Synchronization Stability of Grid-Tied VSC With Control Limiters: Models, Solutions, and Boundaries

Abstract

Existing studies on large-signal synchronization stability of the grid-following voltage-source converters (VSCs) have been focused on the phase-locked loop (PLL). Saturation nonlinearities introduced by current or voltage limiters, which can emerge during large disturbances, could greatly complicate the instability mechanism of PLL-synchronized converters. However, limited attention has been paid to such saturation-induced instability issues in prior work. This paper investigates the large-signal synchronization stability of grid-tied VSCs with full consideration to two typical kinds of saturation nonlinearities in the VSC's control system: the current hard limiter and the voltage circular limiter. The conditions under which the VSC may enter a saturated state are revealed. The impacts of hard and circular limiters on the system transient dynamics are analyzed separately. A unified differential equation (UDE) model that combines both kinds of limiters is proposed for quantitative analysis of synchronization stability. Subsequently, the averaging-theory-based analysis approximately solves the UDE model and provides the analytical large-signal stability boundaries of the system. The impacts of various factors, including operating conditions, PLL parameters, limiter thresholds, and active/reactive current control strategies, on the synchronization stability are investigated based on the proposed model. The effectiveness of theoretical analyses is validated via time-domain simulation and control hardware-in-the-loop tests.