A New Control Method for Suppression of the DC Magnetic Flux Induced in Series Transformers Used for a Dynamic Voltage Restorer

This paper proposes a new suppression method of magnetic-flux deviation in series transformers used in a dynamic voltage restorer. A dc flux as well as an ac flux appears in the series transformers at the beginning of voltage-sag compensation. Since an amount of dc flux depends on a phase angle of the source voltage at the beginning of compensation, the sum of an ac flux and a dc flux may reach twice the rated flux. Therefore, the cross section of a series-transformer core should be designed twice as large as that of a generally-designed transformer, in order to avoid flux saturation. The proposed method to eliminate dc flux from the series transformer makes the compensating voltage zero for a short duration. As a result, only an ac flux appears in the series transformer, thus allowing us to use a generally-designed transformer. Experimental results obtained from a 200-V, 5-kW laboratory system are shown to verify the viability of the proposed method.