Fault ride through in a DFIG grid connect system.

Experiment Name: –

Influence of grid faults on a wind power plant’s feed point.

Experiment objective:

  • Understand the influence of voltage dips on wind power plants.
  • Understand the influence of faults on the grid.

Exercises:

  • Operating the wind power plant under the rated conditions.
  • Studying the response to a three-phase grid fault involving a voltage dip of 40%

Setup:

Theory: –

In DFIG (Doubly Fed Induction Generator) the generator is connected to the electrical grid through power electronic converters. When disturbances such as grid faults (voltage dips, short circuits, etc.) occur, the generator must remain connected and continue operating instead of disconnecting. This capability is called Fault Ride Through (FRT) or Low Voltage Ride Through (LVRT).

FRT is the ability of a wind turbine of DFIG based generator to stay connected to the grid and support it during short-term voltage faults.

During Grid Fault:

  • Grid voltage suddenly drops.
  • Stator flux changes rapidly.
  • Large voltage is induced in the rotor.
  • Rotor current increases sharply.
  • Converters may be damaged.

Rotor Induced voltage is proportional to

\[\color\red{V_r\propto\dfrac{d\psi_s}{dt}}\] \[\color\green{Where:}\] \[\color\red{V_r}\;=\;\color\green{Rotor\;induced\;voltage}\] \[\color\red{\psi_s}\;=\;\color\green{Stator\;flux}\]

During a fault, the rate of change of stator flux becomes large, causing high rotor voltage and current.

Procedures: –

  1. Switch on the machine test stand and control unit for the doubly-fed asynchronous generator.
  2. On the dynamic grid fault simulator, set the voltage dip to 40% by inserting the appropriate jumpers.
  3. Switch on the three-phase mains voltage.
  4. Switch on the dynamic grid fault simulator.
  5. Make sure that the transformer’s circuit breaker is not tripped.
  6. Open the CONTROL CENTRE instrument (the USB interface must be connected to the PC). Select the POWER CHAR mode. Set the pitch control to AUTOMATIC, and actuate the AUTO CONNECT button. Actuate the DRIVE button and set the wind speed to 12.5 m/s.

7. The generator is then connected to the network and outputs an active power of about 750 W.

8. Perform the following settings on the dynamic grid fault simulator:

  • MODE: 3-PHASE
  • ANGLE: 0
  • TIME: 150 ms

9. Select FRT monitor as the reading display mode.

10. Perform the following settings in the FRT monitor.

  • Gain IQ: 2
  • Deadband: 0.1
  • Time: 300 ms

11. Select the following display options in the FRT monitor.

  • Diagram 1: Instantaneous values (INST) Ugrid
  • Diagram 2: Instantaneous values (INST) Igrid

12. Actuate the RECORD button on the virtual instrument.

13. Press the START button on the dynamic grid fault simulator.

14. The grid fault is then activated for the set period.

15. Via the axis scaling, adjust the parameters so as to obtain meaningful readings.

    Result: –

    1. How do the voltage and current respond to faults ?
      • The voltage does not dip in the event of a fault.
      • The voltage dips by about 40% in the event of a fault.
      • The voltage dips by about 60% in the event of a fault.
      • The current rises in the event of a fault. The current drops in the event of a fault.
    2. Does the wind power plant supply reactive current in the event of a fault? Refer to the measured curves.
      • The current in the event of a fault appears to be purely active.
      • The current in the event of a fault appears to be purely reactive.
      • The displayed variables do not indicate whether reactive current is fed in the event of a fault.
    3. Analyze the voltage display in the positive sequence.
      • The voltage at the mains lies at the d-level.
      • The display in the positive sequence clearly indicates a voltage dip of 40% in the event of a fault.
      • The voltage at the mains lies at the q-level.
      • The display in the positive sequence clearly indicates a voltage dip of 60% in the event of a fault.
    4. Analyze the signal characteristics for the negative sequence.
      • The voltage and current do not exhibit any negative sequence in the event of a fault.
      • Only a negative sequence for the current is evident in the event of a fault.
      • Negative sequences do not occur given symmetric components.

    5. Transfer the time characteristics of the mains voltage and current to the placeholders provided below.

    6. Display the measured mains voltage and current in the positive sequence (POS).

    7. Display the measured mains voltage and current in the negative sequence (NEG).

    Conclusion:

    To be written by student.