Closed Loop Multi-Path Common Mode Circulating Currents Mitigation in Paralleled Transformerless Soft Open Points

Paralleled operation effectively increases the power capacity of soft open points (SOPs), while introducing more complex coupled common mode circulating currents (CMCCs) compared to single SOP system. For this operation, conventional methods to reduce common mode voltage (CMV) are often used to mitigate CMCCs. However, these methods are typically open-loop and passive. To overcome these drawbacks, this paper presents a unified closed loop control and modulation strategy for multi-path CMCCs mitigation in both separate and common dc bus paralleled configurations. First, the relationship between CMCCs and CMVs is conducted, and the low-order components of the CMCCs are identified as the dominant components. Then, an improved space vector modulation (SVM) with direct average CMV regulation is proposed. Furthermore, a unified closed-loop multi-path CMCCs mitigation method is developed for both paralleled configurations. In addition, a coordinated average CMV control strategy is introduced to improve the average CMV output capability. Finally, verification results demonstrate the effectiveness of the proposed method in both paralleled configurations.