Analysis and Suppression of Circulating Current in Thyristor Rectifier With Two Cascaded Active Power Filters in Parallel

The thyristor rectifier with two cascaded active power filters in parallel has a larger capacity and a higher power quality. However, there is a circulating current between the thyristor rectifier and the cascaded active power filters. This article proposes an analysis and suppression of the circulating current. First, based on switching functions or driving pulses, equivalent circuits of circulating current loops are obtained, and the circulating current contains a low-frequency component and a high-frequency component, in which the low-frequency circulating current is caused by the source voltage and the on-off states of thyristors in the thyristor rectifier, and the high-frequency circulating current is determined by the dc-link voltage and the loop inductance. Secondly, a suppression method of the low-frequency circulating current is presented without changing the switching frequency and the switching losses through adding additional two modulation voltages to the output voltages of the dc side active power filter. The proposed suppression method prevents the generation cause of the circulating current. Then, an improved suppression performance can be obtained for the dynamic process of circulating current suppression. And, passive suppression methods of the high-frequency circulating current are also presented based on the analysis of the value of the high-frequency circulating current.