highvoltage dclandandsubmarine cablesystem ......with dc, the things for the cable system are much...

Spring 2010 ICC Education Subcommittee – 24 March, 2010 Nashville, USA

HIGH VOLTAGE DC LAND AND SUBMARINECABLE SYSTEM

Ernesto Zaccone

Practical Considerations


HVDC cables are mainly used for submarineapplications were overhead lines cannot be used

HVDC overhead lines are more common for landapplications but some important HVDCunderground cables land connections have beenrealized and are also planned for the nearfuture.

THE USE OF HVDC CABLES


WHY TO USE HVDC TRANSMISSION

The electric power transmission started more than 1century ago with DC but AC soon offered some betterpractical applications.

The approximative relation for the transmissiblepower is:

sin21

X

VVP

R

VVP

2

22

21

For AC

For DC

The line factor that is limiting the DC power transmissionis the conductor resistance R


Cables arecylindricalcapacitors

A cable under AC voltage is subject to a capacitive current that is proportionalto the frequency f[Hz], to the voltage V[V], to the unitary capacitance C[μF/km] and to the cable length L[km]: I = 2·π· f · C · V · LCables for HV-AC transmission typically have a capacitance of the order of0,2-0,3 [μF/km] therefore require capacitive currents of 10 to 25 [A/km],depending on system voltage and frequency.

For short lengths (few kilometers) this is not a problem, but for long lengths,e.g. above 60-80 km depending on the voltage, the capacitive current becomesimilar in magnitude (even if in quadrature) to the active current that the cableis asked to transmit: losses are very much increased and consequently actualcable rating is reduced.

AC TRANSMISSION


With DC, the things for the cable system are much simpler: f = 0;Consequently, capacitive current and main effects relevant to reactances areeliminated. Only conductor resistance plays the major role.

Transmission (Joule) losses are: W [W] = R · L · I 2 (+ W Earth Return)and Voltage Drop: ΔV [V] = R · L · I (+ ΔV Earth Return)

Practically, there are no limits for the Transmission Length, quite independentlyfrom transmission Voltage and Power.

DC TRANSMISSION


Systems are operated in AC; therefore DC transmission shall be associatedwith AC-DC Converter Stations at both ends.

The two networks are not required to be syncronised; they can have differentfrequency and voltage.

The system, overall, acts like aGenerating Power Station that isinjecting power into the receivingnetwork.

PP

AC Networke.g. 345 kV, 60 Hz

PG

AC-DC CONVERSION


Conventional High-Power Converters useTyristors (controlled Diodes): the currentflows in one direction only and the polarityreversed Line Commutated Converter (LCC).

Therefore, when the power flow is reversed, also the polarity on the HVDCcable is reversed: here an example:

+HVDC CABLE

P

i

i

GROUND RETURN

+HVDC CABLE

P

i

i

GROUND RETURN

+HVDC CABLE

P

i

i

GROUND RETURN

Transferring power from side A to B,clockwise direction of current, cableis at positive voltage (+)

Transferring power from side B to A,to keep same direction of current,cable is at negative voltage (-)

_

i

iA B

+_

i

i+_

+A B

_

i

iA B

+_

_

i

iA B

+_

ii

iiA B

+_

ii

ii+_

++A B

LINE COMMUTATED CONVERTER


The New Generation of Converters (VSC – VoltageSource Converters) use use IGBT Transistors. TheAC voltage is ‘built’ as liked; there are no constraintson current direction and therefore there is nonecessity to reverse the polarity when the power flowis reversed

Therefore, when the power flow is reversed, the direction of current isreversed but the polarity of the HVDC cables is the same: here an example:

Transferring power from side A to B,clockwise direction of current, onecable is at positive (+) and one atnegative (-) voltage

Transferring power from side B to A,to keep same polarity of cables butwith anticlockwise direction ofcurrent

i

iA B

ii

ii+A B

VOLTAGE SOURCE CONVERTERS

P/2

P/2HV_

HV+

P/2

P/2HV_

HV+

+

-

-


Higher conversion losses

Limited experience

Limited power

Can feed isolated loads (oil platforms,wind parks, small islands, etc.), mediumpower

