Offshore grids and HVDC

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					Offshore grids and HVDC

Janaka Ekanayake and Tim Green
Content

• Main drivers for offshore grid

• Technologies
   – Current Source Converter based
   – Voltage Source Converter based


• Multi-terminal
Current Offshore Wind Farms in the UK

                      972MW installed capacity, further
                      3.5GW being implemented in
                      2010/11
                      All AC Radial connections to shore
                      Wind turbine rating of between 2 –
                      3.6 MW (except Beatrice – 5 MW)
                      Nine offshore wind farm zones
                      licences which could generate upto
                      32 GW was recently announced.
                      Some of them are situated as far as
                      195 km
    Transmission Possibilities
               Turbines Generate at ~1kV with a
                  rating of between 2 to 5MW

                  Transformers inside Turbines
                     step voltage up to 33kV

                       Maximum Rating of 33kV
                           Cable is 40MW

                      Large Offshore wind farm
                         capacity > 100MW

    Multiple                                                      High Voltage
  33kV cables                                                         AC
                                       High Voltage
                                           DC

                   Wind farm
                connecting point   Onshore                     Offshore
                                   converter                   converter
                                               DC link cable
Main AC grid

                                                                                 Offshore wind farm
   Reasons for choosing HVDC transmission
   offshore                The break-even point between AC and
                                                                                                                      DC transmission depends on the
                      Capital Costs                                                                                   distance to shore and the power rating.
                                              Wind generator and its
                                                power electronic
                                                    interface
                                                                                                                            Main costs:
                                                                                                            Grid


                                                                                                                            HVAC – cables
                                                                                                        transformer




                                                                        Wind farm side
                                                                          converter
                                                                                         Network side
                                                                                          converter
                                                                                                                            HVDC – converter stations

                                                                                                                            3-core 132kV AC cable x5
                          Increasing cable length and ratings                                                               (with compensation)
                                  HVDC more economical

                   Comparison of Distance vs. Cost for a 1 GW offshore wind farm



                         Break-even point
                         for 132 kV AC and
                         2x500 MW DC
Cost per MW (£k)




                         Transmission


                                                                                                                                                  Ref: Econnect, East Coast
                                                                       Break-even point                                                           Transmission Network
                                                                       for 220 kV AC and
                                                                                                                                                  Technical Feasibility Study.
                                                                       2x500 MW DC
                                                                       Transmission                                                               2008, Crown Estate.




                                     Distance from shore (km)                                                         2x 1core 300kV bipolar HVDC
Planned offshore connections - UK




 • Two HVDC offshore transmission links to strengthen the north
   to south interconnection are proposed.
 • Alternatives are on-land reinforcement of North South circuits
   as well as more effective management of existing assets
 • Offshore wind farm connections on a multi-terminal
   configuration is possible
European Super Grid
‘Oceans of Opportunity’, published Sept 2009, EWEA
                                                       A widespread electricity
                                                          network across Europe
                                                          would have a variety of
                                                          advantages:
                                                       • Deregulation of energy
                                                         markets
                                                       • Regional balancing of
                                                         energy generation
                                                       • Increased security of
                                                         supply
                                                       • Reduced dependency on
         Currently existing        Currently planned
                                                         fuel imports
         Under study                                   • Smoothing of load
         Under study-EWEA recommendation                 spikes due to the
         EWEA recommended grids by 2020
         EWEA recommended grids by 2030
                                                         continental spread of
                                                         time zones
                                                                            7
HVDC technologies
                    Current Sourced
                    Converter (CSC)
                    Thyristor based




                    Voltage Sourced
                    Converters
                    IGBT based
                                                 Bipole 1

                         Control
              Bipole 2
                         Building


                                                                Filters




Filters




          Cable Route
                         SVC 2      400kV GIS
                                    Substation          SVC 1
Inside a thyristor valve hall
Naptune HVDC
station
Long Island
Current Source versus Voltage Source



