AC Fault Ride-Through Strategy of a Transformerless Soft Normally Open Point

This paper focuses on the soft normally open point (SNOP) which has no transformers and connects to the 10 kV distribution network through the arc suppression coil grounding mode. A mathematical model containing positive-, negative-, and zero-sequence components under AC fault is established by the circuit analysis and the sequence component decomposition. Then the voltage fluctuation mechanism at the DC side and the non-fault AC side caused by the zero-sequence voltage on the fault side is analyzed under the case without transformers. Taking the single-phase grounding fault as an example, the fluctuation formula of the DC voltage and AC voltage is deduced in detail. Based on the theoretical analysis, the zero-sequence control strategy and the sub-module improvement strategy of the modular multilevel converter (MMC) are proposed. A back-to-back SNOP simulation model is established in PSCAD/EMTDC. The simulation results verify the accuracy of the theoretical analysis, as well as the validity of zero-sequence control and MMC topology improvement strategies.