Abstract
This paper proposes an 18th-order nonlinear dynamic model for grid-connected droop-controlled inverter with local RL load. The control system comprises of a cascaded pulse-with-modulation (PWM) driver, phased-lock loop (PLL), and saturable power, voltage, and current controllers. This paper's focus is on saturable power controllers. Saturation is modeled using algebraic sigmoid saturation functions. Lyapunov-based large-signal stability analysis method and the modified Takagi-Sugeno (TS) algorithm with computational time complexity O(n) provide an estimate of conservative regions of attraction (ROA). Under decreasing saturation levels, the time-domain converters' responses to perturbations show an increase in settling time delay and large oscillation amplitudes. The estimated ROAs shrink as the constant levels decrease, which are also captured in the phase portrait figures produced by the stability boundary test. The proposed saturable nonlinear models offer a tool to investigate the system response under large-signal disturbances to gain insight about the system behavior when limitations are accounted for.