Discrete-Time Domain Modal Analysis of Oscillatory Stability of Renewables Integrated Power Systems

Modal analysis in the continuous-time state-space is a mature and effective method for analyzing the subsynchronous oscillation (SSO) problems of power systems. But it has limitations on oscillation traceability analysis due to the loss of system topology information. To address this issue, a discrete-time domain modal analysis method is developed in this paper. It represents the target system as a networked discrete-time equivalent circuit and uses the nodal method to generate the state matrix, from which the discrete-time eigenvalues in the z-plane can be solved. Based on the oscillation frequency and damping ratio defined in the discrete-time domain, the stability of the oscillatory mode can be quantitatively analyzed. Then, participation factors analysis is carried out similarly to the continuous-time method to locate the contributive components. Most importantly, the distribution coefficients of branch currents/ node voltages are finally defined and derived for in-depth analysis of the oscillatory mode. With this index, the propagation path of the oscillation in the mesh power system is expected to be identified.The proposed method has been applied in several case systems with wind farms integrated. Theoretical analysis results are highly consistent with the electromagnetic transient simulation results in PSCAD/ EMTDC, thus verifying its effectiveness.