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The adoptions of medium voltage in ac collection networks of large dc-connected wind farms significantly increase the ac current magnitudes during normal and fault conditions. Controlling fault currents at zero during asymmetric ac faults is possible, but it has several drawbacks such as increased risk of protection mal-operation due to the absence of fault currents, which also tends to prevent the recovery of ac voltage in postfault. Therefore, this paper presents an enhanced control strategy that exploits the induced negative sequence voltages to facilitate controlled injection of negative sequence currents during asymmetric ac faults. The proposed control not only defines a safe level of fault current in the offshore ac network but also instigates immediate recovery of the ac voltage following clearance of ac faults, which avoid protection mal-operation. In addition, the positive sequence voltage set-point of the offshore modular multilevel converter (MMC) is actively controlled by considering the negative and zero sequence voltages, which effectively avoids the excessive overvoltage in the healthy phases during asymmetrical ac faults. The theoretical basis of the proposed control scheme is described and its technical viability is assessed using simulations.
Shi et al. (Mon,) studied this question.