Overcurrent and its Restraining Method of PQ-Controlled Three-Phase Four-Wire Converter Under Asymmetrical Grid Fault

Three-phase four-wire converter can eliminate power oscillation by introducing the zero sequence current, but the output unbalanced currents are likely to cause overcurrent problem under asymmetrical grid fault. This paper investigates the fault current characteristics of PQ-controlled three-phase four-wire converter and proposes a new overcurrent protection method based on the maximum phase current amplitude estimation. First, the expression of the reference current with active and reactive power oscillation suppression is formulated by the instantaneous reactive power theory. Second, amplitudes of the three-phase unbalanced currents are calculated. The relationships between amplitudes and grid voltage drop depth, as well as that between amplitudes and power factor, are analyzed, respectively. Finally, the reference currents which could simultaneously suppress the fault overcurrent and power oscillation are regulated by detecting the fault phase of power grid and estimating the maximum phase current amplitude. Since the maximum phase current amplitude is obtained by only one phase, the calculation process can be very simplified. RT-LAB experimental results verify the validity of the theoretical analysis by comparisons with the existing methods.