This paper investigates the fundamental principles of voltage regulation and commutation failure mitigation for flexible power transformer (FPT). By introducing a voltage source converter (VSC) with adjustable amplitude is inserted between the high-voltage winding terminal and the ground of the main transformer

dynamic voltage regulation is achieved

which enhances the commutation voltage and mitigating commutation failure. The steady-state control strategy of the VSC and a compensation strategy for DC commutation failure are developed. When a voltage drop caused by grid disturbances is detected and a commutation failure occurs in the DC transmission system

the converter activates its compensation function to inject a voltage either in phase or in opposition to the transformer's high-voltage winding voltage

thereby increasing the commutation voltage

reducing the commutation angle

extending the arc extinction period and improving the overall reliability of DC transmission. Furthermore

simulation studies are conducted for a typical ± 800 kV/10

000 MW ultrahigh-voltage direct current (UHVDC) transmission project

validating the effectiveness and accuracy of the proposed control strategy.