Impedance/admittance models (IM/AM) have been widely used to analyze the stability of grid-tied power electronic devices, such as voltage source converter (VSC). However, generally it is very difficult to predict AM of VSC under realistic black/gray-box conditions. This article provides a novel method to efficiently estimate the AM of the VSC under variable operating conditions. This article extends the nearly-decoupled AM (NDAM) for sequence AM to the dq domain and simplifies the algorithm by an iteration approach. A curve (transfer function) fitting method is further proposed to obtain the AM based on the information of a few disturbance frequencies, which could expedite the algorithm greatly. Finally, the above algorithms are verified by wide simulations under different control strategies. The whole approach can be called as improved nearly-decoupled AM (INDAM), which is expected to be applied to stability analysis of power-electronic-based power systems in practicality.

impedance model,variable opearting points,VSC,system identification,transfer function identification