Impedance-Based Stability Analysis for Interconnected Converter Systems with Open-Loop RHP Poles

Small-signal instability issues of interconnected converter systems can be addressed by the impedance-based stability analysis method, where the impedance ratio at the point of common connection of different subsystems can be regarded as the open-loop gain, and thus the stability of the system can be predicted by the Nyquist stability criterion. However, the right-half plan (RHP) poles may be present in the impedance ratio, which then prevents the direct use of Nyquist curves for defining stability margins or forbidden regions. To tackle this challenge, this paper proposes a general rule of impedance-based stability analysis with the aid of Bode plots. The method serves as a sufficient and necessary stability condition, and it can be readily used to formulate the impedance specifications graphically for various interconnected converter systems. Experimental case studies validate the correctness of the proposed method

Small-Signal Modeling of AC Power Electronic Systems: Critical Review and Unified Modeling

The harmonic state-space (HSS), the dynamic phasor (DP), and the generalized dq (GDQ) modeling are three widely used methods for small-signal analysis of ac power electronic systems. By reviewing their principles and deriving their mathematical relationships, this paper proposes a unified framework for all the three approaches. The unified modeling reveals that the linearization and transformation can be exchanged flexibly in the modeling process, and the initial phase takes a role in transforming the GDQ model into the HSS or DP model. Case studies on a three-phase voltage-source converter in unbalanced power grids are provided for validation. The relationships of three modeling methods are verified by mathematical proofs and time-domain simulations. The unified frequency-domain model is further validated through the frequency scan in experiments. Insights of the unified modeling framework and recommendations from engineering perspectives are finally discussed

Design Oriented Analysis of Control Loops Interaction in Power Synchronization-based Voltage Source Converter

Power synchronization mechanism is coupled with the dc-link, since both synchronization and dc-link dynamics depend on the active power. The active power filter used in the frequency droop is correlated with inertia emulation, which affects the dc-link voltage. Furthermore, the coupling between active and reactive power in the low-voltage grid suggests a possible interaction between dc-link and reactive power control. The strong coupling between all the control loops and the dc-link in power synchronization-based converters represents a challenge both for the stability and for the controller tuning. This paper addresses the tuning of power synchronization-based converter with consideration of all the control loops interactions. A complete converter state-space model is derived, and a step-by-step tuning procedure based on eigenvalue analysis is proposed. Experimental results are provided to demonstrate the method