Modularity, short deliv.time

Small space and envir.impact

No polarity reversal

Standard equipment

Needs strong AC networks

Cannot feed isolated loads

Polarity reversal

Large space occupied

Special equipment (trafo, filters)

Less no. of cables, lighter

No limits in length

Low cable and conv. Losses

Power flow control

Very high transmiss. power

Heavy cable

Length (50-150 km)

Rigid connection/Power control

Require reactive compensation

High short circuit currents

Simple

No maintenance

High Availability

Drawbacks/LimitationsAdvantagesTransmission Solution

ACAC

AC

ACAC

DC - LCCConventional

ACAC

DC - VSC

Higher conversion losses

Limited experience

Limited power

Can feed isolated loads (oil platforms,wind parks, small islands, etc.), mediumpower

Modularity, short deliv.time

Small space and envir.impact

No polarity reversal

Standard equipment

Needs strong AC networks

Cannot feed isolated loads

Polarity reversal

Large space occupied

Special equipment (trafo, filters)

Less no. of cables, lighter

No limits in length

Low cable and conv. Losses

Power flow control

Very high transmiss. power

Heavy cable

Length (50-150 km)

Rigid connection/Power control

Require reactive compensation

High short circuit currents

Simple

No maintenance

High Availability

Drawbacks/LimitationsAdvantagesTransmission Solution

ACAC

AC

ACAC

DC - LCCConventional

ACAC

DC - VSC

Some Considerations on Transmission Systems


TYPICAL HVDCCONFIGURATIONS

MONOPOLE (WITH METALLIC RETURN)

MONOPOLE

+

CABLEi

M.V. RETURN CABLE

Laid Separatedor bundled

(Hokkaido-Honshu 1;

Moyle;SVE-POL;Basslink;Neptune)

P+

CABLEi

M.V. RETURN CABLE

Laid Separatedor bundled

(Hokkaido-Honshu 1;

Moyle;SVE-POL;Basslink;Neptune)

P

BIPOLE WITH EMERGENCY ELECTRODES

BIPOLE WITHOUT METALLIC RETURN

( Majority ofOld Systems:

SA.CO.I;ITA-GREECE;

Fennoskan;Baltic Cable )

+

CABLE i

Cathode Anode

+i

PSEA RETURN

( Majority ofOld Systems:

SA.CO.I;ITA-GREECE;

Fennoskan;Baltic Cable )

+

CABLE i

Cathode Anode

+i

PSEA RETURN

HV

(Cook-Strait;Vancouver 1;Skagerrak;

Haenam-Cheju)

HV+P/2

2 . v

P/2

_ HV

(Cook-Strait;Vancouver 1;Skagerrak;

Haenam-Cheju)

HV+P/2

2 . v

P/2

_

BIPOLE WITH METALLIC RETURN

HV+P/2

2 . v

P/2HV

v

v

(Hokkaido-Honshu 2;Gotland 2)

_

BIPOLE WITH METALLIC RETURN

HV+P/2

2 . v

P/2HVHV

v

v

(Hokkaido-Honshu 2;Gotland 2)

_

P/2

P/2HV_

HV+ (CrossChannel;Nor-Ned;

Transbay)

P/2

P/2HV_

HV+ (CrossChannel;Nor-Ned;

Transbay)


300

400

525

600

D.C.Fluid FilledCable Systems

ROUTE LENGTH kmA.C. one 3-phase system D.C. one bipole

SYSTEM

VOLTAGE

kV

1200 MW

1000 MW

800 MW

600 MW

400 MW

No Theoretical limit for D.C.

