HVDC Power Links

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HVDC Power Links

• Development of HV Converters
• Vast Majority of Electric Power Transmission is
  A.C.
• Why HVDC over A.C.?
• Lower Total Investment
• Lower Losses for Long Line Length
• Technically more Desirable

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Advantages Disadvantages of HVDC

• Advantages
• Lower line costs
• Load sharing capabilities
• Connect systems of different frequencies
• No upper limit on possible line length

• Disadvantages
• Higher terminal costs
• Only viable for transmission lengths over 600-800
  km

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Comparison of Costs of AC DC Transmission
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Cables

• Laying cables
• Structure of a cable
• Conductor
• Armor
• Insulation
• Critical design factors
• DC voltage rating
• Polarity reversal
• Transient over voltage

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HVDC Operation
Three Phase Donor Network
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Power Electronics

• AC to DC conversion
• DC to AC conversion
• High power thyristors
• Insulated gate bipolar transistors (I.G.B.Ts)
• Active filtering of harmonics

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Insulated gate bipolar transistors

• Advantages of I.G.B.T for DC to AC conversion
• High gate impedance
• Ratings up to to 2.5kV and 60 amps
• Switching speeds in the kHz range

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DC to AC conversion using I.G.B.Ts

• Gate drive generated by a microprocessor based
  controller
• Phase and amplitude of output can be altered
  instantly

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Thyristors

• Thyristors are used in AC to DC conversion
• Replaced mercury arc rectifiers from 1970 onwards.

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Thyristor converters for AC to DC conversion
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AC and DC Filtering

• Filtering reduces harmonic content
• Active filters
• Smaller size
• Higher efficiency
• Tolerate slight changes in frequency

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AC active filtering
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Fenno-Skan HVDC Submarine Cable.

• Sweden - Finland
• Monopolar Link
• 500 MW
• 400 kV
• Nordic Power System

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Simplified diagram of the Fenno-Skan Submarine
HVDC Link

• 400 kV AC grid
• Converter Stations
• Converter Transformers
• AC filters
• DC filters

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Fenno-Skan HVDC Submarine Cable

• Swedish Converter Station
• Foreground - Shunt Capacitors
• Background - AC filter

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Fenno-Skan HVDC Submarine Cable

• Thyristor Valves
• Three Thyristor Valves suspended from the
  ceiling in Finland
• 7 kV Forward Blocking Voltage
• Water Cooled
• Light Guides

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Fenno-Skan HVDC Submarine Cable

• A.C. Filters
• Sweden - 80 MVAr AC filter, 160MVAr Shunt
  Capacitor Bank
• Filters banks in both stations similar
• Sweden - 11th 13th
• Finland - 24th 36th
• Automatic switching of Reactive Banks

• DC Filters
• No HV DC filter on Swedish station
• Filter to reduce telephone interference
• Finland - DC filter consisting of two 0.9uF
  branches
• Smoothing Reactor

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Fenno-Skan HVDC Submarine Cable

• Control System Features
• Remotely Controlled
• Modes of Operation
• Constant Power Control
• Normal Mode
• DC voltage kept at maximum and the current is
  controlled to give desired power
• Emergency Power Control

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HVDC Power Links

• Long Term Feasibility
• Future Improvements
• Capacitor Commutated Converters (CCCs)
• Optical DC Transducers
• Question Time