Impedance-Based Dynamics Analysis for DC-Link Voltage-Synchronized Voltage-Source Converters

This article presents ac- and dc-side impedance-based dynamic analyses for voltage-source converters (VSCs) using dc-link voltage-synchronization control. Stability impacts of different types of dc-ink loads and sources are examined. It is found that the constant power load exhibits higher instability risk than the constant-current and constant-resistance loads at the dc link of VSC. Further, it is shown that in the rectifier mode, all the dc-link loads lead to negative resistance at the d-d channel of ac impedance matrix in the low-frequency range, whereas in the inverter mode, the dc-link constant-current and constant-power sources cause a negative resistance at the q-q channel of ac impedance matrix. These negative resistances may lead to oscillations when interacting with grid impedance. To mitigate instability risks, the active power-frequency feedforward control and the q-axis voltage-frequency feedforward control are used, respectively, in parallel with the dc-link voltage control of VSC. Experimentally measured impedance profiles and dynamics testing results corroborate the theoretical findings and the effectiveness of the control methods.