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S C I E N C E P A S S I O N T E C H N O L O G Y
Institute of High Voltage Engineering and System PerformanceGraz University of TechnologyAustria
Michael Muhr
HVDC Transmission
Definition
HV … High Voltage• AC Voltage > 60kV ≤ 220kV• DC Voltage > 60kV ≤ 220kV
EHV … Extra High Voltage• AC Voltage > 220kV ≤ 800kV• DC Voltage > 220kV ≤ 600kV
UHV … Ultra High Voltage• AC Voltage ≥ 800kV• DC Voltage ≥ 600kV
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UHV Transmission
• Historical review & status quo
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1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010
1200
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600
500
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Time
Tran
smis
sion
Vol
tage
in k
V
AC
DC
HVDC Transmission
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DCSystem
System 1 System 2
ACAC
• Long Overhead Lines with high Transmission Capacity and limited Right-of-Way
• Long Cable Transmissions• Asynchronous Interconnections• New Links in Grids where Short-
Circuit Currents are at upper Limits• Fast Control of Power Flow
Siemens EM TS 2 HVDC
Technical aspects of a HV DC transmission
• Power transfer in UHV DC
• Limiting components• Transmission lines• Components in converter stations• Other limiting factors
• Losses in UHV DC• Transmission losses (I2R)• Losses in converter stations• Corona losses
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RUU
RUUUUIUP DDDDDD
DCDDC ⋅−
=−
⋅+
=⋅=22
22
212121
HVDC Transmission
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Trans Bay Cable 400 MW5 x TenneT Offshore 576 – 900 MW
HVDC Classic HVDC PLUSLine-commutated Self-commutatedcurrent-sourced Converter voltage-sourced Converter (VSC)
Thyristor with turn-on Capability Semiconductor Switches with turn-on/ turn-off Capability, e.g. IGBTs
Direct-light-triggered Thyristor (LTT) Up to 10000 MW MI/PPL Cable up to 600 kV OHL up to 800 kV
Western Link 2,200 MWChina projects 8,000 MW
Siemens EM TS 2 HVDC
XPLE Cable up to 320 kV DC Half bridge up to 1,56 kA Full bridge up to 2 kA
HVDC Transmission
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System A System B
DCAC AC
To/ fromotherterminal
21 3 4 5
Controls, Protection, Monitoring
6
1. AC Switchyard
2. Transformers
3. Star Point Reactor
4. Insertion Resistor
5. Power Modules
6. Converter Reactor
Siemens EM TS 2 HVDC
Technical Aspects of HVDC Transmission
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• Power control• Discrete control (tap changers of HVDC transformers)• Continuous control (thyristors)
• Reliability
• Transmission medium• Overhead lines - up to ± 800 kV• Cable - up to ± 500 kV
UHV DC Technology I
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Advantages- Higher rate of use for transmission corridors
compared with AC systems. This is the reasonwhy costs for overhead lines and theenvironmental effects are less
- Conductor can be used till to their thermal limits- There are no problems with long distance
transmission- Simple design of conductors, because there are
only two (bidirectional)- If one phase is defect, the bipolar system can
be used as a monopole system- Only active power will be transmitted, so there
is no need for compensation stations- Connection of asynchronous systems- Fast regulation of power flow by the use of
rectifier valves- Fast current regulation also during an error- Stability support of an AC system with a parallel
used HVDC system
Disadvantages- No direct transformation of direct current- High costs for converter stations- Complexity of rectifier control- Necessity of filters in converter stations,
because of harmonics caused by rectifiervalves
- There is a need for an active AC system whichprovides reactive power for commutation. Thisis not necessary for a HVDC system with switchoff valves called HVDC Light. These systemsare available up to 1000MW
- DC power circuit breakers for multi terminalsystems are difficult to build because there isno zero-crossing of the current
UHV DC Technology II
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Advantages- No skin effect and no dielectric losses- Corona losses and radio interferences are less
than a comparable AC system. Specially duringbad weather
- Less transmission losses as a comparable ACsystem
- The limits for magnetic fields can be easiersatisfied than a low frequency magnetic field
- Long distance see cable transmission possible- Insulation of a DC cable can be thinner than an
insulation of a comparable AC cable. This is forXLPE insulations and also for insulation withimpregnated paper
- Transmission of alternative produced energy(for example off- and onshore wind parks,photovoltaic plants, etc.) with variablefrequency and simultaneous decoupling of windgenerators from the AC system
Disadvantages- Multi terminal systems are difficult to manage- Load flow reversal by using a HVDC system
means voltage reversal. XLPE cables can’t beused cause of the space charge effects (exceptby HVDC Light systems)
- High DC fields make it easy to pollute insulatorsand conductors
Developments
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Costs of HV transmission
Quelle: SIEMENS PTD SE NC - 2002
AC / DC – Hyprid Systems
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R T
S
0 +
-
Required Air Distances when Dimensioning
AC / DC – Hyprid Systems
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Maximum Operating Field Strength
400 kV - AC - Circuit DC - Circuit
Emax (kV/cm)(peak)
EDC (kV/cm) Emax (kV/cm)(peak)
EAC (kV/cm)(peak)
400 kV DC 23,2 3,2 -27,1 3,0
500 kV DC 24,4 4,0 -33,1 3,0
Transmission Lines
- Overhead line (OHL)up to 1200 kV
- Underground cableup to 500 kV
- Gas-insulated line (GIL)up to 600 kV
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Nanotechnology
Cable technology – alternating voltage- and direct voltage use of the medium voltage range up to 500kV
• Reduction of the space charge
• Improved partial discharge behaviour
• Raise of the break-down field strength
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Nanotechnology
