A Seamless Integrated Control Method for SNOP Based on BTB-VSC

In order to adapt to the increasing renewable energy, soft normally open point (SNOP) is used to increase the flexibility of ac distribution network. The back-to-back voltage source converter (BTB-VSC) is the most commonly used SNOP topology, and the current control method is that one end controls the dc voltage and the other end controls the power flow or ac voltage. However, this strategy relies on the synchronous switching of grid breakers (BKs) and control modes. In this article, an integrated control method is proposed, which treats SNOP as an equivalent series-parallel module. In the grid-connected condition, SNOP is modeled as unified power flow controller (UPFC) for power flow control. However, in short-circuit or islanding conditions, SNOP is modeled as impedance, which can effectively limit the current during short circuit and adaptively support islanding voltage in the islanding case. The corresponding pre-synchronization strategy is designed during the grid-connected process and small-signal models are built to design the parameters. Finally, an experimental platform was established to verify the effectiveness of the control method under the conditions of uninterrupted transmission and grid-connected operation.