The cascaded system is composed of two converters in series whose stability depends on not only the stability of the single converter

but also the minor loop gain between the two converters. We proposed a three-phase back-to-back converter cascaded system to research the impedance characteristics. Moreover

we established mathematical models

respectively

to calculate the output impedance of the rectifier with double-closed-loop control of voltage and current as well as the input impedance of the inverter with constant power control in a DC terminal. Based on mathematical models

the influences of PI parameters and output power on impedance were analyzed by Bode Plot. Meanwhile

the perturbance of PI parameters and output power on cascaded system stability were studied through Nyquist curve. Moreover

in view of the effect of parameter variation on the dc terminal impedance

adding feedforward voltage improved input impedance of inverter effectively and cascaded system stability was optimized.