Impedance Modeling of CLLC Converter and Stability Analysis In the DC System

The DC power distribution network (DC-PDN) has gained more attention in recent years. And electric vehicle (EV), act as the energy storage systems (ESS), can help enhance the reliability of the DC system. The EV connected to the grid through a three-level CLLC resonant converter, which can achieve wide output range. The DC-PDN composed of DABs is descripted and divided into two small DC systems. The solid-state transformer (SST), served as the main connection. The traditional dual active bridge converter is connected to the ordinary battery in the 400V DC system, while the CLLC is connected to the EV battery in the 750V DC system. This paper discussed the DC system stability, an important task due to the combination of different kinds of DABs. By analyzing the equivalent circuit with first harmonic approximation (FHA), the small signal model of the three-level CLLC resonant converter can be obtained. Based on the control schemes, the output impedance models of the DABs are established. And frequency response of the impedance models with different voltage levels are derived. There are three characteristics of the output impedance, inductive, capacitive and resistive. Considering the stability criterion, the stability of the system can be evaluated according to the trajectory of the Nyquist plot. Finally, the simulation results are provided to illustrate the stability of the system.