical Flux – Belted Cable Construction

As the cores are not individually screened, the electrical field has radial andtangential components at many points.

The tangential components stress the paper tapes in their weakest axis. Inaddition, the equipotential cross the fillers, and as these areas often have airvoids entrapped in them and are usually made from a dielectrically weakermaterial, there is a high probability that discharging will occur in the fillers.

Belted Cable (without conductor screens)

March 26, 2008page 14 /

Electrical Flux in a “Screened” Cable Construction

The metallic screens applied over the insulation are grounded and are in effectan earth plane, and therefore at zero potential. This eliminates the electricalfield effect, thus preventing any discharge problems outside the screen.

March 26, 2008page 15 /

Design - Medium Voltage “Screened” Cable

Metallic screen

Cotton Woven Fabric

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March 26, 2008page 16 /

Lead Sheath

The lead sheath is impervious to moisture, petroleum fluids and gases.Problems associated with lead such as resistance to fatigue cracking and

extrusion defects. Fractures associated with internal pressure and corrosion

have been eliminated by choice of manufacturing techniques:

For example some cable manufacturers use alloy E for increased fatigue &

resistance. Alloy E is a percentage of 0.4% tin (Sn) and 0.2% antimony (Sb).

The lead sheath is also used for earth fault current, and this rating can be

increased by the use of steel wire armour.

March 26, 2008page 17 /

Mechanical Protection

Both wire or tape armour were common in the MV cable industry.

Steel wire armour (SWA) or aluminiuim wire armour (AWA) for single

core cables is added for increased tensile rating.

Steel Tape Armour (STA) is added for impact resistance and can be

replaced by high density polyethylene (HDPE) if not required for fault

current (this option is in accordance to AS 4026).

For submarine installation, two steel wire armour layers can be used to

minimise the twisting effect of one layer and this is called torque

balancing. Protection from the toredo worm can also be incorporated withthe application of copper/ brass tape which is placed onto the leadsheath.

March 26, 2008page 18 /

Jacket Materials – (Oversheath)

• PVC,

• LDPE,

• MDPE,

• HDPE,

• EPR,

• Jute/Bitumen.

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March 26, 2008page 19 /

MV Polymeric Design

March 26, 2008page 20 /

MV Polymeric Cable Design

Below are some examples of cables that are used in our industry today;

Corrugated Aluminuim Sheath/AWA

Lead Sheath

Aluminium Sheath or Welded AluminiumLaminate

Copper Tape/Copper Wire Shield

(commonly used on voltages from 11 to 33 kV)

March 26, 2008page 21 /

Cross-Sectional View of Triple Head

Insulation

Temp control –

Heating/Cooling

Inner semi-conducting

material

Outer semi-conducting

material

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March 26, 2008page 22 /

MV Polymeric Cable

March 26, 2008page 23 /

MV Polymeric Cable

The general make up of a three core XLPE cable construction is three single

core XLPE cables laid up in trefoil. An inner sheath is then extruded over the

three cores, and if required, armouring is wound over the inner sheath.

Usually armoured XLPE cables have a PVC jacket. Unarmoured cables used

today have a combination of HDPE, MDPE or LLPE depending on design

criteria.

March 26, 2008page 24 /

Insulation Screen TypesThere are three common constructions of insulation screen for polymeric

insulation.

Each one has it’s advantages in manufacture, or under particular

circumstances, for laying or jointing the cable.

The cable preparation tools and skills available at the job site are still a

decisive factor in the reliability of a completed cable installation.

The ease of removing the insulation screen correctly with the tools available

can therefore be just as important as the electrical qualities of the

manufactured cable.

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March 26, 2008page 25 /

Below are the three common types of insulation screens.

We commonly use what we call “easy strip” here in New Zealand,

for voltages ranging from 3.3 up to 33 kV.

