Delay-Dependent Stability Analysis of Grid-tied Voltage Source Converters

The preordained time delay in digital controlled power electronic converters has a substantial impact on the overall system's stability as it will vary the phase-frequency characteristic, further accounting for unstable system dynamics. The stability of a 3-phase grid-interfaced converter with an LCL filter is analyzed in frequency domain considering the time delay of the digitally controlled system, inner current and voltage, and outer power control loops. The indication of a low-phase margin in the minor loop gain of the Nyquist plot reveals the existence of harmonics in the system. Delay compensation and stabilization is also carried out using a Third Order Generalized Integrator (TOGI). Simulation results verify the efficacy of the proposed analysis.