An approach to eliminating dc magnetic flux from the series transformer of a dynamic voltage restorer

This paper proposes a control method for eliminating dc magnetic flux from the series transformer of a dynamic voltage restorer. The method is characterized by intentionally injecting no compensating voltage during a one-sixth line cycle (=3.3 ms at 50 Hz). The control method is independent of an initial phase angle of a voltage sag, thus requiring no flux detection. The period, during which no compensating voltage is injected, is one-third as short as that of an existing method. This paper confirms the effectiveness and viability of the proposed method. A 200-V 5-kW laboratory system verifies experimentally that the proposed method brings no dc magnetic flux to the series transformer. Moreover, the proposed method brings a smaller dc voltage drop to the dc load of a three-phase diode rectifier than the existing method does in the same conditions. Index Terms--Compensating voltage, dc magnetic flux, dynamic voltage restorers (DVRs), magnetic saturation, series transformers.