Mass-impregnatedTraditional or PPL insulatedD.C. Cable Systems

> 2400 MW3500 MW

120 140

A.C./D.C. Fluid FilledCable Systems

A.C. Extruded Insulation Cable Systems10

60

150

0 40 60 80

230

100

Extruded D.C. Cable Systems(or conventional MI)

AC vs. DC - TRANSMISSION OPTIONS

A.C. Extruded or Fluid Filled Cable Systems


CABLES

A (FUNDAMENTAL) COMPONENTOF HVDC SYSTEMS


Copper conductor

Semiconducting paper tapes

Insulation of paper tapes impregnated with viscous compound


Lead alloy sheath

Polyethylene jacket

Metallic tape reinforcement

Syntetic tape or yarn bedding

Single or double layer of steel armour (flat or round wires)

Polypropylene yarn serving

Typical Weight = 30 to 60 kg/m

Typical Diameter = 110 to 140 mm

Copper conductor


Insulation of paper tapes impregnated with viscous compound


Lead alloy sheath

Polyethylene jacket


Syntetic tape or yarn bedding

Single or double layer of steel armour (flat or round wires)




Mass Impregnated Cables are the most used; they are in service for morethan 40 years and have been proven to be highly reliable. At present used forVoltages up to 500 kV DC. Conductor sizes typically up to 2500 mm2.


Self Contained Fluid-Filled Cables are used for very high voltages (they are qualifiedfor 600 kV DC) and for short connections, where there are no hyd raulic limitations inorder to feed the cable during thermal transients; at present used for Voltages up to 500kV DC. Conductor sizes up to 3000 mm2.

Conductor of copper or aluminium wires or segmental strips


Insulation of wood-pulp paper tapes impregnated with lowviscosity oil

Semiconducting paper tapes and textile tapes

Lead alloy sheath


Polyethylene jacket

Syntetic tape or yarn beddings

Single or double layer of steel armour (flat or round wires);sometime copper if foreseen for both AC and DC use, in orderto reduce losses in AC due to induced current




Conductor of copper or aluminium wires or segmental strips


Insulation of wood-pulp paper tapes impregnated with lowviscosity oil

Semiconducting paper tapes and textile tapes

Lead alloy sheath


Polyethylene jacket


Single or double layer of steel armour (flat or round wires);sometime copper if foreseen for both AC and DC use, in orderto reduce losses in AC due to induced current





In fact, an Extruded Insulation(generally PE based) can besubjected to an uneven distributionof the charges, that can migrateinside the insulation due to theeffect of the electrical field.

It is therefore possible to have anaccumulation of charges inlocalised areas inside the insulation(space charges) that, inparticular during rapid polarityreversals, can give rise to localisedhigh stress and bring toaccelerated ageing of theinsulation.

Extruded Cables for HVDC applications are still under development; atpresent they are used for relatively low voltages (up to 300 kV DC), mainlyassociated with Voltage Source Converters, that permit to reverse the powerflow without reversing the polarity on the cable.

Conductor

Semiconducting compound

Extruded insulation


Lead alloy sheath

Polyethylene jacket


Steel armour


Typ. Weight = 20 to 35 kg/m

Typ. Diameter= 90 to 120 mm


Conductor


Extruded insulation


Water swellable tape

Metallic screen

Polyethylene jacket

Typ. Weight = 10 to 30 kg/m

Typ. Diameter= 40 to 120 mm

EXTRUDED INSULATION HVDC LAND CABLE


- Cu Conductor: 1500 mm2- Insulation: Mass impregnated paper- Armour: Galvanized steel- Overall diameter: 121 mm- Weight of cable: 43 kg/m

CABLE SYSTEM - HVDC 400 KV MI CABLES

- Cu Conductor: 2000 mm2- Insulation: Mass impregnated paper- Overall diameter: 121 mm- Weight of cable: 38.5 kg/m

Submarine HVDC Cable Land HVDC Cable


TURNTABLE

CONDUCTOR STRANDING

TURNTABLE

IMPREGNATION VESSEL PAPER LAPPING MACHINE

LEAD EXTRUDER

PE SHEATH EXTRUDERARMOURING MACHINE

TURNTABLES

Typical Manufacturing Flow Diagram of a submarine cables. Main differenciesbetween Mass Impregnated Cables and Extruded Cables are highlighted inthe yellow coloured area.

FactoryJoint

EXTRUSION LINE (CCV)DE-GASSING TANK

PAPER CABLE

EXTRUDED CABLE

OR


CONDUCTOR ON TURNTABLE BEFORE INSULATION


Paper lapping line


Impregnation vessel


MI COMPOUND VISCOSITY

10

100

1000

10000

100000

0 20 40 60 80 100 120 140

temperature °C

visc

osity

cSt

Properties of the MI Compound

The compound used for the mass impregnated HVDC powercables is solid at working temperatures


.

