Dual Active Compensation for Voltage Source Rectifiers Under Very Weak Grid Conditions

Abstract

DC subgrids consisting of modern active loads (ALs) and local dc distributed generation (DG) units are normally interfaced with the main ac grid by utilizing bidirectional voltage source converters (VSCs). Under the very weak grid (VWG) conditions, the integration of voltage-oriented controlled (VOC) VSCs in the inversion mode becomes very challenging and therefore undamped oscillations in the power and angle responses are yielded. Most of the existing works address this issue for VSCs in the inversion mode of operation. However, integration of VSCs in the rectification mode with the consideration of the outer loop controllers into the VWGs has not been reported. To fill up this gap, a state-space model of the bidirectional VSC-to-weak grid (VSC-WG) system is developed in this work with an emphasis on the rectification mode of operation. A modal-sensitivity analysis is then utilized to evaluate small-signal stability of the system, identify the dominant modes, and investigate the system states that have a major influence on these modes. The results reveal two pairs of unstable complex modes that are correlated with the dynamic interaction between the VOC-based VSCs and the VWG impedance. It is also shown that the stability margin of VSCs in the rectification mode is less than that of the inversion mode under the same VWG conditions. To enhance the integration of the VSCs in the rectification mode, a dual-active compensation (DAC) scheme is proposed to mitigate the instabilities under VWG conditions. Several time-domain simulation results are presented to verify the validity of the small-signal model and demonstrate the effectiveness of the DAC scheme under the VWG conditions. Finally, hardware-in-the-loop (HIL) real-time experimental results are presented to validate the simulation results.INDEX TERMS Dual active compensation (DAC), dynamic interaction, rectification mode, short circuit ratio (SCR), small-signal stability, voltage source converter (VSC), very weak grid (VWG).

Comprehensive Analysis and Stabilization of a B2B HVDC System Connecting Two Extremely Weak Grids Considering the Impact of Power Feedforward Compensation

“…However, this study only investigated the low-frequency instability mechanism and the CSCR for inverter or rectifier operation was not identified. In [34], it was shown that VSC stability under a weak grid is more critical under rectifier operation. A VSC is stable for a wider range of SCR under inverter operation than rectifier operation.…”