A control strategy for soft open points based on adaptive voltage droop outer-loop control and sliding mode inner-loop control with feedback linearization

A soft open point (SOP) is a flexible power electronic device that can enable accurate active and reactive power flow control to balance the power flow in a distribution network. Due to the inclusion of renewable energy resources with intermittent characteristics in distribution networks, frequent power adjustments and parameter perturbations present great challenges for the control systems of SOPs. Thus, to fully use the regulatory capabilities of the converters in SOP systems and avoid reaching the voltage limits, an adaptive voltage droop control scheme for the outer control loop that considers both a power sharing factor and a DC voltage deviation factor is proposed. Subsequently, a sliding mode control strategy with feedback linearization of the modular multilevel converters (MMCs) is introduced to the inner control loop, thereby improving the robustness of the system and reducing the difficulty of parameter tuning relative to proportional-integral (PI) control. In addition, this paper investigates the zero-dynamics stability of the proposed controller. By combining the two parts of the control system, a control strategy for SOPs based on adaptive voltage droop outer-loop control and sliding mode inner-loop control with feedback linearization is proposed. Case studies of a three-port SOP in a distribution network are conducted to verify the effectiveness and efficiency of the proposed controller.