Effect of Unbalanced Grid Voltage Conditions on The Performance of a Variable Frequency Transformer

Power grids are interconnected for economical operation, reliable power supply, and maintaining power supply under contingencies. The grid interconnection can be realized using AC tie lines and HVDC systems. Like the HVDC link, a variable frequency transformer (VFT) is another option available for grid interconnection. The VFT is considered a better option due to its natural damping capability, considerable overloading limit, and lesser requirement of reactive power compensation. This paper analyzes the VFT system connecting two asynchronous power systems under different unbalanced voltage sag conditions. The system performance under symmetrical voltage sag is analyzed for light voltage sag (10% dip), medium voltage sag (30% dip), and severe voltage sag (80% dip). Moreover, it is also analyzed for asymmetrical voltage sag of Type-C, Type-D, and Type-F. The effect of symmetrical and asymmetrical sags on the sending end current, receiving end current, and real power are discussed in detail. The VFT system under various sag conditions is simulated in PSCAD/EMTDC environment.