Control Strategy for Non Grid Side Converter-based Variable Frequency Transformer under Dual-Side Asymmetrical Conditions

In view of the problem of uncontrollable rotor voltage of variable frequency transformer, a non grid side converter-based variable frequency transformer (NGSC-VFT) configuration is proposed. Based on this configuration, an integrated mathematical model of NGSC-VFT under dual-side symmetrical and asymmetrical grid conditions is established, and the control strategy of NGSC-VFT is further studied. The stator side series compensation converte (SCC ) aims to maintain the DC-link voltage, independently control reactive power, and eliminate the stator’s negative sequence voltage, while the objective of the rotor side SCC is to eliminate the rotor’s negative sequence voltage. The simulation results show that, with the proposed NGSC-VFT configuration, the DC-link voltage can be regulated without GSC with faster dynamic tracking effect of the active power and lower DC capacitor voltage, and the reactive power can be independently controlled. In addition, the fluctuations of torque and power under dual-side asymmetrical conditions can be further suppressed. The novel configuration improves the asymmetrical fault ride-through capability of VFT system.