Transient modeling and switching logic analysis of a power-electronic-assisted OLTC based Sen transformer

A transient model of a power-electronic-assisted on-load tap-changer (POLTC) based Sen transformer (POST) and its switching logic analysis are presented in this paper. Firstly, a thyristor switch model considering the reverse recovery process (RRP) is developed. Furthermore, the transient model of POST is constructed by integrating the proposed thyristor switch model, which incorporates the RRP with the transient model of the Sen transformer (ST) considering the multi-winding coupling (MWC) effect. Secondly, the fundamental switching logic is established according to the topology of the POLTC. Thirdly, the commutation overlap angle (COA) and the short-circuit current (SCC) of the POLTC are evaluated by the proposed transient model. Finally, a method for selecting the optimum switching angle (OSA) is illustrated by analyzing the switching processes under different power factors. With the help of MATLAB, ANSYS/Simplorer, and PSCAD/EMTDC, analytical calculations and time-domain simulations have been carried out to verify the effectivenesses of the proposed transient model of POST, the suggested switching logic, and the proposed OSA selection method. The results also show that the switching process can be completed in less than one power cycle. Moreover, the magnitude of RRC ranges from 2.13 % to 7.08 % of the transmission line current. The change in the amplitude of the short-duration (safe) SCC during switching is about 5.90 % due to MWC.