Iterative approach to impedance model for small-signal stability analysis

Impedance analysis has been widely used in small-signal stability analysis of power systems and power electronic device. This study proposes an iterative approach to impedance model of individual device. The proposed method applies numerical iteration in the calculation of impedance model instead of complex mathematical derivation. It is easy to be implemented in computer programs. First, initial value of a key impedance/admittance of the target device is set. Second, an iterative algorithm is designed according to structure of the device and transfer functions of its components. Finally, the equivalent impedance is computed and stability characteristics of the whole system are analysed. The obtained model could include all elements of the target device, such as phase-locked loop and shaft system. Thus, it could also be applied to the analysis of sub-synchronous control interaction and torsional interaction. The proposed method could provide impedance contribution from different parts of the target device. This character will benefit the analysis and solution of system instability.