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This paper proposes an ac fault ride-through (FRT) method for high-voltage dc (HVDC) transmission systems that serve as an interconnector between the onshore ac grid and an islanded offshore ac grid. In conventional HVDC systems for offshore wind farms FRT is realized by consuming excess power with braking resistors. However, the same method cannot be applied if the offshore ac grid includes not only wind farms but also various loads, which are being considered for integration into multipurpose interconnectors or energy islands. In this article, a novel FRT method based on voltage control for HVDC interconnectors operating with bidirectional power flow is proposed. The proposed method curtails the ac voltage formed by the offshore converter according to the converter’s dc current and voltage. The effectiveness of the proposed FRT method is established through experiments on a mini model.

This paper proposes a novel circuit configuration for a high-voltage direct-current circuit breaker (HVDCCB) test bench that differs significantly from conventional test benches. The proposed test bench consists of a modular multilevel cascaded converter (MMCC) that is based on H-bridge cells, an output inductor, and an auxiliary capacitor bank. The proposed test bench is capable of generating controllable output currents up to several kiloamperes and output voltages up to several hundred kilovolts because each MMCC cell is operated by phase-shifted pulse-width-modulated (PSPWM) signals. Consequently, the proposed test bench can simulate a wide range of fault conditions within hardware limitations to test different HVDCCB types with various current and voltage ratings. The flexibility of the proposed test bench is complemented by a longer service lifetime with inherent circuit protection in the case of operational failure of the HVDCCB. The concept of the proposed test bench is verified experimentally on a downscaled test bench that consists of nine H-bridge cells and operates at an equivalent switching frequency of $\mathrm {92.5 kHz }$ .