Godfrey, Paul - Presentation

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October 25, 2007

Brief overview

MV Cable Construction/Design

Paul Godfrey

October 25, 2007page 2/

Contents

• Requirements

• Design of Paper Cables

• Design of Polymeric Cables

• Switchgear trends

• Experiences


Operation Requirements• Life time

Expected > 35 years

• Carry electrical stresses

Short circuit - Thermal impact secondsDynamic (mechanical strain)Emergency Thermal impactVoltage 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 arepaper insulated, laid up, and a paper wrapping called belt papers appliedover the laid up cores. The electrical field is contained within thecombination of insulation and belting, and the thermal mechanical

movement between these layers is restricted by the cable operatingtemperature of 65oC


Secondly, screened construction, patented by Hoshstadter is used for 11kVand above for both single core and multi core cables.

The individual cores are paper insulated with a metallic paper screen, and laidup with a conductive wrap to tie the screens electrically together. Theelectrical field is now uniformly contained within the insulating papers and thedesign 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 arelapped onto the conductor. During this process the cable is being spooledinto the impregnation tank.

2. The drying process of impregnation under vacuum and heat removes themoisture 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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October 25, 2007page 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


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).


Design - Medium Voltage “Belted” Cable < 17.5kV

conductor screen

insulation paper

bedding

carbon paperfiller

lead sheath

steel wire armour

oversheathconductor

belt insulation



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Electrical Flux – Belted Cable Construction

As the cores are not individually screened, the e lectrical 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 a re usually made from a dielectrically weakermaterial, there is a high probability that discharging will occur in the fillers.

Belted Cable (without conductor screens)


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.


Design - Medium Voltage “Screened” Cable

Metallic screen

Cotton Woven Fabric



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Lead Sheath

The lead sheath is impervious to moisture, petroleum fluids and gases.Problems associated with lead such as resistance to fatigue cracking andextrusion defects. Fractures associated with internal pressure and corrosionhave 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.


Mechanical Protection

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

Steel wire armour (SWA) or aluminiuim wire armour (AWA) for singlecore 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 faultcurrent (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 torquebalancing. Protection from the toredo worm can also be incorporated withthe application of copper/ brass tape under the armour wires.


Jacket Materials – (Oversheath)

• PVC,

• LDPE,

• MDPE,

• HDPE,

• EPR,

• Jute/Bitumen.



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MV Polymeric Design


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


CCV – (Continuous Catenary Vulcanisation)



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VCV – (Vertical Catenary Vulcanisation)


Cable Manufacturing


Cross-Sectional View of Triple Head



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MV Polymeric Cable


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 HDPE or MDPE jacket.


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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Below are the three common t ypes of insulation screens.

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

“easy strip”

“graphite and conductive” papertape

“bonded” extruded screen

Insulation Screen Types


Experience has shown that cable connections without stress control 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


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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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 resistive and c apacitive properties of the stress

control material.

Controlled Electrical Field at Screen End


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 anAC electrical field).

There are two classes of water trees:

- Bow Tie

- Vented


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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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.


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

radius of 750 mm.


Cable Core Bending Radius

• Min. Bending Radius r = 10 x D

• Special care on safety during thecable bending.



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

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

Recommended for XLPE

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


Sheath Testing

In order to detect damages or weak points on PVC/PE cable jackets, sheathtesting is carried out at the cable by applying a voltage (mostly negative DC) tothe 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” Thisvoltage is sufficient. Higher voltages will only increase the danger of damaging

the oversheath.


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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Projected Lifetimes

• Paper Insulated Cables 80 yrs

• First generation XLPE (60’s/70’s) 20 yrs

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

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


Changes of Switchgear Designs Over The Years


Terminal Boxes – Compact DesignAs 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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What The….???

Right angle bends in cores of PILCcable termination will result inbroken insulation papers.

Cable configuration in terminal box

could hardly be worse…..


What The……..??

Breakout mounted too high

in terminal box:Badly crossed cores & small

radius bends.


The material used for the red HV sleevings and “sheds” used in these

terminations include a complex additive package.

One of the key components is alumina trihydrate - Al(OH)3. Since its initial

use for the first heat shrinkable cable terminations, it has become a standard

as an essential ingredient for non-tracking materials.

This breaks down at high temperatures and has the ability to remove

conducting contamination and prevent carbon deposits that would result in

tracking.

The additional oxygen is available from the a tmosphere. The resultant

moisture has a further benefit of assisting in cooling the surface. The only

remaining solid is alumina Al2O3, which is a white deposit and is an excellent

insulating material.

Explanation of White Deposits



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The arrow shows a white area on

termination – Caused by insuffient

core clearance. This is an indication

of a high level of surface discharge

activity.

The white material is alumina, this is

the result of the activation of one key

ingredient in the non-tracking

additive package in the red core

protection sleeving and sheds.

Explanation of White Deposits


Production of Acidic Gases by Corona/Discharges

The level of discharge noted is excessive and is not desirable.

Discharge of this type gives rise to acidic gases that may have a detrimental

effect on this type of equipment. The mechanism for the formulation of thesegases due to discharges in air is shown below;

3O2 + N2 = NO2 + NO + O3

The resultant nitric oxide and nitrous oxide in the presence of moisture, formnitric acid and nitrous acid.

Ozone is also a contributory factor to the formulation of corrosion and earlydeterioration of many materials.


Termination (Water Ingress)This failure was caused by not having thecorrect sealed lug at the outdoor

termination end.

The outdoor termination had been inservice for 15 years.

The water had migrated down the cable asyou can see in the top picture.

The water then migrated through to theindoor termination.

There are definite signs of erosion of theinsulation at the base of the lug.

Eventually the water migrated down

towards the semi-con cut which caused itto fail.




Thermal Runaway

These two examples, are proof that not

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

correct compression dies to theconnector is a very expensive exercise.

Using the incorrect dies and

compression link, caused thisconnector to over heat, often known as

(thermal run-away).


Thank you for your time

Godfrey, Paul - Presentation

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