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High Voltage DC Transmission HVDC

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... converter operated as a line-commutated inverter converts DC power into AC power ... The converter requires alternating AC voltage to operate as an inverter. 6 ... – PowerPoint PPT presentation

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Title: High Voltage DC Transmission HVDC


1
High Voltage DC Transmission (HVDC)
  • IEE Review
  • March 2000
  • Engineers Australia
  • February 2003

2
When did it become feasible?
  • In 1954 when high voltage (80 KV) mercury-arc
    valve was developed
  • Currently more than 50 GW of power is transmitted
    via HVDC
  • Australia is now in the league

3
The basic structure of HVDC
4
How does it operate?
  • AC power is fed to a converter that operates as a
    rectifier
  • The output is DC power independent of the AC
    frequency and phase
  • DC power is transmitted via an overhead line, a
    cable or a busbar
  • The second converter operated as a
    line-commutated inverter converts DC power into
    AC power

5
Thyristor operation
  • A gate pulse is applied during positive half of
    VTHY (between the anode and cathode) current
    will flow
  • As long as current flows in the forward direction
    conduction continues
  • When the current tries to reverse the thyristor
    turns off
  • The converter requires alternating AC voltage to
    operate as an inverter

6
The rectifier
  • Graetz three-phase bridge (this on the left is a
    single-phase bridge)
  • There are higher harmonics on both DC and AC side
  • Special techniques are required to reduce
    harmonics connection schemes, harmonic AC
    filters (capacitive compensation of reactive
    power)

7
12-pulse thyristor bridge
8
Valves within the bridge
  • Many series connected thyristors
  • The maximum peak voltage of a thyristor is some
    8.5 kV
  • Maximum DC voltage is 600 KV
  • Therefore one valve can contain more than 100
    series connected thyristors
  • The next figure 14 series connected 100mm
    diameter thyristors giving maximum power rating
    of over 100 MW

9
Single semi-tier of ALSTOM thyristors
10
Why HVDC overhead lines
  • Power electronics adds to the cost compared with
    AC only substation
  • Transmission towers are simpler only two
    conductors like in a single-phase AC, sometimes
    earth return
  • The ability of DC power control benefits AC
    system
  • The break-even distance of HVDC line is usually
    in the range 800-1000 km

11
Why HVDC cables
  • In AC cables a capacitive current changing
    direction every half cycle limits the power
    transmitting capability of a cable and its
    practical length
  • In HVDC cables there is no a steady-state
    capacitive current and the limit to the cable
    length is the cost and I2R losses

12
Other advantages of HVDC
  • Transmission between unsynchronised systems (ie
    50 and 60 Hz) and between two independent AC
    systems without applying any operational
    restrictions
  • Parallel AC and DC transmission helps to increase
    transmitting capacity
  • HVDC minimises short-circuit capacity increases
  • Environmental advantage no more power lines
    when AC lines modified to DC transmission

13
New power semiconductor devices
  • GTO (gate turn off thyristor) and IGBT (insulated
    gate bipolar transistor)
  • Self-commutating devices with turn-off and
    turn-on capability
  • The figure shows four-level, capacitor-clamped,
    voltage- sourced converter and its output voltage
    waveform

14
Basslink interconnector
  • The project approved in September 2002
  • Will link Loy Yang in Gippsland, Victoria, across
    Bass Strait to Bell Bay in northern Tasmania
  • 360 km long (295 km undersea)
  • Continuous capacity 480 MW with 10 hours per day
    overload of 600 MW
  • Both sides will benefit (peak hydro power for
    Victoria, reliable supply to Tasmania, especially
    in drought)
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