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Technical Development
High Temperature Superconductivity (HTS)
Cable technology – new developments for applications in the medium high voltage range
• Less losses
• Lower weight
• Compact arrangment
• Current temperatur 138º K (-135º C)
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HTS – Cables
• For distribution networks• For transmission networks
only as pilot projects (e.g. 138 kV, 600 MVA)• For very large power
no very high voltages can be used (eg. 200 kV for 5 GW)• Cooling important
liquid nitrogen (-195 °C)• Current density 200-300 A/mm²
(Cu 1-5 A/mm²)• Low losses
HTS-cable, 110 kV, 38 A, 1000 mlosses 112 kW
2-VPE-cables parallel, 110 kV, 38 A, 1000mlosses 194 kW
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HTS-AC vs HTS-DC
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HTS1
Alternative Insulating Gases
Gas Mixtures with SF6
• 80%N2 and 20% SF6
Gas Mixtures without SF6
• N2 and O2
• N2 and O2 with additive gases
Dry Air
Compressed AirGas Mixtures with Fluorine
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High Temperature ConductorsRaising Energy Transmission in Existing Lines
Hot conductors
Requirements:
• Higher current load-bearing capacity
• No changes of mechanical characteristics
• Lower linear expansion by higher temperatures
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High Temperature ConductorsInterpretation of Conductor Temperatures
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0
50
100
150
200
250
1930 1960 2000
Con
duct
or te
mpe
ratu
re [C
]
TAL-conductor
HT-TAL-conductor
Al/St-conductor
DC Application – Topics
- Requirements of network conditions and insulation coordination
- Dimensioning of insulators and equipment
- Special features and operational behaviour of systems and components
- Test technique with direct voltage
- PD detection
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DC Application – Topics
- Investigations and discussion in specific topics
- Technology selection
- Risk of error
- System features
- Environmental, cost, acceptability
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HVDC Behaviour
- Dielectric behavior under DC stress (breakdown, polarization, conduction processes, electrostatic force, temperature) at different insulation systems (gaseous, liquid, solid)
- Charging of solid insulation f.e. in oil-paper-systems
- Detection of particles in gas insulated DC systems (moving particles, particles on the insulator surface)
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HVDC Behaviour
- Evaluation of the time constant of transient AC-DC field distribution of DC GIS
- Dielectric strength of alternative insulation gases by DC stress and different pressures
- Influence of climatic conditions and surface contamination at DC stress
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Test, Measurement and Standardization
- Special recommendations for testing of HVDC systems
• PD detection with AC voltage• PD detection with DC voltage• Rated DC withstand voltage test• Superimposed impulse voltage test• Polarity reversal test
- Standardization for tests of electrical equipment stressed by direct voltages
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Test, Measurement and Standardization
- Special problems for the UHV range
• Lightning impulse (LI) (front time, overshoot)
• Chopped LI voltage (no deviations from IEC 60060-1)
• Switching impulse voltage (SI) (no deviations from IEC 60060-1)
• Combined and composite voltages (a wide range of voltage drop)
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Test, Measurement and Standardization
- Special requirements for dielectric testing of UHV equipment II
• Artificial rain tests (problems with deviations from IEC 60060-1)
• Artificial pollution tests (pollution tests at complete UHV insulations are not common)
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Test, Measurement and Standardization
- Specific electrical testing of transformers for HVDC transmission (especially tests for the valve winding)
- Test requirements on MO surge arresters for HVDC converter stations
- Test procedures of HVDC cables
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Test, Measurement and Standardization
- PD detection under DC stress (IEC 60270 + Annex 1 - H)
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Trichel Discharges Igniting at Negative DC Voltage
Point-to-plane electrode arrangement
Trichel and Streamer Discharges at AC Test Voltage
40 ms/div
4 ms/div
Lemke HPMT02
Test, Measurement and Standardization
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Lemke HPMT02U. Fromm, Ph.D. Delft University
Recurrence of Cavity Discharges at AC and DC Test Voltage
Test, Measurement and Standardization
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Lemke HPMT02
The mean PD current (slope of the cumulative charge – green trace) remains almost constant, even if magnitudes and repetition rate are scattering over an extremely wide range
Individual PD Pulses (pink) and Accumulated Charge (green) Recording Time: 20 s (left) and 100 s (right)
2 s/div 10 s/div
Test, Measurement and Standardization
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Lemke HPMT02
Reproducibility of PD Pulse Repetition Rate vs. Test Voltage
After 1 hour conditioning at 3.5 kV the recovery time between consecutive PD pulses became well reproducible, where the pulse magnitude was nearly independent on the test voltage level
Test, Measurement and Standardization
IEC and CIGRE have installed many working groups (WG) andmaintenance teams (MT) for preparing standards and recommendationsfor testing and measuring of HVDC systems
• IEC – StandardizationTC 14, TC 20, SC 22, TC 28,TC 42, TC99, TC 115
• CIGRE – RecommendationsSC A2, SC A3, SC B1, SC B2,SC B3, SC C4, SC D1
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Comparison DC - AC
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Investment costs
• AC versus DC transmission cost over distance
• Economical application of DC voltages
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Thank you for your Attention!Thank you for your Attention!