A new insulation system consisting of PaperPolypropylene Laminate (PPL) has beendeveloped for HVDC applications (after longexperience of this kind of insulation for ACapplications).

Extensive qualifications carried out forsystem voltages up to 600 kV havedemonstrated capability to safely operate ata temperature of 85 °C

Test loop atCESI, h=21m

High Performances MI Cable


SUBMARINE CABLES – TYPES OF CONDUCTORS


I c amb W d 0.5 T 1 T 2 T 3 T 4

R ac 10 3 T 1 1 1 T 2 1 1 2 T 3 T 4

I c amb W d 0.5 T 1 T 2 T 3 T 4

R ac 10 3 T 1 1 1 T 2 1 1 2 T 3 T 4

AC AND DC CABLES CURRENT RATING

The current rating of underground and submarine cables is mainly affected bythe losses in the conductor. For the AC cables there are additionally losses inthe other cable components that may strongly affect the cable current rating.

I c amb

R dc 10 3 T 1 T 2 T 3 T 4

AC cables

DC cables• Rac: the AC resistance of conductor is approx 5-20%

higher than the DC resistance• Wd: the dielectric losses are voltage depending and may

be 5-10% of the conductor losses• λ1: The losses in the metallic screen may be of 10%

and may be higher both for submarine and land cablesdepending on the cable design and installation mode.

• λ2: The losses in the metallic armor are applicable tosubmarine cables only and may be very high, generallythe design of the cable is selected in order not tooverpass the conductor losses.


BASIC REQUIREMENTS OF SUBMARINE CABLES

• Long continuous lengths

• High level of reliability with practical absenceof expected faults

• Good abrasion and corrosion resistance

• Mechanical resistance to withstand all laying andembedment stresses

• Minimized environmental impact

• Minimized water penetration in case of cable damage


• Power to be Transmitted

• Route Selection - Seabed Geology, Thermal Resistivity of Seabed

• Length of Cable

• Water Depth

• Protection Requirements - Burial Depth, Fishing Activity, Marine Activity

• Security of Supply

• Environmental Considerations

• Economic Viability

KEY POINTS TO CONSIDER WHEN

SELECTING A SUBMARINE CABLE


Mechanical Protection – Armour Design

SINGLE ROUND WIRE ARMOUR(it covers the vast majority of the submarine installation requirements,including windmill applications)

DOUBLE ROUND WIRE ARMOUR (uni-directional)

DOUBLE ROUND WIRE ARMOUR (contra-directional)

ROCK ARMOUR

PLASTIC COATED WIRES

STEEL TAPE ARMOUR PLUS WIRES

DOUBLE STEEL STRIP ARMOUR

NON-MAGNETIC ARMOUR


CONSIDERATION ON SUBMARINE CABLES ARMOUR DESIGN

Key requirements: Robustness, abrasion and corrosion resistance

The cable shall be capable of withstanding all the mechanical stresses due to storage,handling, installation, burial on the sea bottom but also recovery in case of damage andre-deployment and burial/protection after repair.

Traditionally and confirmed by the experience, the submarine cables shall bearmoured with one layer, called SWA (more common design for shallow waterapplications), or two layers, called DWA (deep water applications and special increasedprotection against outer injuries, bottom roughness and abrasion) of metallic wires.

Mostly used materials for armour are:

- Hot dip galvanised low carbon steel (BL~400-500 Mpa) for HVDC cables and low-rating 1-core AC cables

- Hard drawn copper for 1-core high rating AC cables

- rarely, stainless steel wires or high carbon, high tensile steel


Rock Armour; 7mmangle 50degRock Armour; 7mmangle 50deg

Individually PE covered 4mm wiresIndividually PE covered 4mm wires

Sometime armour wires are required tobe covered by a plastic sheath, eitherindividually (each wire) or overall, inparticular for offshore use when theplatform is actively cathodicallyprotected.

