godfrey, paul - presentation
TRANSCRIPT
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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
October 25, 2007page 3/
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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October 25, 2007page 4/
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
October 25, 2007page 5/
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.
October 25, 2007page 6/
Conductor Screens - Electrical Field Distribution
• Electrical field in a cable
“unscreened conductor”
• Electrical field in a cable
“screened conductor”
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October 25, 2007page 7/
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
October 25, 2007page 8/
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”
October 25, 2007page 9/
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
October 25, 2007page 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).
October 25, 2007page 12 /
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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October 25, 2007page 13 /
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)
October 25, 2007page 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.
October 25, 2007page 15 /
Design - Medium Voltage “Screened” Cable
Metallic screen
Cotton Woven Fabric
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October 25, 2007page 16 /
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.
October 25, 2007page 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 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.
October 25, 2007page 18 /
Jacket Materials – (Oversheath)
• PVC,
• LDPE,
• MDPE,
• HDPE,
• EPR,
• Jute/Bitumen.
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October 25, 2007page 19 /
MV Polymeric Design
October 25, 2007page 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
October 25, 2007page 21 /
CCV – (Continuous Catenary Vulcanisation)
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October 25, 2007page 22 /
VCV – (Vertical Catenary Vulcanisation)
October 25, 2007page 23 /
Cable Manufacturing
October 25, 2007page 24 /
Cross-Sectional View of Triple Head
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October 25, 2007page 25 /
MV Polymeric Cable
October 25, 2007page 26 /
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.
October 25, 2007page 27 /
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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October 25, 2007page 28 /
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
October 25, 2007page 29 /
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
October 25, 2007page 30 /
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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October 25, 2007page 31 /
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
October 25, 2007page 32 /
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
October 25, 2007page 33 /
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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October 25, 2007page 34 /
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.
October 25, 2007page 35 /
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.
October 25, 2007page 36 /
Cable Core Bending Radius
• Min. Bending Radius r = 10 x D
• Special care on safety during thecable bending.
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October 25, 2007page 37 /
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.
October 25, 2007page 38 /
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.
October 25, 2007page 39 /
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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October 25, 2007page 40 /
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
October 25, 2007page 41 /
Changes of Switchgear Designs Over The Years
October 25, 2007page 42 /
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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October 25, 2007page 43 /
What The….???
Right angle bends in cores of PILCcable termination will result inbroken insulation papers.
Cable configuration in terminal box
could hardly be worse…..
October 25, 2007page 44 /
What The……..??
Breakout mounted too high
in terminal box:Badly crossed cores & small
radius bends.
October 25, 2007page 45 /
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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October 25, 2007page 46 /
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
October 25, 2007page 47 /
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.
October 25, 2007page 48 /
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.
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October 25, 2007page 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 theconnector is a very expensive exercise.
Using the incorrect dies and
compression link, caused thisconnector to over heat, often known as
(thermal run-away).
October 25, 2007page 50 /
Thank you for your time