“easy strip”

“graphite and conductive” paper

tape

“bonded” extruded screen

Insulation Screen Types

March 26, 2008page 26 /

Experience has shown that cable connections without stress control 3.3kV and

above will fail particularly frequently at the end of the insulation screen. This is

because the removal of the screen causes a change in the distribution of the

electrical potential.

Uncontrolled Electrical Field at Screen End

March 26, 2008page 27 /

When the electrical field strength is too high, the insulating medium, in this

case air, breaks down. The critical value for air is approximately 2.5kV/mm, at

approximately 70% humidity.

Video

Uncontrolled Electrical Field at Screen End

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March 26, 2008page 28 /

With stress control tube installed the lines of constant voltage are spread out,

reducing the intensity to the desired level.

This is achieved by the unique r esistive and capacitive properties of the stress

control material.

Controlled Electrical Field at Screen End

March 26, 2008page 29 /

Water Treeing Phenomena

imperfection points

insulation

insulation screen

conductor

conductor screen

Water trees are a phenomenon found in polyethylene insulated cables

(crosslinked and uncrosslinked).

They develop and propagate in the presence of moisture (water) and an

AC electrical field).

There are two classes of water trees:

- Bow Tie

- Vented

March 26, 2008page 30 /

Bow Tie Water TreesBow tie water trees are always found in the body of the

primary insulation, and by themselves will not cause the

cable to fail.

They normally propagate from either a micro-void, or an

inclusion in the insulation.

They remain dielectric in character.

They appear at points of highest mechanical stress with

the cable.

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March 26, 2008page 31 /

Vented Water Trees

Vented water trees grow into the insulation from either the

conductor or insulation screens.

They usually propagate from an irregularity at the

screen/insulation interface.

Although initially dielectric in character once they bridge

across both screens they rapidly become conductive and

the cable fails. The bridging effect can be caused by one

vented tree, two vented trees meeting, or two trees

bridging through a bow tie tree.

March 26, 2008page 32 /

Cable Bending Radius

r

cable

The following bending radii for various types of cables:

- 11kV Paper Insulated Single Core – 18 x O.D

- 11kV Paper Insulated Three Core – 15 x O.D

- 11kV XLPE Single and Three Core – 12 x O.D

Example: For three core paper lead cable with an

overall diameter of 50 mm; multiply by 15 = a bending

radius of 750 mm.

March 26, 2008page 33 /

Cable Core Bending Radius

• Min. Bending Radius r = 10 x D

• Special care on safety during the

cable bending.

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March 26, 2008page 34 /

Testing After Installation

Insulation test HD 620.S1:1996

DC testing 15 min > 2U0 - Not recommended to be performed on XLPEcable. DC hi pot testing on XLPE can cause space charges in the primary

insulation which will lead to accelerated ageing and treeing in XLPE cables.

AC at 45 to 65Hz for either at Um or U0 up to 24 h

Recommended for XLPE

VLF 0.1 hz at 3U0 1h need more time for detection of any potential weak

points – 500 times less charge.

Insulation test acc to IEC 60502-2

AC test at U0 for 24h

DC test 15 U0 for 15 minutes is under consideration.

March 26, 2008page 35 /

Sheath Testing

In order to detect damages or weak points on PVC/PE cable jackets, sheath

testing is carried out at the cable by applying a voltage (mostly negative DC) to

the armour or concentric neutral in accordance with several specifications.

Specifications which are common in the industry are;

IEC 229 (up to 10kV DC)

AS 1429 (3.5kV AC

VDE 0298 (5kV DC)

Overall we suggest the most “common practice of applying 1 to 5kV DC” This

voltage is sufficient. Higher voltages will only increase the danger of damaging

the oversheath.

