Control scheme and implement of a unified power flow controller

This paper describes the theory, the modeling technique, control scheme of unified power flow controller. Based on park transformation, it designs decoupled control scheme of UPFC shunt branch and cross-coupling control scheme of UPFC series branch in d and q-axis which eliminates the dynamic interaction between active and reactive power flow through the line. The control system of UPFC series branch consists of outer power loop, middle voltage loop and inner current loop. The response of the inner current loop is much faster than response of the outer voltage loop, which ensures that input current follows input voltage and improves dynamic performance of UPFC. As a step closer to practical application of UPFC, the paper analyzes individually the functions of STATCOM and SSSC. Experimental results verify the viability and effectiveness of applying the proposed control scheme in UPFC