Introduction to HVDC in GB. Ian Cowan Simulation Engineer 12 March 2018

Transcription

1 Introduction to HVDC in GB Ian Cowan Simulation Engineer 12 March 2018

2 Contents 1) History of Electricity Networks 2) Overview of HVDC 3) Existing Schemes 4) Future Schemes 5) Regulation and Ownership 6) Challenges for Development 7) Summary Page: 2

3 History of Electricity Networks

4 Origins of Modern System War of the Currents AC vs DC Edison vs Tesla / Westinghouse Streetlighting Development of networks Page: 4

5 Network of the Last 50 Years Large generators Transmission Distribution Users Page: 5

6 Introduction of HVDC Drivers for HVDC Penetration of non conventional generation Connecting offshore wind farms extensive cable lengths. improve security of supply integrated grids manage regional fluctuations enhance competition Power supply for remote areas Replace old conventional power plants Page: 6

7 Overview of HVDC

8 What is HVDC? Sending End Transformer Rectifier DC Line R I DC Inverter Transformer Receiving End I AC I AC FILTER FILTER I AC I DC I AC - AC voltage/current is fed to a HVDC converter acting as a rectifier - DC voltage/current is produced, independent of AC frequency and phase and transmitted over a conduction medium (overhead lines or cables) - DC quantities are transformed back into AC by the 2 nd converter acting as an inverter and fed into the receiving AC system Page: 8

9 Classification Interconnector Connection between to distinct AC networks (usually cross-border) Offshore wind Embedded Alternative to traditional AC upgrade AC Network 1 Location 1 Interconnector AC Connections AC Network 2 Location 2 Embedded HVDC Page: 9

10 Basic Conversion Six-Pulse Rectifier L d I d Basic conversion based on power electronic switching at various points in the 3-phase AC waveforms U A U B U C L a L b L c I a I b I c V 1 V 3 V 5 U dpn U d Suitable quasi-dc signals produced that are smoothed by additional components Valve conduction sequence V 4 V 6 V 2 I d Additional control of switching allows control over system variables (DC voltage magnitude etc.) Pole to Ground voltage, U dn U dpn U dnn Pole to Pole voltage, U d Page: 10

11 HVDC Topologies Monopole design with metallic return path with no redundancy in case of line loss Symmetrical monopole design can withstand temporary faults on either DC line (pole), with careful consideration of over-voltage on remaining pole Bipolar design adds redundancy to the system for permanent faults. One pole can still carry reduced power in the case of permanent fault on the other pole Monopole Symmetrical Monopole Bipolar Page: 11

12 Valve Development Page: 12

13 LCC vs. VSC Type: Line Commutated Converter (LCC) or Voltage Source Converter (VSC) LCC - Current-source converter (inductors used as current source) - Based on thyristor technology with controlled turn-on only VSC - Voltage-source converter (capacitor used as voltage source) - Based on IGBT technology with controlled turn-on and turn-off Self-Commutated Current-Source Few applications - VSC-HVDC - Electrical drives - LCC-HVDC - Industrial rectifiers Few applications Voltage-Source Line-Commutated Page: 13

14 LCC vs. VSC Function LCC VSC Valves Thyristor: Higher voltage & current ratings turn-on only DC Voltage ±800kV ±500kV IGBT: Lower voltage & current ratings, turn on & off DC Power ~3000MW per block (800kV) ~1000MW per block (±320kV ) Reactive power Consumes up to 60% of rated power No requirements; independent PQ control FIltering Requires filter banks Moderate to no filter banks Harmonics Generates harmonic distortion Negligible when using MMC technology Blackstart Limited application Can feed passive systems Min. short circuit level Critical in design Not critical Commutation failure Transformer Losses Common at inverter with AC faults & critical in design Converter transformer (DC stresses) Typically 0.8% of rated power per station Not critical as self-commutating Standard AC transformer (symmetrical monopole) Typically 1.0% of rated power per station Footprint Large Smaller for comparable LCC system rating Control Simpler control due to natural commutation More complex control due to self commutation Page: 14

15 Advantages of HVDC Lower losses than AC for comparable voltage level Suitable for long distance transmission Fast and flexible control of active and reactive power; Islanding control Break-even distance Additional MW capacity with controllable contribution to short circuit power Auxiliary controls available; Emergency Power Control (EPC), Power Oscillation damping (POD) etc. Connect asynchronous areas and block cascading outages Page: 15

16 Advantages of HVDC Rights of way: ± Typical transmission line right of way for 3000MW capacity HVDC based on two poles, as opposed to HVAC where three phases are needed For the same capacity, HVDC offers smaller towers and reduced rights of way compared to AC Offers less visual impact and lighter lines/cables Page: 16

17 Advantages of HVDC DC capital cost less that AC after break-even distance: Large capital expenditure required on HVDC converters. HVDC lines are cheaper than AC lines as they are smaller for comparable voltage Break-even distance Over a certain line distance, the costs break even and HVDC becomes a cheaper option in terms of capital investment The breakeven distance is even shorter for cables due to AC cable capacitance charging Page: 17

18 Advantages of HVDC Fast power electronic control in HVDC enables auxiliary services: Emergency Power Control (EPC) Fast rate of change possible to respond to fast power deviations on AC system Power Oscillation Damping (POD) Active modulation of P,Q to counteract and damp range of oscillations in AC grid, generally in the range 0.1Hz 4Hz Droop Virtual Inertia controller SubSynchronous Torsional/Control Interaction (SSTI/SSCI) Active modulation of AC voltage or P/Q to damp SSTI phenomena in the <50Hz range, based on grid LC and controller interaction Page: 18

19 Disadvantages of HVDC Technical disadvantages: AC system like a river where water flows naturally based on flow resistance Power flow needs to be actively controlled as opposed to AC where impedance and phase angle response naturally control power flows Lower reliability and availability due to complexity of protection & control More complex maintenance as spares need to be kept for specific HVDC links as there is less standardisation compared to AC DC grids are technically challenging due to lack of natural zero-crossing and fast current rates of change DC system like a pipe network were water flows need to be controlled by pumps to ensure smooth operation Page: 19

20 Existing Schemes

21 Overview of HVDC: Existing Schemes Current Interconnectors 1)Cross Channel (IFA) 2)Moyle 3)BridNed 4)EWIC 5 minute power flow data across 5 years Data from on 05/04/2017 Page: 21

22 Future Schemes

23 Overview of HVDC: Future Schemes Embedded HVDC 5)Western Link 6)Caithness - Moray 7)Western Isles 8)Eastern Link 9)Wylfa Pembroke Offshore Wind farms 20)East Anglia 21)Hornsea 22)Dogger Bank 23)Firth of Forth 24)Moray Firth HVDC ~ New Interconnectors 10)Nemo 11)ElecLink 12)NSN 13)Viking 14)IFA 2 15)FABLink 16)North Connect 17)Shetland 18)Greenwire North 19)Greenwire South Page: 23

24 Regulation and Ownership

25 Regulation and Ownership Ofgem Embedded links TOs CATOs Interconnectors Wind farms Other Networks Page: 25

26 Challenges for Development

27 Challenges for Development Technology gaps : DC breakers Power flow control Automatic network restoration High voltage DC/DC converters Global rules and regulations for operation Limited number of vendors Control interactions Generation and regulatory uncertainty Page: 27

28 Summary 1) Technology advancing quickly 2) Many new schemes in near future 3) Marketplace evolving 4) Many technical challenges still remain Page: 28

29 QUESTIONS? Page: 29