Low voltage ride through control of a cascaded high power converter for direct-drive permanent magnet wind generators

Today´s wind turbine power is approaching 5MW and above, where medium voltage power transfer is preferred for applications in such power range. This paper has identified three generator and converter topologies suitable for medium-voltage large wind turbines. In particular, the cascaded modular converter is investigated regarding the converter topology and control strategy with a focus on low voltage ride through (LVRT) capability. The generator-side rectifier can automatically reduce the generator output power during grid voltage drop through the dc-link voltage control loop. Hence, the converter cell dc-link voltage is maintained and the active power balance is achieved. The guideline to determine the rectifier control bandwidth and to predict the generator speed increase during grid fault is given in the paper. The grid-side cascaded H-bridge converter can generate reactive power as required to support the grid voltage during grid voltage drop and the active power output is limited by the current rating of power devices. A voltage-oriented vector control scheme allows independent regulation of the active and reactive power transferred to the grid. The phase-shifted PWM used for modulating the cascaded H-bridge converter may cause unintended active power flow into and out of individual power module, which could result in the dc-link over-voltage, especially when the converter only delivers reactive power to the grid as during the grid faults. The analytical expression of the active power flow due to the modulation is derived and a solution to avoid dc-link over-voltage is given. The simulation results of a 3.3kV, 3MW system has validated the proposed converter topology and control scheme for LVRT.