Analysis and Damping of Low-Frequency Oscillation for DC-Link Voltage-Synchronized VSCs

For grid-connected voltage-source converters (VSCs), the dc-link voltage control can be merged with the power-based-synchronization control, leading to the dc-link voltage-synchronization control. This article analyzes the low-frequency oscillations of dc-link voltage-synchronized VSCs, which is due to the impact of constant-power load/source (CPL/CPS) in the dc link. An active damping method, based on the active power-frequency feedforward (PFF) control with a notch filter, is proposed to dampen the oscillations. It is revealed that the PFF control loop exploits the phase angle dynamic to resist the frequency deviation, which can be essentially treated as an active damper against the dynamics of dc-link CPL/CPS. Furthermore, a loop-gain shaping method is developed for designing the notch filter, which dampens the synchronous oscillations that is inherent with active power dynamics. Experimental tests are performed under different grid frequencies and strengths, considering both the rectifier and inverter operation modes. The results validate the theoretical analysis and the effectiveness of the control approach.