hv dc prysmian

Eindhoven, 27 March 2006

HIGH VOLTAGE CABLES FOR

DIRECT CURRENT TRANSMISSION

Eindhoven, 27 March 2006 Property of Prysmian

Presentation is based on answering to four main questions:

1. Why Cables for HVDC Transmission ?

2. Which are the main characteristics of an HVDC Cable system ?

3. How is an HVDC Cable made ?

4. Which are the critical issues to be considered in HVDC cable design ?

And will finish with few remarkable examples.


1st question: Why Cables for HVDC Transmission?

Firsts of all, Cables are used when Overhead Lines (that are simple and cheap but with a significant impact on ambient) cannot be built forenvironmental reasons or when power shall be transmitted underwater(through sea, lakes or rivers).

In first case we have the so called Underground High Voltage Cablesystems, in the second case Submarine Cable systems.


In general the power is transmitted using Alternating Current (AC) by simplyconnecting the two networks.

The two networks must be SYNCHRONOUS: same frequency, same phasing(different voltages can be managed with transformers).

Rigid Connection:

Disturbances are also transmitted between the two networks. Power flow control is difficult, lead by impedance of transmission lines (mainly reactances).


Cables are cylindricalcapacitors

A cable under AC voltage is subject to a capacitive current that is proportional tothe 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 of 0,2-0,3 [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 the capacitive current become similar in magnitude (even ifin quadrature) to the active current that the cable is asked to transmit: lossesare very much increased and consequently actual cable rating is reduced.


With DC transmission, the things for the cable system are much simpler: f = 0;Consequently, capacitive current and main effects relevant to reactances are eliminated. 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.


However, systems are operated in AC; therefore DC transmission requiresConverter Stations at both ends to convert AC to DC at sending point and DC to AC at receiving end.

The two networks are not required to be syncronised; they can have differentfrequency and voltage. The power flow is simply controlled by voltage drop.

The system, overall, acts like a Generating Power Station that is injectingpower into the receiving network.

Flexible Connection

P

PG

AC Network 345 kV, 60 Hz


+HVDC CABLE

P

i

i

GROUND RETURN

Conventional High-Power Converters use Tyristors (controlled Diodes): the current must flow in one direction only.

Therefore, when the power flow is reversed, also the polarity on the HVDC cable is reversed: here is a simple example:

i

i+_

+A B

_

i

i

A B

+_

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

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


TYPICAL HVDC CONFIGURATIONS

MONOPOLE (WITH METALLIC RETURN)

MONOPOLE

+

CABLE i

M.V. RETURN CABLE

Laid Separated or bundled

(Hokkaido-Honshu 1;

Moyle; SVE-POL;Basslink)

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

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)

_

P/2

P/2HV _

HV +(Cross

Channel;Nor-Ned)


2nd question: Which are the main characteristicsof an HVDC Cable system ?

In general, an HVDC system can be composed by various sections, sometimeincluding OHL lines, land and submarine cable. Here is an example for the Basslink Interconnection (Tasmania-Victoria, AUS):


The HVDC Cable system is typically made by:

End TerminationsCable

Intermediate Joints


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

Unloadingfrom Drum

Lay in Trench

PullingWinch


For Submarine Cables, the Installation is done by laying the cable on the seabottom by using suitable Ships, that can accomodate large quantity of cableon board, stored on rotating platforms.

GIULIO VERNE SHIP FEATURES:

•Length Overall 133 m

•Moulded Breadth 30 m

•Draft 8.5 m

•Tonnage (tons) 10617

•Dynamic Positioning Control

•Propulsion Power 5,710 kW

•Capstan 6 m 50 tons

•Turntable capacity 7,000 tons


Very often, the cable is protected on the sea bottom against possibledamages caused by fishing tools and anchors by various methods.

Jetting Machinefor Burial

Cast IronShells

Sand/CementBags

Concrete Block Mattresses


3rd question: How is an HVDC cable made ?

Cables used for HVDC transmission are mainly of three types:

• MI: Insulated with special paper, impregnated with high viscosity compound• SCFF: Insulated with special paper, impregnated with low viscosity oil• Extruded: Insulated with extruded polyethylene-based compound

Self-Contained Fluid Filled ExtrudedMass Impregnated


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

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


Typical Manufacturing Flow Diagram of a Mass Impregnated Cables. Lengthsof up to 30-50 km of cable can be lapped and impregnated, without need of intermediate joints. For very long lengths, factory joints are included.

TURNTABLE

CONDUCTOR STRANDING

TURNTABLE

IMPREGNATION VESSEL PAPER LAPPING MACHINE

LEAD EXTRUDER

PE SHEATH EXTRUDERARMOURING MACHINE

TURNTABLES


Self Contained Fluid-Filled Cables are used for very high voltages (they are qualified for 600 kV DC) and for short connections, where there are no hydraulic limitations in order to feed the cable during thermal transients; at present used for Voltages up to 500 kV 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

Metallic tape reinforcement

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


Typical Weight = 40 to 80 kg/m

Typical Diameter = 110 to 160 mm


Extruded Cables for HVDC applications are still under development; at present they are used for relatively low voltages (up to 150 kV DC), mainly associated with Voltage Source Converters, that permit to reverse the power flow without reversing the polarity on the cable.

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

It is therefore possible to have anaccumulation of charges in localised areas inside the insulation(space charges) that, in particularduring rapid polarity reversals, can give rise to localised high stress and bring to accelerated ageing of the insulation.

