AC and DC Interaction Analysis and Transient Control Strategy for Soft Open Points in Unbalanced Distribution Networks

Soft open points (SOPs) can play a key role in hybrid AC/DC distribution networks (DNs) due to their flexible power control ability. However, the sequence-domain behavior of SOPs in unbalanced DNs cannot be ignored from the perspective of system stability and safety. Therefore, this paper first investigates the interaction between AC and DC components in unbalanced DNs by considering SOPs with the typical $V_{dc}$ /Q and P/Q control strategies in medium-voltage DNs. The analytical results indicate $2^{\mathrm { nd}}$ frequency oscillations in q-axis voltage, power, and DC voltage are present in unbalanced DNs. This deteriorates the transient dynamics and synchronization of SOPs and may result in the disconnection of loads and resources. To address this, a robust sequence-decomposed control strategy is proposed for SOP. To ride through unbalanced DNs, an adaptive virtual impedance-based control is activated in the positive sequence to improve restoration capability, while the negative-sequence currents are suppressed by the inner control. The DC synchronization loop is implemented by using the transferred DC power and internal energy for frequency stability enhancement. The case studies in a 40-node medium-voltage DN indicate that the proposed control enables SOPs to ride through both unbalanced load and unbalanced fault conditions in a short transient time.