Low-Frequency Oscillation Damping of Grid-Forming STATCOMs With Phase-Compensated DC-Link Voltage Synchronization

The dc-link voltage synchronization (DCVS) can be applied in the grid-forming static synchronous compensator (STATCOM) to realize dc-link voltage regulation and synchronization with the grid simultaneously. However, DCVS may lead to low frequency oscillation (LFO) of grid-forming STATCOM. To analyze and resolve the LFO issues of grid-forming STATCOM, this article built the small signal model of grid-forming STATCOM with consideration of DCVS, ac voltage and current dual close loop control (VCDC) and reactive power control (RPC). It is revealed that the LFO is mainly caused by the −180° phase characteristic of DCVS and phase lag introduced by VCDC. Besides, RPC may bring right half plan poles to the close loop and lead to subsynchronous oscillation. Based on the analysis, a lead-lag compensator is utilized and designed in this article to compensate phase lag in the close loop, which is simple but can effectively damp the LFO of grid-forming STATCOM. Besides, the proposed strategy is compared with the existing methods and shows the advantage of inertia provision to decrease the rate of change of frequency in the synchronization. Simulation and Experimental results validate the stability analysis and effectiveness of the proposed method.