Evaluation of the Influence of Fixed-ratio Control of DC Receiving Transformer on Grid Strength and System Stability

Line commutated converter based high voltage direct current(LCC-HVDC) transmission of new energy power is highly volatile, resulting in frequent switching of the converter tap-changer. For this reason, some scholars have proposed flexible control of LCC-HVDC inverter with fixed tap-changer position, but the influence of this control on the system needs to be analyzed. Therefore, the impact of this control on grid strength and system stability is quantitatively evaluated in terms of reactive power demand, static voltage stability, and commutation failure risk in this paper. Firstly, the basic principle of flexible control with fixed tap-changer position is described, and then the difference of reactive power demand between this control and the traditional control is analyzed based on the quasi-steady-state model of the system. Secondly, the critical short circuit ratio(CSCR) is derived from the voltage stability Jacobian matrix(VSJM) of the AC/DC system under flexible control, and then the influence of this control on grid strength and the static voltage stability margin is analyzed. Finally, the difference between flexible control and traditional control in the ability to resist commutation failure is compared by time domain simulation. The results show that the flexible control increases the reactive power demand of DC converter station under steady state, but reduces the CSCR of LCC-HVDC converter, improves the grid strength, and reduces the risk of commutation failure, and therefore has a good prospect for engineering applications.