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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.
Eindhoven, 27 March 2006 Property of Prysmian
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.
Eindhoven, 27 March 2006 Property of Prysmian
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).
Eindhoven, 27 March 2006 Property of Prysmian
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.
Eindhoven, 27 March 2006 Property of Prysmian
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.
Eindhoven, 27 March 2006 Property of Prysmian
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
Eindhoven, 27 March 2006 Property of Prysmian
+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 (-)
Eindhoven, 27 March 2006 Property of Prysmian
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)
Eindhoven, 27 March 2006 Property of Prysmian
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):
Eindhoven, 27 March 2006 Property of Prysmian
The HVDC Cable system is typically made by:
End TerminationsCable
Intermediate Joints
Eindhoven, 27 March 2006 Property of Prysmian
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
Eindhoven, 27 March 2006 Property of Prysmian
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
Eindhoven, 27 March 2006 Property of Prysmian
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
Eindhoven, 27 March 2006 Property of Prysmian
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
Eindhoven, 27 March 2006 Property of Prysmian
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
Semiconducting paper tapes
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
Eindhoven, 27 March 2006 Property of Prysmian
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
Eindhoven, 27 March 2006 Property of Prysmian
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
Semiconducting paper tapes
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
Polypropylene yarn serving
Typical Weight = 40 to 80 kg/m
Typical Diameter = 110 to 160 mm
Eindhoven, 27 March 2006 Property of Prysmian
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
Polypropylene yarn serving
Typ. Weight = 20 to 35 kg/m
Typ. Diameter= 90 to 120 mm
Eindhoven, 27 March 2006 Property of Prysmian
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
Eindhoven, 27 March 2006 Property of Prysmian
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 +=
Eindhoven, 27 March 2006 Property of Prysmian
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 +=
Eindhoven, 27 March 2006 Property of Prysmian
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)
Eindhoven, 27 March 2006 Property of Prysmian
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.
Eindhoven, 27 March 2006 Property of Prysmian
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).
Eindhoven, 27 March 2006 Property of Prysmian
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.
Eindhoven, 27 March 2006 Property of Prysmian
SOME EXAMPLES OF SUBMARINE
PROJECTS
Eindhoven, 27 March 2006 Property of Prysmian
BasslinkBasslink (Victoria(Victoria--Tasmania)Tasmania)
Loy Yang / Victoria
Georgetown / Tasmania
Eindhoven, 27 March 2006 Property of Prysmian
Eindhoven, 27 March 2006 Property of Prysmian
BasslinkBasslink: Installation: Installation
Eindhoven, 27 March 2006 Property of Prysmian
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
Eindhoven, 27 March 2006 Property of Prysmian
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