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 MW >100 MW

PWM Rectifiers and Voltage Sags Overcurrent and overvoltage/undervoltage at the dc link must be avoided during voltage sags. Balanced sags “Phase angle jumps” Unbalanced sags Space Vectors Single-line-to-ground fault Two-lines-to-ground fault Line-to-line fault

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, P s, is treated as a disturbance Sags “transformed” to disturbance in P s Disturbance rejection of P s of great importance ”Active damping” in an inner feed-back loop Inner and outer feed-back loops tuned for the same bandwidth Analytical result  Load step Reference step (2.5 pu)

Experimental Results 1(2) -45º “phase angle jump” and 50% balanced voltage sag Balanced voltage sag “Phase angle jump” Analytical results phase angle jump balanced voltage sag (2.6 pu) (2.76 pu)

Experimental Results 2(2) -45º “phase angle jump” and 50% unbalanced voltage sag Ripple due to negative sequence Reduction due to positive sequence Analytical results phase angle jump unbalanced voltage sag (2.6 pu) (0.1 pu) (0.08 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 negative- sequence voltage introduces ripples in dc-link voltage and q-component of the grid current. “Phase-angle jumps” are not critical for PWM rectifiers.