Small-signal Stability Analysis Method for Hybrid AC-DC Systems with Multiple DC Buses

Hybrid AC-DC systems integrate ac bus and various dc buses as well as multiple power electronic converters, and such systems are consequently vulnerable to instability issues. In order to investigate the stabilities of hybrid AC-DC systems with multiple dc buses and identify which converters are mainly responsible for instability, this article proposes a stage-by-stage stability analysis method. The small-signal models of AC-DC and DC-DC converters are established, based upon which the whole system is separated into multiple stages according to its structure, and checked stage-by-stage. Make sure that equivalent input admittance (or impedance) of stage $k(k=2, 3, \ldots)$ has no right-half-plane poles so that it is self-stable, then consider the equivalent input admittance (or impedance) of stage $k$ as the equivalent load admittance (or impedance) of stage $k-1$ , and check the stability of stage $k-1$ ; and repeat the process until the whole system has been checked. All stage being stable can ensure the stability of overall system. The converters that are mainly responsible for instability can be clearly identified by this stage-by-stage method, and it could be beneficial for stability enhancement of the system. The effectiveness of the proposed method is verified by simulations in PSCAD/EMTDC and experiments of the control hardware in the loop (CHIL).