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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

Institute of High Voltage Engineering and System Performance

1

UHV Transmission

• Historical review & status quo


2

1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010

1200

1100

1000

900

800

700

600

500

400

300

200

100

Time

Tran

smis

sion

Vol

tage

in k

V

AC

DC

HVDC Transmission


3

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


4

RUU

RUUUUIUP DDDDDD

DCDDC ⋅−

=−

⋅+

=⋅=22

22

212121

HVDC Transmission


5

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


XPLE Cable up to 320 kV DC Half bridge up to 1,56 kA Full bridge up to 2 kA

HVDC Transmission


6

System A System B

DCAC AC

To/ fromotherterminal

21 3 4 5

Controls, Protection, Monitoring

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1. AC Switchyard

2. Transformers

3. Star Point Reactor

4. Insertion Resistor

5. Power Modules

6. Converter Reactor


Technical Aspects of HVDC Transmission


7

• 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


8

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


9

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


10

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


12

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


13

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


14

Nanotechnology


15


16

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


18

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


20

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


21

High Temperature ConductorsInterpretation of Conductor Temperatures


22

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


23

DC Application – Topics

- Investigations and discussion in specific topics

- Technology selection

- Risk of error

- System features

- Environmental, cost, acceptability


24

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)


25

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


26

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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- 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)


28


- 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)


29


- 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


30


- PD detection under DC stress (IEC 60270 + Annex 1 - H)


31

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



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Lemke HPMT02U. Fromm, Ph.D. Delft University

Recurrence of Cavity Discharges at AC and DC Test Voltage



33

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



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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


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


36

Investment costs

• AC versus DC transmission cost over distance

• Economical application of DC voltages


37

Thank you for your Attention!Thank you for your Attention!

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