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Voltage Sag Response of PWM Rectifiers
for Variable-Speed Wind Turbines
by
Rolf Ottersten, Andreas Petersson and Kai Pietiläinen
financial supported by
Sydkraft AB & Swedish National Energy Agency
PWM Rectifiers
Main advantages (PWM Rectifiers)
• Bidirectional power flow.
• Controllable dc-link voltage.
• Good power quality.
New Grid Codes
0.3-100 MW
>100 MW
PWM Rectifiers and Voltage Sags
• Balanced sags
• “Phase angle jumps”
• Unbalanced sags
• Single-line-to-ground fault
• Two-lines-to-ground fault
• Line-to-line fault
• Space Vectors
Overcurrent and overvoltage/undervoltage at the dc
link must be avoided during voltage sags.
Control System Structure
Main characteristics
•
“Fast” synchronous-frame current control loop.
•
“Slow” dc voltage control loop.
•
“Slow” estimator (PLL) for grid-voltage
synchronization.
DC-Link Control Structure
• Generator power, Ps, is
treated as a disturbance
• Sags “transformed” to
disturbance in Ps
Load step
Reference step
• Disturbance rejection of
Ps of great importance
• ”Active damping” in an
inner feed-back loop
Analytical result 
(2.5 pu)
• Inner and outer feed-back
loops tuned for the same
bandwidth
Experimental Results 1(2)
-45º “phase angle jump” and 50% balanced voltage sag
phase angle jump
balanced voltage sag
Analytical results
“Phase angle jump”
(2.76 pu)
Balanced voltage sag
(2.6 pu)
Experimental Results 2(2)
-45º “phase angle jump” and 50% unbalanced voltage sag
phase angle jump unbalanced voltage sag
Analytical results
Ripple due to negative sequence
(0.08 pu)
(0.1 pu)
Reduction due to positive sequence
(2.6 pu)
Experimental Results Summary
• The reduction in the positive-sequence voltage is critical,
since the q-component of the grid current is increased with
the size of the sag.
• The negative-sequence voltage causes ripple in dc-link
voltage and q-component of the grid current.
• “Phase-angle jumps” are not critical for PWM rectifiers.
• Analytical results in agreement with experiments
Ride-Through Capabilities
It may happen that the SG power exceeds its maximum value during a
voltage sag. The SG power must then be somehow stored or dissipated.
• Rotor energy storage. If the pre-sag power needs to be restored
moments after the sag has been cleared, then blade pitching is
preferably avoided until the rotor approaches over speed.
• A “braking” chopper at the dc link can dissipate the excess wind
energy
• DC-link energy storage. Mainly applicable for small and short-duration
voltage sags.
Conclusion
• The reduction in the positive-sequence voltage is critical.
• The worst-case scenario is a balanced sag with zero
remaining voltage.
• For the candidate control system structure: the negativesequence voltage introduces ripples in dc-link voltage and
q-component of the grid current.
• “Phase-angle jumps” are not critical for PWM rectifiers.