The Small-Signal Stability Analysis of the Droop-Controlled Converter in Electromagnetic Timescale

Recently, the converters controlled by droop controller with phase-locked loop were observed in islanded ac microgrids. However, only the state-space-based approach was applied to investigate the stability of this converter in the previous works. Compared with the state-space-based approach, the characteristic equation approach has plenty of advantages, such as convenient stability margin analysis (phase margin, gain margin, etc.) and simple stability criterion (Routh criterion). Thus, a novel small-signal modeling approach based on characteristic equation for converter-dominated ac microgrids is proposed to assess the system low-frequency stability in this paper. First, considering zero-order holder and time delay, the small-signal characteristic equation of this converter is presented by Pade approximation and dynamic phasor model. Furthermore, the implementation and parameter design of the converters are studied under the practical considerations. Compared with the existing characteristic equation methods, the proposed approach can verify that the performance is significantly improved. Eventually, simulations and experimental results are presented, indicating that the proposed approach can assess the system low-frequency stability conveniently and accurately.