Impedance-based Stability Analysis of Current Controlled Alternate Arm Converter in dq Frame

The alternate arm converter (AAC) is a modular converter configuration well-suited for high-voltage power trans-mission and large-scale integration of renewables. To realize energy balancing, fixed or variable overlap period control is implemented in AAC systems. However, the control of overlap period is linked to instability issues that might impede the safe operation of AAC systems, which are yet to be reported. In this paper, the stability of an AAC system is demonstrated based on measured grid input impedance and converter output impedance, in conjunction with impedance-based stability criterion in the dq frame. The interaction between harmonic sources at AC and DC side of the AAC system is analyzed to determine resonant frequencies in the AC current when any potential resonance is identified in dq frame. Novel results with respect to the impact of overlap period on the system stability are obtained by depicting and comparing the eigenvalue loci in the polar plot, which are validated by real-time simulations using RSCAD and RTDS.