A specially resistance armour to abrasionand crushing is the so called ‘rock type’.The outer layer is applied with a short pitch(typically angle of 45 to 60 deg), made ofbig wires (e.g. 6-7 mm), over a thick PPyarn bedding (4-5 mm).


The HVDC Cable System is typically made by:

End Terminations,Outdoor or Indoor type

Submarine andLand Cable

Intermediate andtransition Joints

In general, an HVDC System includes converter stations and a transmission linewhich can be composed by various sections, sometime including OHL lines, landand submarine cable.


MONOPOLE LAND INSTALLATION

BIPOLE LAND INSTALLATION

TYPICAL HVDC LAND CABLE INSTALLATION


Road Level

500

1400

500

600

100

300

400 kV DC SCFF Cable

Weak Mix

MV Return Cables

Triple Duct

Pilot Cable

Backfilling

ITALIAN LAND SECTION (43 KM LONG) 100% UNDERGROUND CABLEITALIAN LAND SECTION (43 KM LONG) 100% UNDERGROUND CABLE

1 HV CABLE, 2 MV RETURN CABLES, 1 PILOT CABLE, 1 TRIPLE DUCT IN1 HV CABLE, 2 MV RETURN CABLES, 1 PILOT CABLE, 1 TRIPLE DUCT IN THE SAME TRENCHTHE SAME TRENCH

MECHANIZED LAYING SYSTEM USED OUTSIDE URBAN AREAS, WITH THE ADVAMECHANIZED LAYING SYSTEM USED OUTSIDE URBAN AREAS, WITH THE ADVA NTAGE OF:NTAGE OF:

LIMITED IMPACT ON PUBLICLIMITED IMPACT ON PUBLIC TRAFFICTRAFFIC

NO PULLING TENSION EVEN FOR LONGNO PULLING TENSION EVEN FOR LONGLENGTHS OF CABLE (UP TO 1200 m)LENGTHS OF CABLE (UP TO 1200 m)

SAFE CABLE HANDLINGSAFE CABLE HANDLING

SIMULTANEOUS LAYINGSIMULTANEOUS LAYINGAND PROTECTIONAND PROTECTION

ItalyItaly –– Greece, land cable installationGreece, land cable installation


ItalyItaly –– Greece, land cable installationGreece, land cable installationMechanized layingMechanized laying


In the Land (Underground) sections, Installation is generally done from largedrums, in excavated trenches, being the cable directly buried or pulled inplastic pipes.

Unloadingfrom Drum

Lay inTrenchLay inTrench

PullingWinchPullingWinch


Limit is determined by transportation, both in terms ofdimensions and weight. Usually for trucks on roadmax. width is 2,5 m and height 3,5 m. Using specialcarries it is possible to use 4,2 m flange. For largedrums, there are no protection battens. For drums ofland cables, or everytime a protection is required,battens are applied (either steel or wood for thesmallest); increase of dimension is from 0 to 0,1 mmax. on overall diameter.

Wooden battensWooden battens


LAND SECTIONS INSTALLATIONLAND SECTIONS INSTALLATIONrural and urban zonesrural and urban zones

Transportation and trenchingTransportation and trenching

Neptune: New JerseyNeptune: New Jersey –– Long Island (NY)Long Island (NY)


Parameters:T = tension at ships sheave, (kN)To= bottom tension, (kN)S = length of suspended cable, (m)X = distance to touch down point (m)H = the water depth, (m)W = unit weight of cable (in water) (kN/m) = the angle of the cable at sheaveC = min bending radius at touch down

(catenary constant)

T

To

Xθ

H

C

S

T

To

Xθ

H

C

S

Practical formulae to use:T = W·H + To = W·(H + C) ·To= W·CS = T·sin / w = C·tanX = C·sinh -1 (S/C) = tan -1(S/C) = cos -1(C/(H+C)) = cos -1(S·C/T) = tan -1(W·S/T)C = T·cos /W

Nominal Laying tensile forces. The cable suspended from the shipassumes the configuration of a Catenary :

HVDC SUBMARINE CABLE INSTALLATION


Nominal Laying tensile forces: Test forces according to Cigre ELECTRA 171:


SeaSea--Trial (example of the SAPEI 500 kV project)Trial (example of the SAPEI 500 kV project)

• Lay of 6 km of cable including a repair joint and an earthingconnection at maximum depth (1620 m)

• Stay for 6 days st-by with cable suspended at max.depth• Recovery of all cable and un-load back to factory• HV test at 720 kV• Inspection of most significant parts of cable and accessories


HVDC MONOPOLE INSTALLATION

Typical bundle installation of a monopoleHVDC cable system in shallow water


GIULIO VERNE

B0040030

BUNDLE OF TWO SUBMARINE CABLES

ACCURATEPOSITIONING

SIGNALFROM

SATELITEFOR

POWER CABLE

ROPEPOLYPROPYLENE

POWER CABLE

HVDC BIPOLE BUNDLE INSTALLATION

• The cables will be simultaneously laid and buriedin a Bundle configuration wherever possible

• Minimal environmental impact

• The Cable bundle is taken to the trench bottom bya stinger

200 kV dcXLPE Cable

Fibre OpticCable

Lead+PE Sheath


Shore Crossings - Horizontal Directional Drilling

BoreHole

Duct

HVDC Cable

Bore Hole

DuctElectrode cable

Duct

F.O. cable

5-6 m

Drilling the pilot hole Hole reaming

Duct installation


Planning of installation activities

• Offshore activities and hazards

• Weather and oceanographic factors (climatology, tides, currents, waves,water temperature, etc.)

• Seismicity

• Identification of local facilities, local hazards at landing sites

• IS and OOS utilities location

• Bathymetry

• Morphology and nature of the seabed

• Sub-bottom characteristics (mainly necessary in case cable burial isforeseen)

• Permitting (planned developments along the route, marine delimitations,permits and regulations)


Survey Charts


First Panel: Bathymetry


Second Panel: Superficial Features


Third Panel: Profile


Initial and Final Cable landing


Inshore & Shore End Works


Burial EquipmentBurial Equipment -- HydroplowHydroplow


Burial EquipmentBurial Equipment -- Jetting MachinesJetting Machines


Burial EquipmentBurial Equipment –– Trenching MachinesTrenching Machines


Sand/Cement Bags ProtectionSand/Cement Bags Protection


Mattresses ProtectionMattresses Protection


Cast Iron Shell ProtectionCast Iron Shell Protection


Submarine Cables Repair Tecniques


SUBMARINE CABLE REPAIR SEQUENCE

Note: The availability of a spare cable strongly reduce the repair time


FINAL LAYING OFTHE JOINTEDOR REPAIRED CABLE BY USINGTHE QUADRANT


MAINTENANCE

HVDC submarine MI and Extruded cables aremaintenance free the only termiantions may needsome inspections

For HVDC land cables it may be convenient but notmandatory to periodically check the integrity of theouter jacket in order to identify eventual third partiesdamages


MAJOR HVDC SUBMARINE

PROJECTS


World’s Major HVDC Submarine Cable Links


SA.PE.I (Sardinia-Peninsula Italiana)

VOLTAGE 500kV

POWER 1000 MW

Bipolar configuration (2x500 MW)

WATER DEPTH 1650 m

CABLE LENGTH 2x420 km

CABLE TYPE Mass Impregnated

RFS 2008 Pole 1 and

2011 Pole 2

The Deepest HVDC Cable


Main reason for choosing HVDC:Long submarine cable distance andnon-synchronous AC systems

NorNed: Norway – The Netherlands HVDC cableTransmission capacity: 700 MWDC Voltage: ± 450 kVLength of DC cable: 2*580 km

The Longest HVDC Cable


0

100

200

300

400

500

600

0 200 400 600 800 1000Power per cable [MW]

Ope

ratin

gVo

ltage

[kV]

HVDCExtru

ded HVDC M.I.

(incl. PPL)

HVDC TRANSMISSIBLE POWER – TRENDS


THANK YOU FOR YOUR ATTENTION

any questions?

highvoltage dclandandsubmarine cablesystem ......with dc, the things for the cable system are much...

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