March 26, 2008page 36 /

Additional Tests to Consider;

• Polarisation Index

• Step Voltage

• Conductor & Screen Resistance (continuity)

• Phasing

• HV DC

• TDR (Time Domain Reflectometry)

• VLF HV AC

• Tan Delta

• Partial Discharge

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March 26, 2008page 37 /

Projected Lifetimes

• Paper Insulated Cables 80 yrs

• Fir st generation XLPE ( 60’s/70’s) 20 yr s

• S econd generation XLPE (80’s) 30+ yrs

• Third generation XLPE (90’s/00’s) 40 yr s

March 26, 2008page 38 /

Changes of Switchgear Designs Over The Years

March 26, 2008page 39 /

Terminal Boxes – Compact Design As soon as the distance between phases and phase to ground becomes less

than the required air clearance for a given voltage class, the connection area

between termination and bushing stud has to be adequatetly insulated.

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March 26, 2008page 40 /

What The….???

Right angle bends in cores of PILC

cable termination will result in

broken insulation papers.

Cable configuration in terminal box

could hardly be worse…..

March 26, 2008page 41 /

What The……..??

Breakout mounted too high

in terminal box:

Badly crossed cores & small

radius bends.

March 26, 2008page 42 /

Third Party Damage to Cable sheath &

Metallic Screen

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March 26, 2008page 43 /

Repairs to Damaged Sheath & Metallic Screen

Breakdown of repair revealed incorrect application and recommended jointing

practices of the repair.

March 26, 2008page 44 /

Incorrect Repairs to Sheath resulting in localised

heating

Due to incorrect fitting sequence and recommended jointing practice

between the cable shield wires and the earthstrap/braid, created a high

resistive connection causing localised heating effect. Heating effect would

have been caused by induced current and circulating currents in the area ofrepair.

The heat generated caused the softening of the primary insulation and

insulation screen, thus allowing the shield wires to imbed themselves into

the insulation screen eventually leading to failure.

March 26, 2008page 45 /

Potential Failure - 33kV Termination

33kV termination breakdown showing discoluration, and a corona ring aroundthe circumference of the primary insulation.

Cause of potential failure was insufficient heat applied during installation, thus

leading to internal discharge. This installation would have eventually failure

causing puncture to the primary insulation.

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March 26, 2008page 46 /

Introducing a shorter termination into the mix……

Before the new “short” termination was added to this

mix the problem was likely to have been minor. As

leakage travelled down the terminations it would have

taken the path of least resistance to earth.

It is likely that current travelled across rain-sheds to

adjacent terminations which, because of more pollution

or some other factor, presented an easier path.

The effect of bolting these two very different

types of termination in such close proximity wasto create an area of electrical discharge where

the surface of the existing terminations, is sitting

at approximately 19kV, came into contact with

the bottom of the much shorter OXSU

termination, which was effectively at earth

potential.

existing earth plane

new earth

plane

March 26, 2008page 47 /

Introducing a shorter termination into the mix……

As you can see the resulting effect…Not

Good…

The instructions for the installation of these

terminations clearly specify minimum

clearances atht the components must have

from phase/phase & phase/ground.

Unfortunately this scenario could not have

been worst…Having a large number of

terminations bolted in close proximity to each

other in an area of high pollution from salt mistand introducing a low impedance path to earth

over a very short length, resulting in high

concentration of leakage current eventually

eroding the high voltage outer tubing causing

a catastrophic failure.

March 26, 2008page 48 /

Termination (Water Ingress)

This failure was caused by not having the

correct sealed lug at the outdoor

termination end.

The outdoor termination had been in

service for 15 years.

The water had migrated down the cable as

you can see in the top picture.

The water then migrated through to the

indoor termination.

There are definite signs of erosion of the

insulation at the base of the lug.

Eventually the water migrated down

towards the semi-con cut which caused it

to fail.

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March 26, 2008page 49 /

Thermal Runaway

These two examples, are proof that not

matching the correct connector to thecable cross sectional area, or the

correct compression dies to the

connector is a very expensive exercise.

Using the incorrect dies and

compression link, caused this

connector to over heat, commonly

known as (thermal run-away).

March 26, 2008page 50 /

Thank you for your time

15 Godfrey 3

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