AC Stability Analysis and dq Frame Impedance Specifications in Power Electronics-Based Distributed Power Systems
This paper shows that under decoupled dq frame dynamics ? a characteristic of power electronics-based DPSs using active front-end converters ? , as well as voltage source inverters and grid-tied inverters to process power flow, the system impedances seen at ac interfaces feature a diagonally dominant structure in the dq frame. As a result, stability at these interfaces can be determined by the single-input and single-output impedance return ratios seen across the dq frame axes, namely,ZddSource, YddLoad, ZqqSource, YqqLoad, as the trajectory they describe in the complex plane matches the trajectory, or characteristic loci, of the eigenvalues of the return-ratio matrix L(s) , itself a product of the dq frame impedance and admittance matrices ZdqSource and YddLoad, which ultimately determine the stability at the boundary per generalized Nyquist criterion. The relevance of this result is threefold. First, it enables the direct use of impedances to assess stability in power electronics-based ac DPSs, thus recovering the physical insight of the original impedance-based analysis developed for dc systems. Second, it enables the use of straightforward impedance specification criteria to aid in the design and integration of these systems while discarding the conservativism of existing design criteria. Third, it identifies the prominence of the dynamic interactions resulting from the synchronization of power converters, formulating the proposed method in both dq frame axes. Experimental results with a laboratory scale power electronics-based distributed power system are used to validate the approach.