                                                •   AC side
 •   AC side                                        –   Inductor to couple two voltage sources
     –   Established voltage required               –   Reactive power supply not required
     –   Reactive power required by converter   •   DC side
 •   DC side                                        –   Capacitors holds the voltage constant
     –   Inductor for constant current              –   Direct current can reverse giving power
     –   Direct current is unidirectional               reversal
     –   Power reversal through voltage             –   DC capacitor (& converters) feeds the
         reversal                                       fault
     –   Inherent fault (dc side) current       •   Switches
         limiting                                   –   Self-commutated (IGBT, GTO)
 •   Switches                                   •   Power power ratings up to 400 MW
     –   Line commutated (Thyristor)
                                                •   Several converters can operate on
 •   Power ratings up to 7,000MW                    the same DC network
Why Use Voltage Source HVDC
 •   Freedom to operate at any P and Q combination
 •   Ability to operate in a weak grid and even black-start
 •   Fast acting control
 •   No commutation-failure regions to be avoided
 •   Can use polarised cables
 •   Good sine wave-shapes; reduced filter components

 But
 • Ratings presently very much lower than classic current
   source
 • Power losses are higher
Evolution of HVDC technologies
 MW or kV




            source: IEEE Power and Energy Magazine
VSC Operation

                         Electronic                         • Voltage source
    DC source
                           switch                             converter creates an
        vdc                                                   AC voltage from a DC
         2                S1                                  voltage (using pulse-
                                              Load            width modulation or
                                                              similar)
        vdc
                                                       vL
         2                S2
                                                            • The voltage
                                                              difference across the
                         Squire wave
                                                              interface reactor is
                                    Fundamental
                                                              the key to operation
vdc 2                           sinusoidal component


vdc 2

                S1-ON           S1-ON
                S2-OFF          S2-OFF
Pulse Width Modulation
           Modulation signal
               (50 Hz)



                                                                   to S1


                                                                   to S2

                               Comparator               Inverter
       Carrier triangular
       waveform (2 kHz)




                                                Fundamental
   vdc 2                                    sinusoidal component




    vdc 2
    Power Flow Control with VSCs
                                                                        Reactive
                                                                         power
      Converter 1               Converter 2
                      DC link
AC                                            Q
                           C                            Converter
                                                  AC   current rating
                    vdc
                                                                                   Active
                                                                                   power
                                              P
P




     • With a controllable voltage source, in-phase and in-
       quadrature current can be created at will
     • Proper choice of converter AC voltage leads to any
       combination of P and Q on AC side.
P-Q capability
                                           • Full real power rating
                                             available in either
                                             direction

                                           • Independent reactive
                                             power at both
                                             converters

                                           • But reactive power
                                             limited by voltage
                                             considerations and
                                             overall current rating
                                             of converter


  source: IEEE Power and Energy Magazine
Series Valves and Multi-Level
A Series Valve             • Voltages required of
                             HVDC are far above
                             the rating of
                             semiconductor
                             devices (approx
                             ±150kV against 5kV)
                           • Some sort of series
                 1
                     VDC
                 2
                             connection is
                             needed
                              – Direct series
                                connection (ABB
                                approach)
                              – Multi-level
                                converters (Siemens
                                approach)
Multi-Level Power Converters
    Multi-terminal HVDC
                      • Might be used with
                        collection from
                        multiple offshore wind
                        farms in Round 3
                      • Might use multiple in-
               ii       feed sites onshore

i
 Multi-Terminal Configurations
• Provides some redundancy; but
  does the cost-benefit analysis
  support investment
• It is not certain that interconnecting
  DC links will be economic:
   – Cables and cable laying are very expensive
     so shortest routes to shore are favoured
   – The redundancy offered by multi-terminal
     systems may not bring significant savings in
     “spilled” energy in the event of an outage

• Three technical issues
   – Power flow control
   – Fault management
   – Grid-code compliance
VSC HVDC With DC Faults
           • A fault on the DC side draws
             uncontrolled currents through the
             converter diodes
                – The fault could be “cleared” with AC
                  circuit breakers but this would be
                  needed on all terminals and require a
                  shutdown of the whole system
                – Solution might be DC breakers or in
                  different converter topologies that can
                  block the fault current themselves



                 I Fault




             Fault
Concluding Remarks
• HVDC connections are now considered to
  strengthen the existing network and for offshore
  connections.
• VSC HVDC has several key operational
  advantages and allows greater network control
• Unrestricted control of DC power at each node
  makes multi-terminal DC feasible
• DC Fault-blocking required for multi-terminal
  operation
• However, use of off-shore DC network to support
  onshore AC network opens a great many
  questions

				
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posted:2/21/2011
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