On system modeling and analysis using DQ-frame impedance models

The use of dq-frame impedance models for the analysis of ac power electronics systems is examined in this work. The method was originally proposed for small-signal stability analysis of shipboard power systems. In recent years, there have been increasing interests in the use of the method for stability and resonance analysis of multi-converter systems, such as wind and PV farms, as well as offshore wind with high-voltage dc (HVDC) transmission, as an alternative to the sequence impedance method. However, an important feature of the dq-frame impedance that has been neglected in the literature is its dependency on the local reference frame that is used to develop individual converter models. To apply the method to multi-converter systems will require converting individual models from their own and different dq reference frames into a common reference frame, which is virtually impossible in practice. This paper uses dq-frame impedance models of a typical two-level voltage-source converter (VSC) to explain the importance of angle dynamics introduced by the dq transformation for both the measurement and use of dq-frame impedance models. Practical difficulties in building system impedance models in a dq-frame are discussed to demonstrate the limitations of the method.