HVDC Transmission. Michael Muhr. Institute of High Voltage Engineering and System Performance Graz University of Technology Austria P A S S I O N

Transcription

1 S C I E N C E P A S S I O N T E C H N O L O G Y HVDC Transmission Michael Muhr Graz University of Technology Austria

2 1 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

3 2 UHV Transmission Historical review & status quo AC DC Transmission Voltage in kv Time

4 3 HVDC Transmission AC DC System AC System 1 System 2 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

5 4 Technical aspects of a HV DC transmission Power transfer in UHV DC P DC = U D I DC = U D1 + U 2 D2 U D1 U R D2 = U 2 D1 U 2 R 2 D2 Limiting components Transmission lines Components in converter stations Other limiting factors Losses in UHV DC Transmission losses (I 2 R) Losses in converter stations Corona losses

6 5 HVDC Transmission HVDC Classic Line-commutated current-sourced Converter Thyristor with turn-on Capability HVDC PLUS Self-commutated voltage-sourced Converter (VSC) Semiconductor Switches with turn-on/ turn-off Capability, e.g. IGBTs Direct-light-triggered Thyristor (LTT) Up to MW MI/PPL Cable up to 600 kv OHL up to 800 kv Western Link China projects 2,200 MW 8,000 MW XPLE Cable up to 320 kv DC Half bridge up to 1,56 ka Full bridge up to 2 ka Trans Bay Cable 400 MW 5 x TenneT Offshore MW Siemens EM TS 2 HVDC

7 6 HVDC Transmission AC System A DC AC System B Controls, Protection, Monitoring To/ from other terminal 1. AC Switchyard 2. Transformers 3. Star Point Reactor 4. Insertion Resistor 5. Power Modules Converter Reactor Siemens EM TS 2 HVDC

8 7 Technical Aspects of HVDC Transmission 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

9 8 UHV DC Technology I Advantages - Higher rate of use for transmission corridors compared with AC systems. This is the reason why costs for overhead lines and the environmental 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 rectifier valves - There is a need for an active AC system which provides reactive power for commutation. This is not necessary for a HVDC system with switch off valves called HVDC Light. These systems are available up to 1000MW - DC power circuit breakers for multi terminal systems are difficult to build because there is no zero-crossing of the current

10 9 UHV DC Technology II Advantages - No skin effect and no dielectric losses - Corona losses and radio interferences are less than a comparable AC system. Specially during bad weather - Less transmission losses as a comparable AC system - The limits for magnetic fields can be easier satisfied 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 for XLPE insulations and also for insulation with impregnated paper - Transmission of alternative produced energy (for example off- and onshore wind parks, photovoltaic plants, etc.) with variable frequency and simultaneous decoupling of wind generators 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 be used cause of the space charge effects (except by HVDC Light systems) - High DC fields make it easy to pollute insulators and conductors

11 10 Developments Quelle: SIEMENS PTD SE NC Costs of HV transmission

12 11 AC / DC Hyprid Systems 0 + S - R T Required Air Distances when Dimensioning

13 12 AC / DC Hyprid Systems Maximum Operating Field Strength 400 kv - AC - Circuit DC - Circuit E max (kv/cm) (peak) E DC (kv/cm) E max (kv/cm) (peak) E AC (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

14 13 Transmission Lines - Overhead line (OHL) up to 1200 kv - Underground cable up to 500 kv - Gas-insulated line (GIL) up to 600 kv

15 14 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

16 15 Nanotechnology

17 16

18 17 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)

19 18 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 A/mm² (Cu 1-5 A/mm²) Low losses HTS-cable, 110 kv, 38 A, 1000 m losses 112 kw 2-VPE-cables parallel, 110 kv, 38 A, 1000m losses 194 kw

20 19 HTS-AC vs HTS-DC HTS1

21 20 Alternative Insulating Gases Gas Mixtures with SF 6 80%N 2 and 20% SF 6 Gas Mixtures without SF 6 N 2 and O 2 N 2 and O 2 with additive gases Dry Air Compressed Air Gas Mixtures with Fluorine

22 21 High Temperature Conductors Raising Energy Transmission in Existing Lines Hot conductors Requirements: Higher current load-bearing capacity No changes of mechanical characteristics Lower linear expansion by higher temperatures

23 22 High Temperature Conductors Interpretation of Conductor Temperatures 250 Conductor temperature [C] Al/St-conductor HT-TALconductor TAL-conductor

24 23 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

25 24 DC Application Topics - Investigations and discussion in specific topics - Technology selection - Risk of error - System features - Environmental, cost, acceptability

26 25 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)

27 26 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

28 27 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

29 28 Test, Measurement and Standardization - Special problems for the UHV range Lightning impulse (LI) (front time, overshoot) Chopped LI voltage (no deviations from IEC ) Switching impulse voltage (SI) (no deviations from IEC ) Combined and composite voltages (a wide range of voltage drop)

30 29 Test, Measurement and Standardization - Special requirements for dielectric testing of UHV equipment II Artificial rain tests (problems with deviations from IEC ) Artificial pollution tests (pollution tests at complete UHV insulations are not common)

31 30 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

32 31 Test, Measurement and Standardization - PD detection under DC stress (IEC Annex 1 - H) Trichel and Streamer Discharges at AC Test Voltage 4 ms/div Point-to-plane electrode arrangement Trichel Discharges Igniting at Negative DC Voltage 40 ms/div Lemke HPMT02

33 32 Test, Measurement and Standardization Recurrence of Cavity Discharges at AC and DC Test Voltage U. Fromm, Ph.D. Delft University Lemke HPMT02

34 33 Test, Measurement and Standardization Individual PD Pulses (pink) and Accumulated Charge (green) Recording Time: 20 s (left) and 100 s (right) 2 s/div 10 s/div 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 Lemke HPMT02

35 34 Test, Measurement and Standardization 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 Lemke HPMT02

36 35 Test, Measurement and Standardization IEC and CIGRE have installed many working groups (WG) and maintenance teams (MT) for preparing standards and recommendations for testing and measuring of HVDC systems IEC Standardization TC 14, TC 20, SC 22, TC 28, TC 42, TC99, TC 115 CIGRE Recommendations SC A2, SC A3, SC B1, SC B2, SC B3, SC C4, SC D1

37 36 Investment costs AC versus DC transmission cost over distance Comparison DC - AC Economical application of DC voltages

38 37 Thank you for your Attention!