Conductor

Semiconducting compound

XLPE extruded insulation

Semiconducting compound

Lead alloy sheath

Polyethylene jacket

Syntetic tape or yarn beddings

Steel armour


Typ. Weight = 20 to 35 kg/m

Typ. Diameter= 90 to 120 mm


4th question: Which are the critical issues to be considered in HVDC cable design ?

In AC, and in general for rapid applications or changes of the voltage, the electrical stress is led by a capacitive distribution. The insulation can be supposed as divided in concentric capacitors, all in series. It results:

Conductor

Insulation

E [kV/mm]

r [mm]

Ei

EeV

⎟⎠⎞

⎜⎝⎛⋅

=

rirer

VrEln

)(

Being therefore:

EeEi >Typical value for EHV (AC) Cables are:

Ei = 10 to 14 kV/mmEe = 5 to 7 kV/mm


In DC the things are a bit more complicated.

Let’s suppose from time t0 a DC voltage V is applied across insulation:In the first period, the stress distribution is capacitive, but after some time, under static conditions the charges can move and the stress distribution becomes resistive.

V [kV]V

t0 t

The resistive distribution is led by the insulation ‘conductivity’ ơ , that is not similar to the capacitive one (led by ‘permittivity’ε), because ơ varies, as a function of the stress E and temperature θ:

Where stress is higher, insulation conductivity is better (lower resistance) and the charges are moved away from the high stress zone to the low one.

Outer

Insul.

r [mm]

E [kV/mm]

Inner

Insul.

Capacitive Stress

Resistive Stress

E0

βαθσσ +=


If we now circulate a current I in the conductor, then Joule losses W in the form of heat are produced.

Conductor θC

WOuter Insulation θI

Temp. Drop Δθ

I

The heat must cross the insulation to be dispersed outside, thus causing a temperature drop Δθ across the insulation. The inner part of the insulation is hotter than the external one, therefore the conductivity is futher increase by the temperature effect,

and consequently the charges are futher moved away from theinner to the outerinsulation layer.

In conclusion, depending on stresses and temperatures, there could be a stress inversion, with outer stress on insulation higher than the inner one: Ee > Ei

Outer

Insul.

r [mm]

E [kV/mm]

Inner

Insul.

Resistive Stress, COLD

Resistive Stress, HOT (Loaded Cable)

Ei

Ee

Typically α= 0,1 ; Β= 0,03

E0

βαθσσ +=


Another cause of electrical stress is when an impulse voltage Vp arrives from the OHL line or is generated internally due to equipment manouvre or malfunctioning (switching surge). The worst case is when the impulse is of opposite polarity with respect to the cable charging voltage Vo.

In this case the Electrical stress on the cable E is calculated as due to the whole voltage variation, and subtracting the pre-existing resistive stress at nominal voltage:

Vo

- 400 kV

Vp

+ 900 kV

Vo+Vp

1300 kV t

V E (Vp) = ECAPACITIVE (Vo+Vp) – ERESISTIVE (Vo)


Then, we have seen that Electrical design and Thermal design of the cable are very much related. Other important aspects to be considered in the cable system design are the following:

Maximum conductor temperatureThis is related to the insulation performance and expected cable life (in general 30 to 40 years). The calculations must take into account installation configuration and environmental parameters, like thermal properties of the surrounding ground and of the trench backfill, temperatures, etc.

Mechanical designThe cable shall be capable to withstand the pulling forces during installation, bending stresses, the fatigue due to dynamic thermo-mechanical forces (e.g.in unfilled pipes), etc.


Mechanical aspects are very important in submarine cable systems, where special tests are carried out to simulate the cable installation from the ship and cable recovery from the bottom and repairing operations. The picture shows the bending/pulling line capable of a pulling force up to 200 ton (2 MN).

For cables impregnated with low viscosity oil, hydraulical aspectshave to properly be taken into account

Valve

Gauge

Expansion Tank

Electromagnetic Field calculations are sometime required to comply with Country regulations or laws (more frequently for AC transmission rather than for DC).


The final performance of the cable and its accessories, based on the sound design, manufacturing technology and materials used is checked with Type Tests carried out on a miniature circuit including all the parts that will constitute the real cable system: cable, joints and terminations.

Tests are very severe, including thermal daily cycles, polarity reversals and impulse. They are recommended by CIGRE and last several weeks.


SOME EXAMPLES OF SUBMARINE

PROJECTS


BasslinkBasslink (Victoria(Victoria--Tasmania)Tasmania)

Loy Yang / Victoria

Georgetown / Tasmania


BasslinkBasslink: Installation: Installation


Italy Italy -- GreeceGreecePOWER 500 MW

VOLTAGE 400 kV DC

ROUTE LENGTHS:- Submarine 163 km- Land 43+1 km

WATER DEPTH 1000 m

IN SERVICE FROM 2000

NR. OF CABLES 1 HV

CABLE TYPE Paper, MI

HVDC CABLE SIZE 1250 mm2

SEA ELECTRODES


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

POWER 660 (750) MW

VOLTAGE 500 kV DC

ROUTE LENGTHS:- Submarine 82 km- Land 20 km

NR. OF CABLES 1 HV + 1 MR

HVDC CABLE SIZE 2100 mm2

MET.RETURN SIZE 2000 mm2

CABLE TYPE Paper, MI

RFS July 2007

345 & 230 kV XLPE AC Systems

hv dc prysmian

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