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Fault Calculations for Variable Speed Turbines

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Full Inverter Variable Speed ... The inverter is a controlled current injection ' ... Full Inverter wind turbines should be treated as regenerative drives ... – PowerPoint PPT presentation

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Title: Fault Calculations for Variable Speed Turbines


1
Fault Calculations for Variable Speed Turbines
  • Four general generator types
  • Asynchronous (single fed) - known
  • DFIG (doubly-fed) - ½ known
  • Synchronous - known
  • Full Inverter - ?!
  • What are their fault current contributions?
  • Can they be modelled using standard tools?

2
Full Inverter Variable Speed
  • Power electronic converter isolates the generator
    from the grid, typically IGBT 4-quadrant PWM
  • Very short thermal time-constants
  • Current controlled and current limited
  • Very fast response times, typically lt5ms
  • Three-wire connection so no significant
    earth-fault current contribution compared to
    sync. machines
  • Peak Make (10ms) is a physical characteristic
  • RMS Break (50-100ms) is a controlled behaviour

3
Typical Inverter Control System
Converter with Brickwall Limiting
Turbine Control
PWM Switching
PWM reference voltages
Physical converter currents
Active and Reactive current references
4
Controlled Behaviours
  • Assume worst case i.e. FRT compliant
  • Fault contribution is then predominantly dictated
    by FRT or Grid Code requirements
  • The inverter is a controlled current injection
  • Reactive Current is fault current
  • All this assumes that the Phase Lock Loop can
    maintain lock against a faulted voltage

5
Detailed Fault Response (sim)
  • Looks familiar?
  • sub-transient
  • then transient

6
Equivalent Synchronous Machine
  • Is an equivalent synchronous machine an
    acceptable fault calculation representation?
  • There are existing saturated/unsaturated methods
    for local and remote faults
  • May need to be pessimistic for local faults when
    inverter is a constant current injection

7
Summary
  • VS WTG fault behaviour is controller dependent
  • Fault current is low by normal standards
  • Different response for local/remote faults
  • What data should manufacturers provide?
  • Are existing calculation methods sufficient?
  • Should we consider equipment failure?

8
Fault Ride Through Requirements
  • Fault Characteristics
  • Impedance like for shallow dips
  • Constant current like for deep dips

9
IEC 60909
  • Full Inverter wind turbines should be treated as
    regenerative drives
  • This is could result in very pessimistic
    assessments and so needs to be reviewed
  • Better information from manufacturers is required
    to make such assessments
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